View source for Hydrodesulfurization/Citable Version

{{subpages}}

'''Hydrodesulfurization''' (HDS) or '''Hydrotreater''' is a [[catalytic]] chemical process widely used to remove [[sulphur]] compounds from [[Petroleum refining processes|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 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><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]]  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]]. 

Another important reason for removing sulfur from the intermediate product [[Petroleum naphtha|naphtha]] streams within a petroleum refinery is that sulfur, even in extremely low concentrations, 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.

The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting [[hydrogen sulphide]] gas. In [[Petroleum refining processes|petroleum refineries]], the hydrogen sulfide gas is then subsequently converted into byproduct elemental [[sulphur]]. In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from petroleum refining and [[natural gas 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>

==History== 

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 double [[bonds]] (e.g., ([[ethylene]], [[benzene]])  was discovered by the [[France|French]] 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.

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 schematic flow diagram below have been commercialized.  Currently, virtually all of the petroleum refineries world-wide have one or more HDS units.

By 2006, miniature HDS units had been developed for treating jet fuel to produce clean feedstock for [[hydrogen reformer|synthesizing hydrogen]] for a [[fuel cell]].<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>

==The process chemistry==

Hydrogenation is a class of [[chemical reaction]]s that results in the addition of hydrogen. [[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 (S), [[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 refining streams, as an example, the hydrodesulfurization reaction can be simply expressed as:

Ethanethiol + Hydrogen → [[Ethane]] + Hydrogen Sulfide

::&nbsp;C<sub>2</sub>H<sub>5</sub>SH + H<sub>2</sub> → C<sub>2</sub>H<sub>6</sub> + H<sub>2</sub>S

===Catalysts===

Many metals catalyse the HDS reaction, 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> 

In practice, most HDS units in petroleum refineries use catalysts based on cobalt-modified [[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> Aside from the cobalt-modified MoS<sub>2</sub> catalysts, nickel and [[tungsten]] are also used, depending on the nature of the feed.  For example, nickel-[[wolfram]] (Ni-W) catalysts are more effective for hydrodenitrification (HDN).

Metal sulfides are "supported" on materials with high surface areas.  A typical support for HDS catalyst is [[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.

==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.

The image below is a schematic depiction of the equipment and the process flow streams in a typical refinery HDS unit.<ref name=Gary/><ref name=Leffler/>
[[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 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 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 hydrogen sulfide gas (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]] (i.e., gas containing H<sub>2</sub>S) from the stripping of the reaction product liquid.

The liquid from the gas separator vessel is routed through a reboiled stripper [[Continuous distillation|distillation]] tower.  The bottoms product from the stripper is the final desulfurized liquid product from the hydrodesulfurization unit. 

The overhead sour gas from the stripper contains hydrogen, [[methane]], ethane, hydrogen sulfide, [[propane]] and perhaps some butane and heavier components (i.e., higher [[Molecular mass|molecular weight]] 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 sulfur compounds, including [[thiols]], [[thiophenes]], organic [[sulfides]] and [[disulfides]], and many others. These organic sulfur compounds are products of the degradation of sulfur during the natural formation of the 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 of America]], [[Europe]] and elsewhere requiring what is referred to as ''ultra-low sulfur diesel'' (ULSD) requires that very deep hydrodesulfurization is needed. In the 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://npra.org/issues/transportation/?fa=dieselSulfur ''Diesel Sulfur''] published online by the National Petrochemical & Refiners Association (NPRA)</ref>

==Other uses==

The basic hydrogenolysis reaction has a number of uses other than hydrodesulfurization.

===Hydrodenitrogenation===
{{Image|Hydrodenitrogenation of pyridine DEV.jpg|right|300px|Hydrodenitrogenation of pyridine}}
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. In fact, units may be designed for both hydrodesulfurization and hydrodenitrogenation.

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/doi/abs/10.1021/i260073a027 ''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>
| &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>
| &nbsp;→&nbsp; 
|align="center"| C<sub>5</sub>H<sub>12</sub> + NH<sub>3</sub>
|}

===Saturation of olefins===

The hydrogenolysis reaction may also be used to 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>
| &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.

===Hydrogenation in the food industry===

The food industry uses hydrogenation to completely or partially saturate the 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}}