Figure 12.28 Ternary with inerts temperature profile.


Figure 12.28 Ternary with inerts temperature profile.

The composition of A on tray 6 (30 mol% A) is measured and controlled by the flowrate of fresh feed F0A. Throughput is set by flow controlling fresh feed F0B. The reflux-drum level is controlled by manipulating the distillate flowrate, and the base level is controlled by manipulating the bottoms flowrate.

Two alternative structures are evaluated with this control scheme. In the first, the reflux flowrate is simply fixed. In the second (Fig. 12.29), the reflux ratio is controlled. This is achieved by measuring the distillate flowrate, multiplying this signal in the "ratio" element by the desired reflux ratio. The output signal from the ratio changes the setpoint of the reflux flow controller. The steady-state reflux ratio is 70/12.58 = 5.58. The bottoms composition is 98mol% C. The distillate composition is 97.19 mol% I, which means that there are only small losses of reactants A and B in the distillate.

Two 0.5-min first-order composition measurement lags are inserted in both composition control loops. Composition transmitter spans are 20mol%. All valves are 50% open at steady state. The controllers are tuned by running relay-feedback tests. The values of ultimate gains and periods are given in Table 12.3. Tyreus-Luyben tuning is used in most cases, but some loops are detuned to give larger closed-loop damping coefficients.

Figure 12.30 gives the response of the system with the fixed reflux flowrate to a +20% change in the production rate handle F0B. The bottoms composition is maintained at its specified value, but the bottom left graph shows that the inert composition of the distillate drops drastically to about 87 mol% I. This means large amount of the reactants are being lost out the top of the column. The distillate flowrate increases, as does fresh feed F0A and vapor boilup. The production of product C only increases by 8% (bottoms flowrate

increases from 12.39 to 13.5 mol/s) despite the 20% increase in the amount of B being fed. The losses of both reactants are very large.

Implementing the reflux ratio strategy improves the situation, as shown in Figure 12.31. The distillate inert composition only drops to about 96.8 mol%. The bottoms flowrate increases to 14.9 mol/s, which should be compared with the 13.5 mol/s seen in the fixed reflux control structure. Figures 12.32-12.35 give the response to other disturbances in production rate and feed compositions.

In Figure 12.33 fresh feed F0A contains less inert, so the distillate rate decreases and less F0A is fed for a fixed value of F0B. In Figure 12.34 more inert is included in fresh feed F0A, so the distillate rate and F0A both increase. Production rate, as reflected in the bottoms flow-rate, stays constant at the new steady state. The feed composition disturbance in Figure 12.35 is changing F0B from pure B to 5 mol% A. The bottoms purity is not affected, but distillate drops about 1 mol% I. Because less B is fed, less F0A is fed and both bottoms and distillate flowrates decrease.

12.2.3 Control Structure CS2

Figure 12.36 shows a control structure in which the temperature on tray 3 is controlled instead of direct composition control of the bottoms composition. Figures 12.37-12.41 show that this "inferential" control scheme does a fairly good job of maintaining product purity and keeps reactant losses low for throughput changes. However, for feed composition disturbances, both the distillate and bottoms compositions move away fairly significantly

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