Future developments in desalination distillation processes

Desalination, 50 (1984)

61-70 Elsevier Science Publishers B.V., Amsterdam

61 - Printed in The Netherlands

FUTURE DEVELOPMENTS IN DESALINATION DISTILLATION PROCESSES* LEON AWERBUCH

Research and Engineering, Bechtel Group, Inc., Fifty Beale Street, San Fmncisco, (Received

CA (USA)

March 16,1982;

in revised form March 15, 1984)

SUMMARY

Desalination is the fastest growing sector of the water industry - worldwide capacity has doubled in the last four years. There are slightly more than 2200 land-based desalting plants of 250001 gpd capacity or larger in operation or under construction throughout the world as of June 30,198O. These plants are capable of producing close to two billion gallons of water daily. This compares to the worldwide capacity of a little less than one bgd produced by 1500 plants that were in operation or under construction as of January 1,1977. This presentation is based on recent visits to major desalting centers around the world, discussions with manufacturers of desalting equipment actively involved in new developments, and summarizes developments presented at recent international desalting conferences. The future goals of development work going on today were found to be directed toward reduction of energy costs and capital costs of desalting equipment. The presentation briefly describes developments in multistage flash (MSF), vertical tube evaporator (VTE), combination systems (VTE/RIF), rising filmfalling film evaporator (RF-FF), horizontal tube evaporator multiple effect (HTME), multistage flash fluidized bed evaporator (MSF/FBE), and low temperature waste heat desalination.

INTRODUCTION

Distillation processes are the most widely used in the desalination of seawater. They account for about 80% of the total capacity, with the balance utilizing the rapidly growing membrane processes.

*Based on a presentation October 1981. OOll-9164/84/$03.00

at the 2nd World Congress of Chemical Engineering, 0 1984 Psevier

Science Pub&hers

B.V.

Montreal,

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L. AWERBUCH

Distillation processes account for 76% of the total plant capacity, but for only 44% of the total number of plants. The balance is almost entirely in membrane processes with freezing processes accounting for less than l/10 of 1%. The most widely used distillation process is muhistage flash, accounting for 67% of worldwide desalting capacity. The most widely used membrane process is reverse osmosis which accounts for 42% of the total number of plants. This information is included in the new edition of the Desalting Plants Inventory - Report No. 7 compiled by Nabil El-Ramly and Charles Congdon and sponsored by the Water Supply Improvement Association El]. About 30% of the plants listed in this report are located in the United States and its Territories, but their combined capacity totals only 15% of the worldwide plant capacity. One half of the reported 637 plants in the U.S. are located in California, Florida, Texas, and Arizona. A little more than 35% of plants worldwide are in the Middle East and North Africa (mostly in’ OPEC countries), but their combined capacity amounts to about 65% of the total. After OPEC and the U.S., is Europe with 15% of the total number of plants, then Southeast Asia with 8%. The plants in Europe and Southeast Asia account for 12% of worldwide capacity. About 3.5% of the capacity is located in the U.S.S.R., and about the same percentage is shared among the Caribbean, Mexico, and South America. The remaining one percent are scattered throughout the rest of the world. Table I, taken from the Desalting Inventory Report, shows the breakdown of desalting plants by process. The total potential desalting markets for the year 2000 are projected at 29.06 bgd for the U.S. and 4.8 bgd for the Middle East [2] . With such great market potential it is not surprising to see interest in developing new distillation processes.

BASIC TRENDS IN DISTILLATION DEVELOPMENTS

To determine the potential areas of development it is important to review the present and projected costs of desalting. The breakdown of all direct and indirect costs, operating costs, and water costs were recently updated in a study prepared by Oak Ridge National Laboratory [3]. The following water desalination costs are taken from the Oak Ridge Report: Small plants, e.g. 3785 m3 /day (1 mgd), cost from a high of $1.69/m3 using the plant in combination with oil-fired or coal-fired boiler to a low of $1.45/m3 using a VTE plant and nuclear power/steam supply. At the largest plant size considered, 378,500 m3/day (100 mgd), product water costs range from $0.75/m3 for MSF plants using oil-fired boilers to a low of $0.57/m3 for VTE plants with a steam generator using nuclear fuel. The cost of energy varies widely depending on the type of steam generator,

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TABLE I DESALTING

PLANTS BY PROCESS

Type of process

Distillation Single stage flash Mukistage fIash Thin film vertical tube Vertical tube - multistage flash Thin film horizontal tube Submerged tube Vapor compression Vertical tube - vapor compression

