The basic idea behind
fractional distillation is the same as simple distillation. The difference
between simple and fractional distillation is the number of times that the
liquid is vaporized and condensed. Simple distillation condenses the liquid
once, so the boiling points of the two liquids must be far apart to make it
efficient. Simple distillation is performed on a mixture of liquids with
similar volatilities and the resulting distillate is in a more concentrated
form in the more volatile compound than the original mixture and it may contain
a significant amount of the higher boiling compound. If the distillate of
simple distillation is distilled again, the resulting distillate is again of a further
more concentrated form of the lower boiling compound, but still, a portion of
the distillate contains the higher boiling compound. The number of simple
distillations in a fractional distillation depends on the length and efficiency
of the fractionating column.
Fractional distillation
is a process in which vaporization of a liquid mixture gives rise to a mixture
of constituents from which the desired one is separated in pure form. This
method is also known as rectification because a part of the vapour is condensed
and returned as a liquid. This method is used to separate miscible volatile
liquids, whose boiling points are close, using a fractionating column.
Fractional distillation is different from simple distillation. In simple
distillation, the vapour is directly passed through the condenser. In
fractional distillation, the vapour must pass through a fractionating column in
which partial condensation of vapour is allowed to occur. In simple
distillation, condensate is collected directly into the receiver, while in
fractional distillation condensation takes place in the fractionating column so
that a part of the condensing vapour returns to the still.
Principle of Fractional Distillation
When a liquid mixture is
distilled, the partial condensation of the vapour is allowed to occur in a
fractionating column. In the column, ascending vapours from the still are
allowed to come in contact with the condensing vapour returning to the still.
This result is an enrichment of the vapours with the more volatile component.
By condensing the vapour and reheating the liquid repeatedly, equilibrium
between liquid and vapour is set up at each stage, which ultimately results in
the separation of a more volatile component.
Fig.1: Laboratory Set of Fractional Distillation
Construction of Fractional Distillation
The equipment used for
fractional distillation consists of a special type of still-heads known as
fractionating columns. In still-heads condensation and revaporisation are
affected continuously. A fractionating column is essentially a long vertical
tube in which the vapour moves upward and partially gets condensed. The
condensate flows down the column and is returned to the flask. The columns are
constructed to provide a large cooling surface for the vapour to condense and
obstruct the ascending vapour to allow easy condensation. The obstruction also
retards the downward flow of liquid, which is a high boiling component.
Fractionating columns used are packed columns and plate columns.
Packed columns:
In this column, some form of packing is used to affect the necessary
liquid/vapour contact. The packing consists of single turn helices (spirals) of
wire or glass, glass rings, cylindrical glass beads, stainless steel rings etc.
The column consists of a tower containing a packing that becomes wetted with a
film of liquid, which is brought into contact with the vapour in the
intervening spaces. The same type of fractionating columns can be obtained in
various lengths. A long fractionating column is necessary when the boiling
points of the constituents are lying fairly close together. A short
fractionating column is necessary when the boiling point of the constituents
differ considerably. Packing must be uniform to obtain proper channels. If
packing is irregular, the mass transfer becomes less effective. An example is the
Widmer column.
Plate columns:
Many forms of plates are used in the distillation columns. These can be divided
into Bubble cap plates and Turbo grid plates. The bubble cap column is used in
large distillation plants. The column consists of several plates mounted one
above the other. Caps are present on each plate, which allow the vapour to
escape by bubbling through the liquid. Ascending vapour still passes through
the bubble-caps on plate A and the rising vapour will be richer in the more
volatile component. This vapour passes through the liquid on plate B and is partially
condensed. The heat of condensation partially vaporizes the liquid. The process
of condensation and vaporization is repeated at plate C and so on up the
column. Each bubble-cap plate has the same effect as a separate still. The
bubble cap plate is effective over a wide range of vapour-liquid proportions.
There is an excellent contact as the vapour bubbles through the liquid. The
major limitation is a layer of liquid on each plate that results in the considerable
hold-up of liquid over the entire column. There is a need to force the vapour
out of the caps, through the liquid that leading to a large pressure drop
through the column. In addition, the column does not drain when it is not in
use. The structure of the bubble plate is so complicated that makes
construction and maintenance expenses.
