EPA-600/2-77-067
March 1977
Environmental Protection Technology Series
     EVALUATION OF  MOLTEN SCRUBBING  FOR  FINE
                                   PARTICULATE  CONTROL
                                     Industrial Environmental Research Laboratory
                                          Office of Research and Development
                                         U.S. Environmental Protection Agency
                                    Research Triangle Park, North Carolina 27711

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               RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:

     1.   Environmental Health Effects Research
     2.   Environmental Protection Technology
     3.   Ecological Research
     4.   Environmental Monitoring
     5.   Socioeconomic Environmental Studies

This report has been  assigned  to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new  or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
                    EPA REVIEW NOTICE

This report has been reviewed by  the U.S.  Environmental
Protection Agency, and approved for publication.  Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
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recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.

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                                   EPA-600/2-77-067

                                   March 1977
              EVALUATION

       OF MOLTEN SCRUBBING

FOR FINE  PARTICULATE CONTROL
                     by

G.G. Poe, L.R. Water-land, andR.J. Schreiber

    Aerotherm Division/Acurex Corporation
              485 Clyde Avenue
         Mt. View, California 94042
       Contract No, 68-02-1318, Task 24
            ROAPNo.  21ADL-029
         Program Element No. 1AB012
    EPA Task Officer:  Dennis C. Drehmel

 Industrial Environmental Research Laboratory
   Office of Energy, Minerals, and  Industry
      Research Triangle Park, NC  27711


                Prepared for

U.S. ENVIRONMENTAL PROTECTION AGENCY
      Office of Research and Development
            Washington, DC 20460

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                                          FOREWORD





       This document contains a review and evaluation of molten scrubbing as a novel concept for



the control of fine particulate.




       The study was performed for the Environmental Protection Agency, Research Triangle Park,



North Carolina.  Dr. D. C. Drehmel was the EPA Task Officer.  The Aerotherm Program Manager was



Mr. Fred Moreno.  Acting as Technical Advisor for the task was Dr. C. B. Moyer.  The study was



performed during the period December 1975 through February 1976.

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                                     TABLE OF CONTENTS


Section                                                                                 Page

   1       SUMMARY	       1

   2       INTRODUCTION  ......... 	  ......  	       3

   3       LITERATURE SEARCH .....  	  .  	       5

   4       PROCESS'DESCRIPTIONS	       6

   5       PERFORMANCE DATA	      10

   6       THEORETICAL DISCUSSION  	 .  	      12

           6.1  Particle Collection Mechanisms 	  .  	      12
           6.2  High Temperature/Pressure Effects  ....  	      18
           6.3  Pressure Drop and Paniculate Collection  Efficiency  ........      22

   7       PROCESS EVALUATION		      24

           7.1  Material Corrosion 	      24
           7.2  Mist Elimination	      25
           7.3  Molten Salt Reactions	      25
           7.4  Particulate Build-Up in the Molten Salt	      25
           7.5  Process Applications and Economics 	      26



           REFERENCES	      29

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                                   LIST OF ILLUSTRATIONS
Figure                                                                                  Page
   1       Schematic of Battelle-Northwest Molten  Scrubbing Process  	       7
   2       Operation of a Venturi  Scrubber  	      13
   3       Fluid Streamlines  and Particle Trajectories  Around a  Sphere  	      14
   4       Experimental  and Calculated Target Efficiencies  for Spheres  	      16
   5       T^ Versus K .  for f = 0.25	      19
   6       Penetration Versus Aerodynamic Particle Diameter for  Q./Q  =
          1.19 x 10'3	  ?	      20
   7       Penetration Versus Aerodynamic Particle Diameter for  u  = 9310  cm/sec   .  .      21
   8       Penetration Versus Temperature for Q0/(L = 1.19  x 10"3; u_  =  9310
          cm/sec	*.  ?	?	      23
   9       Theoretical Power and Pressure Drop Versus Aerodynamic  Cut  Diameter  ...      27
                                              iv

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                                       LIST OF SYMBOLS


C1          Cunningham correction factor (dimensionless)


CD         drag coefficient (dimensionless)


d          diameter (cm)


d          aerodynamic diameter (cm /g/cm3)
 "a

f          empirical proportionality constant (dimensionless)
K          inertia! impaction parameter (dimensionless)


AP         pressure drop (cm hLO)


P.         penetration (dimensionless)


Q          flowrate (cm3/sec)


r          radius  (cm)


Re         Reynolds number (dimensionless)


t          time (sec)


u          velocity (cm/sec)


y          distance from collector axis to limiting streamline (cm)


Z          axial distance (cm)


n          target efficiency (dimensionless)


y          viscosity (poise)


p          density (g/cm3)


a          surface tension (dyne/cm)

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Subscripts





d          drop





9          gas




&          liquid





p          particle





r          relative





t          throat





0          reference





Superscripts





—          average





           dimensionless
                                             VI

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                                          SECTION  1
                                           SUMMARY

