Fresh Water from Sea by Solar Distillation MARIA TELKES Massashusefts lnsfifufe o f Technology, Cambridge, Mass.
I
S TROPICAL, arid regions the production of fresh water is often a serious problem. Many tropical islands are devoid
of natural fresh water because wells produce only salty or brackish water. Collection of rain water and its storage is costly and often not sanitary, because the dry season may last for several months. During the dry tropical season, the sun's energy is overwhelming and represents a potentially great fuel resource in these regions which are generally also devoid of fuel. In the tropics solar energy received on a horizontal surface is nearly constant in intensity throughout the year, and the water yield will be proportional to the horizontal area covered by solar distillation equipment. It is obviously important that the efficiency of the solar distillation process be high and the initial cost of construction be as low as possible. When these aims are attained, fresh water can be produced by solar distillation for consumption and for irrigation, augmenting the amount of water supplied as rain.
The United States Weather Bureau collects records of the total solar energy received on a horizontal surface. These data by Hand (16-17) are shown in Table I for selected southern locations.
Amount of Solar Energy Received on a Horizontal Surface Sortb
Latitude San Juan, P.R. Honolulu, T.H. Miami, Fls. La Jolla, Calif. Fresno, Calif.
18;
21
;
25 33 36"
Solar Energy Received B.t u./Sq. Ft.
Daily,
December
June
Yearly average
1540 1450 1100
2000 2400 2200 1900 2800
1500
1000 560
8
=
Q = S/Q =
Available Solar Energy
Table 1.
Table I1 indicates that somewhat less heat is required to vaporize water at the lower temperature. The table also shows that the total heat that would be released during condensation could serve to preheat the feed water to the boiling point. The preheating lvould represent a fuel saving of 11%a t the boiling point but only 6.5% a t the 150' F. vaporization temperature, and it is doubtful that i t would be economically advantageous. If solar energy could be used completely for the (single-effect) distillation of water, the average tropical solar energy received on a square foot of horizontal surface (2000 to 2500 B.t.u. per square foot per day) could produce a quart of distilled water daily. The theoretical heat requirement for the evaporation of 1 quart (approximately 2 pounds) of water is 2154 to 2204 B.t.u. according to Table 11. The efficiency of a solar distiller is defined in the following Kay:
D
=
Eff. =
amount of solar energy incident on the distiIIer, B.t.u./ day total heat required-1077 to 1102 B.t.u./lb. water amount of water a t 100% efficiency, Ib./day distillate actually obtained, Ib./day actual efficiency as a decimal fraction,
The water yield will be proportional to the efficiency and to the amount of solar energy received by the distiller. In tropical regions, the sun being "overhead" most of the time, the amount of solar energy received will be proportional to the area occupied by distillers. The efficiency on the other hand will be determined by the construction of the distiller.
1940 1900
Table II.
1540 1720
Solar energy maps of the United States by Fritz (12, 13) show the monthly distribution of the amount of energy received on a horizontal surface on clear days and on average days. The southwestern desert regions receive about 2500 B.t.u. per square foot as a daily average from May to August, while on clear days the maximum may reach 3000 B.t.u. per square foot. During the winter 1000 B.t u. per square foot is received by southern border states, and the clear day maximum is around 1300 B.t.u. per square foot. It i8 therefore probable that the arid tropical regions received 2000 to 2500 B.t.u. per square foot daily during the period when the water requirements are most severe.
Solar Energy Required for Distillation The Steam and Entropy Tables (22)show the amount of heat needed for heating and vaporizing water. The water to be distilled (sea water) may be a t an average temperature of 80' F. The evaporating temperatures of 212" F. and 150" F. are selected as examples, and the results are shown in Table 11.
Solar Energy Required for Distillation
Water initially a t SOo F. Vaporized a t 212" F. Vaporized a t 150' F.
Heat Required, B.t.u./Lb. Water Heat of Total Heat of liquid vaporization heat 132 70
970 1007
1102 1077
Historical Review Solar distillers can be classified into two groups: stationary installations, which receive solar energy directly, and focusing devices, which have facilities for solar energy concentration, such as lenses or reflectors. Distillers Using Lenses or Reflectors. "Burning glasses" were used centuries ago to concentrate solar energy and to heat the contents of flasks or alembics. Lavoisier ($4) used a large lens supported in an elaborate mounting (in 1770). Silver- or aluminum-coated glass reflectors have been described by Mouchot (85) and Kausch (21). Pasteur ($6) and others concentrated solar energy onto a copper boiler, using a conventional water cooled condenser, but the efficiency was not higher than 50%. Abbot's solar distiller (1) used a cylindrical parabolic reflector with aluminum foil surface, concentrating
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I N D U S T R I A L A N D E N G r N E E R I N G CHEMISTRY
solar rays to a vacuum-jacketed focus tube. The reflector was rotated by clockwork. The surging, boiling water in the focus tube caused some difficulties. This device, used in Florida in 1938, produced 2 to 3 gallons of water with a reflector intercepting 11 square feet of solar radiation. The efficiency must have been rather high, approaching 80%. Figure 1 shows Abbot’s solar distiller; its weight and construction are typical of conce‘ntrating devices. The initial cost of the reflector and its mounting, neeied for precise focusing, are inherent in this design.
