- Email: [email protected]
A novel 2500 GPD 5-effects wiped-film rotating-disk vapor-compression module; preliminary results
Desalination, 74 (1989) 289-303 Elsevier Science Publishers B.V., Amsterdam -
289 Printed in The Netherlands
A NOVEL 2500 GPD 5-EFFECTS WIPED-FILM ROTATING-DISK VAPOR-COMPRESSIONMODULE; PRELIMINARY RESULTS by Badawi Tleimat & Maher Tleimat Tleimat & Associates 75 Ina Court Alamo, California 94507
For presentation at the IDA 4th World Congress on Desalination and Water Reuse, State of Kuwait, November 4-8, 1989
ABSTRACT A laboratory model wiped-film rotating-disk (WFRD) vaporcompression evaporator with a capacity of 300 liters/day (80 gpd) was conceived, designed, and built in the late sixties at the Sea Water Conversion Laboratory, University of California at Berkeley. Preliminary data obtained from this model showed exceptionally high overall heat transfer coefficient,U, in excess of 28 kW/m2C (5000 Btu/hft2F). Results from later data with precise instrumentation confirmed the earlier results and showed even higher values for U. Thermoeconomicanalysis of multieffect vapor-compression systems to reclaim 95 percent of agricultural drainage water at the San Joaquin Valley in California showed that the use of multieffect VC systems instead of conventional single effect VC systems reduces energy consumption. It also showed that the ratio of U in the evaporator to its cost per unit area of heat transfer has very strong influence on distilled water cost; the higher the ratio, the lower the distilled water cost. The analysis also showed that because of the exceptionallyhigh value of U in the WFRD evaporator and based on data obtained from the laboratory model at Los Banos, California using agricultural drainage water, its use in a 5-effect
290
VC system will reduce product water cost by a factor of 2 and energy consumption by a factor of 3 as compared to off-the-shelf single effect conventional VC systems. A semicommercial2500 gal/day five-effect vapor-compression WFRD evaporator module was designed, built and tested with tap water. Preliminary data were obtained. The results from these data show an average value of U to be about 20 kW/m2C (3500 Btu/hft2F) and energy consumption by the compressor between 3.6 and 5.1 kWh/m3 (13.6 and 19.3 kWh/lOOO gal) of product water.
INTRODDCl'ION The use of vapor-compression(VC) distillers to reduce the volume of waste brine streams from water recovery plants is gaining acceptance at inland locations (1, 2, 3, 4). Thermoeconomic analysis (5) of multiple-effectvapor-compression(MFVC) distillation shows that its use can reduce energy consumption significantlyin comparison to conventional VC systems for the same type of evaporator and the same feed and brine conditions. The analysis was applied to the recovery of agricultural waste drainage water in the San Joaquin Valley in California to recover 95 percent of the 10,000 ppm feed as distilled water. The analysis showed that by using a five effect vapor-compressionsystem instead of single effect VC system the energy consumption was reduced by about 40 percent in this particular case. The California Department of Water Resources (DWR) funded the Water Technology Center (formerly Sea Water Conversion Laboratory), University of California at Berkeley, to test this analysis using an existing small vapor-compressionunit at the Center. The evaporator in this small unit was the Tleimat wiped-film rotatingdisk (WFRD) evaporator. Tests were conducted (5) at the DWR Desalting Facilities at Los Banos, California, using agricultural drainage water from the San Luis drain. The tests were designed to
291
test the effectivenessof this evaporator for treating the waste water as well as obtaining data on MEVC systems by simulation. The results from these tests using softened agriculturalwaste water showed very high heat transfer coefficient and low energy consumption. The overall heat transfer coefficient was about 31.4 kW/m2C (5530 Btu/hft2F, 27,000 kCal/hm2C) and the energy consumption by the compressor was about 4.1 kWh/m3 of product water (15.5 kWh/lOOO gal) based on a 60 percent isentropic efficiency for the compressor. At that time the California Department of Water Resources contracted with Bechtel National to install and test a 50,000 gpd vapor-compressionunit at Los Banos California. The evaporator was of the horizontal tube spray-film type. It was built by Israel Desalination Engineering and erected and tested under the supervision of Bechtel Engineers. The unit was run between April and September 1986 using agricultural drainage waste water and calcium sulfate slurry seeding technique for scale prevention. Results from these tests (6) showed the overall heat transfer coefficient to be about 1.4 kW/m2C (250 Btu/hft2F, 1200 kCal/hrm2C) and energy consumption by the compressor to be about 25 kWh/m3 (95 kWh/lOOO gal). A proposal was submitted by the Water Technology Center to DWR to fund the construction and testing of a 2500 gal/day five-effect module using the WFRD evaporator as its element. Due to budget constraints DWR declined to fund the proposal and the Center was closed at the end of June 1987 due to lack of funds. Upon closing of the Center, the inventor of the WFRD evaporator, Badawi Tleimat, decided to take early retirement from the Center at the University of California and undertook to build the module using his own funds.
THE WIPED-FILM ROTATING-DISK EVAPORATOR Figure 1 is a schematic cross section of the evaporator. Fig.
292 PRODUCT TUBES
r
WIPERS SHOWN
i BLOWDOWN
FIG. 1.
ROTATING-01%
UIPEO-FILM
I
NOT
BLOW’DOWN (RESIDUE)
BRINE
FIG. 2.
EVAPORATMI CROSS SECTION 'SCHEHATIC"
WIPER DISTILLAND
FIG.).
