Single Reactive Column Control Structures

Two different control structures are explored for the single reactive column process. The first is the two-temperature structure considered in Chapter 10. This system uses only temperatures and does not require a direct composition measurement. The second control structure uses a composition analyzer to measure a composition of one of the reactants on a selected tray in the column to adjust one of the fresh feeds. 

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SINGLE REACTIVE COLUMN CONTROL STRUCTURES 267 RDSVD... 

The reactive distillation column described in the previous section was designed to operate neat (precisely the correct amounts of reactants are fed to the column to satisfy the stoichiometry of the chemistry and there are only small amounts of unreacted reactants that leave in the streams leaving the column). Only a single column is required, so both capital investment and energy cost are minimized. However, it can be difficult to control a reactive column that operates in this neat mode. The problem is the need to feed in exactly enough of both reactants, down to the last molecule, to make sure that there is no excess of either reactant. If the balance is not absolutely perfect, the reactant that is in excess will gradually buUd up in the column, and it will not be possible to maintain product purities. This build-up may take hours or days, but eventually the column control structure will not be able to hold the products at their specified compositions. 

Figure 11.15 shows an alternative control structure for the single reactive column operating in neat mode. Instead of controlling two temperatures by manipulating the two fresh feed-streams and setting the production rate by vapor boilup (Fig. 11.4), this alternative scheme... 

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