       The purpose of this study was to analyze and evaluate molten scrubbing as a new fine
(£ 3 microns) particulate control concept and to investigate its applicability to high tempera-
ture, high pressure processes.
       The concept of molten scrubbing for particulate capture is indeed a novel one.   Previous
experience in molten scrubbing was almost exclusively concerned with sulfur removal, and placed
no emphasis on particulate collection.  The literature search for the study revealed the pau-
city of data on this subject.  Only two projects were found to have any application to this sub-
ject at all.  IGT's molten metal scrubbing process, however, is proprietary and their test rig
is currently shut down.  The work of Battelle Northwest appears to be the only applicable study
with available data.  Their process involves the scrubbing of both hydrogen sulfide and par-
ticulate with a molten liquid of alkali metal salts for application to hot fuel gas from a coal
gasifier.
       Existing theories provide a reasonably detailed description of the particle collection
mechanisms involved in atomized liquid scrubbers.  These theories should apply equally well at
the high temperatures found in molten scrubbers, so the effects of the high temperatures on
particle collection can be predicted.  The collection efficiency for a given particle size de-
creases as the temperature of the gas increases, according to the best available design equa-
tions.  These predictions, however, have not been confirmed experimentally for molten scrubbers.
       Thus far, the experimental work at Battelle has revealed the following:
       •   Overall collection efficiency is not as great as expected (this may be attributed
           to poor design and/or operational problems).
       0   The high corrosiveness of the scrubbing medium presents a difficult materials hand-
           ling problem.
       •   Carryover of molten liquid droplets in the scrubbed gas was much greater than the
           allowable specs on gas turbine inlets.

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       •   Molten salt reactions with other gaseous pollutants can potentially degrade the
           scrubbing liquid.
       •   Decomposition of the molten salts occurs at temperatures around 900°C; thus, limit
           controls for very high temperatures are necessary.
       •   Insoluble particulate build-up necessitates the use of in-line filters in any re-
           generable scrubbing process.
       •   Energy requirements for molten scrubbers are estimated at 7 to 10 kW/Mcfm, which
           are typical for scrubbing processes at more conventional temperatures.
       Even though it has not been adequately demonstrated as yet, the application of molten
scrubbing to high temperature/high pressure advanced energy processes for fine particulate con-
trol appears  possible.  However, before effective molten scrubbing systems for particulate re-
moval are available, the developmental problems described above need to be solved.

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                                          SECTION  2
                                        INTRODUCTION
       One important proposed use of low Btu fuel  gas derived from coal gasification processes
is the generation of electric power by combustion and expansion of the gas through a gas turbine.
Process temperatures associated with coal gasification are typically near 3000°F, which dictates
that the product gas be cooled to the 1800°F gas turbine inlet temperature.  However, planned
improvements in turbine blade technology may allow inlet temperatures up to 3000°F.  Combining
the high temperature gas turbine with a repowered boiler may result in an overall cycle efficiency
of greater than 40 percent.
       Successful operation of such a system is predicated on the conservation of sensible heat
of the fuel gas.  However, the raw gas from the gasification process contains impurities, notably
sulfur compounds and particulate, which are incompatible with turbine operation and must there-
fore be removed.  Since the fuel gas must remain at elevated temperatures, a high temperature
clean-up process is required.  Such technology is not immediately available, but many novel
concepts have been proposed.  In addition to conserving the sensible heat of the hot gas stream,
many of these concepts have been proposed as fine (<_ 3 microns) particulate cleanup processes.
Fine particles have recently come to be recognized as being much more significant air pollutants
than larger particles.
       One proposed concept, molten scrubbing, utilizes venturi or spray scrubbing with molten
fluids that remain in the liquid state at the cleaning temperature.  This report presents a
review and evaluation of the molten scrubbing processes currently under development in the U.S.
       The work presented in this report was performed by Acurex/Aerotherm for the EPA's Indus-
trial Environmental Research Center as Task 24 on Contract 68-02-1318.  The scope of work was
defined by EPA as follows:
       1.  Conduct a limited literature search to determine if similar scrubbing devices have
           been developed or proposed by others.  Determine if the device is truly novel and
           define the mechanisms which are responsible for particulate capture.
       2.  Determine the reliability and significance of any experimental data submitted.

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       3.  Assess the practicality of the device for collection of fine particulate.  Estimate
           the capital and operating costs for the device evaluated.
       4.  Determine which sources might be controlled by the system and estimate the probability
           of successful  application.
       The evaluation of the molten scrubbing concept is presented in the following sections of
this report.

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                                         SECTION 3
                                     LITERATURE SEARCH
       The search for literature concerned with molten scrubbing occurred  in  three steps:
       •   In-house literature review
       •   Telephone interview with molten scrubbing researchers for information on  their
           processes
       •   Computer-aided literature search of NTIS, COMPENDIX, and Physical  and Chemical Ab-
           stracts resources, performed by the Lockheed DIALOG service.
       The first step produced very little in the way of useful information.   The  telephone
calls uncovered reports, papers, and the general status of the Battene, I6T, and  Atomics In-
ternational processes.  The final step using the computer search produced several  papers of
general interest, all of which are listed at the end of the reference section of this report.
       In all, the literature search clearly revealed that molten scrubbing is a very new and
developing process, and that there is a growing recognition of its potential  as a  possible
high temperature fuel gas purification process.

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                                          SECTION 4
                                    PROCESS DESCRIPTIONS

