Dual-Composition Control

The majority of distillation columns are designed to attain a specified separation between the two key components. The two steady-state design degrees of freedom are usually specified to be the impurity of the heavy-key component in the distillate and the impurity of the light-key component in the bottoms. Therefore, in the operation and control of a distillation column, the ideal control structure would measure the compositions of the two products and manipulate two input variables (e.g., reflux flow rate and reboiler heat input) to maintain the desired amounts of the key-component impurities in the two product streams. 

However, very few distillation columns use this ideal dual-composition control stmcture. There are a number of practical reasons for this. Composition analyzers are often expensive to purchase and have high maintenance costs. Their reliability is sometimes inadequate for on-line continuous control. They also introduce deadtime into the control loop if chromatographic methods are used. 

In addition, it is often possible to achieve very effective control without using direct composition measurements and without controlling both products. Single-end control stmctures are widely used because of their simplicity and effectiveness. 

Distillation Design and Control Using Aspett Simulation, Second Edition. William L. Luyben. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

STEADY-STATE CALCULATIONS FOR CONTROL STRUCTURE SELECTION 

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To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

Whenever the feed composition is unpredictable, one must directly control the compositions of both products. The main benefit of dual composition control is minimized energy consumption. The main limitation is caused by the interactions between the two composition loops. On the left of Figure 2.93 an example of a feedforward dual composition control system is shown. In this configuration, the distillate flow is manipulated to control the distillate composition by maintaining the relationship ... 

Let us consider a CSTR/separator/recycle system, where the first-order reaction A —> P takes place. Figure 4.3(a) presents the conventional control of the plant. The fresh feed flow rate is kept constant at the value F0. The reactor holdup V is controlled by the effluent. The reaction takes place at a constant temperature, which is achieved by manipulating the utility streams. Dual-composition control of the distillation column ensures the purities of the recycle and product streams. 

The control structure discussed in this section is presented in Figure 4.4(a). The reactor-inlet flow rate is fixed at the value l. Reactor effluent controls the reactor holdup V, while the coolant flow rate controls the reactor temperature. Dual composition control is used for the distillation column. The reactant is fed on level control. For illustration purposes, a buffer vessel was considered. This increases the equipment cost and might be unacceptable due to safety or environmental concerns. An alternative is to feed the reactant in the condenser dram of the distillation column. This strategy achieves the regulation of reactant inventory, because any imbalance is reflected by a change of the holdup. 

Distillate purity is controlled by manipulating reflux flow. Note that we have chosen to use dual composition control (controlling both distillate and bottoms purities) in the distillation column, but there is no a priori reason for holding the composition of the recycle stream constant since it does not leave the process. It may be useful to control the composition of this recycle stream for reactor yield pur-... 

Let s look again at the simple reaetor/column process in Fig. 2.5. In Sec. 2.4.2 we proposed two control structures where both the bottoms composition xB it he plant product) and the distillate composition xD (the recycle stream) are controlled, i.e., dual composition control. Bottoms composition must be controlled because it is the product stream leaving the plant and sold to our customers. However, there is a priori no reason to control the composition of the recycle stream since this is an internal flow within the plant. 

Our plantwide control perspective may push us to use a dual composition control system on the column. We would have to loosen up the bottoms composition loop tuning. But smoother reactor operation may reduce disturbances to the column and result in better product quality control. 

The simultaneous control of two compositions or temperatures is called dual composition control. This is ideally what we would like to do in a column because it provides the required separation with the minimum energy consumption. However, many distillation columns operate with only one composition controlled, not two. We call this single-end composition control. 

Dual composition control is not recommended for high-purity columns. It is better to select one end or the other for control and provide sufficient reflux to handle the worst-case conditions such that the purity of the uncontrolled product is always at or above specification. 

For the reversible case, we are interested in the composition of the bottoms stream from the DIB column. We may consider dual composition control (controlling both the n.C-4 impurity in the top and the zC impurity in the bottom). Logic would dictate that the distillate composi-... 

Step 8. The previous steps have left us at this point with two unassigned control valves, which are the reflux flows to each column. As discussed in Chap. 6, these are independent variables and can be fixed by flow controllers. We do not need dual composition control for the irreversible case because only one end of both columns is a product stream leaving the process. These two reflux flowrates are available in Step 9 to use as optimizing variables or to improve dynamic response. However, we may need dual composition control in the DIB column for the reversible case as mentioned in Steps 4 and 5. 

We could also change production rate by keeping the recycle flowrate constant and only changing DIB reflux flowrate. This changes the bottoms product purity and affects the reactor inlet composition. We also could use dual composition control in the DIB column, in which case reflux is automatically adjusted. 

The aim in the design of the composition ANFIS estimator is to use together with ANFIS-GA for dual composition control of the distillation column. Therefore, the composition estimator is tested by using the SIMULINK model before it is used for control. The performance of the control structure is checked for set-point and disturbance rejections, as is shown in figure 5. 

Usually the column pressure is controlled by the condenser duty Q. Reflux drum level can be hold by either distillate D or reflux L. Here the so-called Richardson rule is useful use the largest flow to control a level. Base level can be hold with either the bottoms, or with the boilup (reboiler duty). Finally, there are two compositions left, of top x/j and of bottoms Xb, respectively, which can be controlled by the remaining manipulated variables. If both are simultaneously controlled, we speak about dual composition control. If only one is held constant, we have single-end composition control. Basic structures are depicted in Fig. 13.8. The first input controls Xo and the second xb. 

