Oil refining is the process whereby crude oil is split — refined — into commercially useful products. Distillation is the primary means of separating the constituents, which may be sold directly, or be used as feedstock for further processes. These secondary processes may involve separation by extraction or may use catalysts to change the chemical species such that a further range of products are produced.
The objective of refining crude oil is to meet the marked demand in the most economical manner. The nature of the market (for example, whether there is strong demand for motor gasoline or for kerosene) and the relative values of the individual products (their marginal values) largely dictate the mix of refinery processes that are used. The relative values of products differ, with the high value materials typically occurring in the mid-boiling range materials such as motor gasoline, kerosene and diesel fuel. Values are influenced by geographical location, market profile and by the seasons.
Whereas a few refineries process a single source crude, most process a range of crude oils, the choice being dependent partly on the price of the crude cargo, and partly on the ability to most economically meet the product range demand.
Figure 1illustrates the flow scheme of processing units that might be found in a complex refinery, although not all refineries necessarily operate with all the units shown.
Crude oil entering the refinery is first distilled in the crude distillation unit, operating at nominally atmospheric pressure with a crude preheat furnace temperature in the range 370-380°C, any higher temperature causing excessive thermal cracking. The residue from the atmospheric distillation stage is then redistilled under a vacuum of typically 10–50 mb absolute to recover heavier distillates using a heater temperature up to approximately 400–440°C depending on crude type and distillate demand.
Liquid petroleum gases (LPG) are either sold directly, or converted to heavier high octane products for motor gasoline. Propane is converted by catalytic polymerization, and butane by isomeration or alkylation, to produce high octane liquid products for motor gasoline blending. Naphtha is catalytically converted (reformed) to convert naphthenes into higher octane aromatic components, also for motor gasoline (see also Hydrocarbons). Gas oil is used, after sulfur removal, for diesel fuel and heating oil.
Distillates from the vacuum distillation unit are used variously as hydrocracker or catalytic cracker feedstock, both these units giving products ranging from LPG gases to heavy gas oils. Heavy distillates from vacuum distillation are also used as direct feed for the manufacture of lubricating oils in which case the distillates are further processed to remove aromatic components and wax. The residue from vacuum distillation is normally used as fuel, road bitumen or petroleum coke.
Additional processes are used to remove sulfur compounds from both liquid and gaseous streams. Between approximately 2–8% of the feed on an energy basis, depending on refinery complexity, is used to provide fuel to the refinery.