Fig.2: Fractional Distillation Schematic for Crude Oil Separation
Working of Fractional Distillation
In fractional distillation, the vapours formed from the boiling mixture rise into the fractionating column where they condense on the column's packing. This condensation is similar to a single run of simple distillation; the condensate is more concentrated in the lower boiling compound than the mixture in the distillation flask. As vapours continue to rise through the column, the condensed liquid revaporizes. Each time this occurs the resulting vapours are more and more concentrated in the more volatile substances. The length of the fractionating column and the material it is packed with the impact the number of times the vapours will recondense before passing into the condenser. The number of times the column will support this is referred to as the number of theoretical plates of the column. The procedures of simple distillation are so similar to those involved in fractional distillation; the apparatus that is used in the procedures are also very similar. In fractional distillation, a packed fractionating column is attached to the top of the distillation flask and beneath the condenser. This provides the surface area on which rising vapours condense, and subsequently revaporize.
The fractionating column
is used to supply a temperature gradient over which the distillation can occur.
In an ideal situation, the temperature in the distillation flask would be equal
to the boiling point of the mixture of liquids and the temperature at the top
of the fractionating column would be equal to the boiling point of the lower
boiling compound; all of the lower boiling compounds would be distilled away
before any of the higher boiling compounds. In reality, fractions of the
distillate must be collected because as the distillation proceeds, the concentration
of the higher boiling compound in the distillate being collected steadily
increases. Fractions of the distillate, which are collected over a small
temperature range, will be essentially purified; several fractions should be
collected as the temperature changes and these portions of the distillate
should be distilled again to amplify the purification that has already
occurred.
The efficiency of
Fractional distillation: The efficiency of separation by
fractional distillation of a mixture may depend upon various factors that
include fractionating column, reflux ratio, heat input and column temperature
etc. The reflux ratio is the quotient of the amount of liquid returning through
the column to the amount collected into the receiver during the same interval
of time. A column that operates under total reflux will not yield distillate
and thus it should be high. It is controlled by selecting proper still. Other
experimental conditions necessary for good separation are a comparatively large
amount of liquid continuously returning through the column, thorough mixing of
liquid and vapour and a large active surface of contact between liquid and
vapour. The number of vaporization-condensation cycles that can occur within a
fractionation column determines the purity which can be attained. The
efficiency of a column depends upon column length and composition. Generally, the
column is packed with a copper sponge. This increases the surface area that the
ascending vapour encounters and results in more vaporization-condensation cycles
compared to an empty column.
Theoretical Plates:
A measure of the efficiency of a column is known as the number of theoretical
plates of that column. One theoretical plate is equivalent to one vaporization condensation
cycle which is equivalent to the one simple distillation. Thus a fractionation
column that can attain the equivalent of three simple distillations would be
said to have three theoretical plates.
Applications of Fractional Distillation
- Fractional distillation is used for the separation of miscible liquids such as acetone and water, chloroform and benzene.
- Fractional distillation is suitable for a system when the boiling point of the mixture is always intermediate between those of pure components.
- There is neither a maximum nor a minimum in the composition curves (zeotropic mixtures). Examples include benzene and toluene, carbon tetrachloride and cyclohexane, and water and methanol.
Advantages of Fractional Distillation
- Frictional distillation gives good solvent recovery
- Fractional distillation is easy to use and operate.
- Fractional distillation is also highly efficient, especially for systems that use stacked distillation columns, which produce more output at lower costs.
- It helps to produce much-needed fuel.
Disadvantage of Fractional Distillation
- Fractional distillation cannot be used to separate miscible liquids, which form pure azeotropic mixtures.
- It is expensive because it requires large structures, heavy-duty materials, and specialized machinery.
- It also requires staff to be fully trained in the operation of systems to ensure they know how to use the distillation equipment and won’t make mistakes.
- It presents a wide range of risks for the people who are involved in it. For example, explosion.
- It can contribute to environmental pollution. Fractional distillation in itself is not harmful but the types of mixtures that are distilled disturbs ecology. For example, refining crude oil.
Make sure you also check our other amazing Article on : Methodology of Simple Distillation