EPA-650/2-74-108
OCTOBER  1974
Environmental  Protection Technology Series
•       .- •_• •. .'. .*.*.•.v.v.*.' .   . - •

                                                  S^li-:?^^??^

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                                         EPA-650/2-74-108
WET  SCRUBBER  LIQUID  UTILIZATION
                           by

              Kunrad Semrau and Clyde L. Witham

                  Stanford Research Institute
                   333 Ravenswood Avenue
                 Menlo Park, California 94025
                   Contract No. 68-02-1079
                    ROAP No. 21ADJ-005
                 Program Element No. 1AB012
             EPA Project Officer:  Leslie E. Sparks

                 Control Systems Laboratory
             National Environmental Research Center
          Research Triangle Park, North Carolina 27711
                       Prepared for

            OFFICE OF RESEARCH AND DEVELOPMENT
           U.S. ENVIRONMENTAL PROTECTION AGENCY
                  WASHINGTON, D.C. 20460

                       October 1974

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This report has been reviewed by the Environmental Protection Agency
and approved for publication.  Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
                                  11

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                               ABSTRACT

     A bench-scale experimental investigation was made of the ways in
which energy consumption, liquid-to-gas ratio, and condensation of water
vapor affect the collection efficiency of a conventional cocurrent-
contact particulate scrubber of the orifice type.  The test aerosol was
composed of spherical particles of ammonium fluorescein with a mass-
median diameter of approximately 0.6 micron.  The collection efficiency
of the scrubber was determined and correlated as a function of the effec-
tive friction loss across the scrubber.  In the absence of condensation
or evaporation effects, the scrubber efficiency was dependent only upon
effective friction loss, with no independent influences of gas velocity,
liquid-to-gas ratio, or contactor orifice size, except possibly in the
range of very low gas velocities, liquid-to-gas ratios, and pressure
drops.  Evaporation of large amounts of water reduced collection effi-
ciencies, but scrubbing hot, humid gas with cold water to produce con-
densation significantly increased the collection efficiency.  The most
favorable results were obtained by presaturating the hot, humid gas
stream before contacting it with cold water.
     This report was submitted by SRI in fulfillment of Contract No.
68-02-1079,  under the sponsorship of the Environmental Protection Agency.
Work was completed as of July 13, 1974.
                                  iii

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                              CONTENTS


ABSTRACT	   lii

LIST OF FIGURES	    vi

LIST OF TABLES	    ix

ACKNOWLEDGMENTS  	     x

  I  SUMMARY AND CONCLUSIONS	     1

 II  RECOMMENDATIONS	     5

III  INTRODUCTION  	     7

 IV  BACKGROUND	     9

  V  METHOD OF APPROACH	    14

 VI  EXPERIMENTAL EQUIPMENT AND PROCEDURES	    19

     Bench-Scale Scrubber System 	    19
     Aerosol Generation  	    22
     Aerosol Sampling  	    28
     Aerosol Analysis	    30
     General Test Procedure	    31

 VII RESULTS AND DISCUSSION	    33
     Aerosol Generation and Characterization	    34
     Ambient Scrubbing Tests 	    41
     Adiabatic Saturation Tests  . . . .	    54
     Condensing Tests  	    62
     Vaporizing Tests  	    69
     General Discussion  	    74

     REFERENCES	    86

     GLOSSARY	    87

     CONVERSION FACTORS  	    90

                                 iv

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Contents (concluded)

APPENDIX A  LITERATURE REVIEW	      91

            REFERENCES  . „	      99

APPENDIX B  ANALYSIS OF EXPERIMENTAL DATA   	     102

APPENDIX C  POTENTIAL COMMERCIAL APPLICATION OF CONDENSATION
            SCRUBBING	     109

TECHNICAL REPORT DATA   	     116

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                                FIGURES
 1   Flowsheet of Experimental Scrubber System	20

 2   Electron Micrograph of Aerosol  	  27

 3   Scrubber Performance at Fixed Operating Conditions
     (Tests EA-26 through EA-47) 	  38

 4   Scrubber Performance Curves for Ambient Scrubbing Tests
     EA-6 through EA-22 (Aerosol Generator at 35 C)	39

 5   Scrubber Performance Data for Ambient Scrubbing Tests
     EA-48 through EA-128	46

 6   Scrubber Performance Curve for Ambient Scrubbing Tests—
     Aerosol Generation Rates 350-570 Mg/min 	  47

 7   Scrubber Performance Curve for Ambient Scrubbing Tests—
     Aerosol Generation Rates 750-1000 |jg min	48

 8   Scrubber Performance Curve for Ambient Scrubbing Tests—
     Aerosol Generation Rates 1000-1200 pg/min 	  49

 9   Scrubber Performance Curve for Ambient Scrubbing Tests—
     Aerosol Generation Rates 1200-1500 |jg/min	50

10   Scrubber Performance Curve for Ambient Scrubbing Tests—
     Aerosol Generation Rates 1500-1900 (Jg/min	51

11   Summary of Scrubber Performance Curves for Ambient
     Scrubbing Tests   	  52

12   Correlation of Scrubber Pressure Drop Data for Ambient
     Scrubbing Conditions  	  55

13   Scrubber Performance Curve for Adiabatic Saturation
                                                            o
     Scrubbing—Aerosol A.  Gas Temperatures:  Inlet = 128.9 C;
     Outlet = 71.7 C	60
                                  vi

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Figures (continued)
14   Scrubber Performance Curve for Adiabatic Saturation
                                                            o
     Scrubbing—Aerosol B.  Gas Temperatures:  Inlet = 128.9 C;
     Outlet = 71.7°C	    61

15   Scrubber Performance Curves for Condensation Scrubbing
     (Series I).  Gas Temperatures:  Inlet = 130.6 C; Outlet =
     54.4°C	    65

16   Scrubber Performance Curves for Condensation Scrubbing
     (Series II).  Gas Temperatures:  Inlet = 129.4 C; Outlet =
     37.8°C	    66

17   Scrubber Performance Curve for Condensation Scrubbing
     (Series I and II)—Aerosol A  ..'...'	    67

18   Scrubber Performance Curve for Condensation Scrubbing
     (Series I and II)—Aerosol B	    68
19   Scrubber Performance Curve for Condensation Scrubbing
     (Series III).  Gas Temperatures:  Inlet = 80.0 C; Outlet =
     37.8°C	    71
20   Scrubber Performance Curves for Condensation Scrubbing of
                                      o        o
     Gases at Inlet Temperatures of 80  and 130 C—Aerosol B . .    73

21   Scrubber Performance Curve for Vaporization Scrubbing.
     Gas Temperatures:  Inlet = 125.6 C; Outlet = 58.3 C . . . .    75

22   Summary of Scrubber Performance Curves for Adiabatic
     Saturation and Condensation Scrubbing—Aerosol A  	    76

23   Summary of Scrubber Performance Curves for Adiabatic
     Saturation, Condensation,  and Vaporization Scrubbing—
     Aerosol B	    77

24   Performance of Condensation Scrubbers—Literature Data
     (Figure 12, Ref. 6A).  Range of Present Investigation
     Added	    98

25   Analysis of Residual Deviations—Ambient Scrubbing Tests—
     Aerosol Generation Rates 750-1000 pg/min  	 . .   103

                                 vii

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Figures (concluded)
26   Analysis of Residual Deviations — Ambient Scrubbing Tests —
     Aerosol Generation Rates  1000-1200 (j,g/min .........   104

27   Analysis of Residual Deviations — Ambient Scrubbing Tests —
     Aerosol Generation Rates  1200-1500 Vg/min .........   105
28   Analysis of Residual Deviations — Ambient Scrubbing Tests —
     Aerosol Generation Rates  1500-1900 M-g/min .........   106

29   Analysis of Residual Deviations — Adiabatic Saturation
     Scrubbing — Aerosol A ....................   108
                                  viii

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                              TABLES
 1   Summary of Average  Scrubber Test Conditions  	   16

 2   Scrubber Performance  Data for Ambient Scrubbing Tests—
                                    o
     Aerosol Generator Temperature 35 C	   35

 3   Scrubber Performance  Data for Ambient Scrubbing Tests at
     Fixed Scrubber Operating  Conditions	   37

 4   Scrubber Performance  Data for Ambient Scrubbing Tests  ...   43

 5   Scrubber Pressure Drop Data for Ambient  Scrubbing Tests  .  .   56

 6   Scrubber Performance  Data for Adiabatic  Saturation
     Scrubbing Tests 	   59

 7   Scrubber Performance  Data for Condensation Scrubbing Tests—
     Series I	   63

 8   Scrubber Performance  Data for Condensation Scrubbing Tests—
     Series II	   64

 9   Scrubber Performance  Data for Condensation Scrubbing Tests—
     Series III	   70

10   Scrubber Performance  Data for Vaporization Scrubbing Tests.   73

11   Selected Comparisons  of Scrubber  Performances at Different
     Operating Conditions   	   78
                                ix

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                          ACKNOWLEDGMENTS







     Mr. Charles E. Lapple, Senior Scientist, provided major assistance



in the development of the aerosol generation system and the design and



procurement of the bench-scale scrubber system, and valuable advice on



the conduct of the investigation.  Mr. William W. Kerlin, Senior Chem-



ical Engineering Technician, provided valuable assistance throughout



the design, procurement, and assembly of the system, and in the sub-



sequent conduct of the experimental work.





     The authors wish to express their appreciation for the assistance



and cooperation of the Project Officer, Dr. Leslie E. Sparks of the



Particulate Technology Section, Control Systems Division of the



Environmental Protection Agency.

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                              SECTION I
                      SUMMARY AND CONCLUSIONS

     An experimental investigation was made under laboratory conditions
of the performance of a bench-scale orifice scrubber that was generally
a replica of many commercial scrubbers of the orifice or modified
venturi types.  Studies were made of the effects of gas pressure drop,
gas velocity, liquid-to-gas ratio, and vaporization and condensation of
water vapor.  Tests were made at both ambient and elevated temperatures.
     All the tests were made with nominally the same test aerosol,
which was prepared by nebulizing a solution of ammonium flucrescein and
drying the mist particles in a stream of heated air.  This procedure
produced a spherical, nonhygroscopic aerosol composed almost entirely of
submicron particles and having a mass median particle size on the order
of 0.6 micron.  The performances of the scrubber under various operating
conditions were compared on the basis of gas pressure drop (or, more
strictly, of "effective friction loss"), and the influences—if any—of
other operating variables were inferred from differences in the relation-
ship of collection efficiency to the gas pressure drop.
     In fact, the particle-size characteristics of the aerosol generated
varied sufficiently to introduce another variable into the experiments,
complicating the interpretation of the results.  Fortunately, it was
found that a mass generation rate of the aerosol generator was a reason-
ably good correlating factor for the apparent particle-size character-
istics of the aerosol, and it was therefore possible to correlate the
scrubber collection efficiency data and to interpret their significance.
     It appears that the functional relationship between efficiency
(actually expressed in terms of transfer units) and the effective friction
                                  1

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loss is probably a more sensitive indicator of relative aerosol


particle-size characteristics than any of the available methods for


measurement of particle size in the submicron region.



     Scrubber efficiency tests were carried out under the following


conditions:



     »  Scrubbing of ambient air with water at ambient temperatures


        Scrubbing of hot, humid air (32-35% by volume of water vapor)


        with hot water at the temperature of adiabatic saturation of


        the gas stream.  This condition simulated the common practice


        of scrubbing gases with recirculated water, resulting in


        essentially adiabatic saturation of the gas streams



     .  Scrubbing of hot, humid air (32-35% by volume of water vapor)

                                                             o        o
        with cold water to reach final gas temperatures of 54 C and 38 C,


        resulting in condensation of most of the water vapor initially


        present



     .  Scrubbing of hot, humid, nearly saturated air (32-35% by volume


        of water vapor) with cold water to reach a final gas temperature


        of 38°C



     .  Scrubbing of hot, dry air with hot water, resulting in flashing


        and evaporation of part of the water and humidification of the


        air



     Results of the foregoing tests yielded the following conclusions:



     .  Over very wide ranges of gas velocity, liquid-to-gas ratio, and


        gas pressure drop, the collection efficiency of the scrubber was


        dependent only upon effective friction loss (essentially equal


        to the gas pressure drop) and independent of gas velocity and


        liquid-to-gas ratio (except as these latter factors affected


        gas pressure drop). (This conclusion did not apply in cases where

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vaporization or condensation effects were involved.) There may still
be some independent effects of gas velocity or liquid-to-gas ratio
at very low values of these variables, but if so, the operational
region is not one of much practical significance.
    The size of the orifice gas/liquid contactor had no effect on
    collection efficiency,  independent of effective friction loss.
•   Condensation scrubbing of the test aerosol produced significant
    but not very large increases in collection efficiency at a
    given effective friction loss.  The results were qualitatively
    similar but quantitatively much less than indicated by earlier
    pilot-plant studies of condensation scrubbing of Kraft recovery
    furnace fume under similar operating conditions.
                                   o
•   Cooling the outlet gas below 54 C did not increase the condensa-
    tion scrubbing effect measurably, presumably because only a
    small additional quantity of water vapor was condensed•
    Adjusting the inlet gas to a. nearly saturated condition before
    contacting with cold water markedly improved the effectiveness
    of condensation scrubbing.
•   Vaporization of scrubbing water tended to reduce scrubber col-
    lection efficiency, particularly in the lower range of effective
    friction loss.
•   In practical cases, condensation collection is not an alternative
    to conventional high-energy scrubbing, but is a supplement that
    can reduce the scrubbing energy requirements to a greater or
    lesser degree.  Promoting diffusional mass transfer (such as
    vapor condensation) in practical equipment also requires energy
    for gas/liquid contacting.

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Because of the requirements for water cooling and possible gas
humidification, the net saving of energy in condensation scrub-
bing may not often outweigh the other associated costs.
Condensation scrubbing may be, and probably is, warranted under
the following conditions:
(a)  Where the waste gas already has a high water vapor content
and there is use for low-level heat in the associated process
system (e.g., Kraft pulp mills).
(b)  Where the gas must be cooled and dehumidified anyway, for
process reasons (e.g., cleaning of blast furnace gas; purifica-
tion and conditioning of feed gas to contact sulfuric acid
plants).

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                              SECTION II





                            RECOMMENDATIONS







     We recommend that priority be given to the following topics for



extended study of scrubber performance characteristics:





     *  A study of the performance characteristics of the orifice



        scrubber with aerosols of particle sizes ranging from about



        2.0 microns down to about 0.1 or 0.2 micron.   The aerosols



        can be produced by the same general technique used in the



        present investigation:  producing a mist of ammonium fluo-



        rescein solution and drying the droplets to leave solid



        particles.  Even though the aerosols will not be monodis-



        perse, the median particle sizes can be varied over a



        fairly wide range by changing the concentration of the solu-



        tion.  The practical lower particle size is set  by the



        coincident lowering of the quantity of aerosol.   This pro-



        gram should provide badly needed information about the



        relationship of scrubber collection efficiency to both



        contacting power (effective friction loss) and aerosol



        particle size.  We expect that it should also elucidate,



        or help to elucidate, some currently unexplained phenomena



        that sometimes appear in the scrubbing of very fine



        aerosols—most specifically, a rapid falling off of collec-



        tion efficiency in the low contacting power range.





     .  An investigation of the comparative performances of dif-



        ferent classes of gas/liquid contactors as functions of



        contacting power.  The same test aerosol, or aerosols,



        should be used throughout.

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     Particular attention should be given to two classes of

contactors:  (1)  those deriving contacting power from the

energy of the liquid stream, and (2), those deriving contact-

ing power from a mechanically driven rotor.  The tests should

aim at establishing unequivocally whether contacting power

so derived is fully equivalent to that derived from the gas

stream in the form of friction loss (or pressure drop).

     It is possible that with mechanically driven rotors, a

larger part of the total power input can be effective as con-

tacting power than in the other classes of contactors.  This

should afford saving in total power input if contacting power

is fully equivalent for all the types of contactors.


If a field demonstration of condensation scrubbing in a potential

commercial application is desired, we recommend that a study be

made of the scrubbing of Kraft recovery furnace fume.  The test
                                                            3
scrubber unit should have a gas flow capacity of about 850 m /hr
       o                             •>            3
(500 ft /min) or, at most, of 1700 m /hr  (1000 ft /min), which

is small enough to permit ease of experimental operation, or

modification without inordinate costs or time requirements.  The

fan capacity should be sufficient to handle scrubber pressure

drops up to 1800 mm WC (70  inches WC).  Heat exchangers should

be included for cooling recirculated scrubbing water.


     Two scrubber units should be provided in series, each with

gas/liquid contactor and liquid entrainment separator.  The first

unit should be designed for low-energy contacting, primarily to

cool and saturate the gas by  scrubbing with recirculated water.

The second scrubber should  be equipped for high-energy contacting

with cold water or cold recirculated brine.  The scrubbers should

be of the variable-orifice  type with cyclone entrainment  separa-

tors.  However, there should  be provisions  for  substituting a

pipeline contactor for the  variable  orifice.


                           6

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                             SECTION III

                             INTRODUCTION


GENERAL

     In the operation of particulate wet scrubbers, the collection of
progressively finer particles and an increase in the efficiency of col-
lection of particles of any size are both associated with increased

power consumption.  Current trends in the requirements for control of

particulate air pollutants emphasize both the collection of fine parti-
cles (primarily those in the submicron range) and higher efficiencies
in the collection of particles of all sizes.  It can therefore be anti-
cipated that future particulate scrubber installations will have increas-
ing power requirements.  Hence, it is important to investigate any
possible means for minimizing those power requirements.


OBJECTIVES

     The broad objective of this investigation was to study the ways in
which energy consumption, liquid-to-gas ratio, and condensation of water
vapor affect the collection efficiency of particulate scrubbers.  The
original contract called for emphasis on the effects of liquid rate and
energy consumption.  However, at the request of EPA, the emphasis of the
program was shifted to the effects of water vapor condensation on the
collection efficiency of conventional scrubbing equipment.

     The revised scope of work included the following tasks:

     (1)  Review and assessment of the published literature and avail-
          able unpublished information on the effects of liquid-to-gas
          ratio; method of water injection, energy consumption, and
          water condensation and evaporation on the efficiency of
          particulate scrubbers.
                                  7

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(2)  Conduct of a bench-scale experimental study of particle
     collection of a conventional coccurrent-contact particu-
     late scrubber of the modified venturi or orifice type.
     Emphasis was to be placed on enhancing the collection effi-
     ciency by use of condensation techniques.

(3)  Recommendation of a potentially suitable commercial appli-
     cation for condensation-scrubbing techniques,  using basic-
     ally conventional scrubbers of the venturi or similar types.

(4)  Development of a plan for a pilot-plant study of the scrub-
     bing process in the recommended commercial application,
     including preparation of a process design, an experimental
     program, and a cost estimate.
                             8

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                             SECTION IV





                             BACKGROUND








     It is now widely,  though not universally,  accepted that an increase



in the efficiency of particulate scrubbing is associated with an in-



crease in power consumption,  and that the power consumption associated



with attainment of a given collection efficiency increases  with decrease



in the particle size of the particulate matter.  Recognition and accept-



ance of these facts, and use of them in design  procedures,  have formed



the basis for the recent greatly increased use  of scrubbers for col-



lection of fine particles.  Relatively high power consumption is thus



characteristic of scrubbers,  although they may  exhibit definite ad-



vantages in other respects for numerous applications.   As requirements



grow for higher collection efficiencies on finer aerosols,  the associated



increases in scrubber power consumption are a growing economic burden,



and it becomes increasingly important to seek ways to decrease the power



consumption if such can be found.





     The number and diversity of scrubber designs is such as to defy



any entirely self-consistent or truly descriptive system of classifi-



cation, and it is even more difficult to try to describe the various



devices in terms of performance mechanisms. However, since power con-



sumption is a major characteristic of scrubbers, it is possible to



characterize various units according to the way in which power is



applied to the scrubbing  process.





     There are basically three ways of applying power in scrubbers:



     (1)  From the energy of the gas stream



     (2)  From the energy of the liquid stream



     (3)  From a mechanically driven rotor.

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     When the energy is derived from the gas stream, the amount consumed
takes the form of gas pressure drop, and must be supplied by a fan.
Energy may be supplied to a liquid stream by three methods:  (1)  apply-
ing pressure to the liquid stream, (2)  applying to the liquid the
energy of a compressed gas (e.g., compressed air, stream), and (3) apply-
ing to the liquid energy from a mechanically driven spray generator.
The net effect in each of the three cases is that kinetic energy is
imparted to the liquid stream being injected into the gas stream being
scrubbed.
     In a mechanical scrubber, both the gas and liquid streams pass
through the rotating member.  This member, which may be a modified
fan, frequently serves also to provide draft for the scrubber.  In such
case, the power input to the drive shaft can be considered to have
two components, only one of which is related to scrubber efficiency.

