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Fractional distillation

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Fractional distillation is the separation of a mixture of compounds by their boiling point, by heating to high enough temperatures.

Fractional Distillation in a Laboratory

Apparatus

Method

As an example, consider the distillation of a mixture of water and ethanol. Ethanol boils at 78.5°C whilst water boils at 100°C. So by gently heating the mixture, the most volatile component will boil off first. Some mixtures form azeotropes, where the mixture boils at a lower temperature than either component. In this example, a mixture of 95% ethanol and 5% water boils at 78.2°C, being more volatile than pure ethanol, so the ethanol cannot be completely purified by distillation.

The apparatus is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few anti bumping granules, and the fractionating column is fitted into the top. As the mixture boils, vapor rises up the column. The vapor condenses on the glass platforms, known as trays, inside the column, and runs back down into the liquid below, refluxing distillate. The column is heated from the bottom. The hottest tray is at the bottom the coolest 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 gaseous form all the way to the top. The vapor at the top of the column, then passes into the condenser, which cools it down until it liquefies. The separation is more pure with the addition of more trays (to a practical limitation of heat, flow, etc.) The condensate that was initially very close to the azeotrope composition becomes gradually richer in water. The process continues until all the ethanol boils out of the mixture. This point can be recognized by the sharp rise in temperature shown on the thermometer.

The Liebig condenser is characterized by a straight tube within a water jacket construction, it is the simplest form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Alhin condensor is a series of large and small constrictions on the inside tube, each increasing the surface area that the vapor constituents may condense upon. Being more complex shapes to manufacture they are more expensive to purchase, hence condensors are usually sold by the mm. :100mm, 200mm., 400mm. are common lengths and are connected to the other vessels with ground glass fittings.

Industrial uses of Fractional Distillation

Main article: Oil refinery

Distillation is the most common form of speparation technology in the chemical industry. In most chemical processes, the distillation is continuous steady state. New feed is always being added to the distillation column and products are always being removed. Unless the process is disturbed due to changes in feed, heat, ambient temperature, or condensing, the amount of feed being added and the amount of product being removed are normally equal. This is known as continuous steady state fractional distillation.

The most important industrial application of continuous steady state fractional distillation is the distillation of crude oil. The process is similar in principle to the laboratory method described above except for scale, continuous feed and operation, and the fact that crude oil has many different compounds mixed together. The fractionating column has outlets at regular intervals up the column which allow the different fractions to run out at different temperatures, with the highly volatile gases coming out the topmost outlet graduating to the less volatile road tar (bitumen) coming out at the bottom.

Fractional distillation process is also used in air separation, producing liquid oxygen, liquid nitrogen, and high purity argon. Distillation of chlorosilanes also enable the production of high-purity silicon for use as a semiconductor.

In industrial uses, sometimes a packing material is used in the column instead of trays. The packing material is normally small equally shaped pieces that are poured into the column. Liquid and vapor pass between the pieces of packing in the column. The vapor pressure keeps the liquid suspended in the packing material as the vapors bubble through the liquid. Unlike conventional tray distillation in which every tray represents a separate point of vapor liquid equilibrium, the vapor liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns it is useful to compute a number of "theoretical trays" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different of surface area and void space between packings. Both of these factors affect packing performance.