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Fractional Composition
and the vapor has a composition determined by the chemical properties of the mixture. Distillation
of a given component is possible, if the vapor has a higher proportion of the given component than
the mixture. This is caused by the given component having a higher vapor pressure and a lower boiling point than the other components.
The minimum in distillation is flash distillation, where either the temperature is rapidly
increased or pressure reduced, and vapor and liquid fractions are thus obtained, which may be
processed as such. The device used in distillation is referred to as a still and consists at a minimum of a reboiler (pot) in which the source material is heated, a condenser in which the heated
vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid
is collected.
The equipment may affect separation by one of two main methods. First, the vapors given off by
the heated mixture may consist of two liquids with significantly different boiling points. Thus, the
vapor that is given off is in the vast majority of one or the other liquid, which after condensation
and collection effects the separation. Second, fractional distillation may be necessary and is more
effective at separating liquids that have similar boiling points. This method relies upon a gradient
of temperatures existing in the condenser stage of the equipment. Often in this technique, a vertical
condenser, or column, is used and by removal of the distillation products that are liquid at different
heights up the column, it is possible to separate liquids that have different boiling points. The greater
the distance over which the temperature gradient in the condenser is applied, the easier and more
complete is the separation.
The basic idea behind fractional distillation is the same as simple distillation, only the process
is repeated many times. If simple distillation was performed on a mixture of liquids with similar
volatilities, the resulting distillate would be more concentrated in the more volatile compound
than the original mixture but it would still contain a significant amount of the higher boiling
compound. If the distillate of this simple distillation was distilled again, the resulting distillate
would again be even more concentrated in the lower boiling compound, but still a portion of
the distillate would be the higher boiling compound. If this process is repeated several times, a
fairly pure distillate will eventually result. This, however, would take a very long time. In fractional distillation, the vapors formed from the boiling mixture rise into the fractionating column
where they condense on the column’s packing. This condensation is tantamount 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 vapors continue to rise through the column, the liquid
that has condensed will vaporize. Each time this occurs the resulting vapors are more and more
concentrated in the more volatile substances. The length of the fractionating column and the
material it is packed with impact the number of times the vapors will condense 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.
Since the procedures of simple distillation are so similar to those involved in fractional distillation, the apparatus that are used in the procedures are also very similar. The only difference
between the equipment used in fractional distillation and that used in simple distillation is that with
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 vapors condense, and
subsequently vaporize.
Thus, the fractionating column supplies a temperature gradient over which distillation occurs.
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 compound would be
distilled away before any of the higher boiling compound. 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
Fractional Composition
and the vapor has a composition determined by the chemical properties of the mixture. Distillation
of a given component is possible, if the vapor has a higher proportion of the given component than
the mixture. This is caused by the given component having a higher vapor pressure and a lower boiling point than the other components.
The minimum in distillation is flash distillation, where either the temperature is rapidly
increased or pressure reduced, and vapor and liquid fractions are thus obtained, which may be
processed as such. The device used in distillation is referred to as a still and consists at a minimum of a reboiler (pot) in which the source material is heated, a condenser in which the heated
vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid
is collected.
The equipment may affect separation by one of two main methods. First, the vapors given off by
the heated mixture may consist of two liquids with significantly different boiling points. Thus, the
vapor that is given off is in the vast majority of one or the other liquid, which after condensation
and collection effects the separation. Second, fractional distillation may be necessary and is more
effective at separating liquids that have similar boiling points. This method relies upon a gradient
of temperatures existing in the condenser stage of the equipment. Often in this technique, a vertical
condenser, or column, is used and by removal of the distillation products that are liquid at different
heights up the column, it is possible to separate liquids that have different boiling points. The greater
the distance over which the temperature gradient in the condenser is applied, the easier and more
complete is the separation.
The basic idea behind fractional distillation is the same as simple distillation, only the process
is repeated many times. If simple distillation was performed on a mixture of liquids with similar
volatilities, the resulting distillate would be more concentrated in the more volatile compound
than the original mixture but it would still contain a significant amount of the higher boiling
compound. If the distillate of this simple distillation was distilled again, the resulting distillate
would again be even more concentrated in the lower boiling compound, but still a portion of
the distillate would be the higher boiling compound. If this process is repeated several times, a
fairly pure distillate will eventually result. This, however, would take a very long time. In fractional distillation, the vapors formed from the boiling mixture rise into the fractionating column
where they condense on the column’s packing. This condensation is tantamount 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 vapors continue to rise through the column, the liquid
that has condensed will vaporize. Each time this occurs the resulting vapors are more and more
concentrated in the more volatile substances. The length of the fractionating column and the
material it is packed with impact the number of times the vapors will condense 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.
Since the procedures of simple distillation are so similar to those involved in fractional distillation, the apparatus that are used in the procedures are also very similar. The only difference
between the equipment used in fractional distillation and that used in simple distillation is that with
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 vapors condense, and
subsequently vaporize.
Thus, the fractionating column supplies a temperature gradient over which distillation occurs.
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 compound would be
distilled away before any of the higher boiling compound. 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