After the separation of the casing-head gases, the petroleum is refined by distillation to obtain individual products. The industrial distillation of petroleum is accomplished in continuously-operating plants. These consist of two main installations: a pipe still for heating the petroleum and a fractionating column for dividing the petroleum into cuts. The cuts are separate mixtures of hydrocarbons according to their boiling points: the petrol cut, the naphtha cut, the kerosene cut, etc.
The pipe still is lined inside with fire brick. In it there is a long steel coil pipe. The still is heated by burning residual oil or gas. Petroleum is pumped continuously through the coil after preheating in heat exchangers by the distillation products. Inside the coil the petroleum is quickly heated to 300-325 degrees C; as a liquid-vapour mixture it then goes on to the fractionation column.
The fractionation column is a cylindrical structure up to 40 m in height. Inside it has several horizontal partitions with orifices known as bubble plates. The petroleum vapour, on entering the column, rises upwards and passes through the orifices in the bubble plates. In the course of this upward movement the vapour is gradually cooled and condenses on this or that bubble plate, according to its boiling point. The less volatile hydrocarbons condense on the very first plates, producing the gas oil cut; more volatile hydrocarbons are collected higher and form the kerosene cut; the naphtha cut is collected still higher, and the most volatile hydrocarbons of all issue from the column at the top, forming petrol. Part of the petrol is fed back into the column to trickle downwards and in this way aid the cooling and condensation of the rising vapour.
The liquid part of the petroleum entering the column drips downwards through the bubble plates, forming residual oil. Superheated steam is supplied at the bottom of the column and rises upwards to facilitate the evaporation of the volatile hydrocarbons retained by the residual oil.
The processes of evaporation and condensation, repeated on plate after plate, make for the better fractionation of the petroleum. The cuts collecting at certain levels are piped out of the column. As they pass heat exchangers, they give up their heat to petroleum about to enter the pipe still. They are then purified by means of sulphuric acid and alkali, as well as by other techniques, to remove impurities. After this, they are stored as commercial products: petrol, kerosene, gas oil, etc.
By processes similar to those we considered earlier the residual oil vapour is fractionated in the column into cuts that, after purification, yield various lubricants. The undistillable heavy residue collected from the bottom of the column is oil tar. The primary and secondary fractionation plants are often built together as an atmospheric and vacuum distillation complex. The atmospheric unit serves to distill the petroleum, while the residual oil formed is distilled by the second, vacuum unit.
14. Ответьте на следующие вопросы:
1. What process follows the separation of the casing-head gases? 2. What is distillation used for? 3. Where is the industrial distillation of petroleum accomplished? 4. What does a continuously-operating plant consist of? 5. The cuts are separate mixtures of hydrocarbons according to their boiling points, aren’t they? 6. Is the pipe still lined inside with fire brick? 7. How is the still heated? 8. What temperature is the petroleum heated to inside the coil? 9. What does the fractionation column look like? 10. What are bubble plates? 11. What cuts are produced in the fractionation column? 12. What are sulphuric acid and alkali used for? 13. Where is the residual oil formed distilled?