Seawater evaporation

FIG. 11-21 Heat- transfer coefficients in LT - seawater evaporators, = ( F — 32)/l,8 to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply hy 5,6783,... 

Seawater Evaporators The production of potable water from saline waters represents a large and growing field of application for evaporators. Extensive work done in this field to 1972 was summarized in the annual Saline Water Conversion Repoi ts of the Office of Sahne Water, U.S. Department of the Interior. Steam economies on the order of 10 kg evaporation/kg steam are usually justified because (1) unit production capacities are high, (2) fixed charges are low on capital used for pubhc works (i.e., they use long amortization periods and have low interest rates, with no other return on investment considered), (3) heat-transfer performance is comparable with that of pure water, and (4) properly treated seawater causes httle deterioration due to scahng or fouhng. 

FIG. 11-125 Flow sheets for seawater evaporators, a) Multiple effect (falling film), (h) Multistage flash (once-through). (c) Multistage flash (recirculating). 

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation. 

In a series of papers, Harvie and Weare (1980), Harvie el al. (1980), and Eugster et al (1980) attacked this problem by presenting a virial method for computing activity coefficients in complex solutions (see Chapter 8) and applying it to construct a reaction model of seawater evaporation. Their calculations provided the first quantitative description of this process that accounted for all of the abundant components in seawater. 

We see rivers flowing continuously into the sea. We know that with the aid of the sun, seawater evaporates and joins the clouds. Clouds, with the aid of winds (generated largely by differential heating by sun) reach mountain peaks and build up further the ice caps already present. In another stage, sun helps to melt the ice caps, which maintain the flow of water in perennial rivers. The situation appears so well balanced so much in equilibrium so orderly. Is it really so ... 

Current economic and ecological analyses of the various processes available for recovery of minerals from seawater (evaporation, solvent extraction, sorption, ion exchange, flotation, fractional precipitation, distillation, electrolysis, electrodialysis, and electrocoagulation) favor ion exchange and sorption technology. 

The solute and salt sequence for the final stages of seawater evaporation has been determined computationally (Eugster et a/., 1980), and in commercial solar salt ponds (Hermann et al., 1973). The only equivalent information on a saline lake system of considerable size has been provided for the Great Salt Lake (GSL) area, also from evaporation ponds (Jones et ah, 1997) and by direct computation. Kohler (2002), utilized the computer model of Moller et al. (1997), allowing for the dominance of halite in the GSL system, and worked out a precipitation sequence essentially of Ca-sulfate to Mg-sulfate to MgCl2,... 

The reported boron concentrations in Gulf Coast (USA) brines range from a few to —700mgL (Kharaka et al., 1987 Land and Macpherson, 1992a). Dissolved boron shows no correlation with chloride concentration but does show some increase with depth and temperature (Kharaka et al., 1985). The B/Br ratios are highly elevated relative to the seawater evaporation trend for boron and bromide, reflecting derivation of almost all of the boron from rock and/or organic sources. 

Figure 13 Elucidation of saline sources by using the variations of Br/Cl and B/Cl ratios. Note the expected geochemical distinction between seawater, evaporated seawater, brines (e.g.. Dead Sea), hydrothermal fluids, sewage effluents, agricultural drainage (e.g., Salton Sea), and evaporite dissolution.

Raab M. and Spiro B. (1991) Sulfur isotopic variations during seawater evaporation with fractional crystallization. Chem. Geol. 86, 323-333. 

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