Unit 1 B      

The complexity of the present-day automation in a pro­cess plant can be characterized by the large number of recor­ding controllers interconnected by the flow diagram on the cen­tralized control panel board in a process control room. On the control board, the process is divided into several interconnec­ted unit operations, such as reactor, heat exchanger, etc. Asso­ciated with each unit operation, there are a number of measu­red variables, such as temperatures, pressures, and flows. Some of these variables are also controlled by individual contro­llers and can be controlled within a certain range by adjus­ting the controller set points on the control panel. Although the number of variables being controlled, this way is relatively large, there are still process disturbances which cannot be controlled, such as ambient temperature, variation of feed qua­lity, and aging of catalyst. The optimum way to operate the process is generally a function of these disturbances. It is the operator's job to sense these disturbances, estimate the new optimum operating conditions, and readjust the set points of the controllers accordingly. This describes the function of the present-day controllers in a process plant and their relation to the operator.

A. Optimization. The fundamental objective of a process operation is to make a profit. The optimum operating condition is defined as a particular combination of process variables which causes the process to yield maximum profit. Because if the uncontrolled disturbances, the optimum operating condition may change from time to time. The conventional regulating control philosophy is based on the assumption that either the optimum operating condition does not change much or the effect on profit resulting from the change of the optimum is small. B.   Multivariable System. The basic control phylosophy in applying a process control computer is to control the process as a whole. Since the performance of a process is generally dependent on a number of important process variables which are not independent of each other, the control problem falls into the category of multivariable systems. The multivariable problem distinguishes itself from the single-loop control pro­blem by the fact that each output variable is a function of all the input variables, and a change in any input variable causes changes in all the output variables. One of the control problems the process computer must solve is how to handle this multivariable operation in a logical and optimal manner. C. Constraints on Process Variables. Constraints are defined here as the upper or lower limits set for each process variab­le under normal operating conditions. There are two basic ty­pes of constraints to be considered. One kind is the constrai­nt due to physical or equipment limitations and the other is that due to product specifications. For example, the maximum wall temperature of a furnace is a physical constraint and the minimum octane number of gasoline is a product specification constraint. During normal operation, a basic requirement of the process control system is that neither type of constraint be violated.

D. Estimation of Process Variables. One of the basic requi­rements of a good feedback control system is accurate measu­rement. The measurement problem encountered in process con­trol has been a continuing challenge to the instrument indus­try for a long time. To date, there are still serious limita­tions or the scope of process control due to lack or inadequ­acy of measurements. E. Disturbances.  The need for any feedback control system

arises mainly because there are unpredictable and uncontrol­lable disturbances affecting the operation of the process. Therefore, the characteristics of the disturbance have a very important effect on the nature of the computer control prob­lem.