        Although some pilot- and lab-scale experience with molten scrubbing has been obtained by
 the  Pacific  Northwest Laboratory of the Battelle Memorial Institute (Battelle-Northwest), the
 Atomics International Division of Rockwell International (AI), and the Institute of Gas Tech-
 nology (IGT),  in all cases the data reported have been, at best, incomplete, especially with
 respect to particulate scrubbing.
        The Institute of Gas Technology (IGT) in Chicago recently operated a small lab-scale mol-
 ten  metal droplet contactor designed for sulfur and particulate removal.  The metal in this case
 was  presumably molten iron.  The device is currently shut down, and since the process is pro-
 prietary, no further information is available (Reference 2).
        AI investigated a pilot scale molten scrubbing process aimed at removing S02 from
 boiler stack gases (Reference 3).  The AI scrubber used the ternary eutectic of (Li,K,Na)2CO,
 (M.P.  396°C) as the working fluid.  Gas scrubbing was effected with a spray contactor operating
 at 455°C, and  mist elimination was accomplished with a York wire mesh demister.  However,
 particulate  collection was of secondary concern in the AI process; in fact, an electrostatic
 precipitator was used before the scrubber to reduce the flyash load of the gas before scrubbing
 the  S02.
        BatteHe-Northwest has developed a molten scrubbing process designed to remove H2S and
 particulate  from fuel gas produced by coal gasification (Reference 1).  Because the Battelle-
 Northwest data is the most comprehensive, this process will be singled out for more thorough
 discussion below.   Observations and conclusions based on Battelle-Northwest's experience should
 be generally applicable to all molten scrubbing processes, as other proposed processes should
differ  in, at most,  choice of working fluid or gas-liquid contacting scheme.  The  underlying
principles of operation supporting the Battelle-Northwest process, and the problems encountered
should be universal  to all  molten scrubbing processes.
       The Battelle-Northwest pilot-scale scrubbing process is shown schematically in  Figure  1.
Fuel  gas from a gasifier is heated to 850°C and flows through a venturi where  it  atomizes  a stream

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             Temperature

             Flow
             Gas sample

             Pressure P & AP
                                          Salt  transfer
                                              line
From
gasifier
-CxJ-r—
             Purge
                          Gas
                         heater
                                            Venturi
        Salt
        level
_Q  and press
                                   H2S
                                injection
                                           Deimster and
                                         de-entrainment
                                             section
                                                                                         «•  To  burner
                                                                (Waste  to drums
                                                                or drain)
                                                                                        Pump
                 Figure  1.  Schematic of Battelle-Northwest molten  scrubbing process (Reference 1).

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 of molten salts entering from a reservoir.   Good gas-liquid contact is obtained and both sulfur
 compounds and particulates are entrained in the liquid droplets.   Gas-liquid separation is ac-
 complished in a de-entrainment, mist eliminator section,  with the cleaned gas flowing to a bur-
 ner, and the spent molten salt returning to the reservoir.
        The working fluid used in the Battelle scrubber was  a salt mixture of approximately 36 wtl.
 Na2C03, 37 wt% K2C03,  13 wt% Li2C03, and 14 wt% CaC03-   CaC03 was added to facilitate H2$ scrub-
 bing; and the amount of Li2C03 was  decreased from the  1:1:1  weight ratio of alkali  metal  carbo-
 nates in a eutectic mixture, since  the  lithium salt was by  far the most expensive  of the three
 carbonates used.   The  above mixture is  a noneutectic and has a melting range of 393°C to 541°C.
 Above 541°C the mixture consists of one liquid phase.
        No provision was made in the Battelle pilot facility  for either insoluble particulate re-
 moval or the continuous regeneration of metal  sulfides  to metal carbonates in the  spent salts,
 although batch carbonate regeneration was accomplished  by blowing CCL  and steam through the spent
 melt.  In a continuous regeneration system, insoluble  particulates presumably could be filtered
 from used salts and the batch carbonate regeneration scheme  could be adapted to a  continuous sys-
 tem.
        Gas-liquid  contacting in the Battelle pilot plant was accomplished by a venturi  atomizer.
 As discussed in Section 6,  particulate  removal  in  a venturi  scrubber is effected by particle
 inertia!  impaction onto liquid droplets.  The Battelle  venturi  was designed to remove 55 percent
 of the  particles 4.0 microns or larger.
        A key consideration  in any scrubbing scheme is  the final gas-liquid phase separation after
 the  scrubbing is complete.   Mist elimination in the Battelle pilot plant was accomplished in two
 stages.   A  de-entrainment section consisting of a  column  packed with 1-inch diameter ceramic
 (A1203)  balls  was  designed  to remove 50 percent of the  droplets 3.5 microns in diamter and 95+
 percent  of  the  droplets  greater than 10 microns.   This  section was followed by a demister device
 consisting  of packed rolls,  3/4 inch in diameter,  made  of 3-inch  wide  stainless steel screen.
       Perhaps  the most  important consideration in the  design of  a scrubbing process utilizing
molten salts  is materials choice.   The  combination of  high  temperature (850°C in the Battelle pro-
cess) and the high corrosive  nature  of  molten salts makes the choice of materials  of construction
quite difficult.  Both AI and Battelle  conducted short-term static corrosion screening studies  on
a wide range of metals,  metal  alloys, and ceramics.  In addition, AI  conducted long-term corrosion
tests in rotating capsules.   AI  found that  Type 347 stainless steel was suitable for service below
500°C, while Battelle found  aluminized  Type 304 stainless steel sufficiently corrosion resistant
                                                8

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at 760°C.  However, neither material was exceptionally resistant to attack by the hot molten
(Li.KjNaJgCaCOj; CaS mixture.  Alumina ceramics proved quite resistant but these materials were
rejected as not being readily amenable to conventional fabrication and assembly techniques.

