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'''Hydrodesulfurization''' (HDS) is a [[catalytic]] chemical process widely used to remove [[sulfur]] (S) from [[natural gas]] and from [[oil refinery|refined petroleum products]] such as [[gasoline|gasoline or petrol]], [[jet fuel]], [[kerosene]], [[diesel fuel]], and [[fuel oil]]s.<ref name=Gary>{{cite book|author=Gary, J.H. and Handwerk, G.E.|title=Petroleum Refining Technology and Economics|edition=2nd Edition|publisher=Marcel Dekker, Inc|year=1984|id=ISBN 0-8247-7150-8}}</ref><ref>[http://www.theicct.org/documents/Yamaguchi_Mexico_2003.pdf ''Hydrodesulfurization Technologies and Costs''] Nancy Yamaguchi, Trans Energy Associates, William and Flora Hewlett Foundation Sulfur Workshop, Mexico City, May 29-30, 2003</ref>  The purpose of removing the sulfur is to reduce the sulfur dioxide (SO<sub>2</sub>) emissions that result from using those fuels in automotive [[vehicles]], [[aircraft]], railroad [[locomotives]], [[ships]], gas or oil burning [[power plants]], residential and industrial [[furnaces]], and other forms of fuel [[combustion]].  
'''Naphtha''' is an intermediate hydrocarbon liquid stream derived from the [[Petroleum refining processes|refining]] of [[crude oil]].<ref name=Handwerk>{{cite book|author=Gary, J.H. and Handwerk, G.E.|title=Petroleum Refining Technology and Economics|edition=2nd Edition|publisher=Marcel Dekker, Inc|year=1984|id=ISBN 0-8247-7150-8}}</ref><ref name=Leffler>{{cite book|author=Leffler, W.L. |title=Petroleum refining for the nontechnical person|edition=2nd Edition|publisher=PennWell Books|year=1985|id=ISBN 0-87814-280-0}}</ref>  It is most usually [[Hydrodesulfurization|desulfurized]] and then [[Catalytic reforming|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]]).  


Another important reason for removing sulfur from the [[naphtha]] streams within a petroleum refinery is that sulfur, even in extremely low concentrations, [[catalyst poisoning|poisons]] the [[noble metal]] catalysts ([[platinum]] and [[rhenium]]) in the [[catalytic reforming]] units that are subsequently used to upgrade the [[octane rating]] of the [[naphtha]] streams.
Some refineries also produce a small amount of specialty naphthas for use as solvents and those specialty naphthas are subjected to purification processes other than catalytic reforming.  


The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting [[hydrogen sulfide]] (H<sub>2</sub>S) gas. In [[oil refinery|petroleum refineries]], the hydrogen sulfide gas is then subsequently converted into byproduct elemental sulfur. In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and other hydrocarbon processing plants.<ref>[http://minerals.usgs.gov/minerals/pubs/commodity/sulfur/sulfumcs06.pdf Sulfur production report] by the [[United States Geological Survey]]</ref><ref>[http://www.agiweb.org/geotimes/july03/resources.html Discussion of recovered byproduct sulfur]</ref>
== Where the naptha is obtained ==


An HDS unit in the petroleum refining industry is also often also referred to as a '''Hydrotreater'''.
The first unit process in a petroleum refinery is the [[Petroleum refining processes#The crude oil distillation unit|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]] of about 35 °C and a final boiling point of about 200 °C, and it contains [[paraffin]], [[naphthene]] (cyclic paraffins) and [[aromatic]] hydrocarbons ranging from those containing 4 [[carbon]] [[atom]]s to those containing about 10 or 11 carbon atoms.


==History==
The virgin naphtha is often further distilled to produce a ''light'' naphtha containing most (but not all) of the hydrocarbons with 6 or less [[carbon]] [[atoms]] and a ''heavy'' naphtha containing most (but not all) of the hydrocarbons with more than 6 carbon atoms. The heavy naphtha has an initial boiling point of about 140 to 150 °C and a final boiling point of about 190 to 205 °C.  
Although reactions involving catalytic hydrogenation of organic substances were known prior to 1897, the property of finely divided nickel to catalyze the fixation of hydrogen on hydrocarbon (ethylene, benzene) double bonds was discovered by the [[France|French]] [[chemist]], [[Paul Sabatier (chemist)|Paul Sabatier]].<ref>C.R.Acad.Sci. 1897, 132, 210</ref><ref>C.R.Acad.Sci. 1901, 132, 210</ref> Thus, he found that unsaturated hydrocarbons in the vapor phase could be converted into saturated hydrocarbons by using hydrogen and a catalytic metal. His work was the foundation of the modern catalytic hydrogenation process.


