Fractional distillation
Overview
 
Fractional distillation is the separation of a mixture
Mixture
In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...

 into its component parts, or fractions, such as in separating chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s by their boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

 by heating them to a temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 at which several fractions of the compound will evaporate. It is a special type of distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

. Generally the component parts boil at less than 25 °C from each other under a pressure of one atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 (atm
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

). If the difference in boiling points
Boiling Points
Boiling Points is a prank reality television show, much like the format used on Candid Camera. It is broadcast on MTV in the United States. In each half-hour episode, annoying situations are set up and deliberately inflicted on one or more young adults who are unaware that they are being tested...

 is greater than 25 °C, a simple distillation is used.
Fractional distillation in a laboratory makes use of common laboratory glassware and apparatuses, typically including a Bunsen burner
Bunsen burner
A Bunsen burner, named after Robert Bunsen, is a common piece of laboratory equipment that produces a single open gas flame, which is used for heating, sterilization, and combustion.- Operation:...

, a round-bottomed flask and a condenser
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...

, as well as the single-purpose fractionating column
Fractionating column
A fractionating column or fractionation column is an essential item used in the distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities...

.
  • heat source, such as a hot plate
    Hot plate
    A hot plate is a portable self-contained tabletop small appliance that features one, two or more gas burners or electric heating elements. A hot plate can be used as a stand alone appliance, but is often used as a substitute for one of the burners from an oven range or the cook top of a stove...

     with a bath, and ideally with a magnetic stirrer.
  • distilling flask, typically a round-bottom flask
    Round-bottom flask
    Round-bottom 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; and in modern days, they are usually made of heat-resistant borosilicate glass...

  • receiving flask, often also a round-bottom flask
  • fractionating column
    Fractionating column
    A fractionating column or fractionation column is an essential item used in the distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities...

  • distillation head
  • thermometer
    Thermometer
    Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

     and adapter if needed
  • condenser
    Condenser (heat transfer)
    In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...

    , such as a Liebig condenser, Graham condenser or Allihn condenser
  • vacuum adapter (not used in image to the right)
  • boiling chip
    Boiling chip
    A boiling chip or boiling stone is a small, irregularly shaped piece of material added to liquids to make them boil more smoothly. Boiling chips are frequently employed in distillation and heating. When a liquid becomes superheated, a particle of dust or a stirring rod can cause flash boiling...

    s, also known as anti-bumping granules
  • Standard laboratory glassware
    Laboratory glassware
    Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...

     with ground glass joint
    Ground glass joint
    Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may be rapidly fitted together to reflux a reaction mixture...

    s, e.g.
Encyclopedia
Fractional distillation is the separation of a mixture
Mixture
In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...

 into its component parts, or fractions, such as in separating chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s by their boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

 by heating them to a temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 at which several fractions of the compound will evaporate. It is a special type of distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

. Generally the component parts boil at less than 25 °C from each other under a pressure of one atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 (atm
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

). If the difference in boiling points
Boiling Points
Boiling Points is a prank reality television show, much like the format used on Candid Camera. It is broadcast on MTV in the United States. In each half-hour episode, annoying situations are set up and deliberately inflicted on one or more young adults who are unaware that they are being tested...

 is greater than 25 °C, a simple distillation is used.

Laboratory setup

Fractional distillation in a laboratory makes use of common laboratory glassware and apparatuses, typically including a Bunsen burner
Bunsen burner
A Bunsen burner, named after Robert Bunsen, is a common piece of laboratory equipment that produces a single open gas flame, which is used for heating, sterilization, and combustion.- Operation:...

, a round-bottomed flask and a condenser
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...

, as well as the single-purpose fractionating column
Fractionating column
A fractionating column or fractionation column is an essential item used in the distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities...

.

Apparatus

  • heat source, such as a hot plate
    Hot plate
    A hot plate is a portable self-contained tabletop small appliance that features one, two or more gas burners or electric heating elements. A hot plate can be used as a stand alone appliance, but is often used as a substitute for one of the burners from an oven range or the cook top of a stove...

     with a bath, and ideally with a magnetic stirrer.
  • distilling flask, typically a round-bottom flask
    Round-bottom flask
    Round-bottom 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; and in modern days, they are usually made of heat-resistant borosilicate glass...

