The purification of oil is carried out in the following three steps-
Demulsification
Crude oil is a stable emulsion of oil in water and water in oil types. The emulsion is stabilized by emulsifying agents. Oil in water emulsion is stabilized by certain chemicals like sodium salts of naphthenic acid whereas water in oil types is stabilized by asphaltene compounds.
Demulsification process is carried out by various mechanical, thermal, chemical or electrical methods Of them, all electric methods are mostly used by applying high voltage alternating current.
Removal of harmful impurities
The alkali chlorides and sulfur compounds corrode the stainless steel machine parts and so have to be removed.
Alkali chlorides such as NaCl and MgCl2 are removed by desalting and dehydration process.
Sulfur is removed by trading crude oil with copper oxide whereby CuS formed is removed as a precipitate.
Fractional distillation
- crude oil free harmful impurities are heated in an iron at 4000Cwhere all volatile components except asphalt and coke are evaporated.
- Hot vapours are sent to the fractionating column.
- Fractionating column or bubble tower a tall cylindrical tower having horizontal stainless trays at regular intervals. Each tray has a number of chimneys covered by loose caps. Each tray is connected to the immediate lower tray with downspout pipe.
- As vapours go up, they become cooler and fractional condensation takes place at the different height.
- Higher boiling fraction condenses on the subsequent upper trays (lower temperature).
- Thus, liquids having lower boiling points are obtained at higher ports.
- The heavy oil fraction is heated and the molecular weight various fraction is obtained. Each faction has its unique properties and uses.