478 Fermentation and Biochemical Engineering Handbook 2. Liquid-Vapor Separation. Liquid droplets carried througl the evaporator system, known as entrainment, may con- tribute to product loss, lower product quality, erosion of metallic surfaces, and other problems including the ne cessity 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 ne with a very low or negligible level of entrainment 3. Energy Efficiency. Evaporators should be designed to make the best use ofavailable energy, which implies us- ing the lowest or the most economical net energy input Steam-heated evaporators, for example, are rated steameconomy-pounds of solvent evaporated per pound 2 The process scheme or flow sheet is a basis for understanding evaporation and what an evaporator does. Since 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 l 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,tempera- tures, pressures, etc. negligible entrainment of nonvolatile or solid particu- lates 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 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? Water. lb/hr 1,200 1.113 MAS and solids. 2,800 2.800 lb/hr Total. lb/hr 4,000 2.887 l.113478 Fermentation and Biochemical Engineering Handbook 2. Liquid-Vapor Separation. Liquid droplets carriedthrough the evaporator system, known as entrainment, may con￾tribute to product loss, lower product quality, erosion of metallic surfaces, and other problems including the ne￾cessity 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 costeffective evaporator is often one with a very low or negligible level of entrainment. 3. Energy Eficiency. Evaporators should be designed to make the best use ofavailable energy, which implies us￾ing 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 The process scheme or flow sheet is a basis for understanding evaporation and what an evaporator does. Since 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, tempera￾tures, pressures, etc.; negligible entrainment of nonvolatile or solid particu￾lates 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 Ibh, and contains 30% by weight water. If the desired bottoms product is 97% solids, how much water is evaporated? Feed Bottoms Distillate Water, lb/hr 1,200 87 1,113 MAS and solids, 2,800 2,800 None Total, lb/hr 4,000 2,887 1,113 lbh