  • receiving flask, often also a round-bottom flask
  • fractionating column
    Fractionating column
    A fractionating column or fractionation column is an essential item used in the distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities...

  • distillation head
  • thermometer
    Thermometer
    Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

     and adapter if needed
  • condenser
    Condenser (heat transfer)
    In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...

    , such as a Liebig condenser, Graham condenser or Allihn condenser
  • vacuum adapter (not used in image to the right)
  • boiling chip
    Boiling chip
    A boiling chip or boiling stone is a small, irregularly shaped piece of material added to liquids to make them boil more smoothly. Boiling chips are frequently employed in distillation and heating. When a liquid becomes superheated, a particle of dust or a stirring rod can cause flash boiling...

    s, also known as anti-bumping granules
  • Standard laboratory glassware
    Laboratory glassware
    Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...

     with ground glass joint
    Ground glass joint
    Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may be rapidly fitted together to reflux a reaction mixture...

    s, e.g. quickfit apparatus
    Quickfit apparatus
    Quickfit is a brand of laboratory glassware, produced under the Pyrex trademark and distributed through various fronts in differing countries; e.g. Corning in North America. The glassware is well known worldwide for its build quality and it appears in numerous laboratories...

    .

Discussion

As an example, consider the distillation of a mixture of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

. Ethanol boils at 78.4 °C while water boils at 100 °C. So, by heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid. Some mixtures form azeotrope
Azeotrope
An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....

s, where the mixture boils at a lower temperature than either component. In this example, a mixture of 96% ethanol and 4% water boils at 78.2 °C, being more volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

 than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.

The apparatus is assembled as in the diagram. (The diagram represents a batch apparatus, as opposed to a continuous apparatus.) The mixture is put into the round bottomed flask along with a few anti-bumping granules
Boiling chip
A boiling chip or boiling stone is a small, irregularly shaped piece of material added to liquids to make them boil more smoothly. Boiling chips are frequently employed in distillation and heating. When a liquid becomes superheated, a particle of dust or a stirring rod can cause flash boiling...

 (or a Teflon coated magnetic stirrer bar if using magnetic stirring), and the fractionating column is fitted into the top. The fractional distillation column is set up with the heat source at the bottom on the still pot. As the distance from the stillpot of the distillation column increases a heat gradient is formed in the column where it is coolest at top and hottest at the bottom. As the mixed vapor ascends the temperature gradient, some of the vapor condenses and revaporizes along the temperature gradient. Each time the vapor condenses and vaporizes, the composition of the more volatile liquid in the vapor increases. This distills the vapor along the length of the column, and eventually the vapor is composed solely of the more volatile liquid. The vapor condenses
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 on the glass platforms, known as trays, inside the column, and runs back down into the liquid below, reflux
Reflux
Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations...

ing distillate. The efficiency in terms of the amount of heating and time required to get fractionation can be improved by insulating the outside of the column in an insulator such as wool, aluminium foil or preferably a vacuum jacket. The hottest tray is at the bottom and the coolest is at the top. At steady state conditions, the vapor and liquid on each tray are at equilibrium
Vapor-liquid equilibrium
Vapor–liquid equilibrium is a condition where a liquid and its vapor are in equilibrium with each other, a condition or state where the rate of evaporation equals the rate of condensation on a molecular level such that there is no net vapor-liquid interconversion...

. Only the most volatile of the vapors stays in gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

eous form all the way to the top. The vapor at the top of the column then passes into the condenser, which cools it down until it liquefies. The separation is more pure with the addition of more trays (to a practical limitation of heat, flow, etc.) The condensate that was initially very close to the azeotrope composition becomes gradually richer in water. The process continues until all the ethanol boils out of the mixture. This point can be recognized by the sharp rise in temperature shown on the thermometer
Thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

.

Typically the example above now only reflects the theoretical way fractionation works. Normal laboratory fractionation columns will be simple glass tubes (often vacuum jacketed, and sometimes internally silvered
Silvering
Silvering is the chemical process of coating glass with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were made of an amalgam of tin and mercury, but by the 19th century mirrors were commonly made through a process by which silver...

