Multicomponent distillation is a chemical process in which a mixture of volatile compounds separates based on their boiling points. The process occurs in a distillation column, a vertical stacking of trays or stages upon which components in their liquid and vapor phases coexist. As the mixture moves up the column, high boiling compounds concentrate on lower stages, while low boiling compounds concentrate on higher stages.
Distillation columns use basic principles of liquid-vapor equilibrium mixtures. As heat is applied to a liquid, the liquid’s temperature rises until reaching the boiling point (BP). At the BP, additional energy does not cause a temperature increase; rather, the molecules use it to escape the liquid phase and become a gas.
The energy absorbed by the new gas molecule is no longer available to heat the next liquid molecule. As the liquid boils, it is also cooling from the absence of the energy keeping the gas molecules in a gas phase. This cooling will cause some of the gas molecules to condense into liquid again, releasing the energy of vaporization. The energy released is able to reheat the liquid. At equilibrium conditions, the rate of vaporization equals the rate of condensation.
In a multicomponent distillation column, an equilibrium mixture is established on each stage. There is a constant source of new energy supplied to the bottom of the column. This heat causes some of the gas-phase molecules of the most volatile compound on each stage, the one with the lowest BP, to rise to the next stage. At this higher stage, the mixture will attempt to come to equilibrium again. The molecules coming from below may have too high a BP to vaporize on this stage, so they accumulate in the liquid phase.
Eventually, the liquid in each stage becomes concentrated in one or more of the components. Side streams may be taken off at one or more stages. The liquid streams will be concentrated in one or more components, and additional distillations steps may be necessary.
Petroleum is typically distilled into fractions via multicomponent distillation. A fraction is a range of similar compounds with close BP that can be treated as a single compound. Gasoline is an example. A low BP gas may be taken off the top of the column and not require further processing.
The design of a multicomponent distillation requires complex calculations. The column design parameters include specifying the number of stages, the feed stage of the raw material, the stages at which product streams are removed, and the heat needed to drive the column. Specialized multicomponent distillation computer programs perform these calculations, but engineers still learn graphical solution methods to understand the process.