ASPEN PLUS TUTORIAL

Yoel SanchezMcMaster University

Office: 27429Email: sancheya@mcmaster.ca

INTRODUCTIONAspen Plus is a process modeling tool

for conceptual design, optimization, and performance monitoring for chemical processes. Aspen Plus is a core element of AspenTech’s aspenONE® Process Engineering applications.

Some other tools offered by Aspen One: Dynamic Simulation, Heat exchanger design, Production planning, interface with Microsoft excel (Workbook).

Documentation. Where to find it?

C:\Program Files\AspenTech\Documentation

-AspenPlusProcModel2006_5-Start-AspenPlusSolids2006_5-Start- AspenPlusPetroleum2006_5-Start-AspenSimulationWkbook2006_5-Usr.-“Analysis, Synthesis, and Design of Chemical Processes”. Turton, Bailie, Whiting, Shaeiwitz.-“Chemical Engineering Design”, Towler, Sinnot.-“Working Guide to Process Equipment”, Lieberman, Lieberman.-“Distillation Design and Control using Aspen Simulation”, Luyben.

Manuals and textbooks of interest for this course:

Types of Process simulation program

Sequential-modular programs: in which the equations describing each process unit operation (module) are solved module-by-module in a stepwise manner. Iterative techniques are then used to solve the problems arising from the recycle of information..

Simultaneous (equation-oriented) programs: in which the entire process is described by a set of equations, and the equations are solved simultaneously . Simultaneous programs can give faster convergence when multiple recycles are present.

The Structure of a Process Simulator

Component Database

Thermodynamic Model Solver

Flowsheet Builder

Unit Operation Block Solver

Data Output Generator

Flowsheet Solver

Basic Computational Elements in Process Simulator

Select Chemical Components

Select Thermodynamic

Model

Input Topology of Flowsheet

Select Units and Select Feed

stream properties

Select Equipment Parameters

Select Output Display Options

Sequence of Input Steps for a Simulation Problem

Select Convergence

Criteria and Run Simulation

Turton, Bailie, Whiting, Shaeiwitz, “Analysis, Synthesis, and Design of Chemical Processes”, Second Edition, 2003

Problem Description Perform a Flash Calculation for the following stream:

Composition:

Ethane: 40%

Propane: 40%

n-Butane: 20%

Pressure: 15 bar

Temperature: 25 Celsius

Flow: 100 kmol/hr

Beginning the Simulation Start the Aspen program. It can be found in the All Programs menu

under:All Programs/AspenTech/Aspen Engineering Suite/Aspen Plus 2006.5/Aspen Plus

User Interface Choose what type of simulation you would like to use. We will use this time

the template option. The display below will appear and for this time we will choose General with Metric units.

Navigating the Aspen windows

Toolbar Feature

Equipment Model Library

Stream Library

Status Bar Simulation Status

Next Button

Creating a Process Flow Sheet To place a unit operation (or piece of equipment) into the flowsheet window,

select it from the Equipment Model Library and then click on the flowsheet window where you would like the piece of equipment to appear.

Streams can be added by clicking on the process flowsheet where you would like the stream to begin and clicking again where you would like the stream to end

Required Stream

Optional Stream

Some Aspen FeaturesColor Code:- Red: Information is required / simulation has finished with errors- Blue: Everything is ok.- Yellow: Input Information has changed / simulation has finished with

warnings.

Next Button:

Displays the next required input specification

Next Button

Status Indicators

Data BrowserAll of the data input for Aspen is entered in the Data Browser window. This window can be opened by clicking on the eyeglass icon or by going to Data/Data Browser in the Menu Bar. Aspen highlights the areas where the input has been complete and has not been completed with the use of either a blue check mark or a half filled red circle, as seen in the Figure.

However, you can not always assume that all of the required input has been entered, especially if you are simulating a more complex problem.

This feature will only track the minimal data input required to run a simulation and may cause problems in getting simulations to converge successfully.

It is recommended going through each icon on the left hand side one by one to make sure that you input all of the desired data for your particular application.

Specification of Chemical Components This consist of choosing the components that will be included in the mass

balance and deciding which models to use for the prediction of physical properties and phase equilibrium.