Soon after Sabatier's work, a [[Germany|German]] chemist, [[Wilhelm Normann]], found that catalytic hydrogenation could be used to convert unsaturated fatty acids or glycerides in the liquid phase into saturated ones. He was awarded a patent in Germany in 1902<ref>[http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=DE141029&F=0 DE Patent DE141029 (Espacenet, record not available)]</ref> and in Britain in 1903,<ref>[http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=GB190301515&F=0 UK Patent GB190301515 GB190301515 (Espacenet)]</ref> which was the beginning of what is now a worldwide industry.
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.<ref>[http://www.ec.gc.ca/CEPARegistry/regulations/detailReg.cfm?intReg=1 Canadian regulations on benzene in gasoline]</ref><ref>[http://www.ukpia.com/industry_issues/environment_air_quality_health_safety/benzene_in_petrol.aspx United Kingdom regulations on benzene in gasoline]</ref><ref>[http://www.washingtonpost.com/wp-dyn/content/article/2006/03/01/AR2006030102113.html USA regulations on benzene in gasoline]</ref>  


In the mid-1950's, the first [[noble metal]] catalytic reforming process (the [[Platforming|Platformer process]]) was commercialized. At the same time, the catalytic hydrodesulfurization of the naphtha feed to such reformers was also commercialized. In the decades that followed, various proprietary catalytic hydrodesulfurization processes such as the one depicted in the [[Process flow diagram|flow diagram]] below have been commercialized.  Currently, virtually all of the petroleum refineries world-wide have one or more HDS units.
== Types of naphthas ==


By 2006 miniature [[microfluidic]] HDS units had been implemented for treating [[JP-8]] jet fuel to produce clean feed stock for a [[fuel cell]] [[hydrogen reformer]].<ref>[http://www.greencarcongress.com/2006/03/microchannel_de.html Microchannel HDS (March 2006)]</ref>  By 2007 this had been integrated into an operating 5kW fuel cell generation system.<ref>[http://www.pnl.gov/topstory.asp?id=282 Fuel cells help make noisy, hot generators a thing of the past (December 2007) Pacific Northwest National Laboratory]</ref>
It should be noted that there are a great many petroleum crude oil sources worldwide and each crude oil has its own unique composition or [[assay]]. Also, not all refineries process the same crude oils and each refinery produces its own virgin naphthas with their own unique initial and final boiling points. In other words, naphtha is a generic term rather than a specific term.


==The process chemistry==
The table just below lists some fairly 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:


[[Hydrogenation]] is a class of [[chemical reaction]]s in which the net result is the addition of [[hydrogen]] (H). [[Hydrogenolysis]] is a type of hydrogenation and results in the cleavage of the C-X [[chemical bond]], where C is a [[carbon]] atom and X is a sulfur, [[nitrogen]] (N) or [[oxygen]] (O) atom. The net result of a hydrogenolysis reaction is the formation of C-H and H-X chemical bonds. Thus, hydrodesulfurization is a hydrogenolysis reaction. Using [[ethanethiol]] (C<sub>2</sub>H<sub>5</sub>SH), a sulfur compound present in some petroleum products, as an example, the hydrodesulfurization reaction can be simply expressed as
{| class="wikitable"
:{| cellpadding="0" cellspacing="0"
|+ Typical Heavy Naphthas
|align="center"| Ethanethiol + Hydrogen
| &nbsp;→&nbsp;
|align="center"| [[Ethane]] + [[Hydrogen sulfide]]
|-
|-
|align="center"| C<sub>2</sub>H<sub>5</sub>SH + H<sub>2</sub>
! Crude oil name <math>\Rightarrow</math><br>Location <math>\Rightarrow</math>  
| &nbsp;→&nbsp;
! Barrow Island<br>Australia<ref>[http://www.santos.com/library/barrow_crude.pdf Barrow Island crude oil assay]</ref>
|align="center"| C<sub>2</sub>H<sub>6</sub> + H<sub>2</sub>S
! Mutineer-Exeter<br>Australia<ref>[http://www.santos.com/library/refining_characteristics.pdf Mutineer-Exeter crude oil assay]</ref>
|}
! CPC Blend<br>Kazakhstan<ref>[http://crudemarketing.chevron.com/overview.asp?cpc CPC Blend crude oil assay]</ref>
 
