EPA-600/7-77-063
U.S. Environmental Protection Agency Industrial Environmental Research EPA'600/7
Office of Research and Development Laboratory *\^-»
Research Triangle Park, North Carolina 27711 J11116 1977
RENOVATION OF POWER
COOLING TOWER SLOWDOWN
FOR RECYCLE BY EVAPORATION:
CRYSTALLIZATION WITH
INTERFACE ENHANCEMENT
Interagency
Energy-Environment
Research and Development
Program Report
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2. Environmental Protection Technology
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EPA-600/7-77-063
June 1977
RENOVATION OF POWER PLANT
COOLING TOWER SLOWDOWN
FOR RECYCLE BY EVAPORATION:
CRYSTALLIZATION
WITH INTERFACE ENHANCEMENT
by
Hugo H. Sephton
The University of California
Campus Research Office
M-ll Wheeler Hall
Berkeley, California 94720
Grant No. R803-257-01-3
Program Element No. EHE624
EPA Project Officer: Fred Roberts
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, N.C. 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D.C. 20460
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ABSTRACT
This work confirms the effectiveness of interface enhancement* a
novel method applied to the evaporation of liquids; it reduces the energy
and capital cost requi rements for the renovation-recycle of industrial
wastewaters. Interface enhancement* depends upon foamy two-phase vapor-
liquid flow induced during the evaporation of a liquid flowing over a heat
transfer surface; this mode of flow provides a substantial increase in the
rate of evaporation of the liquid, after the addition of a selected surfac-
tant. The objectives of this work included the construction of two new
vertical tube evaporation (VTE) pilot plants. A two-effect upflow-downflow
VTE of 10,000 gallons per day capacity was constructed by adding an
identical second effect to an existing single effect, upflow VTE pilot
plant; the second effect was operated in the downflow VTE mode for the
work reported here. This pilot plant was used to obtain comparative
data on the concentration of saline water by upflow VTE and downflow VTE,
and by interface-enhanced upflow and downflow VTE. These data indicate
that while conventional downflow VTE has a higher heat transfer performance
than upflow VTE, the interface-enhanced method of upflow VTE operation
provides a higher performance than both the interface-enhanced and conven-
tional downflow operations.
A second pilot plant facility assembled for this work, was a 5>000
gallon per day vertical tube evaporator-crystal 1izer (EC), tested in the
downflow mode. This facility was first used with low temperature steam
heating, for crystallizing sodium sulfate and for reducing Mohave power
plant cooling tower blowdown to a 30-fold concentrate, at about 51.5°C
(125°F). The objectives of these tests were to determine the feasibility
of renovating cooling tower blowdown with waste heat available within a
conventional power plant cooling cycle; feasibility was indicated by the
test results. Secondly, this EC was operated with a vapor compressor (VC)
in the evaporation temperature range 101.5O-107°C (215"224°F). This series
of tests were for the concentration, by both conventional and interface-
enhanced modes of operation, of saline agricultural drainage water and
industrial cooling tower blowdown. In each of these cases, it was found
that the heat transfer performance of the VTE was increased while its
energy requirements were simultaneously reduced, by applying interface
enhancement (1) , except in the case of crystallizing sodium sulfate.
f
U.S. Patent No. 3,846j2§4, Nov. 5, Wk and foreign counterpart patents,
ii
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CONTENTS
ABSTRACT
LIST OF FIGURES
LIST OF TABLES
ACKNOWLEDGEMENTS
SECTIONS
SECTION I
SECTION II
SECTION I II
SECTION IV
SECTION V
SECTION VI
SECTION VII
SECTION VII
SECTION IX
SECTION X
SECTION XI
- CONCLUSIONS
- RECOMMENDATIONS
- INTRODUCTION
- BACKGROUND
- OBJECTIVES AND SCOPE OF WORK
- UPFLOW-DOWNFLOW VTE TEST FACILITY
- COMPARATIVE UPFLOW-DOWNFLOW VTE AND
VTFE TESTS
- FACILITY FOR EVAPORATION-CRYSTALLIZATION WITH
INTERFACE ENHANCEMENT
- EVAPORATION-CRYSTALLIZATION TESTS
- DISCUSSION OF RESULTS
- REFERENCES
Page
• *
11
iv
v
vi
1
3
4
7
11
12
18
25
33
48
52
Hi
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LIST OF FIGURES
Figure No. Page
1. RENOVATION AND RECYCLE OF POWER PLANT COOLING
TOWER SLOWDOWN 9
2. SIMPLIFIED FLOW DIAGRAM, TWO-EFFECT UPFLOW-
DOWNFLOW PILOT PLANT 13
3. TWO-EFFECT UPFLOW-DOWNFLOW VTE PILOT PLANT 14
4. STEAM-SIDE FLOW PATTERN 15
5. AXIALLY AND SPIRALLY DOUBLE-FLUTED TUBES USED 19
6. INTERFACE ENHANCEMENT EFFECTS ON HEAT TRANSFER
PERFORMANCE AND PRESSURE DROP IN VTFE OF SEAWATER 21
7. INTERFACE-ENHANCED UPFLOW AND DOWNFLOW VTE
PERFORMANCE AT SEAWATER EVAPORATION TEMPERATURES
120 TO 220°F 22
8. DOWNFLOW EVAPORATOR-CRYSTALLIZER (DIAGRAM) 26
9. DOWNFLOW EVAPORATOR-CRYSTALLIZER (PHOTO) 27
10. VTFE FEED ORIFICE PLATE 28
11. UPFLOW VAPOR-COMPRESSION EVAPORATOR WITH CENTRIFUGE 31
12. DOWNFLOW VAPOR-COMPRESSION EVAPORATOR-CRYSTALLIZER 32
13. .SAMPLES OF COOLING TOWER SLOWDOWN AND THEIR 39
CONCENTRATES
14. SLOWDOWN CONCENTRATION FACTOR 41
15. EVAPORATION-CRYSTALLIZATI ON OF WASTEWATER BRINES;
HEAT TRANSFER ENHANCEMENT BY ADDITION OF SURFACTANT 42
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LIST OF TABLES
Table No. Page
I. CRYSTALLIZING EVAPORATION OF SODIUM SULPHATE 35
II. CRYSTALLIZING EVAPORATION OF SODIUM SULPHATE WITH 36
NEODOL 25-3A ADDED
III. CRYSTALLIZING EVAPORATION OF COOLING TOWER SLOWDOWN 38
IV. VAPOR-COMPRESSION EVAPORATION-CONCENTRATION **3
V. VAPOR-COMPRESSION EVAPORATION-CRYSTALLIZATION WITH **5
COOLING TOWER SLOWDOWN FROM CHEMICAL INDUSTRY
VI. VAPOR-COMPRESSION EVAPORATION-CRYSTALLIZATION WITH *&
SLOWDOWN FROM PETROLEUM REFINERY
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ACKNOWLEDGEMENTS
This work was supported by Grant No. R-303257 from the Environmental
Protection Agency, and in part by the University of California. The author
gratefully acknowledges the helpful suggestions of Fred Roberts of the EPA,
the technical assistance of Carl Freel and Sandy Hensley of this Laboratory,
and typing of the manuscript by Amy Adelson.
vi
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SECTION I
CONCLUSIONS
The following conclusions were drawn from, or confirmed by the work
presented in this report:
(1) Renovation of cooling tower blowdown by vertical tube foam eva-
poration (VTFE) can be carried out with a heat transfer performance substan-
tially enhanced in comparison to conventional evaporation. This enhanced
performance VTFE is sufficiently effective at relatively low temperatures
to substantially improve the prospects of performing this renovation with
waste heat available within the usual power plant cooling cycle.
(2) The upflow mode of VTFE operation is more effective in terms of
heat transfer performance and energy requirements than the downflow VTFE
mode or conventional downflow VTE. The level of surfactant additive required
to provide an adequate foaminess in the wastewater feed flow for maximal
heat transfer enhancement is about 10 parts per million (ppm) for upflow
VTFE and 20 ppm for downflow VTFE. At these conditions the upflow VTFE pro-
vides about a 100 percent heat transfer rate enhancement, and the downflow
VTFE about 50 percent enhancement over the conventional vertical tube evapo-
ration (VTE) procedures.
(3) Multieffect upflow and downflow VTFE, over the usual evaporation
temperature range, offer a basis for substantial capital and energy cost
savings over conventional multieffect VTE. The increased hydrodynamic sta-
bility of upflow VTFE permits an increased performance ratio, compared to
VTE.
(4) Preconcentration of industrial wastewaters such as cooling tower
blowdown should be considered before evaporation-crystallization (EC) of the
concentrate. The addition of a surfactant for VTFE was beneficial in each
preconcentration carried out in this work, and in each but one EC. The
latter4was the case of crystallizing a relatively soluble salt (sodium sul-
phate) from aqueous solution at low temperature. In such cases removal of
the surfactant additive from the preconcentrate for recycle to the preconcen-
trator VTFE is indicated to be advantageous. Centrifugation of the slurried
saline-crystalline blowdown from the EC was effective for the separation of
crystalline product.
(5) The use of a surfactant additive and imposed foamy flow for the
evaporation-concentration of cooling tower blowdown by vapor compression
(VC) VTFE has significant advantages. The evaporation rate is increased
while the power required is simultaneously reduced. This holds generally
but not without exceptions, for evaporation-crystallization by VC-VTFE;
1
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it was found to be beneficial for each cooling tower blowdown examined in
this work.
i
(6) In the case of cooling tower blowdown obtained from the Mohave
power plant, a 30-fold concentration was readily obtained at low temperature
(125°F) with high heat transfer coefficients by VTFE and EC after the addi-
tion of 15 ppm surfactant to the blowdown. This indicates that waste heat
should be sufficient to convert this wastewater into 96 percent pure distil-
led water of potable or reuse quality, and k percent slurried concentrate
for disposal.
(7) Evaporation-concentration of presoftened agricultural wastewater
by VC-VTFE provides a 20-fold concentrated solution for disposal or possible
regeneration of the ion-exchange softeners while 95 percent of it becomes
available for reuse, as pure distilled water. The addition of a surfactant
to this wastewater provided a 37 percent increase in the rate of its evapora-
tion, and a simultaneous reduction in the energy requirements.
(8) Concentration, followed by evaporation-crystallization of indus-
trial cooling tower blowdown by vapor compression VTFE, provided clear
advantages over conventional procedures. In one case, a 27-fold concentra-
tion was obtained with increased performance, providing pure distillate and
a slurried concentrate of salts for disposal. In another case, a 20-fold
concentration was obtained by VC-VTFE with an increased performance through-
out the concentration range. This is clear evidence of the beneficial
effects of surfactant additives and the interface-enhanced mode of VTFE
operation for the renovation of cooling tower blowdown.
(9) The dispersant effects of the heat transfer enhancement additive
can be of significant advantage during distillation of wastewaters. It
serves to retain particulates in suspension, keeping heat transfer surfaces
clean, and thereby contributes toward maintaining high heat transfer perfor-
mance and reducing downtime for cleaning or descaling of evaporators.
(10) At a quoted cost of about 25 cents per Ib in bulk delivery of
60 percent active solutions, the use of surfactant for heat transfer enhan-
cement (1, 2) in the range of 50 to 100 percent with surfactant concentra-
tions of 10 to 20 ppm added to wastewater feeds for VTFE, it is clear that
this technology should be adopted for wastewater renovation, in the interest
of both capital and/or energy cost reduction.
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SECTION I I
RECOMMENDATIONS
(1) It is recommended that this work be followed up by the design,
construction and field testing of a pilot demonstration facility, to pro-
vide field test data on the renovation of power plant cooling tower blowdown
of several different types. Such a project, ideally carried out with the
collaboration of the electric power utility industry, should provide final
feasibility and cost data, and provide confidence in the adoption of VTFE
for the complete renovationrrecycle of such wastewaters. It should also
focus attention on the prospects of integrating blowdown renovation-recycle
into the power plant cooling cycle, using low-grade, waste heat for this
purpose, and complying with zero-discharge guidelines with minimal cost.
(2) It is recommended that the very promising results obtained in
this work on the application of interface enhancement to vapor compression
vertical tube evaporation be followed up by the design and construction of
a mobile field test pilot plant unit for operation on several industrial
wastewaters. Such a project should include as one of its objectives to
satisfy potential industrial users of this technology of the merit of this
approach to wastewater renovation-recycle. The proposed field tests should
be designed to provide a clear view of the feasibility and cost involved.
Ideally this field test series should be carried out with the collaboration
of industry.
(3) It is recommended that work be carried out to demonstrate regene-
ration of ion-exchange resins, used for softening irrigation return flows,
using a concentrate of the softened water.