     It should be noted that the power consumption that is directly
related to scrubber efficiency is not the gross power input to the
scrubber, which may include power consumption due to motor, pump, or
fan inefficiencies, or power input that leads to increases in the kin-
etic energy or pressure head of the gas stream.  The criterion of
scrubber efficiency is that portion of the power consumption that is
consumed in the gas-liquid contacting process and is ultimately con-
verted to heat through turbulence;  this is termed "contacting power".
(A review of the pertinent background literature is presented in Appen-
dix A.)
     The "contacting power rule" proposes that energy applied to the
scrubbing process by any of the three basic methods is equivalent,
and that the efficiency obtained on a given aerosol at a given contact-
ing power should be essentially independent of the scrubber design and
of the way, or combination of ways, in which the contacting power is
applied.   A very substantial body of information that supports this
                                 10

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hypothesis has now been accumulated, particularly for the types  of



scrubbers in which contacting power takes the form of gas pressure



drop.  There is considerable evidence to indicate that contacting power



derived from spray nozzles is also equivalent.  The equivalence  of



contacting power from mechanically driven rotors has not been fully



demonstrated, but is mostly inferred by analogy.





     A scrubber design that could attain significantly higher effi-



ciency at a given power consumption than other competitive designs  would



have a clear advantage.  However, no such advantages for specific de-



vices have so far been clearly demonstrated, although they are frequently



claimed.  A large part of the remaining uncertainty about the validity



of such claims results from the lack of test results for different  de-



vices taken under precisely the same conditions, which are currently



the only available unequivocal basis for comparison.





     One of the uncertainties in scrubber performance is the degree



to which liquid-to-gas ratio may affect efficiency, independent  of



contacting power.  It must be assumed that some lower liquid rate will



exist below which scrubbing efficiency will be lost.  Some investi-



gators have reported that liquid-to-gas ratio has important—or, at



least, significant—effects on the performance of venturi scrubbers.



Others have found little or no independent effect.  (See Appendix A.)





     In the case of spray scrubbers using hydraulic spray nozzles,



the presumption is that a given contacting power and collection effi-



ciency may be attained alternatively by use of a relatively small



volume of spray at high pressure, or of a larger volume of spray at



correspondingly lower pressure.  The possible limitations of this



hypothesis have not been systematically studied, although such data



as are available tend to confirm it.





     The possibilities of channeling of liquid and gas flows appear to





                                  11

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be much more serious in the case of some spray scrubbers, than in the



case of the venturi, orifice, or pipeline types.  In the latter three



types, good liquid dispersion appears generally to be inherent, except



possibly at very low liquid rates or gas velocities.  In the case of



spray scrubbers, it is possible to visualize arrangements in which



channeling effects might be severe.





     The actual mechanisms through which power consumption leads to



particle collection can, for the present, only be inferred in very



general terms.  It seems reasonable that the actual deposition of



particles involves primarily the collision of the particles with



spray droplets formed in the scrubber, and that the primary deposition



is inertial.  The correlation of collection efficiency with power dis-



sipation strongly indicates that fluid turbulence governs the droplet



and particle motions that present the opportunities for collisions.



The observation that the contacting power-collection efficiency relatioi



ship is so similar even for radically different types of devices suggest



in addition that the scrubbing process is essentially the same in all



of the scrubbers.  The dissipation of the power is evidently a gross,



overall measure of the atomization of the liquid and the turbulent



circulation of drops and particles in the gas stream.





     The experience to date suggests that there is no way to increase



scrubber collection efficiency significantly at given levels of con-



tacting power so long as only the inertial mechanism can be used.  It



may be practical under some circumstances to increase the scrubber col-



lection efficiency by superimposing additional particle collection



mechanisms on the process through use of condensation phenomena.  One



such phenomenon is the condensation of moisture on the particles as



nuclei, which will increase the effective particle size and, hence,



the effectiveness of inertial deposition.  The other known phenomenon



in this category is Stefan flow.  Both of these phenomena have been




                                 12

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demonstrated (see Appendix A).  The major uncertainty has been in



the Quantitative increase in collection efficiency attainable by their



use, particularly in conventional scrubbing equipment.  Improvements



in the efficiency of conventional equipment resulting from condensation



effects have been reported fairly frequently on a qualitative basis.



However, it has not often, been demonstrated clearly that part or all



of the reported effects may not have resulted from other, coincident



factors.





     It has also been known from theoretical considerations and labora-



tory experiments that where a liquid is evaporating at a surface, the



resulting Stefan flow tends to inhibit particle deposition.  Hence,



there should be a corresponding tendency to reduction of collection



efficiency in scrubbers in which the scrubbing liquid is vaporizing.



Apparently there has been no effort made prior to the present investi-



gation to determine how important this phenomenon may be in practical



scrubbing systems.
                                 13

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                             SECTION V





                         METHOD OF APPROACH









     The basic method of approach used in this investigation was em-



pirical, although it was guided qualitatively by theoretical considera-



tions.  The particulate scrubbing process is obviously extremely com-



plex, and presenting an adequate and detailed picture of it is a highly



intractable problem from both theoretical and experimental viewpoints.



A number of workers have attempted to develop practical models of



scrubber operation, but have necessarily been obliged to simplify



the models and to introduce assumptions and estimates that are of more



or less questionable validity.  In general, it has not yet been possible



to subject the models to adequately critical experimental verification.





     So far, the correlation of the experimentally determined collectiol



efficiency as a function of contacting power has proved to be the most



reliable as well as the most practical approach to design of actual



installations.  There has been no satisfactory experimental determinatii




of the relationship between collection efficiency and contacting power



for particles of specific, discrete sizes.  Attempts to do this have



been largely frustrated by the problems of measuring the size and



quantity of particulates in a mixture of particles of different sizes,



and by the difficulties of generating particles of discrete and control*



led sizes.  In fact, the collection efficiency/contacting power correlai



curve for a given scrubber appears to be a much more sensitive indicator



of the particle size and particle-size distribution of an aerosol—or



of changes in these quantities—than are the available methods for measii



ing low-micron and submicron particles.  Of course, the scrubber does



not "measure" the particle size.  However, if scrubber performance



curves can be established experimentally for discrete particle sizes,




                                14

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it should then be possible to make a reasonable quantitative or semi-


quantitative interpretation of the size characteristics of an aerosol


from the corresponding scrubber performance curve.



     In the present investigation, the approach was to use a single


aerosol in tests of a bench-scale scrubber over a range of gas pressure


drops at various controlled operating conditions (see Table 1).  For


each such operating condition, a correlation (graphical) was developed


between the collection efficiency (actually expressed as the number of


transfer units) and the contacting power. (See Glossary.)  From compari-


son of the different correlation curves, the incremental effects of


condensation and vaporization were deduced.



     In the general case, contacting power is best expressed in terms

                                              4
of power per unit of volumetric gas flow rate.   However, in many


scrubbers of the venturi or orifice types, including the one used in


this investigation, the contacting power is essentially equivalent to


the gas pressure drop, and can be expressed conveniently in the same


units (e.g., millimeters of water).  However, the contacting power is


not in principle (or in fact, in some cases)  necessarily equal to

                                          4
the total pressure drop across a scrubber.   To emphasize the distinction,


the quantity equivalent to contacting power has been termed the "effective


friction loss" in this report, even though it has been taken to be equal


to the gas pressure drop and has been expressed in the same units;  mm


WG (millimeters of water column).



     The average operating conditions used for the various scrubber


test series are summarized in Table 1.  In the "Ambient" tests, ambient


air was in most cases scrubbed with water from the supply line, without


recirculation.   In a few tests, the scrubbing water was recirculated.


The entering air was actually heated slightly above the ambient tempera-


ture by its mixing with the small flow of heated air containing the


aerosol (see Section VI).  When the scrubbing water was recirculated,

                                  15

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                  Table 1
SUMMARY OF AVERAGE SCRUBBER TEST CONDITIONS
Operating
Condition
Ambient
Adiabatic
saturation
Condensing
I
II
III
Vaporizing
Gas Temperatures
<°C and °F)
Inlet
25-40° C
77-1 04° F
128. 9° C
26 4° F
130. 6° C
267° F
129. 4° C
265° F
80.0°C
176°F
125. 6° C
258° F
Outlet
19-38° C
67-100°F
71.7°C
161° F
54.4°C
130° F
37.8°C
100°F
37 . 8° C
100°F
58. 3° C
137°F
Water Temperatures
(°C and °F)
Inlet
11-39°C
51-102°F
73. 9° C
165°F
13.3°C
56° F
12.2°C
54° F
12. 8° C
55° F
73. 3° C
164° F
Outlet
11-38° C
51-100°F
73.9°C
165° F
54.4°C
130° F
33 . 9° C
93° F
33.9°C
93° F
58,3°C
137°F
Gas Moisture Content
(g/g dry air)
In
0.006-0.010
0.317
0.327
0.315
0.322
0.007
Out
0.008-0.043
0.369
0.117
0.0404
0.0481
0.149

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it also was heated by contact with the warmer air as well as by the
turbulence generated in the water circulating pump.  The gas and water
temperatures and the gas moisture contents are shown as ranges rather
than as averages.
     For the test series designated as "Adiabatic Saturation", "Condensing ,
and "Vaporizing",  the temperatures and gas moisture contents shown are
averages.  The temperatures were actually read to 1° F, and the Fahren-
heit temperatures  were converted into Centigrade values to the nearest
0.1° C.  The tabulated Centigrade temperatures do not imply a precision
of measurements.

     In the Adiabatic Saturation tests, an attempt was made to feed the
scrubbing water at a temperature as nearly equal as practical to the
adiabatic saturation temperature of the entering gas stream;  the scrub-
bing process was thus intended to approximate an adiabatic humidification
(and saturation) of the gas stream, simulating the scrubbing of the gas
with recirculated water.  The inlet water vapor content was selected
as representative of that in a considerable number of waste gases of
industrial importance.
     In the Condensing tests, Series I and II, the inlet gas was nominally
the same as in the Adiabatic Saturation tests, but it was scrubbed with
sufficient cold water to condense water vapor and bring the exit gas
temperature down to desired levels.  In Series III, the procedure was
the same except that the inlet gas was only of the order of 5  C
above  the adiabatic saturation temperature, so that the tests approx-
imated scrubbing of a presaturated gas.
     In the Vaporizing test series, the hot, dry inlet gas was scrubbed
with hot water.  Upon injection into the hot gas, some of the water
flashed to vapor because of its level of sensible heat content.  More
of it was thereafter evaporated by the hot gas.  The Vaporizing tests

                                 17

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were designed to indicate the possible extent to which particle col-



lection may be inhibited by Stefan flow effects.
                                 18

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                              SECTION VI



                EXPERIMENTAL EQUIPMENT AND PROCEDURES





BENCH-SCALE SCRUBBER SYSTEM




      The bench-scale experimental  scrubber  system used in the  investiga-


 tion is portrayed schematically in Figure 1.   This system was  designed


 to accommodate inlet gas flows  of  up to about 4200 liters/min  (150 CFM)

                      o    o    o    o
 at temperatures of 20 -150  (70 -300 F),  and  with gas  pressure drops


 across the scrubber of up to  3000  mm WC (120  inches WC).   The  scrubber


 itself consisted  of a simple  orifice contactor followed by a cyclone


 entrainment separator.  The orifice contactor was mounted in a horizontal


 plane between flanges in a vertical line through which the gas and scrub-


 bing water flowed downward.  The gas and water leaving the orifice


 contactor passed  through a mitre elbow before flowing  horizontally into


 the cyclone separator.  Interchangeable orifice plates were used to vary


 the diameter of the contacting  orifice.   The  scrubbing water was intro-


 duced through a single tap in the  wall  of the inlet line, two  pipe-


 diameters above the orifice plate.



      The scrubber design thus followed on a small-scale the general con-


 figuration currently being used for most commercial scrubbers  of the


 orifice or modified venturi types.



      Gas was drawn through the  system by a  rotary positive-displacement


 blower of the Roots type.  The  main air flow  through the system was drawn


 directly from the room, metered, heated by  a  direct-fired gas  burner,


 humidified by steam injection,  then drawn through the  scrubber.   The  air


 flow through the  system was controlled by bleeding air from the room


 directly to the inlet of the  positive displacement blower.   The air bypass


 intake was fitted with a filter-silencer and  the blower with a discharge



                                  19

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          Air
      Natural
§       Ga,
                             Natural
                              Gas
                              Inlet Gas
                              Sample
                    Main
                   Air Flow
                     fit
[fll
                   Steam
                     I


                    T
     BURNER
                           Aerotol
                  BURNER    Orving
                             Air
               AEROSOL
               DRIER AND
               MIXER
                  Compressed
                    Air
              L
                                               COARSE
                                               SPRAY
                                               TRAP
                           ULTRASONIC
                           NEBULIZER
                                               Aerosol
                                               Sweep
                                               Air
                                          Compressed
                                             Air
                            Contacting
                              Orifice
 MANOMETERS
w
 Air
Bypass


ng
— 1 1 —
!» !
r i
\ 2
i ^ — *^*
M"M Outlet
^-^ Gas
Sample
CLONE (


                                                                                                                               _ Room
                                                                                                                               ' Air
                                                                                          To
                                                                                          Stack
                                                                            BLOWER
               r—XI——-
                                                         Steam
                                                                                                   STEAM
                                                                                                   EJECTOR
                                                                                          To        Water
                                                                                        Recycle       Bypass
                                                                                                                                           SA-2745-22
                                         FIGURE 1     FLOWSHEET OF EXPERIMENTAL  SCRUBBER  SYSTEM

-------
silencer.  However, the main air stream entering the system was not


filtered; since the test aerosol was measured by a specific fluorometric


method, the presence of atmospheric dust in the room air did not inter-


fere with the determinations of scrubber efficiency.



     The scrubbing water system was so arranged that water could be sent


through the system in a single pass; in such a case the water supply


line pressure was usually sufficient to overcome line resistance without


use of the pump.  Alternatively, the scrubbing water could be recircu-


lated by use of the water holding tank and the pump.  When hot water


was used in scrubbing, the steam ejector could be used to heat the water


whether it was used in a single pass or was recirculated.  The water in


the holding tank could be heated by recirculation through the steam


ejector and also by another steam siphon not indicated in Figure 1.



     The scrubber and cyclone were elevated so that the water could be


discharged by gravity at the highest gas pressure drops used in the


tests.  The water was discharged through a seal loop either directly to


the sewer or to the holding tank for recycle.



     Contactor orifices of three different diameters were used in the


investigation:  2.54, 3.81, and 4.45 cm (1.0, 1.5,  and 1.75 inches).


The line in which the contactor orifice was located was made from 2j-inch


stainless steel tubing.  The cyclone separator, which was originally


built and used in an earlier investigation, was 45.7 cm  (18 inches) in


diameter, with an inlet 7.6 cm  (3 inches) in diameter and an outlet


10.2 cm  (4 inches) in diameter.  It was actually larger than necessary


to handle the gas flows used in the present investigation.  The general

                                                     3
design was the same as that used by Lapple and Kamack  for a semiworks


scrubber unit.  The main inlet air header, including the orifice meter


section was made from 15.2-cra  (6-inch)  stainless steel tubing.



    The main air flow was measured with a standard-design orifice meter


equipped with square-edged orifices and fitted with radius taps located



                                  21

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1  pipe diameter upstream and \ pipe diameter downstream of the orifice

plate).  Steam flows were measured with critical-flow orifice meters,

with Bourdon gages used to measure the steam pressures.  The water flowi

were measured with rotameters of appropriate ranges.


     System pressures and pressure drops were measured with U-tube mam*

meters filled with xylene or mercury, as demanded by the ranges of pres-

sures involved.  The gas and water temperatures were measured with dial

thermometers of the bimetal type.



AEROSOL GENERATION


     The aerosol generation system is also indicated schematically in

Figure 1.  The original cloud of droplets was generated from a solution

with an ultrasonic nebulizer.  A small flow of air swept the mist upward

through the chimney above the nebulization chamber and through a trap fd

removal of any residual coarse spray droplets.  The fine mist then rose

straight upward through a section of 1-inch IPS pipe and entered the lot

end of a conical diffuser section which was located concentrically with!

the 15.2-cm (6-inch) riser that carried the main airflow upward to the

scrubber.  Another small flow of heated drying air also entered the lowd

end of the conical diffuser section through the annular space around thd

pipe carrying the mist.  The mist and heated air mixed with the diffuse*

section, where the water was evaporated from the droplets, leaving solid

aerosol particles.  The resulting aerosol stream emerging from the uppei

end of the diffuser was then mixed with the main air stream, which flowe

through the annular space between the upper end of the diffuser and the

6-inch riser.


     The sweep air to the nebulizer was taken from a compressed air line
                                                 3
and metered to a constant 14.2 liters/min (0.5 ft /min) in all tests

with a critical-flow orifice.   The aerosol drying air was also metered
                                  22

-------
with a critical-flow orifice to a constant rate of 84.9 liters/rain

     3
(3 ft /rain).  It was heated by a small direct-fired gas burner to a

                                   o
temperature in the range of 170-200 C.  The aerosol generation system


was developed in separate but parallel and continuing effort supported


by Stanford Research Institute.  An ultrasonic type of nebulizer was


chosen to produce the original mist of solution droplets, because previous


experience at Stanford Research Institute and elsewhere  had indicated


that the ultrasonic device produced a narrower band of droplet sizes


(with a median diameter of about 6 microns) than other types of atomizers,


and should therefore produce a more uniform aerosol.  The ultrasonic


nebulizer is used commercially to prepare mists of therapeutic agents in

                                                       i fi
a particle-size range for effective inhalation therapy. '   It has also


been employed to prepare sodium chloride aerosols for testing respirator

        2
filters,  a use comparable to that in the present investigation.



     The basic element of the generator system was a commercial ultra-


sonic nebulizer (DeVilbiss Model 35A).  The original plastic nebuliza-


tion chamber and chimney were replaced by one made from stainless steel.


The height of the chimney was increased from that of the original to


provide a greater space in which oversized spray droplets could disengage


from the air stream and settle back into the liquid sump.  The solution


to be nebulized was supplied to the nebulization chamber from an over-


head reservoir fitted with a gravity feed system so vented as to maintain


a constant liquid level in the chamber (not indicated in Figure 1).



     Reports by other users and some preliminary experiments had indica-


ted that increases in the temperature of the liquid being nebulized also


increased the output of the generator.  After some of the preliminary


experiments with the scrubber had been carried out, it was decided to


stabilize the temperature of the liquid at a temperature somewhat above


any room temperatures expected to be encountered.  The metal chimney of


the nebulizer was therefore wrapped with heating tape, and the current to



                                 23

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the tape was regulated by a controller operating off the output of a
thermistor immersed in the liquid in the nebulizer.  The liquid tempera-
ture was set at 35 C.  It was then found that operating at the higher
fixed temperature neither stabilized the output of the generator nor
clearly increased it  (see Section VII).  The heating system was there-
fore removed and the generator was operated thereafter at room temperature,
     In another attempt to stabilize the generator output, a harmonically
compensated voltage regulator was inserted in the electrical power supply
line to the generator.  The voltage regulator was continued in use, but
apparently had no effect in stabilizing the generator output.
     Despite the attempts to stabilize generator output by fixing the
temperature and the level of the liquid in the generator and by regula-
ting the voltage of the electrical supply, fluctuations and shifts in
the generator output continued.  The nature and consequences of these
changes are discussed below in Section VII.
     The maximum liquid nebulization rate for the generator was about
    3
3 cm /min.  Since it was desired to produce a well-dispersed aerosol
with a mass-median size of about 0.5 - 0.8 micron, it was necessary to
use a dilute solution of the aerosol material.  Hence, the concentration
of the aerosol in the scrubber air stream was necessarily low.  This
condition was desirable for minimizing flocculation of the aerosol, but
also required a sensitive method of analysis if the necessity of long
sampling tiroes to collect large samples was to be avoided.  For this
reason, a fluorescent dye was highly advantageous.  Reports in the
literature Indicated that ammonium fluorescein produced a spherical,
                                             7 8
nonhygroscopic, and readily analyzed aerosol.  '
     In a preliminary investigation a 10% solution of ammonium fluores-
cein was nebulized in the ultrasonic nebulizer and the droplets were
dried in an air stream to produce dry particles which were collected on

                                 24

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a membrane filter and examined under a light microscope.  The particles
were easily discerned to be spherical, and showed no sign of hygro-
scopicity when left exposed to atmospheric air.  The particle sizes fell
mostly in the range of 1 to 5 microns with an apparent mass-median
diameter of about 3 microns (as judged by visual observation in the
microscope).  From these observations it was judged that nebulizing a
0.1% ammonium fluorescein solution should produce an aerosol with a mass-
median diameter of about 0.6 - 0.8 micron that would be suitable for the
scrubber testing program.
     The ammonium fluorescein solution was first prepared at a concentra-
tion of 1% by weight.  The appropriate amount of fluorescein (acid form)
was weighed out and dissolved in a quantity of ammonium hydroxide solu-
tion equivalent to four moles of ammonium ion per mole of fluorescein
(acid form).  Water was added to make up the solution to the appropriate
total weight (and volume).  This stock solution was stored in glass and
in darkness.  The 0.1% solution for experimental use was made up from
the stock solution by dilution.
     One possible factor in the variation in aerosol generation rates,
which would also affect particle size, is concentration of the solution
in the nebulizer by evaporation.  The sweep air to the nebulizer, taken
from a compressed air source and not humidified after expansion, was dry.
Although it had been expected that most of the moisture evaporated into
the sweep air would be absorbed from the aerosol mist being formed, it
was later found that the concentration of ammonium fluorescein in the
nebulizer solution did increase, demonstrating that at least part of the
water was evaporated from the liquid remaining in the nebulizer.  It was
estimated that the concentration in the nebulizer should reach an equi-
librium level about 30% above that of the feed solution, assuming that
the liquid remained at room temperature.  When analyses revealed that
the nebulizer liquid was being concentrated, some values approximating
                                  25

-------
 the estimated equilibrium concentration were  obtained.   Thereafter,
 each day's operation was  started with fresh feed  solution in  the
 nebulizer.
      Some of the variation in  the aerosol  generation  rate may have been
 related to changes in the solution concentration,  but the pattern of
 the variations did not correspond to the reasonable course of gradual
 concentration of the solution.   A series of measurements of aerosol
 generation rate was also  made  with known nebulizer solution concentra-
 tions ranging up to 16% above  the nominal  feed  concentration.   (This
 work was done as part of  the Institute-supported  program on aerosol
 generation.)  The observed variations in generation rate were much
 greater than could be accounted  for by the changes in the nebulizer
 solution concentration and did not,  in fact,  appear to be significantly
 correlated with them.
      Overall,  it does not appear likely that  variations  in the concen-
 tration of the nebulizer  solution were responsible for more than a minor
 part of the observed variations  in the aerosol  generation rate.
      Near the  end of the  investigation, when  the generator was producing
 "Aerosol  B"  (see Section  VII), samples were taken  of  the  aerosol col-
 lected  on Nuclepore  membrane filters with  0.4-micron  pores.  A gold-
 palladium film (less than 100 Angstrom units  thick) was vacuum deposited
 on the  filter  to provide  a conducting  surface,  and photomicrographs were
 obtained  with  a  scanning  electron microscope.    Photomicrographs of one
 sample  taken at magnifications of 5,000 X  and 10,000 X are  shown in
 Figure  2.  These  show  a range of particle  diameters from about 0.2 to
 0.8 micron, with  a probable mass median of about 0.6 micron.  Examina-
 tion of the same membrane under a light microscope, covering a very much
 larger  field,  indicated the presence of a  small number of  larger Particles
up to perhaps 1.5 microns in diameter.  These few large particles appa-
 rently contributed only a small fraction to the total size distribution
by mass.
                                 26

-------
                    Id)
                    (b)
                                   SA-2 746-20

FIGURE 2  ELECTRON MICROGRAPH OF AEROSOL
                    27

-------
     At  the  time  that  the  photomicrographs  of Figure  2 were taken,  it
 was  known  that  the particle  size  of the  aerosol  produced by the genera-
 tor  was  variable, but  the  variations were probably  too small to be
 readily  detectable by  any  of the  available  methods  of size measurement.
 They were, however,  revealed by the performance  of  the scrubber.  The
 photomicrographs  do  illustrate that the  particle size of the aerosol was
 in the expected range.

