What fractionating columns are and why they are useful

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What fractionating columns are and why they are useful

In: Chemistry

Fractionation is a process which takes a mixture and spreads that mixture out over a space in an attempt to purify it. If done properly one “fraction” – maybe the portion that moves the least from the starting point – will be one pure substance and another “fraction” – maybe the portion that moves the farthest from the starting point – will be another pure substance. There may be any number of these fractions in between the extremes and that’s really why fractionation is useful: you can separate all the components of a mixture in a single process.

As a hypothetical example let’s say you had a mixture of propane, ethane, and methane gasses. You could pump this mixture at a carefully chosen speed down a pipe that was room temperature at one end but got colder and colder and colder as it went along, falling to 100 K. If you collected liquid condensation near the front of the pipe it would be almost all propane. Liquid condensing in the middle of the pipe would be almost all ethane and liquid condensing at the far end of the pipe would be almost all methane. This is an example of fractional distillation and it’s basically how raw mixtures of petroleum products are refined. Oil refineries have such tall towers because fractions are being separated all the way up.

Fractionating with a chromatography column is similar in practice but, well, it’s chromatography: it relies on the differences in solubility between mixture components rather than differences in boiling point. You run your mixture, dissolved in a solvent, through a column of silica gel and mixture components which are more soluble in the gel move slowly and stay at the top which components which are less soluble in the gel race to the bottom. On a small scale, this is cheaper, safer, and more efficient than working with large temperature differences.