TECHNOLOGY
Chemists Sum Up Water Desalting Progress Experts report advances in distillation, electrodia lysis, freezing, and other potentially economic ways to freshen saline water 137TH
ACS
NATIONAL
MEETING
Water and Waste Chemistry
"Man has been trying for a long time to brew a drink of fresh water from the sea. For centuries people generally have known how to turn the trick: Just distill it." But "how do you do this on a large scale at a cost cheap enough to substitute for or augment water from conventional sources?" With these opening remarks, J. W. O'Meara of the Office of Saline Water, Department of the Interior, set the scene for a two-day symposium sum ming up progress in converting sea water to fresh water. The symposium was sponsored by the Division of Water and W'aste Chemistry. Water problems of availability, salinity, quality, and the like touch all 50 states, Mr. O'Meara told the sym posium. We are now using about 240 billion gal. of water per day. And in just 20 years, we may have to face a demand that has more than doubled —to 597 billion gal. per day, accord ing to the U.S. Geological Survey. Thus, we must find new sources of fresh water and, obviously, the most likely sources are the ocean and brack ish inland waters, Mr. O'Meara says. Of the saline water conversion plants now in operation, a plant on the island of Aruba in the Caribbean is the most efficient. It is now mak ing 2.7 million gal. of fresh water per day at a cost of about $2.00 per 1000 gal., Mr. O'Meara points out. To develop ways of producing fresh water at a lower cost, OSW has been authorized to build and operate five saline water conversion plants. Each of the Άνβ will use a different process. Distillation. W. L. Badger Associ ates' approach to sea water conver sion is long tube vertical evaporators 56
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—a nest of tubes about 2 in. in diam eter by 20 to 35 ft. long, with liquid boiling inside the tubes and steam condensing outside. In an initial study for OSW, the company designed a unit that it expects to turn out fresh water at about 35 cents per 1000 gal. The method: Build evaporators of the cheapest metal (steel), operate them at a relatively high 250° F., and keep them free of scale, Badger's Ferris C. Standiford, Jr., told the symposium. For the past two years, Badger has operated a pilot plant in North Caro lina to study heat transfer, scale, and corrosion in the evaporators. This pilot work has shown that steel can be used, that the method works at 250° F., and that scale formation can be prevented (by causing incoming scale to deposit on scale particles sus pended in the sea water instead of on the tubes). Based on these results, OSW chose the process for the first of its five demonstration plants. Badger has completed the design of this plant, which will be built near Dow Chemi cal at Freeport, Tex. (C&EN, July 20, 1959, page 19). Planned * ca pacity is 1 million gal. of fresh water a day. Although this is less than a tenth the size of a plant needed to make water at about 35 cents per 1000 gal., the water cost will be only about $1.00 per 1000 gal., Mr. Standiford says. This is less than for any existing conversion plant, he adds. The Freeport plant will use a 12 effect evaporator. Steam given off by sea water in the first effect (operating at 250° F.) will be condensed in the second effect to boil sea water at about 240° F. Steam from this effect will heat the third one, and so on to the 12th effect, boiling at about 120° F. One pound of steam bought from Dow at 45 cents per 1000 lb. will make almost 12 lb. of distilled water. There fore, heat energy should only cost
about 30 cents per 1000 gal. of water produced, according to Mr. Standiford. The plant will include a deaerator to take care of oxygen dissolved in the raw sea water. Badger plans to use a thickener to remove the sus pended scale particles from the con centrated waste sea water that carries away the salt and other dissolved par ticles. The scale particles will be mixed back into the incoming sea water so that chemical treatment to prevent scale deposits can be avoided. West Coast Plant. OSW's No. 2 demonstration plant will be a combi nation nuclear reactor-saline water dis tillation unit. It will also have a ca pacity of 1 million gal. of water a day and will use a multistage flash evapo ration method engineered by Fluor Corp. (C&EN, May 25, 1959, page 33). OSW selected San Diego, Calif., as the site for this plant, but has learned recently that this location is not acceptable to the AEC. The office is now looking at other proposed sites in the same general area. With a 50 million gal.-per-day evaporation unit, Fluor estimates the cost of potable water will be about 42 cents per 1000 gal., senior research en gineer Donat B. Brice told water and waste chemists. This estimate is based on an energy cost of 37 cents per 1 million B.t.u. The steam will be made in a 370 thermal megawatt, pressur ized water nuclear steam generator, using slightly enriched uranium. The 42 cent cost includes all other costs associated with operating the evaporator. This includes, for in stance, amortization, utilities, supplies, labor, and overhead, plus the cost of energy jto pump the product water to 100 p.s.i.g. The capital cost estimate covers preparing the site, the sub marine sea water lines, and the equip ment to make the water available at the site boundary at 100 p.s.i.g. Fluor is also studying the effect on
NO. 1 DEMONSTRATION PLANT. Badger displayed this scale model of its proposed sea water conversion plant at the Cleve-
land meeting. The facility, based on Badger's long tube vertical evaporator process, will be installed at Freeport, Tex.
