Petroleum naphtha is an intermediate hydrocarbon liquid stream derived from the refining of crude oil. It is most usually desulfurized and then catalytically reformed, which re-arranges or re-structures the hydrocarbon molecules in the naphtha as well as breaking some of the molecules into smaller molecules to produce a high-octane component of gasoline (or petrol).
There are hundreds of different petroleum crude oil sources worldwide and each crude oil has its own unique composition or assay. There are also hundreds of petroleum refineries worldwide and each of them is designed to process either a specific crude oil or specific types of crude oils. That means that it is virtually impossible to provide a definitive, single definition of the word naphtha since each refinery produces its own naphthas with their own unique initial and final boiling points and other physical and compositional characteristics. In other words, naphtha is a generic term rather than a specific term.
In addition, naphthas may also be produced from coal tar, shale deposits, tar sands such as in Canada, the destructive distillation of wood and coal gasification or biomass gasification to produce a syngas followed by the Fischer-Tropsch process to convert the syngas into liquid hydrocarbon products. For that reason, this article is entitled Petroleum naphtha and deals only with naphthas produced by the processing of crude oil in petroleum refineries.
The major source of petroleum naphtha in a petroleum refinery
The first unit process in a petroleum refinery is the crude oil distillation unit. The overhead liquid distillate from that unit is called virgin or straight-run naphtha and that distillate is the largest source of naphtha in most petroleum refineries. The naphtha is a mixture of very many different hydrocarbon compounds. It has an initial boiling point (IBP) of about 35 °C and a final boiling point (FBP) of about 200 °C, and it contains paraffin, naphthene (cyclic paraffins[disambiguation needed]) and aromatic hydrocarbons ranging from those containing 4 carbon atoms to those containing about 10 or 11 carbon atoms.
- a virgin light naphtha with an IBP of about 30 °C and a FBP of about 145 °C containing most (but not all) of the hydrocarbons with 6 or less carbon atoms
- a virgin heavy naphtha containing most (but not all) of the hydrocarbons with more than 6 carbon atoms. The heavy naphtha has an IBP of about 140 °C and a FBP of about 205 °C.
It is the virgin heavy naphtha that is usually processed in a catalytic reformer because the light naphtha has molecules with 6 or less carbon atoms which, when reformed, tend to crack into butane and lower molecular weight hydrocarbons which are not useful as high-octane gasoline blending components. Also, the molecules with 6 carbon atoms tend to form aromatics which is undesirable because governmental environmental regulations in a number of countries limit the amount of aromatics (most particularly benzene) that gasoline may contain.
Types of virgin naphthas
The table below lists some typical virgin heavy naphthas, available for catalytic reforming, derived from various crude oils. It can be seen that they differ significantly in their content of paraffins, naphthenes and aromatics:
|Crude oil name
|Initial boiling point, °C||149||140||149||150|
|Final boiling point, °C||204||190||204||180|
|Paraffins, liquid volume %||46||62||57||38|
|Naphthenes, liquid volume %||42||32||27||45|
|Aromatics, liquid volume %||12||6||16||17|
Some refinery naphthas also contain some olefinic hydrocarbons, such as naphthas derived from the fluid catalytic cracking, visbreakers and coking processes used in many refineries. Those olefin-containing naphthas are often referred to as cracked naphthas.
In some (but not all) petroleum refineries, the cracked naphthas are desulfurized and catalytically reformed (as are the virgin naphthas) to produce additional high-octane gasoline components.
Some petroleum refineries also produce small amounts of specialty naphthas for use as solvents, cleaning fluids, paint and varnish diluents, asphalt diluents, rubber industry solvents, dry-cleaning, cigarette lighters, and portable camping stove and lantern fuels. Those specialty naphthas are subjected to various purification processes.
Sometimes the specialty naphthas are called petroleum ether, petroleum spirits, mineral spirits, paraffin, benzine, hexanes, ligroin, white oil or white gas, painters naphtha, refined solvent naphtha and Varnish makers' & painters' naphtha (VM&P) . The best way to determine the boiling range and other compositional characteristics of any of the specialty naphthas is to read the Material Safety Data Sheet (MSDS) for the specific naphtha of interest.
On a much larger scale, petroleum naphtha is also used in the petrochemicals industry as feedstock to steam reformers and steam crackers for the production of hydrogen (which may be and is converted into ammonia for fertilizers), ethylene and other olefins. Natural gas is also used as feedstock to steam reformers and steam crackers.
- This article incorporates material from the Citizendium article "Petroleum naphtha", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.
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- Fuel Chemistry (scroll down to "What is naphtha")
- Canadian regulations on benzene in gasoline
- Briefing on Benzene in Petrol From website of United Kingdom Petroleum Industry Association (UKPIA)
- USA regulations on benzene in gasoline
- Barrow Island crude oil assay
- Mutineer-Exeter crude oil assay
- CPC Blend crude oil assay
- Draugen crude oil assay