Operational considerations reflux effects

In batch distillation, as the overhead composition varies during operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main-cuts are usually determined by the market or downstream process requirements but the amounts recovered must be selected based on the economic trade off between longer distillation times (hence productivity), reflux ratio levels (hence energy costs), product values, etc. Increasing the recovery of a particular species in a particular cut may have strong effects on the recovery of other species in subsequent cuts or, in fact, on the ability to achieve at all the required purity specifications in subsequent cuts. The profitable operation of such processes therefore requires consideration of the whole (multiperiod) operation. 

Increased reflux ratio has the effect of decreasing the required number of theoretical contacts, but it increases the internal flow, equipment diameter, and energy requirements. It is frequently necessary to solve the separation problem a considerable number of times, imposing various operating conditions and carefully ascertaining the effect on the total cost of the particular solution proposed. Optimum design conditions are discussed in Chapter 13. 

Hydration of ethylene oxide to ethylene glycol has also received considerable attention because of the potential benefits that RD can offer for this reaction [40-42]. The reaction is exothermic and the heat can be utilized effectively in the RD moreover, the control over temperature that is desired for avoiding diethylene glycol formation, can easily be achieved in RD. It is a multi-feed operation in which ethylene oxide is fed at the bottom of the column and water feeds can be introduced at several locations in the upper section. The column is operated under total reflux. 

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