Membrane Electrodialysis Reverse osmosis EIectrodiaIysis-reversing

Number of plants

77 415 104 7 51 127 168 16 965

11.3 1,292.6 75.8 4.7 22.7 21.6 22.1 8.3 -1,459.1

82 929 228

35.7 390.5 36.8

1,239 Freezing Absorprtion

freezing vapor compression

AU types of process

Plant capacity (mgd)

1 2,205

463.0 1,922.2

single or dual purpose usage, and temperature and pressure of motive steam. The relative cost of supplying process steam from extraction points as a function of exhaust temperatures are analyzed in [4,5]. The cost distribution indicates that energy accounts for from 60% for single purpose distillation plants to 20% in most efficient dual purpose nuclear desalting stations. It should be noted that the capital cost of desalting equipment represents 65% of the water cost in dual purpose plants and 20% in single purpose installations. Increased prices for heat transfer surfaces forced developments in enhancement of heat transfer or, alternatively, reduction in the costs of materials by employing aluminum and plastic rather than titanium and copper alloys. The continued increases in fuel prices make the application of alternative energy sources for distillation more economical. Developments in combining new heat sources such as solar panels or solar ponds [6,7], geothermal distillation [8], ocean thermal energy conversion (OTEC) [ 91, and the use of waste heat coukl substantially reduce water costs [W-12].

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Objectives of developments in distillation processes were recently outlined by A. Veenman [13] . These objectives are divided into the following general categories: 1. Reduction of investment costs by: a) reduction of the cost of materials, b) reduction in the heat transfer area by enhancement, c) reduction in volume and weight of equipment, d) process simplification. 2. Reduction of energy costs by: a) increasing performance ratios, b) reduction in nonequilibrium losses, c) reduction of heat transfer fouling, d) usage of new, dual purpose coupling, extracting steam at lower temperatures, e) usage of waste heat, f) usage of alternative energy sources, g) reduction in pumping requirements. 3. Reduction of operation and maintenance costs by: a) ease of operation and increased plant factor, b) improvements in make-up treatment, c) increase in concentration factors, d) reduction in price and improvement in effectiveness of chemicals, e) increase in flexibility, allowing onstream cleaning and minimization of maintenance. These objectives are implemented in the new distillation concepts which are now under rapid development and demonstration. Future distillation plants are briefly described in the following sections.

DESALTING

ADVANCES

VTE systems Vertical tube evaporator (VTE) systems represent a revival of multi-effect systems. The VTE takes advantage of higher heat transfer rates, resulting in smaller dimensions. Envirogenics Systems Company was primarily responsible for the development of VTE systems, both horizontally arranged and vertically stacked. As developed by Envirogenics, the combination systems VTE/RIF allows an increase in the performance ratio of the plant by using a regeneratively integrated feed heater (RIF). Currently Envirogenics and Incon, subsidiaries of Sogex International, constructed and initiated operation in a joint venture with Thyssen Nordselwerke, a hybrid MSF/VC/VTFE floating demonstration plant called MEDA (Meerwasser Entsalzungs Demonstrations Anlage). This hybrid seawater desalting plant, capable of producing 5000m3 /day (1.3 mgd), increases the efficiency of the VTE by topping effects with mechanical vapor compression. The MEFF desalination process was developed by Toyo Engineering Corporation of Japan. The Toyo process represents new ideas in feedwater preheating, direct contact condensation and a final barometric condenser. The falling film preheater allows seawater to heat up to 90°C at ambient conditions, in one vertical bundle. The shell side of the preheater is divided into several stages, condensing steam at correspondingly higher temperatures.

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At the upper part of the preheater where raw seawater is entering, the vertical tube bundle is open to the atmosphere allowing onstream inspection and cleaning. Alkaline scale is prevented by injecting carbon dioxide into the sump from the vacuum system. Further preheating of makeup is achieved in a two-stage, direct contact, barometric condenser. The second stage is used as a high temperature deaerator. The final condenser is also arranged as a vertical tube, falling film condenser, which is open to the atmosphere for cleaning and inspection. The vertical tube bundles of VTE vessels utilize double fluted tubes. Toyo Engineering built and fully tested a 600,000 gpd demonstration plant and in Abu Dhabi. Snamprogetti of Italy developed a seawater desalination VTE process in a vertical column. Thanks to its vertical layout, the Snamprogetti model requires only one seawater feed pump; their design provides a self-regulating siphon and uses special distribution nozzles for double fluted tubes. Snamprogetti constructed and extensively tested the 1440m3/day plant at the Taranto Refinery in Italy. Ishikawajima-Harima Heavy Industries Co., Ltd. of Japan has been developing a Rising Film-Falling Film Evaporator (RF-FF). In the high temperature effects of the vertical tube evaporator, IHI utilizes rising film in order to reduce overall pumping power requirements and minimize operational maintenance problems. IHI recently described their new 1700 m3/day plant as using rising film (RF) heat exchange for all ten effects. In this process, the preheated seawater enters the bottom of the first effect tube bundle, where it is introduced to the vertical, double fluted tubes. In the lower part of the vertical tubes the seawater is boiled and the mixture of the vapor and seawater rises to the top inside of the tubes by means of the thermo+iphon effect. All of the advanced VTE systems use fluted tubes for heat transfer enhancement. The heat transfer performance depends on the mode of operation (upflow-downflow) and the evaporation temperature. The heat transfer is significantly improved in comparison with performance of a plain downflow tube. Heat transfer can be further improved by the introduction of interface enhancement (foam flow) as developed by H. Sephton [13].

