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Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation. Generally the component parts boil at less than 25°C from each other under a pressure of one atmosphere (ATM). If the difference in boiling points is greater than 25°C, a Distillation#Simple distillation is used.
Laboratory setup
Fractional distillation in a laboratory makes use of common laboratory glassware, as well as some single-purpose items like a fractionating column and "pigs" and "cows" used to cut fractions.
Apparatus
. A conical flask is used as a receiving flask. Here the distillation head and
fractionating column are combined in one piece.
Method
As an example, consider the distillation of a mixture of
water and ethanol. Ethanol boils at 78.5 °C while water boils at 100 °C. So, by gently heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid. Some mixtures form azeotropes, 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 Volatility (chemistry) than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.
The apparatus (the diagram represents a batch apparatus, as opposed to a continuous apparatus) is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few Boiling chip, and the
fractionating column is fitted into the top. As the mixture boils, vapor rises up the column. The vapor condenses on the glass platforms, known as
theoretical tray, inside the column, and runs back down into the liquid below, refluxing distillate. The column is heated from the bottom. 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
vapor-liquid 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, 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.
In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a
water jacket, 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.
Vacuum distillation systems operate at reduced pressure, thereby lowering the boiling point of the materials.
Industrial distillation
Distillation is the most common form of separation technology used in
oil refinery, petrochemical and chemical plants and
natural gas processing plants. In most cases, the distillation is operated at a
Continuous distillation steady state. 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 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters 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 or boiling ranges. 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 refinery to separate
crude oil into useful substances (or fractions) having different
hydrocarbons of different boiling points. The crude oil fractions with higher boiling points:
Large-scale industrial towers use reflux 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 plates, 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 nitrogen, and high purity
argon. Distillation of
chlorosilanes also enable the production of high-purity silicon for use as a semiconductor.
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 rings or structured packing. 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 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 plate 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 or the
Fenske equation can be used. For a multi-component feed, simulation 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 2, 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 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 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
External links
- Validating Your Binary VLE Data
- ACS Distillation Tower Packing
- Chemistry Activity: Fractional Distillation
References
Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their
boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of
distillation. Generally the component parts boil at less than 25°C from each other under a pressure of one atmosphere (ATM). If the difference in boiling points is greater than 25°C, a
Distillation#Simple distillation is used.
Laboratory setup
Fractional distillation in a laboratory makes use of common laboratory glassware, as well as some single-purpose items like a fractionating column and "pigs" and "cows" used to cut fractions.
Apparatus
. A
conical flask is used as a receiving flask. Here the distillation head and
fractionating column are combined in one piece.
- heat source, such as a hot plate with a bath
- distilling flask, typically a round-bottom flask
- receiving flask, often also a round-bottom flask
- fractionating column (Vigreux column)
- distillation head
- thermometer and adapter if needed
- condenser
- vacuum adapter (not used in upper right image)
- boiling chips, also known as anti-bumping granules
- rubber bungs, unless laboratory glassware with ground glass joints is used, such as quickfit apparatus.
Method
As an example, consider the distillation of a mixture of
water and ethanol. Ethanol boils at 78.5 °C while water boils at 100 °C. So, by gently heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid. Some mixtures form azeotropes, 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
Volatility (chemistry) than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.
The apparatus (the diagram represents a batch apparatus, as opposed to a continuous apparatus) is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few Boiling chip, and the
fractionating column is fitted into the top. As the mixture boils, vapor rises up the column. The vapor condenses on the glass platforms, known as
theoretical tray, inside the column, and runs back down into the liquid below,
refluxing distillate. The column is heated from the bottom. 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
vapor-liquid 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, 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.
In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a water jacket, 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.
Vacuum distillation systems operate at reduced pressure, thereby lowering the boiling point of the materials.
Industrial distillation
Distillation is the most common form of separation technology used in oil refinery,
petrochemical and chemical plants and
natural gas processing plants. In most cases, the distillation is operated at a Continuous distillation steady state. 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 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters 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 or boiling ranges. 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 refinery to separate
crude oil into useful substances (or fractions) having different hydrocarbons of different boiling points. The crude oil fractions with higher boiling points:
Large-scale industrial towers use
reflux 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 plates, 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 nitrogen, and high purity argon. Distillation of chlorosilanes also enable the production of high-purity silicon for use as a semiconductor.
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 rings or
structured packing. 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 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 plate 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 or the
Fenske equation can be used. For a multi-component feed,
simulation 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 2, 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 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 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
External links
- Validating Your Binary VLE Data
- ACS Distillation Tower Packing
- Chemistry Activity: Fractional Distillation
References
fractional distillation
Process used to split complex mixtures (such as petroleum) into their components, usually by repeated heating, boiling, and condensation; see distillation
Fractional distillation
A diagram of laboratory fractional distillation apparatus. ... round-bottomed distillation flask is underneath the fractionating column, which bears a stillhead and a thermometer ...
Fractional distillation
Chemistry: Fractional distillation - VIEW FULL SCREEN: Periodic Table Ionic bonding Mass number Atomic Structure pH scale Fractional distillation
BBC - GCSE Bitesize - Chemistry | Usefulproducts oil | Fractional ...
For the exam, you need to remember the different properties of hydrocarbons with different lengths of molecule - and to remember the main fractions derived from crude oil, together ...
BBC - GCSE Bitesize - Chemistry | Usefulproducts oil | Distillation ...
Distillation ... Distillation and fractional distillation. Distillation . The distillation process can be used to separate a pure liquid from a mixture of liquids with different ...
Fractional distillation - Wikipedia, the free encyclopedia
Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a ...
fractional distillation
Fractional Distillation
Purpose To separate a mixture of two miscible liquids (liquids that mix in all proportions) with different boiling points. Your mixture will consist of water and 2-propanol ...
Introduction to Distillation
A distillation tutorial from Ming T. Tham, of the University of Newcastle upon Tyne, UK.
Fractional Distillation Apparatus
Fractional distillation apparatus by BR International. New Fractional Distillation for. the Laboratory and Industry . B/R International's fractional distillation ...
