Crude oil is a complex hydrocarbon mixture that mainly includes the following hydrocarbons:

  • Alkanes (paraffins)
  • Cycloalkanes (naphthenes)
  • Aromatic

The specific composition of crude oil varies depending on the geological source and affects its physical and chemical properties. Linear alkanes, while abundant, generally have lower octane ratings, making them less desirable for gasoline formulation.

The process of oil conversion in oil refineries is necessary to convert crude oil into a variety of valuable products such as gasoline, diesel, jet fuel, and petrochemicals.

Oil conversion processes include the following processes:

  • Distillation (Atmospheric and Vacuum)
  • Cracking (thermal and catalytic)
  • Reforming
  • Polymerization and Alkylation
  • Isomerization
  • Coking
  • Visbreaking

Distillation (atmosphere and vacuum)

Crude oil is heated in a furnace to a temperature of about 350-400 degrees Celsius and then fed into a distillation column. The key deductions obtained are:

  • Gases: Propane and butane.
  • Naphtha: Used for gasoline production
  • Kerosene: Jet fuel and solvents
  • Diesel: Fuel for vehicles
  • Residuum: Heavy oils for lubricants or asphalt
Crude Oil Distillation Unit

The heavier fractions remaining after atmospheric distillation are subjected to vacuum distillation. This process produces vacuum gas oil (VGO) and other heavier products.

Cracking (thermal and catalytic)

Cracking is a critical step that breaks down larger hydrocarbon molecules into smaller, more valuable molecules. For higher molecular weight fractions such as atmospheric residues (AR) and vacuum gas oils (VGOs), cracking in the presence of hydrogen is required to get light products. In this case a dual function catalyst is used. It is composed of a zeolite catalyst for the cracking function and rare earth metals supported on alumina for the hydrogenation function. The main products are:

  • Kerosene
  • Jet fuel
  • Diesel
  • Fuel oil
Fluid Catalytic Cracking (FCC) unit

Reforming

Reforming process converts naphtha into high-octane gasoline components. It rearranges molecular structures through heat and catalysts, enhancing the performance of gasoline.

Polymerization and alkylation

Polymerization and alkylation processes are used to combine small petroleum molecules into larger molecules.

Alkylation Processes Unit

Isomerization process

Isomerization of light naphtha is the process in which low octane number hydrocarbons (C4, C5, C6) are transformed to a branched product with the same carbon number.

This process produces high octane number products. One main advantage of this process is to separate hexane (C6) before it enters the reformer, thus preventing the formation of benzene which produces carcinogenic products on combustion with gasoline. The main catalyst in this case is a Pt-zeolite base.

The purpose of this process: to convert low-octane n-paraffins into high-octane isoparaffins. Primary Process Technique: Isomerization is carried out in a fixed bed reactor with chloride where n-paraffins are converted to isoparaffins. The catalyst is sensitive to incoming pollutants (sulfur and water).

Process steps:

  • Desulfurized feed and hydrogen are dried in fixed beds
  • The mixed feed is heated and passed through a hydrogenation reactor to saturate the olefins to paraffin and benzene.
  • The hydrogenation effluent is cooled and passes through an isomerization reactor
  • The final effluent in the form of hydrogen and LPG, which usually goes to gas fuel, and the isomerite product is cooled and separated for gasoline blending.

Feedstocks for isomerization typically consist of straight light naphtha containing C5 to C6 hydrocarbons. Before isomerization, this raw material is pre-purified to remove impurities such as sulfur and nitrogen compounds.

Catalysts, usually platinum or palladium, are used in the isomerization process on acidic substrates such as alumina or zeolites. This catalytic system facilitates molecular rearrangement at high temperatures (150-250 °C) and pressure (up to 20 atmospheres).

Separation and purification

After the reaction, the product mixture contains unbranched linear hydrocarbons. Distillation is used to effectively separate these components and ensure that the final product adheres to gasoline composition specifications.

Coking

Coking is a thermal cracking process used to convert low value residual fuel oil into higher value gas oil and petroleum coke. Vacuum residues and thermal filaments are cracked in the coking process at high temperature and low pressure. Products are petroleum coke, gas oil and lighter oil reserves. Delayed coking is the most widely used process today, but fluid coking is expected to become an important process in the future.

Delayed Coking Process

Visbreaking

Topped crude or vacuum residuals are heated and thermally cracked in the visbreaker furnace to reduce the viscosity, or pour point, of the charge. The cracked products are quenched with gas oil and flashed into a fractionator. The vapor overhead from the fractionator is separated into light distillate products. A heavy distillate recovered from the fractionator liquid can be used as either a fuel oil blending component or catalytic cracking feed.