operation. Thus, another control structure CS5 that includes one composition analyzer is explored in this section.
This control structure differs from control structure CS7 only in how the fresh feed-streams and the reboiler heat input are manipulated. Figure 16.16 shows that the temperature of a tray near the bottom of the column is used to infer bottoms product purity, and it is controlled by manipulating the vapor boilup. Tray 16, which has the biggest steady-state gain (KVS) in the stripping section, is selected. Distillate purity is not controlled. Fresh feed Fqb is flow controlled and serves as the production rate handle.
A composition analyzer is used to detect the inventory of component A in the system. Feedback can be used to prevent the gradual buildup or depletion of reactant A. Thus, the flowrate of the other fresh feed F0A is manipulated by a composition controller that maintains the concentration of component A on a selected tray.
In an attempt to see which tray is the best for detecting the component A concentration, a disturbance sensitivity approach is explored. The compositions of both products are driven to their correct values by varying the reflux ratio and heat input at different feed flowrates. The composition and temperature profiles have to change to give "perfect" composition control of the two products. The changes of the composition of component A through the column at +20% of the original feed flowrates are shown in Figure 16.17. The figure shows that the tray with the least changing composition (tray 20) is selected as the preferred one to control by manipulating the flowrate of
the other fresh feed F0A. This tray is also the effluent of the second (middle) reactor returned to the column.
Figure 16.18 gives relay-feedback test results for both loops: F0A/xA 20 and VS/T16. The unusual sharp spikes in the composition on tray 20 are caused by the step changes in F0A, which is fed directly on tray 20. The amplitudes and periods look reasonable for both loops. These results are also confirmed by the controller gain and reset time values listed in Table 16.4. There is no inverse response (huge reset times) problem. There is also no action related problem because both loops have the same action.
To check the effectiveness of this control structure, quite large +20% step changes in the production rate (fresh feedstream F0B) are applied to the system. Figure 16.19
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