Fundamental Composition-Control Manipulated Variables

In the context of partial control, as we discussed for reactors, we must also identify the dominant variables in the unit and their relationship with the available manipulators. 

A useful way to think about the remaining two degrees of freedom that can be utilized for composition control is to classify them as two fundamental manipulated variables fractionation and feed split. 

Feed split means the fraction of the feed that leaves in one product stream, e.g., the DIF ratio. Feed split can be set directly and explicitly by using either D or B to control one composition. Or it can be set indirectly and implicitly by using reflux or vapor boilup to control one composition and removing D or B to hold reflux drum or base level. 

However we choose to look at it, a basic distillation column has two control degrees of freedom. When we turn to more complex column configurations with sidestreams, side strippers, side rectifiers, intermediate reboilers and condensers, and the like, we add additional control degrees of freedom. These more complex systems are discussed in Sec. 6.8. 

One of the challenging aspects of distillation column control is the many limitations imposed on the operation of the column. There are hydraulic constraints, separation constraints, heat-transfer constraints, pressure constraints, and temperature constraints. We recommend the excellent books by Kister (1992 and 1990) on distillation design and operation. 

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No matter what manipulated variables are chosen to control what controlled variables, there are basically two fundamental manipulated variables that affect compositions. These are feed split and fractionation. ... 

Distillation is one of the most important unit operations in chemical engineering. It forms the basis of many processes and is an essential part of many others. It presents a more difficult control problem then with many other unit operations, as at least five variables need to be controlled simultaneously and there are at least five variables available for manipulation. Thus, a distillation column provides an example of a multiple-input-multiple-output control problem. It is critical that variable pairing is done appropriately between controlled and manipulated variables. The overall control problem can usually be reduced to a 2 x 2 conposition control problem since the inventory and pressure loops frequently do not interact with the composition loops. This workshop will highlight some fundamental mles of distillation control and show how a basic distillation control scheme can be selected. 

Before delving into the feed rearranging control structure, we first construct the fundamental control configuration for the reactive distillation with two feeds. Recall that, unlike the control of conventional distillation systems, we need to control the internal composition (or temperature) to maintain stoichiometric amounts of the two fresh feeds. For the purpose of illustration in this work, we choose to control the composition of reactant A on tray 13 where a large change in the composition of A is observed (Fig. 18.5b). Thus, we have three compositions to be controlled top composition of C, bottoms composition of D, and composition A on tray 13. For the manipulated variables, the ratio scheme is used these three ratios are reflux ratio, boilup ratio, and feed ratio. Figure 18.12 shows the control structure. 

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