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Dyn-001 Revised: Oct 17, 2012

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Dynamic Analysis of a CSTR with Aspen Plus V8.0

1. Lesson Objective:

Understand the basic workflow to create and run a dynamic simulation using Aspen Plus Dynamics Setup a simple dynamic simulation of a CSTR Observe the effect of perturbations through changes in the controller settings

2. Prerequisites

Aspen Plus V8.0 Aspen Plus Dynamics V8.0 File Dyn_001_CSTR_Start.bkp

3. Background

Dynamic Simulation in Chemical Engineering

Dynamic simulation is an extension of steady-state process simulation whereby time-dependence is builtinto the models via derivative terms i.e. accumulation of mass and energy. The advent of dynamicsimulation means that the time-dependent description and control of real processes in real or simulatedtime are possible. This includes the description of starting up and shutting down a plant, changes ofconditions during a reaction, holdups, thermal changes and more. Dynamic simulations require increasedcalculation time and are mathematically more complex than steady-state simulations. They can be seen asrepeatedly calculated steady-state simulations (based on a fixed time step) with constantly changingparameters. Dynamic simulation can be used in both an online and offline fashion. The online case beingmodel predictive control, where the real-time simulation results are used to predict the changes that wouldoccur for a control input change, and the control parameters are optimized based on the results. Offlineprocess simulation can be used in the design, troubleshooting and optimization of process plant as well asthe conduction of case studies to assess the impacts of process modifications.

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Aspen Plus Basic Workflow for Dynamic Simulation Study

The examples presented are solely intended to illustrate specific concepts and principles. They may notreflect an industrial application or real si tuation.

4.

Problem Statement and Aspen Plus SolutionProblem : Use the provided Aspen Plus file Dyn_001_CSTR_Start.bkp , prepare a dynamic simulation flowsheetby adding required dynamic data, and export the simulation to Aspen Plus Dynamics. Then, perform thefollowing studies to investigate how the reactor system behaves dynamically:

Manipulate the level controller set point Vary the reactor feed flowrate

Step 1: Build Steady-State Simulation in Aspen Plus

Step 2: Prepare Dynamic Simulation; Add Dynamic Data

Step 3: Export Simulation to Aspen Plus Dynamics

Step 4: Simulation in Aspen Plus Dynamics

Step 5: Change control system, apply disturbanes, and more....

Export to Dynamics

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Aspen Plus Solution:

4.01. Start Aspen Plus V8.0 . Open Dyn_001_CSTR_Start.bkp .

4.02. Go to the Dynamics tab on the ribbon, and press the Dynamic Mode button.

You wil l then notice that the simulation requires more data to prepare the dynamic simulation:

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4.03. Go to Blocks | CSTR | Dynamics and enter 1 me ter for the Length of the reactor.

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(FAQ) Vessel Geometry Dynamic Data

Vessel Type: - Instantaneous: Default vessel type for most vessels requires no input for

vessel geometry- Vertical- Horizontal

Vessel Geometry: - Head Type

Elliptical Hemispherical Flat

- Length- Diameter

4.04. Press the Next Input ( F4) button and the following dialog window will appear. Then, press OK to runthe simulation.

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4.05. The simulation should complete without warning or error. Go to the Dynamics tab and make sure thatall the buttons are enabled. Find the Flow Driven button and press it.

4.06. You will then be prompted to specify the Aspen Plus Dynamics file to be exported. Enter the file nameDynamics_CSTR_Fdriven.dynf , and press the Save button. After a few moments Aspen Plus Dynamics

wil l automatically open.

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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 bi t more complex to specify (because you need to balance the pressures in Aspen Plus

with valves, pumps, ...) but more rigorous

4.07. From this point on, we will be working in Aspen Plus Dynamics V8.0, which should have automaticallyopened after saving the *.dynf file. In Aspen Plus Dynamics, change the run mode to Initialization , andpress Run (F5) .

The Run complete dialog will appear to notify you that the initialization run has completed withconvergence. Press OK.

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4.08. Display stream results plot. Right click on the PRODUCT stream to open the context menu, go to Form and select TPFmPlot and Results . TPFmPlot is an built-in plot form to show Mass Flow, Pressure, andTemperature versus time. The Results form is a table form which shows key results in the stream at thecurrent time.

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4.09. Change the run mode to Dynamic .

4.10. We wi ll run fol lowing scenario to investigate the reactor dynamics:

Run the dynamic simulation for 1 hour (Note: this is in simulation time, not in real time) Change the Level Controller (CSTR_LC) set point = 0.5 meter Run the dynamic simulation for 1 hour; f ind how the product stream results are being affected

(1) Press F9 or go to Run menu | Run Options . Check the Pause at optionand enter 1 hour for the value. Press OK.

(2) Run the dynamic simulation. It will be paused after 1 hour of simulationtime.

(3) On the flowsheet, right click on CSTR_LC and select Forms | Configure .Enter 0.5 meter for the new set point. Go to Run Options (F9), and enter 2hours for the next pause time.

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(4) Start the dynamic simulation. It wil l be paused after 1 hour. You will see the results below.

4.11.

Change in feed rate . We will now experiment with another scenario to investigate the reactor dynamics.

Linear ramp up the feed rate of component CIS from 100 to 200 kmol/hr in 1 hour Linear ramp down the feed rate of component CIS from 200 to 0 kmol/hr in 0.5 hour

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(1) For a ramp change in feed, right click on the FEED stream, and select Forms | Manipulate . In theManipulate table, right click FcR(CIS) value, select Ramp . It will open Variable Ramp window.

(2)

The scenario (100 200 0kmol/hr) will give following results in the Product stream.

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5. Conclusion

You should now be familiar with how to take a simple Aspen Plus simulation and convert it to an AspenDynamics simulation. In Aspen Plus Dynamics you should be familiar with how to ini tialize a simulation, createcustom plots, display results, and make changes in process conditions. Changes in controller set points or otherprocess conditions can have large effects on the overall process and it is important to understand these effectswhen designing or operating a process.

6. Copyright

Copyright 2012 by Aspen Technology, Inc. (AspenTech). All rights reserved. This work may not bereproduced or distributed in any form or by any means without the prior written consent ofAspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITRESPECT TO THIS WORK and assumes no liability for any errors or omissions. In no event will AspenTech beliable to you for damages, including any loss of profits, lost savings, or other incidental or consequentialdamages arising out of the use of the information contained in, or the digital files supplied with or for use with,this work. This work and its contents are provided for educational purposes only.

AspenTech, aspenONE, and the Aspen leaf logo, are trademarks of Aspen Technology, Inc.. Brands andproduct names mentioned in this documentation are trademarks or service marks of thei r respective companies.

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