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CONFIDENTIAL EH/JAN 2013/CHE624/642

UNIVERSITI TEKNOLOGI MARAFINAL EXAMINATION

COURSECOURSE CODEEXAMINATIONTIME

PROCESS CONTROL AND INSTRUMENTATIONCHE624/642JANUARY 20133 HOURS

INSTRUCTIONS TO CANDIDATES1. This question paper consists of five (5) questions.2. Answer ALL questions in the Answer Booklet. S tart each answer on a new page.3. Do not bring any material into the examination room unless permission is given by the

invigilator.4. Please check to make sure that this examination pack consists of:

i) the Question Paperii) a one - page Appendix 1iii) a o n e - page Appendix 2iv) a one - page Appendix 3v) an Answer Booklet - provided by the Faculty

DO NOT TURN THIS PAGE UNTIL YOU ARE TOLD TO DO SO

This examination paper consists of 7 printed pages Hak Cipta Universiti Teknologi MARA C O N FI D EN TI A L

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CONFIDENTIAL 2 EH/JAN 2013/CHE624/642

QUESTION 1(P02 , C0 3, C3, C4)a) A unit step ap plied to a system generates the response in Figure 1 (Appendix 1).

i) Determine the type of response.(2 marks)

ii) Use the plot to label and determine the values of settling time , time of first maximum ,rise time, decay ratio and overshoot. Show your work in Appendix 1 and submit ittogether with your answ er's booklet. (10 marks)3

752 5

252

1.7$1.5

1.251

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0'1

UNIT Step Res ponsei / \ I i i ' 1 ' 1 ' 1 ' 1 i i i i / \ i i i i i i i i i i.i \r i i i i i i i i t I \ I l i I i i I i i .p I i i i i i i i i i

/ 1 I \ I / i I N__*j -^ i i i i i .i i I \ I / l i l i i I I iT i T \ 7 7 i i i i i 7 T i1 ' ' \ y i i i i i i i i 'I i i i i i i i i i i i

/ i i i i r i < i i f tJf i i i t i i i i i i1 l l l l f I l l l f I- 4 - i - + 1 1 1 - i . - 1 i * . - - + v -f t I I i I I I I I I I .L I i i i i I a i i i iJT i 7 7 i i i i i i i i' J i i i i i i i i i i i 1

1 2 3 4 S S 7 8 9 10 11 12Time (sec)

Figure 1 Process responseb) Graph in Figure 2 shows a response curve for a process with the following transferfunction ( 5 / y - ^p (1/^1)^+1

Determine how increment the value of A affects the response curve. Compare theresponses and justify your answer. (8 marks)

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CONFIDENTIAL 3 EH/JAN 2013/CHE624/642

5 t i m * 1 0t ime15 t

Figure 2 Process response

QUESTION 2( P 0 2 , P 06 , C 01 , C0 2 , C4, C5 )a) Consider the typical control system for the double-effect evaporator show n in Figure 3.

The eva porator is equipped w ith simple feedback co ntrol strategies.

Concentratedsolut ion

Condensate Condensate

Figure 3 P rocess Control for Double Effect-evaporatori Hak Cipta Universi t i Teknologi MARA CONFIDENTIAL

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CONFIDENTIAL 4 EH/JAN 2013/CHE624/642

i) List ALL controlled variables and the corresponding m anipulated variables for thedouble-effect evaporator. (4 m arks)ii) De termine the correct pair of the valve fail-position (in conside ration of safety) andcontroller a ction for control loop A and B. Justify your answer. (6 marks)

b) Given the CSTR process in Figure 4. Develop the feedforward/feedback controller fordisturbance rejection taking CA as controlled variable, Tc as manipulated variable and CAiand T| as disturbances. Draw the P&ID and block diagram. (10 marks)

F, C A i, T.

F T Jc* CI 0 0V [ >

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CONFIDENTIAL 5 EH/JAN 2013/CHE624/642

< Ej > ; jft tDS S'B a I -i -.1 .: 3-

Figure 5 Open loop test of pressure control loop

i) Calculate the respons e rates (RR) for process responses A, B and C. Show your workin Appendix 1 and subm it it together with your answe r's booklet.

ii) Ana lyze the RR value s in 3a (i) and justify why they have different values.(9 marks)

(3 marks)iii) Propose an advanced control system that can be used to overcome problem in 3a (ii).Discuss how this advanced control system improves the controllability of controller.(5 marks)

b) Consider the distillation column shown in Figure 6. In this column, the bottom productcomposition is regulated by adjusting the steam flows to the reboiler. D esign a feedbackcontrol schem e that ensures the bottom product composition is maintained. (3 marks)

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CONFIDENTIAL 6 EH/JAN 2013/CHE624/642

c) For the distillation column in 3(b), the level in the column mu st be maintained below themaximum level to avoid column flooding. It is more important that the column level bebelow its maximum than that the bottom product composition be at its set point. Proposea suitable co ntrol scheme for the distillation co lumn. (5 marks)

_ J kR Bottom Product

Figure 6 Distillation ColumnQUESTION 4( P 0 2 , P 0 6 , C 0 1 , C 0 2 , C 3, C5 )a) A flow control system need to be designed to m aintain the inlet flowrate to a reactor. Theflow control loop, consisting of an orifice meter in series with the control valve, adifferential pressure transmitter, and a controller is to be designed for a nominal processflow of 180,000 scfh of air. Valve inlet conditions are 100 psig and 60F, and the outletpressure is 80 psig. The valve has linear has linear characteristics and a square rootextractor is built into the transmitter so that its output signal is linear with flow. The valvetime constant is 0.06 min and the transmitter time constant is negligible. A proportional-integral (PI) controller controls the flow. (C f=0.9)

i) Propose a feedba ck flow control system and draw a complete P&ID of the controlsystem.(4 Marks)

ii) Obtain the valve capacity factor, Cv and size it for 1 00% o vercapacity. (9 marks)iii) If the installed control valve is an air-to-close valve and has constant valve pressuredrop, determ ine the transfer function of the control valve. (4 marks)iv) Calculate the gain of the transmitter if it is calibrated for a range of 0 to 250,000 scfh.

(3 marks)

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CONFIDENTIAL EH/JAN 2013/CHE624/642

QUESTION 5( P 0 6 , C 0 1 , C 4 , C5)

*** Flue 0B9

FURNACE

2 JLITT ]OIL IN

- * O IL OUT

( F C ) . / T C- (FT)

GasFigure 7 Close-Loop System

a) D evelop a block diagram for the closed-loop system of F igure 7 for the specific situation inwhich the transfer functions are given as follow:G v(s) = 0.5* + l' H^s) = 4, H 2(s) = 6r (^ -5 r i\ X

The inner controller is specified as a proportional controller with gain Kc2=5.0 and theouter controller is under PI control with Kc1=3.0 and Ti = 0.2.

(6 marks)b) Determine the closed-loop transfer function equation relating changes in the temperature

setpoint TSP (s ) to the outlet temperature T(s).(9 marks)

END OF QUESTION PAPER Hak Cipta Universiti Teknologi MARA CONFIDENTIAL

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CONFIDENTIAL APP ENDIX 1 EH/JAN 2013/CHE624/642

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Figure 1 Process response

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CONFIDENTIAL APPENDIX 2 EH/JAN 2013/CHE624/642

Figure 5 Open loop test of pressure control loop

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CONFIDENTIAL APP ENDIX 3 EH/JAN 2013/CHE624/642

Related Equation

f = C WG fft (scfh) =836 C v Cf -L=(y-0.148 / )TJGT

1.63 APvy= - 'Cf V P

Kv=T -(lna) % \ -100 (\ + kLclr l2iG fK v=+T^r(^a)Cv iAPv100 VA G,K v=+-^-cv^d + k Lc2vy^ iAp100 v,maxv " ' \GfK =+ C \^~

100 v'm\Gf

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