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SECTION II I
RENOVATION OF POWER PLANT COOLING TOWER SLOWDOWN FOR RECYCLE
BY EVAPORATION-CRYSTALLIZATION WITH INTERFACE ENHANCEMENT*
INTRODUCTION
The novel interface-enhanced method (1, 2) of evaporating liquids
applied in this study relies on the addition of a few parts per million of a
selected surfactant to the liquid to be evaporated, followed by causing the
feed liquid to flow as a foamy layer over a heated surface. As a result of
this foamy layer flow, heat transfer from the surface to the feed liquid is
augmented by several mechanisms to provide for a significant increase in the
rate of evaporation, or heat transfer coefficient (3, 4).
In the work described here, the method of interface enhancement for
evaporation of liquids was applied to the vertical tube evaporation (VTE)
procedure. VTE accounts for more industrial evaporation in the U.S. than
all other methods used and its further improvement by the vertical tube
foam evaporation (VTFE) method (5) should be of significant utility. This
foam-ehanced method of operation is applicable to both the upflow and the
downflow modes of operation, whereby the feed liquid is caused to flow
either upward or downward through a bundle of parallel tubes while steam
condensing on their outside walls provides the heat of evaporation (6).
Industrial evaporators are usually operated either in single effect'
or in multi-effect series. In single effect operation the source of
condensing steam (heat) is derived by vapor recompression, whereby the vapor
generated during evaporation of the feed is simply compressed to a rela-
tively higher pressure and temperature, and caused to condense on the tube
outside surfaces. In multi-effect plants the source of heat is usually
steam from a boiler condensed on the tubes of the first effect; each of the
subsequent effects in, for instance, a typical sextuple effect plant
receives heating steam as the vapor from the previous effect. Such a series
of evaporators must therefore be operated at a stepwise reduced evaporation
temperature series. Good steam economy provides the main advantage of such
a series of VTE effects; each pound of boiler steam used provides for
about 6 pounds of distillate from a sextuple VTE. This ratio of product to
heating steam is referred to as the performance ratio or economy ratio.
The number of effects that can be used for an upflow VTE series is
limited by the available temperature span between the steam temperature
* U.S. Patent No. 3,846,254, Nov. 5, 197^ & foreign patent counterparts.
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and the coolant used to condense the vapor from the final effect; typically
three to six effects are used. Another factor that tends to limit the number
of upflow effects in a VTE series is the minimal effect AT, or the
temperature difference required to vaporize the feed as it flows upward
through the tubes (3, M. Part of the available AT is expended to pump the
liquid through the tube by a vapor-lift mechanism. The minimal effective
AT therefore depends on the liquid static head or the hydrodynamic pressure
drop (AP) through the upflow VTE tubes. If the typical pressure drop could
be reduced by a significant factor, the minimal intereffect AT could be
reduced proportionately and the number of effects could then be increased
correspondingly to thereby increase the performance or economy ratio of the
upflow VTE. The interface enhancement method, or VTFE, provides the means
of reducing the AP quite substantially. This AP reduction ranges from a
factor of about 3 to a factor of about 10 over the usual evaporation
temperature range which spans about 66°C (150°F), between the condenser
temperature of about 32 C (90°F) and the steam-side temperature of the first
effect at about 116°C (240°F). This AP reduction effect is the most signi-
ficant single consequence of interface enhancement applied to upflow VTE
(VTFE). This reduction of the tube-side pressure drop (AP) by induced
foamy flow (VTFE) also affects the operation of a single effect, vapor
compression (VC) VTE to similarly reduce the minimal AT that is required,
and thereby to save power.
The second significant effect of imposed foamy evaporative flow is
applicable to both upflow and downflow VTFE; it provides a substantial
increase in the evaporation-side heat transfer coefficient. This effect,
apparently the result of a thin-film, wiped-film mode of flow imposed over
the heat transfer surface, has been observed to be consistently associated
with continuous foamy layer flow of the evaporating liquid in VTFE (1-6).
This enhancement is lost or reduced if an antifoaming agent is added to the
feed or if the vapor flow becomes excessive, causing foam breaking (4).
In the case of upflow VTFE, the combination of tube-side pressure drop
reduction and foamy layer flow was shown to provide a 100 to 200 percent
increase in the overall heat transfer coefficient, U (7); the downflow
performance enhancement was not unequivocally established in comparison with
the upflow enhancement at the outset of this work, but the effectiveness of
VTFE had been demonstrated for both upflow and downflow modes individually.
One of the major objectives (and main results) of this work, was to
establish, on a simultaneous, comparative basis the relative merits of
upflow
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objective of this part of the work was to observe the effects of surfactant
additives on the performance, and the fouling and scaling tendencies of a
crystallizing evaporator operated in the slurry-feed mode. Since the latter
is typically performed by the vapor compression (VC) mode of VTE operation
(8), a VC-VTFE mode was used for some of this work. The usual preference
for single effect operation is the consequence of the limited evaporation
temperature range ideally suited for the crystallization of a particular
species; the VC mode is preferred because of its economy over the steam-
heated mode for single effect VTE.
Another objective of this work was to develop design data and
criteria for a possiblefol low-on project that would demonstrate the techni-
cal and economic advantages of the interface-enhanced method of VTE opera-
tion for the renovation-recycle of power plant coolant blowdown. The need
for developing an acceptable and economical means of cooling tower blowdown
renovation was underlined by EPA guidelines for the steam electric power
industries. This also increased industry interest in such a development.
The earlier development work on interface-enhanced VTE for seawater desali-
nation (9) indicated it to be a promising candidate for such applications.
Not only is the use of surfactant additives quite compatible with most
wastewater evaporation, but the antifouling and antiscaling properties of
such additives should provide added benefits.
In response to the author's proposal to evaluate the use of interface
enhancement for industrial wastewater renovation, the EPA supported an
earlier project, the results of which have been published (5). These
studies showed that, for three different industrial wastewaters examined,
VTFE provides clear technical and economic advantages over existing
evaporation-distillation technologies. The types of wastewater examined
were industrial boiler blowdown, acid drainage from pyrites slag piles,
and power plant cooling tower blowdown. The objective of -the initial
study was to obtain heat transfer performance data on these three types of
wastewater with both typical, best state-of-the-art VTE and with the
interface enhanced VTFE mode of operation, under otherwise identical process
conditions. In each case, it was shown that VTFE provided clear and subs-
tantial advantages, especially in terms of increased heat transfer
performance. Generally, these wastewaters were subjected to a 10-fold
concentration; recovery of solids or a maximal recovery of distilled water
was not an objective; both preconcentration by VTFE and the follow-on
crystallizing evaporation of cooling tower blowdown in the presence of
surfactant additives were to be examined here.
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SECTION IV
BACKGROUND
Industrial cooling accounts for more industrial water use in the U.S.
than all others combined; about 80 percent of industrial water use is for
heat rejection. Much of this use is on a once-through basis, the slightly
heated water being returned to the environment. Since the temperature
of the rejected coolant is slightly above its previous equi1ibrium with the
environment, proportionately more will evaporate to readjust to equilibrium.
This in effect degrades the water quality by concentrating dissolved solids.
To reduce the adverse impact of industrial heat rejection, the EPA has
promoted the use of cooling towers operated with zero discharge; the
objective in this case is to reject heat directly into the atmosphere by
evaporative cooling, using a much smaller flow of coolant but concentrating
it to a dry or semi-dry residue during the evaporative cooling process.
The concept of cooling tower operating with zero discharge is not
new; it is however more expensive than both once-through cooling and partial
evaporation followed by discharge of a somewhat concentrated warm effluent.
Development of VTFE promises economic improvements that should make
adoption of cooling towers and zero discharge more acceptable to industry.
Improved economy follows from the increased heat transfer performance of
VTFE. This permits either a reduction in the heat transfer surface area
required, or increased steam economy (increased performance ratio).
Considering that heat transfer surface represents about 40 percent of the
overall cost of evaporators, the approximately 100 percent improvement in
heat transfer performance by VTFE provides up to 20 percent in capital cost
savings. Alternatively, if the advantage of VTFE is realized in terms of
an increased economy ratio, the cost of steam can be reduced significantly
(by up to 30 percent). Ideally one would optimize for maximal overall cost
savings by utilizing the increased design flexibility of VTFE. This
increased flexibility permits improved trade-offs between capital cost
reduction and fuel cost reduction, to best advantage for a particular
industrial situation. In a situation where steam is available at a site at
relatively low cost, one would design in favor of a smaller number of
effects having an increased per-effect production. Where the cost of steam
is relatively high, one would increase the number of effects to increase,the
ratio of product distillate to steam consumed. A similar situation pertains
to the vapor compression VTE. When the cost of power is low, such as from
a hydro-electric source, one would increase the size of - a single-effect
VTFE; on the other hand, a two-effect VTFE could prove advantageous where
the cost of power to drive the compressor is high.
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In addition the VTFE process permits the adoption of novel modes of
operation previously only marginally feasible, or could add new capabilities
to older VTE applications. One of these possible improvements addressed
in this work is a novel power plant coolant flow diagram that permits the
use of waste heat available within conventional coolant flow diagrams to
renovate the cooling tower blowdown, producing distilled water for boiler
feed, potable use or recycle as coolant, and a dry or slurried salt concen-
trate. Such an improved coolant flow diagram, presented in an earlier
proposal to EPA for a scope of work germane to the work described here for
VTFE applications development, is shown in Figure 1. The work reported
here was planned to provide test data and design information needed as
a preliminary to the earlier proposal. Work under that proposal is to
proceed in the immediate future, building on the data and developments of
this report. The overall objective of these two projects have been to
develop, on an experimental basis, an advanced method of cooling tower
blowdown renovation-recycle.
Figure 1 shows the propdsed flow diagram that, if proven feasible by
the work undertaken in this and the fol low-on project, could be utilized by
a pre-existing power plant. The main objectives of adopting such a flow
diagram would be to renovate the cooling tower blowdown at low cost for
recycle with zero blowdown, and to utilize a VTFE facility that would
interface readily with normal power plant operation. Although at present
still unproven, this flow diagram is presented here because it rationalizes
much of the work undertaken in this as well as in the fol low-on project.
One of the significant features of the flow diagram is that it
permits almost instantaneous revision to the normal, pre-existing power
plant mode of operation, should the need arise. This is done by simply
closing four motorized valves (1 to 4) and opening valve number 5; the
time required is only about 30 seconds. This responds to the concern of
power plant operators about the possible adverse effects of a more complica-
ted flow diagram in emergency situations. In actual fact this flow diagram
reduces the potential for emergency situations by increasing the performance
of the power plant. This occurs because of the additional heat dissipation
capacity provided by a small cooling tower (forced draft), which reduces
the normal heat load on the pre-existing cooling tower by about 5 percent.
The proposed add-on coolant blowdown renovation-recycle loop is
shown in the lower half of Figure 1, with a pre-existing coolant flow
diagram represented in the upper half. These two loops interface via four
valves (1 to 5) shown aeeross the middle of the diagram. The upper loop
could be reverted to the pre-existing mode of operation at any time by
closing valves 1 through k and opening valve 5 to resume the usual discharge
of blowdown, for instance to a reservoir for later renovation. During the
renovation-recycle mode of operation proposed, part of the turbine exhaust
steam is diverted via valve number 2 and condensed in the first effect of
the blowdown evaporation system; this condensate is returned to the boiler,
through a pump and valve number 1. A portion of the heated coolant is
diverted, as blowdown, to this vertical tube evaporator (VTE-1) via valve
number 3. The vapor produced in the VTE-1 serves as heating steam for the
VTE-2 and the forced circulation evaporator-crystallizer (FC-VTE). These
8
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RENOVATION AND RECYCLE OF POWER PLANT COOLING TOWER SLOWDOWN
Figure 1
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two units operate as a combination second effect, and the VTE-1 vapor
condensed in these units provides distilled water available for a beneficial
use such as boiler feed makeup, or potable quality fresh water, or for
recycle to the cooling tower. Vapors produced in the VTE-2 and FC-VTE are
returned to the main coolant loop as condensates from the barometric
condenser, via valve number 4; dilution of this main coolant stream with
condensates alleviates scale formation and reduces coolant makeup pretreat-
ment requirements. The coolant makeup for the barometric condenser and
small cooling tower need not be softened since it is continually diluted
by the condensates in the barometric condenser followed by cooling through
re-evaporation of essentially the same condensates in the small cooling
tower. The nett effect of this blowdown renovation loop is to increase
the capacities of the condenser and cooling tower, thereby increasing
turbine generation capacity, and to produce some distilled water while
reducing blowdown to a residue of solids, thus accomplishing zero liquid
discharge by utilizing waste heat. The author presented this scheme in an
earlier paper (10).
