OIL REFINING PROCESSES
Fractional Distillation –
It is the oldest and most common refining process and entails boiling crude oil up to temperatures of about 400°c at atmospheric pressure in a fractionating tower. Different fractions of crude oil having different boiling points are tapped at stages. The greater the number of Carbon atoms in a hydrocarbon molecules, the higher is the boiling point, crude oil is composed of thousands of different hydrocarbon compounds, each with its own boiling point.
Vacuum Distillation and Visbreakinq Process -
It is a modified version of straight run method of distillation, in which the distillation is carried out under partial vacuum to reduce the boiling points to obtain additional heavy distillates. Vaccum distillation produces residual oils that are feed stocks for other refinery processes.
Vaccum distillation bottoms can be further refined by the use of a secondary process such as viscosity breaking. In this procedure the feedstock, vacuum bottoms, is heated to a higher temperature and pressure for cracking. Subsequently the higher viscosity feedstock is broken down to further distillates and to a residuum', which is considerably lower in viscosity than the original feedstock. However, it has an increase density and less-desirable characteristic such as slower ignition quality, higher carbon residence and higher asphaltenes content. It is usually less stable and less compatiable with other residuals. It is therefore not generally used as marine fuel but only as a blend.
Thermal Cracking -
When heat is applied to hydrocarbon molecules in the liquid phase, some of them boil-off or vaporise. However, if the heat is applied when the oil is under pressure, there is a reduced tendency of hydrocarbon molecules to break free as vapour. If the temperature is high enough', the energy developed by the molecules is such that the Carbon-hydrogen and Carbon-carbon bonds are ruptured, and they crack into two or more smaller molecules having different chemical properties and structures. During the cracking process, which is essentially chemical, there is shortage of hydrogen so that some of the molecules formed are unsaturated, with their smaller number of carbon atoms per molecule, are more volatile.
In thermal cracking, it requires temp, of the order of 520-560°c and high pressure (usually about 13 bar) to split the molecules.
Catalytic cracking is carried out at about 490°c and 2 bar, usually on raw distillate feedstock.
Catalytic Cracking -
Catalytic cracking is carried out in a large processing plant, employing a fluidized catalyst, usually a silica-alumina based material in direct contact with the feedstock. A catalyst is a substance which aids and accelerates the chemical reaction but without undergoing any change in its composition. The very fine powdered catalyst flows like a fluid when kept in constant agitation and circulated by steam, air or vapour. In the plant, as the preheated heavy gas oil feedstock enters a reactor, it is met by a stream of hot regenerated catalyst. This vaporizes the feedstock, which in turn 'fluidizes' the catalyst. Cracking occurs in the reactor, and the cracking oil vapours pass over to a fractionating tower. The cracked vapours, composed of new, smaller molecules, are separated into gases, high quality gasoline components, light gas oil used as heating oil and a heavy gas oil known as cycle oil.
Refinery Blending and Storage -
Residual oils bunkered as marine fuels are, in fact, blends of various types of cracked residuum and lighter-distillate cutter stock. By blending, less desirable cracked residual oil can be made more suitable for use as fuel oil. This is accomplished by adding, to a given quantity of residual oil, a small amount of cutter stock, such as light or heavy distillate or cycle oil. Due to this blending, there can be problem of incompatiability which leads to build up of sludge and slurry during storage, and differences in handling. This occurs when asphaltene or high molecular weight compound suspended in.the fuel is precipitated by the addition of cutter stock or other dilutents.
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