Article: Reflux

This article is about reflux, a technique in chemistry and in chemical engineering. For information on the reflux of acid from the stomach, see articles on heartburn, acid indigestion, and gastroesophageal reflux.

Diagram of typical reflux apparatus.

Reflux is a technique used in chemistry to apply energy to reactions over an extended period of time.

A liquid reaction mixture is placed in a vessel open only at the top. This vessel is connected to a Liebig condenser, such that any vapours given off are cooled back to liquid, and fall back into the reaction vessel. The vessel is then heated vigorously for the course of the reaction.

The advantage of this technique is that it can be left for a long period of time without the need to add more solvent or fear of the reaction vessel boiling dry as any vapour is immediately condensed in the condenser. In addition, as a given solvent will always boil at a certain temperature, you can be sure that the reaction will proceed at the same temperature; by careful choice of solvent, you can even control what that temperature is.

The diagram also includes an optional beaker of water between the reactants and the heat. This is often used as a safety precaution when using flammable reactants and a Bunsen burner in order to keep the flame away from the reactants.

Use of reflux in industrial distillation

Overhead system of an industrial fractionator

The term reflux ^[1][2] is very widely used in industries that utilize large-scale distillation towers and fractionators such as petroleum refineries, petrochemical plants, chemical plant and natural gas processing plants.

In that context, reflux refers to the portion of the condensed overhead liquid product from a distillation tower or fractionator that is returned to the upper part of the tower as shown in the schematic diagram of the overhead system for a typical industrial fractionator. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux provided, the better is the tower's separation of lower boiling materials from higher boiling materials.

Use in beverage distillation

By controlling the temperature of the condenser a reflux still may be used to ensure that higher boiling point components (which are also of higher molecular weight) are returned to the flask while lighter elements are passed out to a secondary condenser. This is useful in producing high quality alcoholic beverages, while ensuring that less desirable components (such as fusel alcohols) are returned to the primary flask. This is particularly effective in the production of alcoholic beverages in which it is appropriate to retain the flavors and aromas of the source fruit - such as applejack. For high quality neutral spirits (such as vodka), or post distillation flavored spirits (such as gin) a process of multiple distillations and/or charcoal filtering may be applied to obtain a product lacking in any suggestion of its original source material for fermentation.

See also

• Distillation
• Fractional distillation