SOLAR POWERED. Du Pont's air-supported plastic still makes an average of 0.06 gal. of fresh water per sq. ft. per day
FREEZE SEPARATION. Carrier's 15,000 gal.-per-day pilot plant at Syracuse, N.Y., has been making salt-free ice from salt water since late 1959. The company's process is one of the methods under consideration for OSW's No. 5 demonstration plant
GLASS COVERED. Designed by Dr. George O. G. Lof, this deep basin still has been operating for about seven months APRIL
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late this year or early in 1961, he adds. And similar units with a 10 million gal.-per-day capacity can be built immediately, would have about 2 / 8 the cost estimated for 2 million gal.-perday plants, Mr. Katz predicted in his closing remarks.
Vapor Compression and Freezing.
ELECTROCHEMICAL APPROACH. University of Oklahoma's Dr. G. W. Murphy (right) discusses his demineralization equipment with Dr. G. W. Reid (left) and R. Matthews
cost of possible improvements in design and operation of a multistage flash evaporator plant and a heavy nuclear steam generator using natural uranium. A plant projected for 1972 would consist of a 130 million gal.-perday evaporator and a 950 thermal megawatt generator. Cost of converting sea water with such a plant: 24 to 31 cents per 1000 gal., Dr. Brice predicts.
Electric Membrane. Webster, S.D., is the site of OSW's No. 3 demonstration plant. This facility will use electrodialysis to purify brackish water (C&EN, June 29, 1959, page 29). Leading contender: Ionics, Inc. In electrodialysis, a d.c. electrical field acts on charged ions in solution to move all the cations in one direction, all the anions in the opposite direction. Saline water can be demineralized by controlling this movement of the ions, and stopping them from reentering the purified water. The key: ion exchange membranes. These membranes are thin sheets of cross-linked organic polymers with ion exchange properties—for example, sulfonated polystyrene-divinylbenzene polymers. The membranes are spaced alternately between a cathode and an anode, and the compartment between each pair of membranes is filled with saline water. In operation, ion motion is controlled so that one set of compartments—for instance, the odd numbered ones—will lose ions, and the even numbered compartments will gain ions, Ionics' vice president Wil58
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liam E. Katz explained to the symposium. Result: potable water. Ionics has made a number of improvements in electric membrane demineralization in 1959, Mr. Katz says. The company has developed: • A pair of improved, commercially available cation and anion membranes. The membranes have a lower electrical resistance than those formerly used, but retain the structural, mechanical, and chemical stability needed for operation at high current densities and at high rates of demineralization, Mr. Katz claims. • A new continuous flow, two stage, single brackish water demineralizer. The unit will give up to 93rir desalinization, depending on the capacity, Ionics says. • The Mark III—a large membrane stack with more than three times the available area of any formerly used, commercially proven stack. Ionics has projected costs for 2 million gal.-per-day brackish water demineralization plants, based on the Mark III stack. Total water cost (including 25 year-57r amortization of the equipment) would be 23.5 cents per 1000 gal. for water with 1000 p.p.m. total dissolved solids, according to Mr. Katz. For 2000 p.p.m. solids, the cost would be 40.3 cents; for 4000 p.p.m., 63.5 cents. These figures are based on design principles which have been developed and proven in the field, Mr. Katz stresses. Plants achieving such water costs could be installed
The No. 4 plant, also for brackish water conversion, will be at Roswell, X.M. It will have a capacity of 100,000 gal. per day and will use vapor compression. In the running: Cleaver-Brooks, Griscom Russell, Mechanical Equipment, Bethlehem Steel, Whiting Corp. OSW hasn't picked the site of its East Coast demonstration plant yet, is looking over sites from Maine to Florida. This plant will use a freezing process to convert sea water. One of the approaches under consideration is Carrier Corp/s freeze separation method. Carrier designed a 15,000 gal.-perday pilot plant in 1958, fabricated and assembled it in 1959 (C&EN, Jan. 12, 1959, page 28). The plant began making ice in late 1959, but hasn't operated yet at design capacity as a complete unit for any long periods of time. However, results from 12- to 20-hour runs look good, senior chemist C. M. Bosworth told the division. Carrier has used OSW's standard procedure to estimate capital and operating costs per 1000 gal. of fresh water for its method. Potential costs: 60 cents to S 1.00, Mr. Bosworth estimates. The figures include a number of ways for handling the water v a p o r absorption, compression, and the like— and for handling capacities of 100,000 to 10 million gal. per day. Osmosis. Osmosis as a saline water conversion method is still in the lab stage, a couple of years away from actual plant application. OSW has contracts with Monsanto and Radiation Applications to develop osmotic membranes. And engineers at University of California (Los Angeles) are studying osmosis through a vapor gap. Osmosis is the spontaneous flow of water into a solution—or from a more dilute to a more concentrated solution when these are separated from each other by a membrane. To get fresh water from salt water, however, the flow has to be reversed, UCLA's Joseph W. McCutchan told the meeting. Therefore, he and Dr. G. L. Hassler call their project reverse osmosis. According to H. L. Callendar's theory of osmosis, the liquid passes