! Draugen<br>North Sea<ref>[http://www.statoil.com/STATOILCOM/crude/svg02659.nsf/UNID/C9AC3EF9CE76B0DFC1256B5600528D6D/$FILE/Dra4kv02.pdf Draugen crude oil assay]</ref>
For the mechanistic aspects of, and the catalysts used in this reaction see the section [[#Catalysts and mechanisms|catalysts and mechanisms]]
|-
 
| Initial boiling point, °C ||align=center|149||align=center|140||align=center|149||align=center|150
==Process description==
|-
In an industrial hydrodesulfurization unit, such as in a refinery, the hydrodesulfurization reaction takes place in a fixed-bed [[chemical reactor|reactor]] at elevated [[temperatures]] ranging from 300 to 400 °C and elevated [[pressures]] ranging from 30 to 130 [[atmosphere (unit)|atmosphere]]s of absolute pressure, typically in the presence of a [[catalyst]] consisting of an [[alumina]] base impregnated with [[cobalt]] and [[molybdenum]].
| Final boiling point, °C ||align=center|204||align=center|190||align=center|204||align=center|180
 
The image below is a schematic depiction of the equipment and the process flow streams in a typical refinery HDS unit.
[[Image:HDS Flow.png|frame|center|Schematic diagram of a typical Hydrodesulfurization (HDS) unit in a petroleum refinery]]
 
The liquid feed (at the bottom left in the diagram) is [[pump|pumped]] up to the required elevated pressure and is joined by a stream of hydrogen-rich recycle gas. The resulting liquid-gas mixture is preheated by flowing through a [[heat exchanger]]. The preheated feed then flows through a [[furnace|fired heater]] where the feed mixture is totally [[vaporized]] and heated to the required elevated temperature before entering the reactor and flowing through a fixed-bed of catalyst where the hydrodesulfurization reaction takes place.
 
The hot reaction products are partially cooled by flowing through the heat exchanger where the reactor feed was preheated and then flows through a water-cooled heat exchanger before it flows through the pressure controller (PC) and undergoes a pressure reduction down to about 3 to 5 atmospheres.  The resulting mixture of liquid and gas enters the gas separator [[pressure vessel|vessel]] at about 35 °C and 3 to 5 atmospheres of absolute pressure.
 
Most of the hydrogen-rich gas from the gas separator vessel is recycle gas which is routed through an [[amine gas treating|amine contactor]] for removal of the reaction product H<sub>2</sub>S that it contains. The H<sub>2</sub>S-free hydrogen-rich gas is then recycled back for reuse in the reactor section. Any excess gas from the gas separator vessel joins the [[sour gas]] from the stripping of the reaction product liquid.
 
The liquid from the gas separator vessel is routed through a [[reboiler|reboiled]] stripper [[Continuous distillation|distillation]] tower.  The bottoms product from the stripper is the final desulfurized liquid product from hydrodesulfurization unit.
 
The overhead sour gas from the stripper contains hydrogen, [[methane]], [[ethane]], hydrogen sulfide, [[propane]] and perhaps some [[butane]] and heavier components. That sour gas is sent to the refinery's central gas processing plant for removal of the hydrogen sulfide in the refinery's main [[amine gas treating]] unit and through a series of distillation towers for recovery of propane, butane and [[pentane]] or heavier components. The residual hydrogen, methane, ethane and some propane is used as refinery fuel gas. The hydrogen sulfide removed and recovered by the amine gas treating unit is subsequently converted to elemental sulfur in a [[Claus  process]] unit.
 
Note that the above description assumes that the HDS unit feed contains no [[olefin]]s.  If the feed does contain olefins (for example, the feed is a naphtha derived from a refinery fluid catalytic cracker (FCC) unit), then the overhead gas from the HDS stripper may also contain some [[ethene]], [[propene]], [[butene]]s and [[pentene]]s or heavier components.
 
It should also be noted that the amine solution to and from the recycle gas contactor comes from and is returned to the refinery's main amine gas treating unit.
 
