Aspen Plus - Getting started

What is Aspen Plus? - Aspen Plus is a powerful software tool enabling engineers to

quickly simulate plant behaviour and analyse data. The process simulation capabilities of

Aspen Plus allow engineers to predict the behaviour of a process using basic engineering

relationships such as mass and energy balances, phase and chemical equilibrium, and

reaction kinetics. With reliable thermodynamic data, realistic operating conditions and

rigorous Aspen Plus equipment models, it is possible to simulate actual plant behaviour.

1. Starting Aspen

You should be able to find Aspen through the start Menu, under the folder “AspenTech”

then “Aspen Engineering Suite”, “Aspen Plus 2004.1” and finally clicking on the “Aspen

plus user interface” icon. There are a number of Aspen products licensed to the university

however we will at this stage only deal with Aspen Plus.

.

You should then be presented with a screen as follows

From here you can choose a Blank simulation, template or an existing file. We shall

begin by using a template which will display results of the flow-sheet simulation in

metric units.

We will pick “General with metric units”. Notice the units which will be used in the

simulation. Click the “OK” button to connect to the server and start Aspen.

Your Aspen should look something like the following:

Using Aspen

Building a model in Aspen consists of the following steps:

Drawing the flow diagram of the model.

Specifying the variables of the model, such as stream compositions, equipment

temperature/pressure, etc.

Executing the model and analyzing the results.

When creating the flow diagram, you may employ a variety of equipment. These icons

may be brought up from the model library and put on the flow-sheet from the bottom of

the main Aspen window. This is the way the bottom part of the screen looks when you

first create a new blank window:

Title bar

Toolbar

Menu bar

Status bar

Workspace

Next button

Model

Library

Mode

Button

Title bar

Toolbar

Menu bar

Status bar

Workspace

Next button

Model

Library

Mode

Button

Note that there are nine different categories of units shown as ‘Tabs’. The first category:

Mixers/Splitters comes up as being selected when the window first opens, but you can

select any of the others with the left mouse button. Each of the categories then has several

types of units shown as icons below the name of the category. Each type can also be

represented by a different icon. Clicking on the down arrow to the right of the type will

show each of the icons. The material streams button can be changed to work or energy.

The three types in the first category are the mixer and two types of splitters. The default

icon for these is simply a shaded arrow with the name shown below it. If you click on one

of the icons, it will become lighter in color and you can then make a copy of it in the

workspace window by clicking at any position in the workspace. You can continue to add

that same unit by additional clicks in the flow-sheet space. Right clicking should cancel

the copy command. For example clicking three times in the flow-sheet after selecting the

Mixer icon might produce:

When finished, if you click on one of the icons it will be selected (the corners will have

colored squares on them and black squares will circle the icon) and you can then do

several things to it. Click the right button to bring up a menu that lists the operations and

select among the one you want:

The most frequently used of the operations are: Delete, Rename, Rotate and Exchange

Icon. If you select Exchange Icon, you will find there are lots of other ways to represent a

mixer such as:

Individual units: Mixers/Splitters

Mixer - Mixes streams together.

FSplit - Divides feed based on splits specified for the outlet streams.

SSplit - Divides feed based on splits specified for each sub-stream that may be

entering the splitter. For example, an entering stream may be a mixed solid and

liquid in the form of a slurry. The solids in the inlet may wind up preferentially in

one of the exit streams from the unit.

If you want to see a brief message describing the unit represented by one of the icons,

place your cursor on the icon and you will find the message at the bottom like the

following one for SSplit:

If you want more information about the unit, place your cursor over the icon and (without

clicking on it) hit the "F1" key. This should bring up a help window that will tell you

more.

Separators - There are five different separators:

Flash2 - Two-outlet piece of equipment that allows you to model flash separations

based on vapor-liquid equilibrium.

Flash3 - Three outlet flash for systems that might produce a vapor and two liquid

streams

Decanter - Separator with two liquid outlet streams

Sep & Sep2 - General separators of components in a stream

Exercise 1: Go through all the remaining equipment items to identify what each does.

Setting up a basic model:

Start my deleting any objects in the workspace. It is possible to drag a window around

any object or group of objects and then press the delete button.

What we will do in this very simple exercise is use the mixer module to blend two

material streams into one product stream. We will therefore set up Aspen to mix two

streams of water (one at 25 kg min-1 the other at 75 kg min-1) at the same temperature and

pressure (i.e. 25 °C and 1 atm). Even though we can calculate this very quickly in our

heads we will use this example to demonstrate how to set up an Aspen flowsheet.

Step 1: The first thing we do is place the equipment items required (i.e. a mixer) in the

workspace. Therefore click on the mixer icon and place one in the workspace.

