BACK-OFF CALCULATIONS IN OPTIMIZING CONTROL - A DYNAMIC APPROACH

The ultimate goal in the operation of chemical plants is to work at th e possible optimal conditions. However, must of the time, a plant is f aced with uncertain conditions during its operation. To efficiently ha ndle these uncertainties, chemical plants must have the flexibility to achieve feasible operation over a range of uncertain conditions. One way to accomplish this, is by moving the nominal optimum to some perma nently feasible operating point inside the feasible region (back-off p oint). In a previous study (Bahri et al., 1994), an iterative approach to solve this problem at steady-state has been proposed (Steady State Open-Loop Back-Off Calculation). In order to consider the transient b ehaviour of a system responding to disturbances, the back-off calculat ion should be based upon a dynamic model of the system. Having determi ned the economic penalty associated with the dynamic open-loop back-of f, the next step is to estimate the potential recovery that various co ntrol schemes of varying complexity might provide. The methodology to solve the Dynamic Back-Off problem and a flowsheet example are present ed in this paper.