Figure 1. Abbot’s Solar Distiller, 1938
-
Stationary Solar Distillers. Stationary distillers use a black evaporating pan as a “boiler,” charged with a shallow layer of the distilland. A tightly fitting transparent surface is built over the pan in the form of inclined plates. This is the condenser, cooled bythe ambient air. The sun is transmitted by the transparent surface, heats the distilland in the black pan, and vaporizes it. The vapor condenses on the inner surface of the transparent cover, and it trickles down into narrow channels around the lower edge of the condenser plates. The channels drain the distillate to a discharge tube. The largest solar distiller of this type was designed in 1872 (18) and operated a t Las Salinas in Chile, south latitude 24O, 4300 feet above sea level. The distilland brine contained 14% solids and was pumped from local wells. The black pans were made of wood; the total occupied area was 51,000 square feet. The highest water yield was GOO0 gallons per day or approximately l pound per square foot of pan area. Considering the high altitude of the location, the amount of solar energy may have been 3000 B.t.u. per day and the efficiency around 35%. Distillation started about 10 A.M., considerable heat being required to warm up the pan and the liquid in it. The maximum temperature in the pan was 150” F., and the surrounding air was around 80” F. at noon. Distillation continued after sunset at a slow rate, until 10 P.M. The difficulties were considerable. The wood pans leaked and needed constant attention, but the distiller was still in operation in 1908. The “inclined-plate” type solar distiller has been rediscovered several times ( 3 2 ) . I n 1926 the French
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Government offered a prize for the design of a portable solar still (5, 6). Maurain’s solar still of 3 square feet pan area used an inclined glass plate facing south, supported by wood walls. The tests conducted near Paris resulted in an efficiency of 15%. Richards’ solar still ( 2 7 ) , of similar design and of 15 square feet pan area, produced an efficiency of 25% a t Monaco. These experiments led to the premature statement that the inclined-plate type solar still is inherently inefficient, although i t is very simple in construction and operation and obviously could be constructed a t a relatively low cost. Attempts have been made to improve its yield by using porous black wicks of cotton cloth to increase the evaporating surface (10) or to cool the condenser plate with water (S), but the literature does not reveal any facts concerning the yield or efficiency of such devices. Developments during World War 11. Experimental work sponsored by the Godfrey L. Cabot Solar Energy Project at the Massachusetts Institute of Technology iyas started during the past war. The aim of this work v a s to investigate the fundamental principles of solar distillation and to develop a portable solar distiller suitable for life rafts. This work was in part supported by the OSRD (29, SO). An inflatable, floating type solar still for life raft use has been designed by the writer and was tested in Cambridge during the summer of 1943. This device, shown in Figure 2, had no metallic or rigid parts; it could be folded into a small volume of GO cubic inches (about 1 quart) and weighed 1 pound. The transparent envelope made of Vinylite sheet material, supported a black porous pad that was saturated with sea water. The rays of the .sun transmitted by the envelope heated the sea water, and the vapor condensed on the cold surfaces and was collected in the bottom part of the still. Figure 2A shows $he simplest form of the inflatable, floating model. A sea water-filled “ballast tank” was added to prevent overturning of the distiller when it was floating at sea, and a feeding device served to supply sea water to the porous pad. At the end of the day the salt deposited had to be flushed away. This device operated with a 50 to 60% efficiency, and the temperature of the porous pad was around 150” F. during the noon hour.
Figure 2.
Life Raft Type Solar Distiller
INDUSTRIAL AND ENGINEERING CHEMISTRY
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The patent literature contains numerous examples of the solar stills designed for life raft use during World War 11. Aluminum reflectors have been suggested, with provisions for a windmill, t o generate electricity for distilling purposes in case the sun did not shine (4). Submerged copper coils have been used (88). Various devices were designed of rigid and flexible plastic materials, x i t h transparent windows, which could be rolled up when not in use (9). Other inflatable floating models were designed, similar to that shown in Figures 2 and 2A (51). Some of these devices were manufactured in larger quantities for the Armed Forces and the solar still became standard equipment for life rafts. The specialized requirements of the life raft type solar still are rather exacting. Light weight8and small volume are essential for an inflatable, floating model; the stills must be easy to operate and have to withstand storage in a small package for long times, until they might be needed in an emergency. Experimental Work
I t was the writer’s task to develop the basic principles of the operation of the solar still as a heat transfer device. The causes of heat losses had to be analyzed and limited to a minimum. A complete mathematical treatment of the solar distillation process has been included in the miter’s OSRD report (SO) and only a simplified treatment is presented here.
