Closed Loop Performance

Next, the closed-loop performance of both control structures (with and without feed rearrangement) is evaluated. We first consider the case of a 40% production rate increase.

7W. L. Luyben, Process Modeling, Simulation and Control for Chemical Engineers, Second Edition, McGraw-Hill, New York, 1989.

Figure 18.13 Control structure of reactive distillation with variable feed locations as production rate changes; OPi, operating variable or fraction of valve opening to feed tray i.

The control structure with feed tray manipulation ("coordinated control") gives fast dynamics in the product composition, as can be seen in Figure 18.14 (solid lines) where the top and bottoms compositions return to setpoint in less than 10 h. In contrast, the conventional control structure shows a little slower dynamic response with the product compositions taking more than 10 h to return to their setpoints. Of more importance, the coordinated control structure results in a 21% energy savings compared to the conventional control structure, which can be seen from the smaller vapor rate in Figure 18.14.

Note that a 21% energy savings is smaller than the 28% from steady-state analysis, and the reason is that we fix the tray 13 composition of A to the nominal value. Nonetheless, the amount of energy saved is still quite significant. Figure 18.15 shows the responses for a

Figure 18.14 Closed-loop responses for +40% production rate increase with fixed feed locations (dashed) and coordinated feed trays (solid; with 21% energy savings).

Figure 18.14 Closed-loop responses for +40% production rate increase with fixed feed locations (dashed) and coordinated feed trays (solid; with 21% energy savings).

Figure 18.15 Closed-loop responses for 40% production rate decrease with fixed feed locations (dashed) and coordinated feed trays (solid; with 7.5% energy savings).

Figure 18.15 Closed-loop responses for 40% production rate decrease with fixed feed locations (dashed) and coordinated feed trays (solid; with 7.5% energy savings).

40% step decrease in the production rate. Faster dynamics for top and bottoms product compositions are again observed for the coordinated control structure. Moreover, a 7.5% energy savings can be achieved with this improved dynamics.

The results presented in this section clearly show that the concept of optimal feed tray location can be carried over to process operation and control. With a simple modification in the control structure, improved closed-loop performance can be achieved while realizing substantial energy savings at the same time.

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