Several alternative subcycles are available within the generic
coolant recycle system of Figure 1. Al1 of those generic cycles are
facilitated or rendered more economical by applying interface enhancement
to the blowdown evaporation-concentration steps, by reducing the temperature
difference (AT) required for cost-effective evaporation, or by increasing
the rate or capacity of the evaporators. One alternative would be to use
a backward feed mode, by condensing the turbine exhaust steam in the VTE-2
and FC-VTE where evaporation is subject to higher boiling point elevation
penalties than in the VTE-1, and to use VTE-1 as the second, distillate-
producing effect. Yet other alternatives available are combinations of
VTE and FC-VTE in a single vessel or in a steam-side continuous vessel (11),
and separate upflow and downflow effect arrangements. In the above flow
diagram the evaporator and crystallizer of minimal capacity sufficient for
zero discharge has been considered, especially for the case of a pre-existing
power plant. An extension of this technique, if successful, is its
application to future power plants, where the opportunity exists for
designing the optimal coolant loop for zero liquid discharge. In such
future designs, one of the three condensers shown in Figure 1 could be
omitted and replaced with a vertical tube evaporator, to perform essentially
the same condensation function while it simultaneously concentrates
coolant blowdown (10, 11). If more than one conventional condenser could
be replaced by evaporator-condensers (VTFE) of similar heat transfer duty,
more useful distilled water can be produced. Considering that the heat
transfer performance of a VTFE is several-fold higher that the conventional
condenser heat transfer coefficient, the replacement of part of the conden-
ser duty by a VTFE condenser can provide significant capital cost savings
on copper-nickel heat transfer tubing. In addition, the need for coolant
softening is reduced and may indeed be obviated if a sufficient portion of
the coolant is diverted, as blowdown, to the VTFE and if the distilled water
product is recycled to the coolant stream.
10
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SECTION V
OBJECTIVES AND SCOPE OF WORK
The specific objectives of this work can be summarized as follows:
(a) To design, construct, install and instrument a second downflow
VTE effect comparable to the pre-existing upflow effect of 5,000 gal. per
day capacity.
(b) To obtain comparative upflow and downflow VTE data with the
pilot plant facility using best state-of-the-art process conditions, to
afford abasisfor determining the relative merits of upflow and downflow
VTE for wastewater renovation-recycle.
(c) To obtain comparative upflow and downflow VTFE performance data
after the addition of a selected surface active agent to the feed, using
process conditions consistent with those used under (b), to provide a
basis for evaluating up- and downflow VTE and VTFE for industrial wastewa-
ter renovation.
(d) To design and assemble an evaporator-crystal 1izer (EC) facility
capable of the seed-slurry feed mode of operation and to provide instru-
mentation of sufficient sensitivity for definitive test data.
(e) To obtain heat transfer performance data during crystallization
by evaporation, under state-of-the-art process conditions.
(f) To obtain heat transfer performance data when a selected surfac-
tant has been added to the seed-slurry feed.
(g) From the above series of tests, provide design criteria for a
fol low-on project that would have as its objective the design, construction
and field test operation of a mobile pilot plant, for the renovation-recycle
of cooling tower blowdown at an existing power plant site, utilizing VTFE.
This mobile pilot plant would comprise an evaporator for preconcentration of
the blowdown, an evaporator-crystal1izer and means for separating crystal-
line or solid residues from the concentrated blowdown, during evaporation
crystal 1ization.
11
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SECTION VI
UPFLOW-DOWNFLOW VTE TEST FACILITY
The two-effect upflow-downflow VTE pilot plant of 10,000 gpd capacity,
assembled under this project is represented by the simplified flow diagram
of Figure 2, and as a photographic print in Figure 3- It utilized
full-sized 2-inch OD x 10-foot long commercial aluminum-brass distillation
tubes of the double-fluted type supplied by Yorkshire Imperial Metals Ltd.,
England in 18-tube bundles, removably installed with 'O'-ring seals.
Aluminum-brass tubes were selected over equally-acceptable 90-10
copper-nickel tubes on the basis,of availability and cost. Materials used
for constructing the brineside of the VTE effects were stainless steel,
brass, bronze, copper and Pyrex glass, to reduce fouling by corrosion
products. Two identical VTE effects were tested, in series connection with
two end effects serving respectively as heat source and heat sink, one on
either side of the two real effects. The second of the two real effects
was rotatably mounted to provide the consecutive upflow-downflow test data
reported here. Temperature and flow controls permitted steady-state
operation of this 4-unit evaporator at any selected temperature level within
the usual multieffect VTE temperature span of 100 to 240°F, to simulate
the most significant VTE multieffeet process conditions. Instrumentation
allowed data and observations for defining the most significant process
parameters and modes of two-phase flow for any effect of both upflow and
downflow multieffect operation, on a comparative basis.
Clean steam was allowed to the upflow (first) effect at a selected
temperature from the reboiler loop shown on the right of Figures 2 and 3.
A pneumatic control valve passed 45 psi steam from a boiler into the
tubeside of a vertical U-tube heat exchanger to maintain the reboiler
steam, generated on the shell-side, at the selected temperature (t 0.1°F).
Condensates from the first effect VTE were collected simultaneously from
four individual distillation tubes and, collectively, from the remaining
14 tubes in this bundle. These condensate flow rates were determined by
stopwatch, timing the intervals required to fill calibrated cylindrical
collection vessels; and heat fluxes (Q.) were derived directly from these
flow rates. Steamside design and tube spacings were as shown in Figure 4.
Indicated are the positions of the four individually drained tubes, the
remaining 14 tubes of the bundle, and dummy tubes and baffles to ensure
proper steamside flow and venting Into a 1.5-inch x 10-foot long vent
collecting tube. All condensates, including the vent condensate, were
collected, their flow rates measured and returned to their appropriate
liquid cycles while being maintained under air-free (vacuum) conditions.
Noncondensibles were continuously removed by bleeding some vapor to a
separate condenser and vacuum pump.
12
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STEAM-SIDE
CONDENSATES
HEAT REJECT LOOP
DOWNFLOWVTE EFFECT
LJPFLQW VTE EFFECT
REBOILER LOOP
SIMPLIFIED FLOW DIAGRAM, TWO-EFFECT UPFLOW-DOWNFLOW PILOT PLANT
Figure 2
-------
TWO-EFFECT UPFLOW-DOWNFLOW VTE PILOT PLANT
Figure 3
-------
STEAM SIDE FLOW PATTERN
Disti11lation tubes were spaced 0.5-inch on a triangular pattern, with dummy tubes (shaded)
and baffles to ensure continuous steamside flow from the plenum to the vent tube along
the entire bundle length. The vent tube had 0.125-inch holes drilled at 6-inch intervals
along its entire length for adequate vent collection.
15 Figure 4
-------
Accurate temperature measurements were obtained for both effects,
with calibrated platinum resistance probes that provided a digital display
and direct printout. These probes were located in the steams ides (Ts)
within the tube bundles, in the brine (Tb) flowing from the effect outlets,
and in the feed (Tf) liquid before flashdown and distribution to the tubes.
These temperature data were checked by vapor pressure measurements with an
absolute manometer (mercury-filled), corrected for boiling point elevation
by reference to literature data, and by mercury-in-glass thermometers.
Heat transfer performances were determined as the overall heat transfer
coefficients:
Q_ Where A is the nominal tube outside surface area based
MT on nominal OD, 2-inch; and AT = Ts - Tb.
Vapor produced in the first effect VTE served as heating steam for the
second effect. Venting of the second effect was similar to the first effect,
and all condensates (distillates.) were collected and returned to the brine
cycle under evacuated conditions.after their flow rates were measured as
above.
Vapor produced in the second effect was condensed in the heat reject
loop, with continuous bleed-off of non-condensibles as above. Heat
rejection at constant flux, was to a large volume of preheated coolant
recycled at about 1,000 gpm through the vertical U-tube heat exchanger shown
on the left of Figures 2 and 3» with a continuous addition of cold water
to and simultaneous rejection of hot water from the recycled coolant.
This coolant was maintained under A-0 psi pressure, and the rate of addition
of cold water and rejection of hot coolant were controlled to provide the
desired temperature difference over both effects (2AT), with reference to
the temperature selected for the reboiler cycle.
The evaporation of several types of distil lands was examined with
this VTE facility, including seawater and industrial wastewater, under
both conventional and interface-enhanced process conditions. The feed was
deaerated-decarbonated by recycling it under vacuum through both effects in
series, and by maintaining the pH at 6.6-6.8 by the addition of sulphuric
acid as required (50-100 ppm). Feed flow to the first effect was controlled
at one gallon per minute per tube, passed through a heat exchanger to
adjust its temperature to within 1°F of the steamside of the upflow effect,
and discharged i-nto a standing leg that was vented to the first effect
vapor dome. Feed flow through the upflow effect orifice plate distributor
was subject only to the level in the standing leg (measured with a sight
glass), the effect-to-effect thermal heat and the 18 orifices of £-inch
diameter, one per tube. Flashing of the feed occurred as it passed through
the orifice plate in all tests; this condition was important for good
upflow, two-phase flow stability and high heat transfer performance. Upflow
of the feed .through the distillation tubes was by vapor lift due to
continuous further evaporation along the tube length. The orifice plate
was adjustable in elevation, to provide an experimentally variable gap
(plenum) between its upper surface and the lower tube sheet, and could also
be rotated to direct feed either directly into the distillation tubes or to
16
-------
impinge against the tube sheet between the tubes. The tubeside pressure
drop for the bundle was obtained from a sight glass connected to the tube
inlet plenum (post-orifice) and the vapor dome. Vapor-liquid disengagement
in the first effect dome was by impingement against a saddle-shaped splash
plate, and the vapor was passed through a stainless steel demister screen
to the steamside of the second effect. Residual feed (brine) cascaded over
the upper tube sheet and into a surge tank, and on to the second effect.
For the downflow VTE tests, the second effect was provided with a
pump to recycle the feed at a measured rate (2 to 2.5 gpm per tube) and to
discharge it into a standing leg, vented back to the surge tank. Feed to
the downflow effect was thus mainly by recycle (55 to 65%), with first effect
blowdown and recycled distillates as makeup. Feed flow to the downflow
bundle was controlled by the level in the standing leg, the degree of
superheat in the feed and the orifice diameters. The downflow orifices
were interstitial to the distillation tube positions, comprising 27
drilled holes of 0.2-inch diameter through the orifice plate, for feed
distribution at three locations peripheral to each distillation tube.
Flashdown of the feed as it passed through the orifice plate occurred in
all tests using this distributor. Vapor generated in the downflow effect
was separated from the residual liquid phase in the vapor dome below,
passed through a demister screen and condensed in the heat reject loop.
The distillate flow rate, and the temperatures of the steamside (Ts),
blowdown brine (Tb) and feed (Tf) were determined as above, to provide
heat transfer performance data. Residual brine and distillates were
recycled to the downflow standing leg, with continuous removal of noncon-
densibles by bleed-off of some vapor to a condenser and vacuum pump as
above.
17
-------
SECTION VI I
COMPARATIVE UPFLOW-DOWNFLOW VTE AND VTFE TESTS
Earlier EPA-sponsored work (5) demonstrated that the use of interface
enhancement provided a substantial increase in the heat transfer performance
of upflow VTE of Mohave power plant cooling tower blowdown; increases in U
of about 125 percent were obtained under realistic upflow VTFE process
conditions, with double-fluted aluminum-brass tubes, and significantly
enhanced heat transfer performance data during the 10-fold concentration
of 800 gallons of this blowdown by upflow VTFE, was also obtained. In the
present work the objective was to provide reliable comparative data on a
basis of which one could elect when to use downflow VTFE in preference to
upflow VTFE. These tests could be carried out with any aqueous feed water
having evaporation properties similar to cooling tower blowdown. The most
readily available source was clean seawater, collected off the California
coast at the U.C. Bodega Marine Laboratories site; about 1,500 gallons3 were
pumped into two clean wooden wine barrels and transported to this Laboratory
by truck, for the tests reported here. Seawater contains 35,000 ppm of total
dissolved salts (TDS), about three times as high as the TDS in Mohave
blowdown. Some VTE tests were also carried out with freshwater feed during
preliminary, shake-down runs with this pilot plant, and served to also
confi rm the seawater data reported here.
Interface-enhanced performance data were obtained with two types of
double-fluted tubes, with seawater feed. Tubes were axially double-fluted
aluminum-brass (supplied by Yorkshire Imperial Metal Industries Limited,
England) of 2-inch OD x 10-foot heated length, 0.042-inch wall, and
spirally double-fluted titanium tubes (Timet, supplied by Oak Ridge National
Laboratories and fluted by Yorkshire Imperial Metals) of 2-inch OD x 10-foot
heated length x 0.020 wall. The titanium tubes were in an apparently clean
but previously used condition, and were installed as received without
cleaning; the aluminum-brass tubes were in an oxidized-annealed and
stabilized condition after four years of intermittent testing in the upflow
VTE effect without reconditioning. Their steams ides were in a dull, dark
brown condition. Their brinesides were in a fairly clean condition because
of the dispersant and flushing action of surfactant in seawater at a pH of
6.6-6.8. These double-fluted tube profiles are shown in Figure 5.