 AEROSOL  SAMPLING
     The aerosol was sampled at the inlet and outlet of the scrubber
 (see Figure  1), using  25-mm  Nuclepore membrane filters in Nuclepore
 holders.   The membrane holders were made of polycarbonate plastic,  for
 which the  upper limit  of operating temperature was  about 140 C (285°F).
 The  inlet  piece of the filter holder was machined out to expose almost
 the  entire effective diameter of  the membrane directly to the incoming
 gas.  The  holders were turned to  face directly into the gas stream
 being sampled,  but no  effort was made to sample  isokinetically, because
 the  aerosol was too  small  for sampling to be appreciably affected by
 inertial effects.
     Initially, tests were made with two filter  discs placed in series
 in each holder, with both  discs being analyzed.  Tests were made with
membranes with  1.0,  0.8, 0.6, and 0.4-micron pores.  The standard pro-
 cedure finally  adopted was to use a single  disc with 0.6-micron pores to
provide an acceptable combination of efficiency  and resistance to flow.
     The filter holders were immersed in the gas stream to be sampled
so that the filter would be at the gas stream temperature and water vapor
condensation would be avoided until the gas sample had passed through
the filter.  The gas stream was then drawn through a condenser for removal
of water vapor and cooled to ambient temperature before being metered in
                                 28

-------
a critical flow orifice. Vacuum was supplied by a mechanical vacuum
pump.  Sampling rates in the range of 2-10 liters/min were used, with
the highest rates at the scrubber outlet where the aerosol concentra-
tion was lowest.
     The precision of aerosol sampling was checked by operating two
samplers in parallel at each of the sampling points.  At the scrubber
outlet, 11 pairs of runs were made; the deviations between the paired
samples ranged from 1.1 to 8.9%, and averaged 3.5%.  At the scrubber
inlet, only three pairs of runs were made; the deviations ranged from
1.6 to 12.5% and averaged 6.1%.  Another four paired samples were taken
with one sampler operated at each position, inlet and outlet, and with
no water fed to the scrubber.  In two of these paired runs, the measured
outlet aerosol concentration was higher than the inlet concentration.
The deviations ranged from 1.9 to 5.6%, with an average of 2.9%.
Evidently, the scrubber, operated dry, did not collect any of the
aerosol, at least within the precision of measurement.
     Subsequently, material balance runs were made as part of an Insti-
tute-supported program for development and evaluation of the aerosol
generation system.  The inlet and outlet air streams were sampled in the
usual manner, and integrated samples of the outlet scrubbing water were
also obtained.  The quantity of inlet aerosol measured by sampling the
inlet air was compared with the sum of the quantities leaving the scrub-
ber in the exit air and collected in the water.  This comparison involved
not only the precision of sampling but that of the measurement of the air
and water flow rates as well.  Positive and negative deviations were
about equal in number, ranging from 0.9 to 16.7% with an average of 5.0%.
     From these results, it appears that the average error in aerosol
sampling did not exceed 5 to 6% and that individual errors seldom exceeded
10%.
                                  29

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 AEROSOL  ANALYSIS

      Fluorlmetric  analyses  of  the  ammonium  fluorescein  aerosol material

 were  made with  an  Aminco-Bowraan  spectrophotofluorometer,  with a  xenon

 lamp  (Catalog Numbers  4-8202 and 4-8202B).  The unit was  equipped with

 monochrotneters  for both exciting and emitted light.   Instrument  settings
 used  in  the  analyses were as follows:

             Excitation wave length—482  nm
          .   Emission wave length—540 nm
          .   Primary slits—full  open
          .   Secondary  slits—5 mm
             Photomultiplier slit—0.1 mm
          .   Meter  multiplier setting—to give transmittance  reading
             between 30 and  9O*;
          .   Filter used—Corning 3-69 (to eliminate interferences
             with excitation wave length)

      The transmittance of the ammonium fluorescein solutions followed
                                            _7      3           —10      3
 Beers' Law at concentrations between 4 x 10  gn/cra  and  1 x 10    gm/cm .

 The fluorescence was markedly a  function of pH, increasing by a  factor

 of 25 as the pH was increased from 4 to  10.  Samples were dissolved in

 a solution of ammonium hydroxide having  an  initial pH of  11.

                                      3
      Samples for analysis (1.5 to  2 cm ) were placed in a clean, dry

 cuvette  (fused quartz) of rectangular cross section.

      It was  found  that a single  washing  of  a N'uclepore filter was suffi-

 cient to remove the sample  deposit.  Therefore, filters were simply
                              3
 immersed and agitated  in 10 cm   of the ammonium hydroxide solution having

 a pH  of 11,   If the resulting concentration of ammonium fluorescein was

 above the preferred range for analysis,   the sample was diluted 10 to 1

with  the ammonium  hydroxide solution of  pH  11.

      Experience with replicated  analyses indicated that the error in

 analysis did not exceed 1"  except  for the very smallest samples, for

which the error was somewhat larger.
                                  30

-------
GENERAL TEST PROCEDURE
     Scrubber performance tests were commonly made in groups, with the
scrubber and aerosol generator being allowed to operate continuously
between tests while the scrubber operating conditions were being changed.
The aerosol generator commonly required a warm-up period of about
15-20 minutes before the aerosol output (observed in the glass spray
trap) appeared visually to reach its maximum level.  Except in a few
exploratory tests, the generator was adjusted to give its maximum output.
     In general, it was relatively easy to adjust the scrubber operating
conditions except when it was desired to operate with the inlet gas near
saturation with water vapor.  When hot water was used in scrubbing, the
water was frequently used on a once-through basis, with the temperature
being adjusted by addition of steam at the ejector.  However, control of
the water temperature was somewhat more satisfactory when the water was
recirculated and its temperature was adjusted at the holding tank.
     Before the start of a test, the sampling filter holders were pre-
heated, then inserted into the gas stream at the inlet and outlet of
the scrubber and permitted to come up to temperature with the filter
faces turned out of the gas stream.  The holders were then turned to face
the filters into the gas stream, and the gas flow was started and stopped
simultaneously.  Once the procedure was reduced to a routine, the sampling
period was usually five minutes.  During this period, the quantities of
aerosol collected on the filters were mostly in the range of 0.1 to 15 ^g,
which could be analyzed with precision.  Aerosol concentrations in the
inlet gas to the scrubber ranged from about 0.2 to 0.8 ^/liter of dry
air (25°C and 1 atm.), depending on the gas flow in the scrubber and on
the generation rate.  All gas samples were referred to the selected
standard gas conditions:  25 C,  1 atm. pressure and the dry basis.  The
corresponding aerosol concentrations measured at the inlet and outlet
of the scrubber were used to calculate the collection efficiency.
                                  31

-------
      During the period of the sampling run, readings were made of the
 flowmeters and system temperatures and pressures.
      Analyses of the ammonium fluorescein in the scrubbing water were
 apparently interfered with by residual chlorine from the water purifi-
 cation.   Recirculating water through the scrubber with a flow of ambient
 air apparently stripped out the interfering chlorine.   Hence,  where some
 check runs were made at Ambient scrubbing conditions to determine
 material balances  on the ammonium fluorescein,  a supply of water that
 had been air-stripped to remove chlorine was first accumulated in the
 holding  tank and then pumped through the scrubber in a single  pass during
 the scrubbing tests.

      Except in a few instances,  no direct measurement  was  made of the
 water vapor content  of  the  air  at  the  outlet of  the  scrubber.   If the
 gas was  sampled for  a sufficient period,  the moisture  content  could be
 determined  from the  amount  of condensate  collected in  the  condenser.
 However, the  standard sampling  period was too short  for collection of a
 volume of condensate  that could  be measured  with  reasonable precision.
 Hence, the  outlet gas moisture  content was calculated by assuming satura-
 tion at  the outlet gas temperature if the outlet water temperature was
 equal to or greater than the outlet gas temperature; if the outlet water
was colder than the outlet gas,  the gas was assumed to have a water vapor
 partial pressure equal to the vapor pressure of the water at its outlet
temperature.  Allowance was made in the calculation for the total pres-
sure (subatmospheric) at the outlet of the scrubber.
                                 32

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                              SECTION VII
                         RESULTS AND DISCUSSION

     As  soon as  the  experimental  equipment had undergone a preliminary
 shakedown and appeared  to be  in suitable operating condition, a series
 of  "screening" tests was made to  provide a quick  indication of the range
 of  collection efficiencies  that might be obtained under adiabatic satura-
 tion, condensing,  and vaporizing  conditions.  The scrubber was operated
 at  a single  level  of effective friction loss throughout the tests,
 although setting the other  operating conditions made  it necessary to use
 varying liquid-to-gas ratios  in the different experiments.  Two or three
 replicate tests were made at  each operating condition.  (The results are
 presented under  the  appropriate headings below.)
     Although the  aerosol generation rates observed during the tests
 varied by as  much  as +35% of  the  mean value, the agreement between
 replicate tests was  generally very good or excellent, and it therefore
 was considered that  the  variations in aerosol generator output probably
 did not coincide with any substantial variations in the particle size of
 the aerosol.   However,  it was recognized that such variations might take
 Place.  As additional data  were obtained, difficulties in replicating
 results appeared,  and strong  evidence of variations in the aerosol particle
 size did emerge.   Since  the interpretation of scrubber performance at
 various conditions was dependent  on constancy of the aerosol character-
 istics,  it became  necessary to devote a major effort to attempts to
 stabilize the generator operation (see Section VI) and characterize the
aerosol.  In  the end, it was  not  actually found possible to prevent the
variations in the aerosol characteristics.  Nevertheless,  the observation
that these variations were correlated with the aerosol generation rate

                                  33

-------
made it possible to select comparable collection efficiency data for


different scrubber operating conditions, and thus to interpret the


effect of the operating conditions on scrubber performance.  However,


it was necessary to obtain many more data to establish the correlations


than was originally anticipated.





AEROSOL GENERATION AND CHARACTERIZATION



     The measures taken in the effort to stabilize the performance of


the aerosol generator (see Section VI)  apparently had essentially no


effect.  There was a preliminary indication that raising the tempera-

                                         o
ture of the liquid in the nebulizer to  35 C increased the aerosol


generation rate.  Later, it became evident that the generation varied


over about the same, wide range whether the nebulizer was operated at

  o                                      o
35 C or at ambient temperatures (about  25 C).  Surveyed over a period of


days, the generation rate appeared to follow no consistent or systematic


trends.  The rate showed apparently random short-term variations about


the average value, and within periods of from several hours to a day or


more, shifts in the average generation  rate appeared.


                                                              o
     After the aerosol generator was equipped to operate at 35 C, a


series of scrubber efficiency tests was made at ambient operating con-


ditions (see Table 2), all In the course of one day.  Tests were made at


a number of liquid and gas rates and gas pressure drops.  All the aerosol


generation rates were high, ranging from 1747 to 4097 jjg/min., the latter


value being the highest observed during the entire investigation.  Never


on. any other occasion were the generation rates uniformly so high, nor


wore so many very high individual values observed.  The collection effi-


ciency data, studied in the relation to the effective friction loss,


showed a wide scatter.
                                  34

-------
                                                                                 Table 2



                                                          SCRUBBER PERFORMANCE DATA FOR AMBIENT SCRUBBING TESTS

                                                                  (Aerosol Generator Temperature 35 C)
Test
Code
No.
EA
6
7
8
9
10
11
12
13
14
15
16
17
IS
19
20
21
22
Aerosol
Generation
Rate
(tig/min)

1864
1747
1784
2743
2634
1898
1926
1889
2803
4097
2348
1988
2799
2138
3444
1749
1783
Contactor
Orifice
Size
(cm)

3.81
















Inlet Gas
Flow Rate
(std llters/min)

1140
1130
1130
1130
2110
2120
2110
2110
2940
2770
2770
2770
2770
2770
3530
3520
3S20
Gas Moisture
Content
(g/g dry air)
In

0.01b
















Out

0.008
0.010
0.013
0.008
0.008
0.008
0.008
0.009
0.010
0.009
0.008
0.008
0.008
0.008
0.008
0.009
0.010
Gas
Temperature
<°0
In

33.3
32 C
32 c
32 c
32 c
29.4
38.9
32.8
31.1
31.7
32.2
30.6
30,6
30.6
29.4
30.0
29.4
Out

25.6
27.8
28.9
26.1
22.8
22.2
22.2
23.3
26.7
23.3
22.8
22.2
21.1
20.6
20.6
22.2
22.8
Water
Temperature
C°0
In

10.6
14.4
18.9
11.7
11.7
10.6
11.1
11.1
13.9
13.3
11.7
11.7
11.1
10.6
11.1
12.2
13.9
Out

10.6
15.0
18.3
11.7
11.7
10.6
11.1
12.2
15.0
12.2
11.7
11.7
10.6
10.6
11.7
13.3
15.0
Gas
Velocity
in Orifice*
(m/sec )

16.7
16.5
16.5
16.5
30,8
31.0
30.8
30.8
43.0
40.5
40.5
40.5
40.5
40.5
51.6
51.5
51.5
Liquid-to-
Gas Ratio
/liters\
(•*)

25.5
42.8
42.8
57.2
30.7
19.1
9.53
5.40
1.29
2.05
5.60
5.52
10.3
16.9
4.50
1.56
0.699
Effective
Friction
Loss
(mm WC)

274
535
535
810
1480
870
394
272
259
279
535
535
810
1380
843
429
325
Collection
Efficiency
(%)

56.3
69.7
72.3
90.63
96.95
88.9
65.6
52.1
65.2
78.3
82.9
76.5
92.01
96.19
93.90
68.9
57.3
Transfer
Units
(Nt)

0.828
1.19
1.28
2.37
3.49
2.20
1.07
0.736
1.06
1.53
1.77
1.45
2.53
3.27
2.80
1.17
0.851
w
en
          Calculated at standard gas conditions.

         b
          Estimated.
          Estimated.  Inlet scrubbing water apparently splashed  on  thermometer.

-------
     The scrubber system was then used to study changes in the aerosol.
The scrubber was operated on ambient air at a fixed set of conditions—
air rate, water rate, and gas pressure drop.  Repeated determinations
were made of the collection efficiency, with the aerosol generator
operated at ambient temperatures and at 35 C (see Table 3).  The measured
aerosol generation rates ranged from 662 to 1705 pg/min., and the collec-
tion efficiencies from 67.3 to 77.5% (1.12 to 1.49 transfer units).  The
plot of transfer units vs. aerosol generation rate (Figure 3) suggested
a trend to increasing collection efficiency with increase in the aerosol
generation rate, which inferred that the increase in the generation rate
coincided with an increase in the average particle size of the aerosol.
     The data shown in Figure 3, together with other observations in the
                                                         o
same period, indicated that operating  the generator at 35 C neither
stabilized its output nor clearly increased the output above that obtained
during operation at ambient temperature.  Therefore, use of the higher
temperature was abandoned, and all subsequent operation of the generator
was carried on at ambient temperatures.
     Subsequent studies of scrubber performance at various operating
conditions (see below) developed additional strong evidence that the
variations in aerosol generation rate  did coincide with changes in the
average particle size of the aerosol,  and possibly—if not probably—in
the particle-size distribution as well.  With the aid of this background,
the data from the early tests (Table 2) were re-examined and plotted in
groups according to relatively narrow  ranges of aerosol generation rate
(see Figure 4).  When treated in this  manner, the data show correlations
that are actually quite striking in their excellence and are in agreement
with earlier experience that indicated that scrubber performance data for
a consistent aerosol can yield very close correlations.  The performance
curves for the generation rates of 1864-2138 and 2348-2803 M€/min are
generally consistent with the trends in the performance data later obtained
                                  36

-------
                           Table 3

       SCRUBBER PERFORMANCE DATA FOR AMBIENT SCRUBBING
        TESTS AT FIXED SCRUBBER OPERATING CONDITIONS3
Test
Group
No,
1



2



3

4



5



6



Test
Code
No.
EA
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
Aerosol
Generator
Temperature
(°C)
Ambient



35



35

Ambient



Ambient



35



Aerosol
Generation
Rate
(jig/min)
710
662
688
715
1041
1024
1051
1029
1149
1168
1002
1023
1044
1276
1178
1136
1085
1128
1462
1705
1521
1514
Collection
Efficiency
(%)
70.4
67.3
67.5
68.4
69.5
68.9
69.3
69.3
69.9
70.4
70.9
70.2
68.8
74.6
77.5
72.5
71.5
71.9
71.8
77.0
74.0
73.9
Transfer
Units
(Nt>
1.22
1.12
1.12
1.15
1,19
1.17
1.18
1.18
1.20
1.22
1.24
1.21
1.17
1.37
1.49
1.29
1.25
1.27
1.27
1.47
1.35
1.35
Contactor orifice diameter  =  3.81 cm.
Gas rate  =  2800 std. liters/min.
Water rate  =  15.1 liters/min.
Liquid-to-gas ratio  =  5.39 liters/Sm
Effective friction loss  =  520 mm WC.
                              37

-------
1.8


1.7


1.6

z"
I
1
v> 1.5
2
D
£T
in
& 1.4
1
u_
O
NUMBER
w
1.2


1.1
1.0
4(
1 1 1 1 1 I !
TEST GROUP GENERATOR
NUMBER TEMPERATURE (°C)
~ O 1 Ambient ~~
A 4 Ambient
D 5 Ambient
— • 2 and 3 35 —
• 6 35

~ O
•

— —
A

Q ~
a •
A °
3P *
o
00

i i i i I i i





















X) 600 800 1000 1200 1400 1600 1800 2000
AEROSOL GENERATION RATE — pg/min
SA-2745-1
FIGURE 3  SCRUBBER PERFORMANCE AT FIXED OPERATING CONDITIONS
          (Tests EA-26 through EA-47)
                              38

-------
   10
S
z
  2348-2803
                                                 —— Q  3444-4O97
                        t     1   I   1   1  I  I  I
     100
              1000
EFFECTIVE FRICTION LOSS — mm WC
    torooo
SA-2745-23
    FIGURE 4   SCRUBBER PERFORMANCE CURVES FOR  AMBIENT SCRUBBING TESTS
                EA-6 THROUGH EA-22
                (Aerosol generator at 35°CJ
                                        39

