Evaporation is a process that involves the removal by vaporization of part of the solvent from a solution, with the objective being to concentrate the solution. In the evaporation of solutions containing biological compounds, the volatile solvent can be water or an organic solvent. Organic solvents are frequently used for antibiotics, steroids, and peptides. Often the solution is under a moderate vacuum, at pressures down to about 0.05 atm absolute , which is especially important for heat-sensitive biologicals where the temperature should be as low as possible to minimize degradation. The energy source for evaporation is usually steam at a low pressure, below 3 atm absolute . Evaporation processes typically occur after the processes used for the removal of insolubles. They are often used to concentrate a solution just prior to the bioproduct being crystallized or precipitated. Evaporation can often be coupled with extraction: for example, a bioproduct is extracted from an aqueous stream with an organic solvent, and the extract is sent to an evaporator for concentration. In this chapter, the basic principles of evaporation are discussed, followed by a description of the most common types of evaporators for heat sensitive biological products and a discussion of scale-up and design methods. After completing this chapter, the reader should be able to do the following: • Explain the different types of resistances to heat transfer in an evaporator. • Take into account the boiling point elevation in heat transfer calculations for evaporators. • Calculate the heat transfer resistances and residence time for the concentration of a heat-sensitive bioproduct in a falling film evaporator. • Estimate the fouling factor in an evaporator. • Calculate the maximum allowable vapor velocity from an evaporator. • Select an appropriate type of evaporator to use based on the specific operational and product characteristics. • Size evaporators based on specific operating conditions and the expected overall heat transfer coefficient. The main principles to consider for evaporators are heat transfer and vapor-liquid separation. The theoretical basis of these principles will be discussed.
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