through the semipermeable membrane as a vapor and condenses at the oppo site surface of the membrane. On this basis, the engineers are using a mem brane consisting of two sheets of un treated cellophane separated by a water repellent powder—for instance, silicone-coated pumice powder. To maintain a vapor gap, Mr. McCutchan and Dr. Hassler are using air pressure in excess of the pressure on the salt water. The cellophane sheets support the capillary surfaces, which can withstand up to 1500 p.s.i. In a series of experiments, more than 95% desalinization was obtained, Mr. McCutchan says. Yields with cur rently used lab equipment: about 1 ml. of fresh water per hour on a mem brane surface 10 in. in diameter. To increase the yield, the engineers are currently studying ways to keep the membrane permeable while support ing the high air gap pressure. Electrochemical Way. Research workers at the University of Oklahoma are studying an electrochemical ap proach to desalting water. The proc ess can be compared to ion exchange bed demineralization, except that elec trical regeneration replaces chemical regeneration, the university's Dr. George W. Murphy explains. The electrodes themselves are ion exchang ers that are also electrical conductors. The method differs from conventional electrolytic demineralization, Dr. Mur phy told water and waste chemists, in that no gases form at the electrodes. Therefore, considerably less voltage is needed to carry out the operation. The process has two phases. First, the electrically charged particles— mostly sodium and chloride ions—are attracted to charged graphite elec trodes and thus removed from solu tion. In the second part of the proc ess, the adsorbed sodium and chloride are given up to a more concentrated reject solution by reversing the elec trodes' polarity, Dr. Murphy says. Then the electrodes are ready for an other demineralization cycle. The biggest problem has been to develop two types of highly porous electrodes, Dr. Murphy and coworkers found. They prepared both electrodes initially by depositing finely divided graphite on a synthetic felt carrier. One type must attract sodium when it is charged negatively; the other must permit adhesion of chloride when it is charged positively. The research workers did this by chemically treating commercial graphite with various or
ganic compounds, such as tetramethylphenylenediamine and neocyanine. Because the process is still in the lab stage, Dr. Murphy didn't give any cost figures. However, the electrochemi cal process does hold promise as an efficient and economical route to fresh water, Dr. Murphy says. Sun Power. Direct solar distilla tion is another method of salt water conversion OSW is taking a good look at—with some reservations. OSW feels that the application of the method will be limited to small com munities or isolated areas where no fuel or energy is readily available. One phase of the study—field evalu ation of promising stills—is being car ried out by Β attelle Memorial Insti tute. Three stills, representing two basic designs, are in operation at OSW's Florida Solar Distillation Re search Station. One is a 2500 sq. ft., glass-covered, deep basin still de signed by Dr. George O. G. Lof, an engineering consultant. The other two are 500 and 2300 sq. ft. air-sup ported plastic stills developed by Du Pont, Battelle's James A. Eibling told the symposium. The deep basin still has been operat ing under batch control for about seven months. To determine its per formance, incident solar radiation and distillate production are measured every day. Results: Rate of distillate production depends almost entirely on the amount of solar radiation; average monthly efficiencies vary between 26% and 35%, Mr. Eibling says. Big gest areas of possible improvements: heat losses from reflection on the glass and radiation from basin water; prod uct losses from flie re-evaporation of the distillate. Some possible solutions may be the use of treated glass and ab sorptive foam or mat on the water's surface, Mr. Eibling told saline water chemists. And, he adds, re-evapora tion losses could be reduced by shad ing the distillate collection troughs from direct sunlight. Du Pont's air-supported plastic stills have been operating since January 1959. The 2300 sq. ft. one consists of rows of separate· channels; each channel has a waterproof plastic basin and an air-supported, transparent plas tic cover. The design uses a relatively shallow basin containing only a few inches of water. Efficiency, though erratic, tends to go up with increased solar radiation, Mr. Eibling says. Average production for the still: 0.06 gal. per sq. ft. per day.
