Question

How is the effectiveness of a distillation changed by each of the following?

a) Rapid heating.

b) Selecting liquids with identical boiling points.

c) Using a long packed column.

d) Using a short unpacked column.

Answer 1

How is the effectiveness of a distillation changed by each of the following?

a) Rapid heating.

b) Selecting liquids with identical boiling points.

c) Using a long packed column.

d) Using a short unpacked column.

In distillation process if we heat the mixture rapidly there are the chances that purity of the distillate will be compromised. This is because when we heat the mixture too rapidly less volatile liquid also get vaporise along with the more volatile liquid and it will contaminates the purity of more volatile liquid. Purity of the distillates changes depending upon the composition and concentration of mixture. For example ethanol and water mixture, when heated slowly all ethanol comes out with minimum water content but if we heat the mixture rapidly water content of ethanol increase considerably.

In simple distillation short unpacked column is used it gives effective separation between volatile organic liquids and non-volatile organic liquids , boiling point difference between two liquids in the mixture should be more than 80^oC theoretically but in practical the difference is very small to use simple distillation technique.

Fraction distillation is use in such cases where boiling difference is very small. Fractional distillation runs on the principle that, the vapour phase in equilibrium with the liquid phase will have a higher percentage of the more volatile (lower boiling) liquid than the liquid phase will. In fractional distillation long packed column has proven to be effective since the numbers of theoretical plates are more in long packed column effective separation occurs. When mixture is heated and get vaporized it will be enriched with the lower boiling point liquid. As this moves upward in the column it comes in the contact with glass walls of column and the packed material and cools down recondenses. At the same time some more hot vapours rise up and heat up the condense vapours enriching it with lower boiling point liquid. This is like mini fractional distillation happening at small scale in at every unit area of column i.e theoretical plates. So, when finally liquid comes out of the column it is highly enriched with the lower boiling point liquid. Hence we can say that more the theoretical plate more effective will be fractional distillation. Therefore if we pack the column with say glass beads/ glass wool it increases surface area inside the column proving more theoretical plates for distillation.