A SYSTEM FOR CONTROLLING WATER EVA PORAT ION CASTLE 0 . R E I S E R
Chemical Engineeving Department, Arizona State University, Tempe, Arir. 8528 I
A wind-regulated system has been developed to apply long-chain alcohols for suppression of water evaporation from reservoirs. Film characteristics and chemical requirements were determined from the operation of a prototype and a large water-wind tunnel. A dilute water suspension produced by metering molten alcohol into a jet mixer is pumped into a peripheral pipe supplying spray lines on the windward shore. The film is removed at a rate proportional to ' / a 1 of the wind speed. At winds above 9 miles per hour alcohol additions must be increased as the square of the wind speed in order to suppress waves which would submerge the film. With a 50% reduction in evaporation, the cost of the water saved on an ideal 2500acre circular lake is about $6 per acre foot and decreases with increasing reservoir size.
HE evaporation loss from large lakes and reservoirs in the
T17 western states has been estimated a t over 14,000,000 acre feet, an amount which could supply the annual water needs of 84,000,000 suburbanites (Florey, 1966) ; and losses from Lake Mead are nearly equal to the water consumption of the Los Angeles metropolitan area. The possibility of suppressing evaporation by a monomolecular film was suggested by Langmuir early in this century and in 1932 he received the Nobel prize for his research on the nature of chemical films. Controlling the evaporation of water by using surface films has been studied extensively in Australia, the United States, Israel, the U.S.S.R., and India (Frenkiel, 1965). In this country, the Bureau of Reclamation has conducted many laboratory and field tests and has supported research a t a number of universities. I t has sponsored research at Arizona State University for the design and development of an automatic prototype system using wind-regulated controls (Reiser, 1966). Large scale tests applying long-chain alcohols in a batchwise manner have achieved questionable results. Since the surface film is blown off at a rate of about I / ~ Oof the wind speed, continuous application on the windward shore is needed to maintain an unbroken cover. Because surface films reduce evaporation losses, there is a corresponding increase in water temperatures beneath the film; and this effect has been used by the Japanese to increase the water temperature of rice fields. When the film is blown off, the increased surface temperatures result in higher evaporation rates and the net evaporation savings are greatly reduced. I n a controlled test with continuous application of a water suspension of hexadecanol on a small pond (Crow, 1961) the observed evaporation reduction was 48.7y0 for the first three days, 4o.5yOfrom the 3rd to 13th day, and 35% from the 13th to 18th day. When application of this film was stopped during the night, the 25y0 evaporation reduction achieved in a longterm continuous test dropped to 6.5y0. Field tests conducted by the Bureau of Reclamation a t Lake Hefner in Oklahoma, Sahuaro Lake in Arizona, and Lake Cachuma in California (Garstka, 1962) were unsuccessful in maintaining a surface film a t wind speeds much above 12 miles per hour and the cost of the water saved was estimated to be between $60 and $70 per acre foot for evaporation reductions ranging from 8 to 22%. Water-insoluble, long-chain organic molecules with a hydro-
phylic end group tend to produce a vertically oriented monomolecular film that is very effective in suppressing water evaporation. The resistance to water transport through a compressed film increases with the length of the molecule (LaMer, 1962), but the healing and spreading ability decreases with chain length. Ethylene oxide ethers of long-chain alcohols have enhanced spreading properties, and monolayers of ethoxylates as well as mixtures of ethoxylates and alcohols are reported to form a more durable and effective film than pure alcohols (Frenkiel, 1965). Evaporation reductions of 90% have been measured in laboratory tests of docosanol ethoxylate films. Surface films have been generated by applying alcohols in a variety of forms. The fast spreading rate of alcohol in water dispersions has been attributed to the prewetting of the small particles (Dressler, 1964). Goldstein (1963) and others have obtained similar rates for alcohol in suspensions and powder form which produced rates of 9 to 17 cm. per second a t 35OC. However, the amount of powdered alcohol required was many times greater than if applied in the form 6f a suspension or hydrocarbon solution. Spreading rates of about 12 cm. per second were obtained from gasoline and kerosine solutions, but the increased cost and hazards of hydrocarbon solutions generally preclude their consideration. The highest evaporation suppression efficiencies have been obtained from the application of alcohol suspensions (Cluff, 1966). I n the design of an evaporation-control system, continuous maintenance of the surface film is paramount. Since the film is blown off by the wind, film-forming material must be continually applied from the upwind shore with provision for changing the points of application with shifting wind directions. Its addition rate must be sufficient to suppress waves and form a resistant surface layer. Application by boats or airplanes generally does not satisfy these requirements. The economy of the process is greatly affected by the amount of material used and addition of the dry powder as a dust or spray from sources that are spaced close enough to give a complete film at higher wind speeds usually uses much more than the minimal amount. Since metering the small quantities required to form a monolayer is difficult if not impossible, dilution of the alcohol approximately a thousandfold before its application as a finely divided suspension in water seems the most feasible method. The criteria selected for the design of a feasible system were: application of the chemical in a continuous process VOL. 8 NO.