) filled with a packing, often small glass helices of 4 to 7 mm diameter. Such a column can be calibrated by the distillation of a known mixture system to quantify the column in terms of number of theoretical plates. To improve fractionation the apparatus is set up to return condensate to the column by the use of some sort of reflux splitter (reflux wire, gago, Magnetic swinging bucket, etc.) - a typical careful fractionation would employ a reflux ratio of around 10:1 (10 parts returned condensate to 1 part condensate take off).

In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a water jacket
Water jacket
A water jacket is a water-filled void surrounding a device, typically a metal sheath having intake and outlet vents to allow water to be pumped through the void. The flow of water to an external heating or cooling device allows precise temperature control of the device.-Applications:Water jackets...

, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense.

Alternate set-ups may utilize a "cow" or "pig" which is connected to three or four receiving flasks. By turning the "cow" or "pig", the distillates can be channeled into the appropriate receiver. A Perkin triangle
Perkin triangle
A Perkin triangle is a specialized apparatus for the distillation of air-sensitive materials. It is named after William Henry Perkin Jr., whose design was approximately triangular in shape. The image opposite shows a more modern version in which the glass taps have been replaced with more air-tight...

 is versatile piece of apparatus that can also be used to collect distillation fractions which does not require a "cow" or "pig" adapter. A Perkin triangle
Perkin triangle
A Perkin triangle is a specialized apparatus for the distillation of air-sensitive materials. It is named after William Henry Perkin Jr., whose design was approximately triangular in shape. The image opposite shows a more modern version in which the glass taps have been replaced with more air-tight...

 is most often used where the distillates are air-sensitive or where the fractions distill and are collected under reduced pressure, but can be used for a simple and fractional distillation.

Vacuum distillation
Vacuum distillation
Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure causing evaporation of the most volatile liquid...

 systems operate at reduced pressure, thereby lowering the boiling points of the materials. Note that the use of anti-bumping granules
Boiling chip
A boiling chip or boiling stone is a small, irregularly shaped piece of material added to liquids to make them boil more smoothly. Boiling chips are frequently employed in distillation and heating. When a liquid becomes superheated, a particle of dust or a stirring rod can cause flash boiling...

 will not work at reduced pressures.

Industrial distillation

Distillation is the most common form of separation technology used in petroleum refineries
Oil refinery
An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...

, petrochemical
Petrochemical
Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....

 and chemical plant
Chemical plant
A chemical plant is an industrial process plant that manufactures chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials. Chemical plants use special equipment,...

s, natural gas processing
Natural gas processing
Natural-gas processing is a complex industrial process designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.-Background:...

 and cryogenic air separation
Air separation
An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen sometimes also argon and rarely other inert gases. There are various technologies that are used for the separation process, the most common is via cryogenic distillation. This process was...

 plants. In most cases, the distillation is operated at a continuous
Continuous distillation
Continuous distillation, a form of distillation, is an ongoing separation in which a mixture is continuously fed into the process and separated fractions are removed continuously as output streams. A distillation is the separation or partial separation of a liquid feed mixture into components or...

 steady state
Steady state
A system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...

. New feed is always being added to the distillation column and products are always being removed. Unless the process is disturbed due to changes in feed, heat, ambient temperature, or condensing, the amount of feed being added and the amount of product being removed are normally equal. This is known as continuous, steady-state fractional distillation.

Industrial distillation is typically performed in large, vertical cylindrical columns known as "distillation or fractionation towers" or "distillation columns" with diameters ranging from about 65 centimetres to 6 metres and heights ranging from about 6 metres to 60 metres or more. The distillation towers have liquid outlets at intervals up the column which allow for the withdrawal of different fractions or products having different boiling points
Boiling Points
Boiling Points is a prank reality television show, much like the format used on Candid Camera. It is broadcast on MTV in the United States. In each half-hour episode, annoying situations are set up and deliberately inflicted on one or more young adults who are unaware that they are being tested...

 or boiling ranges. By increasing the temperature of the product inside the columns, the different hydrocarbons are separated. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column.

For example, fractional distillation is used in oil refineries
Oil refinery
An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...

 to separate crude oil into useful substances (or fractions) having different hydrocarbons of different boiling points. The crude oil fractions with higher boiling points:
  • have more carbon atoms
  • have higher molecular weights
  • are more branched chain alkane
    Alkane
    Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...

    s
  • are darker in color
  • are more viscous
    Viscosity
    Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

  • are more difficult to ignite and to burn
    Combustion
    Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...



