Continuous Binary Fractional Distillation

Reflux is a technique involving the

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory

distillation Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...

s. It is also used in

chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

to supply

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law *Chain reaction (disambiguation). Biology and me ...

s over a long period of time.

Reflux in industrial distillation

The term ''reflux'' is very widely used in industries that utilize large-scale distillation columns and fractionators such as

petroleum refineries An oil refinery or petroleum refinery is an industrial process plant where petroleum (crude oil) is transformed and refined into useful products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, liquefie ...

petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sou ...

and

chemical plant A chemical plant is an industrial process plant that manufactures (or otherwise processes) chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transform ...

s, and

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...

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

of the upflowing vapors thereby increasing the efficiency of the distillation column. The more reflux provided for a given number of

theoretical plate A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Such equilibrium stages may also be referred to as a ...

s, 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 chemical reactions

A mixture of

reactants In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

and

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

is placed in a suitable vessel, such as a

round bottom flask Round-bottom flasks (also called round-bottomed flasks or RB flasks) are types of flasks having spherical bottoms used as laboratory glassware, mostly for chemical or biochemical work. They are typically made of glass for chemical inertness; an ...

. This vessel is connected to a water-cooled condenser, 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. the

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...

) and ambient pressure without losing large quantities of the mixture. The diagram shows a typical reflux apparatus. It includes a water bath to indirectly heat the mixture. As many solvents used are

flammable A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...

, direct heating with a

Bunsen burner A Bunsen burner, named after Robert Bunsen, is a kind of ambient air gas burner used as laboratory equipment; it produces a single open gas flame, and is used for heating, sterilization, and combustion. The gas can be natural gas (which is main ...

is not generally suitable, and alternatives such as a water bath,

oil bath An oil bath is a type of heated bath used in a laboratory, most commonly used to heat up chemical reactions. It's essentially a container of oil that is heated by a hot plate or (in rare cases) a Bunsen burner. Use These baths are commonly used ...

sand bath A sand bath is a common piece of laboratory equipment made from a container filled with heated sand. It is used to evenly heat another container, most often during a chemical reaction. A sand bath is most commonly used in conjunction with a hot ...

, electric hot plate or

heating mantle A heating mantle, or isomantle, is a piece of laboratory equipment used to apply heat to containers, as an alternative to other forms of heated bath. In contrast to other heating devices, such as hotplates or Bunsen burners, glassware containe ...

are employed.

Reflux in laboratory distillation

The apparatus shown in the diagram represents a batch distillation as opposed to a

continuous distillation Continuous distillation, a form of distillation, is an ongoing separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams. Distillation is the sep ...

. 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 A fractionating column or fractional column is an essential item used in the distillation of liquid mixtures to separate the mixture into its component parts, or fractions, based on the differences in volatilities. Fractionating columns are used in ...

is fitted into the top. As the mixture is heated and boils, vapor rises up the column. The vapor

condenses Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

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

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

eous 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

, the feed mixture enters in the middle of the column.

Reflux in beverage distillation

By controlling the temperature of the condenser, often called a dephlegmator, a ''reflux 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 quality

alcoholic beverage An alcoholic beverage (also called an alcoholic drink, adult beverage, or a drink) is a drink that contains ethanol, a type of alcohol that acts as a drug and is produced by fermentation of grains, fruits, or other sources of sugar. The c ...

s, while ensuring that less desirable components (such as

fusel alcohol Fusel alcohols or fuselol, also sometimes called fusel oils in Europe, are mixtures of several higher alcohols (those with more than two carbons, chiefly amyl alcohol) produced as a by-product of alcoholic fermentation. The word ''Fusel'' is Ger ...

s) are returned to the primary flask. For high quality neutral spirits (such as

vodka Vodka ( pl, wódka , russian: водка , sv, vodka ) is a clear distilled alcoholic beverage. Different varieties originated in Poland, Russia, and Sweden. Vodka is composed mainly of water and ethanol but sometimes with traces of impuritie ...

), 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 for

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...

. The geometry of the still also plays a role in determining how much reflux occurs. In a ''

pot still A pot still is a type of distillation apparatus or still used to distill liquors such as whisky or brandy. In modern (post-1850s) practice, they are not used to produce rectified spirit, because they do not separate congeners from ethanol as eff ...

'', if the tube leading from the boiler to the condenser, the ''lyne 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 a ''column still'', the addition of inert materials in the column (e.g., packing) creates surfaces for early condensation and leads to increased reflux.

Reflux in industrial distillation

Reflux in chemical reactions

Reflux in laboratory distillation

Reflux in beverage distillation

Gallery

See also

References

Further reading