| What is Crude Oil?
Crude oil is very complex indeed. It is made up mainly of carbon and hydrogen atoms, collectively called hydrocarbons. It is primarily formed of paraffins, naphthenes and aromatic hydrocarbons, the proportions of which vary considerably depending on origin. Crude also contains sulphur, nitrogen, oxygen and some metals.
There are many different types of crude oils, and their make-up varies from oil field to oil field and area to area. Fields can vary between pure gas and almost pure bitumen. The term 'sweet crude' is used to describe crude that is non-sulphurous. 'Sour crude' is used to describe crude that is sulphurous.
At Lytton, crudes predominantly come from South East Asia, Papua New Guinea and Australia (off-shore fields in the Timor Sea and South West Queensland). These are 'sweet crudes', containing low levels of sulphur compared with the high sulphur Middle East crudes which the refinery is unable to process.
At Kurnell, both 'sweet' and 'sour' crudes from Bass Straits, Indonesia, Vietnam, Malaysia and New Zealand are processed. The sour crude from the Middle East is used to manufacture lubricating oils, bitumen, fuel oil and waxes. The basic components for these products do not exist in Australian or South East Asian crude oils.
The Refining Process
Crude oil consists of hydrocarbons that boil anywhere from room temperature to 650°C and higher. Crude distillation is a process that separates various components or fractions as defined by their boiling range. 'Boiling the oil' is a simple, though not strictly accurate, definition of oil refining.
The lightest fractions, at lowest boiling point, are known as liquefied petroleum gases (LPG). Some of these are burned as fuel in the refinery to complement natural gas, but the majority are treated and sold as propane and butane products.
The next fractions boil in the petrol range of 0°C to 200°C and are split into two components known as light-straight-run naphtha (LSR) and heavy-straight-run naphtha (HSR).
HSR has a low octane rating, unsuitable for direct blending into high octane petrols. The molecular structure of this fraction is changed under high temperature and pressure in a hydrogen-filled catalytic reformer to provide high octane reformate suitable for petrol manufacture.
LSR has a higher octane rating than HSR and can be blended directly into petrol. However, to improve the refinery's ability to produce petrol, low octane components of LSR can be removed - or sieved out - in the IsoSiv unit, leaving higher octane iso-naphtha for petrol blending. The lower octane normal naphtha is an excellent petrochemical feedstock.
The next range of fractions - kerosene, jet fuel, heating oil and automotive or industrial diesels - are known as middle distillates, and boil between 200°C and 350°C. Generally, these require very little additional treating after distillation.
However, diesels derived from overseas crude oils generally contain high levels of sulphur (Arab Light crude contains 1.8 per cent sulphur; Gippsland crude averages about 0.1 per cent). This sulphur is removed as hydrogen sulphide (H2S) in the diesel hydrotreating unit. The fuel gas containing the H2S is sent to the sulphur recovery unit, where the H2S is recovered from the gas as sulphur.
Material heavier than the middle-distillate range can be used as fuel oil. However, in Australia, where petrol yield required is very high, the heavy hydrocarbons obtained as residue from single stage distillation or as distillate from second or vacuum stage distillation are 'cracked' in fluid catalytic cracking units to gases, catalytic naphtha (a petrol blendstock) and light cycle oil (a component of fuel oil or diesel).
The polymerisation and alkylation units manufacture further petrol components and LPG from the gases produced from catalytic cracking. Many of the units employ catalysts which help maximise yields of desired products. The heaviest (highest boiling range) components of crude oil derived from the second stage or vacuum distillation are known as vacuum residuum.
A further process, known as propane decarbonising, is used to recover additional feedstock from vacuum residuum for catalytic cracking or lubricating oil refining. This leaves an asphaltic material suitable for bitumen manufacture. Otherwise, vacuum residuum is used as fuel oil. Other lubricating oil feedstocks are obtained by vacuum distillation of suitable overseas crudes, such as Arabian Light crude. Gippsland crude is unsuitable for this purpose.
|