BLADE
EVAPORATOR CROSS SECTION BETYEEN 01% PAIRS OF FIG. I "SCHEMATIC'
\
FEED
CROSS SECTION OF THE YlPER ll,AW AW IN A TANGWTIAL DIRECTIDN
BRINE
nETA,. DISK
293
2 is a schematic cross section taken between two disk pairs. Fig. 3 is a cross section of the wiper taken between the inside and outside periphery of the disk showing the relative position of the wiper and disk. The rotor consists of disk pairs joined together at the periphery of the inside holes. Although Fig. 1 shows only four disks forming two cavities, the rotor can be designed to contain more pairs when required. The rotor is mounted on a stationary shaft closed at one end and open at the other end. Steam from a boiler, from a previous effect, or from a compressor, is introduced into the open end of the shaft and is condensed on the inside surfaces of the rotating disks. The condensate, as it forms on the inside surfaces of the disks, is thrown by centrifugal force to the periphery where it enters stationary product tubes (scoops) connected to a central tube, and flows out of the evaporator as distillate product. The rotor rotates inside a chamber into which the aqueous solution is fed along the length of stationary wipers (Figs. 2 and 3), and is deposited as a thin, uniform film on the outside surfaces of the rotating disks in a manner which eliminates formation of dry spots. Unevaporated feed is slung to the periphery of the disks and is withdrawn from the chamber as blowdown (residue) at the bottom. The combination of centrifugal force and wiped feeding achieves a thinning of both condensate and feed films which result in exceptionally rapid heat transfer.
TEST RESULTS FROM A SIMULATED FOUR-EFFECT VC SYSTEM In this test the agricultural drainage water was softened by ion-exchange to remove calcium. Table I shows the raw water composition. The softened water was collected in a storage tank and then was fed into the evaporator to simulate the first effect and the resulting brine was collected in a different storage tank. This brine was then used as the feed to the evaporator to simulate
294
the second effect. The brine collected from the simulated second effect was used as the feed to the evaporator to simulate the third effect. Again, the brine collected from the third effect was then used as feed to the evaporator simulating the fourth effect. The total recovery from the simulated four-effect system was 92.8 percent starting with a feed salinity of 9380 ppm and ending with a final brine salinity of 126,590 ppm. Figure 4 shows a plot of the overall heat transfer coefficient,U, as a function of the overall temperature difference. The dashed line represents theoretical prediction (7) while the solid line was drawn through the experimental results. The two lines at the bottom represent U for new smooth and double fluted tubes. They are drawn for comparison with U from WFRD. Table II shows a summary of the results from this test. The second, third, and fourth columns show, respectively, the total dissolved solids, in ppm, in the feed, brine, and distillate streams. Column 5 shows the overall heat transfer coefficient, U, while column 6 shows the overall temperature difference. Column 7 shows the measured pressure rise, DP, across the compressor, while column 8 shows the energy consumption by the compressor, WK.
This figure was calculated from
the measured pressure rise, DP, across the compressor using an isentropic compressor efficiency of 60 percent. The results on overall heat transfer coefficient and energy consumption confirmed earlier results obtained in the laboratory using deionized water and seawater feeds.
THE 2500 GPD (10 M3/DAY) FIVE-EFFECT WFRD MODULE Figure 5 is a schematic flow diagram for the five-effectWFRD vapor compression module. In the module, heat transfer surfaces A, B, C, D, and E represent, respectively, the rotor in each of the five effects. Here, pretreated and filtered saline water, stream a at point 1, is split into two streams, al, and a2.
Stream al
295
TABLE I AgriculturalWater Analysis: Sample taken on October 13, 1982 from San Luis Drain at Desalting Site, Los Banos. Analysis made by DWR Element
mg/liter
Sodium
2160
Calcium
500
Magnesium
264
Potassium
6.9
Sulfate
4610
Chloride
1440
Boron
16
Silica as Si02
20 169
Total alkalinity as CaC03
2340
Total Hardness as CaC03
11300 micromhos/cm
Specific conductance at 25 C
8.1
PH Total Dissolved Solids
9370 mg/liter
TABLE II Summary of Data from the Simulated Four-effect test. The Evaporator was Operated at an Average Temperature of 60 C. Feed
Brine
1
9,390
2
U
DT
13,213
2
27,475
0.52
9.6
3.48
13,213
19,438
2
27,208
0.53
10.7
3.83
3
19,438
35,534
2
27,140
0,57
12.55
4.05
4
35,534
126,590
2
26,298
0.59
16.54
5.64
U
DP
WK
Product
Set
= Overall heat transfer coefficient; kCal/hm2C
DT = Average temperature difference; Deg C DP = Average measured pressure rise across compressor: cm H20 WK = Average energy consumption by compressor; kWh/m3
296
Total dissolvedsolids %
z Y Y
I .39
SET I
0
SET 2
A
I .96
SET 3
v
3.4
SET 4
0
12.66
I
5 2000 0 II Vertical --------------
1000
Tube Evaporator ----------------_
Flash Evyorator --------- Multistage --------------------2
t
“0
”
0.7
0.8
At = OVERALL Fig.
4
1.0
0.3 TEflPERATURE
DIFFERENCE.
Overall heat transfer coefficient as difference using softened agricultural
1.1 F
a function drainage
of overall water.
temperature
~ I--------~~t--------l
-.
I
*
SALINE WATER
-----VAPOR
b
c
d
@
- .-
DISTILLATE
I
FIVE-EFFECT EVAPORATOR
Fig.
5 Schematic
flow
diagram
for
a five-effect
vapor-compreeelon
qatcn
297
enters the brine heat exchanger where it is heated from point 1 to point 2 by cooling brine stream f from point 11 to point 12 and stream
a2 enters the distillate heat exchanger where it is heated
from point 1 to point 3 by cooling distillate stream n from point 20 to point 21. The two streams, al and a2 are joined back into one stream. The feed then enters the degasser where noncondensible gases are stripped out and then enters the first effect at point 6 where it is brought into contact with heat-transfer surface A. Here, part of the feed is evaporated and the balance, stream b at higher salinity than stream a, is pumped from the first effect and enters the second effect at point 7.
