Editing Contact process

{{Short description|Industrial method of sulfuric acid production}}{{for|the stochastic process used to model population|Contact process (mathematics)}}{{More footnotes needed|date=May 2020}}
The '''contact process''' is a method of producing [[sulfuric acid]] in the high concentrations needed for industrial processes. [[Platinum]] was originally used as the [[catalyst]] for this reaction; however, because it is susceptible to reacting with [[arsenic]] impurities in the sulfur feedstock, [[vanadium(V) oxide]] (V<sub>2</sub>O<sub>5</sub>) has since been preferred.<ref>{{ cite web | title = History | url = http://www.ravensdown.co.nz/Resources/Education/History.htm | accessdate=March 1, 2010 | url-status=dead | archiveurl=https://web.archive.org/web/20100523153700/http://www.ravensdown.co.nz/Resources/Education/History.htm |archivedate=May 23, 2010 | publisher =Ravensdown}}</ref>

==History==
This process was patented in 1831 by British vinegar merchant Peregrine Phillips.<ref>{{cite book| url =https://books.google.com/books?id=xriMAgAAQBAJ&pg=PA229|title = A History of Platinum and its Allied Metals|isbn = 9780905118833|last1 = McDonald|first1 = Donald|last2 = Hunt|first2 = Leslie B.|date = January 1982| publisher=Johnson Matthey Plc }}</ref><ref>{{cite patent|country = UK| number= 6093| status=| title= Manufacturing of Sulphuric Acid |pubdate= | gdate= 1831| fdate = | pridate = | invent1 =  Peregrine Phillips Junior| | class=| url= }}</ref><ref>{{cite book | url = https://books.google.com/books?id=6pvzCAAAQBAJ&pg=PA16|title = Catalysis: Science and Technology|isbn = 9783642932786|last1 = Anderson|first1 = John R.|last2 = Boudart|first2 = Michel|date = 2012-12-06| publisher=Springer }}</ref> In addition to being a far more economical process for producing concentrated [[sulfuric acid]] than the previous [[lead chamber process]], the contact process also produces [[sulfur trioxide]] and [[oleum]].

In 1890 [[John Brown Francis Herreshoff]] developed a form of the contact catalytic process for the company of which he was a partner.<ref name=cuny1>{{cite news |title=Historical Maps, Geospatial Methods, and Brooklyn’s Early Chemicals and Petroleum Industries |first1=Peter |last1=Spellane |first2=Anne |last2=Leonard |publisher=Brooklyn Waterfront Research Center, NYC College of Technology CUNY |date=17 October 2013 |url=https://s3.amazonaws.com/files.commons.gc.cuny.edu/wp-content/blogs.dir/643/files/2013/10/NEWTOWN-CREEK-BWRC-20131017.pdf }}</ref>

In 1901 [[Eugen de Haën]] patented the basic process involving combining [[sulfur dioxide]] and [[oxygen]] in the presence of [[vanadium oxide]]s, producing [[sulfur trioxide]] which was easily absorbed into water, producing [[sulfuric acid]].<ref>{{cite patent|country = US| number= 687834A| status=| title= Method of Making Sulfuric Anhydrid |pubdate= | gdate= 1901-12-03| fdate = 1914-10-9| pridate = | invent1 = Carl Johann Eugen de Haën| url= https://worldwide.espacenet.com/patent/search/family/025346274/publication/US1371004A }}</ref>  This process was improved remarkably by shrinking the particle size of the catalyst (e.g. ≤ 5000 microns), a process discovered by two chemists of [[BASF]] in 1914.<ref>{{cite patent|country = US| number= 1371004| status=| title= Oxidation of sulfur dioxid and catalyst therefor |pubdate= | gdate= 1921-05-8| fdate = 1914-10-9| pridate = | invent1 = Franz Slama| invent2 = Hans Wolf| assign1= General Chemical Company| class=| url= https://worldwide.espacenet.com/patent/search/family/025346274/publication/US1371004A }}</ref><ref>{{cite book | url = https://books.google.com/books?id=6pvzCAAAQBAJ&pg=PA16|title = Catalysis: Science and Technology|isbn = 9783642932786|last1 = Anderson|first1 = John R.|last2 = Boudart|first2 = Michel|date = 2012-12-06| publisher=Springer }}</ref><ref>{{cite book | url = https://books.google.com/books?id=Q6XFfg8IIuAC&pg=SA10-PA816|title = Industrial Chemistry|isbn = 9788187224990|year = 1991| publisher=Krishna Prakashan Media }}</ref><!--10.1007/978-3-642-93278-6_1 https://archive.org/details/in.ernet.dli.2015.4339/page/n3/mode/2up-->

