Large-scale evaporation distillation

Normally, in evaporation the thick liquor is the valuable product and the vapor is condensed and discarded. In one specific situation, however, the reverse is true. Mineral-bearing water often is evaporated to give a solid-free product for boiler feed, for special process requirements, or for human consumption. This technique is often called water distillation, but technically it is evaporation. Large-scale evaporation processes have been developed and used for recovering potable water from seawater. Here the condensed water is the desired product. Only a fraction of the total water in the feed is recovered, and the remainder is returned to the sea. 

Another vacuum distillation technique, including for water removal in large-scale operations, is evaporation. Thin-film evaporators are designed to expose large surface areas of liquid to heat and/or vacuum in order to speed the evaporation process. The design principle is illustrated in Figure 5. 

Dissolved salts can be removed from seawater by distillation, a simple process in which the seawater is boiled to evaporate the volatile water. Pure water vapor is collected and condensed, leaving the nonvolatile salts behind. This process is quite energy intensive and is not practical for a large-scale operation. It is rarely used commercially. 

In large-scale applications, waste streams of BTFs may be safely reclaimed by distillation in an explosion-proof distillation unit or still. Most stills available today have reclaim efficiencies of 90 to 99%. Using a high efficiency, thin-film evaporation still, 95% or more of the BTF maybe recovered from the spent solvent sludge. To further increase the amount of reclaimed solvent and to reduce the volume of waste that must be disposed of, stills can be equipped to employ steam stripping. 

C change of water temperature vs = 2378 kJ/kg, respectively, and that the top temperature is limited by considerations of scaling and corrosion, dictate the requirement for a very large flow rate of the evaporating stream. For example, (in the following subscripts by dy and s refer to distillate, brine, and steam, respectively) in operating between a typical top temperature 90°C at the inlet to the evaporator and an exit temperature of 40°C... 

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