Abstract
The dynamic and control behavior of a depropanizer that consists of 29 ideal trays plus condenser and reboiler, with feed introduced at the 12th tray, has been studied by digital simulation. Open loop dynamic behavior of the column was studied for step changes of up to 30% in the feed and reflux flow rates and heat input to the reboiler. For changes in feed flow rate, the disturbance variable used for the control studies, the column exhibited inverse behavior in the composition response of the light key component (propane) in the distillate and the heavy key component (isobutane) in the bottom product. However, no inverse response was observed for the controlled variables of the concentration of the heavy key in the distillate and the light key in the bottoms. Control behavior of the five component depropanizer column for conventional single product composition control as well as dual quality control was investigated using the DYCONDIST simulator. Results obtained using indirect control strategies by measuring tray liquid temperature(s) and direct control of the distillate and bottoms are presented. Performance of the dual quality control schemes is discussed with respect to predicted behavior based on static information from relative gain array and relative disturbance gain calculations.
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