An oil refinery may be considered as a factory that converts crude oil into a range of useable products. It is designed to produce what the market requires in the most economical and efficient manner. The first step in the manufacture of petroleum products is the separation of crude oil into the main fractions by atmospheric distillation. When crude oil is heated, the lightest and most volatile hydrocarbons boil off as vapours first and the heaviest and least volatile last. The vapours are then cooled and condensed back into liquids.

This distillation process is carried out in a fractionating column. This is divided into a series of chambers by perforated trays, which condense the vapours at each stage and allow the liquids to flow into storage tanks. Pre-heating of the crude oil is limited to 350°C to prevent the oil being thermally cracked.

Atmospheric and Vacuum Distillation

The residue from atmospheric distillation is sometimes referred to as long residue and to recover more distillate product, further distillation is carried out at a reduced pressure and high temperature. This vacuum distillation process is important in maximising the upgrading of crude oil. The residue from vacuum distillation, sometimes referred to as short residue, is used as a feedstock for further upgrading or as a fuel component. Unlike the fractionating column for atmospheric distillation, a system of packed beds instead of trays is used for condensation of the low-pressure vapours.

Refineries based just on atmospheric and vacuum distillation are said to be operating “the straight run” process and the fuel oil is basically either long or short run residue. The percentage of residue varies depending on the composition of crude processed. For a typical “light” North African crude the residue is 28%, whilst for a “heavy” Venezuelan crude it is as high as 85%. The proportion of products produced does not always match the product demand and is primarily determined by the crude oil.

Thermal Cracking

In order to meet the product demand, further refining processes were introduced. Today, a modern refinery, in addition to atmospheric and vacuum distillation, may also consist of secondary refining processes such as cracking, which may be thermal or with a catalyst. A typical modern refinery installation is shown below. Thermal cracking is the oldest and in principle the simplest refinery conversion process. It is carried out over a wide range of temperatures, between 450-750°C and pressures from atmospheric to 70 bar. The temperature and pressure depends on the type of feedstock and the product requirement. At these elevated temperatures, the large hydrocarbon molecules become unstable and spontaneously break into smaller molecules.

Another important factor in the process is the residence time. The feedstock can be either the residue from the atmospheric or vacuum distillation units, or a mixture of the two. In modern refineries, there are three major applications of the thermal cracking process: visbreaking, a thermal gas oil unit and coking. Visbreaking is the most significant process with regard to the manufacture of residual fuel oil. It is a mild form of thermal cracking often used for reducing the viscosity of straight-run residual fuels. Normally such fuels are very viscous and, if required for sale as heavy fuel oil, must be blended with a relatively high value distillate to meet the finished product specification.

Visbreaking reduces the quantity of distillate required as diluent or “cutter stock”. This material can then be profitably diverted elsewhere. The main aim of a thermal gas oil unit is to produce and recover the maximum amount of gas oil. In extreme cases, the viscosity of the residue may be higher than that of the feed stock. Coking is a severe form of thermal cracking. It is designed to convert straight-run residues into more valuable products such as naphtha and diesel oil. In addition, gas and coke are produced and thus this process does not feature in the manufacture of residual fuel oils.

Catalytic Cracking

Catalytic cracking is the major process in the petroleum refining industry for the conversion of heavy hydrocarbon fractions, mainly into high-quality gasoline and fuel oil components. These are lighter, less viscous and more valuable than the feedstock. There are various different catalytic cracker designs but in all cases the product output can finally be separated to: gases, gasoline blending components, catalytically cracked cycle oils and cycle oil slurry. The cycle oils are very important with respect to residual fuel oil since they are used as cutter stocks to reduce the viscosity of residues. Prior to use as a cutter stock, the cycle oil slurry has to be treated to remove entrained cat fines. In a modern refinery, there is a wide range of residues and diluent available for the production of fuel oil. Usually the fuel will consist of visbroken residue diluted with cycle oils and smaller amounts of other distillates.

The figure below shows the main streams of feedstock, fuel oil diluent and fuel oil residues in a modern refinery. Clearly, if a refinery does not have a thermal cracking facility (visbreaker or thermal gas oil unit) then the fuel oil will be based on long or short residue. Additional to the main residual fuel streams in a modern refinery, it should be appreciated that other developments have taken place to further maximise the production of gasoline, kerosene and diesel from a barrel of oil.

One of these is by residue hydroconversion where residual fractions are converted into feedstock, which in turn can be further processed in conventional crackers to yield lighter products. Maximisation of production for the lighter products is carried out at the expense of residual fuel oil.