Step 2: There are obviously two streams which must connect to the inlet of the mixer and

one outlet stream. We can draw these by clicking on the “Material streams” button.

Notice how the mixer icon has changed.

Two red arrows and one blue arrow have appeared. The red arrows are necessary, the

blue are not. Placing the cursor over one of the arrows will give you more information

about that stream (i.e. feed or product stream). We now need to add two inlet streams and

one outlet stream to the mixer. Click on the feed line (on the left) and move the cursor to

the left. A line should appear, on a second click the line will end at the current cursor

position. Notice an arrow on the line indicating the direction of flow and the line name.

Repeat this to add another feed line and finally add a product line. Your completed

picture should look like:

You have now drawn a process flow diagram which represents the problem. However we

have not given the computer any information about the streams involved.

Step 3: We need to provide Aspen with all the details of the materials to be used,

temperature, pressure what they are, the thermodynamic methods to be used etc. We start

this process by clicking “Setup” on the top toolbar.

The left menu contains all the details of the system, some of these are currently red

indicating that they are not complete; others are blue indicating that all of the necessary

information is available. One of the first things we should do in this initial screen to give

the file a title (e.g. Basic mixer) and to set the units to SI. The stream class should be set

to conventional (slurries or mixtures of solids and liquids and electrolytes are not

conventional streams).

Step 4: We now need to add the components i.e. water.

There is a large database of compounds in Aspen; it is possible to add compounds from

this database by clicking on the “find” button. For this simulation we want to find water.

Click on “water” (it will become highlighted) and then on the “add” button. Notice that

water appears in the main Aspen window. When you are finished press “close” and you

will be brought back to the main Aspen window.

The components section is now complete. We now need to set up the thermodynamic

method to be used in the calculation. Click on “properties” and “specifications”

There are a lot of different equations of state which can be used for thermodynamic

calculations. The one which is probably most familiar to you are the ideal laws. This is

sufficient for many processes as long as there is no interaction between molecules. If

there was interaction then you should choose another method such as UNIQUAC. You

can tell Aspen to use IDEAL by selecting it from the “property method” list. This will

automatically use IDEAL as the base method. This section should now be complete.

Step 5: We now need to input the stream data. We do this by clicking on “streams” and

“1” for stream 1, “2” for stream 2 etc. Notice that only stream 1 and 2 are red indicating

that they are incomplete. Stream 3 as a product stream is calculated and we do not

therefore put any data into this stream. If we did put data in then Aspen would use the

input data rather than the calculated data.

In this window we need to set the temperature of the stream to 25 °C, the pressure to 1

atm and the flow-rate to 25 kg min-1. When you have completed stream one input the

details for stream 2, i.e. 25 °C, 1 atm and 75 kg min-1.

Both streams should now have blue ticks beside them.

Step 6: The blocks are the units used in your flow diagram. For your Aspen file we need

to specify information about the mixer e.g. its pressure drop. Each type of block requires

some information. In this example we will set the pressure drop to be 0.

You should now notice that all the required data has been inputted and that the bottom

right hand corner indicates that “Required input complete”. We are now therefore able to

run the simulation.

Step 7: To run the simulation go to “run” on the toolbar and click “run”

You can view the results by clicking on the “results” icon in the tool bar. In the “results

summary” you can now click on “streams” to show the details of each of the three

streams including the calculated values for stream 3. It will also give information on

vapour fraction enthalpy etc.

Clicking on “stream table” will put the information on the process flow-sheet (you can

return to the PFD workspace by clicking on “workspace” in the “window” menu).

Exercises:

1: Repeat the simulation by changing the temperature of stream 2 to 50 °C. If the average

Cp for water is 4.187 kJ kg-1 °C-1 verify Aspens calculation by hand.

2. If 10 kg min-1 of low pressure steam at 150 oC and 2 bar is added to a stream

containing 60 kg min-1 of Methanol also at 2 bar and 20 oC, use Aspen to calculate the

vapour fraction and temperature of the exit stream.

3. On the same PFD add a separator to separate the vapour/liquid mixture. Notice the

options available in the block. What options will you select if we assume that this is an

adiabatic separator with no pressure drop? What are the mass flows of each stream?

Note: It is possible to connect the outlet stream from one module to the inlet stream of

another by reconnecting the destination of the product stream or the source of an inlet

stream. The option for “Reconnect Destination” or “Reconnect source” can be found by

right clicking on a stream.

4. Further add a heat exchanger to condense the vapour fraction from the separator.

Display the stream summary on the worksheet. Again what assumptions must you make?

Find the energy removed by the heat exchanger.

5: Set up an aspen flow-sheet to verify the following information: