Dyn-001H Revised: Nov 13, 2012 1 Dynamic Analysis of a CSTR with Aspen HYSYS® V8.0 1. Lesson Objective: Understand the basic workflow to create and run a dynamic simulation using Aspen HYSYS Dynamics Set up a simple dynamic simulation of a CSTR Observe the effect of perturbations through changes in the controller settings 2. Prerequisites Aspen HYSYS V8.0 File Dyn-001_CSTR_Start.hsc Basic knowledge of controllers 3. Background Dynamic Simulation in Chemical Engineering Dynamic simulation is an extension of steady-state process simulation whereby time-dependence is built into the models via derivative terms i.e. accumulation of mass and energy. The advent of dynamic simulation means that the time-dependent description and control of real processes in real or simulated time are possible. This includes the description of starting up and shutting down a plant, changes of conditions during a reaction, holdups, thermal changes and more. Dynamic simulations require increased calculation time and are mathematically more complex than steady-state simulations. They can be seen as repeatedly calculated steady- state simulations (based on a fixed time step) with constantly changing parameters. Dynamic simulation can be used in both an online and offline fashion. The online case being model predictive control, where the real-time simulation results are used to predict the changes that would occur for a control input change, and the control parameters are optimized based on the results. Offline process simulation can be used in the design, troubleshooting and optimization of process plant as well as the conduction of case studies to assess the impacts of process modifications. 4. Problem Statement and Aspen HYSYS Solution Problem: Use the provided Aspen HYSYS file Dyn_001_CSTR_Start.hsc, prepare a dynamic simulation flowsheet and perform the following studies to investigate how the reactor system behaves dynamically when: Manipulate the level controller set point Vary the reactor feed flowrate
Transcript
Dyn-001H Revised: Nov 13, 2012
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Dynamic Analysis of a CSTR with Aspen HYSYS® V8.0
1. Lesson Objective:
Understand the basic workflow to create and run a dynamic simulation using Aspen HYSYS Dynamics
Set up a simple dynamic simulation of a CSTR
Observe the effect of perturbations through changes in the controller settings
2. Prerequisites
Aspen HYSYS V8.0
File Dyn-001_CSTR_Start.hsc
Basic knowledge of controllers
3. Background
Dynamic Simulation in Chemical Engineering
Dynamic simulation is an extension of steady-state process simulation whereby time-dependence is built into
the models via derivative terms i.e. accumulation of mass and energy. The advent of dynamic simulation means
that the time-dependent description and control of real processes in real or simulated time are possible. This
includes the description of starting up and shutting down a plant, changes of conditions during a reaction,
holdups, thermal changes and more. Dynamic simulations require increased calculation time and are
mathematically more complex than steady-state simulations. They can be seen as repeatedly calculated steady-
state simulations (based on a fixed time step) with constantly changing parameters. Dynamic simulation can be
used in both an online and offline fashion. The online case being model predictive control, where the real-time
simulation results are used to predict the changes that would occur for a control input change, and the control
parameters are optimized based on the results. Offline process simulation can be used in the design,
troubleshooting and optimization of process plant as well as the conduction of case studies to assess the
impacts of process modifications.
4. Problem Statement and Aspen HYSYS Solution
Problem: Use the provided Aspen HYSYS file Dyn_001_CSTR_Start.hsc, prepare a dynamic simulation flowsheet
and perform the following studies to investigate how the reactor system behaves dynamically when:
Manipulate the level controller set point
Vary the reactor feed flowrate
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Aspen HYSYS Solution:
4.01. Start Aspen HYSYS V8.0. Open Dyn_001H_CSTR_Start.hsc
4.02. Observe the flowsheet. This scenario simulates the production of propylene glycol from water and
propylene oxide. The controllers FIC-100, TIC-100, and LIC-100, control the feed flow, reactor
temperature, and reactor liquid level, respectively. Click on the Dynamics tab in the Ribbon. Notice that
the simulation is already in Dynamics Mode.
4.03. Press SHIFT + P. This will display the pressures of all the streams. Notice that input and output streams
(PreFeed, Reactor Vent, PRODUCT) have stars next to their values. This indicates that there is a
pressure specification on the streams. These specifications are needed for the dynamic simulation to be
Pressure Driven, as the dynamic flowrates are determined by pressure drops. The specifications can be
observed by double clicking on a stream and going to the Dynamics tab. Click SHIFT + N to display the
stream names.
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(FAQ) Types of Dynamic Simulations
Flow driven Feed flowrate and pressures are specified Flowrate is not controlled by pressure differences Useful for a first approach of the dynamic behavior of the process Good for liquid processes (usually good flow controllability)
Pressure driven Feed and product pressures are specified Flowrate results from pressure difference A bit more complex to specify (because you need to balance the pressures in Aspen
HYSYS with valves, pumps, ...) but more rigorous
4.04. The PRODUCT stream will be observed to determine the effects on the changing system. Double click
on the PRODUCT, and go to the Dynamics | Stripchart page. In the Variable Set dropdown menu, select
T, P, and F. Click Create Stripchart. This will create a new stripchart called PRODUCT-DL1.
