TECHNOLOGY
Half stage may boost water plant output Initial half stage and reheat, recycle changes can add millions of gallons to salt water desalination plant output ACS NATIONAL MEETING
154TH
Industrial and Engineering Chemistry
As much as 6 million gallons per day of fresh water free. This is just one result of a modification that would increase the heat economy of a sea water desalination plant having specifications similar to those for the 150 million gallon-per-day plant planned by the Metropolitan Water District of Southern California (MWD). And there are other modifications that could bring added efficiency and heat economy to the operation. They are all part of a series of patents (U.S. 3,288,686; 3,306,346; and 3,329,583) granted over the past nine months to Dr. Donald F. Othmer, Distinguished Professor of Chemical Engineering at Polytechnic Institute of Brooklyn. Other patents are pending. All told, they represent several improvements on conventional multistage flash evaporation (MSF), particularly in its use on sea water. Basically, there are three modifications: • A half stage incorporated in the process between the first or hottest stage and the prime heater, along with operation of the prime heater as both heater and evaporator. The net result, Dr. Othmer says, would be a 4% saving in heat for a plant like the MWD project, which has three 39stage units. For plants with fewer stages, the savings would be greater— about 20% for a typical five-stage MSF unit designed for a large industrial plant. The 4% saving for the large plant translates into a capacity increase of 6 million gallons per day at no increase in heat cost. • A vapor reheat modification in which condensed fresh water from one stage is used as a spray to condense vapors in the next hotter stage. This, Dr. Othmer points out, eliminates metallic heat transfer surfaces. For a plant the size of the MWD operation, this would amount to millions of feet of copper alloy tubing. •Withdrawal and recycle of some of the heated brine at the hot end of the process, thus saving the heat otherwise rejected by conventional operation when partly heated sea water 64 C&EN SEPT. 25, 1967
is sent to waste. This, Dr. Othmer says, can increase steam economy by 4 to 10%, depending on temperatures, design specifications, and the like. The MSF operation is essentially one of evaporating-cooling of hot sea water for condensing-heating of the sea water feed. In one stage after the other, the hot brine stream flashes or evaporates, at the same time cooling down to the temperature of the stage. The vapors from this evaporation condense on cooling tubes carrying cool sea water feed to become fresh water. At the same time, latent heat from the condensing vapors transfers to the sea water inside the tubes, heating it as it flows from a cooler to a hotter stage. In conventional practice, the prime heater is essentially the start of the process. Here, preheated brine is further heated by boiler steam to the highest temperature of the system. 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 the cooling tubes. Cool brine—that which has flashed through to the last stage—recycles to that stage upstream where its temperature is that desired for coolant. Here it becomes the coolant in the tubes and is preheated as it flows through the stages to the prime heater. Below the point at which the brine becomes coolant, raw sea water is used as the coolant. Although part of this sea water joins the recycled brine stream to provide makeup for losses to pure water and blowdown, most of it is discharged back to the sea. The heat absorbed by the raw sea water is thus rejected to keep the evaporatingcooling side of the system in balance with the condensing-heating side. The evaporating-cooling side otherwise may produce more vapors than can be condensed. The first step in modifying the process to include a half stage is to replace the prime heater with a heater-evaporator. The unit continues to carry out the function of heating brine for return to the first stage; but, in addition, some of the brine evaporates. In the half stage, these vapors are used to
continue preheating the brine after it leaves the tube bundle of the first stage. At the same time, however, the vapors condense, providing additional fresh water which joins that in the first stage and continues normally through the process. The half stage thus contains only the cooling tubes and fresh water collection of a normal stage and no flashing brine. Heated brine re-enters the process in the first stage, as it normally would. Dr. Othmer uses the design specifications of the 150 million gallon-perday MWD plant to illustrate how the half stage works. Ordinarily, preheated brine leaves the first stage at 235° F. The prime heater heats it 15° to 250° F., at which temperature it re-enters the first stage. With the half stage, the heating increment is still 15°, but the heating is done in two steps. In the half stage, while vapors from the heater-evaporator condense, brine is heated to 240° F. The remaining 10° is supplied in the heater-evaporator and the heated brine stream enters the first stage at 250° F., as it normally would. Dr. Othmer points out that the heater-evaporator uses the same amount of steam that it would normally. But the vapors going to the half stage contain, as latent heat, almost exactly one third of the prime heat supplied. The half stage thus gives an additional amount of condensate equal to one third of the steam supplied by the boiler. For the MWD plant, this would amount to 595,800 pounds per hour of fresh water that would otherwise not have been recovered—3.44% of the total product. This increase in condensate has a further effect throughout the process. If the design conditions of the process remain unchanged, the additional condensate gives 0.51% more heating effect, hence production. Thus, total increase in capacity using the half stage and heater-evaporator is 3.95%, an increase in gain ratio (pounds of water produced per pound of steam) from 9.72 to 10.1. No additional steam is used, so there is no increase in operating cost. The heat transfer surface of the heater-evaporator should be no greater than that of the heater alone, Dr.