Here, again, part of stream b
is evaporated and the balance, stream c at higher salinity than stream b, is pumped from the second effect and enters the third effect at point a. This process is repeated until the fifth effect where the brine, stream f at the desired blowdown salinity, is taken out at point 11, cooled in the brine heat exchanger, and discharged as brine blowdown at point 12. The vapor generated in the first effect condenses on the cooler heat transfer surface B in the second effect and is taken out as distillate, stream h at point 16. The heat released in condensing vapor stream h is transferred across heat transfer surface B and causes the evaporation of an equivalent amount of water from stream b in the second effect. This process is repeated in the second, third, and fourth effect to produce distillate streams j, k, and m.
The vapor generated in the
fifth effect is taken out at point 13, is compressed to point 14 to raise its saturation pressure and temperature, and is then introduced into the condensing side of the first effect where it condenses on heat transfer surface A and is withdrawn as distillate, stream g at point 15. The condensate streams g, h, j, k, and m at points 15, 16, 17, 18, and 19 are taken out as stream n at point 20 which is then cooled in the distillate heat exchanger and taken out as product at point 21.
298
In this module each rotor consists of 10 disks made from 60 cm diameter x 0.8 mm copper sheet resulting in a total net heat transfer area of 2.12 m2 per rotor, therefore, at a capacity of 10 m3/day, the product rate per unit area of heat transfer surface would be about 39 kg/m2h. The compressor in this module is a positive displacement lobe-type blower driven by a 5 horse-power (hp), 1800 rpm electric motor. The rotor assemblies are driven by a 2 hp, 1200 rpm electric motor through timing belts and pulleys with the drive shaft being sealed by a mechanical face seal using distilled water as lubricant.
DATA AND RESULTS FROM THE MODULE Data were obtained from the module using tap water. The temperature of the streams was measured by calibrated platinum resistance sensors placed in the evaporation and condensation vapor spaces in each effect. The energy consumption by the compressor drive motor and rotors drive motor was each measured by a separate Watt-meter. The distillate rate was determined by measuring the time to fill a given calibrated volume. The overall heat transfer coefficient was calculated from these data assuming that the distillate rate from each effect was equal to one fifth the distillate rate from the module. The module was operated during working hours between 8 AM to 5 PM.
At start-up, the water would be circulated over a 2 kW
electric heater in the degasser circuit to heat the system. When the temperature of the circulated water reached about 35 C, the rotor and the compressor were started and the module started producing water immediately; however, at low rate due to the high specific volume of the vapor at the relatively low start-up temperature in the system. Table III shows data and results obtained from the module. Data were taken at hourly intervals for eight days. When the
299
TABLE III DatxandResultsUeingTapwaterFeed t6
t5
WKCR
DesC
DesC
U WA
WKC
DesC
kWh/m3
mm 3
m3/h
Mb2
48.89
43.47 48.84 49.02 50.14 52.21 54.60
1.09 1.26 1.29 1.32 1.47 1.56
18.64 17.85 19.83 19.96 19.10 25.46
3.97
55.15 55.49 56.72 59.57 62.41
3.54 3.63 3.59 3.56 3.39
7.52 6.66 6.86 6.84 6.67 6.58
0.311 0.354 0.367 0.393 0.429 0.474
25.2 33.4 34.6 37.1 40.5 44.6
47.08 50.41 52.44 55.47 57.62 59.63
42.12 45.02 46.77 49.12 50.76 52.40
0.99 1.08 1.13 1.27 1.37 1.43
20.12 19.76 20.81 19.24 20.49 20.82
3.51 3.51 3.63 3.81 3.80 3.53
6.89 7.09 7.32 6.90 7.01 6.57
0.281 0.314 0.336 0.370 0.402 0.433
26.5 29.6 31.6 34.9 37.9 40.8
51.42 53.93 57.74 60.39 65.09
45.86 40.02 50.84 52.77 56.26
1.11 1.18 1.38 1.53 1.77
19.41 20.92 21.29 18.06 23.44
4.05 3.76 3.82 4.16 4.40
7.43 7.04 6.67 7.01 7.45
0.319 0.355 0.432 0.470
30.1 33.4 37.7 40.7 44.3
42.19 49.51 53.42 54.61 55.27
38.10 44.23 47.53 48.45 49.07
0.82 1.06 1.18 1.23 1.24
18.40 20.49 19.00 19.01 19.16
3.59 3.26 3.35 3.66 3.76
7.28 6.23 6.50 6.57 6.63
0.220 0.302 0.352 0.353 0.365
20.8 28.5 33.2 33.3 34.4
44.08 50.14 52.32 53.51 56.06
39.83 44.03 46.79 47.44 49.78
0.85 1.06 1.11 1.21 1.25
18.13 18.58 19.09 17.67 19.66
4.96 5.57 4.76 4.06 4.68
9.10 9.05 8.05 7.39 7.96
0.236 0.309 0.319 0.337 0.378
22.2 29.1 30.1 31.8 35.6
48.86 55.44 57.87 59.09 63.34 64.22
43.58 49.04 51.51 52.49 55.51 55.79
1.06 1.28 1.27 1.32 1.57 1.69
17.31 17.68 22.01 21.31 19.62 17.73
4.67 4.83 4.74 5.31 5.52 6.02
8.59 8.31 7.99 8.83 9.52 10.11
0.293 0.366 0.415 0.410 0.475 0.477
27.6 34.6 39.2 39.5 44.8 45.0
47.30 49.60 52.91
42.51 44.61 47.33
0.96 1.01 1.12
19.34 20.57 21.08
4.94 4.56 4.73
9.60 9.06 8.96
0.279 0.311 0.355
26.3 29.3 33.4
49.46 53.26 56.03 59.46
44.07 47.44 50.57 52.42
1.09 1.16 1.25 1.41
17.52 18.82 20.96 19.07
4.91 5.04 4.97 5.15
8.91 8.96 8.34 8.54
0.300 0.338 0.392 0.435
28.2 31.9 36.9 41.0
Dt
FR
0.400
Flux
300
temperature in the fifth effect reached about 45 C, the 2 kW heater was turned off and the system temperature increased due to energy input from the rotor and compressor drives. In this table the first column shows the measured saturation temperature in the condensation space of the first effect while the second column shows the measured saturation temperature in the evaporation space of the fifth effect. The third column shows the average overall temperature difference, Dt, across the heat transfer surface in each effect. It is taken to be equal to one fifth of the difference between t6 and t5. The fourth column shows the calculated average overall heat transfer coefficient,U, in the evaporator for all the effects. The fifth column shows the measured energy consumption by the compressor and its drive, WKC, in Kilowatt-hour per cubic meter, while column 6 shows the energy consumption, WKCR, also in the same unit, by both the rotor and compressor drives. Column 7 shows the distillate rate from the module, PR, in cubic meter per hour, while column 8 shows the distillate flux in kg per hour square meter of heat transfer surface in the evaporator. The data and results show the energy consumption,WKC, by the compressor motor to be between 3.26 and 6.02 kilowatt hour per cubic meter of distillate while the energy consumption,WKCR, by the compressor and rotor motors to be between 6.23 and 10.11 kilowatt-hour per cubic meter of distillate. This figure is useful because it provides a direct comparison of this type of evaporator with vapor-compressionunits using conventional evaporators regarding energy consumption by the compressor. It is significant to note here that at the design condition the measured energy consumption by the compressor motor varied between 3.6 and 5.1 kilowatt-hour per cubic meter (13.6 and 19.3 kWh/lOOO gal) while at these conditions the energy consumption by the compressor and rotor motors varied between 6.8 and 10.0 kWh/m3 (25.7 and 37.9 kWh/lOOO
301
gal).