==Sulfur compounds in refinery HDS feedstocks==
 
The refinery HDS feedstocks (naphtha, kerosene, diesel oil and heavier oils) contain a wide range of [[Organic compound|organic]] sulfur compounds, including [[thiols]], [[thiophenes]], organic [[sulfides]] and [[disulfides]], and many others. These organic sulfur compounds are products of the degradation of sulfur containing biological components, present during the natural formation of the [[fossil fuel]], petroleum crude oil.
 
When the HDS process is used to desulfurize a refinery naphtha, it is necessary to remove the the total sulfur down to the parts per million range or lower in order to prevent poisoning the noble metal catalysts in the subsequent catalytic reforming of the naphthas.
 
When the process is used for desulfurizing diesel oils, the latest environmental regulations in the United States and Europe, requiring  what is referred to as ''ultra-low sulfur diesel'' (ULSD), in turn requires that very deep hydrodesulfurization is needed. In the very early 2000's, the governmental regulatory limits for highway vehicle diesel was within the range of 300 to 500 ppm by weight of total sulfur. As of 2006, the total sulfur limit for highway diesel is in the range of 15 to 30 ppm by weight.<ref>[http://www.npradc.org/issues/fuels/diesel_sulfur.cfm ''Diesel Sulfur''] published online by the National Petrochemical & Refiners Association (NPRA)</ref>
 
===Thiophenes===
 
A family of substrates that are particularly common in petroleum are the aromatic sulfur-containing heterocycles called thiophenes.  Many kinds of thiophenes occur in petroleum ranging from thiophene itself to more condensed derivatives called [[benzothiophene]]s and [[dibenzothiophene]]s.  Thiophene itself and its alkyl derivatives are easier to hydrogenolyse, whereas dibenzothiophene, especially its 4,6-disubstituted derivatives, are considered the most challenging substrates.  Benzothiophenes are midway between the simple thiophenes and dibenzothiophenes in their susceptibility to HDS.
 
==Catalysts and mechanisms==
The main HDS catalysts are based on [[Molybdenum disulfide|MoS<sub>2</sub>]] together with smaller amounts of other metals.<ref>Topsøe, H.; Clausen, B. S.; Massoth, F. E., Hydrotreating Catalysis, Science and Technology, Springer-Verlag: Berlin, 1996.</ref>  The nature of the sites of catalytic activity remains an active area of investigation, but it is generally assumed basal planes of the MoS<sub>2</sub> structure are not relevant to catalysis, rather the edges or rims of these sheet.<ref> Daage, M.; Chianelli, R. R., "Structure-Function Relations in Molybdenum Sulfide Catalysts - the Rim-Edge Model", J. of Catalysis, 1994, 149, 414-427.</ref>  At the edges of the MoS<sub>2</sub> crystallites, the molybdenum centre can stabilize a coordinatively unsaturated site (CUS), also known as an anion vacancy. Substrates, such as thiophene, bind to this site and undergo a series a reactions that result in both C-S scission and C=C hydrogenation.  Thus, the hydrogen serves multiple roles - generation of anion vacancy by removal of sulfide, hydrogenation, and hydrogenolysis.  A simplified diagram for the cycle is shown:
[[Image:HDS.png|thumb|450px|center|Simplified diagram of a HDS cycle for thiophene]]
 
===Catalysts===
Most metals catalyse HDS, but it is those at the middle of the transition metal series that are most active.  [[Ruthenium disulfide]] appears to be the single most active catalyst, but binary combinations of cobalt and molybdenum are also highly active.<ref>Chianelli, R. R.; Berhault, G.; Raybaud, P.; Kasztelan, S.; Hafner, J. and Toulhoat, H., "Periodic trends in hydrodesulfurization: in support of the Sabatier principle", Applied Catalysis, A, 2002, volume 227, pages 83-96</ref> Aside from the basic cobalt-modified MoS<sub>2</sub> catalyst, nickel and tungsten are also used, depending on the nature of the feed.  For example, Ni-W catalysts are more effective for hydrodenitrification (HDN).
 
===Supports===
Metal sulfides are "supported" on materials with high surface areas.  A typical support for HDS catalyst is &gamma;-[[alumina]].  The support allows the more expensive catalyst to be more widely distributed, giving rise to a larger fraction of the MoS<sub>2</sub> that is catalytically active.  The interaction between the support and the catalyst is an area of intense interest, since the support is often not fully inert but participates in the catalysis.
 
==Other uses==
 
The basic hydrogenolysis reaction has a number of uses other than hydrodesulfurization.
 