-------
in ambient scrubbing tests with lower aerosol generation rates.  Since
only two data points were obtained in the range 3444-4097 pg/min, the
performance curve they yield is merely indicative, but is consistent with
the other observed trends.
     Over the remaining course of the investigation, the generator output
at first continued to show not only random variations over relatively
short periods, but shifts in the average rate from day to day, or some-
times from one period of several hours to another.  The corresponding
scrubber efficiency data showed a wide scatter.  However, the generator
gradually reached a point at which the band of variation of the average
generation rates became relatively narrow, although random fluctuations
about the average values continued.  There was a corresponding reduction
of the scatter in the scrubber efficiency data.  When the generator set-
tled into a mode producing about 750 to 1100 ug/rain of aerosol, scrubber
efficiency data of generally high consistency were obtained.  The corres-
ponding aerosol is termed "Aerosol A" for convenience.
     Still later, the nebulizer apparently malfunctioned briefly, and
the aerosol generation rate thereafter dropped abruptly to about half
that observed previously, or about 350 to 700 pLg/roin.  The "new" aerosol,
designated "Aerosol B," again gave scrubber efficiencies that were con-
sistent, but that were systematically lower than those obtained with
Aerosol A.  These results, combined with further analysis of the results
obtained earlier, indicated clearly that the nature of the aerosol, as
well as the generation rate, had been shifting.
     Some additional observations, made with Institute support since
termination of this experimental program, Indicated a continuing drop in
the average aerosol generation rate.  It appears likely that this behavior
was associated with deterioration of the transducer, or aging of the trans-
ducer crystal, of the nebulizer.  Replacement of the transducer restored
                                  40

-------
 the  aerosol  generation rate  to  the  previous  "normal" range  of about 900
 to 1300  pg/min,

AMBIENT SCRUBBING TESTS
     The  tests of scrubbing  ambient air with ambient water  had  two major
 objectives:
      (1)  to establish the contacting power/efficiency relationship for
          the test aerosol in the absence of appreciable condensation or
          evaporation  of water
      (2)  To determine the degree to which liquid-to-gas ratio  might
          have an influence  on  collection efficiency, independent of
          the contacting power.
     Scrubbing at ambient temperatures involves only small  absolute
changes in the humidities of the air.  Strictly, the attainment of
 "neutral" conditions with respect to vaporization and condensation phe-
nomena requires that saturated  gas  should be scrubbed with water at the
same temperature.  In  practice, this is experimentally difficult to carry
out without  the danger of bringing  in other, undesired phenomena.  If the
incoming gas stream is saturated, even a small temperature drop may result
in some condensation of water vapor and possible growth of  the  test aerosol
upstream of  the gas/liquid contacting section, with consequent  changes in
the contacting power/efficiency relationship.
                                         o        o
     Attempts to make  neutral tests  at 54 C  and 74 C were made  during the
initial series of "screening" tests.  The inlet gases were actually brought
in at temperatures several degrees  above the saturation temperature to
avoid the possibilities of premature condensation.  For those few "neutral"
tests that were not actually abortive, the efficiencies obtained were in
the same range as those obtained at adiabatic saturation and other operating
conditions,  but presented no consistent,  interpretable picture.  In retro-
spect,  it appears that the results  reflected variations in the  aerosol,

                                  41

-------
which were not fully appreciated at the time.  Ambient scrubbing gave
results that were at least similar to those obtained with adiabatic
saturation scrubbing with a comparable aerosol.  There was no clear indi-
cation that temperature as such had any appreciable effect on scrubber
                                         o
performance over the range of about 25-80 C in which most of the scrubbing
action presumably took place under the various scrubbing test conditions.
     Had the liquid-to-gas ratio been found to have independent effects
on scrubber efficiency, it would have made interpretation of test results
especially difficult, since securing different amounts of condensation
involved use of different liquid-to-gas ratios.  The first ambient scrub-
bing tests were made systematically to display the effects of orifice
contactor size, orifice gas velocity, and liquid-to-gas ratio (see
Table 4).  The evidence of aerosol particle-size variations (previously
discussed) appeared speedily in the form of a wide scatter in the col-
lection efficiency data (see Figure 5).  Although the data points showed
a general correlation of transfer units with effective function loss, the
straight-line relationship suggested by previous experience was not in
evidence.  However, there was no indication—within the uncertainty posed
by the scatter in the data—that there were independent effects of gas
velocity or liquid-to-gas ratio on the number of transfer units.
     When evidence mounted that the apparent variations in aerosol particle
size were not purely random, but were correlated with the aerosol genera-
tion rate, the ambient scrubbing data of Table 4 were grouped according
to convenient ranges of aerosol generation rate and replotted (Figures 6
through 10). (The correlation curves from Figures 6-10 are presented
together without data points in Figure 11, for purposes of comparison.)
The ranges of generation rate were chosen on a largely arbitrary basis,
and slightly different choices might produce somewhat better correlations.
Nevertheless,  it is evident that the data are generally quite well ration-
alized by this device.  The poorest correlation is given by the data for
                                  42

-------
                                                                                      Table 4





                                                             SCRUBBER  PERFORMANCE DATA FOR AMBIENT SCRUBBING TESTS
Test
Code
No.
EA
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
Aerosol
Generation
Rate

1370
1366
1503
1497
1396
846
1358
1292
1280
1687
1148
1179
1569
1397
1503
1642
1440
1234
1063
1316
857
885
1771
1029
1115
1080
1004
978
1008
1317
Contactor
Orifice
Size
(cm)

3.81



















2.54







Inlet Gas
Flow Rate
(std liters/in in)

1230


1740


2650



3640



1790
2640
3580
2650
1390
1770
850
864
1220


1770

2390
Gas Moisture
Content
(g/g dry sir)
In

O.009


0.009


0.010



0.010



O.008


0.010


0.010

0.010


O.010

0.010
Out

O.OO8
0.008
0.008
O.008
0.009
0.008
0.008
0.009
0.010
0.009
0.008
0.008
0.012
0.010
0.009
0.008
0.008
0.008
0.009
0.010
0.009
0.009
0.008
0.008
0.008
0.008
0.009
0.008
0.009
0.010
Gas
Temperature
In

33.3
31.1
32 b
32 b
31 b
31 b
31.1
32.2
35.0
31.7
26.7
26.1
27.8
27.8
27.8
26.7
2S.6
23.9
25.6
27.8
31.1
30.6
27.8
28 b
29 b
28.9
34.4
32.2
31.7
30.0
Out

25.6
25.6
25.0
25.6
25 .0
23.3
23.3
26.1
27.2
22.8
21.1
20.6
23.9
22.8
20.0
19.4
22.2
20.6
20.6
23.9
23.9
23.3
26.1
25.0
23.9
23.3
23.9
23.3
22.8
22.8
Water
Temperature
In

11.7
11.1
11.1
12.8
12.2
11.1
11.1
11.7
16.7
11.7
11.1
10.6
18.3
16.7
11.1
11.1
10.6
10.6
11.1
13.3
11.7
11.7
11.1
11.1
11.1
11.1
11.7
11.1
11.7
12.8
Out

11.1
11.1
11.1
12.2
12.2
11.1
11.1
11.7
14.4
11.7
10.6
10.6
16.7
14.4
11.7
11.1
10.6
11.1
12.2
14.4
11.7
11.7
11.1
11.1
11.1
11.1
11.7
11.1
11.7
14.4
Gas
Velocity
in Orifice
(m/sec)

18.0


25.4


38.7



53.2



26.2
38.6
52.6
38.7
20.3
25.9
28.0
28.4
40.5
41.0
4O.8
58.2

78.7
Liquid-to-
Gas Ratio
fl±tera\
( So.3 J

18.7
26.8
36.6
52.8
37.4
25.9
19.0
8.62
1.43
5.66
12.5
17.0
0.52
1.04
4.12
9.07
25.3
12.4
2.63
1.43
10.9
8.6
38.8
53.2
37.1
26,8
15.7
14.8
8,47
1,59
Effective
Friction
Loss
(mm WC)

228
345
516
833
1217
787
516
239
191
442
864
1176
282
345
767
965
802
870
517
191
151
223
1092
1707
2680
1722
963
2300
1247
1087
Collection
Efficiency
(5)

44.3
57.1
72.9
85.6
93.7
78.4
72.7
40.3
35.6
77.7
88.1
93.5
60.9
67.5
88.9
93.1
86.7
88.9
74.1
60.3
12.0
36.9
95.7
97.2
99.24
97.4
90.7
98.7
94.3
94.1
Transfer
Units
(N't)

0.585
0.846
1.30
1.94
2.77
1.53
1.30
0.515
0.441
1.50
2.13
2.74
0.940
1.12
2.20
2.67
2.02
2.20
1.35
0.923
0.304
0.458
3.14
3.57
4.88
3.66
2.37
4.31
2.87
2.83
CO

-------
Table 4 (Continued)
Test
Code
No.
EA
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
1O4
105
106
107
108
109
Aerosol
Generation
Rate

860
1053
1160
1150
1146
1159
976
1487
1165
1176
1212
1243
1278
52 5d
1866
1795
1585
1817
1675
1674
1044
984
1021
937
847
965
967
973
1052
797
789
766
Contactor
Orifice
Size

-------
                                                                          Table 4 (Concluded)
Test
Code
No,
EA
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
Aerosol
Generation
Rate
(yg/min)

765
762
784
766
789
831
868
824
374
355
404
421
443
398
553
541
548
565
510
Contactor
Orifice
Size
(cm)

2.54



3.81









2.54




Inlet Gas
Flow Rate
(std liters/rain)

548
607
767
789
1473
1713
1967
2235
2031
2031
2031
2036
2025
3294
1150
1150
1153
1153
2111
Gas Moisture
Content
(g/g dry air)
In

0.01C



0.01C



0.01C





0.006




Out

0.009
O.OO9
0.010
0.010
O.010
0.010
0.010
0.010
0.010
0.010
0.010
0.009
0.009
O.O10
0.009
0.009
O.OO9
0.009
0.010
Gas
Temperature
C°0
In

38.9
40.0
39.4
38.9
32.8
32.2
32.2
31.7
28.9
28.9
28.3
27.2
25.0
27.2
31.1
31.7
25.0
27 b
29.4
Out

26.1
26.1
26.1
26.1
26.1
26.1
26.1
26.1
24.4
24.4
22.2
21.7
21.7
21.1
23.3
23.3
23.3
23.9
22.8
Water
Temperature
In

12.2
12.2
12.8
12.8
12.8
12.8
12.8
12.8
13.9
—
12.8
12.8
12.2
12.8
12.8
12.8
12.2
12.2
13.3
Out

12.8
12.8
13.3
13.3
13.3
13.3
13.3
13.3
14.4
13.3
13.3
12.8
12.2
13.3
12.8
12.8
12.2
12.2
13.3
Gas
Velocity
in Orifice
(m/sec)

18.0
19.9
25.2
26.0
21.6
25.0
28.8
32.7
29.7
29.7
29.7
29.8
29.6
48.1
37.8
37.8
37.9
37.9
69.4
Liquid-to-
Gaa Ratio
/liters\
V Sffi3 /

10.4
9.36
7.41
7.20
3.84
3.32
2.89
2.54
1.87
5.59
8.29
11.8
16.0
4.54
16.9
22.5
28.1
38.2
1.80
Effective
Friction
Loss
(mm WC)

81.3
97.7
137
161
77.8
103
130
164
115
216
293
444
588
572
897
1271
1587
2440
866
Collection
Efficiency

17.8
11.6
17.8
21.7
6.2
12.0
16.7
22.9
6.5
21.7
33.4
52.1
62.5
64.0
85.5
91.60
94.82
98.25
84.8
Transfer
Units
(Nt>

0.196
0.123
0.196
0.244
O.O64
0.127
0.180
0.260
O.O67
0.244
0.406
0.736
0.982
1.02
1.93
2.48
2.96
4.04
1.88
en
           Calculated at standard gas conditions.

          b
           Estimated.  Inlet water apparently splashed on thermometer.


           Estimated.
           Generator output deliberately attenuated.

-------
QC

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1 1 1
10 100 1000 10,0i
EFFECTIVE FRICTION LOSS — mm
we

                                                                    SA-2746-2
    FIGURE 5  SCRUBBER PERFORMANCE DATA FOR AMBIENT SCRUBBING TESTS

              EA-48 THROUGH EA-128
                                   46

-------
   10
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 DIAMETER
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                                                        3.81
                                           1000
                             EFFECTIVE FRICTION LOSS — mm WC
 ORIFICE
VELOCITY
  (m/sec)
   38
   46
   69
   30
   48
                                        J	L_i
                                                           10,000

                                                        S A-27 45-3
     FIGURE  6   SCRUBBER  PERFORMANCE CURVE FOR  AMBIENT SCRUBBING  TESTS
                 Aerosol generation rates 350-570 /jg/min.
                                          47

-------
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   67
   26
   53
  20-33
  27-29
     100
               1000
EFFECTIVE FRICTION LOSS — mm WC
                                                                                  10.000

                                                                               SA-2745-4
     FIGURE 7   SCRUBBER PERFORMANCE CURVE  FOR AMBIENT SCRUBBING  TESTS
                 Aerosol generation rates 750-1000 fig/min.
                                          48

-------
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VELOCITY
  (m/sec)
  35-38
   58
  26-28
  40-50
   18
   26
   39
   53
     100
              1000
EFFECTIVE FRICTION LOSS — mm WC
                                                                                  10.000

                                                                               SA-2746-5
     FIGURE 8   SCRUBBER  PERFORMANCE CURVE FOR AMBIENT SCRUBBING TESTS
                 Aerosol generation rates 1000-1200 /^g/min.
                                          49

-------
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                                                 A  2.54
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                                                    A
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   ORIFICE
  VELOCITY
   (m/sec)
     46
     58
     79
     18
     26
     39
     53
                                                                            10.000

                                                                         SA-2745-6
  FIGURE  9   SCRUBBER PERFORMANCE CURVE FOR  AMBIENT SCRUBBING TESTS
              Aerosol generation rates 1200-1500 /jg/min.
                                      50

-------
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         35-38
         25-28
          70
          18
          39
          53

i     1    I   1   1  I  1
     100
     FIGURE 10
                          1000
            EFFECTIVE FRICTION LOSS — mm WC
                   10.000
                                                                                SA-2745-7
SCRUBBER PERFORMANCE CURVE FOR AMBIENT SCRUBBING TESTS
Aerosol generation  rates  1500-1900 jug/min.
                                           51

-------
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B
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                                                                    AEROSOL
                                                                  GENERATION
                                                                     RATE
                                                                     350-570
                                                                     750-1000
                                                                    1000-1200
                                                                    1200-1500
                                                                    1500-1900
                        I     I   I   I   I  I  1

              1000
EFFECTIVE  FRICTION LOSS — mm WC
                                                                                10,000

                                                                             SA-2745-8
      FIGURE 11    SUMMARY OF SCRUBBER PERFORMANCE  CURVES FOR  AMBIENT
                   SCRUBBING TESTS
                                         52

-------
the generation rate of 750-1000  pg/min  (Figure 7), which most nearly cor-
responds  to the range producing  the aerosol defined as "Aerosol A."  The
data of Figure 6, which were actually the last obtained (except for one
point) were taken from tests with  "Aerosol B."  The one point from Test
No. EA-91 was obtained by deliberately  attenuating the output of the
nebulizer at a time when it was  producing about 1200 (jg/min of aerosol
at full power (Table 4).  The efficiency for that run checked very closely
the efficiency obtained in Test  EA-90,  made at the same conditions except
with full generator output.
     In Figures 6-10, the straight-line relationship of transfer units to
effective friction loss (in a log-log plot) appears as suggested by expe-
rience in earlier investigations.  However, another phenomenon, not pre-
viously observed, also appeared  in Figures 6, 7, and 8.  This was the
appearance of a sharp drop-off of  transfer units at low values of effec-
tive friction loss.  It appears  that in this region there is an essentially
separate correlation of transfer units  and effective friction loss (see
Figure 6) that may give a second straight line with a slope exceeding unity.
In Figures 7 and 8, the data in  the low-efficiency region are too few and
too imprecise to establish clear correlations.
     The correlations of Figures 6-10,  as well as the other data correla-
tions obtained during the investigation, demonstrate clearly that the
relationship of transfer units to effective friction loss for the orifice
scrubber was independent of gas  velocity,  liquid-to-gas ratio, and con-
tactor orifice size, at least over the  range of importance (See Appendix B).
There may be exceptions in the cases where separate data correlations appear
in the regions of low transfer units and low effective friction losses.  How-
ever, in the principal operating range, orifice velocities were varied from
18 to 79 m/sec <59 to 259 ft/sec) and liquid-to-gas ratio from 0.52 to
80 liters/Sm3 (3,9 to 60O gal/1000 Sft3) without showing independent
influences on collection efficiency.  Few data were taken at liquid-to-gas
                                    53

-------
                        3                 3
ratios under 2 liters/Sm  (15 gal/1000 Sft ),  even in tests at low
effective friction losses.
     A correlation of the scrubber pressure drop data taken during the
ambient scrubbing tests is shown in Figure 12.  The pressure drop is
expressed as the number of velocity heads based on the gas velocity
through the contactor orifice (without allowance for the presence of the
water), and is presented as a function of the liquid-to-gas ratio expressed
            3
in liters/Am .  In this correlation, the gas volumes, velocities, and
densities were computed at the measured scrubber inlet temperature and a
pressure of one atmosphere (see Table 5).  Since the pressure drop measured
was the total across the contactor orifice, the mitre elbow ahead of the
cyclone, and the cyclone itself, deviations were to be expected between
results obtained at different orifice sizes and gas flow rates.  The cor-
relation curve is no more than a rough average for the data as presented.

ADIABATIC SATURATION TESTS
     The Adiabatic Saturation tests were started, fortuitously, at a time
when the aerosol generator first reached a relatively stable operating
mode.  Most of the data (see Table 6) can therefore be considered appro-
priate to Aerosol A.  A few additional tests (No. AS-23 to AS-27) were
taken after the generator reached its second stable mode and are generally
appropriate to Aerosol B.  The tests No. EN-2 to EN-4 were part of the
series of preliminary screening tests.
     The greater stability of the aerosol generator during these testing
periods was reflected in the close correlation of the number of transfer
units with the effective friction loss (Figures 13 and 14); the character-
istic straight-line relationship was clearly defined.  However, the tests
with Aerosol A and Aerosol B each yielded a separate curve, with Aerosol B
showing systematically lower collection efficiencies.  Once again, gas
                                   54

-------
        100
     1
     I
     3
     C
     Q.
     O
     DC
     Q
     111
     DC
     Q-
    ORIFICE
   DIAMETER
     (cm)
  O 2.54
  A
  D
  O
  V
                                                                         3.81
  A
  •
  *
                                                                         4.45
                       10     15     20    25    30     35     40
                                        LIQUID-TO-GAS RATIO  —  liters/Am
45    50
   3
55
      ORIFICE
     VELOCITY
       (m/sec)
       18-23
       25-34
       36-43
       48-55
       5&-61
       68-81
       18-23
       25-34
       36-43
       48-55
       18-23
       25-34
1 1 1 1 I 1
1 1 1 1 1 1 1
60
65
70
                                                                                           SA-2745-21
FIGURE 12    CORRELATION  OF SCRUBBER PRESSURE DROP DATA FOR AMBIENT SCRUBBING CONDITIONS

-------
                        Table 5
SCRUBBER PRESSURE DROP DATA FOR AMBIENT SCRUBBING TESTS
Test
Code
No.
EA
•18
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
Orifice
Diameter
(cm)
3.81





















2.54







3.81





Liquid-to-Gas
Ratio
(liters/Am3)
17.9
26.2
35.7
51.3
36.4
25.4
18.5
8.49
1.38
5.59
12.4
16.9
0.52
1.03
4.12
9.00
25.3
12.4
2.63
1.42
10.7
8.50
38.4
53.0
36.9
26.4
15.2
14.4
8.26
1.56
0.505
1.41
8.38
18.7
18.5
18.6
Gas Velocity
in orifice8
(m/sec)
18.5
18.4
18.4
18.4
26.0
26.0
26.0
26.1
39.9
39.6
39.0
38.7
53.6
53.6
53.6
53.6
26.2
38.4
52.7
39.0
20.7
26.4
28.2
28.7
40.5
40.5
41.5
59.7
59.4
80.2
54.9
39.3
26.4
18.4
18.5
18.5
Velocity
Head
(nun WC)
20.2
20.1
20.2
20.2
40.1
40.1
40.1
40.4
94.0
93.0
91.4
91.2
173
173
173
173
41.7
90.2
168
91.7
25.4
41.4
47.8
49.3
98.8
98.6
100
211
210
384
177
92.5
41.4
20.1
20.3
20.2
Scrubber
Pressure Drop
mm WC
228
345
516
833
1217
787
516
239
191
442
864
1176
282
345
767
965
802
870
517
191
151
223
1092
1707
2680
1722
963
2300
1247
1087
302
200
217
217
217
217
Velocity Heads
11.3
17.2
25.6
41.3
30.3
19.6
12.8
5.92
2.03
4 75
9.44
12.9
1.63
1.99
4.42
5.59
19.3
9.63
3.07
2.08
5.94
5.37
22.8
34.6
27.1
17.5
9.58
10.9
5.93
2.84
1.71
2.16
5.24
10.8
10.7
10.8
                           56

-------
                  Table 5 (Continued)
SCRUBBER PRESSURE DROP DATA FOR AMBIENT SCRUBBING TESTS
Test
Code
No.
EA
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
Orifice
Diameter
(cm)
2.54






