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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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