Energetics
Water can be removed from solutions in ways other than evaporation, including membrane processes, liquid-liquid extractions, crystallization, and precipitation. Evaporation can be distinguished from some other drying methods in that the final product of evaporation is a concentrated liquid, not a solid. It is also relatively simple to use and understand since it has been widely used on a large scale, and many techniques are generally well known. In order to concentrate a product by water removal, an auxiliary phase is used which allows for easy transport of the solvent (water) rather than the solute. Water vapor is used as the auxiliary phase when concentrating non-volatile components, such as proteins and sugars. Heat is added to the solution, and part of the solvent is converted into vapor. Heat is the main tool in evaporation, and the process occurs more readily at high temperature and low pressures.
Heat is needed to provide enough energy for the molecules of the solvent to leave the solution and move into the air surrounding the solution. The energy needed can be expressed as an excess thermodynamic potential of the water in the solution. Leading to one of the biggest problems in industrial evaporation, the process requires enough energy to remove the water from the solution and to supply the heat of evaporation. When removing the water, more than 99% of the energy needed goes towards supplying the heat of evaporation. The need to overcome the surface tension of the solution also requires energy. The energy requirement of this process is very high because a phase transition must be caused; the water must go from a liquid to a vapor.
When designing evaporators, engineers must quantify the amount of steam needed for every mass unit of water removed when a concentration is given. An energy balance must be used based on an assumption that a negligible amount of heat is lost to the system's surroundings. The heat that needs to be supplied by the condensing steam will approximately equal the heat needed to vaporize the water. Another consideration is the size of the heat exchanger which affects the heat transfer rate.
Some common terms: A = heat transfer area, and q = overall heat transfer rate.
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