Heat transfer performance data were obtained after each steady-state
operation had been reached and maintained for a sufficient time, usually
requiring about 30 minutes for each set of data. Figure 6 shows typical
interface enhancement effects on the overall heat transfer performance and
the tubeside pressure drop for the upflow and downflow effects with
seawater feed. Performance and pressure drop data were plotted as a function
of the concentration of surfactant additive in the feed, at an evaporation
35700 Liters 18
-------
vo
2-INCH OD DOUBLE-FLUTED
ALUMINUM-BRASS
.042-INCH WALL
2-INCH DOUBLE-FLUTED
ALUMINUM-BRASS
.042-INCH WALL
3-INCH OD DOUBLE-FLUTED
ALUMINUM-BRASS
.042-INCH WALL
AXIALLY AND SPIRALLY DOUBLE-FLUTED TUBES USED
2-INCH OD SPIRALLY
FLUTED TITANIUM
.020-INCH WALL
F i ijii re S
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r Plot Bg Axial ly fluted Al-Br tubes, downflow
1 Plot CQ Spirally fluted Ti tubes, upflow
L Plot Df Spirally fluted Ti tubes, downflow —
1 Plot E A Tubes ide pressure drop, upflow
1 Plot FA Tubeside pressure drop, downflow
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SURFACTANT ADDITIVE CONCENTRATION, ppm
INTERFACE ENHANCEMENT EFFECTS ON HEAT TRANSFER PERFORMANCE
AND PRESSURE DROP IN VERTICAL TUBE EVAPORATION OF SEAWATER,
UPFLOW AND DOWNFLOW, WITH DOUBLE-FLUTED TITANIUM AND
ALUMINUM-BRASS TUBES
20
Figure 6
-------
temperature of 210a in the upflow effect and 200°Fb in the downflow effect.
It is apparent that interface enhancement is more effective in upflow
than in downflow VTE; it provided about a 100 percent heat transfer
enhancement with the aluminum-brass tubes after 5 ppm of Neodol (Shell
Chemical Co.) addition in the upflow effect versus only about 13 percent
enhancement for the downflow effect. At 20 ppm of foaming agent the upflow
and downflow performances were within 15 percent of one another, and their
tubeside pressure drops were nearly equal. The maximal interface enhancement
for upflow VTE was in this case about 130 percent, and about 50 percent for
downflow.
The upflow tubeside pressure drop was dramatically reduced by the
addition of 5 to 10 ppm surfactant, reaching a minimal value 11% of its
unenhanced value, and rising only marginally with further addition of
surfactant. The downflow pressure drop was gradually increased by surfactant
addition to the feed, rising from 0.5 to about 12 inches of water. It is
apparent that upflow VTE is substantially improved by 5 ppm of Neodol while
downflow VTE required about four times higher concentrations to provide
half the enhancement effect. At 20 ppm of Neodol the upflow and downflow
performances were essentially maximal and were subject to comparable effec-
tive AT values.
The interface enhancement effects on the heat transfer performance
of spirally double-fluted titanium tubes shown in Figure 6 for the upflow
and downflow VTE modes caused responses parallel with those of the double-
fluted aluminum-brass tubes, but at an approximately 40 percent lower
performance level, in both upflow and downflow modes. Tubeside pressure
drops were not determined for these tubes individually.
The interface-enhanced upflow VTE performances of double-fluted
aluminum-brass tubes and of spirally double-fluted titanium tubes through
the usual seawater evaporation temperature range are shown in Figure 7»
in comparison with their downflow VTE performances. It was known that
conventional downflow VTE should have a slightly higher heat transfer
performance than upflow VTE. However, these data show that upflow VTE
outperformns downflow VTE when interface enhancement is applied.
Furthermore, the upflow mode should be credited for the effect-to-effect
pumping of brine by vapor-lift through the tubes, or with the costs of
downflow per-effect recycle pumping.
The upflow tubeside pressure drop was substantially reduced by the
addition of surfactant to the feed, as shown in Figure 6 while the downflow
pressure drop was increased. This upflow pressure drop reduction by
interface enhancement provides for substantially increased upflow stability,
permitting a reduction in the minimal effect-to-effect AT by about AC to 5°F.
This allows an increase in the number of upflow effects by about 50 percent,
for a comparable increase in the steam to product (gain) ratio. Plot F in
Figure 7 shows that the interface-enhanced upflow pressure drop was quite
low, about 6 inches of water, for 2-inch tubes between 160s and 200°F^ It
was shown earlier that 3-inch double-fluted tubes are preferable to 2-inch
tubes for the low temperature effects, both in terms of pressure drop and
398.9°C; b93.3°C; C2.2°C; d2.8°C 21 e71.1°C
-------
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Plot AQ: Axial 1y fluted Al-Br tubes, upflow
Plot B g: Axially fluted Al-Br tubes, downflow
Plot G O: Spirally fluted Ti tubes, upflow
Plot D H: Spirally fluted Ti tubes, downflow
Plot E^>: As in Plot A but using 3-inch OD tubes
Plot F A: Upflow Tubeside pressure drop, Plot A
(2839)
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interface-enhanced heat transfer performance. Plot E of Figure 7 indicates
that 3- inch- double-fluted tubes should be used for effect temperatures up to
about 170°F.a For effect temperatures above 170°Fa the 2-inch tubes would be
more advantageous, as shown by plot A.
The mode of two-phase flow in the upflow tubes changed significantly
upon the addition of a surfactant, as also noted earlier (9). A rather
wet foamy condition was imposed immediately after the orifice, and the
outflow conditions for both upflow and downflow modes were essentially as
foamy annular layers rather than liquid films. The vapor core flow from
the tubes appeared to entrain fewer liquid droplets after surfactant
addition than before it, for equivalent VTE process conditions. Flow from
the upflow tubes was more continuous with than without surfactant; the
latter is normally a distinctly gushing flow (intermittent spurts). The
dramatic reduction in the upflow pressure drop is very significant; it
caused a substantial improvement in the upflow hydrodynamic stability;
it also increased the effective AT available for vaporization (13). This
increase in effective AT is, however, nottheonly cause of increased heat
transfer performance, as evidenced by the fact that in Figure 6, Plots A & E,
the performance was increased further by the addition of surfactant above
the 20 ppm level while the pressure drop also increased slightly.
The significance of pressure drop in interface-enhanced upflow VTE
for distillation tubes of different diameters was extensively examined and
discussed in recent work done in this Laboratory
The pressure drop response to surfactant addition, observed for the
downflow mode, has not been reported earlier, and further work is required
to characterize it. In this case it included the pressure drop due to
channeling the two-phase foamy layer from the top tube sheet upper surface
into the downflow tubes at 2.5 times the rate of feed flow for the upflow
tubes. A slight reduction in the downflow enhancement effect at surfactant
concentrations above 20 ppm was observed in each downflow case.
The mechanism of heat transfer performance enhancement after the
addition of a surfactant was postulated as being the result of causing a
foamy layer to flow over the heat .transfer surface, providing a thin-film,
wiped-film mode of flow over this surface (1, 7, l4). This liquid film
was seen as being continually renewed by liquid traveling in the interstices
between vapor bubbles being sheared over the surface as the annular foamy
layer or network sweeps over it. This mechanism has recently been defined
more precisely, based on definitive work in this Laboratory on the upflow,
two-phase flow morphology under typical VTE process conditions (15).
The interface-enhanced performance data for 3-inch and 2-inch
double-fluted aluminunHsrass tubes (Figure 7, Plots A & E) provide a basis
for a multieffect plant design study. Such data, modified by the appropri-
ate fouling factors, were utilized in the joint study with Kaiser Engineers
of Oakland, California, to arrive at the preliminary technical feasibility
evaluation of VTFE, part of which was published (6).
The heat transfer performance of titanium tubes was somewhat lower
a76.7°C 23
-------
than anticipated from their wall conductance, and was about equal to the
comparable performance obtained with axially double-fluted titanium tubes,
to be reported elsewhere (12). This rather low performance may be attributed
to the less sharply defined, less angular ridge profile obtained when
fluting a titanium tube as compared with an aluminum-brass tube. It is
apparent that titanium tubes may be less economical than aluminum-brass
tubes for VTE use except in cases where the corrosion-erosion characteristics
of the feed are of compelling significance, for instance in the. case of
an acidic wastewater feed.
It is clear, from the above test series, that vertical tube foam
evaporation (VTFE) offers a promising new approach to wastewater renovation-
recycle. Applied to upflow VTE, it improves two-phase flow stability
substantially, reducing the tubeside pressure drop by a factor of 3 to 10 and
increasing the heat transfer performance by about 100 percent. It also
provides the basis for increasing the number of effects normally permissible
within the usual available temperature span, in the interest of fuel
economy; this also improves design flexibility. Interface enhancement
eliminates the main question that has impeded the application of multieffect
upflow VTE in preference to the downflow mode for distillation . When
applied to downflow VTE, interface enhancement provides a relatively lower
but significant heat transfer performance increase, sufficient to merit
an attempt at improving the performance of existing downflow VTE plants.
Existing upflow VTE plants could also benefit from VTFE after installation
of orifice plates. ,
The relatively low performance of double-fluted titanium tubes,
compared with aluminum-brass tubes, indicates that the selection of titanium
must be justified on a basis of erpsion-corrosion considerations and cost.
This will be the case for many industrial wastewaters, especially those
derived from metal industries, acidic mine effluents, and other .corrosive
effluents such as those relatively high in sulfides, ammonia or amines.
It is apparent that, for evaporation to concentrate relatively dilute
wastewaters within a concentration range where precipitation and fouling
are not serious hazards, upflow VTFE is to be preferred over downflow VTFE
and VTE. This preference of upflow VTFE is supported by both its relatively
high heat transfer performance and process simplicity.
-------
SECTION VI I I
FACILITY FOR EVAPORATION-CRYSTALIZATION WITH INTERFACE ENHANCEMENT
A vertical tube evaporator-crystal 1izer, capable of 5,000 to 7,000
gallon per day capacity was used to obtain comparative performance data
for the conventional slurry-feed mode of operation and for the interface-
enhanced method of operation. For this purpose, a pre-existing evaporator
(16) was adapted by providing the necessary pumps, flow controls and
instrumentation. A basket type centrifuge was also provided to separate
crystalline product from the blowdown slurries. The main objectives were
to determine the feasibility of interface-enhanced evaporation-concentration
into the crystallizing concentration range, to determine the heat transfer
performance effects of the enhancement additive and to observe its effects
on crystallization, fouling and scaling of the heat transfer surfaces.
These data and observations were obtained with both synthetic salt solutions
and with real cooling tower blowdowns obtained from several industrial
sources.
The vertical tube evaporator (VTE) used for this work is shown in
a flow diagram sketch in Figure 8 and as a photographic print in Figure 9.
The VTE contained a 49-tube bundle of double-fluted aluminum-brass
distillation tubes (Yorkshire Imperial Metals Ltd.) of 1.5-inch diameter
by 6-foot heated length. Feed distribution to this tube bundle was in the
downflow mode, using an orifice plate distributor for dispensing the feed
through 0.20-inch holes on to the top tube sheet at three positions adjacent
to each tube inlet end. The feed was thus bounced off the top tube sheet,
and directed downward into the tubes by means of short cylindrical deflectors
attached to the orifice plate and entering the distillation tube inlet
ends by about 0.5-inch to provide an annular feed distribution for each
tube. The orifice plate with cylindrical deflectors attached is shown
(upside down) in Figure 10. Provision was made for preheating the feed
with two electrical immersion heaters of 3>000 watt each, located in the
inlet distributor vessel, immediately above the orifice plate. The 49-tube
bundle discharged from its open-ended outlet ends directly into a large
stain1es"s steel vessel for separating the vapor produced from the feed
slurry. This vessel was provided with three windows to permit observation
of the modes of flow at the tube outlet ends. A pair of rectangular windows
(visible in Figures 11 and 12) located in the inlet distributor vessel
similarly permitted observation of the mode of flow between the orifice
plate and the tube inlet ends. The vapor-liquid separation vessel was
provided with an annular stainless steel mesh mist eliminator located in the
upper part of the vessel as a collar surrounding the tube bundle, for the
purpose of removing entrained droplets of liquid phase from the vapor
produced. Vapor was conducted from the mist eliminator to a condenser
cooled with water coolant of controlled temperature, to provide steady-state
25
-------
SCHEMATIC FLOW DIAGRAM
DOWNFLOW EVAPORATOR-CRYSTALLIZER
£ STEAM FROM BOILER
SUPPORT
FEED MAKEUP-
BLOWDOWN
FEED
VTE
TUBE
BUNDLE
7}
VACUUM
I
CONDENSER
DISTILLATE
SUPPORT
UU U LI
inti
UPFLOW VTE
VAPOR; OUTLET
DEMISTER
CONDENSATE TO BOILER
BRINE RECYCLE
Figure 8
-------
ro
DOWNFLOW EVAPORATOR-CRYSTALLIZER, 5,000 GPD LOW PRESSURE STEAM HEATED
Figure 9
-------
ro
CO
VTFE FEED ORIFICE PLATE
Figure 10
-------
evaporation at selected temperatures and temperature differences (AT) in
the evaporator tubes. The rate of flow of the distillate was measured
during its continuous removal from the system, under evacuated, air-free
conditions.
