Fractional Distillation
Introduction
Simple distillation can be used to separate components of a mixture that have a large difference in their boiling points.
If two components have a boiling point difference of less than 40-50oC, simple distillation will not be successful at
separating them. In this case, fractional distillation must be used.
Theory
When two compounds with relatively close boiling points are heated, the vapor above the liquid will be composed of
both components at the boiling point. Consider the simple distillation of a mixture of 50:50 A and B, where A has a
lower boiling point than B. When the mixture begins to boil, the vapor will be richer in A, but still have a significant
proportion of B; let’s say the vapor above the boiling liquid is 66:33 A:B. This vapor would get cooled in the condenser
and drip into the receiving flask. At the end of the simple distillation the receiving flask has been enriched in A and the
liquid remaining in the still pot is enriched in B. To purify A you would take the liquid from the receiving flask composed
of 66:33 A:B, set up another simple distillation, enriching to perhaps 80:20 A:B. After a series of these
vaporization/condensation cycles you would finally get pure A.
Fractional distillation can be considered a series of simple distillations but rather than having many condensers and
receivers, the evaporation/condensation cycles take place in a single distillation column. Distillation columns are high
surface area tubes that allow multiple vaporization/condensation cycles to occur at once. The added surface area in the
column can come from glass protrusions like in Vigreux columns, the ones we will be using, or from a packing material
such as glass beads or steel wool put in a hollow tube which is known as a Hemple column. In fractional distillation,
vapor rises up the column and condenses on the packing, then re-evaporates, rises further up the column and
condenses again. This evaporation/condensation continues up the column and with each cycle the vapor becomes
more and more pure in the lower boiling component. In this way, fractional distillation accomplishes in one apparatus
what would require several simple distillation setups.
The more vaporization/condensation cycles that the mixture goes through, the better the separation. Although more
surface area in the column leads to better separation, it also makes the distillation process slower. Additionally, surfaces
in the column will be covered with condensation, and liquid that sticks to the surfaces will not be distilled. The liquid
condensed on the surfaces in the column is known as hold-up. When using fractional distillation chemists try to strike a
balance between the quality of the separation and the speed of the distillation.
During fractional distillation there is a temperature gradient over the length of the column. The boiling point of a
mixture varies with the composition of the mixture; the higher the percentage of the low boiling component, the lower
the boiling point will be. Thus as the mixture travels up the column and becomes purer, the temperature of the column
decreases. When the distillate finally reaches the still head, the thermometer reading begins to rise. The thermometer
reads the boiling point of the vapor that is condensing on it. (Boiling point and condensation point are the same
temperature.) If the thermometer reads at the boiling point of the lower boiling compound, the distilled liquid is pure.
When the temperature rises above the boiling point of the low boiling compound that means more and more of the high
boiling component is starting to distill over. To ensure a temperature gradient over the column, the still pot should be
heated slowly. Most of the vapor from the boiling still pot should condense and drip back down into the still pot, a
process known as refluxing.
A more detailed treatment of the science behind fractional distillation can be found on these websites:
http://www.chemguide.co.uk/physical/phaseeqia/idealfract.html
http://www.chemguide.co.uk/physical/phaseeqia/idealpd.html