HTE SYSTEMS

Horizontal tube, multi-effect evaporators have won widespread acceptance. Manufacturers of desalination equipment like AquaChem, Inc., Aiton Ltd., Israel Desalination Engineering, Ltd., Sasakura Engineering Co., Sobelco S A, and Mitsubishi Co. all continue to improve their particular concepts of horizontal tube evaporators.

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Israel Desalination Engineering, Ltd. (IDE) developed a horizontal, aluminum tube, multieffect system which operates successfully at very low top temperatures (below 75’C). Operation at temperatures of 55’C allows coupling to the exhaust of a conventional power plant turbine. This significantly reduces the cost of energy necessary for the operation of a distillation plant. The heat transfer surfaces of the horizontal effects are made entirely of relatively inexpensive aluminum alloy 5052 tubes. This allows the design of large heat transfer areas and reduces thermal driving force per effect. The aluminum tubes are arranged in horizontal bundles over which seawater is distributed by spray nozzles. The wetting rates are controlled because it is necessary to avoid dry spots during evaporation. The heat of condensation of steam inside the tubes evaporates a portion of the water falling over the tubes generating vapor which flows axially to the next effect after passing through mist eliminators. Because IDE units operate at low temperatures, the use of a polyphosphate additive is sufficient to inhibit scale formation. An excellent presentation of IDE low temperature distillation plants was recently made by D. Hoffman [ 151. Currently the Joint U.S.-Israel Desalination Project is designing and constructing a large prototype 5 mgd plant in Ashdod, Israel, based on IDE’s process. The goal of the Joint Project is to develop and advance desalting technology as described in a paper by L. Awerbuch, et al. [16] and in the latest project status report by A. Peled and J. Finke [ll] .The Ashdod was constructed, and it completed one year of operation and testing in March 1984, successfully demonstrating the multieffect low temperature technology. Developments in HTE systems also include the multieffect stack type distillation plants (MES) of Sasakura Engineering Co. These include the use of thermocompressors to increase plant efficiency, and specific solutions to bundle arrangements, venting, and condensate drainage of the tubes. Work is also being carried out to improve heat transfer performance through the use of oval shaped tubes.

MSF SYSTEMS

Advances have taken place also in the multistage desalination industry. One example involves the multistage flash/fluidized bed evaporator (MSF/ FBE) developed by Esmil International BV of Amsterdam in work jointly sponsored by the Dutch government. In this process the feed is introduced at the bottom of a vertical tube, fluidized bed heat exchanger in which the tubes are partly filled with glass beads. The tubes pass through a number of flash stages stacked one on top of the other. The vapor from the flash stages condenses on the outside of the heat exchanger tubes, progressively heating the feed. The feed at the top of the tubes is finally heated by prime steam to