- Pure Components: Most of the pure components are organic compounds, but inorganic compounds and electrolytes are also included. The fact that a pure component is listed in a simulator data bank does not guarantee that any of the properties given for that component are based on measured data. If the properties of a compound are critical to process performance, then the scientific literature should be consulted to confirm that the values used in the simulation are realistic. The design engineer needs to consider carefully which components will have a significant impact on process design, operation and economics. If too few components are used, then the model will be inadequate for process design, as it will not correctly predict the performance of reactors and separation equipment. Conversely, if too many components are used, then the model can become difficult to converge, particularly if there are multiple recycles in the design.

Guidelines to build a component list can be found in: Towler, Sinnott, “Chemical Engineering Design” , 2008.

Specification of Chemical Components- Pure Components: are components created by the simulator to

match the boiling curves of petroleum mixtures. For diesel, crude oil, and heavy fuel oils, the number of possible compounds can be from 10^4 to >10^6. It represent an impossible task to include them all in a model. Instead, a large number of possible compounds with boiling points in a given range are “lumped” together and represented by a single pseudocomponent with a boiling point in the middle of that range. A set of 10 to 30 pseudocomponents can then be fitted to any petroleum assay and used to model that oil.

- Solids and Salts: Some solid phase components can be characterized as pure components and can interact with other components in the model through phase and reaction equilibrium. For more information about handling of solid components see AspenPlusSolids2006_5-Start.

- User Components: in Aspen Plus user-defined components are created trough “user-defined component wizard”. The minimum required information is the molecular weight and normal boiling point.

Specification of Chemical Components Aspen has a big database of commonly used (and some not so commonly

used) components and their physical properties. It also has an option where a user can define components that are not included in the database. If you already know the name of the component you can just type it in the define components list or you can use the “find” button tool

Selection of Physical Property Model Selecting the best physical property model is an extremely

important part of any simulation. If the wrong property package or model is used, the simulated results will not be accurate and cannot be trusted. The choice of models is often over looked by the novice, causing many simulation problems down the road.

Important facts:- The result of any process simulation are never better than the input

data, especially the thermodynamic data.- Everything from the energy balance to the volumetric flowrates to

the separation in the equilibrium stage units depends on accurate thermodynamic data.

- If the thermodynamic option used by the process simulator is a mystery, the meaning of the results obtained from the simulator will be equally mysterious.

- One should always resimulate with either another model that is assumed to be good or with different model parameters.

Selection of Physical Property Model In Aspen there is a tool to help in the selection of the property

method. This “expert” system called “Property method selection assistant” can give you the first guess for your property method, but need to be considered that further evaluations are required, including resimulation with different methods.

Selection of Physical Property Model Peng Robinson: This property method is comparable to the RK-SOAVE

property method. It is recommended for gas-processing, refinery, and petrochemical applications. Sample applications include gas plants, crude towers, and ethylene plants.

Selection of Feed stream Properties

Selection of Equipment ParametersPressure = 0 means no pressure drop through the equipment.Heat Duty = 0 means no Heat is added to the equipment.

Flowsheet with recycles For a sequential-modular simulation program to be able to solve a

flowsheet with a recycle, the design engineer needs to provide an initial estimate of a stream somewhere in the recycle loop. This is known as a “tear” stream, as the loop is “torn” at that point. The program can then solve and update the tear stream values with a new estimate. The procedure is repeated until the difference between values at each iteration becomes less than a specified tolerance, at which point the flowsheet is said to be converged to a solution.

The choice of tear stream can have a significant impact on the rate of convergence. Simulation programs automatically identify the best tear stream.

Convergence MethodsMost of the commercial programs include the methods described

below:

- Successive substitution (Direct Substitution): In this method, an initial estimate, x(k), is used to calculate a new value of the parameter, f(x(k).) The parameter is then updated using the calculated value:

x(k+1) = f(x(k))

x(k+2) = f(x(k+1)), etc

This method is simple to code but is computationally inefficient, and convergence is not guaranteed.

Convergence Methods Bounded Wegstein: Is the default method in most of the simulation

programs. It is a linear extrapolation of successive substitution.

The Weigstein method initially starts out with a direct substitution step:

x(1)=f(x(0))

An acceleration parameter, q, can be then calculated:

q= s/s-1

and, s=[f(x(k)) – f(x(k-1))/[x(k)-x(k-1)]

and the next iteration is then

x(k+1) = qx(k) + (1-q)f(x(k)

The bounded Wegstein is usually fast and robust.

