By John Brennan
Updated Mar 24, 2022
In simple distillation, a mixture of liquids is heated to the temperature at which one component boils. The resulting vapor is collected and recondensed into liquid. While fast and straightforward, many mixtures cannot be separated this way and require more advanced techniques.
Because the mixture is boiled and recondensed only once, the final product reflects the composition of the vapor. When component boiling points are close, the distillate can contain significant impurities. In practice, simple distillation is effective only when the boiling point difference is at least 25 °C. Mixtures with closer boiling points are better handled by fractional distillation.
Some liquid mixtures form azeotropes—compositions that boil at a constant temperature and retain the same vapor composition as the liquid. A classic example is 95.6 % ethanol with 4.4 % water, which boils at a lower temperature than either pure component. Simple distillation cannot break an azeotrope; neither can fractional distillation. Other methods, such as azeotropic or extractive distillation, are typically required.
Heating a liquid or mixture to its boiling point demands substantial energy. When powered by fossil fuels, this increases carbon emissions and operating costs. For instance, distilling ethanol on an industrial scale requires large amounts of fuel. Laboratory setups often use a rotary evaporator (rotovap) that applies vacuum to lower the boiling point, but this approach is less practical for bulk production.
Elevated temperatures can trigger unwanted reactions, altering the desired product. For example, heating a mixture of hydrogen bromide and butadiene can shift the ratio between 3‑bromo‑1‑butene and 1‑bromo‑2‑butene. Heat‑sensitive compounds, such as nitroglycerin, pose safety risks if subjected to boiling temperatures. Consequently, the thermal stability of the mixture must be considered before choosing simple distillation.