Again, at the design conditions the average overall
temperature difference per effect was about 1.3 C and the average overall heat transfer coefficient was about 20 kw/m2C (3500 Btu/hft2F; 17,200 kCal/hm2C).
These two figures are useful for
multieffect distillation using steam as the heat source. For example, assume that in a desalting plant steam is available at 90 C and that cooling water is available to allow the final condenser to operate at 40 C, providing a total temperature difference of 50 C between the steam supply and final condenser. For a multieffect wiped-film rotating-disk evaporator using 1.3 C temperature drop per effect, one can install about 38 effects. Assuming that the sum of the boiling point elevation in all the effects amounts to 10 C (this figure changes with feed water chemistry and recovery), then it is possible to build a 30 effect system that could result in a gained output ratio of about 25 kg of distilled water per kg of steam. economy.
This is significant for steam
It is also significant regarding cooling water use in
that it would require only about 40 percent of the cooling water needed for a plant with a gained output ratio of 10.
These
considerations are extremely important at inland locations where raw water is available and low temperature heat sources such as; geothermal hot water, flat plate solar collectors (8), saltgradient solar ponds (9), waste heat from diesel engines (10) and waste heat from power and chemical plants. The modular design of this evaporator lends flexibility in applications requiring large capacity.
By changing the size of the
disks and varying the number of disks per effect and the number of effects per module it is possible to make modules with capacities as low as 4 m3/day and as high as 200 m3/day.
Obviously each
application will provide its own optimum design conditions regarding the number of effects per module as well as the size and number of disks in each effect.
302
REFERENCES 1.
Hervey, D. E. and M. E. Cross, "Water Reclamation, Systematic Approach," Technical Proceedings, Eighth Annual Conference and InternationalTrade Fair of the National Water Supply Improvement Association,July 6-10, 1980, San Francisco, California.
2.
Hancock, J. F. and W. E. Sringer, 'Zero Discharge of Liquid Wastes from Power Plants,' Technical Proceedings,Ninth Annual Conference and InternationalTrade Fair of the National Water Supply Improvement Association,May 31 - June 4, 1981, Washington, D. C.
3.
Leitner, G. F., "Waste ConcentrationVs. Evaporation Ponds for Reverse Osmosis Reject Water - A case Study," Working Papers, First World Congress on Desalination and Water Reuse, Florence, Italy, May 23-27, 1983.
4.
DeMoel, P. J., R. F. M. De Gier, and G. Onderdelinden, "Design of the 50,000 m3/day Treatment Plant for Qasim, Saudi Arabia: Desalination of Brackish Water at 99% Recovery," Proceedings of the Second World Congress on Desalination and Water Reuse, Bermuda, November 17-22, 1985.
5.
Tleimat, B., A. D. K. Laird, and E. D. Howe, "Analysis and Cost Prediction of Reclaiming Agricultural Drainage Water Using Multieffect Vapor-CompressionDistillation,"Final Report to California Department of Water Resources, UC/DWR Agreement B-55037, Task Order 84-1, November 1985.
6.
Bechtel National, Inc. "Final Report, Field Test of a Vapor Compression Evaporator at the DemonstrationDesalting Facility, Los Banos, California," prepared for the State of California, Department of Water Resources, Specification NO. 85-31, Contract No. C-50653, San Francisco, California, August 1986.
7.
Tleimat, B. W. et al., 'Wiped-Film Rotating-DiskEvaporator for Water Reuse," U. S. Department of the Interior, Bureau of Reclamation, Report RU-82/15, 1982.
8.
Tleimat, B. W., 'Optimal Water Cost from Solar-Powered Multieffect Distillation,'First World Congress on Desalination and Water RE-use, Florence, Italy, May 23-27, 1983; Desalination, 44, pp. 153-165, 1983.
303
9.
Tleimat, M. C., and E. D. Howe, "The Use of Energy from SaltGradient Solar Ponds for Reclamation of AgriculturalDrainage Water in California: Analysis and Cost Prediction, Part III", UniversitywideEnergy Research Group, University of California, Berkeley, California, June 1988.
10. Tleimat, B. W., "Prediction of Distilled Water Cost Using Waste Heat from the Jacket-CoolingWater of Diesel Engines," Twelfth Annual Water Supply Improvement Assn. (WSIA) Conference, Orlando, Florida, May 13-18, 1984.