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JANUARY 1 9 6 9
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Figure 1. Prototype jet mixer used to prepare alcohol in water suspensions
WIND VELOCiTY (V),FT/SEC
Figure 3. Film-removal rate vs. wind speed Rater measured during continuous application of c1-8 alcohol suspensions with and without ethoxylde adduct when wind meter 10% 4 feet above waterrurfoce
Figure 2.
l a r g e water-wind tunnel used for film studies
using a dilute water suspension, distribution on the windward shore from a continuous source or point sources spaced close enough to provide an unbroken surface cover, and wind regulation of the make-up rate to compensate for the wind removal. Pumping costs and water evaporation from the sprayed material should be minimized by maximizing the concentration of the alcohol in the water suspension. Experimental Program The design of an evaporation system based on these principles required the initiation of a research program to select application devices and their spacing as determined by the spreading rate of the material, to find a simple means of producing a stable alcohol suspension, and to determine the windremoval rate. The lateral spreading of an alcohol film was measured by spraying a suspension on the surface of water fiowing in an irrigation canal and observing the leading angle of the surface film. Correlation of the film angle and the water velocity gave an alcohol spreading rate of approximately 0.5 f w t per second across the canal. The small jet sprays (Spraying Systems Model '/a K. 50) which were selected because of their low flow rates and nonclogging characteristics produced a fiat spray about 11 feet wide and 5 feet long at 30 p.s.i.g. A vector analysis based on wind removal and lateral spreading rates indicated that a 20-foot jet spacing would provide a continuous film within 16 feet of the shore line a t wind speeds of 25 miles per hour. Stable alcohol suspensions are readily produced in a hatch mixer by cooling a preheated mixture of water, alcohol, and a stabilizer to a temperature below the melting range of the alcohol, An alcohol sulfate amounting to about 1% of the alcohol is an effective stabilizer for slurries produced in this manner. Although this mixing method can be used in a continuous system, it gives control problems and requires 64
l & E C P R O C E S S D E S I G N A N D DEVELOPMENT
excessive heating energy. Better results were obtained with a jet mixer inducting holten alcohol into a high velocity water stream. Injection into cold water produced a finely divided alcohol suspension and provided good control o f the rate of alcohol addition, but agglomeration of the alcohol produced severe plugging problems in the small sprays which had a diameter of 0.024 inch. In laboratory tests using heated water, a stable suspensionof alcohol particles averagingabout 5 microns in diameter was produced when molten alcohol that contained 10% ethylene oxide adduct was inducted into water at 140'F. The jet mixer used for these preparations is sketched in Figure 1. Energy for heating may be conserved by making a concentrated slurry of about 12% alcohol and then diluting with cold water to the desired concentration by passing the concentrated slurry into a second jet mixer or by piping it to the inlet of a centrifugal pump. Wind-removal rates and wave-suppression studies were carried on in a uniquely large water-wind tunnel constructed under the sponsorship of the Bureau of Reclamation. This tunnel, shown in Figure 2, has an &foot square air passage above a water tunnel which is 6 feet deep and 224 feet long. A two-speed fan with adjustable propellers pulls air through the wind tunnel. Excellent views of the wave disturbances are provided in the below-grade observation pit located on one side of the tunnel, An alcohol suspension was applied through a small spray centered at the entrance end. The alcohol film was removed from the outlet fan end by a skimming plank and trap. Although the surface film is generally observed hy its smoothing effect, the advancing and receding fronts are difficult to detect. Lycopodium powder, flowers of sulfur, camphor, and carbon black have been used to help define surface coverage. Film travel rates at different wind speeds during continuous film application were measured with the aid of sulfur and are shown in Figure 3. No difference was detected between the travel rate of the film and an u?covered water surface influenced by a wind. The ratio df the film to wind travel averaged 1 to 31, as compared with the generally used ratio of 1 to 30. O n an uncovered surface, the advancing film rate was increased by the self-spreading of the alcohol. Similarly, the retraction rate when the alcohol application was stopped dropped below the
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window alcohol required appears to increase as the square of the wind speed. The alcohol required at various wind speeds was determined in wind tunnel tests as shown in Fieure 4. and is riven in
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WIND V E L O C I T Y ( V l . M I L E S / H R
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Figure 4. Alcohol application rates required for smooth surface film Rater determined in water-wind tunnel tests with wind meter 4 feet above water surface
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whe... ."_.I ___.__" r-. .._ molecular weight of the film-forming material per bydrophylic group. Over long expanses, the make-up rate may have to be increased to compensate for biological attrition, solution, or evaporation. Addition rates will also have to be increased if the film-forming material is added to a surface with well developed waves. The above data were obtaine:d with thc spray located less than 20 feet from the front edge of the water. When the suspension was sprayed a n a wave-swept s;urface neai the middle of the tunnel. the alcohol reauired for wirvr DuuuIcasion was about doubled, but the limited tunnel length precluded accurate estimates of the quantity required. The turbulent surface generated with a 25-mile wind is shown in Figure 5. Suppression of the waves by an advancing film front formed by the application of a 0.25y0alcohol suspension may be seen in Figure 6 . Smooth surfaces were produced with wind speeds up to 27 miles per hour. These velocities were measured by a cup anemometer located at the center of the wind tunnel 4 feet above the water surface. 1.
n g u r r 5. Waves in water-wind tunnel with 25 mile-perhour wind
Design