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                                          SECTION 5
                                       PERFORMANCE  DATA
        Performance  data  on  particulate removal  from the Battelle molten scrubber  is presented
 below.   This  scrubber  is  the only one  for which  particulate removal data are available, and
 even these  data  are scanty  and  inconclusive because of the start-up and operating problems ex-
 perienced.
        The  Battelle molten  scrubber was operated for six performance runs during  late 1974.
 Of these, only three runs yielded H2S  scrubbing  data and only two of these yielded particulate
 removal  data  with any  degree of reliability.
        Several shake-down problems were encountered, most of which resulted in salt crystalliza-
 tion and subsequent line plugging throughout the process stream.  This occurred in spite of the
 care taken  with  trace  heating in the pilot plant design.
        A gas  flowrate  of 70 scfm at 750°C - 800°C and 1 to 3 psig was used in the two runs
 yielding particulate removal data.  The salt flow for these runs was estimated at 2 gpm.  Par-
 ticulate gas  samples were isokinetically withdrawn using high temperature stainless steel probes
 both prior  to and after the molten salt scrubbing step.  For the preventuri sample, the total
 inlet particulate burden was determined by passing the sample through a heated glass cyclone
 followed by a high-temperature  glass wool filter capable of collecting particles  0.3 microns
 in diameter.  Post-venturi samples were collected and sized using an Anderson stack sampling
 head capable of  sizing particles in the 0.3 to 20 micron range.  Data for the two productive
 runs  appear in Table 1.  During Run No. 3, the wire mesh demister downstream of the alumina
 balls de-entrainment section was not used.
       Table 1 shows that the overall  scrubber collection efficiency was 70 to 90 percent,
 versus a design  efficiency of 99 percent for particles > 4 microns.  However, most  significant
was  the  fact that approximately 30 wt% of the collected particles in the scrubber outlet gas
were entrained salt crystals.  This observation  has ominous implications if turbine quality
gas  is the desired product.  The Westinghouse Company has specified  a maximum allowable limit
for  alkali metals in gases to present-day turbines of 40 ppb.   The  large quantity of salt
                                               10

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carryover experienced in the Battelle pilot plant suggests that closer attention be paid to mist
eliminator design.

              TABLE 1.  PARTICULATE REMOVAL WITH BATTELLE MOLTEN SALT SCRUBBER*

Inlet gas parti cul ate burden
Outlet gas parti cul ate burden
Overall collection efficiency
Run No. 3
0.1 gr/scf
0.0122 gr/scf
60 wt% < 4 pm
88%
Run No. 6
0.0674 gr/scf
0.0223 gr/scf
76 wt% < 4 pm
67%
Reference 1
       The  Battelle  molten  scrubber was  in operation for a total of about 25 hours.  After Run
 No.  6  it was  disassembled,  inspected  and cleaned.  Visual inspection showed no evidence of
 major  corrosion,  though  some  material  erosion was evident immediately downstream of the venturi
 throat.  However, it is  doubtful  that significant material corrosion evidence would exist after
 such a short  period  of operation.
                                               11

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                                          SECTION 6
                                   THEORETICAL DISCUSSION

       The Battelle-Northwest pilot plant employed a verturi scrubber for gas-liquid contacting.
 The  principles of operation and collection mechanisms for venturi scrubbers, the effects of high
 temperature and pressure, and the effects of using a molten scrubbing medium are briefly reviewed
 in this  section.
 6.1    PARTICLE COLLECTION MECHANISMS
       Venturi scrubbers employ gradually converging, then diverging sections of pipe as shown
 in Figure 2.  The venturi throat is the intersection of the converging and diverging sections of
 pipe.  Generally, liquid is introduced into the gas stream at the throat, and is quickly atomized
 into small droplets by the high velocity gas.  Particles entrained in the gas stream are collected
 by the liquid droplets primarily through particle inertial impaction onto the droplets.  This col-
 lection  proceeds until the droplets are accelerated to the gas (and presumably the particle)
 velocity (References 4 and 5).
       Consider a single spherical collector (liquid droplet) in a uniform velocity field  (gas)
 containing a suspension of small particles as shown in Figure 3.  As the fluid stream approaches
 the  collector the streamlines envelop it.  However, due to inertial forces, the particles  tend
 to cross fluid streamlines, impact the collector, and stick to it or become entrained within it.
 Thus a single collector target efficiency may be defined for the spherical collector,
                                                                                           (1)
where y  is the distance from the sphere's axis to the limiting streamline  for impaction  and
r. is the collector (liquid droplet) radius.
                                              12

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                Clean gas
               and liquid
                droplets
                  0° 0.0 •
1
                                     Liquid in
                  Dirty
                   gas
Figure 2.   Operation of a venturi scrubber.
                      13

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Fluid streamlines
  Figure  3.   Fluid streamlines  and particle  trajectories  around  a
             sphere (Reference  4).

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       The dimensionless equations of motion for particles obeying Stoke' s law are:
                                                                                          (2)
where u,v = u/u , v/u ; u  is a reference velocity,
               0' ""0'  0
                                                                                          (3)
and
                                      Kp -  18^rd  •                                     <«>

The quantities u and v are the x and y components of velocity, respectively,  t is  time,  K   is
termed the inertia parameter, and C1 is the Cunningham correction factor for  the drag  on a
sphere in steady motion.  Equation (2) may be solved for a variety of collector shapes and im-
posed velocity fields allowing, eventually, a calculation of y , and hence r\.  Walton  and
Woolcock (Reference 6) experimentally determined n for K  > 0.2 and found that
approximated the data very well.  A comparison of Equation (5) with the theoretical  prediction
of n vs. K  for potential flow appears in Figure 4.
       If it is assumed that:
       1.  Particles move only with the gas stream and are collected only by liquid droplets
       2.  The liquid drop diameter is given by the Sauter mean diameter from Nukiyama and
           Tanasawa (Reference 7),

                                         ,
                                               15

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          Sphere,  theoretical
          potential  flow
         Sphere,  experimental,
         Walton  & Wool cock
                                i,  .c'  "p V •«
                                T> "   18 Mn <•„
Figure 4.   Experimental  and calculated target efficiencies for spheres (Reference 4).
                                   16

