As stated above, the object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy.
In most cases the solvent is water, heat is supplied by condensing steam, and the heat is transferred by indirect heat transfer across metallic surfaces. For evaporators to be efficient, the equipment selected and used must be able to accomplish several things:
- Transfer large amounts of heat to the solution with a minimum amount of metallic surface area. This requirement, more than all other factors, determines the type, size, and cost of the evaporator system.
- Achieve the specified separation of liquid and vapor and do it with the simplest devices available. Separation may be important for several reasons: value of the product otherwise lost; pollution; fouling of the equipment downstream with which the vapor is contacted; corrosion of this same downstream equipment. Inadequate separation may also result in pumping problems or inefficient operation due to unwanted recirculation.
- Make efficient use of the available energy. This may take several forms. Evaporator performance often is rated on the basis of steam economy-pounds of solvent evaporated per pound of steam used. Heat is required to raise the feed temperature from its initial value to that of the boiling liquid, to provide the energy required to separate liquid solvent from the feed, and to vaporize the solvent. The greatest increase in energy economy is achieved by reusing the vaporized solvent as a heating medium. This can be accomplished in several ways to be discussed later. Energy efficiency may be increased by exchanging heat between the entering feed and the leaving residue or condensate.
- Meet the conditions imposed by the liquid being evaporated or by the solution being concentrated. Factors that must be considered include product quality, salting and scaling, corrosion, foaming, product degradation, holdup, and the need for special types of construction.
Today many types of evaporators are in use in a great variety of applications. There is no set rule regarding the selection of evaporator types. In many fields several types are used satisfactorily for identical services. The ultimate selection and design may often result from tradition or past experience. The wide variation in solution characteristics expand evaporator operation and design from simple heat transfer to a separate art.
