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Generate Ethylene Vapor Pressure Curves with Aspen Plus® V8.0

1. Lesson Objectives Learn how to use the Methods Assistant to select physical property methods

Generate vapor pressure curves for Ethylene

2. Prerequisites Aspen Plus V8.0

Introduction to vapor-liquid equilibrium

3. Background Separation processes involving vapor-liquid equilibrium exploit volatility differences that are indicated by the

components’ vapor pressure. Higher vapor pressure means a component is more volatile.

Ethylene is an important monomer for polymers and there are many ethylene plants around the world. A vital

step in ethylene production is separating it from other compounds and as a result the vapor pressure of

ethylene is an important physical property for ethylene production.

The examples presented are solely intended to illustrate specific concepts and principles. They may not

reflect an industrial application or real situation.

4. Problem Statement and Aspen Plus Solution

Problem Statement

Determine the vapor pressure of ethylene at room temperature (25 °C), and its normal boiling point.

Aspen Plus Solution

If you are unfamiliar with how to start Aspen Plus, select components, or define methods, consult Get Started

Guide for New Users of Aspen Plus.pdf for instructions.

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4.01. Create a new simulation in Aspen Plus using the Blank Simulation template.

4.02. The Components | Specifications | Selection sheet is displayed.

4.03. We must first specify which components will be used in the simulation. Enter ETHYLENE into the first

row under the Component ID column and hit enter. The Component ID is a user defined parameter

that Aspen Plus will use to report and identify components in a given simulation. Aspen Plus will also

guess as to which specie the user has entered based on the given Component ID and retrieve physical

properties from its databases. The other fields can be entered manually if the information is incorrect

or was not found. In our case, Aspen Plus successfully retrieved accurate information for ETHYLENE

from the Component ID alone.

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4.04. Next, we will specify the method that Aspen Plus will use to calculate physical properties in this

simulation. We will be using the Methods assistant to help us select an appropriate Base method. Go

to the Methods | Specifications | Global sheet by pressing the F4 key or using the tree view in the

navigation pane. Click the Methods assistant… button.

4.05. The methods assistant window should appear. Click the Next arrow.

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4.06. Click the Specify component type link.

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4.07. Since we know ethylene is a small hydrocarbon, we will select Hydrocarbon system.

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4.08. We are not using pseudocomponents or petroleum assays, so we will select the corresponding option.

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4.09. The methods assistant is now displaying several methods that are well suited to our components. We

will use the Peng Robinson equation of state.

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4.10. Close the Assistant – Property method selection window. On the Methods | Specifications | Global

sheet, set the Base Method field to Peng-Robinson which is listed as PENG-ROB.

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4.11. Upon selecting this option, your window should now appear as follows. You are now ready to beg in

generating vapor curves for ethylene using the Peng-Robinson equation of state.

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4.12. We will now generate a new analysis object. From the Home tab of the ribbon, click the Analysis | Pure

button

4.13. A pure component analysis, PURE-1, is created and the Analysis | PURE-1 | Input | Pure Component

sheet is displayed. Note that the Run analysis button at the bottom of the sheet is not enabled because

the necessary inputs for the analysis have not been entered yet.

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4.14. Next, select the physical property you would like to analyze. In this case, select PL from the Property

drop-down list. Physical properties are given unique identifiers in Aspen Plus. PL signifies vapor

pressure of a liquid. Some commonly used ones are DHVL (enthalpy of vaporization), and PHI (fugacity).

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4.15. Next, we will tell Aspen Plus what to plot. In the Temperature frame, set the Lower Limit to –110 and

the Upper Limit to 25. In the Components frame, select ETHYLENE from the list of Available

components and click the single right arrow. This will move only the selected items from the list on the

left to the list on the right. Clicking the double arrow will move all of the items, which is useful for

moving a large number of items. Note that the Run analysis button is now enabled.

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4.16. Press the Run analysis button. This will generate a plot of the vapor pressure in a new tab. Note that at

25 °C, ethylene’s vapor pressure is about 70 bar.

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4.17. Repeat actions from step 4.12 to step 4.16 except that we use -100 °C as the Upper limit instead of 25

°C. The generated plot is shown below. Note that the normal boiling point is about –104 °C.

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5. Conclusions As we can see from the generated plots, ethylene is a very volatile component. At room temperature (25 °C), its

vapor pressure is about 70 bar. From this analysis, we also see that ethylene’s normal boiling point temperature

is about –104 °C.

6. Copyright Copyright © 2012 by Aspen Technology, Inc. (“AspenTech”). All rights reserved. This work may not be

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