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           where the relative velocity between  gas  and  liquid is assumed to be the gas velocity
           ug (cm/sec),  a is  the liquid surface tension  (dyne/cm), y£ is the liquid viscosity
           (poise),  p£ the liquid density (g/cm3),  and  Q£ and Q  are volume flowrates of liquid
           and gas,  respectively.  Drop diameter, d., is in microns.
       3.  The liquid droplets'  drag coefficient (Cp) is given by Ingebo's (Reference 8)  data
           in the drop Reynolds  number (Re.)  range  of 6  to 400.
                                          CD =
       4.  Particle collection is by inertial  impaction only.
       5.  Particle concentration is uniform in  any plane perpendicular to the gas  flow.
       6.  Liquid is not atomized and distributed  over a cross section until

                                          I*,, = f Ug .                                     (8)

           where u  is the relative velocity between gas and liquid and f is  an empirical  pro-
           portionality constant.
then the single particle size penetration  (one minus efficiency) may be derived for a  venturi
scrubber in the following manner (References 4 and 5).
       For venturi scrubbers the inertial  impaction parameter is defined in terms of the  rela-
tive velocity between gas and liquid droplets.   Thus,


                                   Kp= 9 Vd"= *v£                                (9)

where the aerodynamic diameter of a particle is  defined as
                                     dD = (PC')1/2 d                                    (10)
                                      va     v      v

Taking into account the continuous change  in ur  from f u  to zero, the average target effi-
ciency (n) is expressed in terms of K t =  MO by integrating Equation  (5) over Kp to obtain

                         fVt + .,.Tt^0.M.(^iI)
                     _      r **         ' txn4-               \            /
                                                17

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 The single particle size penetration,  P^.,  is  obtained  from a  material  balance over  a differen-
 tial volume and then integrating.   Thus,
 where Z is the length over which  scrubbing occurs.   Since  particle  impaction  is  operative only
 when a relative velocity exists between  particles  (gas)  and  liquid  droplets,  Z will  be given by
 the length required to accelerate a  liquid droplet to  the  gas  stream  velocity.   Assuming a con-
 stant acceleration given by C, then
                                                                                         (13)
 where n is given  by Equation  (11).  A  plot of r\ vs.  K t  for f  = 0.25  is  shown  in  Figure 5.
        Standard operating  conditions for  the Battelle molten scrubber were  Q^  - 2 gpm and
 Q  = 70 scfm at 750°C  -  800°C and  1 to 2  psig through a  1.5 inch venturi  throat (Reference 1).
 For these values,  Q^/Q  =  1.19 x 10~3, and throat  gas velocity, u  =  9310 cm/sec.  Shown in
 Figure 6 is  a plot of  predicted particle  penetration for f = 0.25 as  a function of aerodynamic
 particle size with gas velocity as a parameter.  Note that Pt  is a strong function of particle
 size in the  1  to  10 micron  range,  and  that scrubber  efficiency (1 - Pt)  is  low for subitvicron
 particles.   Figure 7 shows  Pt as a function of dp  with  0-/Qa  as a parameter.  Note here that
 P.  is a very strong function  of the liquid-to-gas  ratio.  Again, scrubber efficiency is ob-
 served to be quite low for  submicron particles at  all liquid/gas ratios.

 6.2    HIGH  TEMPERATURE/PRESSURE EFFECTS
        The effects of  temperature and  pressure on  predicted venturi performance are quite  im-
 portant (References  9  and 10).  A wealth  of experience has been gained with venturi operations
 at  temperatures <  100°C  using water, however, molten scrubbing requires  operation at tempera-
 tures  up  to  900°C  -  1000°C.   Referring to Equations  (5), (9),  and (13),  it  is  clear that the
 effects of temperature and  pressure on P  are exhibited  largely through  changes in y   and  C1.
 Strauss and  Lancaster  (Reference 9) have  shown that  changes in C1 with temperature ana pressure
 are much  less significant than  changes in the gas  viscosity, so only  viscosity effects will  be
considered here.
       The viscosity of a gas  increases with both  temperature  and pressure, so the effects of
increased temperature or pressure will  be to decrease the inertial impaction parameter K , and
                                              18

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0.4
0.001
                                           10
50
        Figure 5.   n" versus K .  for f = 0.25 (Reference 4).
                             pi
                                  19

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0.01
   0.3
1.0
10.0
                                                               30.0
                            d   (pm
     Figure 6.  Penetration versus aerodynamic particle diameter
               for Q£/Qg = 1.19 x lO'3.
                                    20

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 1.0
 0.1
0.01
0.001
                                     Q£/Qg = 0.25 x 10'3
                               I   \ I    I  I  I  I  I I
    0.3
1.0
10.0         30.0
      Figure 7.   Penetration  versus  aerodynamic particle diameter
                 for ug = 9310  cm/sec.
                               21

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hence, decrease the target efficiency n-  Consequently, n/u  decreases and P  increases with in-
                                                           y                t
creasing pressure or temperature.  The viscosity of air is roughly directly proportional to tem-
perature, however, pressure effects on viscosity are significant only at very high pressures
(> 100 atm) or at low temperatures (< 300°C).  The effect of temperature on predicted single
particle penetration is illustrated in Figure 8 for the Battelle molten scrubber.  This figure
shows P. for air as a function of temperature with the aerodynamic particle diameter as a param-
eter.   The associated calculations considered only variations in gas viscosity with temperature;
changes in the Cunningham correction factor and in the liquid physical properties were neglected.
It is clear from this figure that penetration increases significantly with increasing tempera-
ture for all particle sizes.
6.3    PRESSURE DROP AND PARTICULATE COLLECTION EFFICIENCY
       Assuming the pressure drop across a venturi scrubber is determined solely by the energy
required to accelerate liquid droplets to the gas velocity (wall frictional losses are negligible)
and assuming the gas velocity is constant, the pressure drop is given by