Heating steam, maintained at a preset pressure and temperature, was
provided from the reboiler loop of the VTE pilot plant, and distributed to
the shell-side of the evaporator-crystal 1 izer through a 6-inch pipe-line
with provision for withdrawal under vacuum of line condensates. Steam
condensation in the tube bundle was under air-free conditions and with
positive flow across the bundle diameter. This was arranged by the
wedge-shaped form of the tube bundle and by continuous withdrawal, under
vacuum, of a part of the steam flow and noncondensable gases to a condenser.
The steam condensed on the shell -side of the tube bundle was continuously
removed, under vacuum, by a pump and its rate of flow measured. This
condensate flow rate provided the heat flux data (Q,) needed to measure
evaporator performance. Additional data required were the steam tempera-
ture (Ts) and the brine temperature (Tb) , measured with platinum resistance
probes and converted electronically into degrees Fahrenheit in a digital
printout and display. From these data the overall heat transfer perfor-
mance of the evaporator-crystal 1 izer (U) was determined according to the
usual formulation:
Q. = U A AT
where A is the shell -side surface area of the tubes based on their nominal
OD (1.5-inch) and length (6-foot), and the AT is the temperature difference
between the steam in the shell-side (Ts) and the brine (Tb) as measured
above.
Makeup feed to the evaporator-crystal 1 izer (EC) was preheated to
within a few degrees of the evaporation temperature before introduction
into the EC. Residual brine or brine-crystal slurry, was recirculated
from the bottom of the EC vessel to the feed distributor and orifice plate
with a stainless steel pump at a rate of flow of one gallon per tube per
minute. Provision was made for continuous or intermittent blowdown of
brine-crystal slurry from the reci rculation pump. This slurry discharge
was introduced into the bowl of the centrifuge for separation of the
crystal 1 ine phase.
• After several preliminary runs with aqueous solutions containing
sodium sulphate and calcium sulphate, cooling tower blowdown was examined
in this EC, increasing the concentration of the blowdown into a two-phase
crystal-brine slurry stream, in recirculated flow at different evaporation
temperature conditions.
After operating this EC with reboiler steam as the source of heat,
generally at low evaporation temperatures to favor the crystallization
of sodium sulphate into a brine slurry, this pilot plant was converted to
the vapor compression mode of operation at evaporation temperatures
above 210°F.a For this purpose a Sutorbilt, Roots type compressor cons-
tructed of bronze was acquired having a 920 CFM displacement, capable of
29
-------
producing about 300 gallons of distillate per hour. A 25 horsepower electric
motor was used to drive the compressor at 918 RPM. This provided for about
a 5 psi compression of the vapor from the EC vessel; the compressed vapor
was then conveyed to the shell-side of the EC through stainless steel pipes
of 6-inch diameter. Two bypass lines of 2-inch diameter were provided, for
the optional•bypassing of some compressed vapor to the suction end of the
compressor, to provide an experimental variation of its capacity. This
VC-EC is shown in Figure 11, in the upflow mounting with the De Laval
basket type centrifuge used for recovering crystalline products from the
slurried blowdown in the foreground, and in Figure 12, as used for the
downflow concentration and crystallization data on cooling tower blowdown.
30
-------
UPFLOW VAPOR COMPRESSION EVAPORATOR WITH CENTRIFUGE
31
Figure 1 1
-------
DOWNFLOW VAPOR COMPRESSION EVAPORATOR-CRYSTALLIZER
32
Figure 12
-------
SECTION IX
EVAPORATION-CRYSTALLIZATION TESTS
All evaporation-crystallization (EC) tests were carried out with the
downflow mode of brine flow. Two methods of heating were used, low
pressure reboiler steam for EC of sodium sulphate at about 120 Ff and vapor
compression heat for EC at about 220°F.b These methods were applied to the
evaporation of cooling tower blowdown and several other saline waters.
Evaporator heat transfer performance data were obtained under conventional
EC process conditions and after the addition of a selected surfactant to the
feed. This work was divided into three sections:
(a) Low temperature evaporation with low pressure reboiler steam as
the source of heat for evaporation-crystallization,
(b) Vapor compression evaporation-concentration at elevated tempera-
ture,
(c) Vapor compression evaporation-crystallization at elevated
temperature.
(a) Low Temperature Evaporation-Crystallization, Downflow
Evaporation-crystallization (EC) at about 120°F3 was preferred for the
separation of sodium sulphate from brine also containing sodium chloride,
because of the favorable relative solubilities at low temperature. Several
initial experiments were carried out to obtain heat transfer performance data
and to gain experience with the pilot plant facility. These runs utilized
low pressure reboiler steam, drawn from the VTE pilot plant discussed in
Chapter VI, at preset temperatures held at steady state (t 0.5°F)cduring
these experiments. Condensates from the shell-side of the distillation tubes
were continuously removed under vacuum (air free) conditions, their
combined rates of flow measured, and returned to the VTE pilot plant.
The heat flux (Q.) was calculated directly from these condensate flow
measurements in the usual manner. Brine and slurried feeds were recircu-
lated, under vacuum (air free) conditions, for downflow through the tubes
at I4 gallon per minute per tube. Vapors produced by evaporation were
passed through a stainless steel mesh Demister and conveyed to a water-cooled
condenser; these condensates (distillates) were withdrawn under vacuum
conditions, their rates of flow were measured and their total dissolved
solids (TDS) content determined. Precise temperature measurements were
obtained on the steam in the shell-side (Ts) and the brine flowing from the
vessel (Tb) to determine the temperature difference (AT = Ts - Tb) imposed
for evaporation. Steady-state conditions were maintained on Tb and the
vapor flow to the condenser by rapidly recirculating the coolant water
through the tube^side of the condenser under moderate pressure (25 psi) while
continuously adding fresh coolant to that cycle at a sufficient, steady rate
C±0.3°C 33
-------
with simultaneous reject of hot coolant from this cycle at an equal rate.
Measurements were also made on the steam (Ps) and vapor (Pv) pressures,
with an absolute manometer.
Test data are reported in tabular form below. The steam-side surface
area of the tube bundle was 115.^5 square foot; the condensate and distillate
flow rates are given as the time, in seconds, required to collect 16 liters
of each; the calculated heat transfer performance (U)is reported in Watts
per square meter per °C (and in Btu per hr-ft^-°F).
(a-1) Evaporation-Crystallization of Sodium Sulphate
Anhydrous sodium sulphate (commercial c.p. grade) dissolved in tap
water was recrystal1ized by EC at 120 Ff to gain experience with the pilot
plant and to determine its heat transfer performance with, and without
surfactant additives. Several runs were performed, two of which are
reported in Tables I and II. (After these preliminary runs, a batch of
cooling tower blowdown obtained from the Mohave power plant in Arizona was
examined in the EC.)
Table I shows the change in overall heat transfer coefficient U
during the gradual concentration into the crystallizing regime, of a 27.3%
Na2S04 aqueous solution at an evaporation temperature of 121°F,k The
steamside temperature was maintained close to 127.6°FS and the condenser
was maintained at constant heat reject capacity. As a consequence, the
temperature difference (AT) adjusted in response to the effect of the brine
boiling point elevation, due to the gradual increase in salt content
followed by a gradual decrease after the start of crystallization; this
of course is derived from the fact that the evaporation temperature and
the saturation vapor pressure of the brine adjust downward with increased
boiling point elevation and vice versa. The difference in absolute
pressures of the steamside and the vapor-side also reflect this variation in
AT. Consequently, the overall heat transfer performance, under conditions
of constant heat flux, is first gradually reduced until this trend is
overcome by rapid crystallization and a downward reversion of the boiling
point elevation due to the reduction in total dissolved solids content of
the brine (liquid phase). Thus the observed data shown in Table I can be
rationalized, excepting the minor fluctuations outside of these general
trends, which are due to the usual experimental errors.
Table II reports data obtained under process conditions close to
those of Table I, except that 100 ppm of Neodol 25-3A (Shell Chemical Co.)
were added after steady-state conditions comparable to the start of the
previous run were established. The data that follow this addition of
surfactant are quite interesting and revealing of the dispersant properties
of the additive. Also of interest was the observation that only a very
slight foaminess was induced in the brine flow by the addition of the
surfactant, which is consistent with earlier work done in this Laboratory
(4); the latter showed induced foaminess to be a prerequisite for interface
enhancement rather than merely adding a surfactant or lowering the
surface tension. In this case, the addition of surfactant at 100 ppm
had no significant effect on the heat transfer performance, consistent with
the observed absence of significant foaminess. In fact, when U-values
a48.9°C; b49.4°C; C53.1°C
-------
UT
TABLE I
CRYSTALLIZING EVAPORATION OF SODIUM SULPHATE
Feed: 56.7 Kg (125 lb) of Anhydrous Sodium Sulphate in 208 Liters (55 Gallons) of Water
Evaporation Temperature (Tb) 50°C (121°F)
Rejected Dist.Cond.
Time T& Tb AT Distillate Ps-Pv Flow Flow U
°c
2.15 53.1
2.25 53.1
2.30 53.05
2.35 53.0
2.40 53.2
2.47 53.2
2,55 55.0
2.59 53.2
3.07 53.1
3-12 53.1
3-25 53.2
3-35 53.2
3-55 53.2
(Of)
(127.6)
(127.6)
(127.5)
(127.4)
(127.8)
(127.8)
(127.4)
(127,8)
(127.6)
(127.6)
(127.8)
(127.8)
(127.8)
°C
49.9
49.9
49.85
49.65
49.65
49.4
49.25
49.05
48.95
49.1
49.35
49.35
49.35
(°F) °C (°F) LiterlGal.j
(121.8)
(121.8)
(121.7)
(121.4)
(121.4)
(120.9)
(120.6)
(120.3)
(120.1)
(120.4)
(120.8)
(120.8)
(120.8)
3.2 (5.8)
3.2 (5.8)
3.2 (5.8)
3.35 (6.0)
3.55 (6.4)
3.8 (6.9)
3.75 (6.8)
START
4.15 (7.5)
4.15 (7.5)
4.0 (7.2)
3.85 (7.0)
3.85 (7.0)
3.85 (7.0)
0 (0)
0 (0)
0 (0)
18.9 (5)
37.85(10)
56.8 (15)
75.7 (20)
) CmHg
26.0
26.0
26.7
27-9
28.4
23.0
33.5
(InHg) Sees.
Sees. W/m*-°C
/16L /16L
(10.25)
(10.25)
(10.5)
(11.0)
(11.2)
(13.0)
(13.2)
304
300
304
306
310
316
320
296
296
300
302
305
316
314
(Btu/hr-ft2-o
3708
3708
3657
3515
3259
2918
2981
(653)
(653)
(644)
(619)
(574)
(514)
(525)
OF CRYSTALLIZATION
94.6 (25)
113.6(30)
132.5(35)
132.5(35)
132.5(35)
132.5(35)
38.1
36.8
36.8
36.8
36.1
36.2
(15.0)
(14.5)
(14.5)
(14.5)
(14.2)
(14.25)
370
330
326
329
329
328
322
324
325
325
328
325
3043
2618
2725
2799
2742
2799
(536)
(461)
(480)
(493)
(483)
(493)
.2
.4
.8
1.8
2.2
2.8
2.8
2.8
2.8
-------
TABLE II
CRYSTALLIZING EVAPORATION OF SODIUM SULPHATE
Feed: 56.7 Kg (125 lb) of Anhydrous Sodium Sulphate in 208 Liters (55 Gallons) of Water
Evaporation Temperature (Tb) 50°C (120°F)
Effect of Surfactant Addition on Crystallization of Sodium Sulphate
Time
pro
r
10.30
10.42
Ts
°C
52.7
52.7
(UF)
H26.9)
(126.9)
Tb
°C
49.6
49.55
100 ppm NEODOL 25-3A ADDED
.15
.25
.30
.35
.43
.50
11.57
12.05
12.15
12.25
12.35
52.55
52.5
52.55
52.55
52.6
52.65
52.6
52.5
52.5
52.3
52.6
(126.6)
(126.5)
(126.6)
(126.6)
(126.7)
(126.8)
(126.7)
02-6.5)
(126.5)
(126.2)
(126.7)
49.35
49.2
49.1
49.0
48.6
48.5
48.45
48.0
47.6
48.2
48.5
(UF)
(121.3)
(121.2)
(40 ml
(120.8)
(120.6)
(120.4)
(120.2)
(119.5)
(119.3)
(119.2)
(118.3)
(117.5)
(118.7)
(119.3)
Rejected
AT Distillate Ps-Pv
°C (°F) LlterlGalJ CmHg (
3.1 (5.6) (0) 23.75
3.15 (5.7) (0) 23.75
InHg)
(9.5)
(9.5)
Dist.
