organized summary of the various ideas and criteria generated over the years. The normal procedure is to plot the steady-state temperature profile and then select a tray that is somewhere in the region where the temperature is changing fairly rapidly from tray to tray.
Theoretically, the temperature at the end of the column should be controlled in a binary constant-pressure system to maintain constant product composition. However, the temperature changes at the ends of the column are quite small in moderate- to high-purity columns. Therefore, small changes in pressure or the presence of other lighter or heavier components can affect temperature much more than composition of the key component.
Another consideration is nonlinearity. The relationship between tray temperature and the manipulated variable can be quite nonlinear if a tray near the end of the column is used. For example, suppose we have a system in which reflux is on flow control and a temperature near the bottom of the stripping section is controlled by heat input to the reboiler. An increase in heat input will drive more light components up the column, but the control tray temperature will increase only very slighdy because it is essentially pure high boiler already. However, a decrease in heat input will drop light component down in the column and the control tray temperature can change very drastically. This nonlinear response presents difficult controller tuning problems when conventional linear controllers are used. Thus a control tray should be selected that is as close to the end of the column as possible but not so close that it gives a highly nonlinear response. Nor should it be so close that it is too sensitive to changes in pressure and to light or heavy nonkey components in the feed.
For good speed of response, temperature probes should be installed in the active part of a tray, not in downcomers.
Was this article helpful?