United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-96/008
September 1996
Technology Transfer
&EPA Capsule Report
Evaporation Process
-------
Technology Transfer EPA/625/R-96/008
Capsule Report
Evaporation Process
September 1996
Center for Environmental Research Information
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati OH 45268
Printed on Recycled Paper
-------
Contents
Process Description 3
Applications 3
Equipment 4
Operation and Maintenance 6
Failure Analysis 6
References 10
Introduction
A failure analysis has been com-
pleted for the evaporation process.
The focus was on process failures
that result in releases of liquids and
vapors to the environment. The re-
port includes the following:
A description of evaporation
and coverage of process prin-
ciples.
Applications of evaporation for
treatment of effluent waters
from the metal finishing indus-
Descriptions of equipment and
operating and maintenance
procedures.
Failure analysis that includes
types of failures and causes.
Key questions that can be used
in software development
A bibliography on evaporation
applications in the metal fin-
ishing industry.
-------
Evaporation Process Process Description
In the evaporation process, waste-
waters from metal finishing processes
are heated until a water vapor is
formed. This vapor is continuously re-
moved and condensed as an over-
head product. In this manner, clean
water is recovered and the solutes
contained in the original wastewater
are concentrated. The solutes may
be contaminants, or useful chemicals
or reagents, such as copper, nickel,
or chromium compounds, which are
recycled for further use. The batch
evaporation process, based on the
use of steam as the energy source, is
illustrated in Figure 1.
If the evaporation process is prop-
erly designed and operated, the clean
condensate generally contains no
more than 10-20 ppm contamination
from wastes containing up to several
percent of dissolved solutes. By us-
ing mechanical vapor recompression
(MVR) or multiple stages, evapora-
tion can be made energy efficient;
however, the initial capital investment
tends to be higher to include these
options.
Evaporation is an established tech-
nology. There is little risk and it has a
low capital cost. Using the proper pro-
cess configuration, evaporation can
achieve a high degree of water re-
moval. However, water removal is
normally limited by the ability to pump
the solution. When wastewaters con-
tain volatile organics with boiling points
that coincide with that for water, prod-
uct condensate can be contaminated
with organics. Removing these organ-
ics requires further treatment, con-
sisting of a carbon bed or other
polishing process. Other obstacles
occur at evaporation temperatures
with foaming, scaling, fouling, and cor-
rosion all possible. Finally, pretreat-
ment chemicals may be needed to
reduce scaling and fouling.
Applications
Due to its reliability and econom-
ics, evaporation has proven to be one
of the better processes for treatment
of wastewaters (dragout) from the
metal finishing industry. Evaporators
are inexpensive to purchase, install,
and operate, making them a powerful
Steam
Condenser
"^^ Clean
water
Condensate
Concentrated
solution
DD-388
Figure 1. Jacketed batch evaporator.
-------
tool for reduction in the cost of water
treatment. The greatest problem is
the control of contaminants in the con-
centrated solution. However, pro-
cesses are available to remove
contaminants, allowing evaporative
recovery to be used on many metal
finishing waters.
Process solutions that have been
successfully treated using evapora-
tive techniques include zinc, cadmium,
copper, nickel, and chromium plating
baths, and phosphoric acid from alu-
minum bright wastewaters. Even
though these waters become contami-
nated by excessive drag-in, dropped
work, impure makeup water, or by
chemical reaction resulting from pro-
cess operation, these impurities can
be controlled, reduced or removed
without excessive equipment costs
(Spearot, 1987; Yates, 1986; Brown,
1984).
Applications for evaporators in other
industries include concentration of li-
quors in the paper industry and re-
covery of potable water from salt water
and brines. Evaporation is also used
to recover water from various types
of wastewaters including cooling tower
blowdown, ion exchange regeneran!
wastes, boiler blowdown, and indus-
trial rinsewaters.
Water
(drag out)
Equipment
Evaporation may be accomplished
operating at a vacuum or at atmo-
spheric pressure. Atmospheric evapo-
rators are used when components in
the wastewater are thermally stable.
