255
DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES
in the feed data sheet. For brevity, the faceplate and configure dialog box
included in the Aspen Dynamics window, shown in Figure 5.40.
2
a
are not
7 I Dynamic
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accepted
FIGURE 5.40
It is obvious that the reactor liquid level remains unchanged with a change in feed
temperature since there is no interaction involved On the other hand, the reactor
temperature is disturbed However the TC2 controller provides satisfactory disturbance
.
.
,
rejection performance under this situation.
So far we have studied mainly the closed-loop behaviour of a reactor system coupled
with Aspen-generated control schemes. We did not include any additional controller with
the CSTR model In Section 5.3 we consider a distillation example to elaborate this point.
.
,
5 3 DYNAMICS AND CONTROL OF A BINARY DISTILLATION COLUMN
Problem statement
A partially vaporized binary mixture of benzene and toluene enters a RadFrac
distillation model as displayed in Figure 5 41.
.
he column has total 25 theoretical stages (including condenser and reboiler) and
operates at a pressure in the reflux drum of 18 psia and reboiler of 21 psia. The
ow rate is 285 Ibmol/hr and reflux ratio is 2 2 (mole basis)
.
.
256
PROCESS SIMULATION AND CONTROL USING ASPEN
Feed Specifications
o
TOP
Flow rate = 600 Ibmol/hr
Temperature = 225° F
Pressure = 21 psia
Feed stage = 13 (above stage)
FEED
Mole %
Component
benzene
45
toluene
55
FIGURE 5.41
BOTTOM
A flowsheet of a distillation column
.
In Table 5.1, the reflux drum and the base of the column (the 'sump' in Aspen
terminology) are specified. It is fair to use an aspect ratio (length to diameter ratio) of
2 (Luyben, 2004).
TABLE 5.1
Item
Vessel type
Head type
Height / Length (ft)
Reflux drum
horizontal
elliptical
elliptical
5
25
5
25
Sump
-
Diameter (ft)
.
.
The column diameter is 5 ft. Use default values for other tray hydraulic parameters
(e.g., tray spacing, weir height and weir length to column diameter ratio). Consider logmean temperature difference (LMTD) assumptions for the total condenser. Actually the
LMTD is calculated using the temperatures of process fluid and coolant In the simulation.
assume constant reboiler heat duty and apply the UNIFAC base property method.
,
.
Simulate the column model to obtain the products mole fractions.
Keeping the default level and pressure control algorithms unaltered, inspect
the servo as well as regulatory performance of a proportional integral (PI1
controller that is required to insert to control the benzene composition in the
distillate by manipulating the reflux flow rate.
(0 Devising an another PI control scheme to maintain the benzene composition in
the bottom product with the adjustment of heat input to the reboiler, observe
the interaction effect between the top and bottom composition loops.
(a)
(b)
Simulation approach
(a) Select Aspen Plus User Interface and when the Aspen Plus window pops up.
choose Template and press OK. In the subsequent step, select General with
f
English Units and hit OK button. To open the process lowsheet window, click
OK when the Aspen Plus engine window appears.
Creating flowsheet
From the Model Library toolbar, select the Columns tab. Place the RadFrac model on
the flowsheet window and add the feed as well as two product streams. Renaming all
the streams along with distillation block, we have Figure 5.42.
DYNAMICH AND rnNTKOI, OP KI-OW DIUVKN I'lfOCKHHKH
.
257
imaimAJmm lAdftiauugj*) ji.ojjiIzJII
o:
6
-i
twin
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FIGURE 5,42
Configuring settings
Ah we hit Nt'xf followed by OK button, the ROtUp input Corni appcarH (h«m* Kitfun!
.43).
The diHtillation problem Is tilled oh: 'Cl0S6d-l00p Performanct; of a DihI illation ('olumn'
Iniportuntly, une tin- 'Dynanne' input noxle
mltiMI
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lit
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FIGURE 6 43
258
PROCESS SIMULATION AND CONTKOI, IISINC ASPEN
Figure 5.44 includes the Aspen I'lus (iccon/ilin
Accounting sheet with any name, number and ID.
infornuition We can fill up the
.
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FIGURE 5.44
We like to see the composition of all incoming and outgoing streams in mole fraction
basis in the final results table. Accordingly, we use Stream sheet under the Report
Options of Setup folder (see Figure 5.45).