===Hydrodenitrogenation===
 
The hydrogenolysis reaction is also used to reduce the nitrogen content of a petroleum stream and, in that case, is referred to '''Hydrodenitrogenation''' (HDN). The process flow scheme is the same as for an HDS unit.
 
Using [[pyridine]] (C<sub>5</sub>H<sub>5</sub>N), a nitrogen compound present in some petroleum fractionation products, as an example, the hydrodenitrogenation reaction has been postulated as occurring in three steps:<ref>[https://dspace.mit.edu/bitstream/1721.1/27892/1/03740017.pdf ''Kinetics and Interactions of the Simultaneous Catalytic Hydrodenitrogenation of Pyridine and<br> Hydrodesulfurization of Thiophene''](John Wilkins, PhD Thesis, [[MIT]], 1977)</ref><ref>[http://pubs.acs.org/cgi-bin/abstract.cgi/iepdaw/1980/19/i01/f-pdf/f_i260073a027.pdf?sessid=6006l3 ''Simultaneous Catalytic Hydrodenitrogenation of Pyridine and Hydrodesulfurization of<br> Thiophene''](Satterfield,C.N., Modell, M. and Wilkens, J.A., Ind. Eng. Chem. Process Des. Dev., 1980 Vol. 19, pages 154-160)</ref>
:{| cellpadding="0" cellspacing="0"
|align="center"| Pyridine + Hydrogen
| &nbsp;→&nbsp;
|align="center"| [[Piperdine]] + Hydrogen
| &nbsp;→&nbsp;
|align="center"| [[Amine|Amylamine]] + Hydrogen
| &nbsp;→&nbsp;
|align="center"| Pentane + [[Ammonia]]
|-
|-
|align="center"| C<sub>5</sub>H<sub>5</sub>N + 5H<sub>2</sub>
| Paraffins, liquid volume % ||align=center|46||align=center|62||align=center|57||align=center|38
| &nbsp;→&nbsp;
|align="center"| C<sub>5</sub>H<sub>11</sub>N + 2H<sub>2</sub>
| &nbsp;→&nbsp;
|align="center"| C<sub>5</sub>H<sub>11</sub>NH<sub>2</sub> + H<sub>2</sub>
| &nbsp;→&nbsp;
|align="center"| C<sub>5</sub>H<sub>12</sub> + NH<sub>3</sub>
|}
 
and the overall reaction may be simply expressed as:
:{| cellpadding="0" cellspacing="0"
|align="center"| Pyridine + Hydrogen
| &nbsp;→&nbsp;
|align="center"| Pentane + Ammonia
|-
|-
|align="center"| C<sub>5</sub>H<sub>5</sub>N + 5H<sub>2</sub>
| Naphthenes, liquid volume % ||align=center|42||align=center|32||align=center|27||align=center|45
| &nbsp;→&nbsp;
|align="center"| C<sub>5</sub>H<sub>12</sub> + NH<sub>3</sub>
|}
 
Many HDS units for desulfurizing naphthas within petroleum refineries are actually simultaneously denitrogenating to some extent as well.
 
===Saturation of olefins===
 
The hydrogenolysis reaction may also be used to [[saturation (chemistry)|saturate]] or convert [[olefins]] ([[alkenes]]) into [[paraffin]]s ([[alkane]]s).  The process used is the same as for an HDS unit.
 
As an example, the saturation of the olefin, pentene, can be simply expressed as:
:{| cellpadding="0" cellspacing="0"
|align="center"| Pentene + Hydrogen
| &nbsp;→&nbsp;
|align="center"| Pentane
|-
|-
|align="center"| C<sub>5</sub>H<sub>10</sub> + H<sub>2</sub>
| Aromatics, liquid volume % ||align=center|12||align=center|6||align=center|16||align=center|17
| &nbsp;→&nbsp;
|align="center"| C<sub>5</sub>H<sub>12</sub>
|}
|}


Some hydrogenolysis units within a petroleum refinery or a petrochemical plant may be used solely for the saturation of olefins or they may be used for simultaneously desulfurizing as well as denitrogenating and saturating olefins to some extent.
Some refinery naphthas also contain some [[Olefin|olefinic]] hydrocarbons, such as naphthas derived from the [[fluid catalytic cracking]] and [[coking]] processes used in many refineries. Those olefin-containing naphthas are often referred to as ''cracked'' naphthas.