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between 85’C and 115’C, depending on the process requirements. The heated feed then falls through the successive flash chambers which surround the tubes. The vapor which condenses on the outside of the tubes is collected and discharged from the plant as product water. The Esmil design comprises several novel features. As a result of the fluidization of the glass beads by the feed rising in the tubes, adequate wall-toliquid heat transfer is achieved at flow velocities as low as 0.2 to 1.0 m/s, approximating those achieved in conventional heat exchangers at a velocity of 1.8 m/s. The beads perform the additional function of minimizing scale deposition on the tube walls. As a result, the height of a flashing stage is minimized. A further improvement is the reliance on flashing to generate vapor in the flash stages, avoiding the customary emphasis on pool boiling. As a result of these innovations, a very small flash chamber is adequate, about l/10 the volume of the conventional flash stage. A further advantage is the ability to achieve steam-water separation without mist eliminators. Unusual flexibility and turndown ratio are claimed by Esmil for this design. A “semi-commercial”, 500 m3 /day plant demonstration plant has been in operation in Holland since 1978. A 100 m3/day plant has been designed for an Amsterdam power station and is scheduled for operation early in 1982. A 4,000 m3/day Esmil MSF/FBE plant is currently in prospect for the Middle East. Multistage controlled flash evaporator (MSCF) was developed by Aquanova BV Rotterdam, The Netherlands. This unit allows vertical stacking of MSF stages with drastic reduction in stage height. Flash chambers are in the range of 15 cm in height, this being possible by special venturi type controlled flash devices. Aquanova also introduced the commercial use of aluminum 5052 tubes in the forced circulation condenser. Cross-flow tube design is integrated within the total aluminum structure. The Kogan-Rose direct contact condensation multistage flash process developed in Israel is based on a vapor reheat system. The flashed vapors are condensed in countercurrent flowing pure condensate. The energy of the flashing brine stream is utilized to lift the brine in consecutive flashing stages. A plastic heat exchanger is utilized to recover heat from hot distillate and to preheat incoming seawater. The development of thin-walled plastic heat exchangers combined with direct contact condensation is claimed to reduce both capital and operating costs [ 181. The development of vertical arrangement MSF or cross-flow MSF with double fluted tubes to increase heat transfer performance is an innovation contemplated by several MSF manufacturers. One more example of a new approach to the MSF process is Nord-Aqua Company of Finland which is developing a low temperature waste heat desalination process. Of particular interest are Nord-Aqua’s methods of barometric deaeration, flashing at low temperature, and noncondensible removal with brine educators.

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Under the Joint U.S.-Saudi Arabian Technical Cooperation Program in Desalination, the development of large capacity MSF distillation plants was studied and recently reported by Isam M.R. Jamjoom, et al. [ZO] . In January of 1979, the Sea Water Conversion Corporation of Saudi Arabia (SWCC) awarded contracts to three firms (two architectural/engineering firms and one MSF manufacturer) for conceptual designs of large capacity (up to 66 mgd) MSF plants. The scope of work required investigation not only of innovations related to increasing unit capacity, but also non-capacity related innovations such as process configuration, scale and corrosion prevention, improved heat transfer rates, etc. Upon completion of the three conceptual designs, a committee of SWCC and Joint Team personnel evaluated the features of each design and related them to SWCC’s existing and future plant needs. A test module construction and testing program to obtain design data necessary for the design of a large plant was then outlined. Bechtel [21] was one of the participants in this study and recommended that, although no technical limitations for MSF plants of up to 66 mgd exist, the optimal size of a single MSF unit should be 22 mgd. This 22 mgd capacity selection, resulted from an analysis that indicated that to ensure reliable and continuous water distribution, significantly higher product water storage costs are encountered as the number of units decreases and the size of the largast unit increases at a site. The higher water storage costs, plus the higher costs of reserve or standby plants, reduce the cost benefits derived from unit capacities exceeding the 20-30 mgd range for the regional demands that could be expected in the Kingdom of Saudi Arabia. Barge-mounted construction was recommended by the study participants to lower installed costs and reduce construction time. Additional benefits identified included an increase in manufacturing quality control, a reduction in startup and commissioning problems through system tests prior to transport to site, and a reduction of the demands on the labor force in the Kingdom. A once-through MSF process cycle was recommended by Bechtel and one other study participant, and it was adopted as a goal for the large plant. Once-through plants are attractive because of the smaller number of process streams, pumps and control loops, and their historically higher reliability. Chemical treatment costs and scaling thresholds had previously limited once-through plants to a maximum temperature of 90°C. Indications that higher once-through temperatures would be possible with high temperature additive (HTA) feed treatment make it attractive. Spirally enhanced heat transfer tubing was recommended by Bechtel and one other study participant as a means of reducing the film coefficients both inside and outside the tubes. Tentative adoption of this recommendation was made, recognizing that reliable scale control methods and ease of tube cleaning were essential prerequisites to permit sufficient savings (through reductions in design fouling allowances) to overcome the necessarily higher pump-

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mingcosts. Several tube p&Yes were recommended to be prescreened and two parallel brine heaters (one enhanced, and one smooth for comparison purposes) were recommended for the test module. In late 1980, after evaluation of SWCC’s future plant construction program, SWCC’s Board of Directors decided to postpone further work on this project. Although this decision has stopped work related to the development of larger capacity MSF plants, it is expected that many of the innovations identified during the course of the work will be further investigated within the context of the R &D program of the Joint U.S.-SWCC Project, as they have applicability to new MSF plants of any size and, sometimes, to existing plants.