6
incidence of the solar radiation is greater than 50°, the transmitted radiation diminishes. It is important, therefore, that the transparent surface be positioned in such a way that the angle of incidence is less than 50°, a t least during the best hours of the day (8 A.M. to 4 P.M. solar time). Heat Losses Due to Incomplete Absorption. The transmitted energy is absorbed by the black evaporating pan. A thin layer of pure water does not absorb appreciable amounts of solar energy, although i t reflects some. Most L‘completelyblack” surfaces still reflect a small amount of radiation, and these’ losses amount to 4% of the total energy if the blackening is reasonably effective (80). Deposits or crusts formed by insoluble salts may produce a gray or white surface, reflecting and scattering solar energy. Such deposits must be avoided. Heat Loss through Foundation of Evaporator. The most serious heat loss can occur through the evaporating pan into the ground. If t, is the temperature of the water, f,,is the temperature of the ground, k is the specific heat conductivity of the base on which the evaporating pan rests, and d is its thickness, then the heat loss through the bottom of the pan f& will be k(tm qb =
5 6 10 11
= pad suspension = attaching reinforcement = plus = towing loop
The calculations and experiments for the tropical-zone type solar distiller are considered first, because in this region the sun is overhead most of the time, and a horizontal surface receives nearly the maximum amount of solar energy. This solar still is shown in Figure 3. The heat losses will be the following: Transmission of Solar Energy through Transparent Surface. Measurements of solar energy transmission through glass (window glass grade) and various transparent plastic materials show that nearly 90% of the total solar energy is transmitted, a t normal or near normal incidence (bo). There is an unavoidable loss of 8% due to reflection and an additional small loss due to absorption, in the glass or in the thin film of water that condenses on the inner surface of the transparent inclined plate. It is essential that the condensing water be in the form of a thin film, which invariably forms on clean water-wettable surfaces, such as glass. Most plastic surfaces, unless specially treated, produce drop type condensation and “fogging.” Considerable solar energy is reflected or scattered by the small droplets, diminishing the transmitted solar energy. Untreated Vinylite may reflect and scatter as much as 40% of the incident solar radiation. The antifogging treatment must adhere firmly to the plastic surfaces, because it is incessantly washed by the condensing distillate. Water-soluble wetting agents are useless. When the angle of
B.t.u. sq. f t . hour
Value of Insulating Materials Specific H e a t Conductivity
Life Raft Type Solar Distiller
1 = black porous pad 2 = pad support 3 = transparent envelope 4 = wafer-collecting chamber
- tBr) d
It is obvious that a thicker layer of insulating material of low k value will diminish the heat loss from the pan into the ground. If the distiller is supported off the ground and air can circulate under it, the heat losses may be considerable. The solar distillers built in the past generally used a thin metal pan (sheet iron, painted black) resting on a thin wood base or possibly on sand, soil, or gravel. Evaporating pans made of concrete may appear to be desirable until the heat loss through these materials is calculated. I n Table I11 the k values of these insulating materials are given, and the heat loss is calculated for one square foot of pan surface, resting on a layer of I-inch thickness, Then the temperature t,, = 80’ F. and the temperature difference is 70” F.
Table 111. Figure 2A.