Large-scale industrial towers use reflux
Reflux
Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations...

 to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the reflux liquid flowing downwards provides the cooling needed to condense the vapors flowing upwards, thereby increasing the effectiveness of the distillation tower. The more reflux is provided for a given number of theoretical plate
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 an equilibrium stage, ideal stage or a theoretical...

s, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux provided for a given desired separation, the fewer theoretical plates are required.

Fractional distillation is also used in air separation, producing liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

, liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

, and highly concentrated argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

. Distillation of chlorosilane
Chlorosilane
Chlorosilanes are a group of reactive, chlorine-containing chemical compounds, related to silane and used in many chemical processes. Each such chemical has at least one silicon-chlorine bond.-Synthesis:...

s also enable the production of high-purity silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 for use as a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

.

In industrial uses, sometimes a packing material is used in the column instead of trays, especially when low pressure drops across the column are required, as when operating under vacuum. This packing material can either be random dumped packing (1-3" wide) such as Raschig ring
Raschig ring
Raschig rings are pieces of tube used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic or metal and provide a large surface area within the volume of the column for interaction between liquid and gas or vapour...

s or structured sheet metal
Structured packing
The term structured packing refers to a range of specially designed materials for use in absorption and distillation columns and chemical reactors...

. Typical manufacturers are Koch, Sulzer and other companies. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer
Mass transfer
Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used...

 takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor liquid equilibrium the vapor liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns it is useful to compute a number of "theoretical plates"
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 an equilibrium stage, ideal stage or a theoretical...

 to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.

Design of industrial distillation columns

Design and operation of a distillation column depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method
McCabe-Thiele method
The McCabe-Thiele method was presented by two graduate students at Massachusetts Institute of Technology , Warren L. McCabe and Ernest W. Thiele in 1925. The technique is considered to be the simplest and perhaps most instructive method for analysis of binary distillation...

 or the Fenske equation
Fenske equation
The Fenske equation in continuous fractional distillation is an equation used for calculating the minimum number of theoretical plates required for the separation of a binary feed stream by a fractionation column that is being operated at total reflux .The equation was derived by Merrell Fenske in...

  can be used. For a multi-component feed, simulation
Simulation
Simulation is the imitation of some real thing available, state of affairs, or process. The act of simulating something generally entails representing certain key characteristics or behaviours of a selected physical or abstract system....

 models are used both for design and operation.
Moreover, the efficiencies of the vapor-liquid contact devices (referred to as plates or trays) used in distillation columns, as seen in Figure 3, are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation column needs more plates than the number of theoretical vapor-liquid equilibrium stages.

An indication of numbers: the separation of two compounds with relative volatility
Relative volatility
Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. This quantity is widely used in designing large industrial distillation processes. In effect, it indicates the ease or difficulty of using distillation to separate the more volatile...

 of 1.1 requires a minimum of 130 theoretical plates with a minimum reflux ratio of 20. With a relative volatility of 4, the required number of theoretical plates decreased to 9 with a reflux ratio of 0.66. In another source, a boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

 difference of 30 °C requires 12 theoretical plates and, for a difference of 3 °C, the number of plates increased to 1000.

The reflux ratio is the ratio of the amount of moles returned as refluxed liquid to the fractionating column and the amount of moles of final product, both per unit time.

See also

  • Azeotropic distillation
    Azeotropic distillation
    In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, azeotropic distillation usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous...

  • Batch distillation
    Batch distillation
    Batch distillation refers to the use of distillation in batches, meaning that a mixture is distilled to separate it into its component fractions before the distillation still is again charged with more mixture and the process is repeated...

  • Extractive distillation
    Extractive distillation
    Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. The method is used for mixtures having a low value of relative volatility, nearing unity...

  • Fractionating column
    Fractionating column
    A fractionating column or fractionation column is an essential item used in the distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities...

  • Freeze distillation
  • Raschig ring
    Raschig ring
    Raschig rings are pieces of tube used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic or metal and provide a large surface area within the volume of the column for interaction between liquid and gas or vapour...

    s
  • Steam distillation
    Steam distillation
    Steam distillation is a special type of distillation for temperature sensitive materials like natural aromatic compounds....


External links

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