Multi-component distillation separates volatile compounds based on their boiling points in a distillation column. Heat causes molecules to escape the liquid phase and become a gas, which then cools and condenses back into liquid. Equilibrium mixtures are established on each phase, and liquid eventually concentrates into components. Petroleum is distilled into fractions, and designing a multicomponent distillation requires complex calculations.
Multi-component distillation is a chemical process in which a mixture of volatile compounds separate based on their boiling points. The process takes place in a distillation column, a vertical stacking of trays or stages on which components in their liquid and vapor phases coexist. As the mixture moves up the column, high-boiling compounds concentrate on the lower stages, while low-boiling compounds concentrate on the higher stages.
Distillation columns use the basic principles of vapor-liquid equilibrium mixtures. When heat is applied to a liquid, the temperature of the liquid increases until it reaches its boiling point (BP). At BP, the additional energy does not cause a temperature rise; rather, 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 also cools due to the absence of energy which keeps the gas molecules in a gaseous phase. This cooling will cause some of the gas molecules to condense back into liquid, releasing the energy of the vaporization. The energy released is capable of heating the liquid. Under equilibrium conditions, the rate of vaporization is equal to the rate of condensation.
In a multi-component distillation column, an equilibrium mixture is established on each phase. 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 PA, to rise to the next stage. In this higher stage, the blend will try to return to equilibrium. Molecules from below may have too high a BP to vaporize at this stage, so they accumulate in the liquid phase.
Eventually, the liquid in each phase concentrates into one or more of the components. Side streams can be removed in one or more steps. Liquid streams will be concentrated into one or more components and additional distillation steps may be required.
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 can be taken from the top of the column and require no further processing.
Designing a multicomponent distillation requires complex calculations. Column design parameters include specifying the number of phases, the feedstock phase, the phases in which product streams are removed, and the heat required to operate the column. Specialized computer programs for multicomponent distillation perform these calculations, but engineers still learn graphical solution methods to understand the process.
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