Convergence Methods

Newton and Quasi-Newton Methods: The Newton method uses an estimate of the gradient at each step to calculate the next iteration. Quasi-Newton methods such as Broyden’s method use linearized secants rather than gradients. This approach reduces the number of calculations per iteration, although the number of iterations may be increased.

Newton and quasi-Newton methods are used for more difficult convergence problems, for example, when there are many recycle streams, or many recycles that include operations that must be converged at each iteration, such as distilation columns. The Newton method should not normally be used unless other methods have failed, as it is more computationally intensive and can be slower to converge for simple problems.

Running the simulationAt this point you can notice that the Simulation Status changes to “Required Input Complete”. There are a number of other features in the Data Browser that we will work with over the course, but for now our input is complete.

There are a few ways to run the simulation. The user could select either the Next button in the toolbar which will tell you that all of the required inputs are complete and ask if you would like to run the simulation. The user can also run the simulation by selecting the run button in the toolbar (this is the button with a block arrow pointing to the right). Finally, the user can go to run on the menu bar and select run. After the simulation is run and converged, you will notice that the Results Summary Tab on the Data Browser Window has a blue check mark. Clicking on that tab will open up the Run Status. If your simulation has converged it should state “Calculations were completed normally”

Running the simulation

That our simulation converged normally, it does not necessarily mean that the solution is reasonable.

Verify your results. Are the results reasonable?

Some checks to perform include a quick material balance, a quick heat balance, and a comparison to experimental or operating data if it is available. Further along in your careers, you will be able to use your experience to notice much more quickly if the results do not appear to be reasonable. However, even then you should look at every number that is presented in the results. If your results appear to be acceptable you can move on to adding the simulation results to the process flowsheet for ease of presenting.

Select Output Display Options Stream Table:

Select Output Display Options Adding stream conditions: In a large simulation, it is often useful to

add stream conditions directly to the streams themselves so the user doesn’t have to search through a large stream table for values. While this is not the case in our simulation we will now the mass flow rate and vapor fraction to each of the streams to learn how to do this.

This can be done in the Options window under Tools in the menu bar. When you have opened the Options window, click on the Results View Tab. Select the Mass flow rate and vapor fraction options and hit OK. You will notice those two properties will now be shown on your process flow worksheet as shown in Figure below.

Preliminary Plan for practical sessions.Practical sessions in laboratory:- Flash Calculation.

Spreadsheet configuration.

Generating Txy and Pxy Diagrams.- Impact of the Thermodynamic Property Methods on the simulation results.- Convergence Methods Behavior.

Spreadsheet configuration of a flowsheet with a recycle stream to evaluate convergence methods.

- Analysis of Distillation Columns.

Finding the optimum feed tray and Minimum Conditions/Column sizing.

Comparison shortcut methods with rigorous. - Heat Exchanger Design problem.- Modeling Processes with solid.

Reactors

Solid Library equipments- Sensitivity Analysis and Optimization- Aspen Simulation Workbook

Plan for practical sessionsTopics : (Following the course textbook content):

1. Introduction to design

2. Fundamentals of Material Balances

3. Fundamentals of Energy Balances and Energy Utilization (Pinch Analysis)

4. Flowsheeting

5. Piping and Instrumentation

6. Costing and Project Evaluation

7. Materials of Construction

8. Design Information and Data.

9. Safety and loss preventions

10. Equipment selection, specification, and design

11. Separation columns

12. Heat Transfer equipments

13. Mechanical Design of Process Equipment

14. General site considerations

WorkbookAspen simulation workbook is a tool for

interfacing AspenTech’s process simulation models with Microsoft Excel worksheets. Any number of model variables can be linked to an Excel sheet. Model variables can be input specifications, calculated results, or state parameters (such as number of stages). Once the variables are linked, the end users can follow the behavior of rigorous Aspen plus models through a friendly and easy to use Excel environment.

Workbook Aspen Simulation Workbook adds two toolbar menus to

Excel: the Aspen Simulation Workbook Design Toolbar, and the Aspen Simulation Run Toolbar. Model users using Excel version 2007 will view the toolbar menus as ribbon bar menus as shown below.

Workbook

Workbook

Workbook

Aspen Plus Tutorial

QUESTIONS?