Alcohol Dehydration

Azeotropic distillation for alcohol dehydration has the longest history of use within the industry. Beginning with the work of Young, who demonstrated the first industrial application, azeotropic distillation is still used today in the production and dehydration of alcohol. Alcohol production is present in the CPI, speciality chemicals and food industries, and although the end use and perhaps the alcohol purity requirements vary, the method of production using azeotropic distillation is similar.

Prior to Young's work, absolute alcohol was prepared by distilling the alcohol with a dehydrating agent such as freshly ignited lime. This method required the major component (alcohol) to be boiled overhead leaving the minor component (water) at the bottom. Young's objective was to find a method of removing the minor component as the overhead product. When two different chemicals are distilled together, often a minimum boiling point occurs where a mixture of the two chemicals will distil off first, with the last portion being the compound in excess. Since ethanol is a hydroxyl compound like water and yet an organic compound, it should exhibit analogies to both water and hydrocarbons. Therefore, ethanol should form azeotropes with water and organic compounds. If an azeotroping agent such as benzene is used (in Young's time benzene and heptane were preferred due to their availability), the fractions would come off as follows: ternary water/benzene/ alcohol azeotrope; alcohol and benzene azeotrope; and water and benzene azeotrope. At this point, all the water should be removed, leaving anhydrous alcohol. Young performed his azeotropic dehydration of alcohol by a batch method. While popular for distilling strong spirits, this batch method did not catch on commercially. It was not until Backus et al. and Guinot developed continuous azeotropic distillation processes that it became a commercial success.

As an example of an alcohol dehydration process, let us examine ethanol dehydration. Dilute water/

ethanol solutions can be rectified to produce an ethanol-rich stream containing a maximum of about 89.4 mol% ethanol at atmospheric pressure. The introduction of benzene as a heterogeneous azeo-tropic entrainer to the top of the column, which is fed with an 89.4 mol% aqueous ethanol feed, produces the ternary ethanol/benzene/water azeotrope. This ternary azeotrope boils at 64.9°C and is easily separated from ethanol (b.p. = 78.4°C), which is removed as the bottom product. The ternary ethanol/ benzene/water azeotrope forms two liquid phases with the benzene-rich phase fed back to the column. The aqueous phase contains nearly equimolar proportions of ethanol and water which is rectified to produce water as the bottom product and the binary ethanol/ water azeotrope as the overhead product. The concentrated ethanol/water azeotropic stream is recycled back to the feed for the dehydration column. Figure 7 illustrates a typical ethanol dehydration process.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment