Where it is desired to be able to run a column or train at maximum capacity, certain additional overrides may be needed.
With the previously discussed system, the limitation on any one column is the maximum permissible column AP that was taken to be a known, fixed value. For some columns, however, as one increases feed rate slowly and before flooding starts, a plot of top composition, xD, versus reflux flow, wR, shows a maximum value of xD at a particular reflux rate as shown by Figure 9.13. To put it another way, at some high feed rate, an increase in reflux flow will cause a decrease in overhead purity, xD. This is so because the increase in entrainment offsets the effect of increased reflux. This usually occurs at a column AP considerably higher than the nominal maximum permissible A P. It should be noted, however, that the peak of the xD versus wR curve does not occur at fixed, reproducible values of xD, wR, and A P. By measuring xD and wR and computing dxD/dwR, one can devise an override control system that maintains a positive value of dxD/dwR by pinching the steam or feed valve. This would replace the AP override. Implementation is shown in Figure 9.14.
If the condenser or reboiler limits before the column proper does, one may use either the cooling-water valve position or steam-valve position for override purposes. Generally, for good control, it is not desirable for valves to go more than 95 percent open or less than 10 percent open, except on a momentary basis. Overrides may be provided such that if either valve goes beyond this value, the feed valve is pinched slowly until the utility control valve's position is back to a reasonable value. This requires a slow-acting override incorporating a proportional-reset or floating controller. A particular arrangement is shown in Figure 9.15.
For highly integrated plants, it may be necessary to protect the utility supply during startups. In "bootstrap" plants, for example, it may be necessary during wet startups to avoid putting such a demand on the steam header as to reduce header pressure seriously. One automatic approach is to connect header pressure to overrides on the reboiler steam valves to close them partially as header pressure drops. A possible hookup is shown on Figure 9.16.
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