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Chemical Distillation
Robert Owens 01/15/17

In chemistry, distillation is a process in which a liquid mixture is fed to a distillation column to extract the desired substance(s) from the feed. The column distills the mixture via vapor-liquid extraction, which separates substances by their differences in relative volatilities. The thoroughness of distillation is determined by whether the operator desires the mixture to be fully separated in its nearly-pure components, or a partial separation of the original mixture. During operation, heat is manipulated by the operator in such a way that certain components of the mixture will evaporate to the top of the column, while others will condense and trickle down to the bottom. So, although the process takes advantage of chemical properties, Chemical Distillation does not involve any set of chemical reactions.

Thermodynamics behind Distilling
Distillation of a mixture relies on the difference of each pure-component's relative volatility — which is a way of comparing the vapor pressures of each component in a liquid mixture of chemicals. In other words, it is a method of comparing how volatile the mixture's components are in respect with one another. Relative volatility can be thought of as having an inversely proportional relationship with a substance's boiling point. Therefore, substances with low boiling points will have a high relative volatility and those with high boiling points will have a low relative volatility. As far as Chemical Distillation is concerned, when heat is applied, the components with high relative volatilities will evaporate up and out of the column, while components with low relative volatilities will remain in mixture and condense to the bottom of the column. For this very reason, it is common (and most practical) to use substances in mixture with high relative volatilities.

The Conventional Model for Distillation Columns
The common set up of a distillation column is to have a set of trays (stages) aligned on top of one another for various stages of separation. The number of trays the operator chooses to use in distillation dictates both the operating cost (more trays are more expensive) and the purity of the final product (more trays yields a higher purity). When using a distillation column, the mixture is continuously fed to the middle of the column. In order to maximize both efficiency and purity, the mixture must be constantly fed to the middle of the column, and excess products must be constantly removed from the two product streams.

Within the distillation column, there is a vapor stream, which flows upward to the distillate (at the top), and a liquid stream, which trickles down to the bottom of the column to a separate liquid stream. These two streams are established by the use of a reboiler placed at the bottom of the column which causes a vapor to constantly rise upward away from the liquid stream flowing in the opposite direction. Volatile components flow along with the vapor stream, while substances with low volatility exit with the liquid stream. The operator may choose whether to extract either the distillate or liquid streams, based on which product they are concerned with.