An evaporator in a chemical plant or a fermentation operation is a highly-engineered piece of processing equipment in which evaporation takes place. The process and mechanical computations that are required to properly design an evaporator are many and very sophisticated, but the basic principles of evaporation are relatively simple, and it is these concepts that the engineer or scientist involved in fermentation technology should comprehend.
Often an evaporator is really an evaporation system which incorporates several evaporators of different types installed in series. All evaporators are fundamentally heat exchangers, because thermal energy must be added to the process, usually across a metallic barrier or heat transfer surface, in order for evaporation to take place. Efficient evaporators are designed and operated according to several key criteria:
1. Heat Transfer. A large flow of heat across a metallic surface of minimum thickness (in other words, high heat flux) is fairly typical. The requirement of a high heat transfer rate is the major determinate of the evaporator type, size, and cost.
2. Liquid-Vapor Separation. Liquid droplets carried through the evaporator system, known as entrainment, may contribute to product loss, lower product quality, erosion of metallic surfaces, and other problems including the necessity to recycle the entrainment. Generally, decreasing the level of entrainment in the evaporator increases both the capital and operating costs, although these incremental costs are usually rather small. All these problems and costs considered, the most cost-effective evaporator is often one with a very low or negligible level of entrainment.
3. Energy Efficiency. Evaporators should be designed to make the best use of available energy, which implies using the lowest or the most economical net energy input. Steam-heated evaporators, for example, are rated on steam economy—pounds of solvent evaporated per pound of steam used.
The process scheme or flow sheet is a basis for understanding evaporation and what an evaporator does. S ince the purpose of an evaporator is to concentrate a dilute feed stream and to recover a relatively pure solvent, this separation step must be defined. Figure 1 is a model for any evaporator, whether a simple one-pass unit or a complex multiple-effect evaporation system, which considers only the initial state of the feed system and the terminal conditions of the overhead and bottoms streams. The model assumes: steady-state conditions for all flow rates, compositions, temperatures, pressures, etc.; negligible entrainment of nonvolatile or solid particulates into the overhead, and no chemical reactions or changes in the chemical constituents during the evaporation process.
Example: In the production of Vitamin C, a feed stream containing monoacetone sorbose (MAS), organic salts, and water is to be concentrated. The feed rate is 4,000 lb/hr, and contains 30% by weight water. If the desired bottoms product is 97% solids, how much water is evaporated?
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