289 Printed in The Netherlands
A NOVEL 2500 GPD 5-EFFECTS WIPED-FILM ROTATING-DISK VAPOR-COMPRESSIONMODULE; PRELIMINARY RESULTS by Badawi Tleimat & Maher Tleimat Tleimat & Associates 75 Ina Court Alamo, California 94507
For presentation at the IDA 4th World Congress on Desalination and Water Reuse, State of Kuwait, November 4-8, 1989
ABSTRACT A laboratory model wiped-film rotating-disk (WFRD) vaporcompression evaporator with a capacity of 300 liters/day (80 gpd) was conceived, designed, and built in the late sixties at the Sea Water Conversion Laboratory, University of California at Berkeley. Preliminary data obtained from this model showed exceptionally high overall heat transfer coefficient,U, in excess of 28 kW/m2C (5000 Btu/hft2F). Results from later data with precise instrumentation confirmed the earlier results and showed even higher values for U. Thermoeconomicanalysis of multieffect vapor-compression systems to reclaim 95 percent of agricultural drainage water at the San Joaquin Valley in California showed that the use of multieffect VC systems instead of conventional single effect VC systems reduces energy consumption. It also showed that the ratio of U in the evaporator to its cost per unit area of heat transfer has very strong influence on distilled water cost; the higher the ratio, the lower the distilled water cost. The analysis also showed that because of the exceptionallyhigh value of U in the WFRD evaporator and based on data obtained from the laboratory model at Los Banos, California using agricultural drainage water, its use in a 5-effect
290
VC system will reduce product water cost by a factor of 2 and energy consumption by a factor of 3 as compared to off-the-shelf single effect conventional VC systems. A semicommercial2500 gal/day five-effect vapor-compression WFRD evaporator module was designed, built and tested with tap water. Preliminary data were obtained. The results from these data show an average value of U to be about 20 kW/m2C (3500 Btu/hft2F) and energy consumption by the compressor between 3.6 and 5.1 kWh/m3 (13.6 and 19.3 kWh/lOOO gal) of product water.
INTRODDCl'ION The use of vapor-compression(VC) distillers to reduce the volume of waste brine streams from water recovery plants is gaining acceptance at inland locations (1, 2, 3, 4). Thermoeconomic analysis (5) of multiple-effectvapor-compression(MFVC) distillation shows that its use can reduce energy consumption significantlyin comparison to conventional VC systems for the same type of evaporator and the same feed and brine conditions. The analysis was applied to the recovery of agricultural waste drainage water in the San Joaquin Valley in California to recover 95 percent of the 10,000 ppm feed as distilled water. The analysis showed that by using a five effect vapor-compressionsystem instead of single effect VC system the energy consumption was reduced by about 40 percent in this particular case. The California Department of Water Resources (DWR) funded the Water Technology Center (formerly Sea Water Conversion Laboratory), University of California at Berkeley, to test this analysis using an existing small vapor-compressionunit at the Center. The evaporator in this small unit was the Tleimat wiped-film rotatingdisk (WFRD) evaporator. Tests were conducted (5) at the DWR Desalting Facilities at Los Banos, California, using agricultural drainage water from the San Luis drain. The tests were designed to
291
test the effectivenessof this evaporator for treating the waste water as well as obtaining data on MEVC systems by simulation. The results from these tests using softened agriculturalwaste water showed very high heat transfer coefficient and low energy consumption. The overall heat transfer coefficient was about 31.4 kW/m2C (5530 Btu/hft2F, 27,000 kCal/hm2C) and the energy consumption by the compressor was about 4.1 kWh/m3 of product water (15.5 kWh/lOOO gal) based on a 60 percent isentropic efficiency for the compressor. At that time the California Department of Water Resources contracted with Bechtel National to install and test a 50,000 gpd vapor-compressionunit at Los Banos California. The evaporator was of the horizontal tube spray-film type. It was built by Israel Desalination Engineering and erected and tested under the supervision of Bechtel Engineers. The unit was run between April and September 1986 using agricultural drainage waste water and calcium sulfate slurry seeding technique for scale prevention. Results from these tests (6) showed the overall heat transfer coefficient to be about 1.4 kW/m2C (250 Btu/hft2F, 1200 kCal/hrm2C) and energy consumption by the compressor to be about 25 kWh/m3 (95 kWh/lOOO gal). A proposal was submitted by the Water Technology Center to DWR to fund the construction and testing of a 2500 gal/day five-effect module using the WFRD evaporator as its element. Due to budget constraints DWR declined to fund the proposal and the Center was closed at the end of June 1987 due to lack of funds. Upon closing of the Center, the inventor of the WFRD evaporator, Badawi Tleimat, decided to take early retirement from the Center at the University of California and undertook to build the module using his own funds.
THE WIPED-FILM ROTATING-DISK EVAPORATOR Figure 1 is a schematic cross section of the evaporator. Fig.
292 PRODUCT TUBES
r
WIPERS SHOWN
i BLOWDOWN
FIG. 1.
ROTATING-01%
UIPEO-FILM
I
NOT
BLOW’DOWN (RESIDUE)
BRINE
FIG. 2.
EVAPORATMI CROSS SECTION 'SCHEHATIC"
WIPER DISTILLAND
FIG.).