Vol. 45, No. 5
Pan Resting on 1Inch Thickness Wood pine Sand, ’fine, dry Soil, dry Soil. wet Gravel, d r y
Slag, fine Concrete Insulating materials Suitable type, dry
(k),
B.t.u. (Hi-.-I) (Ft.-2) (Inch) (” F.) 1.0 2.2
2.0 4.6 2.6
Heat Loss/ Sq. Ft. Pan as S ecified, B.t.u. (k-9(Hr.-l) 70 154
140 322 182
6.0
0.7
49 420
0.3
21
The maximum amount of solar energy that can reach the evaporating pan is around 300 B.t.u. (foot-*) (hour-’), and the heat losses can be directly compared with this amount of energy, In this case the use of a 1-inch thick layer of insulating material will limit the heat loss to fiyoof the available energy. If, however, the evaporating pan rests on the ground, more than half the energy will leak into the ground and will be useless. The heat loss through concrete will be larger than the amount of solar energy received during an hour and probably several hours of solar radiation will be required to warm up the concrete pan. The insulating material will prevent heat leakage into the ground, and therefore the evaporating pan will be heated more rapidly. The time required to heat up the evaporating pan will be determined by the heat capacity of the water in the pan and the heat capacity of the pan material and its insulation or foundation. Favorable conditions can be realized if the evaporating pan contains only a shallow layer of water and if the specific heat and
May 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Other Losses. The evaporator should be covered with a shallow layer of water, but the formation of "dry islands" must be avoided. It is obvious that a dry distiller cannot evaporate water. Losses due to cracks or holes in the evaporator pan must be avoided, because they dissipate heat and deteriorate the foundation or insulation of the distiller. The distilled water collecting channel should not absorb additional solar energy and should not influence the quality of the water. The summary of the above losses is shown in Table IV. These calculations clearly indicate that the efficiency of the solar distiller may approach 70% if the still is insulated. The low efficiency of the inclined-plate type solar stills built in the past (maximum 35%) was simply due to the fact that the major part of the energy received from the sun was Figure 3. Solar Distiller for Tropics conducted away by the ground. Distiller rests on 1 -inch thick insulating pad; water-collectins channels and supports are redwood; roo1 Is glass, evaporating pan, black plastic sheet The losses a t the transparent surface, the losses due to imperfect blackness, t o reradiation and to air circulation within the still cannot be avoided; they amount to about 26%. Therefore the maximum efficiency density of the evaporating pan and its foundation are as low as of the inclined-plate type still, using a single-effect operation, possible. Reflective insulation such as air spaced aluminum cannot be higher than 74% of the theoretical maximum. foils may be most effective if it can be kept permanently dry and Multiple-effect stills, operating a t atmospheric pressure, have not tarnished. been designed by Ginnings (14)for life raft use. Here the heat of Heat Loss Due to Reradiation from Evaporator. The evapcondensation of the first stage is used to evaporate the distilland orator pan being a t temperature tzu will radiate heat to the conof the second stage. Two- or three-stage operation may be densing surface, which is a t a cooler to temperature. Both water feasible, but the design is not practical in its present stage of deand glass have a high emissivity for this type of radiation, and velopment. therefore the heat loss due t o reradiation per square foot of pan area, qr, when tu* = tw 460 will be Experimental Results q. = 17.2 X (tw*4 B.t.u. (foot?) ( hour-').
+
-
*
Using the experimentally determined values of t, = 150' F. and 1, = 127O, qr will be around 30 B.t.u. (foot-2) (hour-') or nearly 10% of the 300 B.t.u. (foot-2) (hour-') solar energy received by the evaporator. It should be mentioned here that the transparent condenser surface acts as a heat exchanger. On the outside it is cooled by the ambient air and wind of variable velocity, while on the inside it receives the heat which has been used to evaporate the water. The condenser must be cooled, and it should not be insulated; just as in a conventional still, the condenser must be kept cool t o condense the distillate. In the roof t y p e construction shown in Figure 3, the tilt of the roof is approximately 4 5 O , and the glass area is about 40% larger than the area of the evaporator pan. This amount of surface provides enough cooling even in nearly still air. Artificial cooling is not needed, although devices have been considered using a spray of water on the outside of the transparent surface to cool it. The efficiency was actually lowered as this can be predicted from the calculations. Heat Loss Due to Air Circulation within Evaporator. The evaluation of this loss shows that it is relatively unimportant and generally not more than a few per cent of the total solar energy.
Insulated solar stills, as shown in Figure 3, were tested in Cambridge, Mass., during the summer. The pan area of these stills varied from 2 to 32 square feet. The incident solar energy was measured with a Weather Bureau type pyrheliometer. The stills were positioned with their longer dimensions in easbwest direction. The stills rested on an insulated base. Figure 4 shows i 00
I
I
I
SOLAR
Table IV.
Heat Losses and Efficiency for Uninsulated and Insulated Solar Distillers
Figure 4.
