When there is only one source and one load (see Figure 8.3), control may be both simpler and more flexible. The column that is the source does not need to be operated at a constant pressure—in the scheme shown, it finds its own pressure. For the illustrative example, the overhead composition of the supply column is controlled via reflux; the base composition of the load column is controlled by boilup in the supply column.
The scheme of Figure 8.3 has an interesting dynamics problem. The controls must be so designed that changes in vapor flow from the supply column must reach the condenser—reboiler at about the same time as feed flow changes from the supply column. If there is a serious discrepancy, particularly if the second-column bottom-product flow is small, base level in the second column may experience serious upsets.
Another problem associated with this scheme is the selection and sizing of the feed valve to the first column. This column will run at a low pressure at low feed rates and at a higher pressure at high feed rates. Assuming that the feed comes from a centrifugal pump, one can see that valve pressure drop will be very high at low flow, and low at high flow. The variation in valve pressure drop with flow will be much greater than that normally encountered in a pumped system.
In another version a following column in the train supplies heat to a preceding column as shown in Figure 8.4. In this particular case, the first column gets only part of its heat from the second column; the remainder comes from an auxiliary reboiler. Interactions between the two columns may be severe. Again, for the cases studied, we have found it advantageous to let pressure find its own level in the second column, that is, the one supplying heat.
An interesting practical problem here is how to adjust the auxiliary reboiler on the first column. After examining some complex heat-balance schemes, we decided that the simplest approach was to use column A P. Vapor flow to the first column from the condenser—reboiler will not be constant, but the AP control will provide a rapid method of ensuring constant boilup. The AP control, in turn, may have its set point adjusted by a composition controller for the lower section of the first column.
It should be noted that for the schemes of both Figure 8.3 and Figure 8.4, maximum column pressure occurs at maximum feed rate and boilup rate. For columns we have studied to date, there has been no problem with flooding at lower rates and pressures.
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