ch32b midterm1 2005

UNIVERSITY OF THE WEST INDIES

DEPARTMENT OF CHEMICAL ENGINEERING

CH32B – Process Dynamics and Control II

Midterm Examination: Monday, April 4/05 (Chem. Eng. Design Office)

Answer all questions. Copy the files created in each question onto a floppy disk using the suggested filenames. Label your disk with your name, student number and the number of the computer you used. All written work you wish to have marked must be written on the question paper.

The concentration of dilute liquors by evaporating solvent from the feed stream is an important industrial process used in sugar mills, alumina production and petroleum refining. Figure 1 is a conceptual representation of the first effect of the pilot-scale evaporator system modelled by Newell and Fisher (1972). The solvent is water and the solute is triethylene glycol.

Figure 1 - Evaporator system.

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BottomsProduct

LC1

Feed

OverheadVapour

FC1

PC1

Steam

CC1V4

V3

V2

V1

FC2

The liquid feedstream contains 3.2% triethylene glycol (TEG) by weight and is charged to the evaporator at a rate of 5 ft3/hr. Its temperature and pressure are 190 F and 38.8 psia, respectively. The volume of the evaporator is 1 ft3. At the initial steady-state, the evaporator is half-full and operating at a temperature of 225 F.

Flow control valves V1-V3 are linear-trim and designed to be 50% open at the nominal flowrates. The design pressure drop across V1 and V3 is 20 psi and that of the overhead vapour valve V2 is 11 psi. The steam control valve V4 is manipulated by concentration controller CC1 in order to regulate the mass fraction of TEG in the concentrated product. When V4 is fully-open, the steam coil is capable of transferring 2 105 Btu/hr of heat into the evaporator.

The bottoms product pump delivers a pressure rise of 40 psi with an efficiency of 75%. The flow transmitters are ranged for 0 – 10 ft3/hr and the pressure transmitter for 10 – 30 psia. The on-stream concentration analyzer is calibrated to detect TEG mass fractions in the range 0 – 0.1 and exhibits first-order measurement dynamics with a time constant of 1 minute.

(a) State the action (reverse or direct) required for each of the controllers in Fig. 1. Explain the reasoning behind your choice of action for FC2, LC1 and CC1.

[10 marks]

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(b) Construct a steady-state simulation of this process using HYSYS.Plant. Name the file test1 (ss).hsc. When building this model,

Choose the UNIQUAC property package.

Specify the proportional gain and integral time of the flow controllers as 0.7 %/% and 0.1 minute, respectively. Set the tuning constants of the pressure controller to 2 %/% and 5 min and the parameters of concentration controller CC1 to 2 %/% and 10 min.

Use a value of in the LC which will ensure that the controller output equals 100% when the level rises to 55% and is zero when the level falls to 45%. (For the purpose of computing this controller gain, the setpoint may be assumed constant at the initial liquid level of 50%.) Show your work in the space provided below.

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Select the Signal Processing option in the FC1 and FC2 PID blocks and install PV filters with time constant 5 seconds.

Model the analyzer dynamics using a Lag 1 transfer function with unity gain.

Configure a strip chart for each controller which will sample the PV, SP and OP every six seconds and trend the preceding six hours of operation. Change the span of the PV axes on the flow controller charts to 2 – 7 ft3/hr. Change the span of the PV axis on the concentration controller chart to 0.04 – 0.07.

Uncheck the Active box for Pressure Rise on the Dynamics tab of the pump and select Power instead. Activate the dynamic pressure specifications for each boundary stream on the flowsheet.

Left-click the Dynamics Assistant button on the toolbar to ensure that no problems will be encountered when transferring your model to Dynamics Mode.

[45 marks]

(c) Switch to Dynamics Mode. Place FC1, PC1 and LC1 in Automatic and secondary flow controller FC2 into Cascade mode. (If the level-to-flow cascade system has been properly built, the letter ‘R’ will appear on yellow background on the FC2 controller faceplate.) Run the simulation for one minute and then save your file as test1 (dyn).hsc.

[5 marks]

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(d) Open test1 (dyn).hsc and save it as test1d.hsc. Simulate 2 hours of open-loop operation (i.e. with CC1 on manual), introducing a 20% step increase in the controller output at time t = 10 minutes. Fit a first-order-plus-deadtime model to the reaction curve and describe the method used to estimate each parameter. The steady-state gain should be expressed in dimensionless form, i.e. with engineering units %/%. Save your case file, then close it.

[15 marks]

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(e) Open test1 (dyn).hsc and save it as test1e.hsc. Simulate 2 hours of open-loop operation, introducing a 0.5 ft3/hr step increase in the setpoint of feed flow controller FC1 at t = 10 minutes. Fit a first-order-plus-deadtime model to this reaction curve and describe the method used to estimate each parameter. Express the steady-state gain in dimensionless form. Save your case file, then close it.

[10 marks]

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(f) Open test1 (dyn).hsc and save it as test1f.hsc. Simulate 2 hours of feedforward/feedback control of the bottoms product composition, introducing a 0.5 ft3/hr step increase in the feed flowrate at time t = 10 minutes. The setpoint of the concentration controller is to remain fixed at 0.0485 (mass fraction TEG). Explain the method used to design your feedforward compensator. When the simulation is complete, save your case file and close it.

[15 marks]

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END OF QUESTION PAPER

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ch32b midterm1 2005

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