TECHNOLOGY
OSW lets contracts for desalination module Multistage flash evaporation module foreshadows California plant for 150 million gallons per day
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Multistage flash:
first in the big plants
In multistage flash evaporation, the brine heater is essentially the start of the process. Here, brine is heated by steam to the high temperature of the system (about 250° F. is the current state of the art). It flows into the first flash stage, which is at a slightly lower temperature and pressure. Because of this difference, part of the brine flashes to vapor, which passes through an entrainment separator and then condenses on cooling tubes. Pure water collects in a trough running from stage to stage below the tubes and is withdrawn from the last and coolest stage (about 90° F., depending on sea water temperature). Brine continues flashing from stage to stage, each stage at a slightly lower temperature and pressure than the preceding one. Cool brine from the last stage recycles to that stage upstream where its temperature is that desired for coolant, and here it becomes the coolant in the tubes. It passes upstream counter cur rent to the flashing brine, becoming heated as it moves from stage to stage. It flows from the first stage into the brine heater, where it turns around to again become the flashing brine. The point at which the recycled brine becomes coolant separates the train of stages into two sections. From this point upstream—the heat recovery section—latent heat evolved in the flashing-condensing operation is absorbed by the brine-as-coolant. This reduces the amount of heat addition otherwise required in the brine heater were cool sea water fed directly for a single pass through the stages. Below the brine recycle point—the heat rejection section—raw sea water is fed into the last stage as coolant and passes upstream through the section. Most of the heated sea water is discharged back to the sea. Part, however, undergoes deaeration and scale-control treatment, and then joins the brine stream at the recycle point. This provides makeup for that part of the brine going to pure water, as well as that part discharged to keep brine concentration at desired levels.
56 C&EN NOV. 28, 1966
By Jan. 1, the Office of Saline Water expects to have a construction contractor chosen for its module program. By the following January, it expects the module to be in operation at its San Diego test facility. A forerunner of big sea water desalination p l a n t s most immediately, the project planned by the Metropolitan Water District of Southern California (MWD)-the module will be a major scaleup of multistage flash evaporation toward eventual plants of 50 million gallons per day or larger. Moving along on its tight schedule, OSW has in the past two months: • Awarded a contract for just under $150,000 to Anaconda American Brass Co. for fabricating more than 1 million feet of 3 / 4 -inch tubing. OSW will supply the material— 90 / 10 coppernickel scrap which it is buying from the U.S. Treasury. Shipment of tubes will start Dec. 30 and is to be completed by March 1 next year. • Awarded a contract for just under $67,500 to Aqua-Chem's Davis Engineering subsidiary for fabricating the brine heater. • Awarded a contract for $356,770 to Japan's Mitsui for building the recycle pump and other major pumps. Delivery of the large recycle pump will be in 12 to 14 months. The remainder of the module will come under the construction contract. Plans and specifications for that contract are now out for bid. Total cost will come to abjout $6 million. Financially, OSW's module program isn't a direct part of the MWD project. But since the goals of the two organizations coincide technically, the design relationship is an intimate one. The MWD project is a big dual-purpose desalination/power plant which will produce a total of 150 million gallons per day of desalted water from three 50 million gallon-per-day process units. It will also generate 1800 M w ( e ) . from two nuclear reactors. MWD's justification for going ahead with the project, a decision reached in August, is based primarily on a future need for economic data. Water supplied from conventional sources being developed by MWD is entirely adequate for quite some time. But MWD feels that it will face decisions for future water-source development by
M W D Project:
W h a t it is, how it happened
When the Metropolitan Water District of Southern California resolved on August 9 to build a large dual-purpose desalination /power plant, it put the seal of action on a technology which has been extensively studied, debated, and justified—but only on paper. Though it will be some time yet—the first phase of the MWD project won't go on stream until 1972—actual operation will provide a stronger base for establishing technical and economic feasibility of such plants in the future. This knowledge won't come cheap. Capital cost for combined desalination and nuclear power generation is estimated by MWD at upward of $400 million. What this money will buy is a plant complex producing 150 million gallons a day of desalted water and generating 1800 Mw(e). Multistage flash evaporation will be the process for the desalting plant. Two pressurized or boiling water reactors will be used in a two-unit nuclear power plant. The facility will be located on a man-made island of about 45 acres offshore of Orange County south of Los Angeles. The complete project will be built in two phases. The initial phase includes a single desalination unit of 50 million gallons per day, along with the entire power-generating facility. In the second phase, which will follow the first by five years, two additional 50 million gallon-per-day evaporation units will be added to the desalination plant. The somewhat complex history of the MWD project dates back to 1959. In that year, MWD contracted with Fluor Corp. to make a feasibility study of desalination. Fluor concluded that multistage flash evaporation was the cheapest practicable process and that nuclear fuels constituted the only feasible energy source for a big plant. Then, late in 1964, MWD, the office of Saline Water, and the Atomic Energy Commission jointly sponsored a study contracted to Bechtel Corp. of a combination desalination/power plant of 50 to 150 million gallons per day of water and 150 to 750 Mw(e). Early in 1965, however, the scope changed. The three electric utilities in Southern California—the City of Los Angeles Department of Water & Power, Southern California Edison Co., and San Diego Gas & Electric Co.