Reflux

Reflux

:"This article is about using reflux in chemical engineering and chemistry. For the reflux of stomach acids, see articles on heartburn, acid indigestion, and gastroesophageal reflux. For the reflux of urine, see vesicoureteral reflux."

Reflux is a technique involving the condensation of vapours and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions over a long period of time.

Reflux in industrial distillation

The term reflux [cite book|author=Kister, Henry Z.| title=Distillation Design|edition=1st Edition|publisher=McGraw-Hill |year=1992|id=0070349096] [cite book|author=Perry, Robert H. and Green, Don W.|title=Perry's Chemical Engineers' Handbook|edition=6th Edition|publisher=McGraw-Hill|year=1984| id=0070494797] is very widely used in industries that utilize large-scale distillation columns and fractionators such as petroleum refineries, petrochemical and chemical plants, and natural gas processing plants.

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

Reflux in laboratory distillation

The apparatus shown in the diagram represents a batch distillation as opposed to a continuous distillation. The liquid feed mixture to be distilled is placed into the round-bottomed flask along with a few anti-bumping granules, and the fractionating column is fitted into the top. As the mixture is heated and boils, vapor rises up the column. The vapor condenses on the glass platforms (known as plates or trays) inside the column and runs back down into the liquid below, thereby refluxing the upflowing distillate vapor. The hottest tray is at the bottom of the column and the coolest tray is at the top. At steady state conditions, the vapor and liquid on each tray is at equilibrium. Only the most volatile of the vapors stays in gaseous form all the way to the top. The vapor at the top of the column then passes into the condenser, where it cools until it condenses into a liquid. The separation can be enhanced with the addition of more trays (to a practical limitation of heat, flow, etc.). The process continues until all the most volatile components in the liquid feed boil out of the mixture. This point can be recognized by the rise in temperature shown on the thermometer. For continuous distillation, the feed mixture enters in the middle of the column.

Reflux to apply energy to chemical reactions

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 purpose is to thermally accelerate the reaction by conducting it at an elevated temperature (i.e. the solvent's boiling point.)

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, one can be sure that the reaction will proceed at a constant temperature. By careful choice of solvent, one can control the temperature within a very narrow range. The constant boiling action also serves to continuously mix the solution, although a magnetic stirring rod mechanism is often used to achieve a uniform solution. This technique is useful for performing chemical reactions under controlled conditions that require substantial time for completion.

The diagram shows a typical reflux apparatus for applying energy to chemical reactions. It 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. In modern laboratories, open flames are avoided due to the many flammable solvents often in use, and electrical heating, (i.e., with a hot plate or mantle) is preferred. Furthermore, a high boiling, thermally stable silicone oil is generally used to immerse the reaction vessel, rather than water which evaporates too readily to be useful for lengthy reactions. Using an oil bath, temperatures of up to several hundred degrees can easily be achieved, which is higher than the boiling point of most commonly used solvents. If even higher temperatures are required, the oil bath can be replaced with a sand bath.

Reflux 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, a process of multiple distillations or charcoal filtering may be applied to obtain a product lacking in any suggestion of its original source material for fermentation.

ee also

*Batch distillation
*Fractional distillation
*Fractionating column

References

External links

* [http://lorien.ncl.ac.uk/ming/distil/distileqp.htm Distillation column components]

Gallery


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  • reflux — [ rəfly ] n. m. • 1532; de re et flux 1 ♦ Mouvement des eaux marines qui se retirent à marée descendante. ⇒ jusant. Le flux et le reflux de la mer. « Le flux les apporta; le reflux les remporte » (P. Corneille). ♢ Fig. « Un flux et reflux d… …   Encyclopédie Universelle

  • reflux — REFLÚX, refluxuri, s.n. Fază de coborâre a nivelului mărilor şi oceanelor în cadrul fenomenului de maree. ♦ fig. Dare înapoi, descreştere; regres. – Din fr. reflux. Trimis de IoanSoleriu, 13.09.2007. Sursa: DEX 98  Reflux ≠ flux Trimis de siveco …   Dicționar Român

  • Reflux — (von lat. refluxus „Rückfluss“) steht in der Medizin für folgende pathologische Rückflüsse von einem Hohlorgan in ein anderes: Gastroösophagealer Reflux, Rückfluss vom Magen (Gaster) in die Speiseröhre (Ösophagus), siehe Refluxösophagitis… …   Deutsch Wikipedia

  • reflux — Reflux. s. m. v. Mouvement reglé de la mer qui se retire & qui s éloigne du rivage aprés le flux. Il y a flux & reflux dans l Ocean. il prit le temps du reflux pour passer. en certains endroits de la mediterrannée. il y a flux & reflux. Il se dit …   Dictionnaire de l'Académie française

  • Reflux — Re flux , n. [F. reflux. See {Refluent}, {Flux}.] A flowing back, as the return of a fluid; ebb; reaction; as, the flux and reflux of the tides. [1913 Webster] All from me Shall with a fierce reflux on me redound. Milton. [1913 Webster] …   The Collaborative International Dictionary of English

  • REFLUX — REFLUX, обратный ток, затекание жидкости (мочи или контрастного вещества) в противоестественном направлении; В урологии термин R. применяется 1) в отношении затекания мочи из мочевого пузыря в мочеточник и лоханку пу зырно мочеточниковый R., 2)… …   Большая медицинская энциклопедия

  • Reflux — Re flux (r? fl?ks ), a. Returning, or flowing back; reflex; as, reflux action. [1913 Webster] …   The Collaborative International Dictionary of English

  • reflux — index decrease, outflow Burton s Legal Thesaurus. William C. Burton. 2006 …   Law dictionary

  • reflux — (n.) early 15c., a flowing back (of the sea, etc.), from M.L. refluxus, from L. re back, again (see RE (Cf. re )) + fluxus a flowing (see FLUX (Cf. flux)). Digestive sense is recorded from 1937 …   Etymology dictionary

  • reflux — [rē′fluks΄] n. [ME < ML refluxus < pp. of L refluere: see REFLUENT] a flowing back; ebb; specif., regurgitation of food from the stomach to the esophagus …   English World dictionary

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