reflux is the part of the distillation process where some of the heavier compounds in the alcohol-rich vapor rising from the heated wash condense out of the vapor and fall back into the wash before the rest of the vapor is drawn off or collected for condensation. Distillers generally consider reflux a desirable thing, as it yields a cleaner, stronger spirit, although how much it is facilitated varies greatly according to the type of still used and the spirit being made.
All stills produce some reflux. In pot stills, this can be manipulated in several ways. The most common is by placing a narrow neck in between the pot and the still head. When the vapor passing out of the neck expands to fill the head, it loses pressure, and as a result some of the heavier compounds—water, fusel oils, and other high-boiling-point compounds—fall out of suspension. The same thing happens if the head of the still is made very tall or drawn up into a point, with the vapor exiting from the top. Cooling the still head with water has a similar effect.
Nonetheless, in a basic pot still the ability to separate ethanol from the other organic components and water is limited; for some spirits, such as cognac and malt whisky, the resulting higher levels of congeners and fusel oils in the final distillate are desirable, and that outweighs the still’s inefficiency and the lower distillation proof. On the other hand, distillation equipment that incorporates a rectification column, sometimes referred to as a continuous still, has the capability to produce a product free of extraneous fusel oils and excess water by the amplification of reflux. Accomplishing the same level of purity in a pot still requires the distillate to pass through the distillation process multiple times, which is inefficient.
Schematically, the reflux process in a column still is simple: the still uses the vapor rising through the column from the boiling, mostly spent wash that has collected at the bottom to strip the ethanol from the relatively cool fresh wash, which is fed in at the top and trickles down, in the process condensing the higher-boiling-point compounds in the rising vapor. Inside of the rectifier column, there is insufficient heat for higher-boiling-point components to return to a vapor phase once they have condensed, so they fall back into the boiler. Inside the boiler, there is sufficient energy to convert the water and fusel oils back into a vapor, but they must work their way back up the column before they can make it into the final distillate. The continuous cascade of reflux pushes the fusel oils and water vapor back down into the boiler.
The key to beneficial reflux is the creation of an equilibrium where the fusel oils and water concentrate in the bottom half of the rectifier and the ethanol proceeds to the top. Once the majority of the ethanol is removed from the boiler, the temperature will begin to rise above the boiling point of ethanol, allowing the feints or tails (phenol compounds) to rise inside the rectifier column. This signals the end of the distillation.
Though it is difficult to remove all the fusel oils and water from a distillate, due to things like chemical bonding and the azeotrope effect, a proper rectifying system with steady reflux can attain a nearly pure ethanol concentration of more than 95 percent. See azeotrope. Purification of ethanol beyond that point requires specialized methods.
See also distillation, process; and column still.
Hengstebeck, R. J. Distillation Principles and Design Procedures. New York: Reinhold, 1961.
Miller, Gregory H. Whisky Science: A Condensed Distillation. Cham, Switzerland: Springer Nature Switzerland AG, 2019.
Smith, Gavin. The A-Z of Whisky. Glasgow: Angels’ Share, 2009.
By: Darcy O’Neill