Flow
Sees.
/16L
302
300
Cond.
Flow
U
Sees. W/mz-°C
/16L
296
292
Cone
Fact
" ^™»"^^^
(Btu/hr-ft2-°F)
3838
3821
(676)
(673)
1
1
); VERY SLIGHT FOAMINESS OBSERVED
3.2 (5.7) (0) 25.5
3.3 (5.9) 18.9 (5) 26.3
3.45 (6.2) 37.85 (10) 27.3
3.55 (6.4) 56.8 (15) 28.8
4.0 (7.2) 75.7 (20) 33.8
4.15 (7-5) 94.6 (25) 36.8
START OF CRYSTALLIZATION
4.15 (7.5) 113.6 (30) 40.0
4.5 (8.2) 132.5 (35) 40.0
4.9 (9.0) 141.9 (37.5)40.6
4.1 (7.5) 141.9 (37.5)38.8
4.1 (7.4) 141.9 (37.5)38.8
(10.2)
(10.5)
(10.9)
(11.5)
(13.5)
(14.7)
(16.0)
(16.0)
(16.25)
(15.5)
(15.5)
300
300
304
310
315
324
332
326
-
-
-
298
297
297
304
312
318
328
320
324
317
318
3747
3634
3458
3276
2839
2674
2589
2430
2192
2527
2549
(660)
(640)
(609)
(577)
(500)
(471)
(456)
(428)
(386)
(445)
(449)
1
1.1
1.2
K6
1.8
2.2
2.8
3.1
3.1
3.1
-------
are compared with the previous run at equal brine concentrations, it is
apparent that the additive had a negative effect on heat transfer
performance from the point where crystallization ensued for the first run.
This is rationalized as being the consequence of a retardation effect of
the additive on crystallization; such a threshold-inhibition of crystalli-
zation was also observed in earlier work in this Laboratory (5). Retarding
or inhibiting crystallization is clearly indicated by the data of Table II;
crystallization was first observed at a concentration factor of 1.57 for the
first and 1.83 for the second run. Also, recovery of heat transfer
performance due to the reduction of the boiling point elevation of the
brine, by rapid crystallization of sodium sulphate from the solution, was
clearly retarded in the second run by comparison with the first run. (See
Tables I and II.)
In summary, the above data indicate that the addition of a surfac-
tant during crystallizing evaporation of highly concentrated salt
solutions (27.3% and higher) provides no significant heat transfer
advantage and retards the onset and rate of crystallization. The latter
in turn reduces the evaporator heat transfer performance by the increase
in boiling point elevation due to the increased dissolved salt content
(supersaturation) of the brine in recirculation through the evaporator.
(a-2) Cooling Tower Slowdown Evaporation-Crystallization
Table III reports heat transfer performance data obtained during
the crystallization-evaporation of 6.4-fold preconcentrated cooling tower
blowdown obtained from the Mohave power plant. Preconcentration of
blowdown having a TDS of 10,000 ppm by a factor of 10 was performed by
upflow VTFE (5) at low temperature (130°F)a after the addition of 10 ppm
of Neodol 25-3A. This 10-fold preconcentrated blowdown (45 gallons) was
reconstituted to a 6.43-fold concentrate by dilution to 70 gallons with
distilled water; this dilution was done to facilitate the quantitative
transfer of the 10-fold concentrate, having considerable crystallized solids
in suspension, into the evaporator-crystal 1izer (EC). This suspension was
recirculated through the evaporator under evacuation for about one hour to
remove non-condensibles (deaeration) and to bring the solids in equilibrium
with the brine. The temperature of the recirculated slurry was adjusted
to 120°F and recirculation under vacuum was continued for another hour
to equilibrate suspended solids with the liquid phase (brine). Evaporation-
crystallization was then performed as reported in Table III. In this
operation the steam-side temperature was held steady at about 125.5°F9
andtthe coolant flow into the condenser loop was adjusted to provide a AT
of about 5.5°F.d The EC feed was recirculated at close to 1 gallon per
minute per tube; concentration of the feed was completed in 1 hour and
35 minutes, evaporating it from a 6.43-fold to a 30-fold concentrate,
3.33% of its original volume. The distillate obtained, representing 96.66%
of the original cooling tower blowdown, had a total dissolved solids
content of 19 ppm. Figure 13 shows a sample of the final brine slurry
obtained from the Mohave cooling tower after 30-fold concentration.
The evaporator heat transfer performance during crystallization
fluctuated somewhat more than usual, generally between 750 and 670
Btu per hr-ft2-°F, but did not decrease significantly during this final
954.**°C; b48.9QC; 051.900; 37 d3.05°C
-------
00
Feed:
TABLE I I I
CRYSTALLIZING EVAPORATION OF COOLING TOWER SLOWDOWN
Cooling Tower Slowdown from Mohave Power Plant Having 10 ppm of Surfactant Added and Concentra-
ted 6-fold by VTFE, 265 Liters (70 Gal.).
Evaporation Temperature 50°C (120°F)
Rejected
Time
pm
9
3.00
3.30
3.40
3.45
3.52
3.56
4.05
4.08
4.15
4.22
4.30
4.35
4.40
Ts
°C
52.1
51.9
52.0
51.9
51.5
51.65
52.15
52.05
52.05
51.8
51.7
51.95
51.95
(°F)
(125.8)
(125.3)
(125.6)
(125.3)
(124.7)
(125.0)
(125.9)
(125.7)
(125.7)
(125.2)
(125.0)
(125.5)
(125.5)
Tb AT Disti
°C (°F) °C (°F) Liter
49.35 (120.8) 2.75 (5.0) 0
48.9 (120.0) 3.0 (5.3) 37.9
49.0 (120.2) 3.0 (5.4) 56.8
48.9 (120.0) 3.0 (5.3) 75.7
48.4 (119.1) 3.1 (5.6) 94.6
48.15 (119.6) 3.0 (5.4) 134
49.15 (120.5) 3.0 (5.4) 132
49.1 (120.4) 2.95 (5.3) 151
49.05 (120.3) 3.0 (5.4) 170
48.7 (119.7) 3.1 (5.5) 189
48.55 (119.4) 3.15 (5.6) 199
48.85 (119.9) 3.1 (5.6) 208
48.9 (120.0) 3.05 (5.5) 208
1.
(GalJ
(0)
(10)
(15)
(20)
(25)
(30)
(35)
(40)
(45)
(50)
(52.5)
(55)
(55)
Ps-Pv
CmHg
15.2
16.1
16.5
19.1
17.8
17.8
17.8
18.3
18.4
19.1
19.6
19.8
19.8
(InHg)
(6.0)
(6.35)
(6.5)
(7.5)
(7.0)
(7.0)
(7.0)
(7.2)
(7.25)
(7.5)
(7.7)
(7.8)
(7.8)
Dist.
Flow
Sees.
/16L
287
290
291
296
294
294
290
291
294
298
300
feed
too
low
Dist.
Flow
Sees.
/16L
288
304
306
318
290
290
288
282
294
288
301
302
292
n U
W/m2-°C
Cone.
Fact.
(Btu/hr-ft -F)
4423
3952
3855
3787
3929
4077
4088
4259
4014
4037
3782
3765
3970
(779)
(696)
(679)
(667)
(692)
(718)
(720)
(750)
(707)
(711)
(666)
(663)
(699)
6.43
7.50
8.18
9.0
10.0
11.26
12.85
15.00
18.00
22.5
25.72
30.01
30.01
BRINE RESIDUE COLLECTED: 14 gall
DISTILLATE QUALITY: 19 ppm TDS
-------
VAI
VO
CONCENTRATED
MOHAVE POWER PLANT
COOLING TOWER
SLOWDOWN
30-FOLD CONCENTRATE
SLOWDOWN FROM
PETROLEUM REFINERY
COOLING TOWER
CONCENTRATED
PETROLEUM REFINERY
COOLING TOWER
SLOWDOWN
50-FOLD CONCENTRATE
SLOWDOWN FROM
CHEMICAL INDUSTRY
COOLING TOWER
CONCENTRATED
CHEMICAL INDUSTRY
COOLING TOWER
SLOWDOWN
27-FOLD CONCENTRATE
SAMPLES OF COOLING TOWER BLQWDQWN AND THEIR CONCENTRATES
Figure 13
-------
concentration stage. The heat transfer performance for the entire
concentration operation, including preconcentration by VTFE, with 10 ppm
of surfactant added to the original Mohave blowdown is shown in Figure 14.
(b) High Temperature, Vapor Compression Evaporation-Concentration, Downflow
Vapor compression evaporation was carried out at brine-side tempe-
ratures of 212-224°Fa with and without surfactant addition to the brine.
The downflow mode was used, with brine recirculation at about 1 gallon
per tube per minute. This series of runs were performed with the objective'
of gaining experience with VC-VTE operation. Crystallization was not the
objective here; agricultural wastewater from the San Joaquin Valley was used.
This irrigation drainage water had a TDS of about 7000 ppm but was
presoftened by cation exchange of calcium and magnesium for sodium ions.
It had a pH of about 8 and was used as such without acidification. This
type of wastewater has a typical composition, after softening, as follows
in parts per million: Na = 2200; K = 2; Ca = 12; Mg = 10; SO^ = 4000;
Cl = 500; HC03 = 300; N03 = 50.
In this test series the objective was to determine the effects of
surfactant addition to the downflow VTE concentration of a strongly saline
wastewater by a factor of 20. Since the maximum and minimum holdup
volumes of the evaporator was 55 and 15 gallons, respectively, it was
necessary to add the feed batchwise, after consecutive concentration steps.
The test data are shown in Table IV.
The first line of Table IV reports the steady-state process condi-
tions applied and the VC-VTE heat transfer performance observed with ion
exchange softened agricultural wastewater having a typical wintertime high
TDS of 7000, without any additions to it. The next line shows the
comparative VC-VTFE performance after the addition of 10 ppm of Neodol 25-3A
(Shell Chemical Co.) to the same feed. The AT adjusted downward, while the
distillation rate increased; simultaneously (and consistently) the vapor
compressor ran noticeably smoother under the reduced differential
pressure (AT) (Power consumption by the 25 horsepower VC motor could not
be measured for these tests.) A 31 percent increase in performance was
obtained with 10 ppm of surfactant. The third line of data shows the
comparative overall heat transfer coefficient after the addition of a
further 5 ppm of surfactant. The heat transfer performance was enhanced
to 2188 Btu per hr-ftz-°F, 37.4 percent above the conventional performance
of line 1. The distillates were recombined with the recirculated feed
for each of these above sets of data, maintaining the concentration factor
of the brine at 1, or 7000 TDS. The fourth through tenth lines of data
summarize the procedure of batchwise additions of softened wastewater to
the recirculated feed and the batchwise rejections of distillates, to obtain
a stepwise increase in the concentration factor of the recirculated feed.
The eleventh line reports the VC^VTFE process conditions applied and the
heat transfer performance obtained on 20-fold concentrated wastewater having
a TDS of 140,000 ppm and about 15 ppm of surfactant. The overall heat
transfer coefficient at these concentrations was 33 percent above the
initial performance with unconcentrated wastewater. These data are plotted
in Figure 15.
aiOO°C-106.7°C 40
-------
CONCENTRATION OF MOHAVE POWER PLANT COOLING TOWER SLOWDOWN
o_
BY INTERFACE-ENHANCED VERTICAL TUBE EVAPORATION AT 130 F
o
CM
L.
JT
O
o
3
UJ
o
(681^
1000
600
(2271)
200
START OF CRYSTALLIZATION
VTFE CONCENTRATION
— O —
Preconcentration by VTFE with 10 ppm
Surfactant added to original blowdown.
-Evaporation-Crystallization by VTFE of
preconcentration residues.
APORATION-CRYSTALLIZATION H
1
1
1
1
1
J.
1 ].Qk 1.11 1.25 1.43 1.6? 2.0 2.5 3.33 5
BLOWDOWN CONCENTRATION FACTOR
10
15
20
25 30
Figure \k
-------
TABLE IV
VAPOR COMPRESSION EVAPORATION CONCENTRATION
Feed: Agricultural Wastewater, Softened by Ion Exchange, TDS of 7000 ppm
Additions Rejected Dist. Cone.
to feed Ts J_b AT Distil. Flow U Fact.
Liter (Gal.) Or (°c\ °r toc^ °r /°F) Liter (Gal.) Sees. W/m^-°C
G V h; L V h' L v i-; /1^1 (Btu/hr-ft2-°F)
189 (50.) 109.3 (228.7) 104 (2-19.3) 5.3 (9.4) 0 (0) 68 9045 (1593) 1
Neodol added to Feed, to 10 ppm.
0 (0) 110.5 (230.8) 105.8 (222.4) 4.7 (8.4) 0 (0) 58 11,838 (2085) 1
Neodol added to Feed, to 15 ppm.
