Fractionating column

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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, with male and female ground glass at both ends (usually). Inside the tube, there are a number of glass spurs (known as trays or plates), whose purpose is to return the liquid vapor to the distilling flask, refluxing the rising distillate vapor.

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

To be added

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.

See also

References

Relevant Sciencemadness threads