D

the production rate, and the setpoint of the total B flow controller is ratioed to the flowrate of A. The control scheme features reflux ratio control and temperature controllers in both columns.

The alternative flowsheet used just one column and is more economical, but it presents a much more difficult control problem. The operation is "neat," meaning that the amounts of the two reactants fed are exactly balanced so as to satisfy the reaction stoichiometry. Figure 9.5 illustrates the system for the case when the reaction produces two products (A + B C + D), which go out the two ends of the column. The two temperature controllers achieve the balancing of the reactants. With two products, the column temperature information can be used to detect whether more or less of each reactant should be fed. The production of methyl acetate and water from methanol and acetic acid is an example of this type of reactive distillation system.

A commonly committed error in these two-reactant feed systems is to assume that a control structure with one feed ratioed to the other will provide effective control. This scheme does not work because of inaccuracies in flow measurements and changes in feed composition. Remember in neat operation that the reactants must be balanced down to the last molecule. This can be achieved only by using some sort of feedback information from the process that indicates a buildup or depletion of reactant.

However, consider the case when there is only one product: the reaction A + B C. Now the column temperature information is not rich enough to use to balance the stoichi-ometry. This means that the measurement and control of an internal column composition must be used in this neat operation. An example of this type of system is shown in Figure 9.6. The production of ETBE from ethanol and isobutene produces a heavy product, which goes out the bottom of the column. The C4 feedstream contains inert

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