2.54















4.45


2.54



3.81









Liquid-to-Gas
Ratio
(liters/Am3)
8.22
9.04
10.7
12.6
35.2
25.8
17.7
17.7
38.3
24.0
12.4
3.38
9.37
6.93
78.2
42.4
21.1
9.37
12.8
38.5
9.24
3.17
21.7
3.88
2.98
2.73
9.92
8.92
7.06
6.87
3.74
3.24
2.82
2.49
1.84
5.52
8.20
12.3
16.0
4.51
Gas Velocity
in Orifice
(m/sec)
60.7
59.1
59.1
59.1
40.8
41.1
42.1
42.4
38.7
36.3
39.3
38.7
26.6
72.8
27.2
37.5
50.3
68.0
39.0
38.7
26.9
39.3
39.0
20.9
27.3
29.8
18.8
20.9
26.5
27.2
22.2
25.7
29.5
33.5
30.1
30.1
30.0
30.0
29.6
48.5
Velocity
Head
(mm WC)
214
209
209
209
102
103
105
105
88.4
78.2
89.7
87.6
40.9
307
43.9
83.8
154
277
89.4
89.4
42.4
90.4
89.7
26.4
44.5
53.3
20.5
25.3
40.6
42.9
29.2
39.1
51.6
66.3
54.1
54.1
54.1
54.1
53.1
142
Scrubber
Pressure Drop
mm WC
1290
1413
1633
2000
2532
1768
1224
1224
2590
1237
663
292
209
1854
2946
2642
2291
1902
673
2565
224
272
1250
82.1
136
162
81.3
97.7
137
161
77.8
103
130
164
115
216
293
444
588
572
Velocity Heads
6.02
6.75
7.81
9.59
24.8
17.2
11.7
11.6
29.3
15.8
7.39
3.33
5.12
6.05
67.1
31.5
14.9
6.87
7.52
28.7
5.27
3.00
13.9
3.12
3.04
3.04
3.96
3.85
3.39
3.75
2.67
2.64
2.52
2.48
2.11
3.99
5.40
8.20
11.1
4.03
                          57

-------
                           Table 5 (Concluded)





          SCRUBBER PRESSURE DROP DATA FOR AMBIENT SCRUBBING TESTS
T*»at-
Code
No.
EA
124
125
126
127
128

Orifice
Diameter
(cm)
2.54





Liquid-to-Gas
Ratio
(liters/Am3)
16.5
22.0
28.1
38,0
1.77

Gas Velocity
in Orifice
(in/sec)
38.7
38.7
37.8
38.1
70.4

Velocity
Head
(mm WC)
88.6
88.6
87.1
87.9
297
Scrubber
Pressure Drop
mm WC
897
1271
1587
2440
866
Velocity Heads
10.1
14.3
18.2
27.8
2.92
Calculated at actual inlet gas conditions.
                                     58

-------
                                                                                   Table  6




                                                     SCKUBBER PERFORMANCE DATA FOR AD1ABATIC  SATURATION SCRUBBING TESTS
Test
Code
No.
EN
2
3
4
AS
6
7
9
10
11
12
13
14
IS
16
17
18
19
20
21
22
23
24
25
26
27
Aerosol
Generation
Rate
(M-g/mln)

1060
1245
1370

1319
1010
1225
1371
1176
2063
1226
1074
1190
1007
1083
1072
997
8S4
685
942
704
665
537
661
703
Averages
Contactor
Orifice
Size
(cm)

3.81



3.81




2.54




2.54


3.81


2.54

3.81


Inlet CBS Flow Rate
(std liters/mill)
Air

1670
1670
1670

1580
1580
1580
1580
1580
1240
1230
119O
1190
1190
1150
1120
1120
1580
1580
1580
1250
1240
1590
1560
1570
Water
Vapor

923
923
923

800
8OO
800
800
80O
659
659
573
573
573
542
542
542
783
7S3
783
682
682
787
801
787
Total,

2590
2590
2590

2380
2380
2380
2380
2380
1900
1890
1760
1760
1760
1690
1660
1660
2360
2360
2360
1930
1920
2380
2360
2360

Gas Moisture
Content
(g/B dry air)
In

0.341
0.341
0.341

0.315
0.315
0.315
0,315
O.315
0.329
0.334
0.300
0.300
0.300
0.292
0.301
0.301
0.308
0.308
0,308
0.338
0.343
0.308
0.319
0.312
0.317
Out

0.384
0.384
0.384

0.390
1.369
0.362
0.350
0.342
0.485
0.467
0.372
0.386
0.384
0.369
0.355
0.335
. 0.327
0.325
0.318
0.389
0.371
0.344
0.331
0.323
0.369
Gas
Temperature
<°C)
In

126.1
126.1
126.1

128.9
130
130.6
130.6
127.8
125
129.4
130
138.9
123.3
128.9
121.7
125
128.3
12$. 9
130
12S.9
12S.9
130
130
129.4
128.9
Out

73.9
73.9
73.9

72.2
72.2
72.8
72.8
72.8
71.1
70.6
70.6
71.1
71.7
70.6
71.1
70.6
71.7
71. 7
71.7
71.1
71.1
71.7
71.7
71.7
71.7
Water
Temperature
<°C>
In

74.4
74.4
74.4

73.3
73.3
73.9
73.3
75.6
73.3
72.8
73.3
73.9
75.6
73.3
73.3
73.3
73.9
73.3
73.9
73.9
73.9
73.9
73.9
74.4
73.9
Out

74.4
74.4
74.4

73.9
73.9
73.9
73.9
73.3
73.3
73.3
73.3
73.9
74.4
73.3
73.3
72.8
73.9
73.9
73.3
73.3
72.8
73.9
73.9
73.9
73.9
Gas
Velocity
In
Orifice
(ra/sec)

50.7
50.7
50.7

46.9
47.0
47.1
47.1
46.8
83.5
83.9
78.3
80.0
77.0
75.0
72.3
72.9
46.5
46,5
46.7
85.9
85,3
47.2
46.8
46.4
Liquid- to-
Gas Ratio
AitersX
\ Sm3 /

4.78
4.78
4.78

19.0
13.5
8.16
4.69
1.54
10.2
10.3
11.0
11.0
8.59
11.5
9.02
5.71
4.81
4.01
1.39
8.36
6.24
11.4
6.33
3.13
Effective
Friction
Loss
{mm WC)

432
432
432

1240
837
571
353
197
1750
1750
1620
1620
1370
1550
1150
1000
371
319
186
1660
1390
688
439
253
Collection
Efficiency
(%)

69.05
68.61
70.23

95.98
88.28
77.81
64.86
41.67
98.64
97.67
97.78
97.14
96.06
97.65
95.42
93.21
62.18
53.02
36.62
97.35
96.01
79.23
64.15
44.64
Transfer
Units
fNt>

1.17
1.16
1.21

3.21
2.14
1.51
1.05
0.539
4.30
3.76
3.81
3.56
3.23
3.75
3.08
2.69
0.972
0.756
0.456
3.63
3.22
1.57
1.03
0.591

en
to

-------
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                       I     I   I   I  I  I I  I
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                J	I	1   I   I   I  t I I
                                                                   ORIFICE
                                                                  DIAMETER
                                                                    (em)
                                                                 O 2-54
                                                                 A 3.81
'     I   i
                                         1000
                           EFFECTIVE FRICTION LOSS
                 10.000
                                                 — iwn WC
      FIGURE 13   SCRUBBER  PERFORMANCE CURVE  FOR ADIABATIC SATURATION
                  SCRUBBING—AEROSOL A
                  Gat temperatures:  Inlet * 128.9°C; Outlet - 71,7°C.
                                        60

-------
s
£
   0.1
     100
                       1—I—I   I  I  I  11
                       '     I    I  I  I  11
1000
                   T	1   I   I   I  I  U
                                                                   ORIFICE
                                                                   DIAMETER
                                                                     (cm)
                                                                  V  2.54
                                                                  <>  3.81
                  J	L
                            EFFECTIVE FRICTION LOSS — mm WC
    10,000

SA-2746-10
      FIGURE U   SCRUBBER PERFORMANCE CURVE FOR ADIABATIC SATURATION
                  SCRUBBING—AEROSOL B
                  Gas temperatures:  Inlet « 128.9°C; Outlet = 7t.7°C.
                                        61

-------
velocity and liquid-to-gas ratio were found to exert no independent
influences on collection efficiency (See Appendix B).

CONDENSING TESTS
     In the Condensing tests of Series I and II, the initial tests were
made with Aerosol A, and separate groups of tests were subsequently made
with Aerosol B when it became evident that the aerosol generator mode had
again shifted (Tables 7 and 8).  The inlet gas conditions were regulated
to be the same as for the Adlabatic Saturation tests, but the gas was then
scrubbed with sufficient cold water to bring the exit gas to a selected
final temperature; 54.4°C (130°F) in Series I, and 37.8°C (100°F) in
Series II,  For each series of tests, the liquid-to-gas ratio was there-
fore fairly constant.  However, in Series II there was a trend toward
reduction of the required liquid-to-gas ratio as the scrubber pressure
drop increased, indicating that the efficiencies of heat transfer and
dehumidification, like the efficiency of particle collection, were
increased by increases in the effective friction loss.  In the tests of
Series  I, the average outlet gas and water temperatures were equal, but
in the  tests of Series II, the outlet water stream was always substantially
colder  than the exit gas, and colder at lower effective friction loss than
at high.
     The scrubber performance curves for Test Series I and II are presented
in Figures 15 and 16, respectively.  In both cases, good correlations were
obtained, and the curves for Aerosol A and Aerosol B were systematically
different, as in the Adiabatic Saturation tests.  The corresponding
curves  for Test Series I and  II were actually identical; this is obvious
from Figure 17, in which the data points for collection of Aerosol A in
both Series I and Series II are presented together.  The data points for
collection ol Aerosol B  in Series  I and II are  similarly presented  together
in Figure 18.   It appears that the  coincidence  of  the performance  curves
                                    62

-------
                                                                                  Table 7





                                                   SCRUBBER PERFORMANCE DATA FOR CONDENSATION SCRUBBING TESTS,  SERIES I
Test
Code
No.
EC
4
6
7
27
28
29
30
31
32
33
46
47
48
49
Aerosol
Generation
Rate
(Jig/rain)

867
807
892
808
850
843
801
773
917
932
5S4
555
607
776
Contactor
Orifice
Size
(cm)

3.81


3.81



2.54


3.81


2.54
Inlet Gas Flow Rate
(std liters/min)
Air

2140
2140
2140
1370
1660
2190
2680
1220
1300
1650
1860
2340
2920
1760
Water
Vapor

1230
1230
1230
691
849
1080
1380
629
739
849
960
1170
1450
912
Total

33 7O
3370
3370
2060
2510
3270
407O
1850
2040
25OO
2820
3510
4370
1970
Averages
Gas Moisture
Content
(g/g dry air)
In

O.358
0.358
0.358
0.313
0.317
O.3O6
0.321
O.319
0.353
0.319
0.320
0.310
0.308
0.321
0.327
Out

0.104
0.104
0.104
0.113
0.115
0.118
0.122
0.119
0.122
0.131
0.119
0.119
0.122
0.139
0.117
Gas
Temperature
(°C)
In

130.6
130.6
130.6
130
131.1
131.1
133.3
127.2
127.8
130.6
131.1
131.7
133.3
131.1
130.6
Out

52.2
52.2
52.2
54.4
54.4
54.4
54.4
53.9
53.9
53.9
55.0
54.4
54.4
54.4
54.4
Water
Temperature
(°C)
In

12.2
12.2
12.2
12.8
12.8
12.8
12.8
13.3
12.8
12.8
13.9
13.3
13.9
13.9
13.3
Out

51.1
51.1
51.1
52.2
52.8
54.4
55.0
53.9
54.4
54.4
55.0
55.0
54. 4
55.0
54.4
Gas
Velocity
in
Orifice
(m/sec)

66.7
66.7
66.7
40.7
49.8
64.8
81.1
81.7
9O.2
111
55.9
69.7
87.1
87.9
Liquid- to-
Gas Ratio
/liters\
U3 j

3.37
3.37
3.37
2.94
3.09
2.90
2.41
2.78
2.97
2.73
2,75
2.75
2.25
2.62
Effective
Friction
Loss
(mm WC)

444
444
444
121
234
398
553
798
983
1550
282
467
645
1710
Collection
Efficiency
(%>

74.57
72.94
72.66
45.76
58.79
74.75
83.14
93.85
95.46
97.40
56.04
75.49
84.04
97.71
Transfer
Units
(Nt)

1.37
1.31
1.30
0.612
0.886
1.38
1.78
2.79
3.09
3.65
0.822
1.41
1.83
3.78

0>
w

-------
                              Table 8





SCRUBBER PERFORMANCE DATA FOR CONDENSATION SCRUBBING TESTS, SERIES II
Teit
Code
Mo.
EC
a
9
10
11
12
13
14
13
18
17
18
19
30
21
33
33
23A
24
29
28
43
44
45
50
Aeroaol
Generation
Rate


957
888
1316
1132
1197
111S
801
690
3340
1083
1258
855
915
981
787
789
854
879
875
850
340
430
510
370
Contactor
Orifice
SUe
td Uteri Bin)
Air

1740
1740
1390
1680
2210
2740
1140
1140
1480
1880
1250
1210
1300
1400
1520
1680
1340
1490
1690
1890
1910
2150
2350
1530
Water
Vapor

923
923
690
850
1000
13TO
573
567
739
838
641
603
660
728
7T8
832
634
739
849
960
960
1080
1180
838
Total

2660
2660
1980
2510
3300
4110
1710
1710
2200
2510
1890
1810
1960
2130
2300
2490
1970
2230
2540
2850
2870
3230
3530
2370
Averages
Gu Moisture
Content
(g/f. dry air)
In

0.33O
O.330
0.325
0.317
0.305
0.312
0.313
0.310
0.315
0.311
0.319
0.310
0.315
0.321
0.317
0.311
0.294
0.308
0.312
0.315
0.313
0.312
0.312
0.339
0.315
Out

0.0484
0.0481
0.0440
0.0449
0.0458
0.0470
0.0502
0.0487
0.0516
0.0535
0.0488
0.0471
0.0494
0.0508
0.0926
0.0955
0 .0453
0.0471
0.0447
0.0451
0.0446
0.0454
0.0479
0.0531
0.0404
Cu
Temperature
(9C)
In

129.4
129.4
128.3
128.3
129.4
131.7
127.8
128.3
128.9
128.9
128.3
128.7
127.2
128.3
128.3
129.4
128.3
128.9
130
130.6
134.4
133.9
131.1
130.6
129.4
Out

38,9
38.9
37.8
37.8
37. 8
37.8
38.3
37.8
37.8
37.2
37.8
37.2
37.8
37.8
37. 8
37.8
38.3
38.9
37.8
37.8
37.2
37.2
37.8
37.8
37.8
Water
Temperature
<°C>
In

12.2
12.2
12.2
11.7
11.7
11.7
12.2
11.7
11.7
11.7
11.7
12.2
12.2
12.2
12.2
12.2
12.2
12.2
12.2
12.8
12,8
12.8
12.8
13.3
12.2
Out

32.8
32.8
27.8
32.2
34.4
35.6
36.1
36.1
36.1
35.6
3S.6
34.4
39.0
38.1
35.6
35.8
27.8
—
30
32.2
32.2
33.9
34.4
36.1
33.9
Gai
Velocity
in
Orifice
(a/aec)

52.5
92.5
28.6
36.3
47.9
59.9
75.6
75.7
97.6
111
83.7
79.8
86.6
94.3
102
111
28. 6
32.3
36.9
41.5
57.4
64.5
70.0
106
U quid- to-
Gai Ratio
/lltera\
(~^~)

1.04
7.04
9.78
7.73
6.88
5.68
6.54
6.32
6.64
6.31
6.83
7.31
6.95
6.83
6.67
6.82
9.52
8.S5
8.78
8.26
8.25
7.50
7.13
6.S4
Effective
Friction
Lou
(mm WC)

447
447
183
278
480
666
938
925
1557
2120
1120
1050
1210
1450
1710
2120
176
227
301
379
587
726
866
1790
Collection
Efficiency
«)

79.27
79.65
58.83
66.62
80.41
87.37
95.55
94,98
99.16
99.24
96.46
95.84
96.90
97.87
98.48
99.00
51.02
58.98
68.15
74.22
80.00
85.35
89.17
97.92
Tranifer
Unlti
(Nt>

1.58
1.60
O.S87
1.10
1.63
2.07
3.11
2.99
4.78
4.88
3.34
3.18
3.17
3.85
4.18
4.61
0.714
0.891
1.14
1.36
1.81
1.92
2.22
3.87


-------
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                                                     ORIFICE
                                                    DIAMETER
                                                       (cm)
                                                   O  2.54
                                                   V
                                                   A  3.81
                                                   O
                                                  	I
                                        AEROSOL
                                            A
                                            B
                                            A
                                            B
                                _L   1   I   I   I  I
              1000
EFFECTIVE FRICTION LOSS — mm WC
                                                             10,000

                                                         SA-2745-11
    FIGURE 15   SCRUBBER PERFORMANCE CURVES  FOR CONDENSATION SCRUBBING
                 (SERIES I)
                 Gas temperatures:  Inlet = 130.6°C; Outlet =  54.4°C.
                                        65

-------
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                                     ORIFICE
                                    DIAMETER
                                      (cm)
                                   O 2.54
                                   V
                                   A 3.81
                                   <0
                                   D 4.45

                                  	I
                          1000
            EFFECTIVE FRICTION LOSS — mm WC
                                                                     AEROSOL
                                                                        A
                                                                        B
                                                                        A
                                                                        B
                                                                        A
 I     I   I   I   I  I  I
                  10.000

               SA-2746-12
    FIGURE 16   SCRUBBER  PERFORMANCE CURVES FOR CONDENSATION SCRUBBING
                 (SERIES II)
                 Gas temperatures:   Inlet = 129.4°C; Outlet = 37.8°C.
                                         66

-------
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                                                       GAS TEMPERATURES (CC)

                                                       Inlet            Outlet    —

                                                   O  130.6             54.4

                                                   A  129.4             37.8
                                                      I
              1000

EFFECTIVE FRICTION LOSS — mm WC
    10.000



SA- 2745-13
      FIGURE 17   SCRUBBER PERFORMANCE CURVE FOR CONDENSATION SCRUBBING

                  (SERIES  I AND II)—AEROSOL A
                                        67

-------
   10
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                                     1   I   I  I   I  IJ
                                                       GAS TEMPERATURES CO
                                                       Inlet           Outlet
                                                   O  130.6            54.4
                                                   A  129.4            37.8
                            I   I   I   I  I  I I
              1000
EFFECTIVE FRICTION LOSS — mm WC
    10,000

SA-27 45-14
    FIGURE 18  SCRUBBER  PERFORMANCE CURVE FOR CONDENSATION SCRUBBING
                (SERIES I AND II)—AEROSOL B
                                       68

-------
 for Series I  and II results from the fact that only a relatively small
 amount of additional water vapor condensed as the gas temperature was
 reduced from 54.4° to 37.8°C (130° to 100°F).

      There was no indication of independent effects of orifice size or
 gas velocity on the collection efficiency.

      In Test Series III the inlet gas was adjusted to the same moisture
 content as in Series I and II, but the dry bulb temperature was only about
 8 Centigrade degrees above the saturation temperature (Table 9).   The
 tests were carried out over the period in which the aerosol generator
 shifted from producing Aerosol A to producing Aerosol B.   Not enough
 data points were obtained with Aerosol A to establish a performance curve,
 but it was possible to obtain a curve for Aerosol B (Figure 19),  which
 could then be compared with the similar performance for Test Series I
 and II(Figure 20).   The substantial improvement in performance obtained
 with the  nearly saturated inlet gas probably indicates that condensation
 started almost immediately in the  gas/liquid contacting zone,  whereas
 superheated  inlet gas  had to be cooled substantially before condensation
 could start.

