Separation of Acetic Acid from Water Using Ethyl Acetate Entrainer

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The binary mixture of acetic acid and water does not form an azeotrope. Despite this, obtaining pure acetic acid (with boiling point 118.1 °C) from a solution of acetic acid and water by distillation is very difficult because of the presence of a severe tangent pinch. Indeed, as progressive distillations produce solutions with less and less water, each further distillation becomes less effective at removing the remaining water. Distilling the solution to glacial acetic acid is therefore economically impractical. Ethyl acetate forms a positive azeotrope with water that boils at 70.4 °C (with azeotropic composition 35.9% mole water and 64.1% mole ethyl acetate). By adding ethyl acetate as an entrainer, it is possible to boil off the azeotrope and obtain pure acetic acid as the residue.


This Demonstration simulates the separation process using a batch rectifier operation at atmospheric pressure. Constant molal overflow is assumed for simplicity, and the constant vapor boil-up rate is taken to be . The holdup of the stages and condenser is assumed constant. The still, condenser, and plates are initially filled with the ternary mixture. This initial feed is such that the composition of water and ethyl acetate (on an acetic acid-free basis) is equal to the azeotropic composition. The vapor-liquid calculation uses the UNIQUAC model and takes into account the gas-phase dimerization of acetic acid. Snapshot 1 shows the distillate composition versus time (in minutes) for a user-set choice of the reflux ratio and the number of stages . It is clear that one can recover almost all the acetic acid in pure form at the end of the run, while the positive azeotrope exits the column initially. In snapshot 2, two plateaus are observed: the first corresponds to the positive azeotrope between water and ethyl acetate, and the second to glacial acetic acid.


Contributed by: Housam Binous, Mamdouh Al-Harthi, and Ahmed Bellagi (December 2015)
Open content licensed under CC BY-NC-SA




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