Distillation control fundamental variables

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. 

Chapters 2, 3, and 4 deal with the distillation variables, and Chapter 5 covers distillation process control strategies. Chapter 6 describes some of the constraints on distillation variables and separation capabilities. Chapter 7 introduces the concepts that are critical to product quality and the measurements that evaluate performance criteria such as frequency of failure. Chapter 8 describes the concepts and nomenclature that are fundamental to PID control loops. Chapter 9 covers the concepts of tuning process controllers when they are operating in automatic output mode. Chapter 10 is about measuring the response of process variables when the controller is in manual output mode, that is, with no feedback from the process variable. 

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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