A typical refinery process includes the following major units.
Desalting of Crude Oil
Crude oil is first processed to remove the water, inorganic salts, suspended solids, and water-soluble trace metals. There are two ways of desalting.
- In chemical desalting, water and chemical surfactant (demulsifiers) are added to the crude. The crude is heated so that salts and other impurities dissolve or attach to the water, then held in a tank to settle out.
- Electrical desalting is the application of high-voltage electrostatic charges to concentrate suspended water globules at the bottom of the settling tank. Surfactants are added only when the crude has a large number of suspended solids.
Crude Oil Distillation Unit
After desalting the crude oil, it goes to the distillation unit after pre-heating. Distillation is the core unit of a typical refinery process. Distillation separates the different fractions of crude oil based on relative volatility. The lightest fraction leaves from the top of the column while the remaining residue leaves from the bottom. The separated fractions from the distillation column include fuel gas, naphtha, kerosene, diesel, and gasoline. The atmospheric residue from the main distillation column is then introduced to a vacuum distillation tower operating at about 50 mmHg, where heavier products are obtained.
Naphtha Hydro-treater Unit
The naphtha from the atmospheric distillation column is sent to the naphtha hydrotreater unit to remove the impurities of sulfur and nitrogen compounds. The naphtha hydrotreating unit uses the catalyst of cobalt-molybdenum to remove sulfur chemically by converting it to hydrogen sulfide gas that is removed along with unreacted hydrogen. Reactor conditions for a naphtha hydrotreater unit are around 400-500F and the pressure of 350-650 psi.
Isomerization of light naphtha is the process in which low octane number hydrocarbons are transformed into a branched product with the same carbon number. This process produces high octane number of 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.
Catalytic Reforming Unit
In this process, a special catalyst (platinum metal supported on silica or silica base alumina) is used to restructure naphtha fraction (C6–C10) into aromatics and iso-paraffins.
The produced naphtha reformate has a much higher-octane number than the feed. This reformate is used in gasoline formulation and as a feedstock for aromatic production (benzene–toluene–xylene, BTX).
Alkylation is the process in which iso-butane reacts with olefins such as butylene to produce a gasoline range alkylate. The catalyst, in this case, is either sulfuric acid or hydrofluoric acid. The hydrocarbons and acid react in the liquid phase. Iso-butane and olefins are collected mainly from FCC and delayed Coker.
Fluid Catalytic Cracking (FCC) Unit
Fluid catalytic cracking (FCC) is the main player for the production of gasoline. It upgrades the heavier, higher-boiling fractions from the crude oil distillation by converting them into lighter and lower boiling, more valuable products. The catalyst in this case is a zeolite base for the cracking function. The main feed to FCC is VGO and the product is gasoline, but some gas oil and refinery gases are also produced.
Distillate Hydrotreater Unit
This is one of the major processes for the cleaning of petroleum fractions from impurities such as sulfur, nitrogen, oxy-compounds, chloro-compounds, aromatics, waxes, and metals using hydrogen. The catalyst is selected to suit the degree of hydro-treating and type of impurity. Catalysts, such as cobalt and molybdenum oxides on alumina matrix, are commonly used.
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, and fuel oil.
It upgrades heavy residual oils from the vacuum distillation unit by thermally cracking them into lighter, more valuable reduced viscosity products. This is a mild thermal cracking process used to break the high viscosity and pour points of vacuum residue to the level which can be used in further downstream processes. In this case, the residue is either broken in the furnace coil (coil visbreaking) or soaked in a reactor for a few minutes (soaker visbreaker). The products are gases, gasoline, gas oil, and unconverted residue.
Delayed Coking and Fluid Coker Units
This process is based on the thermal cracking of vacuum residue by carbon rejection forming coke and lighter products such as gases, gasoline, and gas oils.
Three types of a coke can be produced
The vacuum residue is heated in a furnace and flashed into large drums where coke is deposited on the walls of these drums, and the rest of the products are separated by distillation.