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Fractional distillation

Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize, it uses distillation to fractionate. The component parts have boiling points that differ by less than 25 °C from each other under a pressure of one atmosphere. If the difference in boiling points is greater than 25 °C, a simple distillation is used. Fractional distillation in a laboratory makes use of common laboratory glassware and apparatuses including a Bunsen burner, a round-bottomed flask and a condenser, as well as the single-purpose fractionating column. A laboratory setup would include: heat source, such as a hot plate with a bath distilling flask a round-bottom flask receiving flask also a round-bottom flask fractionating column distillation head thermometer and adapter if needed condenser, such as a Liebig condenser or Allihn condenser vacuum adapter standard laboratory glassware with ground glass joints.

As an example consider the distillation of a mixture of water and ethanol. Ethanol boils at 78.4 °C while water boils at 100 °C. So, by heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid; some mixtures form azeotropes. In this example, a mixture of 96% ethanol and 4% water boils at 78.2 °C. For this reason, ethanol cannot be purified by direct fractional distillation of ethanol-water mixtures; the apparatus is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few anti-bumping granules, the fractionating column is fitted into the top; the fractional distillation column is set up with the heat source at the bottom on the still pot. As the distance from the stillpot increases, a temperature gradient is formed in the column; as the mixed vapor ascends the temperature gradient, some of the vapor condenses and revaporizes along the temperature gradient. Each time the vapor condenses and vaporizes, the composition of the more volatile component in the vapor increases.

This distills the vapor along the length of the column, the vapor is composed of the more volatile component. The vapor condenses on the glass platforms, known as trays, inside the column, runs back down into the liquid below, refluxing distillate; the efficiency in terms of the amount of heating and time required to get fractionation can be improved by insulating the outside of the column in an insulator such as wool, aluminium foil or preferably a vacuum jacket. The hottest tray is at the bottom and the coolest is at the top. At steady state conditions, the vapor and liquid on each tray are at equilibrium; the most volatile component of the mixture exits as a gas at the top of the column. The vapor at the top of the column passes into the condenser, which cools it down until it liquefies; the separation is more pure with the addition of more trays Initially, the condensate will be close to the azeotropic composition, but when much of the ethanol has been drawn off, the condensate becomes richer in water.

The process continues. This point can be recognized by the sharp rise in temperature shown on the thermometer; the above explanation reflects. Normal laboratory fractionation columns will be simple glass tubes filled with a packing small glass helices of 4 to 7 millimetres diameter; such a column can be calibrated by the distillation of a known mixture system to quantify the column in terms of number of theoretical trays. To improve fractionation the apparatus is set up to return condensate to the column by the use of some sort of reflux splitter - a typical careful fractionation would employ a reflux ratio of around 4:1. In laboratory distillation, several types of condensers are found; the Liebig condenser is a straight tube within a water jacket, is the simplest form of condenser. The Graham condenser is a spiral tube within a water jacket, the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense.

Alternate set-ups may use a multi–outlet distillation receiver flask to connect three or four receiving flasks to the condenser. By turning the cow or pig, the distillates can be channeled into any chosen receiver; because the receiver does not have to be removed and replaced during the distillation process, this type of apparatus is useful when distilling under an inert atmosphere for air-sensitive chemicals or at reduced pressure. A Perkin triangle is an alternative apparatus used in these situations because it allows isolation of the receiver from the rest of the system, but does require removing and reattaching a single receiver for each fraction. Vacuum distillation systems operate at reduced pressure, thereby lowering the boiling points of the materials. Anti-bumping granules, become in

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