BLADE
EVAPORATOR CROSS SECTION BETYEEN 01% PAIRS OF FIG. I "SCHEMATIC'
\
FEED
CROSS SECTION OF THE YlPER ll,AW AW IN A TANGWTIAL DIRECTIDN
BRINE
nETA,. DISK
293
2 is a schematic cross section taken between two disk pairs. Fig. 3 is a cross section of the wiper taken between the inside and outside periphery of the disk showing the relative position of the wiper and disk. The rotor consists of disk pairs joined together at the periphery of the inside holes. Although Fig. 1 shows only four disks forming two cavities, the rotor can be designed to contain more pairs when required. The rotor is mounted on a stationary shaft closed at one end and open at the other end. Steam from a boiler, from a previous effect, or from a compressor, is introduced into the open end of the shaft and is condensed on the inside surfaces of the rotating disks. The condensate, as it forms on the inside surfaces of the disks, is thrown by centrifugal force to the periphery where it enters stationary product tubes (scoops) connected to a central tube, and flows out of the evaporator as distillate product. The rotor rotates inside a chamber into which the aqueous solution is fed along the length of stationary wipers (Figs. 2 and 3), and is deposited as a thin, uniform film on the outside surfaces of the rotating disks in a manner which eliminates formation of dry spots. Unevaporated feed is slung to the periphery of the disks and is withdrawn from the chamber as blowdown (residue) at the bottom. The combination of centrifugal force and wiped feeding achieves a thinning of both condensate and feed films which result in exceptionally rapid heat transfer.
TEST RESULTS FROM A SIMULATED FOUR-EFFECT VC SYSTEM In this test the agricultural drainage water was softened by ion-exchange to remove calcium. Table I shows the raw water composition. The softened water was collected in a storage tank and then was fed into the evaporator to simulate the first effect and the resulting brine was collected in a different storage tank. This brine was then used as the feed to the evaporator to simulate
294
the second effect. The brine collected from the simulated second effect was used as the feed to the evaporator to simulate the third effect. Again, the brine collected from the third effect was then used as feed to the evaporator simulating the fourth effect. The total recovery from the simulated four-effect system was 92.8 percent starting with a feed salinity of 9380 ppm and ending with a final brine salinity of 126,590 ppm. Figure 4 shows a plot of the overall heat transfer coefficient,U, as a function of the overall temperature difference. The dashed line represents theoretical prediction (7) while the solid line was drawn through the experimental results. The two lines at the bottom represent U for new smooth and double fluted tubes. They are drawn for comparison with U from WFRD. Table II shows a summary of the results from this test. The second, third, and fourth columns show, respectively, the total dissolved solids, in ppm, in the feed, brine, and distillate streams. Column 5 shows the overall heat transfer coefficient, U, while column 6 shows the overall temperature difference. Column 7 shows the measured pressure rise, DP, across the compressor, while column 8 shows the energy consumption by the compressor, WK.
This figure was calculated from
the measured pressure rise, DP, across the compressor using an isentropic compressor efficiency of 60 percent. The results on overall heat transfer coefficient and energy consumption confirmed earlier results obtained in the laboratory using deionized water and seawater feeds.
THE 2500 GPD (10 M3/DAY) FIVE-EFFECT WFRD MODULE Figure 5 is a schematic flow diagram for the five-effectWFRD vapor compression module. In the module, heat transfer surfaces A, B, C, D, and E represent, respectively, the rotor in each of the five effects. Here, pretreated and filtered saline water, stream a at point 1, is split into two streams, al, and a2.
Stream al
295
TABLE I AgriculturalWater Analysis: Sample taken on October 13, 1982 from San Luis Drain at Desalting Site, Los Banos. Analysis made by DWR Element
mg/liter
Sodium
2160
Calcium
500
Magnesium
264
Potassium
6.9
Sulfate
4610
Chloride
1440
Boron
16
Silica as Si02
20 169
Total alkalinity as CaC03
2340
Total Hardness as CaC03
11300 micromhos/cm
Specific conductance at 25 C
8.1
PH Total Dissolved Solids
9370 mg/liter
TABLE II Summary of Data from the Simulated Four-effect test. The Evaporator was Operated at an Average Temperature of 60 C. Feed
Brine
1
9,390
2
U
DT
13,213
2
27,475
0.52
9.6
3.48
13,213
19,438
2
27,208
0.53
10.7
3.83
3
19,438
35,534
2
27,140
0,57
12.55
4.05
4
35,534
126,590
2
26,298
0.59
16.54
5.64
U
DP
WK
Product
Set
= Overall heat transfer coefficient; kCal/hm2C
DT = Average temperature difference; Deg C DP = Average measured pressure rise across compressor: cm H20 WK = Average energy consumption by compressor; kWh/m3
296
Total dissolvedsolids %
z Y Y
I .39
SET I
0
SET 2
A
I .96
SET 3
v
3.4
SET 4
0
12.66
I
5 2000 0 II Vertical --------------
1000
Tube Evaporator ----------------_
Flash Evyorator --------- Multistage --------------------2
t
“0
”
0.7
0.8
At = OVERALL Fig.
4
1.0
0.3 TEflPERATURE
DIFFERENCE.
Overall heat transfer coefficient as difference using softened agricultural
1.1 F
a function drainage
of overall water.
temperature
~ I--------~~t--------l
-.
I
*
SALINE WATER
-----VAPOR
b
c
d
@
- .-
DISTILLATE
I
FIVE-EFFECT EVAPORATOR
Fig.
5 Schematic
flow
diagram
for
a five-effect
vapor-compreeelon
qatcn
297
enters the brine heat exchanger where it is heated from point 1 to point 2 by cooling brine stream f from point 11 to point 12 and stream
a2 enters the distillate heat exchanger where it is heated
from point 1 to point 3 by cooling distillate stream n from point 20 to point 21. The two streams, al and a2 are joined back into one stream. The feed then enters the degasser where noncondensible gases are stripped out and then enters the first effect at point 6 where it is brought into contact with heat-transfer surface A. Here, part of the feed is evaporated and the balance, stream b at higher salinity than stream a, is pumped from the first effect and enters the second effect at point 7.
Here, again, part of stream b
is evaporated and the balance, stream c at higher salinity than stream b, is pumped from the second effect and enters the third effect at point a. This process is repeated until the fifth effect where the brine, stream f at the desired blowdown salinity, is taken out at point 11, cooled in the brine heat exchanger, and discharged as brine blowdown at point 12. The vapor generated in the first effect condenses on the cooler heat transfer surface B in the second effect and is taken out as distillate, stream h at point 16. The heat released in condensing vapor stream h is transferred across heat transfer surface B and causes the evaporation of an equivalent amount of water from stream b in the second effect. This process is repeated in the second, third, and fourth effect to produce distillate streams j, k, and m.
The vapor generated in the
fifth effect is taken out at point 13, is compressed to point 14 to raise its saturation pressure and temperature, and is then introduced into the condensing side of the first effect where it condenses on heat transfer surface A and is withdrawn as distillate, stream g at point 15. The condensate streams g, h, j, k, and m at points 15, 16, 17, 18, and 19 are taken out as stream n at point 20 which is then cooled in the distillate heat exchanger and taken out as product at point 21.
298
In this module each rotor consists of 10 disks made from 60 cm diameter x 0.8 mm copper sheet resulting in a total net heat transfer area of 2.12 m2 per rotor, therefore, at a capacity of 10 m3/day, the product rate per unit area of heat transfer surface would be about 39 kg/m2h. The compressor in this module is a positive displacement lobe-type blower driven by a 5 horse-power (hp), 1800 rpm electric motor. The rotor assemblies are driven by a 2 hp, 1200 rpm electric motor through timing belts and pulleys with the drive shaft being sealed by a mechanical face seal using distilled water as lubricant.
DATA AND RESULTS FROM THE MODULE Data were obtained from the module using tap water. The temperature of the streams was measured by calibrated platinum resistance sensors placed in the evaporation and condensation vapor spaces in each effect. The energy consumption by the compressor drive motor and rotors drive motor was each measured by a separate Watt-meter. The distillate rate was determined by measuring the time to fill a given calibrated volume. The overall heat transfer coefficient was calculated from these data assuming that the distillate rate from each effect was equal to one fifth the distillate rate from the module. The module was operated during working hours between 8 AM to 5 PM.
At start-up, the water would be circulated over a 2 kW
electric heater in the degasser circuit to heat the system. When the temperature of the circulated water reached about 35 C, the rotor and the compressor were started and the module started producing water immediately; however, at low rate due to the high specific volume of the vapor at the relatively low start-up temperature in the system. Table III shows data and results obtained from the module. Data were taken at hourly intervals for eight days. When the
299
TABLE III DatxandResultsUeingTapwaterFeed t6
t5
WKCR
DesC
DesC
U WA
WKC
DesC
kWh/m3
mm 3
m3/h
Mb2
48.89
43.47 48.84 49.02 50.14 52.21 54.60
1.09 1.26 1.29 1.32 1.47 1.56
18.64 17.85 19.83 19.96 19.10 25.46
3.97
55.15 55.49 56.72 59.57 62.41
3.54 3.63 3.59 3.56 3.39
7.52 6.66 6.86 6.84 6.67 6.58
0.311 0.354 0.367 0.393 0.429 0.474
25.2 33.4 34.6 37.1 40.5 44.6
47.08 50.41 52.44 55.47 57.62 59.63
42.12 45.02 46.77 49.12 50.76 52.40
0.99 1.08 1.13 1.27 1.37 1.43
20.12 19.76 20.81 19.24 20.49 20.82
3.51 3.51 3.63 3.81 3.80 3.53
6.89 7.09 7.32 6.90 7.01 6.57
0.281 0.314 0.336 0.370 0.402 0.433
26.5 29.6 31.6 34.9 37.9 40.8
51.42 53.93 57.74 60.39 65.09
45.86 40.02 50.84 52.77 56.26
1.11 1.18 1.38 1.53 1.77
19.41 20.92 21.29 18.06 23.44
4.05 3.76 3.82 4.16 4.40
7.43 7.04 6.67 7.01 7.45
0.319 0.355 0.432 0.470
30.1 33.4 37.7 40.7 44.3
42.19 49.51 53.42 54.61 55.27
38.10 44.23 47.53 48.45 49.07
0.82 1.06 1.18 1.23 1.24
18.40 20.49 19.00 19.01 19.16
3.59 3.26 3.35 3.66 3.76
7.28 6.23 6.50 6.57 6.63
0.220 0.302 0.352 0.353 0.365
20.8 28.5 33.2 33.3 34.4
44.08 50.14 52.32 53.51 56.06
39.83 44.03 46.79 47.44 49.78
0.85 1.06 1.11 1.21 1.25
18.13 18.58 19.09 17.67 19.66
4.96 5.57 4.76 4.06 4.68
9.10 9.05 8.05 7.39 7.96
0.236 0.309 0.319 0.337 0.378
22.2 29.1 30.1 31.8 35.6
48.86 55.44 57.87 59.09 63.34 64.22
43.58 49.04 51.51 52.49 55.51 55.79
1.06 1.28 1.27 1.32 1.57 1.69
17.31 17.68 22.01 21.31 19.62 17.73
4.67 4.83 4.74 5.31 5.52 6.02
8.59 8.31 7.99 8.83 9.52 10.11
0.293 0.366 0.415 0.410 0.475 0.477
27.6 34.6 39.2 39.5 44.8 45.0
47.30 49.60 52.91
42.51 44.61 47.33
0.96 1.01 1.12
19.34 20.57 21.08
4.94 4.56 4.73
9.60 9.06 8.96
0.279 0.311 0.355
26.3 29.3 33.4
49.46 53.26 56.03 59.46
44.07 47.44 50.57 52.42
1.09 1.16 1.25 1.41
17.52 18.82 20.96 19.07
4.91 5.04 4.97 5.15
8.91 8.96 8.34 8.54
0.300 0.338 0.392 0.435
28.2 31.9 36.9 41.0
Dt
FR
0.400
Flux
300
temperature in the fifth effect reached about 45 C, the 2 kW heater was turned off and the system temperature increased due to energy input from the rotor and compressor drives. In this table the first column shows the measured saturation temperature in the condensation space of the first effect while the second column shows the measured saturation temperature in the evaporation space of the fifth effect. The third column shows the average overall temperature difference, Dt, across the heat transfer surface in each effect. It is taken to be equal to one fifth of the difference between t6 and t5. The fourth column shows the calculated average overall heat transfer coefficient,U, in the evaporator for all the effects. The fifth column shows the measured energy consumption by the compressor and its drive, WKC, in Kilowatt-hour per cubic meter, while column 6 shows the energy consumption, WKCR, also in the same unit, by both the rotor and compressor drives. Column 7 shows the distillate rate from the module, PR, in cubic meter per hour, while column 8 shows the distillate flux in kg per hour square meter of heat transfer surface in the evaporator. The data and results show the energy consumption,WKC, by the compressor motor to be between 3.26 and 6.02 kilowatt hour per cubic meter of distillate while the energy consumption,WKCR, by the compressor and rotor motors to be between 6.23 and 10.11 kilowatt-hour per cubic meter of distillate. This figure is useful because it provides a direct comparison of this type of evaporator with vapor-compressionunits using conventional evaporators regarding energy consumption by the compressor. It is significant to note here that at the design condition the measured energy consumption by the compressor motor varied between 3.6 and 5.1 kilowatt-hour per cubic meter (13.6 and 19.3 kWh/lOOO gal) while at these conditions the energy consumption by the compressor and rotor motors varied between 6.8 and 10.0 kWh/m3 (25.7 and 37.9 kWh/lOOO
301
gal).
