Answer:
Well.. Its a small n initeresting question but the answer is too lengthy for this.. I will try briefing on it. Crude oil procured contains many impurities like salts, metals which need to be taken off. Its been taken to desalters and then to the main crude column. In fractionator, the components get separated depending on their boiling points. The bottmom of the fractionator is taken to vaccum column. The vacuum maintained will be approx 15- 20 mmHg at the bottom of the column. Lighters generated in main column are aondensed and used as naptha. The different cuts like kerosene and diesel are routed to respective hydro treaters. The heavier portion (higher C-H) are considered for either hydro cracking or hydro treater. In hydrotreater the main reactant is Hydrogen which is used at very high pressure and temperature which helps in denitrification and desulfurisation respectively.