— jointly expressed to MWD their interest in participating in the project and offered a proposal outlining the basis for their participation. When Bechtel finished its study at the end of 1965, it concluded that the plant concept based on the electric utilities' proposal resulted in the lowest water cost and the least capital investment by MWD. As a result of study, discussions, and negotiations, the MWD project has a host of participants in varying degrees. After MWD decided to go ahead on the project, it continued discussions with OSW and AEC. On August 19, a memorandum of understanding between these parties resulted, providing a basis for Government participation. Subsequently, a bill was passed by Congress and signed by President Johnson authorizing AEC participation. A similar bill for OSW participation passed the Senate, but was deferred by the House until next year, since neither OSW nor the House considered it to be emergency legislation. AEC needed more lead time for its part in the project. Under the authorization, AEC will contribute $13.5 million to the first phase and $1.5 million to the second. It will go for R&D, design, testing, or for specialized features or components of the plant. OSW, for its part, would contribute $35.7 million to capital cost in the first phase, $10 million to the second, and an additional $11.5 million for operating and maintenance costs. The remainder of the costs will be borne by MWD and the uitilities. Both OSW and AEC agreements are with MWD alone. MWD has its own subsidiary arrangements with the utilities. In essence, the arrangements are these: MWD will have control over the desalination part of the facility; the Department of Water & Power will have control over one of the powergenerating units, the private utilities the other; and OSW will furnish technical advice and will have rights to use and disseminate all data and information resulting from the desalination project. AEC has similar rights.
about 1980. Starting now with the desalination project, it will take until then to gather enough economic and operating data so that desalination can be considered realistically as an alternative. One goal of OSW's overall program is to develop big-plant desalination technology by 1971-72. For OSW, the MWD plant represents an opportunity to do it at the least possible cost—far less than if OSW were to go its own way, eventually building a 50 million gallon-per-day prototype. Consequently, OSW is participating financially in the MWD project, keying its contribution to the value of information and data it will gain compared to alternate approaches. The module is part of OSW's bigplant program, regardless of participation in the MWD project. But since that project is now also a big part of OSW's own program, the module is being designed to produce data and experience needed for the M W D plant. The module, as its name suggests, isn't a complete plant in itself, but represents a portion of a larger plant. The larger plant is one designed for a pure-water output of 50 million gallons per day using multistage flash evaporation. It would have 12 parallel trains of 39 flash stages each, with each of three brine recycle pumps feeding four trains. The module is designed as a portion of a third of the large plant unit. Equipment will be full size, and one recycle pump will feed the equivalent of four trains as though the module were a 17 million gallon-per-day unit. However, one of the trains will be a dummy surge tank, and the other three trains will have only nine stages each. Valves will simulate the pressure drop that would occur across the remainder of full-size trains. As a result, the actual product flow will be only 2.5 million gallons per day. Of the nine stages in the trains, six will be heat-recovery stages and three heat-reject stages. The stages will be designed so that with only slight modification they can operate either as the high-temperature end of a full-size unit (the nine stages operating from 250° to 205° F.) or as the low-temperature end (those from 127° to 90° F . ) . NOV. 28, 1966 C&EN 57
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Design of the module, anything but arbitrary, represents the most advanced state of the art. In 1965, OSW contracted for conceptual designs of a 50 million gallon-per-day plant with 15 different contractors—firms like AquaChem, Badger Co., Baldwin—LimaHamilton, Fluor, and Foster Wheeler. Each contractor was free to choose process and design at his discretion, limited only by ground rules set down by OSW—primarily that the plant be capable of operating in conjunction with an electric generating plant, that it be suitable for a West Coast site, and that it use technology based on known concepts or data proved at the production or pilot-plant level. The ground rules also specified costs and costing methods so that the designs would be comparative. The contract for design of the actual module went to Fluor. Able to pick what it considered the best ideas from the 15 conceptual designs, Fluor came up with a reference 50 million gallonper-day plant design. It then designed the module as a cross section of that plant. OSW then held review meetings with representatives of all interested parties—experts from construction contractors, operating contractors, and equipment suppliers. The final module design incorporates revisions made as a result of feedback from these meetings. The module thus represents a concensus of the best available design information. But it will go into operation confronting the big unknown: effect of scaleup. It's not that the process hasn't been proved. More than 800 desalination plants of 25,000 gallons per day capacity or more are now operating or being built throughout the world. Of the processes they use, multistage flash is the most commercially successful, in that it is the one used most often in nonresearch installations of, say, 200,000 gallons capacity or more. Of 11 plants with units producing 1 million gallons per day or more, all but OSW's long-tube-vertical evaporator demonstration plant at Freeport, Tex., use multistage flash. But of these units, the largest one operating—one of two at a plant built by Britain's Weir Westgarth on Curacao—has a capacity of only 1.7 million gallons per day. This will be surpassed by the single-unit plant Westinghouse is building at Key West, Fla., but even that will reach only about 2.6 million gallons per day. The level of experience is thus a far cry from a 50 million gallon-per-day plant. OSW's module will help to close the gap. Experience will be gained with both equipment design and process operation. With equipment, for ex-
