fractional distillation

is a process of separating and isolating compounds within a mixture, based on their specific volatility and boiling points in a continuous distillation process. In principle, simple distillation, as done by a pot still, is very similar, but in practice it is not an effective means of separating and isolating compounds that have relatively close boiling points—for example, ethanol and water, the two compounds of greatest concentration in an alcoholic fermented substrate (i.e., beer or wine). Therefore, to achieve separation, a column fitted with perforated trays is utilized, the trays serving as partial barriers dividing the column into stages. On each tray, vapor is forced to mix with a thin layer of liquid enriching the volatile content throughout the column. This causes each stage to have a slightly different chemical makeup; in relation to ethanol, more volatile compounds will concentrate at the top of the column and less volatile toward the bottom—hence, fractionation. During a steady-state operation, draws can be made and condensed off these trays based on the gradients within the column. The same theory can be applied to a packed column (i.e., with ceramic beads); however, the transition in volatility is streamlined versus a stage-wise shift.An industrial example of fractional distillation is the side-stream collection of fusel oils, which are primarily collected to limit their presence in the final spirit. Since the compounds present within this fraction are less volatile than ethanol (although in close range), they can be removed on trays in a lower section of the column below the spirit collection tray.

See also column still; distillation, process;, fusel oil.

Madson, P. W. “Ethanol Distillation: The Fundamentals.” In The Alcohol Textbook: A Reference for the Beverage, Fuel and Industrial Alcohol Industries, 4th ed., by W. M. Ingledew, 319–336. Nottingham, UK: Nottingham University Press, 2009.

By: Elizabeth Rhoades