ENERGY
I
so
I
.20
,,"'auR
Efficiency and Water Yield of Solar Distillers in Relation to Incident Solar Energy
Loss of Solar Energy, %
Transparent surface Imperfect blackening Reradiation Air circulation Insulated evaporator Uninsulated evaporator Efficiency of insulated still Efficiency of uninsulated still
8 4
10 4
6 33 or more 68 41 or less
the water yield and the efficiency in relation to the incident solar energy. The efficiency is around 65 to 70y0with solar energy values of 200 to 300 B.t.u. (foot-2) (hour-'), confirming the calculations. Such values of solar intensity may be obtained in the vicinity of Cambridge (north latitude, 42'21') during clear summer days, around the noon hours. I n tropical regions 200 to
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INDUSTRIAL AND ENGINEERING CHEMISTRY
300 B.t.u. (foot-2) (hour-I) can be expected as average values during the better part of the day. Electrically heated solar distillers were also used to eliminate the fluctuations of the sun and to compare the results using an exactly measured “heat input.” These tests were carried out in the absence of sunshine. A metal evaporating pan was used which was placed on a thin electrical “heating pad,” specially made to deliver the required amount of heat (100 watt = 341 B.t.u. per hour. Allowance was made for the 8% loss of solar energy a t the transparent surface. The input was measured with a wattmeter, and the temperature of the water and the glass surface was measured with thermocouples. The effect of base insulation, ambient temperature, wind velocity, and heat capacity (due to the wat,er in the evaporator pan) was determined with the electrically heated model. The results are summarized briefly as follows:
1. Insulation: A layer of 1-inch thick insulation (of k = 0.3) or its equivalent is sufficient. 2. Ambient temperature: The efficiency is slightly higher if the air temperature is higher (confirmed by field tests). The temperature of the water in the evaporating pan and the temperature of the condensing surface decreases as the ambient temperature is lowered; the heat loss through the insulation decreases slightly; and the water yield decreases very slightly, a t constant energy input, 3. Wind: Wind does not have an appreciable influence, provided the condensing surface is greater than the evaporating surface. 4. Feed water layer in evaporator pan: The heat capacity of the feed water covering 1 square foot of evaporator pan is proportional to its thickness. The heat capacity of a n inch thick layer of water (covering 1 square foot) is 5.2 (Iw - 20) B.t.u. (foot-2) ( ” F.-1) where t,is the temperature of the water during the distilling process and lo is its initial temperature. Starting with a t o (cold) still a t sunrise (or a t the start of an experimental run), a slight amount of condensate can be observed as soon as the temperature of the water in the evaporator pan is a few degrees warmer than the condensing, transparent surface. Appreciable distillation will occur if this temperature rises, ultimately to near 120’ to 150” F. We may assume that the final temperature difference, t, - to. is about 70” F., and therefore the heat required to warm up an inch layer of water will be 5.2 X 70 = 362 B.t.u. (foot-2). This represents nearly 2 hours of early morning clear sunshine. A thinner layer of water, possibly a quarter inch deep, covering the evaporator pan uniformly, speeds up the warming time. An excess of feed water in the evaporator pan is therefore not desirable. Assuming that a maximum clear day radiation of 3000 B.t.u./square foot evaporator pan is to be expected and the efficiency of the still is 65%>the daily maximum water yield will be 1.8 pounds of water per square foot of evaporator, corresponding to a water layer 0.35 inch deep. It is therefore not necessary to supply more than a half inch layer of feed water. Large solar distillers may be fed a t a continuous slow rate, with a thin layer of sea water flowing through and discharging nearly concentrated brine. Effect of Orientation. The maximum amount of solar energy is received by a surface which “follows the sun,” being tilted or moved in such a way that the rays of the sun are at normal incidence. This could be accomplished by using reflectors, but it appears that the solar energy gain would not be sufficient to warrant the increased construction costs of such a device. Lenses or reflectors concentrate only the direct rays of the sun; the diffuse or sky radiation cannot be concentrated, and this amounts to nearly 10% of the total solar energy on clear day8 and considerably more than 10% on overcast days. Reflecting surfaces of silver or aluminum, even if they could be protected against tarnishing, at best reflect only 85 to 90% of the incident solar radiation. It is obvious, therefore, that the reflectors cannot concentrate to the boiler more than 80% of the total solar energy. Some further losses from the boiler, as mentioned previously, decrease the efficiency to less than 80%. Because of seasonal variations of solar energy incidence (determined by the variations in the angles of solar altitude and azimuth), orientation or tilting of the solar evaporator may appear to be desirable. This could be accomplished only through
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an elaborate design, and it is doubtful that the gain in the water yield would warrant the increased cost of such a design. Table I shows that the amount of solar energy received on a horizontal surface during December decreases rapidly a t higher geographical latitudes. I n the Northern Hemisphere the need of water in winter is less. During rainy periods the distiller cannot produce any water, but the inclined plate surface of the distiller can be used as a rain water catchment surface; the water being collected in a suitable channel a t the lower rim of the distiller.
Figure 5.
