CHE 322 Spring 2002 Final Exam

CHE 322 Spring 2002 Final Exam - Name Student...

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Unformatted text preview: Name: Student #: Q1 :___Q2 :___Q3:__Q4:_Q5:___ Total:___ UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING FINAL EXAMINATION, APRIL 2002 CHE322$ — PROCESS DYNAMICS AND CONTROL Examiner — W.R. Cluett Closed Book All work to be marked must appear on front of page. Use back of page for rough work only. 1. (20 marks) (a) Feedforward and feedback control each has important advantages that compensate for deficiencies of the other. Here is a list of possible advantages and disadvantages. On the next two pages, identify which of these are associated with feedback and which are associated with feedforward. You must also give some explanation for each advantage and disadvantage to show you understand why one control approach has that particular advantage or disadvantage. Advantages: - does not affect the stability ofthe control system (A1) - provides zero steady-state offset (A2) — effective for all disturbances (A3) - compensates for a disturbance before the process output is affected (A4) Disadvantages: - requires a sensor and model for each disturbance (DAl) - affects the stability of the control system (DAZ) - cannot eliminate steady-state offset (DA3) does not take control action until the process output variable has deviated from its setpoint (DA4) loflS Feedback Control (advantages and disadvantages with explanation for each) 20f15 Feedforward Control (advantages and disadvantages with exptanation for each) 3of15 2. (30 marks) Censider the packed-bed chemical reactor shown in the figure below with its feedback control strategy. The control objective is to maintain the reactor outlet concentration close to its setpoint by adjusting the flow through the preheater installed on the reactor feed. The goal here is to design a, feedforward control strategy for this process using the sensors and manipulated variable given. Suppose that reactor feed concentration is a significant disturbance. Note, in the diagram, the symbols stand for: A: analysis (concentration) AC: analysis and control F: Howrate T: temperature (a) Which of the measured variables is the most appropriate variable to be used fer feedforward control? Sketch your proposed feedforward control strategy on the figure. Heading medium Reactor feed 4of|5 (b) The following dynamic response test information is available to you. [t has been observed that a 10% step increase in the preheat valve position causes a steady-state decrease in the outlet concentration of—--l .0 moles/m}. After the step increase, the outlet concentration did “tot respond until 44 minutes had passed, and then the outlet concentration responded in a first order manner taking a further 200 minutes to reach steady-state. It has also been ob5crved that a step change in the feed concentration of 1.0 molest/mJ leads to a steady—state increase in the outlet concentration of0.3 moles/m3. After the step increase, the outlet concentration did not respond until 42 minutes had passed, and then the outlet concentration responded in a first order manner taking a further 200 minutes to reach steady-state. Recommend a feedforward controller equation relating your measured variable to your manipulated variable that can actually be implemented on the real system. State your answer both in the Laplace dOmain and in the time domain in terms of the real physical variables. Give the engineean units for all controller parameters. SoflS (c) Determine an analytical expression for the response in your manipulated variable and your controlled variable to a unit step increase in the feed concontration using, your feedforward controller designed in part (b), and sketch the response in both variables. 60f15 (d) What process changes would you recommend be made in order to improve the performance ofyour feedforward controller? What limitations would there be to making these process changes? (6) A feedforward controller is often used in conjunction with a feedback controller. This is done by simply,r combining (using superposition) the feedforward controller output and the feedback controller output. Sketch the block diagam for this combined feedforward- feedback controlled system using the feedback controller shown in the original diagram and the feedforward controller indicated in part (a), relating the signals and blocks in this diagram to the process variables and equipment. 7oflS (0 Given the combined feedfonvard-feedback control system in part (e), why would it be important in general that “disturbance dynamics (Gd ) not be significantly faster than the process dynamics (GP )”, i.e. do you see any potential conflict in the action ofthe two controllers in response to a disturbance? 80f15 3‘ (25 marks) Cascade control represents an enhancement to single-loop feedback control. Cascade control is desired when single-loop control does not pr0vide satisfactory control ofthe primary control variable and when a measured secondary variable is available. This secondary variable must satisfy the following criteria: 1. The secondary variable must indicate the occurrence ofan important disturbance. 2. There must be a cause-and-effect relationship between the manipulated input and the secondary variable 3. The secondary variable dynamics must be faster than the primary variable dynamics. (3) Explain why each criterion must be satisfied for cascade control to work properly. 90f15 (13) Consider again the packed bed reactor example fr0m Question #1. The objective is to tightly control the outlet concentration. Suppose that the single-loop controller shown in the figure below does not provide adequate control performance and that the most significant disturbance is the heating medium temperature, T2. Select an appropriate secondary variable for ca5cade control, and evaluate your choiCe using the abovementioned criteria. lOoflS (c) On the process flow diagram below, indicate your proposed cascade control strategy. (d) Describe qualitatively how your cascade controller has the potential to improve upon the single-loop controller performance by considering the response to a disturbance in the heating medium temperature. lloflS (e) Consider a separate disturbance in the feed concentration (A2) How would the performance of your cascade strategy compare to the performance ofthe single-loop strategy to a disturbance in feed concentration? lZofiS 4. (20 marks) (a) A negative relative gain between an input and an output variable indicates a poor pairing decision for a multiloop control strategy First, explain what a multiloop control strategy is and, second, explain what a r agative relative gain implies and why this makes for a poor pairing decision. 2 35 + 1 controller for this process with r, = 3. Sketch the root locus diagram for this closed-loop system by examining values of KC > O. (b) Consider a first order process where G p = . Assume that we use a feedback PI 13 0f|5 (c) Sketch the root locus diagram for the closed-loop system using the same PI controller — 2 3s+lD that was examined in part (b) (r, =3 and KC > 0) but with GP =‘ (d) From your results in 133115 (b) and (c), comment on the effect of a change in the sign of the process gain on the stability of the closed-loop system. ldoflS 5. (5 marks) Give an argument to support or reject the following statement: “Information on the variability ofke" process variables is required for evaluating the performance ofa process — average values of process variables are not adequate." lSoflS...
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