Again, at the design conditions the average overall
temperature difference per effect was about 1.3 C and the average overall heat transfer coefficient was about 20 kw/m2C (3500 Btu/hft2F; 17,200 kCal/hm2C).
These two figures are useful for
multieffect distillation using steam as the heat source. For example, assume that in a desalting plant steam is available at 90 C and that cooling water is available to allow the final condenser to operate at 40 C, providing a total temperature difference of 50 C between the steam supply and final condenser. For a multieffect wiped-film rotating-disk evaporator using 1.3 C temperature drop per effect, one can install about 38 effects. Assuming that the sum of the boiling point elevation in all the effects amounts to 10 C (this figure changes with feed water chemistry and recovery), then it is possible to build a 30 effect system that could result in a gained output ratio of about 25 kg of distilled water per kg of steam. economy.
This is significant for steam
It is also significant regarding cooling water use in
that it would require only about 40 percent of the cooling water needed for a plant with a gained output ratio of 10.
These
considerations are extremely important at inland locations where raw water is available and low temperature heat sources such as; geothermal hot water, flat plate solar collectors (8), saltgradient solar ponds (9), waste heat from diesel engines (10) and waste heat from power and chemical plants. The modular design of this evaporator lends flexibility in applications requiring large capacity.
By changing the size of the
disks and varying the number of disks per effect and the number of effects per module it is possible to make modules with capacities as low as 4 m3/day and as high as 200 m3/day.
Obviously each
application will provide its own optimum design conditions regarding the number of effects per module as well as the size and number of disks in each effect.
302
REFERENCES 1.
Hervey, D. E. and M. E. Cross, "Water Reclamation, Systematic Approach," Technical Proceedings, Eighth Annual Conference and InternationalTrade Fair of the National Water Supply Improvement Association,July 6-10, 1980, San Francisco, California.
2.
Hancock, J. F. and W. E. Sringer, 'Zero Discharge of Liquid Wastes from Power Plants,' Technical Proceedings,Ninth Annual Conference and InternationalTrade Fair of the National Water Supply Improvement Association,May 31 - June 4, 1981, Washington, D. C.
3.
Leitner, G. F., "Waste ConcentrationVs. Evaporation Ponds for Reverse Osmosis Reject Water - A case Study," Working Papers, First World Congress on Desalination and Water Reuse, Florence, Italy, May 23-27, 1983.
4.
DeMoel, P. J., R. F. M. De Gier, and G. Onderdelinden, "Design of the 50,000 m3/day Treatment Plant for Qasim, Saudi Arabia: Desalination of Brackish Water at 99% Recovery," Proceedings of the Second World Congress on Desalination and Water Reuse, Bermuda, November 17-22, 1985.
5.
Tleimat, B., A. D. K. Laird, and E. D. Howe, "Analysis and Cost Prediction of Reclaiming Agricultural Drainage Water Using Multieffect Vapor-CompressionDistillation,"Final Report to California Department of Water Resources, UC/DWR Agreement B-55037, Task Order 84-1, November 1985.
6.
Bechtel National, Inc. "Final Report, Field Test of a Vapor Compression Evaporator at the DemonstrationDesalting Facility, Los Banos, California," prepared for the State of California, Department of Water Resources, Specification NO. 85-31, Contract No. C-50653, San Francisco, California, August 1986.
7.
Tleimat, B. W. et al., 'Wiped-Film Rotating-DiskEvaporator for Water Reuse," U. S. Department of the Interior, Bureau of Reclamation, Report RU-82/15, 1982.
8.
Tleimat, B. W., 'Optimal Water Cost from Solar-Powered Multieffect Distillation,'First World Congress on Desalination and Water RE-use, Florence, Italy, May 23-27, 1983; Desalination, 44, pp. 153-165, 1983.
303
9.
Tleimat, M. C., and E. D. Howe, "The Use of Energy from SaltGradient Solar Ponds for Reclamation of AgriculturalDrainage Water in California: Analysis and Cost Prediction, Part III", UniversitywideEnergy Research Group, University of California, Berkeley, California, June 1988.
10. Tleimat, B. W., "Prediction of Distilled Water Cost Using Waste Heat from the Jacket-CoolingWater of Diesel Engines," Twelfth Annual Water Supply Improvement Assn. (WSIA) Conference, Orlando, Florida, May 13-18, 1984.