Vacuum conditions reduce boiling
temperatures and prevent decompo-
sitions. Decompositions can readily
occur in zinc and cadmium cyanide
solutions.
Evaporation Using a
Packed Column
Atmospheric evaporators may con-
sist of a packed tower with a heated
feed mixture fed to the top of the
tower and air (or hot air) fed to the
bottom of the tower. In this process,
as the hot air contacts the water,
evaporation takes place, thereby con-
centrating the wastewater (see Fig-
ure 2). The wastewater is
concentrated and recycled to the pro-
cess. Because contaminants are con-
centrated by this process, special
attention may be required for their
removal.
Air/water
(vented or
condensed)
Packed
column
Heater
DD-641
Figure 2. Evaporation using a packed column.
Concentrated
stream
recycled for
lurther use)
Packaged Evaporators
Packaged evaporators, which are
available from a number of suppliers,
are used in both atmospheric and
vacuum operations (Lavis, 1994).
Most purchases for packaged evapo-
rators are for film evaporators, with
forced-circulation evaporators ranking
next. Film evaporators are normally
used to concentrate solutions up to
the point where the solubility limits of
the solutes are reached and signifi-
cant amounts of suspended solids
develop. Forced circulation evapora-
tors are designed to handle solutions
containing suspended solids (Worral,
1988). A number of good references
are available on how to select and
design evaporators (Lavis, 1994;
Worral, 1988; Mehra, 1986; and APV
Crepaco, 3rd Ed.). Both film and
forced circulation evaporators are de-
scribed below.
Film Evaporators
In film evaporators, the process liq-
uid is distributed as a film on the heat
transfer surface (see Figure 3). The
process liquid occupies only a thin
film on the tube wall, resulting in low
liquid holdup. Film evaporators are
limited to low viscosity fluids, because
at high viscosities the film is thick-
ened and results in low heat-transfer
coefficient. The practical upper limit
of viscosity for film evaporators is 1 00-
500 centipoise (Dedert Corp, 1994).
The amount of heat transferred from
the heating medium to the wastewa-
ter depends on the temperature dif-
ference between the process fluid and
heating medium, area of heat trans-
fer surface, and the heat transfer co-
efficient.
Q = UADt (1)
where
Q = amount of heat transferred
to aqueous wastewater from
heating medium, Btu/hr.
U = overall heat transfer coeffi-
cient, Btu/hr/sq ft/°F.
A = area of heat transfer sur-
face, sq ft.
Dt = t,-tp,°F.
t. = temperature of heat source,
°F.
L = temperature of process, "F.
For a given heat load and tempera-
ture difference, Equation (1) may be
used to estimate the required heat
transfer area of the evaporator. Heat-
transfer coefficients for film evapora-
-------
Steam
Clean water
condensate
(for recycle)
Wastewater
(dragout)
Concen-
trated
solution
(for
recycle)
DD-435
Figure 3. Falling film evaporator.
Recirculation pump
tors are based on operating experi-
ence or pilot plant testing. Assuming
that the heat source is condensing
steam, overall heat transfer coeffi-
cients range from 500 Btu/h/sq ft/°F
when processing water-like materials
to 100 Btu/h/sq ft/°F for high viscosity
fluids.
Film evaporators come in rising or
falling film configurations. In the fall-
ing film evaporator, liquor is supplied
at the top of the evaporator and is
distributed to the tubesheet by
nozzles. The liquor then falls down-
ward by gravity along the tube wall.
Steam supplied on the outside of the
tube in a shell-and-tube configuration
causes evaporation of the film; then
vapors pass along the center of the
tube while the film progresses down
the tube wall. As the liquid-vapor mix-
ture enters the main body of the
evaporator, liquid falls to the bottom
while vapors rise. Following entrain-
ment separation, vapors exit the
evaporator. Liquid is discharged as a
concentrate from the bottom of the
evaporator body.
Forced Circulation
Evaporators
Solutions containing significant
amounts of suspended solids are bet-
ter handled in a forced-circulation
evaporator. In this type of evaporator,
process liquid circulates through the
heat exchanger at a very high rate.
