Single-end composition 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. 

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. 

Control structure CSl shown in Fig. 13.15 makes use of the feed in recycles of both reactants. Recycles flow rates are also fixed on flow control. Note that the make-up of A and B may be done directly in the reflux drum and in the reboiler sump, respectively. The reactor outlet is put on level control. Single-point composition control and fixed reflux are used for distillation columns. Composition controllers can be co-ordinated by the composition measurement for the end product. This structure works well, but has the disadvantage of an indirect setting of production. 

These results indicate that either Stage 8 or Stage 29 do a fairly good job in maintaining product purities in this binary system when single-end temperature control is used. If dualtemperature control was used and the temperatures at the two ends of the column were controlled, product compositions would be held exactly at their desired values under steady-state conditions if pressure changes do not occur. 

In this section a systematic approach is proposed to design the control structures for these three types of reactive distillation flowsheets. Because all five reactive distillation systems (Table 7.5) have almost equal molar feedflows (neat flowsheet), the stoichiometric balance has to be maintained. Here we adjust the feed ratio to prevent accumulation of unreacted reactants attributable to stoichiometric imbalance. The next issue is, how many product compositions or inferred product purities should be controlled For the esterification reactions with A -f B C + D with a neat flowsheet, controlling one-end product purity implied a similar purity level on the other end, provided the product flowrates are equal. Thus, a single-end composition (or temperature control) is preferred. This leads to 2 x 2 multivariable control, as opposed to a 3 x 3 multiple-input-multiple-output system. The... 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

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. 

In single-end control structures, only one composition or one temperature is controlled. The remaining control degree of freedom is selected to provide the least amount of product quality variability. For example, a constant reflux ratio RR can be maintained or the reflux-to-feed ratio R/F can be fixed. The control engineer must find out whether this more simple approach will provide effective control of the compositions of both product streams. One approach to this problem is to use steady-state simulations to see how much the reflux ratio and the reflux flow rate must change to maintain the specified impurity levels in both product streams (heavy-key impurity in the distillate X/>(hk) and light-key impurity in the bottoms Xb(lk)) when changes in feed composition occur. The procedure is call feed composition sensitivity analysis. ... 

If there are significant changes in both of these ratios, single-end control will probably be ineffective. Because the flow ratios have to change, the control structure must be capable of changing both manipulated variables (reflux and reboUer duty). This implies that two-end control is required. The structure could control two compositions, two temperatures or one composition and one temperature. This decision depends on the shape of the temperamre profile, which we explore in Section 6.3. 

If there are only small changes in one of the ratios, a single-end control structure with this ratio fixed may provide effective disturbance rejection in the face of both feed composition and feed flow rate disturbances. 

In the last three chapters, we have developed a number of conventional control structures dual-composition, single-end with RR, single-end with rellux-to-feed, tray temperature control, and so on. Structures with steam-to-feed ratios have also been demonstrated to reduce transient disturbances. Four out of the six control degrees of freedom (six available valves) are used to control the four variables of throughput, pressure, reflux-drum level, and base level. Throughput is normally controlled by the feed valve. In on-demand control structures, throughput is set by the flow rate of one of the product streams. Pressure is typically controlled by condenser heat removal. Base liquid level is normally controlled by bottoms 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. 

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. 

Then the feed composition is changed to 0.6 mole fraction nC4 and 0.4 mole fraction C5 with the product specification held constant. The required reflux ratio and reflux-to-feed ratio are 1.075 and 0.6471, respectively. Next the feed composition is changed to 0.4 mole fraction C4 and 0.6 mole fraction nC5 with the product specification held constant. The required reflux ratio and reflux-to-feed ratio change to 1.665 and 0.6626, respectively. The third and flfth columns in Table 16.1 show the percent changes in the two variables from the design values. Since the changes in the reflux-to-feed ratio are quite small, a single-end control structure may be able to handle feed composition disturbances fairly well. 

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