Solar Distiller for Temperature Z o n e
The water yield of the solar distiller in the temperate Eone can be increased by using the design shown in Figure 5 . This solar distiller for the temperature zone has a north surface made of a thin reflecting material. At low solar altitudes the solar rays are reflected by the north side into the evaporating pan. Simple geometries show that the north reflecting surface should be tilted south, a t an angle mhich is equal to the solar altitude a t noon on June 21. In the Southern Hemisphere the reflecting surface should face north. The construction of this distiller is otherFTise exactly the same as that of the tropical design.
Table V.
Heat Losses and Efficiency for Life Raft Type Solar Distiller Loss of Solar Energy, To
Transparent surface Imperfect blackening Reradiation (two sides) S i r circulation ( t w o sides) Total losses Efficiency
Field tests performed in Cambridge, Rlass., during the summer, on clear days produced a n-ater yield of nearly 1 quart per day per square foot of evaporator pan. It is probable that in the tropics an average of 1 gallon of water per day can be obtained with 4 square feet of evaporator pan. One acre of evaporator pan should therefore produce approximately 10,000 gallons of water daily in the tropics and also in the temperate zone on clear summer days. Preliminary tests sho-ived that this yield can be obtained (19).
Life Raft Type Solar Still This type mentioned above and shown in Figures 2 and 2 8 is a special inflatable, floating adaptation. Figure 2A shows the cross section of this design: The evaporator is a porous pad, sur-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
rounded with a solar energy transmitting and water condensing envelope. There is no insulation under the evaporator; this is not needed because vaporization occurs from both sides of the sea water saturated pad. The heat losses from this model are given in Table V. The solar energy transmission of the plastic surface can be lower than the theoretical value of 8% because of the drop-type condensation, The efficiency will be lower owing t o this effect, as indicated by the symbols ( +) ( - ). The design shown in Figure 2A may be modified to represent the cross section of a long tubular solar still. I n this case a feeding conduit can be provided to supply the absorbent, porous pad with sea water, flowing a t a slow continuods rate and flushing out any solid residue during the night. Stills of this type could be mounted on supporting posts, and the distilled water could be used to irrigate the soil below. In tropical regions the stills could supply daily inch of water over the area occupied by them, equivalent to a monthly rainfall of 10 inches. If a monthly rainfall of 1 to 2 inches be sufficient for agricultural purposes, solar stills may water an area five to ten times larger than the area occupied by them. The success of this arrangement is primarily an economical problem.
for one family. Only 25 to 30% of the rain water can be collected. Brief showers hardly wet the dry concrete and more than a inch rainfall is needed before the rain water begins to flow into the tanks. During the dry, cloudless season of these islands, solar energy is abundant every day to distill water from sea water os other saline water. I n many desert regions wells produce nonpotable water which could be purified by solar distillers. There is no need for an excessively large tank t o hold a supply for several months. The sun will shine relentlessly every day and produce a fresh water supply. When it rains the roof of the solar still acts as a catchment and the rain water can be collected. Catchment basins and tanks are built a t present for a group of houses or an entire settlement. Solar stills may be built for EL community and “solar water works” may be very effective in solving the critical water situation of many tropical locations. Comparison with Fuel Operated Stills. Recent cost escimates of fresh water from the sea have been published by AuItman ( 8 ) and Campobasso (7), and these can be compared with the cost of natural water (8, 11). All cost comparison must account for: 1. Initial investment (capital) costs
2.
Table VI.
Estimated Cost of Distilling Fresh Water from the Sea
[Compared with actual cost of water from natural sources (8, 1 1 ) ] Cost in Dollars per 100 1000 Ton cu. ft. gal. Acre-foot Fuel only 2.60 850 0 62 1.94 Single-effect stills 0.80 1.07 350 Triple-effect stills 0.26 0.40 0.54 175 Compression still 0.13 Total costs 1.25 400 0 30 0.94 Compression still Natural water supply 0.12 0.16 51 Average for 404 cities 0.037
Solar stills of this type, mounted on sufficiently high supports, would not interfere with agricultural operations. The shadow cast by the solar distillers travels during the day and would not impede the growth of the plants. In this way arid land may be used for agricultural purposes. The partial shading effect may even be desirable, protecting the plants from the burning rays of the tropical sun. Economics of Solar Distillation In view of the increasing interest in water supply, it is important to consider the economical aspect of solar distillation, by comparing it with the conventional fuel operated stills. Free solar energy, a t a time of increasing fuel costs, appears to be attractive, but the cost of the fuel is only a part of the total cost. The initial investment, its amortization and interest, the costs of operation, the pumping of sea mTater or brackish well water, the distribution of the fresh water, and the need for it are the determining factors. Large scale solar distillation has never been adequately tested. With the exception of the I-acre installation, built in Chile in 1872, all attempts made were on a small scale. Comparison with Catchment Basins. Obtaining drinking water on islands in the tropical zone is often difficult. At present the water supply is derived from rain water. The catchment basins and storage tanks are familiar t o those who have visited Bermuda and the Bahamas. Water must be stored for a rainless season lasting from 3 to 4 months. Toward the end of this season, the water supply is replete with algae and higher forms of aquatic life and consequently may be unsanitary. The catchment basins and tanks built of reinforced concrete a t an initial. cost of $200 to $400 per household supply 5 to 10 gallons daily
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Yearly operating costs, including: Depreciation and interest Labor and maintenance Fuel (if not free solar energy)
The estimated costs of producing fresh water by fuel burning distillers is compared with the charges made for natural water supply in Table VI. I n many cases the capital investment cost of the natural water source is not included in the cost accounting; therefore the cost basis is not exactly comparable. The cost of compression distillation equipmenta (23) is distributed in the following way: . Initial cost Yearly operating costs t o produce 365,000gallons
Cost per 1000 Gal. Daily Capacityb $2430
530 Distribution of Yearly Cost, % Depreciation and interest 46 $243 Fuel 24 128 Labor 14 74 iMaintenance 16 85 a Initial cost and operating cost of compression distillation system change with size of equipment and unit fuel cost; the figures quoted here are subject t o change. b Prorated from initial cost of single unit of larger capacity.