As the process liquid is heated, boil-
ing is suppressed by back pressure
created by the static head of the pip-
ing at the heat exchanger exit. As the
liquid leaves the heat exchanger, the
pressure is reduced, and the liquid
flashes in the evaporator body. In the
evaporator body, vapor is removed
and concentrated liquid is recirculated.
The elements of the forced circula-
tion evaporator are illustrated in Fig-
ure 4.
High velocities in the heat ex-
changer increase the heat-transfer
coefficient and reduce fouling but in-
crease the pressure drop. Design con-
siderations for forced circulation
evaporators include balancing heat
exchanger requirements versus pump-
ing requirements. Heat exchangers for
forced circulation evaporators are usu-
ally of the shell-and-tube design with
the process fluid nearly always on
the tube side. Plate heat exchangers
offer higher heat transfer coefficients
than the shell-and-tube exchangers,
and a more compact design, but they
are more expensive.
Energy Savings
Because evaporation is an energy
intensive process, it is important to
explore ways for energy conserva-
tion. Common ways to save energy
in evaporation include using mechani-
cal vapor recompression (MVR) and
multiple stages. While both options
increase capital costs, energy sav-
ings may justify the expenditure when
energy is expensive or at remote sites,
where thermal energy may be spar-
ingly available.
Mechanical Vapor
Recompression
Energy for evaporation may be re-
duced by 95% using MVR, though
aa'ding the necessary compressor
adds to mechanical complexity.
Evaporators equipped with MVR are
commonly used in the food industry
(Iverson, 1980 and Centec Corp.,
1980). In MVR, the vapor leaving the
evaporator is compressed to the pres-
sure that corresponds to the satura-
tion temperature required on the
steam side of the heat exchanger. In
most cases, steam is not required
once the system is running. MVR was
popular in the early 1980s when en-
ergy was costly, but lower energy
costs have made new installations
less common. The falling film evapo-
rator equipped with MVR is illustrated
in Figure 5.
Multiple Stages
In multiple-stage evaporation, the
water vapor from one stage is used
as the heating medium for another
stage operating at lower pressure.
Multiple-stage evaporation is com-
monly used for recovery of potable
water from seawater (Darwish et al.,
1989). Most of the separation takes
place in the first stage, but as many
as six or eight stages have been used
-------
Vapor
Heat exchanger
C
I
Compressor
Liquor
Forced
circulation
evaporator
Clean water
(for recycle)
DD-664
Wastewater
(dragout)
iO
/\ Recycle
Pump
Concentrated solution
(for recycle)
Figure 4. Forced circulation evaporator with mechanical vapor recompres-
sion.
in one system. To illustrate energy
savings, three stages require slightly
less than half the energy than a single-
stage operation. The process is illus-
trated in Figure 6 for three stages.
The lower steam requirement of the
multiple-staged evaporator is accom-
panied by a higher equipment cost.
The available temperature difference
at any single heat exchanger is con-
siderably lower than that available in
a single-stage evaporator. Thus the
total heat transfer surface area is
greater. In addition, the multiple-stage
evaporator requires more vessels,
pumps and piping. As in other pro-
cesses, the trade-off between capital
and operating costs is the key con-
sideration.
Operation And
Maintenance
Pretreatment chemicals may be re-
quired for the evaporation process,
depending on the characteristics of
the wastewater. Chemicals are used
to prevent corrosion and fouling of
the evaporator. Other operations in-
clude adding acids to reduce alkalin-
ity, removing carbon dioxide to
enhance performance, adjusting pH
to control precipitation, and removing
oxygen to reduce corrosion. Should
precipitation occur, calcium sulfate (or
other crystals) are needed, solids and
scales deposit on the crystals rather
than on heat transfer surfaces. Mate-
rials of construction must be selected
to minimize corrosion and provide long
equipment life.
For moderate fouling, film evapora-
tors are acceptable if cleaned often.
All film evaporators have a minimum
liquid wetting rate, to assure that the
surface gets coated. If the required
rate is above the product flow rate,
one must recirculate material back to
the heat exchanger to maintain the
minimum wetting rate.