0 (0) 111.5 (232.5) 106.6 (223.9) 4.9 (8.6) 0 (0) 54 12,423 (2188) 1
0 (0) - - - - - - 95 (25) - 2
189 (50) 189 (50) - 4
189 (50) 189 (50) - 6
190 (50) 189 (50) - 8
189 (50) - - - 190 (50) - 10
189 (50) 189 (50) - 12
189 (50) - - - 189 (50) - 14
190 (50) 108.2 (226.8) 103.8 (218.8) 4.4 (8.0) 208 (55) 60 12,054 (2123) 20
189 (0) 1438 (380)
\t -) ftQ crystallization occurred during this 20-fold concentration.
-------
3000
o
I
C>J
I
CNJ
3
•M
OQ
2000
o
LU
O
o
LLl
(8517)
1000
LU
<
g(2839)
DATA, HO SURFACTANT
Chemical Industry Cooling Tower SlowdownTDS 850
Petroleum Refinery Cooling Tower Slowdown TDS 830
A Agricultural Wastewater,.TDS 7000
NTERFACE-ENHANCED CONCENTRATION DATA; SURFACTANT ADDED
O Chemical Industry Cooling Tower Slowdown
with 10 ppm Surfactant
LJ PetroJeum Refinery Cooling Tower Slowdown
with 10 ppm Surfactant
Agricultural Wastewater with 15 ppm Surfactant
I
I
8 12 16
BRINE CONCENTRATION FACTOR
20
EVAPORATION-CRYSTALLIZATION OF WASTEWATER BRINES
HEAT TRANSFER ENHANCEMENT BY ADDITION OF SURFACTANT
Figure 15
-------
(c) Vapor Compression Evaporation-Crystallization of Cooling Tower
Slowdown at 215-220°Fa
Two different industrial cooling tower blowdowns were examined by
downflow VC-VTFE in this section. The first of these, reported in Table V,
was obtained from a chemical manufacturing complex. The second came
from a large petroleum refinery and is reported in Table VI. The evapora-
tion-crystallization temperatures, and the vapor compression mode of
operation were chosen to represent realistic present wastewater renovation
practice (8).
The chemical industry cooling tower blowdown had a TDS of 850 and
a pH of 7-95; it was used as received, without pH adjustment. Evaporation-
concentration was done on a batch-wise-continuous basis as in the previous
case, with surfactant addition to 10 ppm in the residual, recirculated
feed. A 27-fold concentration level was obtained, with some calcium
carbonate being precipitated into the recirculated feed towards the end
of the run.
The first two lines of data in Table V report the VC-VTE process
conditions used, with a heat transfer performance (U) of 8233 W/m^-°C
(1450 Btu/hr-ft2-°F) on the wastewater as received. The second line shows
a slight decline in performance at a wastewater concentration factor of
1.33* Distilled water produced at this condition had a TDS of 5 ppm.
Line 3 shows the shift in process conditions resulting from the addition
of 10 ppm of Neodol 25-3A (Shell Chemical Co.). The steam-side
temperature (Ts) and pressure dropped, the brine temperature (Tb) increased
adjusting the AT downward, and the evaporation or distillation rate
increased. This combination of interface enhancement effects provided
a 53 percent increase (to 12,5^8 W/m2-°C) in the VC-VTFE performance
over the conventional VC-VTE conditions. In addition to increasing the
distillate productivity rate, the vapor compressor ran noticeably smoother
because of the reduction in AT and pressure after surfactant addition
to the feed; this advantage should be reflected in a reduced electric
power consumption by the 25 horsepower motor; it could however not be
measured because of a lack of the necessary equipment (watt-hour meter).
A further minor advantage of the induced foamy vapor-liquid flow through
the tubes was that the vapor and distillate produced had less entrained
brine droplets; the distillate quality was improved to a range of 1.5 to
2.5 ppm of TDS compared to 5 ppm before surfactant addition. This
simultaneously provides a measure of the surfactant transfer (in brine
droplets) into the distillate, at about 18 parts per billion.
A 27-fold concentration of this wastewater was readily obtained
with VC-VTFE, with a heat transfer performance enhancement ranging from
53 to 3^- percent through the 16-fold concentration range, renovating
96 percent of this wastewater into pure disti1 led water, and reducing the
wastewater to a slurry of k percent of its original volume. See Figure 15
for a plot of these performance data.
Another batch of cooling tower blowdown examined was obtained from
a large petroleum refinery; it had a TDS content of 380 ppm, a pH of 6.8
-------
TABLE V
VAPOR COMPRESSION EVAPORATION-CRYSTALLIZATION
Feed: Cooling Tower Slowdown from Chemical -Indus try: TDS-850 ppm, pH-7.75-
Addition Rejected Dist.
to Feed Ts Tb AT Distillate Flow
Liter
189
189
Neodol
0
0
189
190
189
189
189
190
0
1514
(Gal.)
(50)
(50)
added
(0)
(0)
(50)
(50)
(50)
(50)
(50)
(50)
(0)
(400)
°C (°F)
107.8 (226.0)
108.9 (228.0)
to 10 ppm
108.6 (227.5)
'• -
-
-
107.5 (225.5)
-
-
107.1 (224.8)
-
°C
102
103.3
103.8
-
-
-
102.9
-
-
102.7
-
* \ c\
i O c i f*
\ '/ v
(215.5) 5.8
(217.9) 5.6
(218.8) 4.8
-
-
-
(217.2) 4.6
-
-
(216.8) 4.4
-
(0F) Liter (Gal.)
(10.5)
(10.1)
(8.7)
-
-
_
(8.3)
_
(8.0)
0
95
0
189
189
189
190
189
189
189
_18
1457
(0)
(25)
(0)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(10)
(385)
Sees.
/16L
66
70
53
-
-
-
61
-
-
66
-
U
Cone.
Fact.
(Btu/hr-ft2-°F)
8233
8193
12,548
-
-
-
11,458
-
-
10,987
-
(1450)
(1443)
(2210)
-
-
-
(2018)
-
-
(1935)
-
1
1.33
1.33
4
6
8
10
12
14
16
26.7
Distillate quality varied between 1.5 and 5 ppm TDS.
These data were plotted in Figure 15.
Slowdown precipitated off-white layer shown in Figure 13.
-------
TABLE VI
VAPOR COMPRESSION EVAPORATION-CRYSTALLIZATION
Feed: Codling Tower Slowdown for Petroleum Refinery; TDS-380 ppm, pH-6.8, chromate-14 ppm.
Additions
to Feed
Liter
189
Neodol
0
189
189
190
189
189
189
190
189
183
T192
(Gal.)
(50)
Ts
oc
\*
108.6
was added to
(0)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(500)
107.0
109.2
107.9
-
109.5
107.6
-
107.4
106.1
108.1
,0 V
(227. 5)
10 ppm.
(224.5)
(228.5)
(226.0)
-
(229.0)
(225.7)
-
(225.3)
(223.0)
1226.5)
"•
Tb
o /o »
r i F^
u \ r /
102.5 (21.6.5)
102.3 (216.1)
104.0 (219.1)
102.6 (216.6)
M ••
104.5 (220.1)
102.6 (216.6)
. -
102.4 (216.3)
101.3 (214.4)
103.1 (217.5)
"" ™
AT
O Ok
r ( FI
\* \ r )
6.1 (11.0)
4.7 (8.4)
5.2 (9.4)
5.3 (9.4)
-
5.0 (8.9)
5.0 (9.1)
-
6.0 (9.0)
4.8 (8.6)
5.0 (9.0)
™" ^
Rejected Dist.
Distillate Flow
Liter
U
Cone
Fact
(Gal. ) Sees. W/m4-°C
/16L
0
95
189
189
189
190
189
189
189
190
189
vks
(0)
(25)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(50)
(15)
(490)
70
67
62
64
-
64
66
-
64
68
66
™
(Btu/hr-ft2-oF)
7540
10,323
9937
9641
-
1947
9619
-
10,084
9948
9778
••
(1328)
(1818)
(1750)
(1698)
-
(1787)
(1694)
-
(1776)
(1752)
(1722)
••
1
1
4
6
8
10
12
14
16
18
20
50
Distillate Quality — 1 to 2.5 ppm TDS
Slowdown is shown in Figure 13.
Data could not be managed for the terminal concentration steps
because of time constraints.
-------
and contained 15 ppm of chromate. This wastewater was renovated by down-
flow VC-VTFE, similar to the previous case reported; test data are
reported in Table VI and plotted in Figure 15.
Initial, baseline performance data are shown in the first line of
Table VI, using the wastewater (as received) as feed and recycling the
distillate into the feed. Neodol 25-3A was then added to 10 ppm, and the
interface-enhanced data of the second line were obtained. This provided
a heat transfer enhancement of 37 percent; less foaminess was observed
than in the previous run in which an enhancement of 53 percent was
obtained with 10 ppm of Neodol addition. This reduced foaminess may have
been caused by the presence of 15 ppm of chromate in the wastewater used in
this run. This reduced level of interface enhancement was observed through
the entire concentration range. An increased level of surfactant additive
should overcome this adverse effect. Further stepwise concentration of
the feed, added batchwise as before, is reported in Table VI showing that
the heat transfer enhancement remained at about 33 percent through the
remainder of the run, somewhat below the previous run but consistently
above the baseline performance. These data are plotted in Figure 15,
on a comparative basis.
Final concentration of this waslewater by a factor of 50 was
readily obtained, producing 98 percent of high quality distilled water
(TDS ranging from 1 to 2.5 ppm) and reducing the wastewater to a slurry
only 2 percent of its original volume. The final concentration from
20- to 50-fold was obtained in a very short time (4 minutes), by the
rejection of the final 15 gallons of distillate, and heat transfer data
collection could not be managed for this terminal step, because of time
constraints and the shutdown procedure required.
Figure 13 shows samples of wastewater feeds and corresponding
concentrates obtained during evaporation-crystallization with VTFE in this
work.
-------
SECTION X
DISCUSSION OF RESULTS
The objective of this work was to obtain sufficient vertical tube
evaporation (VTE) heat transfer data on the preconcentration, followed
by the evaporation-crystallization of industrial cooling tower blowdown,
to provide a basis for the design of a mobile pilot plant facility for
field testing. This pilot plant would also provide definitive data on the
merit of the interface-enhanced mode of vertical tube foam evaporation
(VTFE) for the complete renovation of power plant cooling tower blowdown,
and for using waste heat available within the usual power plant cycle
to accomplish this evaporation at low temperature (120°F)a For this
objective, we needed to construct a pilot plant to provide both
comparative upflow and downflow data with VTE and VTFE used for the
preconcentration of cooling tower blowdown; we also had to provide a
separate pilot plant for comparative downflow VTE and VTFE evaporation-
crystallization data with the preconcentrated blowdown. This second pilot
plant needed to be operated both at low evaporation temperatures, and
under vapor compression conditions at elevated temperature. Much of the
time and effort under this study was therefore devoted to process and
hardware design, procurement of equipment, and construction of these
facilities.
A two-effect, upflow-downflow VTE pilot plant of 10,000 gallons per
day (gpd) capacity, shown in Figures 2 and 3. was constructed, and
comparative test data were obtained to ascertain the heat transfer
performance with brine of 35,000 ppm TDS (seawater). These data, when
compared with earlier concentration data obtained under EPA sponsorship
(5) with cooling tower blowdown from the Mohave power plant using the
single-effect upflow VTE pilot plant section of the present two-effect
pilot plant, provided information required under the objectives of the
present study. Those earlier data (10) are in part reproduced in Figure
14, and the additional comparative upflow-downflow data of this study
are reported in Figures 6 and 7 of this report.
It is apparent from Figure 6 that 20 ppm of Neodol 25-3A provides
about the optimal foaming agent concentration for downflow VTFE enhance-
ment, while 10 ppm should be adequate for upflow VTFE. This applies for
both aluminum-brass and titanium tubes of the most advanced, double-fluted
types. It is clear that the use of interface enhancement is well justified
for both upflow and downflow VTE; the cost of the surfactant additive
used would amount to about 22 cents per pound for a 60 percent solution
supplied in bulk deliveries. A significant reduction of this relatively
low cost should result frpm the use of foam fractionation for recovery
and recycle of the additive (6). The tube-side pressure drop data
-------
plotted in Figure 6 show that, in the case of upflow VTFE, a significant
further advantage is gained by this foamy mode of flow. The approximately
10-fold reduction in tube-side upflow pressure drop improves the upflow
hydrodynamic stability substantially, thereby permitting an increase in the
number of effects acceptable for a multieffect VTFE plant. Alternatively,
in the case of a vapor compression (VC) VTE, this pressure drop reduction
reduces the load on the compressor, which should reduce its power
consumption. Also significant, is the possibility of driving a second
effect VTFE with the same vapor compressor that would normally manage
only one effect VTE. This could provide a substantial increase in the
usual gain ratio, to reduce power cost by about half, for upflow VTE.
