Many of us think that the quality of heavy fuel oil used on board a ship gets degraded when it comes on the ship or during the transportation process from the fuel oil distillation plant to the ship. However, it is learned that maximum contamination of the fuel oil occurs in the distillation plant or oil refinery itself. Surprised? Don’t be, because here we aren’t talking about adding of impurities from some external source during the storage or transportation process but the spoilage of the oil during the refining or distillation process, and it does take place on a large scale.
How Distillation Process Itself Spoils the Fuel?
Refining of oil is carried out to obtain higher quality of petrol and byproducts. Moreover, in regards to the increasing environmental concern the demand for unleaded gasoline and more refined petroleum products is on the rise. Petroleum refineries are continuously looking for techniques to procure more valuable fuels with acceptable octane ratings from the crude oil stock and also to reduce the generation of valuable residual products.
More yield is obtained when the residual products from the conventional atmospheric distillation process is subjected to a vacuum distillation process. In vacuum distillation process the products are subjected to reduced pressure which lowers the boiling point of the products and thus allows distillation process at reduced temperature. The distillate from the vacuum is then passes through a catalytic cracking process. However, it is then when the main contamination of oil starts.
There are several types of crackling process; however the one that is most widely used is fluidized solid catalytic process. Generally silicon and aluminum oxides are used as catalyst in the powdered form and often breaks up to form catalyst dust. The distillate after the process often contains some of this dust which gets carried over from the reactor to the distillate.
Thermal cracking is yet another type of distillation process which alters the molecular structure of distillates and residue to increase the yield and quality of the fuel oil. The process also helps in reducing the viscosity of residual products. However in doing so as the amount of residue increases, the impurities such as sulphur, vanadium, sodium etc that are already present in the fuel oil becomes much more concentrated in the same unit amount of crude oil, making the oil more impure. This same thing happens in the fluid catalytic process when the catalyst dust shows higher concentration in lesser residual amount.
Blended Fuels Do all the Damage
Generally the fuel supplied to the ships is a blend of residual fuel having high viscosity and distillate fuel having low viscosity. Both the types of fuels are blended according to the viscosity required by the engine. However, if the viscosity of the residual fuel is low then the amount of distillate fuel required to bring the blend to the required viscosity will also be low, thus resulting in an increase in the concentration of impurities.
Thus, the distillation and catalytic process not only help in distilling the fuel oil but in a way also makes the oil impure. Moreover, they also increase the amount of aromatic compounds in the distillate which leads to problems with engine combustion and cleaning of the fuel by purifiers and clarifiers.