Reflux is a technique involving thecondensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial[1] and laboratory[2]distillations. It is also used inchemistry to supplyenergy toreactions over a long period of time.
The termreflux[1][3][4] is very widely used in industries that utilize large-scaledistillation columns andfractionators such aspetroleum refineries,petrochemical andchemical plants, andnatural 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 andcondensation of the upflowing vapors thereby increasing the efficiency of the distillation column.
The more reflux provided for a given number oftheoretical 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.[5]
A mixture ofreactants andsolvent is placed in a suitable vessel, such as around bottom flask. This vessel is connected to a water-cooledcondenser, which is typically open to the atmosphere at the top. The reaction vessel is heated in order to boil the reaction mixture; vapours produced from the mixture are condensed by the condenser, and return to the vessel through gravity. The purpose is to thermally accelerate the reaction by conducting it at an elevated, controlled temperature (i.e. thesolvent'sboiling point) and ambient pressure without losing large quantities of the mixture.[6]
The diagram shows a typical reflux apparatus. It includes awater bath to indirectly heat the mixture. As many solvents used areflammable, direct heating with aBunsen burner is not generally suitable, and alternatives such as a water bath,oil bath,sand bath,electric hot plate orheating mantle are employed.[6]
The apparatus shown in the diagram represents a batch distillation as opposed to acontinuous distillation. The liquid feed mixture to be distilled is placed into the round-bottomed flask along with a fewanti-bumping granules, and thefractionating column is fitted into the top. As the mixture is heated and boils, vapor rises up the column. The vaporcondenses 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 atequilibrium. Only the most volatile of the vapors stays ingaseous form all the way to the top. The vapor at the top of the column then passes into thecondenser, 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. Forcontinuous distillation, the feed mixture enters in the middle of the column.
By controlling the temperature of the condenser, often called a dephlegmator, areflux still may be used to ensure that higher boiling point components are returned to the flask while lighter elements are passed out to a secondary condenser. This is useful in producing high qualityalcoholic beverages, while ensuring that less desirable components (such asfusel alcohols) are returned to the primary flask. For high quality neutral spirits (such asvodka), or post distillation flavored spirits (gin, absinthe), a process of multiple distillations or charcoal filtering may be applied to obtain a product lacking in any suggestion of its original source material forfermentation. The geometry of the still also plays a role in determining how much reflux occurs. In apot still, if the tube leading from the boiler to the condenser, thelyne arm, is angled upward, more liquid will have a chance to condense and flow back into the boiler leading to increased reflux. Typical results can increase production as high as 50% over the basic worm type condenser. The addition of a copper "boiling ball" in the path creates an area where expansion of gasses into the ball causes cooling and subsequent condensation and reflux. In acolumn still, the addition of inert materials in the column (e.g., packing) creates surfaces for early condensation and leads to increased reflux.[citation needed]
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