question_answer 130)

  Draw a diagram of bubble plate type fractionating column. When do we require such type of a column for separating two liquids? Explain the principle involved in the separation of components of a mixture of liquids by using fractionating column. What industrial applications does this process have?

Answer:

  Fractional distillation is used for the separation and purification of organic liquids from non-volatile impurities or for separating two or more volatile liquids from a liquid mixture which have boiling points close to each other. Since, in this process, the distillate is collected in fractions under different temperatures, it is known as fractional distillation. When the liquids present in the mixture have their boiling points close to each other, the separation into pure liquids is difficult. The fractions collected are always contaminated. The fractions are further purified by repeated distillations. To decrease the number of distillations, the separation is done by fitting the distillation flask with a fractionating column*which in turn is connected to the condenser. Various types of fractionating columns are given in Fig. Liquids forming a constant boiling mixture (azeotropic mixture) cannot be separated by this method. Fractional distillation is used these days in industry, especially, in the distillation of petroleum, coal-tar and crude alcohol. A mixture of methanol (b.pt. 338K) and propanone (b.pt.330K) or a mixture of benzene and toluene may be separated by fractional distillation.