Modifications cut cost of desalination using MSF evaporation Steam Sea w a t e r reject
Sea w a t e r Product water Prime heater
First s t a g e Recycle brine Concentrated brine discharge In conventional multistage flash (MSF) evaporation, brine is heated to the high temperature of the system by a prime heater. It enters the first stage, which is at a slightly lower temperature and pressure, and some of the brine flashes, condensing on cooling tubes and collecting in a
Condensate trough running from stage to stage. Brine continues through the system, some flashing at each stage, which is at a temperature and pressure slightly lower than in the preceding stage
Steam Vapors Sea w a t e r reject Half Stage Sea w a t e r Product water
Prime heater evaporator
First s t a g e Recycle brine Concentrated brine discharge By modifying the system through addition of a half stage, heat efficiency can be increased up to 2 0 % , depending on the number of stages—the smaller the number, the greater the increase. In this modification, the first stage operates at the same temperature and pressure that it would without the half stage. However, the prime heater operates also as an
Condensate evaporator, sending some evaporated vapors to the half stage, where they condense, providing more pure water and preheating the brine more than otherwise before it enters the prime heater. The same amount of steam is used in the heater-evaporator as would have been used without the half stage
B r i n e recycle Vapors
Steam
Half stage Condensate
Prime heaterevaporator J
First s t a g e Heat withdrawal Concentrated brine discharge
Product water A further change—modifying the heat reject operation and employing vapor reheat—can increase steam economy frotn 10 to 20% and eliminate the huge amounts of heat transfer surface otherwise required. In conventional practice, sea water used as coolant in the low-temperature ~+2ges is discharged back to the sea. This provides an operational balance
in heat flow. However, the otherwise lost heat can be recovered if, instead, some of the heated brine is diverted before it reaches the prime heater-evaporator and is sent to the evaporating stream at a stage of the same temperature. In the vapor reheat modification, pure water sprays into each stage to provide the cooling needed for condensing flashed vapors
Sea water
Othmer points out, since a higher overall heat transfer coefficient can be expected. As a result, the only increase in capital cost of the plant is substantially that of the heating tube bundle of the half stage. Although the half-stage and heaterevaporator modification is relatively simple and could be incorporated in current plants or designs, the vapor reheat concept involves a redesign of the process. In this case, the flow of condensed fresh water is reversed, moving from low-temperature to hightemperature stages. Also, sea water enters the process at the hot end rather than at the cool end. Essentially, product water, which has been externally cooled, is introduced into the condensing section of the coolest stage as a spray. Vapors from flashing brine contact the spray and condense, mixing with the spray. This stream then goes to the condensing section of the next hotter stage as a spray. As hot condensate leaves the hottest stage, it goes to a heat exchanger where it preheats raw sea water. This sea water then flows to the heaterevaporator. Some evaporates and goes to the half stage, while the remainder becomes hot brine feeding to the first stage. Dr. Othmer points out that a basic advantage of vapor reheat is the very low temperature difference necessary as a driving force for heat transfer when vapors condense directly on water sprays. It allows a much closer approach temperature in all stages. The half-stage modification can also be used with vapor reheat. Advantages are the same as for the conventional process. The heat reject modification—the third of the group—can be applied to conventional MSF or to the vapor reheat version. In this modification, heat which must be withdrawn to maintain the desired heat balance in the system is withdrawn at a high temperature level in the form of a stream of preheated sea water, rather than at the lowest possible temperature by the usual coolant stream. This way, it's possible to recover or utilize the heat otherwise rejected. The stream of preheated sea water is withdrawn just before going to the prime heater. The stream bypasses the prime heater and, in an emergency, it could be diverted to waste. However, it would normally go to the flashing side of a stage at a point where its temperature is that of the stage. The sensible heat of the preheated stream is thus substantially recovered. It is given up in flash evaporation after combining with the main flow of evaporating brine. 66 C&EN SEPT. 25. 1967
Starch xanthates show rubber chemical promise
154TH
ACS NATIONAL MEETING Rubber Chemistry
Starch xanthates show promise as contenders in the multibillion pound market for rubber chemicals and reinforcing agents, according to chemists at the U.S. Department of Agriculture Northern Regional Research Laboratory, Peoria, 111. USDA's Russell A. Buchanan says that zinc starch xanthate, when masterbatched with SBR, natural, or nitrile rubber latexes, acts as a rubber reinforcer, coagulation modifier, and curing accelerator. Starch xanthate masterbatching is compatible with current industrial
masterbatching processes, and reinforcement with starch xanthate is competitive in cost, on an equal volume basis, with some carbon blacks and many nonblack fillers. Mr. Buchanan points out that even though starch and elastomers are incompatible, fine particles of starch xanthates in rubber may be prepared by masterbatching. Such dispersions are analogous to reactive resin-rubber blends in that xanthate groups are chemically reactive toward elastomer molecules during vulcanization. Thus, zinc starch xanthate and starch xanthide accelerate sulfur curing of elastomers at a rate proportional to the loading. When dispersed as colloidal particles in compounded blends, starch xanthates improve the tensile properties of a variety of elastomers. The
Incorporation of zinc starch xanthate into rubber by latex masterbatching Starch xanthate solution
Latex
Water
Antioxidant emulsion
Mixing
Dilute sulfuric acid Zinc sulfate solution
Coprecipitation
Curd
Drying
Filtration
Serum (discarded)
Additional antioxidant
Milling Masterbatch (product)