Figure 7 shows the conventional VTE and VTFE performance for
upflow and downflow operation with the two different types of enhanced
tubes, on a comparative basis. Also shown are the upflow VTE tube-side
pressure drop prof iles confi rming that, for 2-inch diameter distillation
tubes, one can increase the number of effects in the temperature range
above l60°F.a Also apparent, is the advantage of 3- inch diameter tubes
over 2-inch tubes in the evaporation range below 160° Fa on a basis of
their comparative VTFE performances. On this basis, it should be
advantageous to select 3-inch by 10-foot double-fluted tubes for the field
test pilot plant (preconcentrator) planned for the fol low-on EPA-sponso red
project. These data generally confirm the above conclusions regarding
the advantage of upflow VTFE over downflow VTE and VTFE, with both the
aluminum-brass and titanium tubes used in these tests, especially in the
higher temperature region.
Comparing the upflow VTFE data on Figure 7 with the Mohave cooling
tower blowdown data of Figure \k, it is apparent that the addition of
10 ppm of surfactant had about equal effects on the heat transfer
performance at 130°F for these two different saline waters. In addition
the response in upflow VTE heat transfer performance in Figure 6 corres-
ponds closely with similar data obtained under the earlier EPA-sponsored
project (5) with Mohave blowdown. Taken together with other reports from
this Laboratory, these data provide confidence in upflow and downflow
VTFE and VC-VTFE for the concentration of cooling tower blowdown, and
for producing high quality distillate for recycle, boiler feed or potable
use.
The second part of the objective of this work, the evaporation-
crystallization of cooling tower blowdown, or of its preconcentrate
obtained by VTFE, was carried out by using the second new pilot plant.
This pilot plant, a 5,000 gpd rotatably mounted, upf low-downflow VTE
was operated both with low temperature, reboiler steam as the source of
heat, and with a newly. acqui red vapor compressor. This VTE was designed
to provide reliable comparative heat transfer performance data for upflow
versus downflow VTE, and VTFE data for vapor compression evaporation and
for evaporation-crystallization with the downflow crystal -slurry mode
of operation. The work reported here was obtained with the latter mode and
was performed at two temperature levels: 120°FC with low pressure, reboiler
steam, and 220°Fd with compressed vapor as the source of heat energy. Two
types of slurry-feed crystal 1 ization of relatively insoluble salts from
a71.1°C; b54.4oc; C48.9°C;
-------
dilute solution (cooling tower blowdown and sodium sulphate) were examined
at high temperature. The effects of surfactant addition on the heat
transfer performance was investigated in each case.
In the crystallization of sodium sulphate from highly concentrated
solution at low temperature, it was found that the addition of a
surfactant did not enhance heat transfer but tended to reduce it, apparently
because of the inhibition or retardation of crystallization which increases
the boiling point elevation. Also, foamy vapor-liquid flow through the
distillation tubes, previously shown to be required for good heat transfer
enhancement, was not obtained in these low temperature, highly saline runs.
It is therefore apparent that removal of surfactant from wastewaters
preconcentrated by VTFE should be advantageous, if one wants to subsequently
recover sodium sulphate by evaporation-crystallization (EC) with the slurry-
feed mode of operation at low temperature (as proposed to EPA for the
follow-on project). Such removal of the surfactant additives should have
the beneficial effect of clarifying the VTFE preconcentrate, in the interest
of a cleaner EC product. In cases where the slurried feed mode is applied
in EC at high temperature, the more readily imposed foamy vapor-liquid
flow with surfactant additives may overcome the boiling point elevation
effect and the retardation effects of the additive. In such cases the
presence of surfactant could be beneficial for heat transfer performance,
but the crystalline product would contain adsorbed surfactant.
The crystallizing evaporation (EC) of Mohave cooling tower blowdown
concentrate was performed at low temperature by the slurry-feed mode in
the presence of a surfactant. The precipitated solutes (mostly calcium
sulphate) reduced the heat transfer performance enhancement effects of
the surfactant, as could be anticipated because of the apparent loss of
surfactant due to adsorption on the precipitate. Some enhancement effect
remained however, and removal of the surfactant from the VTFE preconcentrate
does not appear advisable in such cases. Adsorption of the fully biodegra-
dable surfactant on to solid residues to be ponded, does not appear
objectionable. The removal of the surfactant additives by foam fractiona-
tion before final EC, and their recycle to the VTFE preconcentrator appear
advisable when the final solid product is to be utilized subsequently.
A photograph of this concentrate is shown in Figure 13.
Vapor compression (VC) downflow evaporation-concentration of agri-
cultural wastewater, presoftened by ion exchange, was carried out at about
220°Fain the presence of 15 ppm of surfactant. The addition provided a
37 percent heat transfer enhancement effect at a TDS of 7000 ppm. This
heat transfer enhancement remained effective through a 20-fold concentra-
tion, to 140,000 PPm; no salt crystallization was noticed during concentra-
tion, but-some flocculent precipitate was noticed in the concentrate after
cooling to room temperature. This concentration is reported in Table IV
and the data are plotted in Figure 15.
Cooling tower blowdown from a large chemical-industrial complex
was examined with VC-EC, to determine the effects of surfactant addition;
data are reported in Table V and plotted in Figure 15. At an EC tempera-
ture of 220°Ff it was found that the addition of 10 ppm of surfactant
-------
provided a 53 percent heat transfer performance Increase. The evaporation
or distillate productivity rate was increased while the AT or power
consumption was simultaneously reduced. A 27-fold concentration was
obtained with a good interface enhancement (minimal of 3k percent) being
maintained throughout in spite of the crystallization of some solutes.
Distillate quality was good and represented 96 percent of this wastewater.
The final concentrate, shown in Figure 13» had a volume of k percent of
the original wastewater and contained the surfactant additive calculated
to be in approximately 270 ppm concentration. Excessive foaminess was
not a problem at any time, and removal of the additive would be feasible
by foam fractionation if the solid residues are to be used as land fill or
as chemicals.
A third cooling tower blowdown, obtained from a large petroleum
refinery, was examined by VC-EC at 220°pf to determine the effects of
surfactant addition during its reduction to a slurry of only 2 percent
by volume and 98 percent distilled water of high purity. Data are
reported in Table VI and Figure 15. This wastewater contained about
14 ppm of chromate; this or another ingredient apparently inhibited
foaminess with the 10 ppm of surfactant added. This adverse effect could
have been overcome by adding more surfactant. The interface enhancement
effect of 10 ppm Neodol 25-3A (Shell Chemical Co.) was however substantial
(at 37 percent) and remained effective through 20 factors of concentration.
As in the previous run, the removal of the additive by foam fractionation
would not be necessary unless the residual solids are to be used
subsequently, for instance as a source of chemicals. For the more usual
case of ponding such residues, the presence of the surfactant additive
shoud not be objectionable. This surfactant is claimed to be fully
biodegradable, and non-toxic, by its manufacturer.
It appears, from these and previous VTFE test data obtained on a
comparative basis with conventional test data, that the use of surfactant
additives during the renovation of cooling tower blowdown to produce
concentrated saline-crystalline slurries and pure distilled water for
recycle, can be recommended.
51
-------
SECTION XI
REFERENCES
1. Sephton, Hugo H. Interface Enhancement Applied to Evaporation of
Liquids. U.S. Patent No. 3,#16,254, November 5, 1974.
2. Sephton, Hugo H. Interface Enhancement for Vertical Tube Evaporators:
A Novel Way of Substantially Augmenting Heat and Mass Transfer.
Presented at the American Society of Mechanical Engineers Heat Transfer
Conference, ASME Publication 71-HT-38, Tulsa, Oklahoma, August 1971.
3. Sephton, Hugo H. Interface Enhancement for Vertical Tube Evaporation
of Seawater. Proceedings of the 4th International Symposium on
Fresh Water from the Sea, Vol. 1, September 1973. pp. 471-480.
4. Fong, H. L., King, C. J. and Sephton, H. H. The Mechanism of Heat
Transfer Improvement in Upflow Vertical Tube Evaporators by, Induced
Foamy Two-^Phase Flow. Presented at the ASME-AlChE Heat Transfer
Conference, ASME Publication 75~HT-1, San Francisco, California, 1975.
5. Sephton, Hugo H. Renovation of Industrial Inorganic Wastewater by
Evaporation with Interface Enhancement. Report No. EPA-600/2-76-017.
Evironmental Protection Technology Series, March 1976.
6. Sephton, Hugo H. Desalination by Upflow Vertical Tube Evaporation with
Interface Enhancement. Proceedings, International Desalting and
Environmental Association Conference, Ponce, Puerto Rico, April 1975-
7. Sephton, Hugo H. Vertical Tube Evaporation Uti1izing Vortex Flow
and Interface Enhancement. Research and Development Report No. 574,
U.S. Department of the Interior, Office of Saline Water, May 1970.
8. Anderson, J. H., Herri gel H. R. and Johansen. Operational Experience
with Brine Concentrators in the Electric Utility Wastewater Treatment.
International Water Conference No. 75-19, Pittsburgh, Pa., 1975.
9. Sephton, Hugo H. Upflow Vertical Tube Evaporation of Seawater with
Interface Enhancement: Process Development by Pilot Plant Testing.
Desalination, Vol. 16, No. 1, pp. 1-13, February 1975.
10. Sephton, Hugo H. Recycle of Power Plant Cooling Tower Slowdown with
Interface Enhancement. Proceedings of the Second National Conference
on Complete WateReuse, Chicago, May 1975.
52
-------
11. Awerbuch, L and Rogers, A. N. Desalination of Cooling Tower
Slowdown. Proceedings of the Second National Conference on Complete
WateReuse, Chicago, May 1975.
12. Sephton, Hugo H. Vertical Tube Evaporation with Fluted Tubes and
Interface Enhancement: Comparative Performance of Upflow versus
Downflow of the Feed. Presented at the ASME-AlChE Heat Transfer
Conference, ASME Publication No. 75-HT-43, San Francisco, 1975.
13. Sephton, Hugo H. New Developments in Vertical Tube Evaporation of
Seawater. Proceedings of the Fifth International Symposium on
Fresh Water .from the Sea, Vol. 2, 1976. pp. 279-287.
14. Fong. H. L., King, C. J. and Sephton, H. H. Upflow Vertical Tube
Evaporation with Interface Enhancement: Pressure Drop Reduction
and Heat Transfer Enhancement by the Addition of a Surfactant.
Desalination, Vol. 16, No. 1, pp. 15-38, February 1975-
15. Fong, H. L., King, C. J. and Sephton, H. H. An Experimental Study
of Heat Transfer in Upflow Vertical Tube Evaporation. Presented
at the ASME-AlChE Heat Transfer Conference, ASME Publication No.
75-HT-46, San Francisco, California, 1975.
16. Sephton, H. H. Rotatably Mounted Upflow-Downflow, Vapor Compression
Vertical Tube Evaporator. To be published.
53
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-77-063
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE Renovation of Power Plant Cooling
Tower Blowdown for Recycle by Evaporation:
Crystallization with Interface Enhancement
5. REPORT DATE
June 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Hugo H. Sephton
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The University of California
Campus Research Office, M-ll Wheeler Hall
Berkeley, California 94720
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
R803-257-01-3
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 AND PERIOD COVERED
Final; 8/1/74-5/31/76
14. SPONSORING AGENCY CODE
EPA/600/13
15.SUPPLEMENTARY NOTES IERL_RTp project officer for this report is Fred Roberts (EPA,
Environmental Research Laboratory, Corvallis, OR 97330, phone 503/757-4715)..
16. ABSTRACT
The report confirms the effectiveness of a novel evaporation method which
reduces the energy and capital cost requirements for the renovation/recycle of indus-
trial was tewaters. Interface enhancement (U.S. Patent 3,846,254) depends on foamy
two-phase vapor/liquid flow induced during the evaporation of a liquid flowing over a
heat transfer surface; this flow mode substantially increases the liquid's evaporation
rate, after adding a surfactant. Two new vertical tube evaporation (VTE) pilot plants
were used; a two-effect upflow/downflow VTE of 10,000 gpd capacity was produced by
adding an identical downflow second effect to an existing single-effect upflow VTE
pilot plant. This pilot plant was used to obtain comparative data on the concentration
of saline water by upflow and downflow VTE, and by interface-enhanced upflow and
downflow VTE (the interface-enhanced upflow VTE operation provides a higher per-
formance than both the interface-enhanced and conventional downflow operations). A
second pilot plant assembled for this work was a 5,000 gpd downflow VT evaporator-
crystallizer. The heat transfer performance of this plant was increased and its
energy requirements reduced, by applying interface enhancement to the concentration
of several cooling tower blowdowns.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Industrial Water
Waste Water
Renovating
Circulation
Electric Power Plants
Cooling Towers
Crystallization
Evaporation
Energy
Surfactants
Pollution Control
Stationary Sources
Industrial Wastewater
Interface Enhancement
13B
14B
13A,07A
20B,07P
UK
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
60
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
54
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