Petrol | Gasoline manufacturing process in petroleum refining
Gasoline is the fuel for small vehicles such as motor cycles, cars, some jeeps and some small machines.
What is gasoline?
Gasoline (petrol) is a very important hydrocarbon fuel in the world. Gasoline contains different hydrocarbon compounds upto ten carbon atoms. It is a liquid at room temperature. Petrol has a characteristic smell. Octane number of petrol is very important fact.
Manufacturing process of petrol
Heavy naphtha is the source for petrol production. Naphtha is separated in crude distillation unit. There are two naphtha types as heavy naphtha and light naphtha.
There are two different steps of gasoline manufacturing
- Naphtha unifiner process
- Platforming unit
Manufacturing process of gasoline can be different according to the design of plant.
There are specifications of gasoline after production. So refinery plants are designed to obtain those specifications. Sulfur content of petrol and other petroleum fuels have as significant effect and it should be reduced to very low level.
Naphtha unifiner process
Crude oil contains sulfur, nitrogen, oxygen and halogens in small quantities. First these impurities should be removed. Rich hydrogen gas (H2) gas stream is fed to the naphtha feed. Reactions are occurred in the catalyst reactor.
In the reactor
Sulfur includes in different forms in crude oils such as mercaptan, sulfide, disulfide, cyclic sulfide and thiophenic. Sulfur is removed as hydrogen sulfide (H2S). A little bit of unsaturated olefins (alkene) get saturated in the presence of H2. and catalysts.
Nitrogen, oxygen and chlorine are removed as ammonia (NH3), water and hydrogen chloride (HCl) respectively. Ammonia and HCl react and produce ammonium chloride (NH4Cl) which is soluble in water.
Next, mixture of products of the reactor, naphtha, unreacted hydrogen gas and other hydrocarbons are sent to the separators. In these separators, H2, HCl, H2S, NH3 are separated.
Separating vessels
There are two separator vessels. First one is high pressure and other one is low pressure separator.
High pressure three phase separating vessel
H2 gas is separated at the top of the vessel. Produced NH3 and HCl react and form NH4Cl which dissolves in water. Water is drained at the bottom of the vessel.
Low pressure separating vessel
H2S gas is separated in this vessel and naphtha mixture is removed from vessel.
Naphtha mixture is fed to the heat exchangers and furnace to gain the heat. Hot naphtha is sent to the columns to obtain the desired separation.
In the first column, LPG is separated from the top of the column and naphtha is obtained from bottom of the column.
Separated naphtha (heavy naphtha + light naphtha + LPG) from first column is fed to the second column. In the second column, light naphtha is separated from the top of the column. Heavy naphtha is separated from the bottom of the column.
Separated heavy naphtha is stored in tanks or directly fed to the octane increasing unit (platforming unit)
Platforming unit
In this unit, octane number of heavy naphtha is increased by reactions in multiple reactors.
Heavy naphtha is heated at heat exchangers and a furnace. Heated naphtha is sent to the reactors. In these reactors following operations are occurred to increase the octane number.
Dehydrogenation
Cyclic compounds are turned to aromatic compounds.
Hydrocracking
Long chain hydrocarbon molecules are cracked to smaller molecules.
Isomerization
Molecular structure is changed while keeping hydrocarbon formula constant.
Cyclization
Paraffins are converted to cyclic compounds.
Final products have very high temperature and used to pre-heat the naphtha feed to the reactors. In these reactors, hydrogen gas is produced.
Now final product contains some LPG gases, hydrogen gas with produced gasoline. This final product is cooled by heat exchangers, fin fan coolers and trim coolers. Then, cooled product is sent to a vessel for separating hydrogen gas.
Next, gasoline mixture is heated again and sent to the distillation column for separating LPG gas. Now pure gasoline is produced and cooled before sending to the storage.