Fractional distillation
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Fractional distillation is the separation of a mixture of miscible compounds by their boiling point, by heating to high enough temperatures.
Image:Fractional distillation lab apparatus.png
Contents |
Fractional Distillation in a Laboratory
Apparatus
- round bottom flask
- conical flask
- fractionating column
- liebig condenser
- graham condenser
- alhin condenser
- thermometer
- anti-bumping granules
- rubber bungs unless quickfit apparatus is used.
Method
As an example, consider the distillation of a mixture of water and ethanol. Ethanol boils at 78.5°C while water boils at 100°C. So by gently heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid. 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. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures. The apparatus (the diagram represents a batch apparatus, as opposed to a continuous 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.
In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a water jacket, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Alhin condensor has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense. Being more complex shapes to manufacture, these latter types are also more expensive to purchase. Condensers are usually sold by the mm: 100, 200, and 400 mm are common lengths, and are connected to the other vessels with ground glass fittings.
Industrial uses of Fractional Distillation
Main article: Oil refinery#operation
Distillation is the most common form of separation technology in the chemical industry. In most chemical processes, the distillation is continuous steady state, where batch fractionation is not as economical. 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 widely used industrial applications Template:Ref label Template:Ref label of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical plants and natural gas processing plants. Image:ShellMartinez.jpg Industrial distillation is typically performed in large, vertical cylindrical columns known as "distillation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more. The distillation towers have liquid outlets at intervals up the column which allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column. Large-scale industrial towers also use reflux to achieve more complete separation of products.
Fractional distillation 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, especially when low pressure drops across the column are required, as when operating under vacuum. This packing material can either be random dumped packing (1-3" wide) or structured sheet metal. Typical manufacturers are Koch, Sulzer and other companies. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. 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 stages" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.
See also
References
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