Fractionating column

A fractionating column is an essential item used in distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities.

General

A fractionating column consists of a long tube, usually with male and female ground glass at both ends. Inside the tube, there are usually elements that increase the surface area for better contact between gas and liquid phase. In some types they may take the shape of glass spurs (known as trays or plates).

Types

Vigreux column

Consists of a borosilicate glass tube, where a glass blower has modified the simple tube to include an abundance of downward-pointing indentations, thus dramatically increasing the surface area per unit length of the condenser. Such columns are often used to add the theoretical plates required in fractional distillation, and present added cost for their manufacture, which can include designs with or without an outer glass cylinder (jacket), open to air or allowing fluid circulation, or, to aid in insulation, an outer vacuum jacket.

Snyder column

It is a single glass tube with a series of circular indentations/restrictions in the walls of the cylinder (e.g., 3 or 6) in which rest, inverted, the same number of roughly tear-shaped, hollow, sealed glass stoppers; above each point where an inverted tear-shaped stopper rests, the cylinder has further Vigreux-type indentations, in this case serving to limit how high the glass stopper can be raised (by vapor flow) above its resting place, where, when not raised, it seals the circular opening created by the circular indentation. These floating glass stoppers act as check valves, closing and opening with vapor flow, and enhancing vapor-condensate mixing.

Widmer column

A complex type of air condenser combining Golodetz-type concentric tubes and the Dufton-type glass rod-and-wound-spiral at its center.

Packed column

More often encountered in chemical engineering and chemical plants, this type of column consists of a cylinder filled with a packing material such as Raschig rings. Packed columns can also used as catalyst reactors.

How a fractionating column works

Fractionating columns allow for good contact between the vapours of the solution and backflowing distillate. The idea is that here the more volatile part of the liquid is evaporated by the vapours, while the least volatile part of the vapours condense back. This leads to an increased proportion of the volatile component in the gas phase and an increased proportion of the less volatile component in the reflowing liquid. Columns with actual physical plates make this miniature redistillation quite clear, but even in packed columns or straight tubes a thermal gradient is formed, along which the more volatile component is enriched in the vapour. Each theoretical plate can be visualized as one step in the McCabe-Thiele-method.

This makes it very clear that reflux is needed for fractionation to work. If nothing is refluxing inside the column, no change in the composition can be expected, making the column useless. For optimal control of the reflux ratio a reflux divider (sometimes called a partial-takeoff stillhead) is used, which returns some proportion of the fresh distillate back into the column.

The thermal gradient in the column is supposed to form on its own, which is why columns tend to be isolated with (mirrored) vacuum mantles or in some other way. However, it should be noted that it is easier to overload/flood the column and so diminish the required reflux inside that is required for proper functioning. Thus it may be the case that a badly insulated column provides better separation because it is more difficult to overshoot and ruin the reflux ratio.

Availability

Fractionating columns can be bought from lab suppliers or online.

DIY fractionating column

You can make a fractionating column by inserting glass wool or some other inert filling (like silica gel) in a straight condenser, though make sure you don't stuff it too dense, otherwise during the distillation you'll flood the column.

Important tips

• Wrapping aluminium foil around the column will help limit heat loss, which is important when distilling liquids with a high boiling point.
• If aluminium foil is not enough, a three layer insulation from aluminium foil, dry kitchen towel and more aluminium foil to hold it in place can provide better insulation.
• Do not crank the heat when distilling volatile solvents, especially if you've wrapped insulating foil around the column, as you may flood it.

See also

• Condenser
• Distillation
• Reflux

References

Relevant Sciencemadness threads

• fractional distillation
• Fractionating colums and application...
• Why fractionating columns should be heated and/or well insulated
• snyder column
• Fractionating column for vacuum distillation
• Insulation on fractional distillation?
• Insulated fractionating column
• fractionating column help?
• Making a fractionating column