Control Scheme for a Distillation Column

In this section, a control scheme will be developed, using Table 33.1 from the previous chapter. 

Distillation is a much-used separation method in the chemical industry. Often, the separation process takes place in continuous distillation columns with one feed (F), one top product (D) and one bottom product (E), as shown in Fig. 34.1. 

Separation is a result of counter-current contact between vapor and hquid on the trays. The driving force is the difference in relative volatility of the eomponents. The vapor flow is generated in the reboiler, whieh is heated by steam or a hot hquid or a waste gas flow (//) If the feed is partly evaporated, the vapor flow is increased by the vapor part of the feed at the point where it enters the eoluum. 

The vapor flow fiom the top of the column is condensed in a condenser, which is cooled by water or air (Q. It is also possible to transfer the heat of condensation to another process. 

The distillate flows to an accumulator, from which part of the hquid is returned to the column as reflux R). The other part of the hquid is top product (D). The internal reflux can be larger than the external reflux R) if it is undercool, thereby condensing some of the vapor. 

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A typical control scheme for a distillation column is shown in Fig. 19. Flow controllers (FCs) regulate the flow rates of the feed and overhead products. Each flow rate is measured by a device such as an orifice plate placed upstream... 

Often a goal of a control scheme for a distillation column is to maintain the quahty of the top product on specification while also maintaining the material balance. Material balance control and energy balance control are two control schemes that can achieve this. Different criteria can be considered for the selection of a control scheme. Since the performance of both control schemes has a large impact on the profitable operation of the distillation column, this choice is not trivial. 

The purpose of a control system for a distillation column is to provide stable operation and to produce products with the desired purity. Figure 21.9 shows a control scheme typically used to regulate a binary distillation system. Flowrates, pressure, stream composition, and liquid levels are regulated. The five control variables to be regulated are shown (column pressure, composition of distillate, composition of bottoms product, liquid level in the bottom of the column, and the liquid level in the reflux drum). 

Lost composition control schemes for a binary column involve manipulating either reflux or distillate to control top composition, and either boilup or bottom produrt to control bottom composition. Changes in either of the two top manipulative variables will affect not only top composition, but bottom composition as well. Correspondingly, changes in either bottom-product flow or boilup will affect both bottom and top compositions. As a minimum, therefore, for feedback control purposes we are interested in two composition gains at each end of the column. 

Figure 6.20 gives a control scheme for a single sidestream column. The flowrate of the sidestream can be manipulated, so we have an additional control degree of freedom. Three compositions can be controlled the impurity of B in the distillate (xDiS). the purity of B in the sidestream (xsS), and the impurity of B in the bottoms xBS). Note that we cannot control the two impurity levels (A and C) in the sidestream xsa and Xs,c because there are not enough degrees of freedom. 

The goal of this book is to help chemical engineering students and practicing engineers develop effective control structures for chemical and petroleum plants. Our focus is on the entire plant, not just the individual unit operations. An apparently appropriate control scheme for a single reactor or distillation column may actually lead to an inoperable plant when that reactor or column is connected to other unit operations in a process with recycle streams and energy integration. 

EXAMPLE 1.3. Our third example illustrates a typical control scheme for a simplified version of an entire chemical plant. Figure 1.5 gives a sketch of the process configuration and its control system. Two liquid feeds are pumped into a reactor, in which they react to form products. The reaction is exothermic, and therefore heat must be removed from the reactor. This is accomplished by adding cooling water to a jacket surrounding the reactor. The reactor effluent is pumped through a preheater into a distillation column that splits it into two product streams. 

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

Figure 21.10 An Advanced Control Scheme for a Binary Distillation Column (Adapted from M. R. Skrokov, Mini- and Microcomputer Control in Industrial Processes Handbook of Systems and / plications Strategies [New York Van Nostrand Reinhold, 1980]. Reproduced by permission.)...

The techniques presented will now be illustrated in the following two examples, namely temperature control of a mixer outlet and control scheme configuration for a distillation column. The RGA will be used to select which controlled and manipulated variables will be paired, the NI will then be used to demonstrate whether or not the resulting control loops are stable, and SVD will be used to test whether the control loop interactions are overly sensitive to slight errors in process gains. 

Devise a control scheme for the column to maintain the distillate... 

Avoid saturation of a manipulated variable. A good example of saturation is the level control of a reflux drum in a distillation column that has a very high reflux ratio. Suppose the reflux ratio (R/D) is 20, as shown in Fig. 8.10. Scheme A uses distillate flow rate D to control reflux drum level. If the vapor boilup dropped ouly 5 percent, the distillate flow would go to zero. Any bigger drop in vapor boilup would cause the drum to run dry (unless a low-level override controller were used to pinch back on the reflux valve). Scheme B is preferable for this high reflux-ratio case. 

For example, suppose that the distillate flowrate from a distillation column is large compared to the reflux. We normally would use distillate to control level in the reflux drum. But suppose the distillate recycles back to the reactor and so we want to control its flow. What manipulator should we use to control reflux drum level We could potentially use condenser cooling rate or reboiler heat input. Either choice would have implications on the control strategy for the column, which would ripple through the control strategy for the rest of the plant. This would lead to control schemes that would never be considered if one looked only at the unit operations in isolation. 

In this chapter we have presented some fundamental concepts of distillation control. Distillation columns are without question the most widely used unit operation for separation in the chemical industry. Most final products are produced from one end or the other of a distillation column, so tight control of product quality requires an effective control system for the column. However, the column is usually an integral part of an entire plant, so its control scheme must also be consistent with the plantwide control structure. 

The control of the separation section is developed unit by unit by applying the standard control schemes for distillation units adapted to take into account the behaviour of the distillation column in a recycle environment. 

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. 

When a side draw is used on a distillation column, there is another degree of freedom introduced into the control scheme. There are actually two material balance splits to keep in balance. One is the ratio of distillate/side draw, and the other is the ratio of side draw/bottoms. The separation power base can be set by the ratio of steam/feed, and then the distillate flow rate can be manipulated by a temperature controller for the MRT point above the side draw. The side draw flow rate can be manipulated by the second temperature controller for the MRT point below the side draw. 

A central issue is when is the model complete When correcting variables have not been taken into account, the system is usually undetermined. If important state variables have been forgotten, they will also not appear in the behavioral model and in any control scheme. For example, if, for simplicity reasons, in the case of distillation the column pressure has not been taken into account as state variable, the pressure will not appear in the behavioral model and pressure control will not be part of the system. 

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]. 

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 ... 

The feed split is simply the amount of feed that leaves as distillate versus the amount that leaves as bottoms. The other variable, fractionation, is the amount of separation that occurs per stage. The overall column fractionation depends on the number of stages, the energy input, and the difficulty of separation. A typical control scheme for this column is shown in Figure 8.3. 

In this example, the RGA analysis is used to find the appropriate pairing for a conventional distillation column. There are typically two control schemes for distillation columns single and dual composition control. The single con tosition control scheme maintains the composition of one of the products at a desired value, whereas in dual composition control both products are regulated. 

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. 

Ratio control can be used where it is desired to maintain two flows at a constant ratio for example, reactor feeds and distillation column reflux. A typical scheme for ratio control is shown in Figure 5.21 (see p. 233). 

Shinskey (1984) has shown that there are 120 ways of connecting the five main parts of measured and controlled variables, in single loops. A variety of control schemes has been devised for distillation column control. Some typical schemes are shown in Figures 5.22a, b, c, d, e (see pp. 234, 235) ancillary control loops and instruments are not shown. 

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