Production of Anhydrous Ethanol Using an Extractive Distillation Column

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Consider a binary mixture of water and ethanol at 1 atm. This mixture exhibits a positive azeotrope ( ethanol), so that anhydrous ethanol cannot be produced by a simple distillation column. If an entrainer (e.g. ethylene glycol) is used in an extractive distillation column, then one can recover pure ethanol as a distillate stream, as shown in the setup snapshot. The extractive distillation breaks the azeotrope. Water exits the column with ethylene glycol at the bottom and a second column is needed in order to regenerate the entrainer. A make-up stream of ethylene glycol is also required to compensate for the small losses of the entrainer.

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The column has 48 stages including a total condenser and a partial reboiler. Pure ethylene glycol is fed into the top of the column at stage 4. A mixture of ethanol and water ( water) is fed at stage 21 (i.e. near the bottom of the column). The reboil ratio is such that the vapor flow rate in the column is .

This Demonstration shows the time behavior of the distillate and bottom stream compositions. In addition, it shows the steady-state composition profile along the column as well as the ternary diagram. It can be seen from the snapshots that with a reflux ratio of 5.0, one gets almost pure ethanol in the distillate stream and a mixture of water and entrainer in the residue.

In the ternary diagram, the mixing line (blue line) and point for the two feeds are shown, as well as the material balance line for the distillation column (green line).

You can see how the outcome of this distillation setup changes with various values of the reflux ratio.

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Contributed by: Housam Binous (March 2011)
Open content licensed under CC BY-NC-SA


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