                                                   u_2 Q.
                                  AP = 1.03 x ID'3  9n  *                                (14)
                                                     W9
where AP is in cm H?0 and u  is in cm/sec (Reference 4).  Since pressure drop is the direct
                   ^       9
measure of the energy consumed in the scrubbing process, it is clear from Equation (14) that
additional  energy would be required to increase the collection efficiency for a given particle
size and to significantly increase the efficiency of collecting fine particles (5 3 microns).
This becomes quite evident after examining the curves in Figures 6 and 7.  This is discussed
further in  Subsection 7.5.
                                               22

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1.0
0.1
0.01
0.00
   200
                        dn, = 0.3 ym (g/cm3
800         900
               Figure 8.   Penetration versus  temperature for Q£/Qg = 1.19 x 10"3;
                          U  = 9310 cm/sec.
                                              23

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                                          SECTION 7
                                      PROCESS  EVALUATION

        Although no molten  scrubbing  schemes have yet  been  suggested with  the  primary  goal of
 particulate removal  (Battelle's  scrubber  treated particulate  removal  as a secondary considera-
 tion and the AI scheme  considered  particulates  to  be  an  unavoidable salt  contaminant),  this
 technique may be advanced  as  such  in the  future, especially in  instances  where  sulfur compound
 removal is simultaneously  desired.   Venturi scrubbers (as  well  as  all  scrubbing methods which
 rely on particle-droplet impaction for  particulate collection)  require considerable energy con-
 sumption for efficient  removal of  submicron particles.   For particles  larger  than  2 or  3 microns,
 gas-liquid scrubbing is proven to  be an efficient  particulate collection  method.   On  the whole,
 this method remains  effective even at the elevated temperatures  required  for  fuel  gas scrubbing
 with molten salts.   However,  the use of molten  salts  at  elevated temperatures presents  a new
 set of problems which warrants further  attention.   Some  of these points of concern are  discus-
 sed below.

 7.1    MATERIAL CORROSION
        Perhaps  the  greatest drawback to the use of molten  salts  as a  scrubbing  liquid lies with
 the high  corrosiveness  of  the medium.   Molten alkali  metal carbonates  at  high temperatures pre-
 sent quite  a severe  environment  to virtually  all materials commonly used  in the construction of
 processing  equipment.   Moreover, the presence of metal sulfides  in the gas compounds  this pro-
 blem significantly.  Based on corrosion tests,  Battelle  found aluminized  stainless steels to be
 an  acceptable material  and reported  no  problems with  severe corrosion during  pilot plant opera-
 tions,  however,  the  pilot  plant  was  in  operation for  less  than  30  hours.   Based on longer term
 dynamic corrosion tests, AI used an  austenitic  stainless steel  as  a balance between  cost,
 availability, and corrosion resistance  in the construction of their stack gas scrubbing system.
 However,  during operation  of  their pilot  plant  AI  experienced many problems with  stress corro-
 sion cracking of piping and transfer lines, even though  the AI  process operated at a  relatively
 low  (455°C)  temperature.  AI  also considered  using aluminized stainless steel but rejected  this
material because (a) the aluminum is  difficult  to  apply  uniformly  thus yielding uncoated patches
and  (b) the aluminizing technique is  quite expensive, especially for  parts with large exposed
                                              24

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surface areas (Reference 12).  At the present  time there is no material that is highly resistant
to corrosion by hot molten salts and readily amenable to the common process equipment fabrica-
tion and assembly techniques at an acceptable  cost.

7.2    MIST ELIMINATION
       An integral part of any gas-liquid  scrubbing scheme is the final separation of entrained
liquid from the cleaned gas.  This is a  special  concern when cleaning fuel gas for eventual
turbine expansion using molten alkali metals since the allowable concentration of alkali  metals
in turbine inlet gases is quite low.  Both Battelle and AI experienced significant problems  with
mist elimination (References 11 and 12).  The  Battelle data on salt carryover was  presented  and
discussed in Section 5.  AI reported similar problems with salt crystallization and subsequent
salt carryover in wire mesh demisters.
       An associated problem is that of  salt vaporization and subsequent salt loss in the clean
gas.  Although the vapor pressures of alkali carbonates are quite low, the vapor pressure of
alkali chlorides are significant (e.g.,  the vapor pressure of NaCl  at 800°C is 395 ppm).   The
use of alkali metal chlorides and other  salts  with significant vapor pressures must definitely
be avoided.

7.3    MOLTEN SALT REACTIONS
       Alkali and alkaline earth metal carbonates begin to decompose to their oxides at temper-
atures around 500°C.  Since the oxides of  these  metals have higher melting points  than the car-
bonates, this decomposition increases the possibility of salt crystallization and line clogging.
       Another concern is the possibility of gas reactions with the molten salt liquids.   Even
though Battelle found no change in the CO, HZ, C02> 02> N2 or CH4 composition of fuel gas passing
through their molten carbonate scrubber, equilibrium thermodynamics suggest this could be a  pro-
blem under other test conditions.

7.4    PARTICIPATE BUILD-UP IN THE MOLTEN SALT
       For a molten particulate scrubbing  process to be economically and environmentally  feasi-
ble, the particulates must be removed from the spent salt melt.  AI found  flyash  sparingly  solu-
ble in molten carbonates at 450°C, however, at 800°C Battelle found that 45 to 50 wt% of carbon-
free flyash dissolved in the molten carbonates.  Certainly the effects of  a long-term build-up
of these soluble inorganics on the properties  of the melt should be studied in greater detail
in any future process development work.

