Write a note on fractional distillation of petroleum.


  1. No.4. Write a note on fractional distillation of petroleum.

Answer. The crude oil is refined in the refineries. Refining process is the separation of crude oil mixture into various  useful  products

(fractions). It is carried out by a process called fractional distillation. The principle of fractional distillation is based upon separation of substances depending upon their boiling points. The substances having low boiling points boil out first leaving behind others. Then next fraction of slightly higher boiling point boils out. This process remain continue until a residue is left behind. The vapours of each fraction are collected and condensed separately. The fractional distillation of petroleum is carried out in a tall fractionating tower as shown in figure below:


The crude oil is heated in a furnace up to a temperature of 400°C under high pressure. Then vapours are passed through a fractionating column from near its bottom as shown in figure. Hot vapours rise up in the column and gradually cool down and condense. Such that vapours of higher boiling point fraction (350—400°G) condense first in the lower part of the tower, while vapours of medium and lower boiling point fractions rise upwards in the tower and condense gradually with respect to their boiling points at different levels. In this way crude oil is separated into six hydrocarbon fractions as discussed below. Each fraction has its specific boiling range, composition and uses.



  1. No.5. How urea is manufactured? Explain showing the flow sheet diagram.

Answer. Manufacture of Urea

Urea is nitrogenous fertilizer. It consists of 46.6% nitrogen. It is white crystalline compound, highly soluble in water. It is used for the manufacturing of important chemicals, but its major (about 90%) use is as a fertilizer.

The raw materials for the manufacturing of urea are:

(i) Ammonia (NH3)      (ii) Carbon dioxide (CO2)

Ammonia is prepared by the Haber’s process. One volume of nitrogen (from air) and three volumes of hydrogen (obtained by passing methane and steam over heated nickel catalyst) are passed over iron catalyst at 450°C and 200 atm pressure.


N2(g) + 3H2(g)         2NH2(g)



Manufacturing of urea involves three stages:

(i)       Reaction of ammonia and carbon dioxide; Carbon dioxide is passed through liquid ammonia under high pressure to form ammonium carbonate

2NH3 + CO2 ——— > NH2COONH4

Ammonium carbonate

(ii)      Urea formation; When ammonium carbonate is evaporated with the help of steam, it dehydrates to form urea.



(iii)    Granulation of urea: At this stage, liquid urea is evaporated to form granules. When liquid urea is sprayed from top of a tower under pressure and a hot current of air is introduced from the base, it evaporates to form granules. This is stored to be marketed.

(Unreacted NH3, CO2 & H2O)

Flow sheet diagram of manufacturing of urea

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