FEEDBACK CONTROL OF DISTILLATION COLUMNS BY DOUBLE DIFFERENTIAL TEMPERATURE CONTROL

A new technique is proposed for indirectly controlling the composition of the product of a distillation column by controlling the difference between two temperature differences in the column—i.e., controlling a double differential temperature. Conventional techniques control a temperature on a tray near the end of the column where the desired product is removed, the difference between two temperatures in a section of the column, or a pressure-compensated temperature on some tray. In the specific example studied, a 25-tray deisobutanizer, it is desired to hold bottoms composition, XB, by manipulation of heat to the reboiler. Steady-state error in XB with double differential temperature control is less than with the other techniques in the face of disturbances in feed rate, feed composition, column pressure, and tray pressure drop. Dynamic performance of double differential temperature control was at least as good as the other techniques, as indicated by the plant transfer functions obtained from a linearized model and the time responses obtained from a digital simulation of the nonlinear equations describing the system. © 1969, American Chemical Society. All rights reserved.