R-V Reflux and Boilup. The levels in reflux drum and base are kept by distillate and bottoms. This structure may be interactive as dual composition control. As a single-end composition control it is one of the most used. In this case either reflux or boilup are kept constant at a value sufficient to ensure an acceptable variation in the composition of the uncontrolled product. 

RR-V Reflux ratio (L/D) and Boilup. This structure is similar with R-V, but works better in dual composition control. 

The set of specifications used in the previous section (Fq, V, Z3, Z4) can be viewed as a conventional plantwide control structure, as displayed in Fig. 13.20a. Plant throughput is set by the reactant feed, the reaction volume is kept constant, and the separation section is dual-composition controlled. For this control structure, the feed disturbances affect the flow rate and composition of the reactor outlet/separation inlet. Hence, manipulated variables internal to separation section are used to reject the disturbances. As a result, disturbances are rejected mainly by changing the reaction conditions. 

Note that the reflux flow rate has been fixed in the all three columns. This solution avoids interactions that could occur if dual-composition control would be Implemented. 

The reflux flow rates on both columns are fixed dual composition control is not used to keep the column control systems as simple as possible. In some situations better control may be achieved by using dual composition control in the columns to prevent the impurity levels in the recycle streams from becoming so large that they affect conditions in the reactor. 

To illustrate the improvement in control that is achieved by using dual-composition control, the CS3 control structure is augmented by two composition controllers, one controlling propane impurity in the bottoms (CCxB) and the second controlling isobutane... 

Figure 8.17 (a) Dual-composition control structure, (b) Dual-composition faceplates. 

Figure 8.18 demonstrates the effectiveness of this dual-composition control structure. Figure 8.18a shows how product purities vary in the face of the same scenario of feed flow rate dismrbances and feed-composition disturbances previously used. A comparison of these results with the CS3 results given in Figure 8.15 reveals a very significant reduction in product quality variability, both dynamically and at steady state. Both products are remrned to the specifications, even for feed-composition disturbances. 

The variability in the vapor distillate flow rate is still much less than with the other control structures, so even with dual-composition control, the downstream unit is not subjected to large and rapid disturbances. 

Modifying the CS3 structure by the addition of dual-composition control provides effective product quality control with essentially the same low variability in distillate flow rate. 

There are a host of conventional control structures, and the best choice depends on a number of factors, some of which we have already discussed the shape of the temperature profile and the sensitivity of the several flow ratios to feed composition. Economics obviously have an impact, mostly in terms of energy consumption versus control structure complexity. The control structure that minimizes energy is dual-composition control. But it is more complex than a simple single-end control structure. Therefore, we must evaluate how much money is lost by using a more simple structure. In areas of the world where energy is inexpensive, both the optimum design and the appropriate control structure are different than in areas with expensive energy. 

Concerning the first question, there are many more distillation columns that use singleend temperature control than use dual-temperature control. This is the standard control structure in the methanol/water separation. Using the reflux-to-feed ratio scheme and a single temperature result in energy consumption that is almost the minimum possible by using dual-composition control in the methanol/water system. Instrumentation complexity and loop interaction are avoided. So the structure used in this paper is widely applied in industrial columns. 

Although the ideal control scheme would control the compositions of both products (dual-composition control), in practice most distillation column use temperature control... 

There are two remaining control degrees of freedom. The ideal dual-composition control structure would control C5 impurity in the distillate by manipulating reflux flow rate and C4 impurity in the bottoms by manipulating reboiler duty. However, this ideal control structure is the exception not the rule in industrial applications. We usually try to find a more simple control structure in which a single-end control scheme provides adequate regulatory control using a suitable tray temperature. 

Figure 4.58 shows the controller faceplates of this dual composition control structure. There are two composition controllers (CCxB and CCxD) and one temperature controller (TC). Note that the temperature controller is on cascade, receiving its setpoint from the OP signal for the CCxB composition controller. 

Figure 4.59 Dual composition control feed composition disturbances.

If dual composition control is required then relative gain analysis, as described in Chapter 8, will help assess the level of interaction. While not an entirely accurate tool, because it only considers steady state interactions, it in indicative of the severity of the problem. Since we need to perform steptests to tune the composition controllers the additional effort involved is minor. 

One of the common applications of MVC is providing dual composition control on distillation columns. It is often the most practical way of resolving interactions. Here we work through a simple example of its design. Figure 12.117 shows the addition of MVC to a column with an energy balance scheme. 

Ryskamp, C. J. (1980) New strategy improves dual composition control. Hydrocarbon Processing, 59(6) 51-59. 

Fig. 34.6. Control schemes for dual composition control a) energy balance control, b) material...

The control schemes are developed for the case of dual composition control. Also other material and energy balance control schemes are possible. The main difference between the control schemes is that in the case of energy balance control the reflux and vapor flow affect the outpoint (distillate-bottom-ratio) as well as the fractionation, whereas in the case of material balance control outpoint and fractionation control are separated. Either the reflux ratio or the vapor flow is manipulated to control the fractionation. As can be seen, the developed control scheme of Fig. 34.5 is similar to the energy balance control scheme. 

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