CONCLUSIONS

The future of distillation processes will depend largely on successful demonstration of new developments in the enhancement of heat transfer, reduction in the cost of materials, moving from titanium and copper alloys to aluminum and plastics, and reduction of energy costs. The continued increases in fuel prices force distillation plants to significantly increase their performance ratios, to adopt advanced multieffect systems, to look for alternative energy sources and to use waste heat or exhaust steam. To reduce operation and maintenance costs, future distillation plants will have to operate at lower temperatures, or use newly developed, high temperature additives, or look at applications of slurry seed systems to eliminate chemical costs. A big challenge to distillation is being mounted during the 1980s by the reverse osmosis (RO) technology; nonetheless, advanced distillation processes will dominate during this decade in dual purpose powerdesalting plants using advanced coupling concepts and waste heat utilization. Finally, it is obvious that increased competition between membrane and distillation processes will advance desalting technology and, hopefully, will contribute materially to low cost desalination and its acceptance around the world.

REFERENCES 1. Desalting PIantsInventory - Report No. b, National Water Supply ImprovementAssociation, May 1981. 2. Desalting Plant and Progress,an Evaulationof the State-of-the-Art and Future R BED Requirements, Final Report, Flour E Q C, Inc., January 1978. 3. S.A. Reed and J.V. Wilson, DetsakingSewater and Brackish Water, A Cost Update, 1979,ORNL/TM-6912, August 1979.

L. AWERBUCH 4. Leon Awerbuch and AIfred N. Rogers, Advanced Desalination Program, Proceedings of the 6th International Symposium on Fresh Water from the Sea, 2 (1976) 15-26. 5. Sameer K. Tadroa, A New Look at Cogeneration Systems for the Production of Powwer and Desalinated Water Economy and Design Features, Proc. NWSIA Ninth Annual Conf.. May 1981. 6. Werner Luft, Solar Energy Water Desalination in the United States and Saudi Arabia, Proc. NWSIA Ninth Annuai Conf., May 1981. 7. B.W. Tleimat and E.D. Howe, Comparative Productivity of Distillation and Membrane Processes Using Energy from Solar Ponds, Proc. NWSIA Ninth Annual Conf., May 1981. 8. Leon Awerbuch, Alfred N. Rogers and Wayne Femeliua, Geothermal Desalination, Proc. 5th Intern. Symp. on Fresh Water from theSea, September 1976. 9. Leon Awerbuch, Hybrid Cycle-Ocean Thermal Energy Convemion Power Desalting Plant, Proc. 6th Inter. Symp. on Fresh Water from the Sea, September 1979. 10. Hugo H. Sephton, H. Rie and K. Someahsarii, Power Plant Waste Heat Used for Desalination to Produce Pure Water While Saving Fuel, Proc. NWSIA Ninth Annual Conf., May 1981. 11. A. Peled and J. Finke, Joint U.S.-Israel Desalination Project Status Report, Proc. NWSIA Ninth Annual Conf., May 1981~. 12. Leon Awerbuch, Use of Alternative Energy Sources in Desalination Technology, Proc. 7th Intern. Symp. on Fresh Water from the Sea, Amsterdam, September 1980. 13. A.W. Veenman, A Review of New Developments in Desalination of Distillation Processes, Desalination 27 (1978). 14. H.H. Sephton, New Developments in Vertical Tube Evaporation of Seawater, Proc. 5th Intern. Symp. on Fresh Water from the Sea, 2 (1976) 279-287. 15. Daniel Hoffman, Low Temperature DiitilIation Plants - A Comparison with Seawater Reverse Osmosis, Proc. Ninth Annual Conference of the NWSIA and WSIA Journal, 8 (2) July 1981. 16. L. Awexbuch, W. Bamea, R. Horowitz and A. Barak, Joint U.S.-Israel Desalination Project, Proc. of the 5th NWSIA Conf., 1977. 17. D.G. Klaren and J. Windt, Design and Construction of a 500 m3/dayMSF/FBE,Proc. 6th Inter. Symp. on Fresh Water from the Sea, 2 (1978) X-30. 18. A. ‘Kogan, Direct Contact Condensation MSF Distillation, Presented at the Fit De salination Congress of the American Continent, 1976. 19. R. Saari, Desalination by Waste Heat, Proc. 6th Intern. Symp. on Fresh Water from the Sea, l(l978) 297-304. 20. Iaam M.R. Jamjoom, Abdul Bahmam, F. Al-Yousef and Ray T. Heizer, Progress under the USDI-SWCC Agreement for Technical Cooperation in Desalination, WSIA Journal, 8 (2) July 1981. 21. Conceptual Design of Large Capacity Desalting Units for the Saline Water Conversion Corporation, Kingdom of Saudi Arabia, Final Technical Report, Bechtel National, Inc., August 1979.