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Calculation of Gasoline Additives with Aspen Plus V8.0

1. Lesson Objectives Learn how to specify a mixer

Learn how to use the Calculator block in Aspen Plus to perform customized calculations for setting

targets and for getting results

2. Prerequisites Aspen Plus V8.0

3. Background Ethyl tert-butyl ether (ETBE) is an oxygenate that is added to gasoline to improve Research Octane Number

(RON) and to increase oxygen content. The goal is to have 2.7% oxygen by weight in the final product. The legal

limit is that ETBE cannot exceed more than 17% by volume. For simplicity, we use 2,2,4-trimethylpentane to

represent gasoline. Since ETBE's molecular weight is 102.18 g/mol, the ETBE in the product stream can be

calculated as following:

This yields 17.243% of ETBE by weight in the product stream. Given this, the Calculator block can be utilized to

target the ETBE feed to achieve the desired oxygen content.

In this tutorial we will calculate:

For a certain flow rate of gasoline (e.g., 100 kg/hr), how much ETBE should be added to achieve the

oxygen content of 2.7% by weight in the blended gasoline.

Check whether or not the legal limit of ETBE content is satisfied.

Two separate Calculator blocks are used to perform calculations on each criterion. Both targets should be met

in the simulation.

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

reflect an industrial application or real situation.

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4. Aspen Plus Solution If you are unfamiliar with how to start Aspen Plus, select components, define methods, or construct a flowsheet,

consult Get Started Guide for New Users of Aspen Plus.pdf for instructions.

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

4.02. The Components | Specifications | Selection sheet is displayed. In the Component ID column, enter

GASOLINE and ETBE. In the Alias column, enter C8H18-13 for component GASOLINE. This sheet should

look like the screenshot below.

4.03. Define methods. Go to the Methods | Specifications | Global sheet. Select PENG-ROB for Base

method.

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4.04. Move to the simulation environment by clicking the Simulation bar in the navigation pane. Add a Mixer

from the Mixers/Splitters tab on the Model Palette to the flowsheet and add two material inlet stream

and one material outlet stream. Rename the block and three streams so that your flowsheet looks like

the one below.

4.05. Navigate to the Streams | ETBE | Input | Mixed sheet. Enter 25 for Temperature and 1 for Pressure.

Select Mass for Total flow basis. Enter 17.243 for Total flow rate. Actually, you can enter a different

value here because it will be overridden by results from a Calculator block defined later. Select Mass-

Frac in the Composition frame and enter 1 for ETBE value field.

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4.06. Go to the Streams | FEED | Input | Mixed sheet. Enter 25 for Temperature and 1 for Pressure. Select

Mass for Total flow basis. Enter 82.757 for Total flow rate. Select Mass-Frac in the Composition frame

and enter 1 for GASOLINE.

4.07. Go to the Blocks | BLENDER | Input | Flash Options sheet. Confirm that Pressure has a value of 0

(interpreted by Aspen Plus as no pressure drop).

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4.08. In the navigation pane, select the Flowsheeting Options | Calculator node. The object manager for

Calculator is displayed. Click the New button to create a new calculator called TARGET. This

calculator block will set the target for the ETBE flowrate. Go to the Flowsheeting Options | Calculator |

TARGET | Input | Define sheet. New variables can be defined by clicking the New button. Here, we

define three new variables: MSETBE, MSGASOLI, and MWETBE.

4.09. For MSTEBE, select the Streams radio button in Category frame. Select Mass-Flow for Type, ETBE for

Streams, MIXED for Substream, and ETBE for Component. Select Export variable in Information flow

frame. This variable should update component ETBEs mass flow rate for in stream ETBE.

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4.10. We define variable MSGASOLI as follows. Select Streams radio button in the Category frame. Select

Mass-Flow for Type, FEED for Stream, MIXED for Substream, and GASOLINE for Component. In

Information flow frame, select Import Variable radio button. This variable receives the value of the

mass flow rate for component GASOLINE in stream FEED.

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4.11. For variable MWETBE, select Physical Property Parameter in the Category frame. Select Unary-Param

for Type, MW for Variable, ETBE for ID1 and 1 for ID2. In the Information flow frame, select Import

variable. This variable stores the value of ETBEs molecular weight.

