Crude oil as produced in the oil field is a complex mixture of hydrocarbons ranging from methane to asphalt, with varying proportions of paraffins, naphthenes, and aromatics. The objective of crude distillation is to fractionate crude oil into light-end hydrocarbons (Q-C4), naphtha/ gasoline, kerosene, diesel, and atmospheric resid. Some of these broad cuts can be marketed directly, while others require further processing in refinery downstream units to make them saleable.
The first processing step in the refinery, after desalting the crude, is separation of crude into a number of fractions by distillation. The distillation is carried out at a pressure slightly above atmospheric. This is necessary for the following considerations:
1. To raise the boiling point of the light-end carbons so that refinery cooling water can be used to condense some of the C3 and C4 in the overhead condenser.
2. To place the uncondensed gas under sufficient pressure to allow it to flow to the next piece of processing equipment.
Crude oil is preheated in exchangers and finally vaporized in a fired furnace until approximately the required overhead and sidestream products are vaporized. The furnace effluent is flashed into the crude column flash zone, where the vapor and liquid separate. The liquid leaving the flash zone still contains some distillate components, which are recovered by steam stripping. After steam stripping, the bottom product, also known as reduced crude, is discharged from the tower. The bottom temperature is limited to 700-750°F to prevent cracking.
The atmospheric resid is fed to a furnace, heated to 730-770°F and next to a vacuum tower operated at a minimum practical vacuum (80-110 mm Hg). The operating conditions are dictated by cracking and product quality required. The objectives of vacuum distillation is generally to separate vacuum gas oil (YGO) from reduced crude. The VGO may become feedstock for FCCU or hydrocracker units or used to make lube base stocks. Depending on the end use, there may be one or more sidestreams. The bottom stream from the vacuum distillation unit may be used to produce bitumen or used for fuel oil production after mixing it with small amounts of cutter stocks (in the diesel/kerosene range).
If the crude contains very high percentages of light-ends, a flash drum or a prefractionator with an overhead condensing system is added ahead of atmospheric tower. The prefractionator is designed to recover most of the light-ends and a part of the light naphtha. The bottom stream from prefractionator becomes feed to atmospheric tower.
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