 VAPORIZING TESTS

      The conditions  for  the  Vaporizing tests were  chosen arbitrarily  to
 be  as similar  as  possible  to  those  employed  in  the  other test  series.
 The  inlet  air  and water  temperatures were  adjusted  to  approximately the
 same  values as  in the Adiabatic Saturation tests  (Table 10).  The water
 rate was adjusted to yield the  same outlet gas  temperature  in  all the
 runs.  The three initial tests  (EV-1,  -2,  and -3) were made as part of
 the original series of screening tests, on an aerosol that was pre-
 sumably equivalent to that later designated as Aerosol A.   The remainder
of the tests were made with Aerosol B, but the data points from Tests

                                  69

-------
                                                                                Table 9



                                                  SCRUBBER PERFORMANCE DATA FOR CONDENSATION SCRUBBING TESTS, SERIES III

Test
Code
Ho.
EC
34
35
36
37
38
39
41
42

Aerosol
Generation
Rate
918
1015
1011
936
481
484
524
586

Contactor
Orifice
Size
2.54




3.81



Inlet Gas Flow Rate
(atd liters/mm)
Air
1160
1230
1240
1660
1450
1930
1670
1310
Water
Vapor
629
629
623
845
739
1060
838
682
Total
1790
1860
I860
2510
2190
2990
2510
1990
Averages

Ga.3 Moisture
Content
In
0.335
0.318
0.311
O.319
0.317
0.34D
0.312
0.323
0.322
Out
0.0477
0.0481
0.0480
O.OS39
0.0517
0.0460
0.0450
0.0443
0.0481

Gas
Temperature
In
77.2
77.2
80.0
80.0
80.6
82.8
82.8
80.6
80.0
Out
37.B
37.8
37.8
37.8
38.3
37.8
37.8
37.8
37.8

Water
Temperature
In
12.8
12.8
12.8
12.8
12.2
12.8
12.8
12.8
12.8
Out
33.3
34.4
34.4
35.6
37,2
33.9
32.2
27.8
33.9
Gas
Velocity
In
Orifice
la/sec)
69.2
71.9
72.5
97.8
85.5
52.2
43.8
34.5
Ll quid- to-
Gaa Ratio
/liters\

6.86
6.32
6.29
6.12
5.80
7,02
7.23
8.74

Effective
Friction
Loss
(ma WC3
922
980
978
1910
1360
549
371
242

Collection
Efficiency
93.04
97.95
97.23
99.23
97.55
87.53
77.61
67.11

Transfer
Units
(Nt)
3.93
3.89
3.59
4.93
3.71
2.08
1.50
1.11

-J
o

-------
   10
D
EC
EC
CD
D
   0.1
     100
                        I    I    I   I
                                 1IIII  1  l_
                                                          ORIFICE
                                                          DIAMETER
                                                            (cm)
                                                         O 2.54
                                                         A
                                                         D 3.81
                                         AEROSOL
                                             A
                                             B
                                             B
                                                         I
                                  I
              1000
EFFECTIVE FRICTION LOSS — mm WC
    10,000
SA-2745-15
     FIGURE 19   SCRUBBER  PERFORMANCE CURVE FOR  CONDENSATION SCRUBBING
                  (SERIES III)
                  Gas temperatures:   Inlet = 80.0°C; Outlet = 37.8°C.
                                          71

-------
   10
z




-------
                                                                        Table 10




                                               SCRUBBER PERFORMANCE DATA FOR VAPORIZATION SCRUBBING TESTS
Test
Code
No.
EV
1
2
3
4
5
6
7
8
9
Aerosol
Generation
Rate
920
990
1050
447
535
495
558
559
653
Contactor
Orifice
Size
(cm)
3.81


2.54


3.81


Inlet Gas Flow Rate
(std liters/rain)
Air
2250
2280
2280
1720
1570
1320
2170
1940
16 4O
Water
Vapor
__
—
—
—
--
—
—
—
—
Total
2250
2280
2280
1720
1570
1320
2170
1940
1640
Averages
Gas Moisture
Content
(g/g dry air)
In
0.01a


0.006
0.006
0.006
0.006
0.006
0.006
0.007
Out
O.150
0.147
0.147
0.178
0.159
0.138
0.147
0.145
O.132
0.149
Gas
Temperature
In
126.1
125.6
125.6
128.3
125
122.8
126.1
126.7
123.9
125.6
Out
58.9 ,
58.3
58.3
58.9
58.3
57.2
58.3
58.3
57.2
58.3
Water
Tempera ture
In
71.1
72.8
72.8
73.9
73.9
73.9
74.4
74.4
74.4
73.3
Out
58.9
58.9
58.9
60.0
60.0
58.3
60.0
60.0
59.4
58.3
Gas
Velocity
in
Orifice
(m/sec)
44.0
44,6
44.6
76.4
69.0
57.6
42.2
38.1
32.0
Liquid-to-
Gas Ratio
/liter s\
V sm3 )
5.04
4.99
4.99
7
6.75
4.30
5.01
4.77
4.38
Effective
Friction
Loss
(mm WC)
430
431
431
1610
1090
609
390
281
159
Collection
Efficiency
55.75
53.69
55.56
96.04
90.29
76.80
46.30
27.43
13.82
Transfer
Units
0.815
0.770
0.811
3.23
2.33
1.46
0.622
0.321
0.149

Estimated.
Estimated from pressure drop.

-------
EV-1, -2, and -3 appeared to fit the correlation curve fairly well
(Figure 21).
     The scrubber performance curve for Vaporizing conditions is notable
for the very marked "break" and the appearance of a quite distinct and
separate correlation curve in the low range of effective friction loss,
similar to that obtained for Ambient scrubbing of Aerosol B (Figure 6).
This phenomenon did not appear in any of the Adiabatic Saturation or
Condensation tests.

GENERAL DISCUSSION
     The scrubber performance curves for tests at Adiabatic Saturation,
Condensing, and Vaporizing conditions are summarized, without data points
in Figures 22 and 23.  The curves for scrubbing of Aerosol A are presented
in Figure 22, and those for scrubbing of Aerosol B in Figure 23.  In
Table 11, the numbers of transfer units and corresponding collection
efficiencies are tabulated for selected values of effective friction
loss, for convenient reference.
     Although condensation scrubbing is seen to have afforded a signifi-
cant increase in the collection efficiency at a given level of effective
friction loss, the improvement was not extremely marked.  The maximum
improvement obtained (comparing condensation scrubbing of the nearly
saturated gas with the adiabatic saturation scrubbing of the hot gas)
was equivalent to reductions of about 25 to 50% in the effective friction
loss required to attain a given collection efficiency.  This is much less
favorable than indicated by previous experience in pilot plant studies of
                                            4,5,9
scrubbing fume from Kraft recovery furnaces,      where condensation
scrubbing of the hot gas appeared to reduce the required contacting power
by about 50 to 70% from that required in adiabatic saturation scrubbing
of the same gas.  However, the qualitative trends in the results from  the
                                   74

-------
   10
u>
DC
IU
u.
O
GS
00
   0.1
                             \   I   IMF)
                             I   I   1   I  II  I
                                                     ORIFICE
                                                     DIAMETER
                                                       (on)
                                                    V  2.54
                                                    A  3,81
     100
     FIGURE 21
                        1000
          EFFECTIVE FRICTION LOSS — mm WC
                                                   AEROSOL
                                                      B
                                                      A
                                                      B
                                                                                10,000

                                                                            SA-27 45-17
SCRUBBER  PERFORMANCE CURVE FOR VAPORIZATION SCRUBBING
Gas temperatures: Inlet = 125.6°C; Outlet = 58.3°C.
                                         75

-------
   10
I
3
I
g
m
   0.1
                                I  I  I  I  I  |
                                         OPERATING
                                         CONDITION
                                        - Condensing
                                        • Adiabatic
                                         Saturation
                         GAS TEMPERATURES <°C)
                          Inlet            Outlet
                         130
                         128.9
37.8 and 54.4
   71.7
                       I     I   I  I   I  M  I
     100
              1000
EFFECTIVE FRICTION LOSS — mm WC
                                                                             10.000

                                                                         SA-2745-18
     FIGURE 22  SUMMARY OF SCRUBBER PERFORMANCE CURVES FOR ADIABATIC
                 SATURATION AND CONDENSATION  SCRUBBING—AEROSOL A
                                        76

-------
    10
                        1—I    I
        1—I   I
12
z
S
u.
O
S
I
                                  OPERATING
                                  CONDITION
                          __ _• Condensing
                          « — «— Condensing
                          i        Adiabatic Saturation
                          .—— Vaporizing
GAS TEMPERATURES (*C)
Inlet            Outlet
80.0             37.8
130.0         37.8  and 54.4   —
128.9             71.7
125.6             58.3
   0.1
     100
                                           1000
                            EFFECTIVE FRICTION LOSS — mm WC
                          10.000

                      SA-2746-19
    FIGURE 23   SUMMARY OF SCRUBBER PERFORMANCE
                 SATURATION, CONDENSATION, AND VAPORIZATION  SCRUBBING—
                 AEROSOL B
                                         77

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                                                            Table 11





                       SELECTED COMPARISONS OF SCRUBBER PERFORMANCES AT DIFFERENT OPERA!ING CONDITIONS
Test Condition
Gas Temp ( °C)
Inlet
Outlet
Aerosol
A


D
Effect ivo
Friction
(mm WC)
250
IOOO
1500
'250
IOOO
1500
Adiaba t ic
S a t u r:i i i o n
129
71.7
Trruis iv r
Unit*
O.G85
2.52
:«.68
2 . 'M
.'3.43
Co 1 Ice t ion
K J 1 if l«MH-y
'19. G
91.95
97.48
14.2
yo.o
96. 7G
Condons inp
Series 1 .md 11
I HO
54.4 »mi :V7.»
Tranclcr
Units
1 .01
2.9:1
•1 . 00
2 . >G
C'o 1 left loll
K I I u ii-ijcy
GM.:t
y'l.oi)
«H . 1 7
Mi . K
91, -Hi
9G.50
Condensing
Series III
80. O
:n.H
Transfer
I'lll Is



\ .14
-H . 1 2
'1.1H
C'o 1 Ice I ion
Ei f icluncy



0^7
Vapor i zmj;
126
&».a
Trims f».ir
Unl vs>



n.ruvj
Col loft ion
K 1 lie icii<-y
c:)



KK.9
<).'». 2 :H
00

-------
 different investigations were similar, and there were wide differences
 in the natures of the aerosols that might account  for the quantitative
 differences in the performances that were observed.
      The scrubber tests clearly demonstrated that  liquid-to-gas ratio,
 gas velocity, and contactor orifice size had no influences on collection
 efficiency that were independent of effective friction loss, at least
 over the operating range of major practical interest (see Appendix B).
 Exceptions to this conclusion may exist in the instances where a separate,
 steep performance curve appears in the region of low effective friction
 1oss and low collection efficiency.  No clear exceptions appeared even
 *« that operating region, but since the pertinent data obtained were
 relatively imprecise and few in number,  and covered relatively narrow
 ranges of variables, some exceptions may have been concealed.  However,
 the operating region is not often one of much practical importance.
      This observation concerning liquid-to-gas  ratio is consistent with
 the  experience obtained  in. other investigations with both similar and
 some  dissimilar types of scrubbers,  although  it conflicts with  reports
 fr°m  some other investigators  (see  Appendix A).  Where  condensation and
 VaPorization phenomena are  controlled  by  liquid-to-gas  ratio,  this  factor
 Cai» be expected to have  independent  effects on  efficiency.  However,  in
 c°«densation tests, Series  I and II, where additional gas  cooling did  not
 effect substantial additional condensation of water, liquid-to-gas  ratio
 ^ain appeared  to have no independent effects.
     The results of the  investigation have thus constituted an additional
 Verification of tne "contacting power rule" as applied to a small replica
°f the generalized class of commercial "venturi" scrubbers.   (Most current
venturi scrubbers are actually orifice scrubbers employing some form of
Va*iable-area orifice.)  Deviations between the various performance curves
8116 therefore  ascribable  to introduction of additional mechanisms—
                                 79

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in the present case, vaporization and condensation.
     Where diffusion and heat transfer effects come into play, as in con-
densation scrubbing, it is not to be expected that performance should be
dependent only on contacting power and be independent of scrubber con-
figuration.  To the extent that particle scrubbing is an inertial phe-
nomenon, efficiency should be—and apparently is—independent of contact
time.  However, where diffusion and heat transfer are involved, contact
time and scrubber configuration should play roles.  Power consumption
does, of course, affect mass transfer as well as particle collection.
A number of investigators have now produced empirical correlations of gas
absorption data for cocurrent contactors (particularly pipeline contactors)
in terms of energy dissipation (see Appendix A).  There should thus be
relationships between contacting power and particle collection by Stefan
flow and probably also between contacting power and the growth of particles
by condensation of water vapor, but these relationships are presumably
quite different from that between contacting power and particle collection
by inertial deposition.  The scrubber performance curve for condensation
scrubbing thus represents the net effects of at least two major particle
collection mechanisms, each being a different function of contacting power.
     Qualitatively, at least, it does not appear that condensation scrub-
bing should be regarded as a possible way to eliminate use of energy in
contacting, since utilization of condensation phenomena will also demand
contacting power.  Rather, it should probably be regarded as a way merely
of reducing contacting power requirements.  Of course, condensation scrub-
bing will, in practice, incur some energy consumption elsewhere (in water
cooling, for example) that will partly counteract the saving.
     Where diffusional phenomena, such as Stefan flow, are involved, it
may be advantageous to use a type of gas/liquid contactor that provides
a relatively long contact time rather than one (such as a venturi or

                                    80

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 orifice)  that gives  an intense but  short contact.  A pipeline contactor
 operating in the  spray-flow regime  appears  to be a promising device
 meeting this objective.
      Since the results of  this investigation clearly indicate a major
 improvement from  presaturating the  gas before condensation scrubbing, a
 high-energy condensation scrubber should be preceded by a. low-energy
 Sas/iiquid contactor designed to operate as an adiabatic saturator by
 ^circulation, of  the water.  The pres&turator would not be designed to
 collect fine particles.
      It was also  indicated that in  condensation scrubbing there is minimal,
                                                                       o
 if any, benefit from reducing the final gas temperature below about 50 C,
 since the additional condensation of water vapor is small.   Hence,  it
 should not be necessary to use very cold scrubbing water unless  there
 are other reasons to dehumidify the gas.   On the other hand,  there may
 an advantage in increasing the saturation temperature of the inlet gas,
 since the quantity of water vapor present in the gas  increases  rapidly with
 temperature.  This approach incurs the extra costs  not  only of humidifying
 the inlet  gas but of removing the increased  load of low-level heat  from the
 scrubbing  water.   Such economic considerations  are  further  discussed  in
 Appendix C.
      A  number of  phenomena  appearing in this investigation  may be of
 wajor significance,  theoretical or practical, but at  this point they  lack
 explanation or—in some instances—complete  confirmation of their reality.
      The  straight-line relationship in log-log  plots of scrubber perfor-
 mance was  generally well confirmed by the results of this investigation.
 However, the relationship is empirical, and  at present  there is no  basis
 other than experience  for assuming that the  performance curves should  be
 straight lines in  the general case.   Calvert's model for the venturi
 scrubber (see Appendix  A) leads to an approximation that, for a monodis-
Perse aerosol of given  diameter,  the number  of transfer units is directly
                                  81

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 proportional to the  pressure drop; hence, the  predicted curve of  transfer
 units versus pressure  drop on  a log-log plot is  a  straight line with a
 slope of unity.  For a polydisperse  aerosol, the overall collection effi"
 ciency would have to be determined by integrating  the collection  effici-
 encies over the range  of particle sizes represented in the particle-size
 distribution.  Presumably, then, the form of the scrubber performance
 curve should be dependent on the nature of the particle-size distribution,
 and  it is not  obvious  that the curve should necessarily be straight.
     For the present,  there is no theoretical  basis for the applicability
 of the "contacting power rule" to various types  of scrubbers that do not
 fit  the models of Calvert and  other  investigators.  If it can be  assumed
 on the basis of experience to  date that scrubber performance curves are
 straight lines on log-log plots, then it also  appears logical that for a
 monodisperse aerosol,  the slope of the curve should be unity, with succes-
 sive equal increments  of contacting  power producing equal increments in
 the  number of transfer units.  There is no experimental confirmation of
 this conjecture, since no appropriate data are available for the  scrubbing
 of a truly monodisperse aerosol.  In scrubbing of  a polydisperse  aerosol,
 larger particles will  be collected with higher efficiency than smaller
 ones, and successive increments of contacting power should yield  succes-
 sively smaller increments of transfer units, leading to a performance
 curve with a slope of  less than unity.  In fact, it has been found that
 the  performance curves for most aerosols are straight lines with slopes
 less than unity.  However, some data developed from the literature have
yielded curves having  slopes greater than unity.4  If the collection
mechanism is assumed to remain the same, such a performance curve
that the aerosol increases in average particle size in passing through the
scrubbing process.  This interpretation, in turn, implies either that the
finer particles are collected selectively, which is contrary to experience
and reasonable expectation, or that the aerosol undergoes a coagulation
                                   82

-------
  Process  in  the  scrubber  that  Is  also  related  to  contacting power, which
  would seem  possible but  unlikely.
      The present  investigation is apparently  the first to indicate that
  the observed  straight-line relationship in scrubber performance curves
 may sometimes break down in some part of the  contacting power range—
  specifically, the low range.  No indication of comparable deviations in
 the high contacting power range has yet appeared.  If it is accepted that
 the performance curve is a straight line,  then it appears reasonable that
 it should remain so over the entire contacting power range,  unless dif-
 ferent particle collection mechanisms predominate in different parts of
 the range.
      In  the present investigation,  there were four occasions  in which
 separate  correlation curves appeared in the low contacting  power region
       es  6,  7, 8,  21).   In two of these cases, the curves were reasonably
      defined (Figures  6,  21),  and appear to be straight, or nearly
 straight  lines.  Also,  the  slopes of these  curves exceeded unity.
 However,  in each case,  the  principal correlation curve  in the  upper
 contacting  power range was  a straight  line  with  a slope less than unity.
 ^Is behavior  suggests  that in the cases reported  in the literature where
 correlation  curves with very steep slopes were encountered, data may have
 be«m taken only  over a range defining  the lower curvej extending the data
 to a higher  contacting power range might have  revealed a second curve of
more familiar character.
     It also appears that the steep  curves  in  the low-energy region were
most pronounced  in those  cases in which the aerosol was presumably the
finest,  at least in average particle size.
     These observations suggest that the appearance of two segments in
    scrubber performance curve  for an aerosol may indicate  a change  in
                                   83

-------
the dominant particle-collection mechanism over the contacting power
range in which the steeper correlation curve appears—for instance,  a
shift from Brownian diffusion to inertia! deposition.  At present, this
hypothesis is—at best—highly speculative, but it does suggest experi-
mental methods of attack on the problem.
     A less rarified explanation for the low-energy-range performance
would be channeling of liquid in the contactor.  Such channeling might
indeed explain behavior with very low water and air rates, but it does
not seem adequate to explain behavior such as shown in Figures 6 and 21,
or the absence of comparable behavior in other test series.
      In cases where scrubbing tests on evidently different aerosols are
involved (Figure 11), crossing of the performance curves appears readily
explainable in terms of differences in particle-size distributions as
well  as average particle sizes.  However,  in cases where the  same aerosol
is being scrubbed at different operating conditions  (Figure 23),  it is
difficult  to offer a satisfying physical interpretation for a crossing of
performance curves.  Differences in collection mechanisms, as in  the
cases of Adiabatic Saturation and Condensation scrubbing, can offer a
reason—if not a detailed explanation—why the performance curves have
different  slopes.  Since the curves do have different  slopes,  they must
at some point either intersect or merge.   If they merge,  the  straight-
line  relationship for at least one of the  curves must  break down.  So f&r
there has  been no evidence  of such breakdowns  in performance  curves at
high  contacting powers  and  high efficiencies.
      The apparent intersection of  the performance  curves  for  Adiabatic
Saturation scrubbing and Condensation  scrubbing in Figure 23  occurs near
the  upper  end of  the range  in which data were  obtained,  so that a merger
of the  curves, rather  than  an  intersection, may have been concealed  by
lack of experimental precision.  Nevertheless,  there is  no unequivocal
evidence  that this was,  in  fact, what  happened.
                                   84

-------
     To illuminate some of these questions, the performance of the scrubber
should be investigated with aerosols of different particle sizes, generated
by atomizing ammonium fluorescein solutions of different concentrations.
Aerosols should be used that are both larger and smaller than the one
used in the present investigation.  Any trends appearing in the observed
phenomena as a result of known changes in particle size should provide
much better bases for interpretation.
                                    85

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                              REFERENCES


1.   Christoforidis,  A. J.,  J. F.  Tomashefski,  and R.  I.  Mitchell,  Use  of
     an Ultrasonic Nebulizer for the Application of Oropharyngeal,  Laryn-
     geal, and Tracheobronchial Anesthesia. CHEST 59 (6), 629-633  (June
     1971).

2.   Ferber,  B. I., F. J. Brenenborg, and A. Rhode, Penetration of  Sodium
     Chloride Aerosol Through Respirator Filters.  Auer.  Ind.  Hyg.  Assoc.
     J. 33 (12), 791-796 (December 1972).

3.   Lapple,  C. E., and H. J. Kamack, Performance of Wet  Dust  Scrubbers.
     Chem. Eng. Prog. 51 (3), 110-121 (March 1955).

4.   Semrau,  K. T., Dust Scrubber Design—a Critique on the State of the
     Art.  J. Air Poll. Control Assoc. 13 (12), 587-594 (December 1963).

5.   Semrau,  K. T., C. W. Marynowski, K.  E. Lunde, and C. E. Lapple,
     Influence of Power Input on Efficiency of Dust Scrubbers. Ind. Eng.
     Chem. 50 (11), 1615-1620 (November 1958).

6.   Stevens, H. R.,  and H.  B. Albregt, Assessment of Ultrasonic Nebuli-
     zation.   Anesthesiology 27 (5), 648-653 (September-October 1966).

7.   Stober,  W. and H. Flachsbart, An Evaluation of Nebulized  Ammonium
     Florescein as a Laboratory Aerosol.   Atmos. Environ. 7_ (7), 737-748
     (July 1973).

8.   Stober,  W., H. Flachsbart, and C. Boose, Distribution Analyses of
     the Aerodynamic Size and the Mass of Aerosol Particles by Means
     of the Spiral Centrifuge in Comparison to Other Aerosol Precipi-
     tators.   J. Coll. Interface Sci. 39 (1), 109-120 (April 1972).

9.   West, P. H., H.  P. Markant, and J. H. Coulter, New Venturi Scrubber
     Developments.  The Steam-Atomized Venturi.  Tappi 44 (10), 710-715
     (October 1961).
                                   86

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                                GLOSSARY
 Actual Conditions (gas) - The actual temperature and pressure of a gas
 at the point of reference.

