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A laboratory fractionating column is a piece of
glassware used to separate vaporized mixtures of
liquid compounds with close volatilities. It can
also be called a fractional column. Most
commonly used is either a Vigreux column or a
straight column packed with glass beads or metal
pieces such as Raschig rings.
Fractionating columns help to separate the
mixture by allowing the mixed vapors to cool,
condense, and vaporize again in accordance with
Raoult's law. With each
condensation-vaporization cycle, the vapors are
enriched in a certain component. A larger
surface area allows more cycles, improving
separation. This is the rationale for a Vigreux
fractionating column or a packed fractionating
column. Spinning band distillation achieves the
same outcome by using a rotating band within the
column to force the rising vapors and descending
condensate into close contact, achieving
equilibrium more quickly.
As shown in Image 1, as a liquid mixture in the
round bottomed flask is boiled, vapor rises up
the fractionating column. The vapor condenses on
the glass platforms (known as trays or plates)
inside the column, and runs back down into the
liquid below and refluxes the upflowing
distillate vapor. The hottest tray is at the
bottom of the column and the coolest tray is at
the top. At steady state conditions, the vapor
and liquid on each tray is at equilibrium. Only
the most volatile of the vapors stays in gas
form all the way to the top. The vapor at the
top of the column then flows through the
water-cooled condenser, which cools the vapor
down until it condenses into a liquid
distillate. The separation may be enhanced by
the addition of more trays (to a practical
limitation of heat, flow, etc.)
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