Effect of Pressure on Temperature Control

Column pressure affects boiling points and, therefore, the control temperature. The temperature controller interprets a pressure change to signal a composition change, and is fooled by it. Consider a drop in column pressure. The control temperature will fall, and the temperature controller will counteract it (e.g., by raising boilup). This will induce heavies to ascend into the control location. Some of the heavies will end up in the distillate. Conversely, lights will be induced into the bottom product upon a pressure rise. 

For adequate composition control, the control temperature must be relatively insensitive to the expected pressure changes. This condition prevails in most superatmospheric separations of components whose boiling points are not close. The tight pressure control practiced in most columns suppresses pressure variations, thereby assisting the 

When the control temperature and the column control pressure are measured at two different locations, the control temperature may also be affected by changes in the differential pressiure between the two locations. For instance, if column top pressure is controlled, and the control temperature is located near the bottom, a rise in differential pressure will increase the pressure at the temperature control location. The control temperature will rise, the controller will interpret the change as a rise in heavy key concentration, and will coimteract it (e.g., by decreasing boilup). 

Changes of differential pressure between the temperature control and pressure control locations are normally small compared to variations in column pressure. For this reason, the control temperature is seldom sensitive to differential pressure variations in other than low-pressure (particularly deep vacuum) services. Even then, the effects of differential pressure changes can be minimized by locating the temperature and pressure control measurement points close together. 

The effect of pressure on the control temperature can be minimized by adequate selection of the temperature control location. Generally, all column temperatures have a similar sensitivity to pressure, but the sensitivity of temperature to composition varies widely from tray to tray. Therefore, locating the control temperature in a region highly sensitive to composition reduces its relative sensitivity to pressure changes (see Fig. 18.4). 

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