Two-Temperature Control Structure

The gains are large and positive for changes in Fqb in the lower part of the column an increase in the heavier reactant raises ternperamres in the stripping section and in the lower part of the reactive zone. However, they are negative and small in the upper part of the column. 

The results from Chapter 10 suggest that the two temperature controllers should have the same action (both direct acting). Therefore, a tray somewhere in the region where the gain between temperature and Fqb is negative should be selected. Tray 12 exhibits a negative gain, but it is small. 

Relay-feedback tests are run individually on both temperature loops with the other loop on manual. Three 1-min temperature measurement lags are included. Temperature transmitter spans are 50 K. Valves are half open at steady state. Controller tuning constants are given in Table 11.1. Tyreus-Luyben tuning constants are used with some reductions in controller gains to give less oscillatory behavior. 

All of the systems and control structures are subjected to a number of disturbances in throughput and feed compositions. Step disturbance are made at 10 min. 

In summary, the two-temperatiu e control structure, which does not use any composition measurement, handles all of these disturbance well except when some of the lightest component C is present in the fresh feed of B. Remember the Fob feed is trying to control the tray 12 temperature, and it is a direct acting controller. Thus, when the light C component enters near the top of the column in the Fob stream, it drops the temperature and the temperature controller decreases Fqb- However, the decrease in Fqb produces an increase in the temperatures in the lower part of the column (remember the T/Fqb gains are positive in the lower part of the column). As a result, the tray 2 temperature increases and the second temperature controller then increases Fqa (see upper left graph. Fig. 11.13). This puts more light material in the column and drives the tray 12 temperature even lower. The result is a system shutdown. 

---

Thus, this two-temperature control structures uses steam flow rate to control temperature during normal operation but switches to using reflux to control temperature when the minimum steam flow rate limitation is reached. 

Figure 15.10 gives a direct comparison of the performances of the three control structures for the same ramp disturbances. The dashed lines are for the two-temperature control structure. The dotted lines are for the VPC control structure. The solid lines are for the recycle control structure. 

The worst temperature control and the largest deviations in methanol product purity (xD) occur with the two-temperature control structure. The VPC control structure does not... 

Figure 15.11 gives results for the two-temperature control structure. When feed flow rate is dropped to 3000kmol/h, temperature rises up to 105 °C, and the TCI controller decreases reboiler heat input. At the same time, the R/F ratio controller produces an immediate drop in reflux flow rate. However, the rise in temperature causes the TC2 controller to increase reflux temporarily until the temperature is returned to the TCI set point of 101 °C. 

Compared to the more complex two temperature control structure CS2QA, the singletemperature control structure CS3 performs very satisfactorily. Both product compositions are maintained at light specihcalions even though the temperature control point is far away from the column bottom. However, control structure CS4 is not able to keep the acetic acid loss out of the column top close to the desired level. 

The dynamic controllability of the ideal quaternary two-reactant, two-product system was explored. Adding more reactive trays improved dynamic controllability. The two-temperature control structure provided fairly effective control of the quaternary system. 

In the two-temperature control structure studied in Chapter 10, the 7 control degrees of freedom are allocated as follows ... 

Figures 11.7 and 11.8 gives responses to positive and negative 20% changes in vapor boilup, the throughput handle in this control structure. These disturbances are handled well by the two-temperature control structure. Stable base-level regulatory control is achieved. The increase in Vs results in increases in both fresh feeds, and the distillate and bottoms streams increase. Product purities xd(q and xb(d> are maintained fairly close to their desired values. Product purities drop slightly below their specifications for the increase in V5 but rise above specifications for the decrease in throughput. Reflux increases because of the reflux ratio control stmcture.

Figures 11.9-11.14 demonstrate the responses of the system with the two-temperature control structure for a variety of feed composition changes. Instead of feeding pure reactant A in the Fqa fresh feedstream and feeding pure reactant B in the Fqb fresh feedstream, the composition of these streams zoa(J) Zobu) changed in various ways. In Figure 11.9...

However, the two-temperature control structure fails when the feed composition disturbance is the introduction of some C in the Fob fresh feed (zob(b) 0.95, Zob(0 = 0.05). Figure 11.13 shows that a shutdown occurs in about 1 h. The tray 12 temperature controller tries to hold the temperature as it begins to fall at about 45 min by cutting back on Fob. However, the tray 2 temperature has increased, so the controller increases Foa- Adding more light reactant tends to drop the temperature on tray 12 even more. The system crashes when the Fqb feed is cut off completely. The distillate purity drops drastically because there is no B being fed to produce it. 

In Chapter 10 we investigated a two-temperature control structure for the quaternary, two-product system. We demonstrated that an internal composition measurement is not required in that system to provide the extremely precise balancing of the stoichiometry of the reaction, that is, feeding exactly the right amount of reactants so that no excess builds up in the column. Will a similar two-temperature control structure be effective in the ternary system without inerts This stracture is shown in Figure 12.14. The two fresh feeds are manipulated to control the temperatures on two trays. 

These results demonstrate that a two-temperature control scheme does not provide effective control of the ternary system with inerts. The two-temperature control structure works for the quaternary system and for the ternary without inerts, but not for the ternary with... 

A two-temperature control structure is considered first. Selecting the vapor boilup and reflux ratio as manipulated variables, the control problem then is to find the best locations for the two temperature control trays. Figure 12.50 shows the steady-state changes in tray temperatures throughout the column for several small changes in vapor boilup and reflux ratio. 

Using the reflux ratio and vapor boilup as the manipulated variables, the two-temperature control structure is shown in Figure 12.80. Two temperature control trays, tray 85 and tray 5, are selected based on SVD analysis. Using relay-feedback tests, values of ultimate... 

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. 

D. Kaymak and W. L. Luyben, Comparison of two types of two-temperature control structures for reactive distillation columns, Ind. Eng. Chem. Res. 44, 4625-4640 (2005). 

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... 

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. 

In practical applications it is desirable to use inferential temperature measurements whenever possible instead of direct composition measurements. Composition analyzers have higher cost, require more maintenance, and can introduce dead time into the control loop. Therefore, we first explore a control structure that does not have any composition analyzer. The performance of this two-temperature control structure will then be compared with a structure that uses a composition analyzer. 

---