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New Route to Phthalic Acid Liquid phase xylene oxidation with no catalyst gives high yields with few side reactions 137TH
ACS
NATIONAL
MEETING
Petroleum Chemistry
Sulfur and water alone are the oxi dants in a new process for converting xylenes to free carboxylic acids. Yields as high as Sor/f can be obtained with complete conversion to hydrocar bon, California Research's Dr. William G. Toland told the Division of Petro leum Chemistry's Symposium on Oxi dation to Produce Petrochemicals. Sulfur and water as oxidants for or
ganic compounds usually require as a catalyst a base such as sodium hy droxide or ammonia. Other forms of sulfur—sulfite, thiosulfate. and sulfate triggered by hydrogen sulfide—also need a base. The products are car boxylic salts; if ammonia is the base used, amides are also produced. Liquid Phase Is Key. But in Cal Research's new process, no base is needed. Starting from methylbenzenes, for example, you get the free carboxylic acids directly: C (; H,CH :! + 3S + 2H.O -» C ( ; H,CO,H + 3rLS
Sulfur, Water Oxidize Xylenes o-Xylene SO,
•*" H,0 ^
Oxidizer (340° C, 2400 p.s.i.g., 15to20min.)
•*"
Phase Separator (100° C.)
/Tv Extractor (100° C.)
*H2S -*- By-products \ o-Xylene plus by-products o-ToIuic acid and thiophthalide
I Crystallizer
ι Filter
I Phthalic Acid
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H20
Success with this oxidant depends on avoiding direct interaction of sulfur with the organic compound. When these interact, you get entirely differ ent products, such as stilbenes, dibenzyls, and tetraphenylthiophenes. The secret: Heat the organic com pound with a large excess of water be fore the compound can react appreci ably with the sulfur. Predissolving the compound in water is the most effective way, but the feed may also be added to a preheated mixture of sulfur and water. Reaction time: 15 minutes to two hours between about 200 c and 400 c C. When toluene is premixed cold, the yield is 39.3 mole per cent; when it is mixed in at reac tion temperature, yield jumps to 85.5 mole per cent. Dr. Toland says. One of the surprising features of the oxidation is its reversibility, Dr. Toland says. The reaction can be driven to completion by using excess oxidant or by removing one of the products. Sulfur dioxide, for example, reacts with the hydrogen sulfide, drives the reaction to completion, and generates more sulfur. It can, in fact, be used as the sole oxidant in place of sulfur. This reaction, previously studied in vapor phase over vanadia catalysts, requires large excesses of hydrocarbon over sulfur dioxide to obtain good yields of benzoic acid. And dibasic acids are hard to obtain. The liquid phase reaction, on the other hand, doesn't need catalysts, gives high yields of phthalic acid. Reason: The liquid phase handles the free liquid sulfur formed by excess sulfur dioxide reacting with product hydrogen sul fide. This free sulfur causes problems in catalytic vapor phase systems. To produce phthalic acid, Cal Re search releases the reaction products before cooling, or uses sulfur dioxide to form sulfur from the by-product hy drogen sulfide. The acid is recovered directly by recrystallizing it from hot water after removing insoluble by products. In the processing scheme the company proposes, o-xylene is fed to an extractor to remove intermediate products (o-toluic acid and thiophthalide). The intermediates are then carried to the oxidizer for further oxidation. Thus far, the process has only been used in lab-scale equipment. oXylene to o-phthalic acid appears most promising, according to Dr. To land. If this process is used commer cially, the acid will be converted to the anhydride for marketing, he adds.