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4.05. We will run following scenario to investigate the reactor dynamics:
Run the dynamic simulation for 2 hours (Note: this is in simulation time, not in real time)
Change the Level Controller (LIC-100) set point to 60%
Run the dynamic simulation for 3 additional hours; find how the product stream results are
being affected
4.06. In the Navigation Pane, click on the Strip Charts folder and press the Display button.
4.07. Right click on the chart and select Graph Control. In the Axes tab check Automatic Auto Scale in the
Auto Scale section and Show All in the Axis Display section.
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4.08. In the Time Axis tab, click the Set-up Logger button. This allows you to change the number of samples
the logger will keep on the plot. The full simulation takes 5 hours, which is equal to 900 20-second
intervals. Enter 900 in the Logger Size field.
4.09. In the Dynamics tab in the Ribbon, click on the Integrator button.
4.10. In the Integrator menu, enter an end time of 120 minutes. Click the Start button to run the first 2 hours
of the simulation.
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4.11. On the Flowsheet, double click on LIC-100, and change the set point to 60%. This specifies a tank level
that is 60% full.
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4.12. Change the End Time of the Integrator to 300 minutes, and press Continue to finish the remaining 3
hours of the simulation.
4.13. When the simulation finishes, right click on the strip chart PRODUCT-DL1 and select Graph Control. In
the Time Axis tab, change Low Time to 0 minutes.
4.14. Resize PRODUCT-DL1 by dragging the corner. Observe the effect that changing the tank height had on
the product stream.
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4.15. Change in feed rate. We will now experiment with another scenario to investigate the reactor dynamics.
(1) Run simulation for 2 hours
(2) Linear ramp up the feed rate from 2.064e+4 lb/hr to 2.8e+4 lb/hr in 2 hours
(3) Linear ramp down the feed rate from 2.8e+4 lb/hr to 0 lb/hr in 1 hour
4.16. Click the Integrator button in the Dynamics tab on the Ribbon. Clear the value for End Time so that it
reads <Non-stop> and press the Reset button. Click Yes on the subsequent window that appears.
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4.17. Change the SP of LIC-100 back to 87.22 and run the simulation until the PRODUCT stream stabilizes. This
will return the simulation to its original state. Reset the Integrator again.
4.18. Under Modeling Options in the Dynamics tab of the Ribbon, click on Event Scheduler.
4.19. The Event Scheduler allows us to set up a series of events that can take place at different times or due
to specific triggers. Click Add under Schedule Options to create a new schedule. Click Add on the right-
hand side of the window to create a new Sequence within the schedule.
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4.20. Click View to open Sequence A. This sequence will consist of three events. Ramping up the controller
set point, ramping down the set point, and terminating the sequence. Click the Add button three times
to create three events.
4.21. Double click on Event 1 to specify the first event. In the Condition tab, click the A Specific Simulation
Time radio button and enter 2 hours for Wait Until.
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4.22. In the Action List tab, click Add to create a new Action. In the Type dropdown menu, select Ramp
Controller. Click Select Target and choose FIC-100 for Controller. Enter 28000 lb/hr for the Target SP,
and 2 hours for Ramp Duration. Close the window.
4.23. In the window titled Sequence A of Schedule 1, double click on Event 2. This is the step where the
controller ramps down, and it occurs 4 hours into the simulation. Once again, click the A Specific
Simulation Time radio button, and this time enter 4 hours in the Wait Until field. Add a new Action in
the Action List tab, and select Ramp Controller for Type. Select FIC-100 for Controller, 0 for Target SP,
and 1 hour for Ramp Duration.
4.24. Double click on Event 3 in the Sequence A of Schedule 1 to specify the final event. Select the A Specific
Simulation Time radio button and enter 5 hours in the Wait Until Field in the Condition tab. In the
Action tab, Add a new Action, and select Stop Integrator for Type.
4.25. In the Sequence A of Schedule 1 window, click Start under Sequence Options.
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4.26. The simulation is ready to begin. Click Run in the Dynamics tab of the Ribbon.
4.27. View strip chart PRODUCT-DL1 to see the effects of changing the flow rate.
5. Conclusion
You should now be familiar with the basic setup of a dynamic simulation in HYSYS. You should also be familiar
with how to initialize a simulation, create custom plots, display results, and make changes in process conditions.
Changes in controller set points or other process conditions can have large effects on the overall process and it
is important to understand these effects when designing or operating a process.
