Neat operation

The control of the coupled reactor-regenerator is challenging because of the interaction between the two vessels and the neat operation in terms of energy. The temperatures in both vessels must be controlled at levels that are just below the metallurgical limits of the equipment materials. 

A commonly made mistake 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 compositions. Remember in neat operation the reactants must be balanced down to the last molecule. This can only be achieved 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 stoichiometry. 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 ethyl tcrt-butyl ether (ETBE) from ethanol and isobutene produces a heavy product, which goes out of the bottom of the column. The C4 feed stream contains inert components in addition to isobutene. These inerts go out of the top of the column. The production rate is set by the flow controller on the isobutene feed stream. The ethanol concentration on a suitable tray in the column is maintained by manipulating the ethanol fresh feed. ReboUer heat input controls a tray temperature in the stripping section to maintain ETBE product quality. 

NEAT OPERATION VERSUS USING EXCESS REACTANT... 

Many industrial reactive distillation systems do not use stoichiometric amounts of reactants. An excess (10-20% above the stoichiometric amount) of one of the reactants is fed to the reactive column. There may be kinetic reasons for using an excess in some systems. These include suppressing undesirable side reactions, reducing catalyst requirements, and increasing conversion. However, even in the absence of kinetic reasons, the use of an excess of one of the reactants makes the control problem easier because the fresh feed flowrates of the components do not have to be precisely balanced in the reactive column. Achieving this exact balance may require the use of expensive and high maintenance on-line composition analyzers in some systems. In addition, the variability of product quality may be larger in the neat operation process because there arc fewer manipulated variables available and there is only one column to contain disturbances. 

Figure 4.16 compares the reactive column composition profiles in the two excess reactant cases with those for base case neat operation. The concentrations of A are lower than the base case when an excess of B is used and larger when an excess of A is used. The opposite is true for the concentrations of B. The concentration of C at the top of the column is lower when an excess of A used. The concentration of D at the bottom of the column is lower when an excess of B used. 

The use of excess reactant has been demonstrated to increase capital and energy costs when compared to neat operation. The dynamic control of these two flowsheets will be compared in Ghapter 11. 

In Parts I and II we explored the steady-state designs of several ideal hypothetical systems. The following three chapters examine the control of these systems. Chapter 10 considers the four-component quaternary system with the reaction A + B C + D under conditions of neat operation. Chapter 11 looks at control of two-column flowsheets when an excess of one of the reactants is used. Chapter 12 studies the ternary system A + B C, with and without inerts, and the ternary system A B + C. We will illustrate that the chemistry and resulting process structure have important effects on the control structure needed for effective control of reactive distillation columns. 

However, if two products are produced, the temperature information may be rich enough to infer compositions with sufficient accuracy so that direct composition measurement is not absolutely necessary for neat operation. The ideal case considered by Roat et al. (A -f B C -f D) and the production of methyl acetate (methanol + acetic acid methyl acetate -f water) are examples of this type of chemistry. In this chapter, we consider the control of the ideal quaternary system. In Chapter 13 the control of the methyl acetate system and other similar real systems are explored. 

There are two reasons for the improvement in control. The first has already been discussed the elimination of inverse response. The second is equally important the two temperature controllers in the CS7-RR structure have the same action (direct). This means that when both control loops see a positive vapor boilup disturbance, which increases both tray temperatures, the two controllers will increase both fresh feeds. This helps to maintain the delicate stoichiometric balance between the reactants that is essential for neat operation of a reactive distillation column. Because a reactive distillation column acts like a pure integrator with respect to the reactants, this similar initial response is very important for CS7-RR, where feedstreams are used as manipulated variables. 

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