tailieunhanh - Lecture Chemical process control - Chapter 5

a | Chapter 5 Dynamic Behavior In analyzing process dynamic and process control systems it is important to know how the process responds to changes in the process inputs. A number of standard types of input changes are widely used for two reasons 1. They are representative of the types of changes that occur in plants. 2. They are easy to analyze mathematically. 1. Step Input A sudden change in a process variable can be approximated by a step change of magnitude M Chapter 5 The step change occurs at an arbitrary time denoted as t 0. Special Case If M 1 we have a unit step change . We give it the symbol S t . Example of a step change A reactor feedstock is suddenly switched from one supply to another causing sudden changes in feed concentration flow etc. Example The heat input to the stirred-tank heating system in Chapter 2 is suddenly changed from 8000 to 10 000 kcal hr by changing the electrical signal to the heater. Thus Chapter 5 Q t 8000 20005 t S t @unitstep and Q t Q - Q 2000S t Q 8000kcal hr 2. Ramp Input Industrial processes often experience drifting disturbances that is relatively slow changes up or down for some period of time. The rate of change is approximately .