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Aspen Plus Tutorial 6_Azeotropic.Extractive Distillation

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					Aspen Plus Tutorial 6

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Aspen Plus Tutorial 6: Azeotropic/Extractive Distillation
This tutorial will guide you though the setup of an azeotropic distillation column using RADFRAC.

Residue and Miscibility
Before setting up the model, the tutorial will show how to use Aspen's ability to create residue curves based on your chose models and the add to that a liquid-liquid miscibility curve.

Flowsheet
Start a new flowsheet in Aspen and add a FLASH3 unit and add stream FEED to the feed stream, VAPOR to the vapor stream, 1-LIQUID to the first liquid stream and 2-LIQUID to the second liquid stream.

Components and Properties
The components are vinyl acetate (VA, VINYL-ACETATE, C4H6O2-1), acetic acid (AA, ACETICACID, C2H4O2-1) and water (H2O, WATER, H2O). We will use the NRTL-RK property set.

http://www.che.utexas.edu/~che473k/aspen/aspentutor6.htm

2005-2-24

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Feed Stream
We will set up Aspen to vary this stream a short bit later. For now define the feed stream as having T=230F, P=65psi, VA = 1 lbmol/hr, H2O = 1 lbmol/hr. This will be the pressure at the top of the column.

Flash3
Define the decanter as shown below. Here we have set the pressure to 0 denoting no pressure drop and heat duty to zero denoting adiabatic phase separation. Thus this unit will have the same conditions as the inlet stream. The key component (the majority component) in the 2nd liquid stream will be water.

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Sensitivity Analysis
We can use Aspen to get an idea of how sensitive (how much our process changes) our process is to a changing variable. We will use this feature to get a list of the liquid-liquid equilibria for a waterVA-AA system. We will later put this on a ternary diagram. To set the sensitivity, expand "Model Analysis Tools" and click on Sensitivity. Add a new object. You will then need to add FORTRAN variables to calculate your data. Don't despair. We won't be using any heavy duty FORTRAN and you shouldn't need much more than what is in this tutorial. Just so you know, basic statements are much like any other programming language. y = 2 (2+ 2x sin(x)/2)x-1 would be written y = 2*(2+2*x-sin(x)/2)**(x-1). But again, you probably won't need all of that. We want to examine 6 variables, the mole fractions of each component in the 2 liquid streams. Select new to enter in each of the variable definitions.

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etc... You should have the following variables entered in the end: Name AA1 VA1 WA1 AA2 VA2 WA2 Type Mole-frac Mole-frac Mole-frac Mole-frac Mole-frac Mole-frac Stream 1-LIQUID 1-LIQUID 1-LIQUID 2-LIQUID 2-LIQUID 2-LIQUID Substream MIXED MIXED MIXED MIXED MIXED MIXED Component AA VA H2O AA VA H2O

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So you know, Aspen uses an old FORTRAN convention of only having six letters for variable names. We will vary the flow of acetic acid. The system forms a two phase liquid up to about 33% acetic acid. We will collect 21 data points varying acetic acid from 0-1lbol/hr.

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We will simply tabulate the variable we defined above.

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Run Simulation
Everything is in, so save and run the simulation. The results under Model Analysis Tools\Sensitivity\S-1\Results will show you how your model reacted. You should notice that the final sensitivity run does not have information for the second liquid. That point is outside of the immiscibility region.

Ternary Diagram
We want to plot this data on a ternary diagram that Aspen will create. To do this, in the menu, go to Tools|Analysis|Property|Residues. Components are VA, AA, H2O and the pressure should be 65psi. Valid phases should be Vapor-Liquid-Liquid. Click Go to make a plot.

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Now you can plot the data from the 3 phase flash sensitivity run on the plot (Print it out and do it by hand).

RADFRAC Setup
Save that project and save as a new name again. This will be your RADFRAC file. Erase the unit and the product streams and delete the sensitivity run.

Flowsheet
Set up the following flowsheet. Be sure that VA-RICH is in the liquid distillate port and the H2ORICH is in the 2nd liquid distillate port. Note that RADFRAC has the appropriate picture this time (mainly for clarity).

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Feed Stream
The feed stream, like the column configuration, is taken from last year's project.

http://www.che.utexas.edu/~che473k/aspen/aspentutor6.htm

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RADFRAC Setup
Set up RADFRAC using the specifications on the following sheets. Some things of note: The valid phases are Vapor-Liquid-Liquid We will be extracting 2683 lbmol/hr from the bottom of the column, the same amount of AA going in. VA-RICH should be 1st liquid, H2O-RICH should be 2nd liquid. Three phase calculations will happen on all stages. Water is the key component in the 2nd liquid stream.

http://www.che.utexas.edu/~che473k/aspen/aspentutor6.htm

2005-2-24

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2005-2-24

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2005-2-24

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Run Simulation
Save and run.

Assignment
Plot a residue curve for the VA,AA,H2O system. On the curve show the liquid-liquid equilibrium curve and tie lines for conditions at the top of the column (230F, 65psi). Draw in the Feed, VARich, Water-Rich, and Bottoms points on this diagram based on the azeotropic distillation from RADFRAC. Also indicate the material balance point, and the distillate mixture point. Draw lines between significant points. Turn this page and a printout of your stream summary in with your group number and name.

http://www.che.utexas.edu/~che473k/aspen/aspentutor6.htm

2005-2-24


				
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