Alternative Temperature Control Structures

Integration of rigorous dynamic modeling and control system synthesis for distillation columns. In J. D. Birdwelland J. R. Cockette, Editors, Chemical Process Control—CPC III, Elsevier, New York, 1986, pp. 99-138. 

TABLE 13.4 Controlled Variables, Manipulated Variables, Process Gain Matrices, RGA, and Tuning Parameters for Three Esterification Systems with Two Different Two-Temperature Control Structures 

CONTROL OF REACRVE DISTILLATIONS FOR ACETIC ACID ESTERIFICATION 

For the BuAc system, the Q-FR control structure outperforms the other two structures for a 20% production rate increase (Fig. 13.10). The top product composition (Xdrjo) takes more than lOh to settle for the F-FR and F-F control structures. The reason for this is that it takes a long time for T2z to come back to its setpoint when two fresh feedstreams are manipulated. 

In summary, the performance of alternative temperature control stractures depends on the system studied. Despite having the advantage of direct production rate handling, the Q-FR control structure performs better only for the BuAc system. The F-FR and F-F control structures give comparable performance, and they outperform the Q-FR control structure for the MeAc and AmAc systems. 

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Figure 13.8 Alternative temperature control structures using manipulated variables (a) Q-FR, (b) F-FR, and (c) F-F.

For the two-temperature control structures studied thus far, one of the fresh feeds is fixed, which provides a direct handle on the production rate. The ratio to the second fresh feed (feed ratio FR) is adjusted to control a tray temperature and subsequently the stoichiometric balance. A second tray temperature is controlled by manipulating the reboiler heat duty. This is called the Q-FR control structure (Fig. 13.8a). Roat et al. ° propose a different two-temperature control stmcture for the control of the MeAc system. Two tray temperatures in the column are maintained using two fresh feedstreams. Production rate changes are achieved by adjusting the vapor boilup (Fig. 13.8c). We call this an F-F control structure, which has been studied by Kaymak and Luyben. A third control stmcture is similar to the F-F control stmcture but, instead of manipulating both feedstreams, one fresh feedstream is used to maintain a tray temperamre and the ratio to the second fresh feed is adjusted to control a second tray temperamre, which we call the F-FR control stmcture shown in Figure 13.8b. The purpose here is to compare the effectiveness of these three alternative control stmctures. A comparison will be made for dynamic responses for production rate changes for the MeAc, BuAc, and AmAc systems. 

Because of steady-state offsets in the product composition, a one-temperature, one-composition control structure was proposed to maintain on-aim produet quality. This offers an alternative when the two-temperature control structure shows unacceptably large offsets in product composition. 

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. 

An alternative control strategy fixes the reactor-inlet toluene flow rate [16]. Fresh toluene is fed into the condenser drum of the last distillation column, on level control. Production-rate changes can be achieved by changing the setpoint of the toluene reactor-inlet flow, or the setpoint of the reactor-inlet temperature controller. When this control structure is used, the whole range of conversion becomes stable. Drawing of this control structure is left as an exercise to the reader. 

We want to see how well the three alternative control structures developed above perform in the face of disturbances, that is, how close to the desired values of temperature and composition are these variables maintained, both at steady state and dynamically. A disturbance is made and the transient responses are plotted. 

Runs are made to compare the dynamic and steady-state performance of the two alternative control structures (temperature control and cascade composition/temperature control) with the R/F and QR/F ratio installed. The column is subjected to disturbances in feed flow rate and then feed composition. 

The control of partial condenser columns is more complex than total condenser columns because of the interaction among the pressure, reflux-drum level, and tray-temperature control loops. Both pressure and level in the reflux dmm need to be controlled, and there are several manipulated variables available. The obvious are reflux flow, distillate flow, and condenser heat removal, but even reboiler heat input can be used. In this section, we explore three alternative control structures for this type of system, under two different design conditions (1) a large vapor distillate flow rate (moderate RR) and (2) a very small vapor distillate flow rate (high RR). 

An alternative simulation was developed using the Flash3 model, as shown in Figure 8.40b after exporting and installing a control structure. In Aspen Plus, a vapor line with a valve is added. The decanter is specified to be adiabatic and at a fixed pressure (0.6 atm). The temperature specified in the upstream condenser HX2 is adjusted to 320 K to give a very small vapor flow rate (3% of the feed). After the file is exported, a pressure controller is inserted on the decanter, but it is put on manual and the vapor valve is closed. Now decanter pressure varies with temperature and composition. Its steady-state value is 0.366 atm with the decanter temperature controller set point set at 313 K so that a direct comparison with the previous case can be studied. 

Many distillation column use reboiler heat input as a primary manipulated variable, usually to control a temperature on an appropriate tray. This means that reboiler heat input is not constrained during normal operation with normal feed flow rates. However, as the feed flow rate to the column is decreased, less vapor boilup is required to achieve the same separation. If the feed drops to the point where the low vapor-boilup limit is encountered, the control structure must change. Three alternative control structures for achieving stable operation at minimum vapor flow rates are discussed in the following. 

Figure 15.2 shows an alternative control structure in which there is only one temperature controller TC that always manipulates the set point of the steam flow controller. Fairly tight control of temperature is maintained under all feed flow rate conditions. 

Figure 15.3 shows a third alternative that avoids the problems of the two control structures discussed above. The steam flow rate can never be less than the minimum, and fairly tight temperature control can be maintained. The basic idea is to use the flow rate of feed to the column to control temperature during periods when plant throughput rates are low. This is achieved by recycling some of the distillate and bottoms streams back to the feed to the column so that the total feed (fresh plus recycles) to the column can be adjusted and is independent of the net feed coming from the upstream unit. 

The alternative control structure of controlling the temperature in the low-pressure column with reflux ratio in the low-pressure column might seem a more logical choice. However, the very low reflux ratio in this column makes this strucmre ineffective. Temperatures are affected much more by vapor boilup (which results from changes in the high-pressure column) than by reflux ratio. Simulations of this alternative stmcture confirmed that it was unworkable with shutdowns occurring for extremely small disturbances. 

An alternative control structure would be to use a Total Q controller. In this scheme the heat transfer in the condenser/reboiler and the heat input in the auxiliary reboiler are added together and become the process variable signal to a Total Q controller. The setpoint of this controller is the output signal of the TCI temperature controller. The output signal of the Total Q controller goes to the control valve in the steam line feeding the auxiliary reboiler. 

An optimum overall control strategy is also proposed for this column system to hold both bottom and top product specifications in spite of +10% feed rate and +10% feed water composition load disturbances. Several alternative control structures are compared using dynamic simulation. The proposed overall control strategy is very simple, requiring only one tray temperature control loop in the column. This simple overall control strategy can easily be implemented in industry. 

Figure 11.15 shows an alternative control structure for the single reactive column operating in neat mode. Instead of controlling two temperatures by manipulating the two fresh feed-streams and setting the production rate by vapor boilup (Fig. 11.4), this alternative scheme... 

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