Systematic top-down economic plantwide control of the cumene process

Plantwide control system design for economic operation over a wide through range (design throughput to maximum throughput) encompassing multiple active constraint regions, is studied for the cumene process. A unique feature of the process is that it recycles the heavy side-product to extinction. A novel top-down control system synthesis approach, where the control objectives for maximum throughput operation are first obtained using steady state optimization followed by control loop pairings with highest priority to economic objectives, is applied. The control structure thus obtained is unconventional with tight active constraint control requiring ‘long’ level loops that maintain the reflux drum and bottom sump levels of a column using the two process fresh feeds. This structure for maximum throughput operation is adapted for economic operation at lower throughputs. Rigorous dynamic simulations show that the structure provides acceptable process regulation for large disturbances despite the long level loops over the entire throughput range. More importantly, no back-off from the active hard equipment capacity constraints also ensure that the loss in throughput from the maximum achievable is negligible. This work is amongst the first reports illustrating the application of the top-down plantwide control system design approach for superior economic performance with robust process stabilization.