===Hydrogenation in the food industry===
== References ==
{{see|Hydrogenation|Wilhelm Normann|Trans fat}}
The food industry uses hydrogenation to completely or partially [[saturated fat|saturate]] the [[unsaturated fat|unsaturated]] [[fatty acids]] in liquid [[vegetable fats and oils]] to convert them into solid or semi-solid fats, such as those in [[margarine]] and [[shortening]].


==References==
{{reflist}}
{{reflist}}
==External links==
*[http://www.albemarle.com/Products_and_services/Catalysts/ Albemarle Catalyst Company] (Petrochemical catalysts supplier)
*[http://www.uop.com/refining/1060.html UOP Company] (Engineering design and construction of large-scale, industrial HDS plants)
*[https://portal.mustangeng.com/pls/portal30/docs/FOLDER/MUSTANGENG/TECHNICAL_ARTICLES_CONTENT/USDLHYDROTREATER.PDF Mustang Engineering Company] (Description and flow diagram of an HDS unit, from an article published in the Oil & Gas Journal)
*[http://members.ift.org/NR/rdonlyres/27B49B9B-EA63-4D73-BAB4-42FEFCD72C68/0/crfsfsv4n1p00220030ms20040577.pdf ''Hydrogenation for Low Trans and High Conjugated Fatty Acids''] by E.S. Jang, M.Y. Jung, D.B. Min, Comprehensive Reviews in Food Science and Food Safety, Vol.1, 2005
*[http://www.akerkvaerner.com/Internet/IndustriesAndServices/Process/Petrochemicals/ChemicalImtermediates/OxoAlcohols.htm Oxo Alcohols]  (Engineered and constructed by Aker Kvaerner)
*[http://www.jmcatalysts.com/pct/marketshome.asp?marketid=10&id=373 Catalysts and technology for Oxo-Alcohols]
[[Category:Chemical engineering]]
[[Category:Oil refineries]]
[[Category:Chemical processes]]
[[Category:Unit processes]]
[[de:Hydrodesulfurierung]]
[[fr:Hydrodésulfuration]]
[[ja:脱硫]]

Revision as of 00:02, 1 February 2008

Naphtha is an intermediate hydrocarbon liquid stream derived from the refining of crude oil.[1][2] 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).

Some refineries also produce a small amount of specialty naphthas for use as solvents and those specialty naphthas are subjected to purification processes other than catalytic reforming.

Where the naptha is obtained

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 of about 35 °C and a final boiling point of about 200 °C, and it contains paraffin, naphthene (cyclic paraffins) and aromatic hydrocarbons ranging from those containing 4 carbon atoms to those containing about 10 or 11 carbon atoms.

The virgin naphtha is often further distilled to produce a light naphtha containing most (but not all) of the hydrocarbons with 6 or less carbon atoms and a heavy naphtha containing most (but not all) of the hydrocarbons with more than 6 carbon atoms. The heavy naphtha has an initial boiling point of about 140 to 150 °C and a final boiling point of about 190 to 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.[3][4][5]

Types of naphthas

It should be noted that there are a great many petroleum crude oil sources worldwide and each crude oil has its own unique composition or assay. Also, not all refineries process the same crude oils and each refinery produces its own virgin naphthas with their own unique initial and final boiling points. In other words, naphtha is a generic term rather than a specific term.

The table just below lists some fairly 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:

Typical Heavy Naphthas
Crude oil name
Location 		Barrow Island
Australia[6] 		Mutineer-Exeter
Australia[7] 		CPC Blend
Kazakhstan[8] 		Draugen
North Sea[9]
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 and coking processes used in many refineries. Those olefin-containing naphthas are often referred to as cracked naphthas.

References

  1. ↑ Gary, J.H. and Handwerk, G.E. (1984). Petroleum Refining Technology and Economics, 2nd Edition. Marcel Dekker, Inc. ISBN 0-8247-7150-8. 
  2. ↑ Leffler, W.L. (1985). Petroleum refining for the nontechnical person, 2nd Edition. PennWell Books. ISBN 0-87814-280-0. 
  3. ↑ Canadian regulations on benzene in gasoline
  4. ↑ United Kingdom regulations on benzene in gasoline
  5. ↑ USA regulations on benzene in gasoline
  6. ↑ Barrow Island crude oil assay
  7. ↑ Mutineer-Exeter crude oil assay
  8. ↑ CPC Blend crude oil assay
  9. ↑ Draugen crude oil assay
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