Figure 8.21c shows what happens to product purities for two types of feed composition changes. In the first, feed composition is changed from 60 mol% methanol to 70 mol%. In the second, the feed composition is changed to 50 mol%. This second disturbance causes the impurities of the distillate and bottoms streams from both columns to climb significantly higher than the desired 0.1 mol%. The problem is a steady-state one, not a dynamic one. The fixed R/F structure with single-end temperature control does not maintain product purities in the face of large feed composition disturbances.
If the process is subject to large and frequent feed composition disturbances and if tight control of product quality is required, it may be necessary to go to a dual-temperature-control structure.
If dual-temperature control is to be used in both columns, we need to select two appropriate trays in each. The SVD method is used to guide in this selection. Figure 8.22 gives the steady-state gains and SVD results for both columns. In the high-pressure column C2, the manipulated variables are the conventional ones: reboiler heat input and reflux flowrate. However, in the low-pressure column, the manipulated variables are reflux flowrate and feed flowrate. The steady-state gains are obtained by making 0.1% changes in the inputs and calculating the changes of the temperatures on all trays.
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