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DYNAMICS \NI) CONTUOI, OK I'l.OW DRIVKN ('UOCICSSKS
259
Specifying components
'
From (In- Data Brow.scr, hoIoc! ( oniponcnta/Speeipcctions to open the componont input
lorm In ill'' lahlc. shown in l- i mc S 1 (> Ihc Ivvo species are dclincd
'
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FIGURE 5.46
Specifying property method
,,
>(. li'i on Hm- Nil
choose Properties/Specifications and gel the property Inpul form.
a property method originall} Includes several models for calculating
the physical properties For the distillation example, set the UNIPAC base method b>
.
in Aspen simulation
,
scrolling down (see Figure 5.47).
Specifying stream Information
,"
next, (.pen Streama IFEED IInput ISpecifications sheel Entering the given
Values lor all State variables and teed eompo
Figure 6 'IH
ion Ihe slream mpnl lorm looks like
.
Specifying block information
,"
lefl pane ol the Data Browser window select Blocks IRADFRACI Setup to open
Configuration sheet and then insorl the required datn (see Figure 5.49)
,
260
.
PROCESS SIMULATION AND CONTROL USING ASPEN
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FIGURE 5.48
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DYNAMICS AND CONTROL OK KLOW-DKiVEN PROCESSES
261
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FIGURE 5.49
In the subsequent step (see Figure 5.50), fill up Streams sheet with informing t\
location 113th tray (above stage)!.
M
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DYNAMICS AND CONTROL OK KLOW-DKiVEN PROCESSES
261
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In the subsequent step (see Figure 5.50), fill up Streams sheet with informing t\
location 113th tray (above stage)!.
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262
PROCESS SIMULATION AND CONTROL USINKi ASPEN
In Figure 5.51, the column pressure profile is defined.
Re
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FIGURE 5.51
Entering heat transfer data for condenser and reboiler
Next select Dynamic under RADFRAC of Blocks folder. There are three heat transfer
options: constant duty, constant medium temperature and LMTD. As mentioned in the
problem statement, the condenser heat duty depends on the log-mean temperature
differential between the process fluid and the coolant. The coolant inlet temperature is
set constant. Here the temperature approach represents the difference between the
process temperature and the coolant outlet temperature at the initial steady state
Note that among the heat transfer specifications the coolant inlet temperature and
temperature approach may vary during a dynamic simulation whereas the specific
heat capacity of the coolant is fixed during a dynamic run (see Figure 5.52).
,
,
For the reboiler
,
simulation. However
it is fair to use constant heat duty computed in the Aspen Plus
the reboiler duty may be changed at dynamic state either by
,
manually or automatically with employing a controller (see Figure 5.53).
Entering geometry data for reflux drum and sump
The reflux drum and sump are specified in Figures 5.54(a) and (b) with their given
geometry data. The information on vessel orientation, head type, length (or height)
and diameter are used to compute the vessel holdup
.
DYNAMICS AND CONTROL OF FLOW DmVKN PROCKSSP S
-
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263
4-
PROCESS SIMULATION AND CONTROL USING ASPEN
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|
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DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES
265
Entering tray geometry
The example column has total 25 stages-Stage 1 being the condenser and Stage 25
the reboiler. We already have inserted the necessary information for stages 1 and
25. Now, we need to inform the simulator the tray geometry specifications for stages
2 through 24. Note that the tray holdups are computed using these geometry data
(see Figure 5.55).
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FIGURE 5.55
Running steady state simulation and viewing results
Hit Afecf button and press OK to run the steady state simulation. Finally, the result."
table
,
shown in Figure 5 56, is obtained. At this time, we should save the work.
.
(b) Exporting dynamic simulation: For process dynamics study, we wish to
export the steady state Aspen Plus simulation into flow-driven Aspen Dynamics
simulation giving a file name of'Ch5
_
window
.
53
.
RadFrac'. Then close the Aspen Plus
_
266
PROCESS SIMULATION AND CONTROL USING ASPEN
E* Vte*v
Data
To*
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Help
1 ±Lil <iJF
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FIGURE 5.56
Starting Aspen Dynamics
Open a blank dynamic simulation window for the example column, following a similar
procedure as previously shown for the CSTR problem. In the next, simply open the
flow-driven dynamic file 'Ch5 5 3 RadFrac.dynf. As a result, the Aspen Dynamics
window appears (see Figure 5.57) accompanying with the closed-loop process flow
.
