BACK-OFF CALCULATIONS IN OPTIMIZING CONTROL - A DYNAMIC APPROACH

The ultimate goal in the operation of chemical plants is to work at the possible optimal conditions. However, must of the time, a plant is faced with uncertain conditions during its operation. To efficiently handle 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 permanently feasible operating point inside the feasible region (back-off point). 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 behaviour of a system responding to disturbances, the back-off calculation should be based upon a dynamic model of the system. Having determined the economic penalty associated with the dynamic open-loop back-off, the next step is to estimate the potential recovery that various control schemes of varying complexity might provide. The methodology to solve the Dynamic Back-Off problem and a flowsheet example are presented in this paper.