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 one or more fractions of the compound will vaporize. 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. If the difference in boiling points 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, a round-bottomed
flask and a condenser, as well as the single-purpose fractionating column.
Apparatus
heat source, such as a hot plate with a bath, and ideally with a magnetic stirrer.
distilling flask, typically a round-bottom flask
receiving flask, often also a round-bottom flask
fractionating column distillation head
thermometer and adapter if needed condenser, such as a Liebig condenser, Graham
condenser or Allihn condenser vacuum adapter
boiling chips, also known as anti-bumping granules
Standard laboratory glassware with ground glass joints, e.g. quickfit apparatus.
Discussion As an example consider the distillation of
a mixture of water and ethanol. 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 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; the mixture is more volatile 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 mixture is put into the round bottomed flask along with a few anti-bumping granules,
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 increases, a temperature gradient is formed in the column; 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 component 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 component. The vapor condenses on the glass platforms, known
as trays, inside the column, and runs back down into the liquid below, refluxing 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.
The most volatile component of the mixture exits as a gas at the top of the column. 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 Initially,
the condensate will be close to the azeotropic composition, but when much of the ethanol
has been drawn off, the condensate 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. The above explanation reflects the theoretical
way fractionation works. Normal laboratory fractionation columns will be simple glass
tubes 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 trays. To improve fractionation the apparatus is set
up to return condensate to the column by the use of some sort of reflux splitter - a typical
careful fractionation would employ a reflux ratio of around 4:1.
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 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 multi–outlet distillation receiver flask to connect three
or four receiving flasks to the condenser. By turning the cow or pig, the distillates
can be channeled into any chosen receiver. Because the receiver does not have to be removed
and replaced during the distillation process, this type of apparatus is useful when distilling
under an inert atmosphere for air-sensitive chemicals or at reduced pressure. A Perkin
triangle is an alternative apparatus is often used in these situations because it allows
isolation of the receiver from the rest of the system, but does require removing and
reattaching a single receiver for each fraction. Vacuum distillation systems operate at reduced
pressure, thereby lowering the boiling points of the materials. Note that the use of anti-bumping
granules will not work at reduced pressures. Industrial distillation
Fractional distillation is the most common form of separation technology used in petroleum
refineries, petrochemical and chemical plants, natural gas processing and cryogenic air separation
plants. In most cases, the distillation is operated at a continuous 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. By increasing the temperature of the product inside the columns,
the different hydrocarbons are separated. The "lightest" products exit from the top
of the columns and the "heaviest" products exit from the bottom of the column.
For example, fractional distillation is used in oil refineries to separate crude oil into
useful substances 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 alkanes
are darker in color are more viscous
are more difficult to ignite and to burn
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 highly concentrated 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 such as Raschig rings or structured sheet metal. 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 plates" 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 used in distillation columns 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 1,000. The reflux ratio is the ratio of the number
of moles returned as refluxed liquid to the fractionating column and the number of moles
of final product, both per unit time. See also
Azeotropic distillation Batch distillation
Extractive distillation Fractionating column
Freeze distillation Raschig rings
Steam distillation References
External links Validating Your Binary VLE Data
Chemistry Activity: Fractional Distillation