Failure Analysis
High Probability
Seals
Seal or o-ring failures may occur in
the evaporator feed pump, chemical
feed pumps, or the air compressor,
which delivers instrument air to in-
struments and control valves. Pos-
sible causes of seal failures include
overheating and mechanical stress.
Visual inspection will confirm spray-
ing or leaking of wastewater at the
pumps or compressor.
Valves and Pipe Fittings
This type of failure is more preva-
lent in older plants than in newer ones.
Generally, leaks in evaporator sys-
tems are likely to be small because
evaporators operate at low pressures
(atmospheric and vacuum). Causes
for this failure include mechanical
stress, improper maintenance proce-
dures, and freezing during cold
weather.
Miscellaneous Spills During
Daily Operations
Spills of pretreatment chemicals and
wastewaters can occur when tanks
are replenished. They may also oc-
cur when the system is shut down for
maintenance. In evaporation systems,
pretreatment chemicals include acids,
bases, and phosphates.
Relief Valves (Vapor)
Tanks are equipped with vapor re-
lief valves to maintain a constant tank
pressure. These valves will release
contaminated vapors to the atmo-
sphere as tank levels (and tank pres-
sures) increase. These releases are
small but they occur frequently.
Evaporator Failures
An evaporator can fail for a num-
ber of reasons but failures are due
mostly to foaming or entrainment.
When foaming occurs, wastewater
foam fills the body of the evaporator
and ultimately contaminates the clean
condensate. Foaming is caused by
the presence of surface active agents,
though such agents are often difficult
to detect and measure. With entrain-
ment, wastewater drops are physi-
cally carried overhead by the flowing
-------
Compressor
Clean water condensate
(for recycle)
Concentrated
wastewater
Concentrated solution
(for recycle)
DD-658
Recirculation pump
Figure 5. Falling film evaporator with mechanical vapor recompression.
vapor with the condensate being con-
taminated by the wastewater drops.
Entrainment is caused by operating
at vapor rates higher than design ca-
pacity.
Moderate Probability
Relief Valves (Liquid)
Relief valves are included in evapo-
ration systems to protect piping and
filings from overpressure, they are
less numerous and less likely to fail
than in reverse osmosis because
evaporators are operated at low pres-
sures (atmospheric and vacuum).
Tank Overflows
Tank overflows can result in a sig-
nificant release of wastewater or
chemicals to the environment. They
occur mostly during startups and shut-
downs.
Low Probability
Tank Ruptures
A tank can rupture, possibly be-
cause of mechanical failure or freeze
damage. Though this type of failure
is rare, a rupture can result in the
release of large quantity of wastewa-
ter or chemicals to the environment.
Piping Ruptures
Possible causes of rupture include
mechanical stress, freezing, and im-
proper maintenance procedures. Sig-
nificant leaks are possible with this
type of failure.
The types and causes of failure
and associated questions for subse-
quent software development are pre-
sented in Table 1.
-------
Boiler
steam
First
stage
Second
stage
Third
stage
Condenser
Vapor
Condensate
Vapor
Vapor
Cooling
water
Liquor
Wastewater
i
i.
B
Concentrated
wastewater
Clean
water
DD-364
Figure 6. Three-stage evaporation process.
-------
Table 1. Failure Analyses for Evaporation System
Failure Cause(s)
Questions for Software Development
High Probability
Seals
Valves and pipe fittings
Miscellaneous spills
during daily operations
Relief valves (vapor)
Evaporator failures
- Overheating
- Mechanical stress
-Abrasive wear
- Mechanical stress
- Improper maintenance procedures
- Freezing
-Spills during filling of tanks (due to
faulty gages and equipment and
mistakes by operators). Spills can
include wastewater and
pretreatment acids and bases.
- Faulty maintenance procedures
- Increases in tank levels
-Changes in ambient temperature
Foaming
Entrainment (operating at above
design capacity)
What is the expected quantity of leaks through seals
(gallons)? What is the disposition of these leaks (i.e.
Do they go to a capture system, process sewer, or
are they lost directly to the environment?
What is the expected quantity of leaks through
valves and pipe fittings (gallons)? What is the
disposition of these leaks?
