ASPEN Tutorial #1ChE 473K

VAPOR-LIQUID EQUILIBRIUM

We will first use Aspen to solve the following problem:

The ethanol(1) – water(2) system forms a non-ideal liquid mixture, which has an azeotrope at 78.17 oC and 4 wt% water at 1.00 atmospheres total pressure. In order to determine the composition of the vapor in equilibrium with a binary liquid mixture of known composition, we will send the liquid stream to a flash vessel, and define either the pressure or temperature, and the vapor fraction. By setting the vapor fraction to extremely small, 0.00001, the composition of the liquid stream will be virtually unchanged. Using ASPEN,

(a) Calculate the vapor composition in equilibrium with a liquid composition of 0.35 mole fraction ethanol and 0.65 mole fraction water with the SYSOP0 option set.

(b) Calculate the vapor composition in equilibrium with a liquid composition of 0.35 mole fraction ethanol and 0.65 mole fraction water using the UNIFAC w/ Redlich-Kwong option set.

(c) Determine the percent error in the vapor composition of the ethanol by Part (a) if the Redlich-Kwong method is assumed to be accurate.

This can be done using the FLASH2 separator model. Create a flowsheet that looks like Figure 1.

Figure 1: Example of flowsheet to be used in vapor-liquid equilibrium calculations.

Enter all of the information as indicated in the problem statement. When you reach the FLASH2 block specifications you must enter the following information.

Vapor Fraction: 0.00001Pressure: 1 atm

We enter the pressure value because we wish to study the system at the pressure specified by the problem. We enter the vapor fraction because we would like to see what the composition of the vapor would be over a liquid with a composition of .35 mole percent ethanol and .65 mole percent water. This will simulate these conditions, the liquid leaving the flash unit will have very close to this composition and the vapor will have the composition we desire to calculate.

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Before running the simulation go to the Setup-Report Options form in the Simulation Environment and select the Stream tab. Then click on the Mole checkbox in the Fraction Basis box (See Figure 2). This will include the fractional composition of the streams in the calculation results.

Figure 2: The Setup-Report Options screen used to include the molar fractional basis in the results.

Run the simulation, and record the results:

Results for SYSOP0:

Ethanol Vapor Fraction: 0.5478Water Vapor Fraction: 0.4522

Rerun for UNIFAC, and record the results:

Results for UNIFAC:

Ethanol Vapor Fraction: 0.5896Water Vapor Fraction: 0.4104

Percent Difference: EtOH: 7.07%Water: 10.16%

As you can see, incorrectly picking the EOS can create significant errors!!

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Now we will look at the following problem:

For the following mixture at 1.75 atmospheres,

Ethanol 58 wt%2-Propanol 9 wt%Water 33 wt%

Use ASPEN to determine the dew and bubble point temperatures with

(a) the UNIFAC option set, and(b) the NRTL option set.

Also determine the K values for each component at the bubble point by both methods.

Use the same simulation setup used for the previous problem, but enter the new information.

The determination of the bubble point temperature requires the vapor fraction of the FLASH2 unit to be set at 0.00001 as in the previous problem. This will give the bubble point composition and temperature. The K value is located in the Blocks-B1 results section under the Phase Equilibrium tab.

The determination of the dew point temperature requires the vapor fraction of the FLASH2 unit to be set at 0.99999. This will vaporize almost all of the incoming stream and allow you to look at the temperature of and composition of the last drop of liquid.

Record the results from your simulation:

For the UNIFAC option set:

Bubble Point Temperature: 95.2 °CDew Point Temperature: 101.4 °CK Ethanol (Dew Point): 4.62K Ethanol (Bubble Point): 1.40K Water (Dew Point): 0.62K Water (Bubble Point): 0.68

For the NRTL option set:

Bubble Point Temperature: 95.25 °CDew Point Temperature: 101.70 °CK Ethanol (Dew Point): 4.6K Ethanol (Bubble Point): 1.42K Water (Dew Point): 0.62K Water (Bubble Point): 0.67

You should now be ready to do your homework. The only additional concept is the vary block. The vary block allows you to vary the one of the input variables in a systematic way and look at its effect on specific output variables. We will use a vary block to look at the effect of varying the FLASH2 pressure on the bubble point temperature. For this simulation please set your option set to UNIFAC.

1. In the Simulation Environment, select Model Analysis Tools- Sensitivity from the Navigation Pane

2. Click the New… button within the Define folder (this is where you define the name and properties of the output variables).

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3. Enter the ID as SENS-1.

4. Click New… button within the Define folder and name the variable “TEMP”.

5. We would like to know the bubble point temperature or the temperature in stream #3. Enter the information as seen in Figure 3.

Figure 3: Sens-1 Define Variable Screen.

6. Next, you must enter the conditions under which you would like to vary the input. Here we are going to vary the pressure in the FLASH2 unit. You can enter this information as shown in Figure 4.

Figure 4: Sens-1 Vary Input Screen.

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7. Next you should enter the output format in the tabulate form, see Figure 5.

Figure 5: Sens-1 Tabulate Input Screen

8. Switch the Unit Set to MET so that the pressure is reported in atm instead of bar. In the Properties Environment, click Setup-Specifications and change the global unit set to MET. To check the units used in each Unit Set, go to Setup - Unit Sets.

9. Now you are ready to run your simulation. The output will have a folder for Model Analysis Tools- Sensitivity- SEN1 which will contain the results of your sensitivity analysis. Those results can be plotted by clicking on Results Curve at the top of the menu. Choose “Temp C” then click OK to see the final curve.

Another way to get bubble point and dew point curves is the following:

1. Select a stream (take S1) by clicking on it. Go to Stream Analysis -> Bubble/Dew on the Home Menu2. In the Bubble-Dew form check both Bubble Point and Dew Point3. Enter a Pressure Range (for our problem 1 atm to 1.75 atm) and the number of points (20)4. Click on GO

A plot will immediately appear with the Bubble point and Dew point curve.

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ASPEN HW Assignment

1. For the following mixture at 250 psia:

Methanol CH4OH 24.3 mole%i-Butane C4H10 18.2 mole%n-Butane C4H10 23.8 mole%i-Pentane C5H12 33.7 mole%

use ASPEN to calculate the bubble point temperature, the dew point temperature, and the entire flash curve (V/F versus T) with:

(a) Ideal physical properties(b) UNIFAC physical properties(c) NRTL-RK physical properties

Do these methods give significantly different answers?

Which method do you think gives the best results? Why?

You must answer these questions, and turn in a V/F versus T graph for each physical properties set. Please highlight the bubble point temperature and the dew point temperature on each graph.

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