The compression distillation method is more advantageous than the other fuel operated stills, and therefore it is used as a yardstick to determine the maximum allowable cost of the solar still. The fuel consumption of the compression still represents 24% of the yearly operating costs, while labor and maintenance are rather high (total 30%) because of shutdown and cleaning requirements. The solar still would definitely save the fuel charges, and it may eliminate the major part of the labor and maintenance charges. For equal daily output of 1 gallon per day, the initial cost of the compression still ($2.43 per gallon per day) could be increased with 10 year’s fuel savings to $3.71 per gallon per day and with 10 year’s fuel saving and half the operating cost savings to $4.51 per gallon per day. Therefore, if the initial cost of the solar still is less than $4.00 per daily gallon of water production, it would be economically more advantageous than other fuel operated stills, including the compression still. Under tropical conditions 1 gallon of fresh water can be obtained with about 4 square feet of solar distiller base area; if the allowable cost is $4.00 (or less) then 1 square foot of solar still should cost $1.00 or less. It is probable that the relatively low cost of the required glass, redwood, insulation, and other parts would not exceed this cost allowance. It is difficult to estimate the anticipated life of the solar still.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1114
The roof-type still is similar to the construction of a greenhouse, which has a much longer anticipated life than 10 years. The basis of amortization for the solar still may therefore be considerably longer than 10 years. The Las Salinas solar still was in continuous operation for 36 years. It may have operated for a longer time. Solar distillation appears to be a very promising method for producing fresh water in semitropical and tropical zones. The writer expects to be able to continue its development. literature Cited (1) Abbot, C. G. Pub. No. 3530, Smithsonian Inst. iWisc. Coll., 98, No. 5 (1930); Vol. 2, Smithsonian I n s t . Ser. 1944; U. S.Patent 2,141,330 (Deo. 27, 1938). ( 2 ) Liultman,W. W., Eng. and Sci., 12, No. 5, 12-16 (1949). Aultman, W. W., J . Am. W a t e r Works Assoc., 42, 775-94 (August 1950). (3) Barnes, W. S.,U. S.Patent 2,383,234 (Aug. 21, 1945). (4) Bohmfalk, B. H., Ibid., 2,332,294 (Oct. 19, 1943). (5) Boutaric, A,, Chaleur & Ind., 11, 69-66, 147-155 (1930). (8) Boutaric, A., Recherches et Inventions, 8 , 205-15 (1927). (7) Campobasso, J. J., J . Am. W a t e r Works Assoc., 40, 547-52 (1948). (8) Chapman, 0. L., R e d a m a t i o n Era, 35, 162 (August 1949). (9) Delano, W. R. P., et al. (to Gallowhur Chemical Co.), U. S. Patents 2,398,291 and 2,398,292 (April 9, 1946); 2,402,737 (June25, 1946); 2,405,118 (Aug. 6, 1946); 2,405,877 (Aug. 13, 1946); 2,412,466 (Dec. 10, 1946); 2,413,101 (Dee. 24, 1946); 2,427,262 (Sept. 9 , 1947). (10) Dooley, G . W,, U. S.Patent 1,812,516 (June 30, 1931). (11) E n g . News-Record, 1 4 4 , 3 2 4 (1May 18,1950). (12) Fritz, S., Heating a n d Ventilating, 46, 69-74 (January 1949). (13) Ibid., 46,85-9 (July 1949). (14) Ginnings, D. C., U. S. Patent 2,445,350 (July 20, 1948) (granted
Vol. 45, No. 5
under the act of March 3, 1883 as amended April 30, 1928; 370 O.G. 757).