 Collection Efficiency - The fraction (IJ) or percentage (100TJ) of an
 aerosol entering a collection device that is removed from the gas
 stream by the device.

 Contacting Power - The power that is dissipated in fluid turbulence
 ^and ultimately as heat) in the gas and liquid phases during gas/liquid
 contacting in a scrubber.  It is commonly expressed in units of power
 per unit of volumetric gas flow rate:

                    3                  3
           KwClOOO m /hr) or Kwh/1000 m               Metric units

           hp/1000 (ft3/min>                          English units

 Diffusiophoresis - Movement of particles through a nonuniform gas
 resulting from  the existence of concentration gradients.   It is produced
 by the uneven molecular bombardment of  the particles due to gradient  in
 gas  composition.

 Affective Friction Loss  - The  equivalent of contacting  power,  expressed
 in terms  of  equivalent  gas  pressure drop (due to friction loss only)
 across the gas/liquid contactor.   In most cases,  it is  approximately
 equal  to  the actual gas  pressure  drop across a device that takes  all  its
 contacting power from the gas  stream.   It is commonly expressed  in  the
 same units as gas  pressure  drop:

          mm WC  (millimeters of water column)        Metric units

           inches WC (inches of water column)         English units

Liquid-to-Gas Ratio - The ratio of the volumes of liquid and gas brought
into contact  in a gas/liquid contactor.   It may be expressed on the basis
°f the gas volume either at actual or at  standard conditions:
                                  87

-------
                   3
          liters/Am   (liters of water per actual       Metric units
                      cubic meter of gas)

                      [liters of water per
                      cubic meter of gas)

                         [gallons of water
                         actual cubic feet of gas)
         3
litors/Sm  (liters of water per standard
            3
gal/1000 Aft  (gallons of water per 100O     English units
                      3
          gal/1000 Sft   (gallons of water per 10OO
                         standard cubic feet of gas)

Scrubber Performance Curve - A correlation curve expressing the functional
relationship between the number of transfer units and the contacting power
(or effective  friction loss).  This is most conveniently presented on a
log-log plot.

Scrubber Pressure Drop - The total drop in gas pressure across a gas
scrubber, whether due to friction loss or to kinetic energy changes.
It is commonly expressed as  the equivalent fluid head loss:

          mm WC  (millimeters of water column)          Metric units

          inches WC (inches of water column)           English units

It may also be expressed in dimensionless form as the number of velocity
heads, based on the gas velocity at the entrance to the gas/liquid con-
tactor or some other suitable entry point to the scrubber.

Standard Conditions (gas) - The selected reference temperature and pres-
sure for specification of gas volumes.  In this report, the standard con-
               o     o
dltions are 25 C (77 F) and 1 atm pressure.

Stefan Flow (also, Stephan Flow) - The net convective gas flow produced
by the diffusion of vapor away from a surface where evaporation of liquid
is taking place, or by diffusion of vapor toward a surface where con-
densation is taking place.  (Definition specific to circumstances of
interest to aerosol deposition.)

Transfer Unit  - Defined by the relationship

          N  -

where N  = number of transfer units and n = fractional collection
efficiency.

                                   88

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Velocity Head - The kinetic energy per unit mass of a  flowing  fluid,
expressed in terms of fluid head:
          mm
WC (millimeters of water column)           Metric  Units
          inches WC (inches of water column)            English  Units
                                    89

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                           CONVERSION  FACTORS
 Gas  Flow
         3                 3
      1  m /hr =  0.58857  ft /rain


      1  liter/min =  0.035316  ft  /min
 Gas Velocity



     1 m/sec =  3.2808  ft/sec



 Liquid Flow



     1 liter/min = 0.26418 gal/min


     1 liter/hr = 0.0044030 gal/min


     1 m /hr =  4.4029  gal/min



 Liquid-to-Gaa Ratio



     1 liter/ra3 = 7.4808 gal/1000 ft3



Contacting Power


                 3                         3
     1 Kw/1000  (ra /hr) = 2.2784 hp/1000 (ft /nin)



 Effective Friction Loss



     1 mm WC = 0.039370 inch WC


     1 mm WC = 2.724 x 10~3 Kw/1000 (ra /hr)

                                   3
     1 inch WC = 0.1575 hp/1000 (ft /min)



Aerosol Concent rat ion


          3                   3
     1  g/m  = 0.43699 grain/ft

           3              —7         3
     1  M,g/m  = 4.3699 x 10   grain/ft

                                 3
     1  g/llter = 436.97 grains/ft

                             —4         3
     1  tig/liter = 4.3697 x 10   grain/ft
                                  90

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                               APPENDIX A
                                            *
                           LITERATURE REVIEW
     Although particulate scrubbers went into use long ago,  sometimes
with marked success, their wider application was long hampered by the

lack of any systematic procedure for designing—or even for selecting—

units for a specific application and specific duty.  In a general way,  a

number of investigators had noted that high scrubber collection effici-
ency generally coincided with high power consumption, but this was regarded
mainly as a disadvantage to be circumvented in some way if possible.   The
                                14
recognition by Lapple and Kamack   that the relationship of scrubber

collection efficiency to power consumption was actually a functional one

did not eliminate the disadvantage of high power consumption,  but did
provide a highly useful if empirical method for designing scrubbers for a
                                 19,20,21
given service.  Later development         of this method for correlating
scrubber performance data led to the "contacting power concept" which has

become, over the past 20 years, a widely accepted basis for practical
scrubber design, employed by many manufacturers as well as users of scrubbers.

     The current demand for higher collection efficiencies for fine parti-
cles has led to increased interest in possible additional particle collec-

tion mechanisms that might accentuate the efficiencies of scrubbers without
further raising the power requirements.  The use of water vapor condensation

phenomena was also suggested many years ago, but there has been very little
information presented to show what might be the magnitude of such effects

in practical scrubbing systems.  At least two quite different mechanisms
             5,19
are involved.       One is the buildup of particles that act as nuclei for

the condensation of water vapor into droplets; this action increases the
 The references cited in this appendix are collected at the end.

                                   91

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effective size of the particles, making their deposition by the inertial


mechanism easier.  This mechanism has, until recently, received the most


attention from research workers and others.  The other mechanisms, diffu-


siophoresis and Stefan flow, are directly diffusional in nature, and have


received appreciable attention only in the last 25 years.



     The available literature on scrubbers to 1962 was reviewed from the

                                                               19,20,21
standpoint of the contacting power correlation by Semrau et al,

                                                    14        27
following the pioneering paper by Lapple and Kamack.    Walker   and

               28
Walker and Hall   also used the contacting power basis in correlating


their own experimental data on scrubber efficiency as well as some data


from the literature.



     Since 1963, most of the papers on scrubbers (other than those that


were purely descriptive) have fallen into two classes.  The first consists


of those papers that treat applications and present some efficiency data,


or correlations of efficiency data, on the empirical basis of gas pressure


drop or some other expression of contacting power.   Such papers have con-


tributed additional confirmation of the validity of the contacting power


concept but have not presented extensions of the earlier work.  A few


empirical or semiempirical studies have attempted more systematic approaches,


and have reported either general agreement with the contacting power concept^


or some exceptions to it.



     The second class of papers in this period has consisted mostly of


theoretical approaches to the problem of scrubber operation and design,


not usually accompanied by much, if any, experimental data for confirma-


tion of the theory.  Most of these theoretical studies have treated the


case of the venturi scrubber.



     Studies of condensation effects to date have been largely theoretical


or speculative in nature, but a number of laboratory investigations have


been carried out in recent years.   Most of the latter have dealt with basic




                                  92

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phenomena and did not attempt to treat applications to practical scrub-


bing equipment.  There has been almost no quantitative information reported


on the magnitude of the effects attainable in actual scrubbing equipment.





POWER CONSUMPTION AND SCRUBBER EFFICIENCY


                                    27
     In experimental studies, Walker   observed that the flooded-disc


(annular orifice) scrubber gave efficiencies on black liquor recovery

                                                            19.20,29
furnace fume that agreed well with those previously obtained


with venturi scrubbers.  He also demonstrated improvements in the effi-


ciences obtained by electrostatic pre-agglomeration of the fume.  In sub-

                                                             28
sequent studies of the flooded-disc scrubber, Walker and Hall   concluded


that collection efficiency was dependent on the gas pressure drop and was


independent of gas velocity and liquid-to-gas ratio.  They also concluded


that there was little effect of scrubber size on the efficiency/pressure


drop relationship.


                      23
     Taheri and Haines   studied the performance of a pilot-plant venturi


scrubber on a test aerosol generated by drying a spray of a solution of


methylene blue.  They investigated the effects of three methods of water


injection: (l) injection through spray nozzles at the throat, (2) injection


through continuous slots at the throat, and (3) injection through a weir


at the beginning of the venturi convergence.   They reported that injection


of the water through the weir produced a given efficiency for about 10%


less pressure drop than did the bther two methods of water injection.


They did not report any effects of gas velocity or liquid-to-gas ratio.


Their data on outlet aerosol concentrations show considerable scatter that


may have resulted from variations in the aerosol particle size.


                 24
     Taheri et al.   studied the efficiency of a pilot plant scrubber using


a butterfly valve as an adjustable orifice contactor.  They used a methy-


lene blue aerosol,  measuring the efficiencies in different particle-size
                                   93

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ranges by sampling at the inlet and outlet of the scrubber with cascade


impactors.  For particles in two size ranges, 1.0-3.0 microns and under


0.6 micron, efficiencies were shown as functions of pressure drop, with


no significant independent effects of liquid-to-gas ratio or butterfly


valve opening.  For the intermediate-sized particles, 0.6-1.5 microns,


there were apparent, marked independent effects of liquid-to-gas ratio


and angle of opening of the valve.  The authors do not appear to suggest


a clear explanation for this behavior, although they report an interaction


of inertial and diffusional collection mechanisms in the intermediate


particle-size range.



     In another study with the same experimental system, but using fixed

                                25
orifice contactors, Taheri et al.   reported that a 1.5-inch contactor


orifice gave higher efficiencies at given pressure drops than did a


1.0-inch orifice, except in the high pressure drop range.  They also


reported that there were independent effects of liquid-to-gas ratio, as


well as of orifice size, in the lower pressure drop range.  However, all


the tests in the lower pressure drop range were apparently made with low


or very low liquid-to-gas ratios,  which might well show independent effects


and lead to Independent effects of orifice size as well.



     A number of investigators have made theoretical analyses of the


venturi scrubber, but few of them have attempted experimental verifica-

                                     2
tion of the theoretical models.  Boll  developed a model for the venturi

                                                      g
scrubber, and attempted to fit the data of Glintheroth,  but acknowledged


that the agreement was only fair.   Among major experimental investigators,


Guntheroth appears to make the strongest claims that venturi geometry,


gas velocity, and liquid-to-gas ratio have major independent effects on

           10
efficiency.



     Behie and Beeckmans  developed a mathematical model of venturi scrub-


ber performance and attempted an experimental verification, but without
                                   94

-------
  attempting to correlate efficiency as a function of gas  pressure  drop.
  Plots  of  their data suggest  that the  actual  particle sizes  of  their test
  aerosols  may have  varied considerably during some tests.
                    3 5
      Calvert et al.'   developed  a  mathematical model  for the operation
  of venturi and other atomizing scrubbers.  They  later extended this
  analysis  to present the efficiency of the scrubber  as a  function of the
  gas pressure drop.   Their design  equations  for  particle penetration
  through the scrubber  depend on  experimental evaluation of one of the
  paraaeters,  which may  then absorb  the  effects of  various actual deviations
  from the  theoretical development.   It  does not appear that the Calvert
 model has  yet  received  extensive experimental verification or evaluation.

 CONDENSATION SCRUBBING
      The possibilities  of using condensation phenomena to assist in scrub-
 bing of particulates have long been considered and were discussed  at  length
 by Harmon   in 1938 with specific reference to the cleaning of  blast  fur-
 nace gas.   The earlier suggestions were made almost entirely with  reference
 to the  buildup of aerosol particle size by using the aerosols as nuclei
 for the condensation of water vapor.  However, more recent  theoretical
                         7 ft  15 1_S 22
 and experimental studies ' '   '   '   have treated the phenomena of dlffu-
 siophoresis and Stefan  flow.   Many  studies have  been entirely theoretical,
 faut in  recent years there have  been a  number  of  laboratory  investiga-
 tions7'8'15'16 that  have clearly  demonstrated the existence  and nature of
 the phenomena.   However, there has  been very  little  information to  indicate
 the magnitude of possible effects in practical or conventional scrubbing
equipment.   Apparently,  the first published data  of  this  sort were those
°f Semrau  et  al,   who  found different  relationships between contacting
Power and collection efficiency in  the  scrubbing of Kraft recovery furnace
                                         29
fume with hot and cold water.  West  et  al<-  later  obtained very similar
data,  although they did not ascribe the differences in scrubber performance
                                  95

-------
to condensation phenomena.  Other reports of the improvement of industrial


scrubber performance by scrubbing with cold water have been qualitative.



     Calvert et al» have recently presented a theoretical analysis of the


use of condensation scrubbing for collection of fine particles.  They


also obtained some data on the efficiency of collection of a submicron


aerosol by a single sieve-plate.  The sieve-plate collector had a low pres-


sure drop and only small amounts of water vapor were condensed during the


tests.  The measured collection efficiencies, which ranged from 11 to 31%,


were compared with the values predicted by the theory developed by the


investigators.  Apparently, no data were obtained on the efficiency of


the device in the absence of water vapor condensation.


                                                                    12 13 17
     It is noteworthy that in recent years a number of investigators  '   '


have successfully correlated mass-transfer data in cocurrent contactors as


functions of energy dissipation.  Most such correlations have been devel-


oped for various forms of pipeline contactors but at least one of the


studies was made of a packed column with cocurrent gas and liquid flows.


The similarities of these correlations to the contacting power correlation


for particle collection is obvious.  The existence of these correlations


strongly suggests that in practical scrubbing equipment, particle collec-


tion by diffusiophoresis and Stefan flow should also be a function of


contacting power.  However, there is no present reason to assume that the


functions will be the same for different types of contactors .



     In both the steel and pulp and paper industries, scrubber systems have


been fitted with cooling towers or other contactors in which the gases are

                        26,30
cooled and dehumidified.       These systems were installed for reasons


other than attempts to increase collection efficiency, and apparently no


effort has been made to determine whether there have been increments of


efficiency resulting specifically from the condensation of water vapor.
                                   96

-------
     In a very recent paper, Calvert et al,  report an extension of
                 f*
the earlier work   on condensation scrubbing by a single sieve-plate


collector.  The investigators studied the collection of iron oxide


aerosols by a three-plate bench-scale scrubber and a five-plate pilot-


scale scrubber.  The units were operated with countercurrent flow of


water (entering cold),  and the entering gas was saturated with water

vapor at its inlet temperature.  Each scrubber was operated at essen-


tially the same gas pressure drop in all tests.  As the quantity of water


vapor condensed (g/g dry air) was increased, collection efficiency


increased markedly.

                  a A
     Calvert et al,  also presented a plot  (reproduced here as Figure 24)


that summarizes the results of a number of condensation scrubbing studies


reported in the literature; penetrations are shown as functions of the


amount of water vapor condensed in or injected into the respective scrub-


ber systems (expressed as g/g dry gas).  Where investigators carried out

tests over ranges of condensation ratio, there are indicated reductions


in the particulate penetration with increasing condensation ratio.  The


operating range observed in the present investigation has been added to


the figure, appearing as the shaded vertical band.  The condensation


ratio used varied only slightly in the tests, but the range of values of


the penetration was determined by the effective friction loss across the

scrubber, which was the other major variable determining the penetration.
                                  97

-------
    1.0
o
I-
o
<
o:
LJ_
   O.I  -
o:
\-
u
z
u
a.
   0.01
                                             PRESENT  INVESTIGATION
8
7,Na2CO,  \   \
                                            ,np=I
 LOW VELOCITY x
              j	
                              4, HIGH VELOCITY



                                     i  t i  i i—
                                                      J	L
     0.01              0.05    O.I               0.5     1.0               5.0



          CONDENSATION OR INJECTION RATIO (g H20/g DRY GAS)



  FIGURE 24  PERFORMANCE OF CONDENSATION SCRUBBERS—LITERATURE DATA (FIGURE 12,

            REF  6A) — RANGE OF PRESENT INVESTIGATION ADDED


            (Number on curves refer to references in Ref. 6A.)
                                    98

-------
                              REFERENCES

                              APPENDIX A
 1.  Behie, S. W., and J. M. Beeckmans, On the Efficiency of  a Venturi
     Scrubber, Can. J. Chem. Eng. 5JL  (4), 430-533  (Aug. 1973).

 2.  Boll, R. H., Particle Collection and Pressure Drop in Venturi
     Scrubbers, Ind. Eng. Chem. Fundam. 12 (1), 40-50  (1973).

 3.  Calvert, Seymour, Venturi and Other Atomizing Scrubbers  Efficiency
     and Pressure Drop, AIChE Journal 16 (3), 392-396  (May 1970).

 4.  Calvert, S., J. Goldshraid, D. Leith, and D. Mehta, Scrubber Handbook,
     Vol. I, A.P.T., Inc., Riverside, California (July 1972), EPA Contract
     No. CPA-70-95, NTIS:  PB-213 016.

 5.  Calvert, S., J. Goldshmid, and David Leith, Scrubber Performance
     for Particle Collection, AIChE Symposium Series 70 (137), 357-364
     (1974).

 6.  Calvert, S., J. Goldshmid, D. Leigh, and N. C. Jhaveri,  Feasibility
     of Flux Force/Condensation Scrubbing for Fine Particulate Collection,
     EPA Rept. No. EPA-650/2-73-036 (Oct. 1973), NTIS:  PB-227 307.

 6A. Calvert, S., and N. C. Jhaveri, Flux Force/Condensation  Scrubbing,
     Proc. EPA-APT Fine Particle Scrubber Symposium (May 28-30, 1974),
     pp. 86-118.

 7.  Fuchs, N., and A. Kirsch, The Effect of Condensation of  a Vapor on
     the Grains and of Evaporation from Their Surface  on the  Deposition
     of Aerosols  in Granular Beds, Chem. Eng. Sci. 20, 181-185 (1965).

 8.  Goldsmith, P., and  F. G. May, Diffusiophoresis and Thermophoresis
     in Water Vapor Systems, Aerosol Science, Davies,  C. N.  (ed.),
     Academic Press, New York  (1966), pp. 163-194.

 9.  Guntheroth, H., Schwebstoff-Nassabscheidung aus Gasen mit dem Venturi
     Scrubber, Fortschritt-Ber. VDI-Z Series 3,  Issue  13  (1966).

10.  Guntheroth, Hans, An Artificially Generated Aerosol for  Wet Scrubbing
     Tests on a Semi-Industrial Scale, Staub-Reinhalt  Luft (in English)
     28 (11), 82-87 (Nov. 1968).
                                    99

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REFERENCES (continued)

11.  Harmon, Robert Rogers, Removal of Suspended Matter from  Industrial
     Gases, J. Inst. Fuel 11_  (60), 514-527  (Aug. 1938).

12.  Jepson, John C., Mass Transfer in Two-Phase Flow in Horizontal
     Pipelines, AIChE Journal 16_  (5), 705-711  (Sept. 1970).

13.  Kulic, E., and E. Rhodes, Chemical Mass Transfer in Co-Current Gas-
     Liquid Slug Flow in Helical  Coils, Can. J. Chem. Eng. 52 (1),
     114-116 (Feb. 1974).

14.  Lapple, C. E., and H. J. Kamack, Performance of Wet Dust Scrubbers,
     Chem. Eng. Prog. 51 (3), 110-121 (Mar. 1955)

15.  Meisen, A., A. J. Bobkowicz, N. E. Cooke, and E. J. Farkas, The
     Separation of Micron-Size Particles from Air by Diffusiophoresis,
     Can. J. Chem. Eng.  19 (4), 149-157 (Aug. 1971).

16.  Melandri, C., V. Prodi, 0. Rimondi, and G. Tarroni, Submicron
     Particle Precipitator Based  on Water Vapor Condensation, Treat.
     Airborne Radioactive Wastes, Proc. Symp. 1968, 541-550

17.  Reiss, L. Philip, Cocurrent Gas-Liquid Contacting in Packed Columns
     I & EC Proc. Des. Develop. 6 (4), 486-599 (Oct. 1967).

18.  Rozen, A. M., and V. M. Kostin, Collection of Finely Dispersed
     Aerosols in Plate Columns by Condensation Enlargement, Int. Chem.
     Eng. 7 (3), 464-467 (July 1967).

19.  Semrau, Konrad T.,  Correlation of Dust Scrubber Efficiency, J. Air
     Poll. Control Assoc. 10  (3), 200-207 (June 1960).

20.  Semrau, Konrad T.,  Dust Scrubber Design—A Critique on the State of
     the Art, J. Air Pollution Control Assoc. ^3 (12), 587-594 (Dec. 1963).

21.  Semrau, K. T., C. W. Marynowski, K. E. Lunde, and C. E.  Lapple,
     Influence of Power Input on Efficiency of Dust Scrubbers, Ind. Eng.
     Chem. 50  (11), 1615-1620 (Nov. 1958).

