10 Evaporation Howard l。 Freese 1.0 INTRODUCTION Evaporation is the removal of solvent as a vapor from a solution or slurry. The vapor may or may not be recovered, depending on its value. The end product may be a solid, but the transfer of heat in the evaporator must be to a solution or a suspension of the solid in liquid if the apparatus is not to be classed as a dryer. Evaporators are similar to stills or re-boilers of distillation columns, except that no attempt is made to separate components of the The task demanded of an evaporator is to concentrate a feed stream by removing a solvent which is vaporized in the evaporator and, for the greatest number of evaporator systems, the solvent is water. Thus, the"bottoms oduct is a concentrated solution, a thick liquor, or possibly a slurry. since the bottoms stream is most usually the desired and valuable product, the overhead"vapor is a by-product of the concentration step and may or may not be recovered or recycled according to its value. This determination may bemade upon incremental by-product revenues for reusable organic solvents or upon minimizing incremental processing costs for water vapor which may be slightly contaminated and must be further treated to meet environmental constraints. The solvent vapors generated in an evaporator are nearly alway condensed somewhere in the process, with the exception of solar evaporation systems(ponds) which evaporate into the local atmosphere 47610 Evaporation Howard L. Freese 1 .O INTRODUCTION “Evaporation is the removal of solvent as a vapor from a solution or slurry. The vapor may or may not be recovered, depending on its value. The end product may be a solid, but the transfer of heat in the evaporator must be to a solution or a suspension of the solid in liquid if the apparatus is not to be classed as a dryer. Evaporators are similar to stills or re-boilers of distillation columns, except that no attempt is made to separate components of the vapor. ”[l] The task demanded of an evaporator is to concentrate a feed stream by removing a solvent which is vaporized in the evaporator and, for the greatest number of evaporator systems, the solvent is water. Thus, the “bottoms” product is a concentrated solution, a thick liquor, or possibly a slurry. Since the bottoms stream is most usually the desired and valuable product, the “overhead” vapor is a by-product of the concentration step and may or may not be recovered or recycled according to its value. This determination may be made upon incremental by-product revenues for reusable organic solvents, or upon minimizing incremental processing costs for water vapor which may be slightly contaminated and must be further treated to meet environmental constraints. The solvent vapors generated in an evaporator are nearly always condensed somewhere in the process, with the exception of solar evaporation systems (ponds) which evaporate into the local atmosphere. 4 76