                                                 25

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        Filtering of the  used  salt melt  is  required  to  remove  the  insoluble components.  Other
 than corrosion,  AI  reported no  problems  in  filtering insolubles from  the molten carbonates using
 sintered stainless  steel  filters.   However,  they  concluded  that continuous removal of the filter
 cake was necessary.
 7.5    PROCESS APPLICATIONS AND ECONOMICS
        Molten scrubbing  has been applied to  two types  of emission problems over the past few
 years.  Atomics  International developed  the  process for removing SOg  from power plant stack
 gases, and both  IGT and  Battelle-Northwest  proposed the use of molten scrubbing for H^S and
 particulate removal  from hot  fuel gas from a coal gasifier.  We have concluded from the present
 study that molten scrubbing seems most ideally suited  and most appropriately applied to H?S and
 particulate removal  from hot  fuel gases, since an additional step in the regeneration of the
 molten liquid is required for SO^ removal.
        At present there  are no  process cost  estimates  for a molten  salt fuel gas  scrubber.  The
 only cost estimates  available were  prepared  by AI for  their flue  gas  S0? scrubbing concept.
 These, of course, would  not apply to a proposed fuel gas particulate  scrubber because of the
 different gas volumes  and salt  regeneration  requirements.   However, for an order  of magnitude
 illustration  AI  estimated in  1971 (Reference 3) that,  for a 800 MW  power plant (3.6 x 106 cfm)
 capital  requirements would be $17/kW and annual operating costs would be 0.95 mills/kWh (3.53
 mills/Mcf).
        In the absence  of other  data, the relative operating costs for gas cleaning systems may
 be  estimated  from the  energy  requirements of these  systems.   For  atomized liquid  scrubbers such
 as  venturi  scrubbers the  energy required for particle  scrubbing is  essentially proportional to
 the gas  pressure drop  through the scrubber.  Calvert (Reference 13) has developed a relation-
 ship between  scrubber  pressure  drop and  the  aerodynamic particle  diameter at which penetration
 is  50 percent (dp   )  for a variety of gas scrubbing schemes.   Figure 9 (Reference 14}  shows
 dp     as  a  function  of gas pressure drop for a sieve plate, impingement plate, packed column,
 and venturi scrubber.  Also shown are the theoretical  power requirements for each of these  tech-
 niques.   The  plot shows that  the energy  efficiency  of  venturi scrubbing is  comparable to  that
 of  other  scrubbing schemes, and  is, in fact, more energy efficient  for collecting smaller,
 submicron, particles.  The figure indicates that, for  collecting  particles  in  the 0.5 to  2.0
micron range, energy requirements for venturi scrubbers are 1 to  10 kW/Mcfm.   It  is  indeed sig-
nificant that the amount of energy required  for scrubbing increases quite strongly as the par-
ticle size decreases.
                                              26

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                                 Theoretical power  (kW/Mcfm)
      5.0
                0.2
      2.0
      i.o
      0.5
 IO
a.
      0.2
      0.1
                     No.
1
2
3
4
       0.5      1.0
                                               2.0
                               5.0      10.0
Sieve
Venturi, f = 0.25
Impingement plate
Packed column
                                      I
                                      I
                            10        20             50

                                     Pressure drop (cm
                                         1
                               100      200
20.0
                                                           500
             Figure 9.  Theoretical power and pressure drop versus aerodynamic cut
                        diameter (Reference 14).
                                            27

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       Typical  energy usages for other particulate control  systems are (Reference 15):
       •   Cyclones — 1  to 7 kW/Mcfm
       •   Fabric filters - 3 to 4 kW/Mcfm
       •   Electrostatic precipitator - 1.5 to 2 kW/Mcfm
Indeed, the energy costs for venturi scrubbing are comparable to those of other gas cleaning
methods, and may be lower for smaller particles.  Therefore, from an economic standpoint,
molten scrubbing appears promising as a high temperature fine particulate clean-up process.
                                            28

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                                           REFERENCES


 1.  Moore,  R.  H.,  Allen,  C.  H.,  Schiefelbaum,  G.  F.,  and Maness, R. F., "A Process for Cleaning
    and  Removal  of Sulfur Compounds  from Low Btu  Gases," OCR/R&D 100/Int. 1, NTIS PB 236 522,
    August  1974.

 2.  Telephone  interview of Mr.  Dennis  Duncan,  Institute of  Gas Technology, Chicago, Illinois,
    January 7, 1976.

 3.  Atomics International, "Development of a Molten Carbonate Process  for Removal of Sulfur
    Dioxide from Power Plant Stack Gases," APTD-0752,  NTIS  PB 203 466, July 1971.

 4.  Calvert, S.,  Goldschmid, J., Leith, D., and Mehter, D.,  "Wet Scrubber System Study, Vol.  1,
    Scrubber Handbook," EPA-R2-72-118a, July 1972.

 5.  Calvert, S.,  "Venturi and Other Atomizing  Scrubbers Efficiency and Pressure Drop," AIChE. J.,
    Vol.  16, No.  3, pp. 392 - 396, May 1970.

 6.  Walton, W. H.  and Woolcock,  A.,  Intern. J.  Air  Pollution, Vol. 3, p. 129, 1960.

 7.  Nukiyama,  S.  and Tanasawa,  Y., Trans.  Soc.  Mech.  Engrs.  (Japan), Vol. 4, p. 86, 1938.

 8.  Ingebo, R., NASA Tech. Note  3762,  1956.

 9.  Strauss, W. and Lancaster,  B. W.,  "Prediction of  Gas Cleaning Methods at High Temperatures
    and Pressures," Atmospheric  Environ., Vol.  2, pp.  135 -  144, 1968.

