Evaporators Solar

Solar evaporation Solarization Solar panels Solar ponds Solar power systems Solar radiation Solar salt... 

The simple control scheme in Figure 2.103 measures the temperature difference (ATC-1) between the solar collector temperature and the solar water tank temperature, starts the water pump when AT > 20°C, and stops it when this temperature difference drops to 10°C. The variable-speed refrigerant pump circulation is similarly controlled. Because the condenser (lake, well, or swamp) temperature is more or less constant, AT control might not be essential and measuring only the evaporator (solar water tank temperature) by a standard thermostat (TC-2) can be sufficient. In this configuration, the refrigerant pump is started whenever the solar tank temperature is above 60°C (140°F), and the pump speed is increased to its maximum, as the temperature in the solar hot water tank reaches, say, 70°C (160°F). 

The economics of a combination sea water evaporator—solar heat collector have been examined on the basis of current technology, using 1960 construction costs. A multistage flash evaporator was selected. A Tedlar-covered solar heat collector was selected to heat the brine. 

Manguo, J.L. and Schwartz, J.C. (1985) Simulation of an evaporative solar salt pond. Industrial and Engineering Chemistry Process Design and Development, 24, 1245-1251. 

Another mining process involves the recovery of sodium carbonate decahydrate from alkaline ponds. EMC mines this material from its solar evaporation pond using a bucket wheel dredge. The decahydrate slurry is dewatered, melted, and processed to soda ash. 

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

Sa.Ia.rs and Lakes. Brines having high lithium concentration are found in salars of northern Chile, southwestern Bohvia, and northwestern Argentina. Brines of lower lithium concentration are found in salars in the western United States and the Tibetan Plateau. Brines pumped from beneath the surface of the Salar de Atacama (Chile) and Silver Peak (Clayton Valley, Nevada) are used for commercial production of lithium uti1i2ing solar evaporation (see Chemicals frombrines). The concentration of selected ions in brines from salars and lakes of potential commercial interest worldwide are shown in Table 1. 

Recovery from Brines. Natural lithium brines are predominately chloride brines varying widely in composition. The economical recovery of lithium from such sources depends not only on the lithium content but on the concentration of interfering ions, especially calcium and magnesium. If the magnesium content is low, its removal by lime precipitation is feasible. Location and avadabiHty of solar evaporation (qv) are also important factors. 

Fig. 1. Principal salt deposits and dry salt production sites in North America, where H represents the salt deposits and D, x, and Q correspond to evaporated, rock, and solar production sites, respectively. Sites in Canada, Hsted by company name (location), are Alberta, , Canadian (Lindberg) Saskatchewan, , Sifto/Namsco (Unity) and Canadian (BeUe Plaine) Ontario, and x, Sifto/Namsco (Goderich) , Canadian (Windsor) x, Canadian (Ojibway) New Bmnswick, x. Potash Corp. of Saskatchewan (Sussex) and Nova Scotia, and x, Canadian (Pugwash) and Mines Seleine (Quebec), and,...

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

Highway. Rock salt, solar salt, and in some cases in Europe, evaporated salt are used to maintain traffic safety and mobiUty during snow and ice conditions in snowbelt regions throughout the world. Sodium chloride melts ice at temperatures down to its eutectic point of —21.12°C. Most snowstorms occur when the temperature is near 0°C, where salt is very effective. More than 40% of dry salt produced in the United States is used for highway deicing. 

SQM Nitratos (Chile) operates two sodium nitrate plants in northern Chile Pedro de Valdivia and Mama Elena, about 30 km distant from one another. The caUche is mined in open-pit areas. A solar evaporation plant, Coya Sur, Hes in between. A flow sheet of the processing operations for sodium nitrate production is shown in Figure 2. 

At Great Salt Lake Minerals Corporation (Utah), solar-evaporated brines are winter-chilled to —3° C in solar ponds. At this low temperature, a relatively pure Glauber s salt precipitates. Ponds are drained and the salt is loaded into tmcks and hauled to a processing plant. At the plant, Glauber s salt is dissolved in hot water. The resulting Hquor is filtered to remove insolubles. The filtrate is then combined with soHd-phase sodium chloride, which precipitates anhydrous sodium sulfate of 99.5—99.7% purity. Great Salt Lake Minerals Corporation discontinued sodium sulfate production in 1993 when it transferred production and sales to North American Chemical Corporation (Trona, California). 

Action of Vacuum on Spacecraft Materials. For service beyond the atmosphere, the vacuum environment allows materials to evaporate or decompose under the action of various forces encountered (1,18,19). These forces include the photons from the sun, charged particles from solar wind, and dust. The action of space environment on materials and spacecraft can be simulated by a source—sink relationship in a vacuum environment. Thus, for example, the lifetime of a solar panel in space operation may be tested (see Photovoltaic cells). 

Because water of depths below about 2 m does not absorb much solar radiation direcdy, the radiation is absorbed and converted to heat primarily in the basin floor, which thus should have high radiative absorptance in the solar radiation spectmm. It is also noteworthy that if the stUl is designed to have low heat losses to the ambient, and if the ambient temperature drops, distillation will continue for some time even in the absence of solar energy input, because the saline water may remain warmer than the condensing glass surface and thus continue evaporating. 

Solar Evaporation. Recovery of salts by solar evaporation (1 3) is favored in hot dry climates. Solar evaporation is also used in temperate 2ones where evaporation exceeds rainfall and in areas where seasons of hot and dry weather occur. Other factors (4,5) affecting solar pond selection are wind, humidity, cloud cover, and land terrain. 

Production of KCl at the Wendover, Utah operation employs a large 7000 acre complex of solar ponds. Both shallow brine wells and deeper wells are used to pump brine into the pond complex. In the preconcentration ponds water is evaporated and sodium chloride is crystallized. Later the brine is transferred to production ponds where sylvinite is deposited. Brine is then transferred to other ponds where camaUite is crystallized. Sylvinite is removed from drained ponds with self-loading scrapers and taken to the plant were KCl is separated by flotation with an amine oil collector. The camaUite,... 

Recovery Process. Figure 5 shows a typical scheme for processing sodium chlodde. There are two main processes. One is to flood solar ponds with brine and evaporate the water leaving sodium chlodde crystallized on the pond floor. The other is to artificially evaporate the brine in evaporative crystallizers. Industrial salt is made from solar ponds, whereas food-grade salt, prepared for human consumption, is mosdy produced in the crystallizers. 

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