Dynamic control analysis of partially heat-integrated pressure-swing distillation for separating a maximum-boiling azeotrope

Abstract

Dynamic control behavior of partially heat-integrated pressure-swing distillation process is investigated by taking a separation of maximum-boiling ethylenediamine/water azeotrope as the example. The feasibility of using a single-end control strategy is affirmed by the method of feed composition sensitive analysis, and a series of novel control structures are developed to eliminate or attenuate the snowball effect, wherein the special control loops are that the flow in bottoms recycle line is controlled by regulating the set-point of a flow controller placing in recycle loop through two alternative ratio controllers including flow and thermal ratios. Both the robust and stable regulatory control are achieved with dual-product composition restored to their steady-state design specifications for handling large (20%) step changes in throughput and feed composition. The effectiveness of control structure with flow ratio option works better than that of another alternative taking the comprehensive consideration of product transient deviations, product offsets and settling time. Besides, there is no product offset for distillate ethylenediamine composition, although only the temperature control loops are used in the low-pressure column for control structure with thermal ratio option, which is the advantage of this control structure since the dual-end product composition control loops are required for that with thermal ratio. Of the particular importance is that the severer and stronger oscillatory effect of product composition is weakened by taking the proportional-only cascaded temperature controller for facing throughput perturbations.