In its natural state crude oil has little practical use, but by refining it we can end up with liquid petroleum gas, petrol, diesel, jet fuel, gas oil, heating oil and residues such as bitumen.

Refining also provides the by-products or feedstocks for lubricants and, most importantly, for petrochemicals which are the basis for plastics, paints, adhesives, detergents, resins, solvents, synthetic fibres and rubber.

The type of crude oil processed - for example lighter and 'sweeter' North Sea Brent blend or Arabian heavy - has an influence upon the mix of products a refinery produces. UK refineries process a range of crude oils, but those from the North Sea predominate (about 70% of the total). This helps refineries produce the high quality, low sulphur road fuels that modern vehicles require to deliver low exhaust emissions.

Typically, a barrel of North Sea crude oil will yield 3% LPG, 37% petrol, 25% diesel, 20% kerosene (jet fuel/heating oil) and 12% fuel oil ( heavy residue for power generation). A heavy crude oil will yield a much smaller proportion of petrol,diesel and kerosene - perhaps 50% - and the balance fuel oil residue which requires further processing to transform it into lighter more useful fuels.

How a refinery works

Typically, the refining processes involves four main stages:

• fractionating or distilling by use of heat
• chemical conversion by cracking & other processes
• treatment and clean up to remove, for example, sulphur
• formulating & blending into finished products

Click here for a diagram of the refining process

Many of the refineries in the United Kingdom came on stream in the late 1950s and early 1960s, reflecting the post-war demand for petroleum products as the economy recovered, road transport expanded and car ownership took off. Since that time, refineries have evolved to meet the growing demand for more complex and environmentally friendlier fuels, as well as the myriad by-products such as solvents and petrochemical feedstocks.

The result is that no two refineries are identical - although the common factors are similar crude distillation and upgrading units - each taking a slightly different route to a common goal of extracting maximum value from each barrel of crude oil processed.

The UKPIA publication 'Meeting our energy needs:The Future of UK Oil Refining' outlines many of the current and future major challenges in refining, such as overall balance and trade- off between product quality and product mix, new specifications,  future demand patterns, types of crude oil and meeting environmental oblectives. A joint industry/Government study, the 'Review of UK Oil Refining Capacity', which examines many of these issues was published in May 2007. Click here to view.

Distillation

The starting point in all refineries is crude oil distillation. As the phrase implies, essentially this involves boiling crude oil in a high tower called a fractionating column, which breaks down the crude oil into more useful components or fractions.

The crude oil enters the tower at the bottom and is heated to several hundred degrees Celsius. The lighter fractions with lower boiling points vapourise and rise up the column until they reach a temperature and pressure corresponding to their boiling point. The heavier, higher boiling point fractions cascade in a liquid stream down the column, again, to a point corresponding to their boiling point.

To aid this continuous process, the column has a whole series of trays with valves set in each layer, which allows vapour to pass up through the liquids as they cascade like a waterfall from level to level. The vapours are cooled, condensed and drawn off in a liquid state at various levels up the tower - the heavier fractions first such as gas oil and diesel, then kerosene and naptha, the building block for petrol, and finally gases propane and butane.

What is left over are the heavy fractions, often referred to as bottoms or straight run residue, which require considerably more processing to turn them into valuable products or blending components.

Cracking

Distillation is just the start of the process as on its own, distillation does not produce enough of the lighter, more valuable products such as petrol. As the name implies, cracking involves breaking the molecular composition of the heavier gas oil and residues into simpler lighter ones.

Crackers split into three broad types: thermal crackers which use heat alone; fluid catalytic crackers which use heat and a catalyst to intensify the chemical conversion process; and hydrocrackers which use heat, a catalyst and hydrogen, more specifically for maximising diesel production.

Alkylation

Alkylation is a process which builds smaller molecules into larger ones, typically to produce high octane blending components for petrol. The process will take light gases from the cracker as well as from other sources, such as isobutane from an isomerisation unit.

Desulphurisation

Increasingly, refineries are concentrating on processes which remove sulphur from fuels, in response to tighter fuel specifications requiring low sulphur but also to improve refinery flexibility. Reliance on low sulphur crude oils alone restricts the flexibility of a refinery.

The process uses hydrogen to remove sulphur by converting it to hydrogen sulphide by use of a catalyst. After treatment, the remaining sulphur is sold to other process industries.

Reforming

Reforming units use a catalyst, usually platinum, to promote a chemical process which produces more high octane blending components by modifying the molecular structure of naptha feedstock.

As with most catalytic processes, some of the catalyst can be lost during transformation of the feedstock. Increasingly, more efficient refineries employ technology which continuously regenerates the catalyst or removes build up of carbon.

Blending

Blending is the process which brings together the various components that make up fuels, to produce a finished product to the correct specification and quality. and with the appropriate properties for the market for which it is intended.