Process dynamic and control Tutorial 1 Mo Shengyong 17/09/2007

Self introduction Mo Shengyong ( 莫胜勇 )  Tutorial   Lab 7119; Tel Zhou Feng ( 周峰 )  Homework   Lab 7119; Tel

Example 1 Consider the following mixing system. Salt solution is fed with q f and c f, and water flow q w. Assume the solution inside the mixer is perfectly mixed. Control purpose:  To maintain the outlet concentration c v q 0, c v qwqw q f, c f Control valve

Question 1 Propose a feedforward control scheme of c v, by manipulating q f with the control valve Features:  Disturbance variable is measured  Control variable is not measured q 0, c v qwqw q f, c f CT CC

Question 2 Propose a feedback control scheme Feedback control features:  Controlled variable is measured  Disturbance variable is not measured q 0, c v qwqw q f, c f CT CC

Question 3: Compare these two schemes  Feedforward control Disturbance variable must be measured No corrective action is taken for unmeasured disturbances Process model is required Act before the output is affected  Feedback control Deal with any kind of disturbance Act after the output deviates from the set point Any other better control scheme?

FF+FB Feedback control for unmeasured disturbances Feedforward control for measured disturbances q 0, c v qwqw q f, c f CT CC CT CC ++

Example 2 There are two flow control loops. Indicate whether each system is either FB or FF control system. Why? Assume the distance between the flow transmitter and control valve is quite small in each system. FTFCFTFC

Answer  Both cases are feedback control  Reason: in both cases, controlled variable (flow rate) is measured, and the controller responds to this measurement.

Example 3 Establish a dynamic model for the system shown in example 1  Procedures: see Table 2.1 on page 20 q 0, c v qwqw q f, c f Control valve

Development of dynamic model Major steps  Modeling objectives  Schematic diagram  Assumptions Perfect mixing and constant density  Conservation equations Mass balance Component balance Energy balance  Equilibrium equations and simplification  Degrees of freedom analysis  Inputs classification: disturbance or manipulated variable

Assumptions:  Perfect mixed  Liquid volume V is constant  Mass balance: