CHEMICAL ENGINEERING 3P04
PROCESS CONTROL, SPRING 2008
ASSIGNMENT No. 5
Note: The purpose of the assignments is to provide you with challenges to help you learn the material in the course. For
this assignment, groups of up to three students are allowed. (Larger groups will receive a grade of 0%.) You may
complete this assignment individually, if you wish.
Your assignments must be neatly prepared and easily read by the grader. Points will be deduced for poor presentation, and
zero grades will be given for illegible papers.
1. (15 points) In this question, you will implement digital PID feedback control. The data for the
process is given in Figure 1.
Note that the controller is executed every 5 minutes. Why would we slowdown the controller
calculation? Usually, the cause would be the use of a slow analyser, that takes 5 minutes in this
case to supply a new measurement.
a.
Determine the proper tuning for this process for a continuous PI controller.
b.
Determine the proper tuning for the digital PI controller with a 5-minute execution period.
c.
Using the S_LOOP program, simulate the closed-loop behavior for a step change in the
set point using the tuning in (b). Discuss the performance.
2. (20 points) Piping and Instrumentation (P&I) Drawing - Figure 2, giving a drawing of two
distillation towers, is from Dr. Wood's textbook. This is an example of a piping and
instrumentation drawing, which is used extensively in engineering practice. Answer the following
questions. You should submit a copy of the drawing as part of your solution and clearly identify
each of your answers (with the question letter).
a.
b.
c.
d.
e.
f.
Locate one flow controller.
Locate one temperature controller.
Locate one pressure controller.
Locate one variable that is measured but not used for control. What variable is
measured, and why is it measured but not controlled?
Identify one variable with an alarm, and explain whether or not it is appropriate.
Locate one fail closed valve and explain whether or not it is appropriate.
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Sub-Menu #1
**************************************************************
*
S_LOOP: SINGLE LOOP CONTROL SYSTEM ANALYSIS
*
MODIFY FEEDBACK PROCESS PARAMETERS, CV(s)/MV(s) = Gp(s)
*
*
Kp (taulead(s)+1) exp (- theta(s))
*
Gp(s) = ------------------------------------------------*
(tau1(s)+1) (tau2(s)+1) (tau3^2(s)+2(tau3)xi(s)+1)
*
*
*
Measurement noise can be added to the dynamic simulation
*
(not the frequency response calculations) by setting kn
*
greater than 0.
*
*
noise standard deviation ~= 1.4 * kn
***************************************************************
SELECT THE APPROPRIATE MENU ITEM
MODIFY...
PRESENT VALUES
1) Process Gain, Kp
1.0000
2) Dead time, theta
5.00
3) Time Constant, taulead (numerator) 0.00
4) Time Constant, tau1 (lag)
0.00
5) Time Constant, tau2
5.00
6) Time Constant, tau3
0.00
7) Damping Coef., xi
1.00
8) measurement noise magnitude, kn
9) Return to main menu
Enter the desired selection:
0.05
Sub-Menu #3 – Enter the tuning constants
Sub-Menu #8
*************************************************************
* S_LOOP: SINGLE LOOP CONTROL SYSTEM DYNAMIC SIMULATION
*
*
*
*
Hints: 1) All inputs are steps at 5% of total time
*
*
2) If Entry 5 is non-zero, PID controller
*
*
is turned off
*
*
3) Select Time step (delta t) to approximate *
*
a) for the smallest time constant,
*
*
(delta t) / tau = 0.05
*
*
b) dead time/ (delta t) = integer
*
*
4) Usually, (total time) / (time step) < 1000 *
*
5) Digital controller exec time integer of
*
*
simulation delta time
*
*************************************************************
SELECT THE APPROPRIATE MENU ITEM
PRESENT VALUES
1) Total simulation time
100.00
2) Time step for simulation
0.100
3) Set point change
4) Disturbance change
5) Process reaction curve MV input
0.50
0.00
0.00
6) Select continuous/digital controller, currently digital
Controller period t = 5.00
7) Execute dynamic simulation
8) Return to main menu
Enter the desired selection:
Figure 1. Menus and data for Question 1.
2
*
*
*
*
*
*
*
*
*
*
*
*
*
Figure 2. Distillation tower from Woods, D.R. Process Design And Engineering Practice,
Prentice-Hall, Englewood Cliff, 1995
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3. (25 points) In this question, you will consider the process with a mixing point and three
mixing tanks (without reaction) shown in Figure 3. You have decided to control the
concentration of component A in the third tank effluent by adjusting the vA.
a.
You have found that single-loop control is not satisfactory. Design a cascade control
system that will achieve the greatest improvement in control performance. You can add
one (and only one) sensor; you can assume that the sensor dynamics are very fast and the
cost is low. Explain your choice and sketch your design on Figure 3.
The control system should respond well in the following situations.



Disturbance in fluid B composition of A
Disturbance in the fluid A pump outlet pressure
Change in the third tank composition controller set point
b.
Describe briefly how your design would function for the three situations above. Be sure
to state whether the cascade performance would be better than single-loop performance.
c.
Calculate the tuning for both controllers. Here, you cannot provide exact values, but you
should be able to estimate reasonable starting values.
KP = 0.040 % A/%open
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fluid B
F = 7 m /min after mix point
V = 35 m3 for each tank
vB
vA
fluid A
A
wgt fract A, C A3
Figure 3. Note that the concentration is dilute.
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4. (15 points) In this question you will evaluate a closed-loop dynamic response. The process is
known to have a dead time of about 3 minutes and a time constant of about 5 minutes; these are
approximate values and could be in error by about 30%.
Using the guidelines in Chapter 9, evaluate the performance in the following figure and if not as
good as expected, recommend which controller tuning constant(s) should be changed, increased
or decreased and by how much.
S-LOOP plots deviation variables (IAE = 11.943)
0.8
0.7
Controlled Variable
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
0
20
40
60
Time
80
100
120
0
20
40
60
Time
80
100
120
1.2
1
Manipulated Variable
0.8
0.6
0.4
0.2
0
-0.2
Figure 4. Dynamic response of closed-loop PI control system.
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