How Distillation Works
Absolute ethanol boils at 78.5°C and water boils at 100°C at standard atmospheric pressure. Ethanol has a higher vapor pressure than water; in other words, it takes less energy to convert ethanol to ethanol vapor than water to steam. When ethanol and water are mixed the boiling temperature varies and falls between the boiling points of pure ethanol and water. When we boil a mixture of ethanol and water, more ethanol vapor rises from the vessel than water vapor. The compositions of liquid and the vapor produced at different temperatures can be plotted against the temperature; this graph is known as the phase diagram, which is shown in Figure 14.1. If we can capture vapor and condense the vapor, the condensate has a higher concentration of ethanol than the original mixture. Now we can boil this condensed liquid and capture the vapor and recondense the vapor. This second condensed vapor will have even
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EtOH 0% EtOH 89.4% 100% EtOH Azeotrope Figure 14.1 Ethanol-water phase diagram showing the minimum-boiling azeotrope of composition of 89.4 mol% ethanol and 10.6 mol% water; the azeotrope mixture will boil at 78.2°C and standard atmospheric pressure. |
a higher proportion of ethanol. This process is called rectification, and can be repeated until most of the ethanol is drawn off. However, things are not that simple in the case of ethanol water, and this solution forms a minimum-boiling azeotrope at the composition of 89.4 mol% ethanol and 10.6 mol% water. At this composition vapor phase and liquid phase both will have the same composition as shown in Figure 14.1. The azeotrope mixture will boil at 78.2°C at standard atmospheric pressure and ethanol cannot be further concentrated by simple fractional distillation when it reaches this composition.
There are several methods that can be used to further enrich ethanol from an azeotrope composition mixture; the two common methods are:
1. Introduction of a third component called an entrainer that will affect the volatility of one of the azeotrope constituents more than the other. When an entrainer is added to a binary azeotrope the mixture will form a ternary azeotrope. Benzene or cyclohexane can be used as an entrainer for further concentration of ethanol.
2. Use of dehydration agents such as molecular sieves to selectively absorb water from the mixture. This is a widely applied technology in drying distilled ethanol to fuel grade ethanol.
Further enrichment of ethanol to ~99.5% fuel grade ethanol is also known as dehydration, and this topic will be discussed in Chapter 15.