A graphical description for mixtures

A picture is truly worth a thousand words, and a few graphs can help create a clear understanding of what happens when mixtures are heated and condensed. These phenomena are the reason that distillation works, and understanding them is essential to designing any type of effective distillation apparatus.

Let's start with a methanol/water mixture with X% methanol by volume. We've chosen methanol because its characteristics are similar to ethanol, but they're not complicated by azeotropism. (Methanol is not what you find in bottles marked 'methylated spirits'. That is ethanol, which has been made poisonous by adding small amounts of methanol and other noxious substances that cannot be removed by distillation).

This chart plots the boiling points of solutions of methanol and water as a function of the methanol content. The top left dot shows that the boiling point of a solution containing X% methanol is Tx°C. The vapor produced contains a higher percentage of methanol, because it has a higher vapor pressure than water (shown by the top right dot at Tx°C) and this vapor will condense at Ty°C.

Subsequent vaporizations and condensations are plotted in this chart. As the concentration of the condensed liquid approaches 100% methanol, the boiling point 64.7°C.

Methanol Vapor Pressure Chart
Fig. 2-3
Dew Point Chart For EthanolMethanol Distillation Curve

Joining up the dots produces two separate curves. The upper one is called the vapor line. Any point above it is vapor, and anything below the lower line, the dew line, is liquid.

Points lying in between the lines represent a dynamic mixture of liquid and vapor. As a droplet of liquid starts at the dew line and slides towards the vapor line to its right at a constant temperature, it gradually grows smaller as it transforms into a vapor. The liquid and vapor in such a situation are said to be in equilibrium, and charts like this one are called Equilibrium Charts.

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