_
_
diagram. The flowsheet actually includes the three default control schemes LCI, PC2
and LC3 to monitor the reflux drum liquid level, top stage pressure and column base
liquid level, respectively.
In the present discussion, we do not want to change anything of the three
automatically inserted control strategies. All data, including timing parameters, ranges,
bias values and controller actions, remain untouched. A little detail of these control
structures is given below.
Loop 1
Controller: LCI
Type of controUer: P-only (since integral time is very large (60000 minutes))
Controlled variable: liquid level in the reflux drum
Manipulated variable: distillate flow rate
Controller action: direct
DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES
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FIGURE 5.57
Loop 2
Controller: PC2
Type of Controller: PI
Controlled variable: top stage pressure
Manipulated variable: coolant inflow rate
Controller action: direct
Loop 3{
Controller: LC3
Type of controller: P-only
Controlled variable: liquid level in the column base
Manipulated variable: bottoms flow rate
Controller action: direct
Adding a new PI controller for top composition loop
Now we wish to include a proportional integral (PI) law to control the benzene
composition in the top distillation product by manipulating the reflux rate. In the top
left of the window the Dynamics library is included within Simulation folder of Ml
Items pane Click on expand (+) button of Dynamics subfolder. Consequently, the expand
,
.
button changes to collapse (-) button as shown in Figure 5.58.
268
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Again hit expand button next to the ControlModels icon Then select PID controller
.
,
drag it to the flow diagram and drop the control block near to the top product stream
.
Renaming the top composition controller as CCT we have Figure 5.59.
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Connecting controller with process variable (Controlled variable)
Ixpand Stream Types under Dynamics subfolder and hold down the mouse button
he ControlSignal icon. As we drag it onto the flowsheet window, many blue an
-
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appear around the process diagram. Interestingly, when we f l I Z S
wiih holding the ControlSignal icon over a port, the name of that P
*ame
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.
CONTROL OF FLOW
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FIGURE 5.60
As we press 0/!l button, the cursor becomes a solid black arrow representing the
input signal to the controller. To transmit this signal to the CCT block, connect the
black arrow with a port marked InputSignal. Since this signal conveys the process
variable (PV) information to the CCT controller select 'CCT.PV by name with 'Process
,
variable
'
by description in the Select the Control Variable dialog box (see Figure 5.61).
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270
PROCESS SIMULATION AND CONTROL USING ASPEN
Hit OK button and obtain Figure 5.62. Obviously the CCT controller is partially
configured. To complete the top composition loop, the controller output should be
,
connected with the manipulated variable to pass on the signal
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Connecting controller with control variable (Manipulated variable)
Again hold the ControlSignal icon, drag it onto the process flowsheet and drop it on
the blue outgoing arrow marked OutputSignal from the CCT block. As Select the
Control Variable dialog box appears (see Figure 5.63), choose 'CCT.OP' by name and
press OK.
Immediately, a solid black arrow representing the controller output signal is
automatically generated. Move the mouse pointer to reflux stream and make a
connection to InputSignal2 port. To use the reflux flow rate as control variable, select
BLOCKSC'RADFRAC). Reflux.FmR' in the dialog box and click OK (see Figure 5.64).
'
Now the binary distillation column is coupled with four control schemes, LCI,
PC2, LC3 and CCT, and the closed-loop process looks like Figure 5.65. The subsequent
discussion includes the modification of different tuning properties of the CCT
controller.
DYNAMICS AND CONTROL OK FLOW DRIVKn PROCESSES
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Modifying controller tuning properties
First we wish to see the default tuning properties. So double-click on the CCT block and
then hit Configure symbol in the faceplate to open the Configure dialog box (see Figure 5.66).
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DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES
273
Obviously, some of the default values set by Aspen Dynamics are not acceptable.
For example, the operator set point value of process variable (benzene composition in
distillate) should not be greater than L Secondly, the CCT controller action must be
Reverse'. In addition, the value of control variable (reflux flow rate) at steady state is
'
usually used as
bias value.
We have two options in our hand to correct the default values. Either manually
we can do it or Aspen Dynamics can automatically initialize the values of set point
process variable, control variable, bias and ranges. Note that the controller action is
,
changed only manually. It is wise to initialize the values by the help of Aspen Dynamics
For this, press Initialize Values button in the Configure dialog box and use 'Reverse'
.
controller action. It is obvious in the window, shown in Figure 5.67, that the values of
SP, PV and OP in the faceplate change automatically to their steady state values. If
this approach fails to initialize the simulation of controller model with the steady state
data, check and replace, if necessary, the values of PV and OP with their steady state
values by double clicking on signal transmission lines (input to the controller and output
from the controller).