22.  Sparks, L. E., and M. J. Pilat,  Effect of Diffusiophoresis on
     Particle Collection by Wet Scrubbers, Atmos. Environ. 4, 651-660 (1970),

23.  Taheri, M., and G.  F. Haines, Optimization of Factors Affecting
     Scrubber Performance, J. Air Poll. Control Assn. 19_ (6), 427-431
     (June 1969).
                                  100

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REFERENCES (concluded)

24.  Taheri, M., S. A. Beg, and M. Beizaie, Gas Cleaning in a Wetted
     Butterfly Valve, J. Air Poll. Control Assn. 22  (10), 794-798
     (Oct. 1972).

25.  Taheri, M., S. A. Beg, and M. Beizaie, The Effect of Scale-Up on
     the Performance of High-Energy Scrubbers, J. Air Poll. Control Assn.
     23 (11), 963-966 (Nov. 1973).

26.  Vegeby, Anders, Hot Water from Recovery Boiler Stack Gas:  SF
     Scrubber—MODO System, Pulp Paper Mag. Can. 69  (9), 68-74 (May 3, 1968).

27.  Walker, A. B., Enhanced Scrubbing of Black Liquor Boiler Fume by
     Electrostatic Pre-Aggloraeration:  A Pilot Plant Study, J. Air Poll.
     Control Assn.  13 (12), 622-627 (Dec. 1963).

28.  Walker, A. B., and R. M. Hall, Operating Experience with a Flooded-
     Disc Scrubber—a New Variable Throat Orifice Contactor, J. Air Poll.
     Control Assn.  18 (5), 319-323 (May 1968).

29.  West, P. H., H. P. Markant, and J. H. Coulter, New Venturi Scrubber
     Developments.  The Steam-Atomized Venturi, Tappi, 44 (10), 710-715
     (Oct. 1961).

30.  Willett, H. P., and D. E. Pike, Venturi Scrubber for Cleaning Oxygen
     Steel Process Gases, Iron & Steel Engr., 38 (7), 126-131 (July 1961).
                                  101

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                             APPENDIX B
                   ANALYSIS OF EXPERIMENTAL DATA

     The scrubber performance curves drawn through the data points in
the various correlations were placed by eye, and not all points were
accorded equal weight.  Rather, judgments were made of the probable
reliability of individual points and some points were essentially
ignored.  In general, the curve was selected to give the best appear-
ing straight line over the widest possible range of pressure drop.
When the data points in the low pressure drop range appeared to fall on
a different curve, they were fitted (so far as possible) by separate
straight lines.
     This procedure involved relatively little uncertainty in those cases
where little scatter appeared in the data points and a straight line was
clearly defined.  For the Ambient Scrubbing tests, considerable scatter
usually remained in the data even after they were rationalized by plot-
ting according to ranges of aerosol generation rate.  In these instances,
the bias injected during curve fitting was substantially greater.
     Although the Ambient Scrubbing tests were originally made with the
intention of displaying systematically the effects (if any) of orifice
gas velocity, liquid-to-gas ratio, and orifice size, the effort was
largely frustrated by the variations in the aerosol.  Thus, the individual
correlations according to aerosol generation rates (Figure 6-10) were
composed of data having somewhat random values of gas velocity and liquid-
to-gas ratio.  The transfer unit data were well rationalized by plotting
them as functions of effective friction loss, without reference to ori-
fice diameter, gas velocity, or liquid-to-gas ratio.  This point is
illustrated more clearly and forcefully by the plots in Figures 25-28,
                                 102

-------
3.0
2.0
1.0
0.9
0.8
0.7
0.6

0.5

0.4

0.3
     a
       o
     -OD-
                                                                  Cl
I   I  I  I
                           4   5678910
                             LIQUID-TO-GAS RATIO
                    20
                  liters/m3
                                                                  40     60    80  100
UJ
a
o
3









3.0

2.0

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
1

I I I I I I I I

. — —
0 D
0
a
n-l PI .. _,
Q
— —
— —
— —
— —
_ —
I I I I I I I I
0 20 40 60 80 10
GAS VELOCITY IN ORIFICE — m/«c
                                                                           S A-2745-26
 FIGURE 25   ANALYSIS OF RESIDUAL DEVIATIONS—AMBIENT SCRUBBING TESTS
              Aerosol generation rates 750-1000
                                      103

-------
z

z
LU
o
g

to
LU

DC
3.0
2.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
1
3.0
2.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
I I II IN
	 1 1 1 Mill
„ —
V Z
V ^ v v V _
— —
_ —
— —
_ —
I I I I 1 1 1 1 1 1 1 1 1 1 1 1
2 3 4 5 6 7 8 9 10 20 40 60 80 10
LIQUID-TO-GAS RATIO — liters/m3
1 1 1 1 1 1 1 1

I ^ * V" I
— —
— —
— —
_ —
1 1 1 1 1 1 I 1







       10          20          40     60   80 100

           GAS VELOCITY IN ORIFICE — m/sec
                                                                            SA-2745-27
     FIGURE  26    ANALYSIS OF RESIDUAL  DEVIATIONS—AMBIENT SCRUBBING TESTS


                  Aerosol generation rates 1000-1200 jig/min.
                                         104

-------
g

<
>
Q
_J
<
g

-------
g
<
UJ
D
CO
UJ
tc.
3.O
2.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
1
3.0
2.0
1.0
0.9
0.3
0.7
0.6
0.5
0.4
0.3
I 1 I 1 1 1 1 i 1 1 1 1 1 1 1 I I
_ —
A * * A A
-
_ A —
— —
— —
— —
— —
1 1 1 f 1 1 1 1 1 1 1 1 1 1 1 1
2 3456789 10 20 40 60 80 10
LIQUID-TO-GAS RATIO — liters/m3
1 1 1 1 1 1 1 1
A jU .
<> /j
~~ A
— A —
— —
— —
_ —
— —
1 1 1 1 1 1 1 1







       10          20          40      60   80  100
           GAS VELOCITY IN ORIFICE — m/sec
                                                                             SA-2745-29
     FIGURE  28   ANALYSIS OF RESIDUAL  DEVIATIONS—AMBIENT SCRUBBING TESTS
                 Aerosol  generation rates  1500-1900 jig/min.
                                         106

-------
where the residual deviations of the data points from the correlation
curves are plotted as functions of liquid-to-gas ratio and gas velocity
in the contractor orifice.
     The residual deviations were determined graphically, and laid off
from the reference axis 1.0 on the same log-log grid used for the origi-
nal plot.  Any trends with liquid-to-gas ratio or gas velocity that are
not reflected in the effective friction loss should thus be made readily
visible.
     The dispersion of the residuals in Figures 25-28 is exaggerated by
the bias introduced in placing the original correlation curves.  Never-
theless, no significant independent trends in performance with varia-
tions in liquid-to-gas ratio or gas velocity are indicated.  The effects
of these variables are evidently completely absorbed in the effective
friction loss.
    No such plots were made for the separate correlations in the low
pressure drop regions of Figures 7 and 8, since the data in these regions
were inadequate to define good correlation curves in the first place.
No residuals were plotted for the correlations of Figure 6, since it was
obvious by inspection that the residuals would be very small except for
the one data point presumed to represent a systematically different
aerosol.
     In Figure 29, similar plots are presented for the residual devia-
tions of the data points for the Adiabatlc Saturation scrubbing tests
with Aerosol A (Figure 13).  In this case, the original correlation was
generally better than those for the Ambient scrubbing tests.  The absence
of any significant trends in the residual deviations is again clear, even
though the liquid-to-gas ratio—in particular—was varied over a range
of nearly fourteenfold.
                                 107

-------
Ul
Q
Q
in
UJ
tr
    3.0
    2.0
     1.0
     0.9
     0.8
     0.7
     0.6

     0.5

     0.4

     0.3
    3.0
2.0
    1.0
    0.9
    0.8
    0.7
    0.6

    0.5

    0.4

    0.3
                                                                           I   I  I  I  I
                                           cP
                           I    I   I  I  M  I
                               456789 1O          20
                                LIQUID-TO-GAS RATIO — liiers/m3
                                                                  40
                                                                         60   80 100
               I
                                                       ORIFICE DIAMETER Icm)
                                                              O   2.54
                                                              A   3.81
       10          20           4O     60   80  100
            GAS VELOCITY  IN ORIFICE — m/sec
                                                                               SA-2746-3O
     FIGURE 29   ANALYSIS OF  RESIDUAL DEVIATIONS—ADIABATIC SATURATION
                  SCRUBBING—AEROSOL  A
                                         108

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                               APPENDIX C
                     POTENTIAL COMMERCIAL APPLICATION
                        OF CONDENSATION SCRUBBING
     The results of the present investigation as well as of earlier ones
have indicated that the contacting power requirement for scrubbing of a
given aerosol may be substantially reduced if condensation phenomena can
be employed to enhance the more conventional scrubbing mechanisms.  In
addition to the reduction in contacting power, there is an additional
saving on power that results from the cooling of the gas and the conse-
quent reduction in the volume of the gas that must be handled by the
induced-draft fan.  Particularly in cases where the original gas has a
high water vapor content, the condensation of the vapor as well as the
reduction in temperature can combine to produce a major reduction in the
gas volume that must be handled.
     The use of gas cooling towers following some scrubbers on steel making
furnaces (see Appendix A) has apparently been intended primarily to gain
public relations advantages by eliminating steam plumes from stacks, but
the saving of fan power may be of equal weight.  Apparently it was (and
perhaps still is) a common practice to discharge the cooling water from
the towers directly to sewers, since the water contained relatively little
particulate matter.  However, if such discharges are stopped by water
pollution control regulations (as they probably will be), the cooling water
will itself have to be treated and recirculated after being cooled by direct
or indirect means.  In such a case, the economics of cooling and dehumidi-
fying the scrubber exit gas will present a different and less favorable
picture.
                                   109

-------
     On  the  other hand,  the  cleaning  and dehumidification of  blast
 furnace  gas  for  immediate  or delayed  use as  fuel  is  a general necessity.
 Hence, the conditions  for  potential use of condensation scrubbing already
 exist  in this  case.  A similar potential application exists where basic
 oxygen furnaces  are  equipped with  noncombustion gas  collection systems.
 When the offgas  is accumulated for later use as fuel, it will—like blast
 furnace  gas—have to be both cleaned  and dehumidified.   Yet another simi-
 lar  situation  exists where gases from smelters, pyrite  roasters, or other
 similar  sources  are  used to  make sulfuric acid by the contact process
 (see Appendix  A).  The feed  gas to the contact plant must be  thoroughly
 cleaned  and  dehumidified,  and requirements for final gas cleanliness and
 dryness  are  more critical  than those  for blast furnace  gas.
     Gas  cleaning and  conditioning systems for blast furnace  gas and
 acid-plant feed gas  are already well  developed and reasonably well stan-
 dardized  after long  experience, even  though  they  may still be far from
 optimum.  Gas  scrubbing systems for unburned EOF  offgas  are in commercial
 use, but  apparently  are not  yet as well developed or standardized as in
 the  other two  applications.  In all three of these applications of scrub-
 bers,  condensation mechanisms of particle collection may already be oper-
 ative, but possibly  not in optimum ways.  If condensation phenomena can
 be used more effectively,  it may require no  more  than relatively minor
 modifications  of the systems to accomplish this.
     A fourth  possible  application of  condensation scrubbing  is in the
 collection of  fume from a  Kraft black  liquor recovery furnace  (see Appen-
 dix A).  This  collection problem is a more conventional  one than the others
mentioned above.  The gases  already have high moisture contents (25 to 35%
by volume),  and in this respect are representative of a  considerable num-
ber of industrial waste gases (e.g.,  gases from wet-process cement kilns
 and from lime  kilns burning precipitated lime; tail  gases  from thermal
phosphoric acid furnaces).   Scrubbers are already extensively used on
                                   110

-------
black liquor recovery furnace gases, either alone or in combination with
electrostatic precipitators.  Pulp mills utilize substantial amounts
of low-level heat, particularly for heating water for pulp washing.
Considerable use is already being made of scrubbers—particularly in
Scandinavia—to heat water by contact with the hot gas from recovery
furnaces, and this results in condensation of water from the gases.  In
such cases, condensation effects may be brought into play inadvertently,
but apparently there are no operating installations that were designed
deliberately to take advantage of condensation.
     Still another possible application for condensation scrubbing is
the cleaning of the flue gases from sewage sludge incinerators.  Because
of the high moisture contents of the sludge filter cakes being burned,
the water vapor content of the flue gas is commonly equivalent to a
                                  o
saturation temperature of about 80 C.  Scrubbing with sufficient water
at ambient temperature will thus produce a large amount of condensation,
which will increase the collection efficiency.  However, the fly ash from
the incinerators apparently contains little material that is fine enough
in particle size to present a difficult collection problem.  Therefore,
there may not actually be a strong economic incentive to employ condensa-
tion scrubbing.

RECOMMENDED APPLICATION FOR TESTING
     Scrubbing of black liquor recovery furnace gases appears to be by
far the most advantageous application for a field demonstration of con-
densation scrubbing in a potential commercial application.  It does not
have the highly specialized aspects of the cleaning of blast furnace and
EOF gases.  The fume is of a relatively uniform nature and composition
and is generated on a comparatively steady basis, and considerable infor-
                                   111

-------
mation is already available on its scrubbing characteristics, including
preliminary indications of possible improvements in performance realizable
from use of condensation.  Also, the use of low-level heat now wasted in
pulp mills is becoming more attractive as the costs of fuel rise.  There
is already extensive experience with the use of commercial scrubbers,
including observations that the performances of small and large units at
the same contacting power level are essentially the same.  Hence, it
should be possible to apply system modifications developed on a pilot-
plant scale to a full-scale commercial unit with minimum risk.

PILOT-PLANT SYSTEM
     If a field demonstration is carried out it is recommended that the
investigation be carried out on a pilot-plant basis with a scrubber unit
                                                     3           3
 (or units)  having a gas flow capacity of about 850 m /hr (500 ft /min),
 measured at inlet conditions.  Such a pilot plant is small enough to be
 operated with relative ease on a slip stream of gas from a commercial
 recovery furnace without interfering with,  or being unduly interfered
 with,  by the operational requirements of the furnace.  Modifications of
 the experimental system can be made quickly and at modest costs.  At the
                                                               3
 largest, the pilot plant gas capacity should not exceed 1700 m /hr
         3
 (1000 ft /min).  The fan capacity should be adequate to handle scrubber
 pressure drops of up to at least 1800 mm WC (70 inches WC).
      A minimum program can be elected, or an optional expanded program
 can be undertaken at higher cost.  If the expanded program is adopted,
 the following major system components will be required:
 A.   A simple baffle chamber with a spray nozzle located at the inlet.
      The chamber should be designed for a maximum superficial gas
      velocity of about 1.0 m/sec (3 ft/sec) and to give a nominal gas
      residence time of up to three seconds.
 B.   A variable-orifice scrubber with a cyclone entrainment separator.
                                   112

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C.   A scrubber with alternative variable-orifice and pipeline contactors
     and a cyclone entrainment separator.

D.   A countercurrent packed tov/er with provisions for installation of
     up to 4 m (13 ft) of grid packing.  The tower should be designed
     for a maximum superficial gas velocity of 1.5 m/sec (5 ft/sec).
     It should be equipped with a louvre- or herringbone-type of entrain-
     ment separator, designed for a maximum superficial gas velocity of
     1.5 m/sec (5 ft/sec).

E.   Induced-draft fan.

F.   Water-circulating system, including pumps and shell-and-tube heat
     exchangers for cooling recirculated scrubbing water.

     If the minimum program is elected, then A, the baffle chamber, and
D, the packed tower, can be eliminated.  The collectors in the system
should be so equipped that they can be fitted together in any desired

combination,  although only certain combinations are contemplated.  Pro-

visions should be made to sample the gas not only at the inlet and outlet

of the system but between any units placed in series.


MINIMUM PROGRAM

     The basic, or minimum program will make use of the two cocurrent-

contact scrubbers B and C in the following steps:

     The reference operating condition will be adiabatic saturation
scrubbing of the gas, using the variable-orifice scrubber C. Sufficient

data will be taken to establish a scrutbar performance curve for a wide

range of effective friction loss (up to at least 1500 mm WC).

         Scrubber C will then be operated with cold water as a
         condensation scrubber, to give an exit gas temperature of

         50 C.  Scrubber performance curves will be established for

         both the orifice and pipeline contactors, if the two devices

         should give different curves.
                                  113

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      •   Scrubbers B and C will be used in series, with B operated
         as a low-energy adiabatic saturator and with C operating
         on the presaturated gas as a condensation scrubber with an
                                   o
         exit gas temperature of 50 C.  The efficiencies of each
         scrubber individually and of the two in combination will
         be determined.  Sufficient data will be taken to establish
         scrubber performance curves over the range of effective
         friction loss up to at least 1500 mm WC total (for both
         scrubbers).

EXPANDED PROGRAM
     The expanded program will incorporate the minimum program, together
with additional experiments using the baffle chamber A and the counter-
current packed tower D.
          The baffle chamber will be used as a gas quencher rather
          than as a scrubber.  It will be followed by Scrubber C,
          used in both adiabatic saturation scrubbing and condensation
          scrubbing.  The baffle chamber will be fed with only enough
          cold water to bring the inlet gas to saturation and—perhaps—
          to cause fume particles to nucleate droplet formation.  If
          significant buildup of the particles takes place, it may be
          revealed by the performance curves for the scrubber.
          The countercurrent packed tower D should represent a
          low-energy device that nonetheless will produce a large
          amount of water vapor condensation.  It will be operated in
          series with,  and downstream of,  Scrubber C, which will be
          operated as an adiabatic saturation scrubber at two levels
          of effective friction loss but at the same gas flow rate.
          The overall efficiency and the separate efficiencies of each
                                  114

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         device will be determined.  For each level of prescrubber

         effective friction loss, the efficiency of the packed tower

         will be studied as a function of water rate and amount of

         water vapor condensed.
PROGRAM REQUIREMENTS


     If the sizes of the pilot-plant units are kept within the capacity
              3
range of 850 m /hr, the cost of the foregoing program can be kept within

a relatively modest figure.  For the minimum program, the costs of the

equipment and experimental work could probably be kept within about

$250-275,000.  Administrative and other costs involved in carrying on a

program at the plant site of a cooperating Kraft mill could add sub-

stantially to this estimate.  The expanded program would perhaps add

$50-75,000 to the cost.
                                 115

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 4. TITLE AND SUBTITLE

 Wet Scrubber Liquid Utilization
 7. AUTHOR(S)

 Konrad Semrau and Clyde L.  Witham
                                 TECHNICAL REPORT DATA
                           (Please read Instructions on the reverse before completing)
 1. REPORT NO.
  EPA-650/2-74-108
                            2.
                                                        3. RECIPIENT'S ACCESSION-NO.
            5. REPORT DATE
             October 1974
            6. PERFORMING ORGANIZATION CODE
            8. PERFORMING ORGANIZATION REPORT NO.
 9. PERFORMING ORGANIZATION NAME AND ADDRESS
 Stanford Research Institute
 333 Ravens wood Avenue
 Menlo Park, California 94025
             10. PROGRAM ELEMENT NO.
             1AB012; ROAP 21ADJ-005
             11. CONTRACT/GRANT NO.

             68-02-1079
 12. SPONSORING AGENCY NAME AND ADDRESS
 EPA, Office of Research and Development
 NERC-RTP,  Control Systems Laboratory
 Research Triangle Park, NC 27711
             t3. TYPE OF REPORT AND PERIOD COVERED
             Final;  Through 7/13/74	.
             14. SPONSORING AGENCY CODE
 15. SUPPLEMENTARY NOTES
  . ABSTRACT
               report gives results of bench-scale experiments on the ways in whicn
 energy consumption, liquid-to-gas ratio (L/G), and water vapor condensation affect
 the collection efficiency of a conventional,  co cur rent -contact, orifice-type particu-
 late scrubber. The test aerosol consisted of spherical particles of ammonium fluo-
 rescein with a mass-median diameter of about 0. 6 micron. Scrubber collection
 efficiency was determined and correlated as  a function of the effective friction loss
 across the scrubber. Without condensation or evaporation effects, scrubber  efficient
 was dependent only upon effective friction loss , with no independent influences of gas
 velocity, L/G. or contactor orifice size, except possibly in the range of very low gas
 velocities, L/G, and pressure drops. Large amounts  of water evaporation reduced
 collection efficiencies; but scrubbing hot humid gas with cold water (to produce con-
 densation) significantly increased collection efficiency. The most favorable results
 were obtained by presaturating the hot humid gas stream before contacting it with
 cold water.
 7.
                              KEY WORDS AND DOCUMENT ANALYSIS
                 DESCRIPTORS
 Air Pollution
 Scrubbers
 Efficiency
 Aerosol
 Liquid Saturation
 Condensing
 8. DISTRIBUTION STATEMENT


 Unlimited
b.lDENTIFIERS/OPEN ENDED TERMS  C. COSATI Field/Group
Air Pollution Control
Stationary Sources
Liquid Utilization
Particulates
Energy Consumption
Liquid-to-Gas Ratio
19. SECURITY CLASS (ThisReport)
Unclassified
20. SECURITY CLASS (Thispage/
Unclassified
13B
07A
14A
07D
21. NO. OF PAGES

  126
                         22. PRICE
EPA Form 2220-1 (9-73)
                                         116

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