4.12. Now, all three variables are displayed on the Flowsheeting Options | Calculator | TARGET | Input |

Define sheet.

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4.13. Navigate to the Flowsheeting Options | Calculator | TARGET | Input | Calculate sheet. Select Excel in

the Calculation method frame. Click the Open Excel Spreadsheet button.

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4.14. An Excel spreadsheet opens in a new window. The user can import or export variables by selecting a

variable in the drop-down list under the Add-Ins tab, as shown in screenshot below. Import both

MSGASOLI and MWETBE to the Excel spreadsheet to cell C3 and cell C6 respectively. Enter the label

and value for Target oxygen content (weight) in cells B1 and C1. Enter the label and value for ETBE

content limit in cells B2 and C2. The label for Target ETBE content (weight) is entered in cell B7. The

formula for calculating its value is =MWETBE/C5*2*C1 and is entered in cell C7. Fill in the remaining

text so your spreadsheet looks like the one below. See the next step for defining cell C8.

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4.15. The formula for Target mass flow rate of ETBE is =C7*C3/(1-C7) and is entered in cell C8. Also, C8 is

linked to variable MSETBE. Thus, the value from C8 is sent to MSETBE. When finished filling out the

spreadsheet, close the spreadsheet and return to the Aspen Plus user interface.

4.16. Navigate to the Flowsheeting Options | Calculator | TARGET | Input | Sequence sheet. Select Before

for Execute, Unit operation for Block type, and BLENDER for Block name.

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4.17. In the navigation pane, select Flowsheeting Options | Calculator. The object manager for Calculator is

displayed. Click the New button to create another Calculator block named CHECK. We will create two

variables: VETBE and VGASOLI. This calculator block will be used to check the results.

4.18. In the Flowsheeting Options | Calculator | CHECK | Input | Define sheet, click the New button to

create a new variable called VETBE. For VETBE, select Streams for Category. Select StdVol-Flow for

Type, ETBE for Stream, MIXED for Substream, and ETBE for Component. Select the Import variable

radio button in Information flow frame. This variable will provide the volumetric flow rate of

component ETBE in stream ETBE.

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4.19. Click the New button on the Flowsheeting Options | Calculator | CHECK | Input | Define sheet again

to create another variable called VGASOLI. For VGASOLI, select Streams for Category. Select StdVol-

Flow for Type, FEED for Stream, MIXED for Substream, and GASOLINE for Component. Also select

Import variable for Information flow. This variable will provide the volumetric flow rate of component

GASOLINE in stream FEED.

4.20. At this point, these two variables are displayed on the Flowsheeting Options | Calculator | CHECK |

Input | Define sheet.

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4.21. Navigate to the Flowsheeting Options | Calculator | CHECK | Input | Calculate sheet. Select Excel for

Calculation method. Click the Open Excel Spreadsheet button to open Excel spreadsheet in a new Excel

window. Link variable VETBE to cell C2 and VGASOLI to cell C3. The formula for calculating ETBE

content in PRODUCT stream (vol %) is =VETBE/(VGASOLI+VETBE)*100 and is entered in cell C5. Enter

text so your flowsheet looks like the following.

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4.22. Close the Excel spreadsheet window. Navigate to the Flowsheeting Options | Calculator | TARGET |

Input | Sequence sheet. Select Last for Execute.

4.23. Press F5 to run the simulation. After the simulation is complete, check the results from both calculators

to confirm that both targets have been met.

4.24. The legal limit of ETBE content of 17% by volume is met according to the calculator block CHECK.

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4.25. For a gasoline mass flow rate of 82.757 kg/hr, the target mass flow rate of ETBE is calculated to be

17.24293 kg/hr according to calculator block TARGET.

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5. Conclusions For a specified gasoline mass flow rate of 82.757 kg/hr, 17.24293 kg/hr of ETBE is needed to achieve 2.7%

oxygen content by weight in the final product. Furthermore, after blending, the product does not exceed the

legal limit for ETBE of 17% by volume. If gasoline contains a single component, manual calculation should be

easy without a simulator. However, real gasoline contains many unknown components and gasolines contents

vary as feedstock or plant operation conditions change. Therefore, manual calculation becomes very difficult

and the use of a simulator such as Aspen Plus can be helpful to carry out the calculation.

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