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60 C&EN NOV. 28, 1966
ample, OSW points out that the recycle pump is a 5000-hp. unit pumping 77,000 gallons per minute. This size isn't uncommon in standard utilities operations. But in such cases, the units are pumping against only about a 40-foot water head. In the module, the total dynamic head is 218 feet. In operation, the module will determine scaleup effects on such aspects of the process as hydraulics, heat transfer, and instrumentation. It will, for example, provide information on how a large stream of flashing brine reacts and determine the effect of size on reaching flashing equilibrium in a stage. It will determine carry-over of impurities into product water. It will study the reflashing of product water as it flows through the process from stage to stage. The knowledge and experience gained from OSW's module and translated into MWD's plant will place multistage flash in the position of being the only process operated on a really large scale. Hardly anyone, however, views multistage flash as necessarily being the ultimate answer for big plants, and much can be said for other processes, particularly distillation. For example, a strong case can be made for long-tube-vertical evaporators on both operational and theoretical grounds. Such a case was made recently by Ferris C. Standiford, president of W. L. Badger Associates, at the Atlantic City meeting of the American Institute of Chemical Engineers. Badger's assessment of the Freeport demonstration plant, which it designed, shows the operation to be more economical than that of OSW's Point Loma multistage flash demonstration plant. The assessment, however, is at variance with that of OSW, which reported the total unit cost of water as the same for each plant. Mr. Standiford, citing unequivalent distribution of capital charges, figures the cost of pure water produced at Point Loma at 12% higher than that at Freeport. On a theoretical basis, multiple-effect evaporation, such as the longtube-vertical type at Freeport, has an edge on multistage flash in thermal efficiency. It makes more effective use of the overall temperature difference, the driving force for heat transfer. On one basis of comparison, Badger shows that the long-tube-vertical evaporator process can utilize 83.6% of the available temperature difference for heat transfer, compared to 61.1%? for multistage flash. All distillation processes have some loss of temperature difference, which results in a loss of heat transfer from that which is theoretically possible. Multistage flash, being a forced circulation system, has some loss of temperature difference resulting from the
failure of the large volumes of circulating brine to flash completely to equilibrium at stage pressure. One loss of available temperature difference common to both multiple-effect evaporation and multistage flash is in boiling-point rise as brine becomes concentrated. Still another loss results from the absorption of heat as sensible rather than as latent heat. This produces a temperature rise that is subsequently lost when the brine flashes. It occurs throughout the multistage flash process, although the more stages there are the lower the loss. In multiple-effect evaporation, the loss of temperature difference in the preheater due to this cause is higher. But it occurs only in the feed preheater section, since each evaporation effect operates at constant temperature. Thus, only a small amount of the total heat being transferred is affected. C. F. Braun has gone beyond the single-process concept. In its conceptual design for OSW, the company developed the idea of a combination plant. The plant takes advantage of the lower temperature-rise loss of multistage flash along with the greater utilization of temperature difference provided by multiple-effect evaporation. In essence, a multistage flash operation acts as the preheat section for multiple-effect evaporation. Putting the processes on a comparable basis, Braun shows that multistage flash would require 40% more heattransfer surface than would the combination plant, and multiple-effect evaporation would require 14% more. Such a savings in heat-transfer surface could have a big effect on water cost. Braun points out that for a 50 million gallon-per-day plant using multipleeffect evaporation, amortization of capital cost amounts to about 40% of the total cost of water. About 7 5 % of the capital costs is in the heat-transfer equipment. These are just a few of the many distillation processes, modifications, schemes, and techniques that have been proposed or are under investigation. In addition, there are the processes other than distillation—freezing and membrane processes, for example. These processes, however promising, have a way to go in their development to catch up with multistage flash. Multistage flash is commercially successful. It has been used in plants built and operated under a wide variety of conditions. It has been used almost exclusively in the largest plants. The only large-scale use of multipleeffect evaporation to date is in the Freeport plant. Other processes are only in pilot plant or at bench-scale levels of development or, indeed, only on paper.
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