EVALUATION OF PLANT-WIDE CONTROL-STRUCTURES BY STEADY-STATE DISTURBANCE SENSITIVITY ANALYSIS

One of the most important problems in process control is how to develop effective control structures for complex multiunit plants. This paper presents a method that is aimed at helping to solve this problem by providing a preliminary screening of candidate plant-wide control structures in order to eliminate some poor structures. Only steady-state information is required. Equation-based algebraic equation solvers are used to find the steady-state changes that occur in all manipulated variables for a candidate control structure when load changes occur. Each control structure fixes certain variables: flows, compositions, temperatures, etc. The number of these fixed variables is equal to the number of degrees of freedom of the closed-loop system. If the candidate control structure requires large changes in manipulated variables, the control structure is a poor one because valve saturation and/or equipment overloading will occur. The method is illustrated on a complex plant that contains four components, a coupled reactor/stripper, two distillation columns, and two recycle streams. A number of alternative structures are quickly explored. The effectiveness of the remaining structures is demonstrated by dynamic simulation. Some control structures are found to have multiple steady states and produce closed-loop instability.