Distillation columns basic control scheme

FIGURE 5.11-1 Some basic control schemes for composition control, distillation columns. 

Tolliver and McCune described four alternative material balance control schemes for distillation columns. Each scheme manipulates a different variable to control a temperature point in the column, that is, the D/F ratio, and this provides a structure for categorizing distillation control strategies (Table 5.1). There can be a number of variations of the four basic control schemes. 

In this chapter, four basic distillation control schemes were introduced along with a number of variations on each scheme. The concept was introduced that there can be more than one way to successfully control a distillation column. In some schemes, the separation power base is controlled by the ratio of steam/feed and then the distillate flow rate or reflux flow rate can be manipulated to control an MRT point in the distillation column. In other schemes, the separation power base is controlled by the ratio of reflux/feed, and the steam to the reboiler or the bottoms flow rate can be manipulated to control an MRT point in the column. Control of reflux drum level and column base level was presented as basic to all control schemes. Control of column pressure was considered to have an overriding effect on the stability of distillation column control. 

Figure 21.9 Typical Basic Control Scheme for a Binary Distillation Column...

In the previous chapter the procedure for the design of control schemes was discussed. The procedure was illustrated on a reactor with recycle. The selection of appropriate combinations of controlled and manipulated variables was relatively simple, since the interactions were limited. In this chapter the procedure will be applied to a distillation column. This is a unit operation with many interactions between the corrections that are made. Using a basic knowledge of the process dynamics, a basic control scheme is designed. Subsequently, two control schemes will be compared a basic control scheme based on material balance control and a control scheme based on "energy balance control. The distillation column can also be used to demonstrate the optimization of the control scheme. The principle is that the control scheme should be designed in such a way, that an objective function can be maximized. 

Steady-state simulation and design methods for separation processes, with emphasis on distillation, have been presented in detail in many references, a few of which are listed in the references for this chapter. This chapter will present a discussion of the basic control schemes for distillation columns. Let us start by stating the obvious tbe amount of literature on separation processes, particularly distillation, is colossal. Particularly readable books and references are those by Buckley and co-workers [1,2], King [3], Tyreus [4], Seborg et al. [5], Shinskey [6], Smith and Corripio [7], Svrcek and Morris [8], and Wilson and Svrcek [9]. 

There are several issues which need to be addressed when designing the basic control for the column. The first is one of pairing. We will see that on our simple column that there are five PVs that we must control - pressme, reflux drum level, column base level, distillate composition and bottoms composition. We normally have available five MVs - distillate flow, bottoms flow, reflux flow, reboiler duty and condenser duty. We need therefore to decide which MV is going to be used to control which PV. Theoretically there are 5 , or 120, possible combinations. While many of these are nonsensical, a large number of feasible schemes are possible. 

Rigorous dynamic simulation is the third important activity in control system desiga A flexible dynamic simulator allows for rapid evaluation of different control stmctures and their responses to various disturbances. In choosing a control scheme, there are several design considerations to take into account. First, it is important to remember that a distillation column performs two basic functions ... 

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. 

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