10.  Thring, M. W.  and Strauss,  W., "The Effects of  High Temperature on Particle Collection
    Mechanisms," Trans. Inst. Chem.  Eng., Vol.  41,  p.  248,  1963.

11.  Telephone  interview of R. H. Moore, Battelle-Northwest  Laboratories, Richland, Washington,
    January 6, 1976.

12.  Telephone  interview of R. D. Oldenkamp, Atomics International Division, Rockwell  Interna-
     tional, Canoga Park, California, January 19,  1976.

13.  Calvert, S., "Engineering Design of Fine Particle Scrubbers," J. APCA, Vol. 24, No. 10,
    October 1974.

14.  Calvert, S. and Yung, S., "Evaluation of Petersen Scrubber," APT,  Inc. report prepared for
    EPA Contract 68-02-1328, Task 12,  June 1975.

15.  Hegarty, R. and Shannon, L.  J., "Evaluation of  Sonics  for Fine Particle Control," EPA-600/
    2-76-001,  January 1976.


                                  Supplementary Bibliography


 1.  Plyler, E. L.  and Maxwell,  M. A.,  "Proceedings, Flue Gas Desulfurization Symposium 1973,
    New  Orleans,  Louisiana," EPA-650/2-73-038  (NTIS PB 230  901), December 1973.

 2.  Singmaster and Breyer, New York, "An Evaluation of the  Atomics International Molten Carbo-
    nate  Process," NTIS PB 207 190,  November 1970.

 3.  Atomics International, Canoga Park, California, "Development of a Molten Carbonate Process
    for  Removal of Sulfur Dioxide from Power Plant  Stack Gases.  Part  I.  Process Chemistry-
    Reduction," NTIS  PB 191  957, October 1968.

 4.  ibid.,  "	 Part II.  Process Chemistry-Regeneration,"  NTIS PB 191 958, October 1968.

 5.  ibid.,  "	 Part III.  Materials Studies,"  NTIS PB 191  959, October 1968.

6.  ibid.,  "	 Part IV.   Contractor  Development," NTIS PB  191 960, October 1968.

7.  Mandelin,  D. J.,  "Removing Sulfur  Dioxide  from Gas Streams using Molten Thiocyanates,"
    Patent  No.  3773893, Occidental Petroleum Corporation, February 1970.

                                                29

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                                 TECHNICAL REPORT DATA
                           (Please read Instructions on the reverse before completing)
 1. REPORT NO.
   EPA-600/2-77-067
                            2.
                                                        3. RECIPIENT'S ACCESSION-NO.
 4. TITLE AND SUBTITLE
  Evaluation of Molten Scrubbing for Fine
     Particulate Control
              5. REPORT DATE
              March 1977
              6. PERFORMING ORGANIZATION CODE
 7. AUTHOR(S)
           G.G.  Poe, L.R. Waterland, and
           R.J.  Schreiber
                                                        8. PERFORMING ORGANIZATION REPORT NO.
 9. PERFORMING ORGANIZATION NAME AND ADDRESS
  Aerotherm Division/A cur ex Corp.
  485 Clyde Avenue
  Mt. View,  California 94042
                                                        10. PROGRAM ELEMENT NO.
              1AB012;  ROAP 21ADL-029
              11. CONTRACT/GRANT NO.

              68-02-1318, Task 24
  12. SPONSORING AGENCY NAME AND ADDRESS
  EPA, Office of Research and Development
  Industrial Environmental Research Laboratory
  Research Triangle Park, NC 27711
13. TYPE OF REPORT AJM
 Task Final;  1
                               PERIpp_CpVERED
              14. SPONSORING AGENCY CODE
               EPA/600/13
  is.SUPPLEMENTARY NOTES IERL.RTP task officer for this report is D.C.  Drehmel, Mail
  Drop 61, 919/549-8411 Ext 2925.
 16. ABSTRACT
               repOrj. gjves results of an evaluation of molten scrubbing for fine parti-
  culate control,  a concept that study results indicate as seeming to be feasible.  Appli-
  cation of the concept to fine particulate clean-up in advanced energy processes seems
  possible.  Molten scrubbing is especially well-suited to processes where simultan-
  eous removal of sulfur compounds is desired.   However,  before effective molten
  scrubbing systems can be developed for particulate removal, two important problems
  need to be solved: (1) finding  construction materials at an acceptable cost which can
  adequately withstand the highly corrosive scrubbing medium presented by hot molten
  liquids;  and (2) improving gas/liquid separation and mist  eliminator designs so that
  liquid carryover satisfies emission standards or gas  turbine inlet specifications.
  Based on the report's observations and on the above conclusions , it appears that
  considerable development work would be required to investigate the aforementioned
  problems before a final assessment of the feasibility of this  concept could be made.
 17.
                              KEY WORDS AND DOCUMENT ANALYSIS
                 DESCRIPTORS
                                            b.lDENTIFIERS/OPEN ENDED TERMS
                           c.  COSATI Field/Group
 Air Pollution
 Dust
 Scrubbers
 Fused Salts
 Desulfurization
 Gas Turbines
  Air Pollution Control
  Stationary Sources
  Particulate
  Molten Scrubbing
             13B
             11G
             07A
             07D

             13G
 3. DISTRIBUTION STATEMENT
 Unlimited
                                            19. SECURITY CLASS (This Report)
                                            Unclassified
                           21. NO. OF PAGES
                               36
 20. SECURITY CLASS (Thispage)
  Unclassified
             22. PRICE
EPA Form 2220-1 (9-73)
30'

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