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Modifying ranges for process variable and controller output
we hit the Ranges tab, the Configure dialog box (see Figure 5.68) shows the
ranges imposed
on process variable and controller output.
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274
PROCESS SIMULATION AND CONTROL USING ASPEN
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DYNAMICS AND CONTROL OF FLOW DRIVEN PROCESSES
-
275
Process variable
Range minimum: 0.85
Range maximum: 10
Output
Range minimum: 10000 Ib/hr
Range maximum: 120000 Ib/hr
It is important to mention that it is a good idea to carry out Initialization as well
as Dynamic run after each new change in the control scheme so that any error in
controller installation can be detected individually.
We have now completed all required control specifications for the top composition
loop In the ongoing study, we prefer to conduct the simulation experiment to observe
the designed controller performance continuously for 5 hours. As done for the previous
f
CSTR problem, similarly either simply press Ctrl+F5 on the keyboard or select Pause
At rom the Run menu and put 5 hours as Pause at time.
In the next, we will inspect the CCT controller performance first dealing with the
servo problem followed by the regulatory problem.
Viewing servo performance of CCT
As we double-click on the CCT controller block in the flowsheet window the faceplate
appears. Then open the Configure as well as ResultsPlot dialog box The second one is
,
.
basically a blank graph sheet that presents the variations of process variable set point
and controller output with respect to time.
Before running the program make sure that all the items in the Configure dialog box
,
,
and faceplate are correct. In the next hit Run button to start the dynamic simulation. The
plots, shown in Figure 5 70, illustrate the servo behaviour of the PI control algorithm with
a step increase (0 9437 - 0 97 at time = 1.51 hours) followed by a step decrease
(0 97 - 0 9 at time = 3 hours) in the set point value of the distillate composition of benzene.
To achieve an improved closed-loop performance we have used the values of proportional
,
.
.
,
gain of 10 %/% and integral time of 10 minutes. These values have been chosen based
on a pulse input test in the distillate composition of benzene and using the trial-anderror approach It should be kept in mind that the objective at this point is not to come
up with the best control structure or the optimum controller tuning. We only need a
control scheme and tunings that provide a reasonably good tracking performance to
drive the simulation to a new steady state.
Remember that to edit the plots, shown in Figure 5 70, double-click on different
elements of the plots and modify them as we like.
Viewing regulatory performance of CCT
In order to investigate the regulatory performance of the CCT controller, we give a step
input change in the feed pressure (21 -» 23 psia) at time = 1.48 hours and that in the feed
temperature (225 -» 230oF) at time = 3 hours. The PI controller tuning set provides good
disturbance rejection performance (see Figure 5.71) although the tuning parameter values
.
gain and integral time) have been chosen based on a pulse set point input change.
276
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(c) Adding a new PI controller for bottom composition loop: We have to
devise another PI control scheme to monitor the bottoms composition of benzene
by adjusting the heat input to the reboiler. As developed, the CCT controller
for the top loop, similarly we can configure the CCB controller for the bottom
loop as shown in Figure 5.72.
DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES
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We have chosen the following tuning properties (see Figure 5.73):
Gain = 10 %/%
Integral time = 10 minutes
Controller action: Direct
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PROCESS SIMULATION AND CONTROL USING ASPEN
In addition, the used constraints are reported below:
Process variable
Range minimum: 0.0
Range maximum: 0.1
Output
Range minimum: 6000000 Btu/hr
Range maximum: 18000000 Btu/hr
Viewing interaction effect between two composition loops
To observe the effect of interaction between two composition loops, the set point value
of bottoms composition of benzene has been changed twice. The simulation result is
depicted in Figure 5.74 for a step increase (0.0033 -> 0.0045 at time = 1.5 hours) followed
by a step decrease (0.0045 -> 0.0025 at time = 3 hours).
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Clearly, the CCB controller shows satisfactory set point tracking performance
against a pulse input change It is observed from Figure 5.74 that owing to strong
.
interaction between the two composition loops of the distillation column the set point
changes in bottom loop affect the top product composition Similarly, when any set
point change is introduced in the top composition loop, the bottom product composition
,
.
will also be affected.