Separation of ethanol and benzene with a pressure swing method

Welcome back to our blog! Previously, we explored the basic concepts of the Pressure Swing method of chemical separation. For more complete material you can visit this page. Today, we will take you deeper into the case of azeotropes, where the composition of a mixture that is sensitive to pressure changes requires a clever solution. Let's discover how we can separate this dynamic duo, ethanol, and benzene, using the advanced Pressure Swing technique. Azeotrope with ethanol and benzene composition is pressure sensitive. So to separate the components of the azeotrope, a pressure swing can be used.

Case example

This case will use two distillation columns, the first column operating under 3 bar and the second column operating at 0.1 bar. A compressor is used to pressure swing the recycle stream from 0.1 bar to 3 bar before it is fed back into the first column. Since the relative volatility is large except for the azeotrope point, there is no need to add a third component (as solvent).

Aspen HYSYS completion

Open Aspen HYSYS and create a new simulation.

Define the component list that will be used by clicking Add in the Component List folder. Next, add Ethanol and Benzene to the component list.

Define the fluid package that will be used. In this simulation, we will use the PRSV fluid package.

Next, we start the Simulation by clicking Simulation at the bottom left of the screen.

Next, we add two Material Streams. Name the two streams as FEED and RECYLE.

Double click on FEED stream then go to Worksheet and enter the value of Vapor Fraction 0.5, Pressure 3 Bar, and Molar Flow 100 Kgmole/h. then go to the Composition section and enter the value of Mole Fraction 0.5 for ethanol and benzene.

Now we will define the RECYCLE stream. This stream will consist of ethanol and benzene vapors coming out of the top of the second column, which we will recycle so we don't waste any product. Double-click on the RECYCLE stream. On the Worksheet tab, enter Vapor Fraction 1, pressure 3 bar, and Molar Flow 200 kgmol/hr. On the Composition tab, enter a Mole Fraction of 0.5 for both ethanol and benzene. These values are initial estimates. They will eventually be replaced by the actual recycled products.

We now add the Distillation Column Sub-Flowsheet from the Model Palette.

Double-click on the Distillation column (T-10). On page 1 follow the information below, then click Next

On page 2 leave the default, just click Next

On page 3 enter the Condenser and Reboiler pressure of 3 bar. For this simulation, we will assume there is no pressure drop through the column. Click Next

On page 4 leave it blank. Just click Next

On the last page enter the Reflux Ratio value of 3. After that click Done

The Column T-10 window will appear. We need to specify the design specifications for the column. We have already determined the reflux ratio, but we still need to determine the mole fraction of benzene in the underflow. First, go to the Specs Summary form and make sure only the Reflux Ratio is checked as active.

Next, we will specify the mole fraction of benzene in the bottom stream. Go to the Design tab then click the Specs section. Click Add, and select Column Component Fraction. Select Stream for Target Type, Benzene for Draw, enter 0.99 for Spec Value, and select Benzene for Component.

Then the Degrees of Freedom will be 0. Click Run to start the calculation and the column should converge quickly.

Next, we will add the second distillation column

Double-click on the second column (T-11) to open Distillation Column Input Expert. On the first page enter the values as below. Then click Next

On page 2 leave the default, click Next

On page 3 enter the Condenser and reboiler Pressure value of 0.1 bar, click Next

On Page 4 distillation Column Input Expert leave it blank. Click Next

On the last page enter the Reflux Ratio value of 1. Next, click Done

Next, go to Specs Summary under the Design tab. We want to determine the reflux ratio and mole fraction of ethanol in the underflow. We have already specified the reflux ratio, but we still need to specify the mole fraction of ethanol in the bottom. First, make sure that the only current active specification is the Reflux Ratio.

Now we will specify the mole fraction of ethanol in the bottom stream. Go to the Design tab next to the Specs section click Add and select Column Component Fraction. Select Stream for Target Type, Ethanol for Draw, and enter 0.99 for Spec Value and Ethanol for Component.

Click Run to start the calculation, then the Column should be converged.

Before we connect the recycle loop, we must first add a compressor to increase the pressure of the REC stream. Add Compressor to the flowsheet from the Model palette

Double-click on the compressor (K-10). Select stream REC as Inlet. Then define the Outlet stream as Rec-HighP and the Energy stream as Q-Comp.

Enter a Pressure value of 3 bar on the Worksheet tab. The compressor should be finished

We can now connect the recycle loop. Add the Recycle block to the flowsheet.

Double-click on the recycle block (RCY-1) select Rec-HighP stream as Inlet and select RECYCLE stream as Outlet.

Check the result. Double-click on the Benzene stream and the Ethanol stream. You will find that the flow rate of each stream is approximately 50 kgmol/hour with a mole fraction of 0.999 for each product.