(15) (16) (17) (18) (19)
Hand, I. F., Hehting and VentiZati.ng, 47,92-4 (January 1950). Hand, I. F., M o n t h l y Weather Reo., 69, 95-125 (1941). Hand, I. F., U . S. W e a t h e r B u r . Tech. Paper, No. 11 (1949). Harding, J., Proc. Inst. Civil Eng., 73, 284-8 (1883). Hollingsworth, F. N., Heati?zg and Ventilating, 45, 99 (August
1948). (20) Hottel, H. C., and Woertz, B. B., Mech. Eng., 64, 91 (1942). (21) Kausch, O., “Die unmittelbare ausnutzung der Sonnenenergie,” Weimar, Carl Steinert, 1920. (22) Keenan, J. H., and Keyes, F. G., ‘‘Steam Tables,” New York, J. Wiley & Sons,Inc., 1944. (23) Latham A., Jr., Mech. Eng., 68, 221-4 (1946). (24) Lavoisier, A. L., “Oeuvres de Lavoisier,” Vol. 3, Table 9, Paris, Son Excellence le ministre de l’instruction publique et des cultes, 1882-1893. (25) Mouohot, A., “La chaleur solaire et ses applications industrielles,” Paris, Gauthier-Villars, 1869. (26) Pasteur, F., Compt. Rend., 30, 187, 1928. (27) Richards, J., Recherches et Inventions, 8 , 474-5 (1927). (28) Simpson, H. S., and Palmer, E. J. (to Allis Chalmers Co.), U. S. Patent 2,424,142 (July 15, 1947). (29) Telkes, M., “Distilling water with solar energy,” unpublished report (January 1943). (30) Telkes, AI., OSRD Rept., KO.5225, OT8, PB 21120 (May 1945). (31) Cshakoff, E. A. (by direct, and mesne assignments, of 35% t o S. A. Baron, Kew Orleans, La., for the benefit of himself and F. A. Middleton), U. S.Patents 2,455,834 and 2,455,835 (Dec. 7, 1948). (32) Wheeler, N. W., and Evans, IV. W., Ibid., 102, 633 (May 3 , 1870). RECEIVED for review April 17, 1961. A C C E P T E D February 24, 1953, Presented before the Division of Agriculture and Food Chemistry, 119th Meeting, AMERICAAC R E m c A L S O C I E T Y , Boston, AIass. Publication No. 22 of the Solar Energy Conversion Project.
0
0 0
Concurrent Production of Fatty Alcohols and Sodium Cyanate
0
FRED 0. BARRETT, J. D. FITZPATRICKI,
RICHARD G. KADESCH
AND
Emery Industries, Inc., Cincinnati, Ohio
T
HE original Bouveault-Blanc method ( 4 ) for the reduction of an ester to the corresponding alcohol employed an excess of sodium and ethyl alcohol. The procedure has been modified by using toluene (6) or petroleum ether ( 3 ) as a solvent. Yields are only 70 to 80% because of side reactions including acetoacetic ester condensation and evolution of hydrogen. Further improvements involving careful adjustment of the ratio and quantities of reactants ( 2 2 ) and the use of a secondary reducing alcohol ( 7 ) have given yields over 90%. This improved process is applied commercially to produce fatty alcohols from tallow, hydrogenated tallow, and coconut oil (9). Methyl isobutyl carbinol is used as reducing alcohol and toluene as solvent. When reduction is complete the mixture is quenched in water, and the solvent-fatty alcohol layer is separated and distilled. The aqueous glycerol-sodium hydroxide layer is conveniently utilized in soap manufacture. As four atoms of sodium are used in the reduction of each ester group, the amount of by-product sodium hydroxide is large:
0
/I
CHzOCR
I
f
i;!
CH-OCR
address, Applied Science Research Laboratory, University of Cincinnati, Cincinnati 21, Ohio.
1
--+
OH
CHzOCR CH20h-a
I I
CHONa
+ 3 RCHzONa + 6 CH3CHCHzCH(CH3)z 1
CHzONa CHzONa
I I
CHONa CHzO?ia CHzOH I
6HOH 1 Present
+ 12 Na + 6 CH3CHCHzCH(CH3)2
I
CHZOH
(1)
OXa
+ 3 RCHzONa + 6 CH&HCH2CH(CH& + 12 H20+ 1
ONa
+ 3 RCH,OH + 6 CH3CHCH2CH(CH& + 12 NaOH 1
OH
(2)