What is the expected quantity of leaks from spills
(gallons)? (Base on plant experience and
operating records). What is the disposition of these
spills?
What is the expected quantity of leaks through vapor
relief valves (standard cubic feet/hour)? What is the
disposition of these leaks?
What is the expected quantity of leaks through
evaporators (gallons)? What is the disposition of
these leaks?
Moderate Probability
Relief valves (liquid)
Tank overflows
- Overpressures during startups,
upsets, and shutdowns (for
evaporators operating at pressures
of atmospheric and above)
-Key control valves failing in
closed position
- Plugging of valves, piping, and
membrane modules due to buildup
of solids. Hollow-fiber and spiral
membrane modules are most
susceptible to fouling.
-Occur mostly during unstable
conditions (during startups and
shutdowns). Overflows can
include wastewater and
pretreatment acids and bases.
What is the expected quantity of leaks through liquid
relief valves (gallons)? What is the disposition of
these leaks?
What is the expected quantity of tank overflows
(gallons)? (Base on plant experience and records).
What is the disposition of these overflows?
Low Probability
Tank ruptures
Piping ruptures
DD-840
Mechanical failures
Freezing
Mechanical failures
Freezing
What is the expected quantity of releases due to tank
failures (gallons)? (Be sure to include the concentrated
waste if it is stored onsite). What is the disposition of
these releases?
What is the expected quantity of losses due to pipe
ruptures (gallons)? What is the disposition of these
losses?
-------
References
APV Crepaco, Inc., Chicago, IL,
Evaporator Handbook, 3rd Edi-
tion, EHB-1189.
Brown, C., "Recovery of Phospho-
ric Acid by Ion Exchange and
Evaporation," 71 st AES Annual
Technical Conference Proceed-
ings, New York, NY, July, 1984,
American Electroplaters' Soc.
Inc., Winter Park, FL (1984).
Centec Corporation, "Energy Sav-
ing Potential of Mechanical
Vapor Recompression," Tech-
nology review sponsored by In-
dustrial Organizations in the
Food Industry and U.S. Dept.
of Energy/Office of Industrial
Programs, April 1980.
Darwish, M. A. et al., "Technical
and Economical Comparison
Between Large Capacity MSF
and RO Desalting Plants," De-
salination, 76:281-304, (1989).
Dedert Corp., Olympia Fields, IL,
"A New Evaporator Gives
Sweet Performance," Chemical
Engineering, May 1994.
Iverson, C. H., G. E. Coury, and
J. H. Fischer, "Evaporation
by Mechanical Vapor Re-
compression," Final Report
#BSDF-38-4Q-80, Beet
Sugar Development Foun-
dation, September, 1979-
October31, 1980.
Lavis, G., "Evaporators How To
Make The Right Choice,"
Chemical Engineering, April
1994, p92.
Mehra, D. K., "Selecting Evapora-
tors," Chemical Engineering,
February 3, 1986.
Spearot, R. M., "Evaporative Re-
covery," Plating and Surface
Finishing, February, 1987, pp
22-29.
Worrall, P., "Tips For Evaporator
Selection," CPI Equipment Re-
potter, May/June 1988.
Worrall, P., "Sorting Out Evapora-
tor Types And Designs," CPI
Equipment Reporter, March/
April, 1988.
Yates, B., "Atmospheric Evapora-
tors: Recovery with Atmo-
spheric Evaporators Shows
Major Benefits - Particularly for
Nickel and Chromium Plating
Lines," Plating and Surface Fin-
ishing, 73:4, April 1986, pp 30-
32.
&U& GOVERNMENT PRINTING OFFICE: 1994.75MOV41053
-------
m
I
M
yi
33
oo
w CD 3
o S Q!
" en
5'
CD
8
O 33 O m C
-1 (U IB
Ğ $ 2
= g, ro
Si §-3
1 Jg
CD n>
m S ĞD
5T w
3 5
en
l\3 5".
o> o
cop
9
-^i
ro
<
S
(D o
'
(Q
ro
o
m
3)
-a
O
en
p
9
CO
tn
------- |