== Process ==

The process can be divided into four stages:

# Combining of [[sulfur]] and [[oxygen]] (O<sub>2</sub>) to form [[sulfur dioxide]], then purify the sulfur dioxide in a purification unit
# Adding an excess of oxygen to [[sulfur dioxide]] in the presence of the catalyst [[Vanadium(V) oxide|vanadium pentoxide]] at 450&nbsp;°C and 1-2&nbsp;atm
# The [[sulfur trioxide]] formed is added to [[sulfuric acid]] which gives rise to [[oleum]] (disulfuric acid)
# The oleum is then added to water to form sulfuric acid which is very concentrated. Since this process is an exothermic reaction, the reaction temperature should be as low as possible.

Purification of the air and sulfur dioxide (SO<sub>2</sub>) is necessary to avoid catalyst ''poisoning'' (i.e. removing catalytic activities). The gas is then washed with [[water (molecule)|water]] and dried with sulfuric acid.

To conserve energy, the mixture is heated by exhaust gases from the [[catalytic heater|catalytic converter]] by heat exchangers.

Sulfur dioxide and dioxygen then react as follows:

:2 SO<sub>2(g)</sub> + O<sub>2(g)</sub> ⇌ 2 SO<sub>3(g)</sub> : Δ''H'' = -197&nbsp;kJ·mol<sup>−1</sup>

According to the [[Le Chatelier's principle]], a lower temperature should be used to shift the chemical equilibrium towards the right, hence increasing the percentage yield. However too low of a temperature will lower the formation rate to an uneconomical level. Hence to increase the reaction rate, high temperatures (450&nbsp;°C), medium pressures (1-2&nbsp;[[atmosphere (unit)|atm]]), and [[vanadium(V) oxide]] (V<sub>2</sub>O<sub>5</sub>) are used to ensure an adequate (>95%) conversion. The catalyst only serves to increase the rate of reaction as it does not change the position of the [[thermodynamic equilibrium]]. The mechanism for the action of the catalyst comprises two steps:

# Oxidation of SO<sub>2</sub> into SO<sub>3</sub> by V<sup>5+</sup>:
#: 2SO<sub>2</sub> + 4V<sup>5+</sup> + 2O<sup>2−</sup> → 2SO<sub>3</sub> + 4V<sup>4+</sup>
# Oxidation of V<sup>4+</sup> back into V<sup>5+</sup> by dioxygen (catalyst regeneration):
#: 4V<sup>4+</sup> + O<sub>2</sub> → 4V<sup>5+</sup> + 2O<sup>2−</sup>

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H<sub>2</sub>SO<sub>4</sub> in the absorption tower to form oleum.

: H<sub>2</sub>SO<sub>4</sub>  + SO<sub>3</sub>  → H<sub>2</sub>S<sub>2</sub>O<sub>7</sub> 

Note that directly dissolving SO<sub>3</sub> in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H<sub>2</sub>SO<sub>4</sub>.

:H<sub>2</sub>S<sub>2</sub>O<sub>7</sub>  + H<sub>2</sub>O  → 2 H<sub>2</sub>SO<sub>4</sub>

== Purification unit ==
This includes the dusting tower, cooling pipes, scrubbers, drying tower, arsenic purifier and testing box. Sulfur dioxide has many impurities such as vapours, dust particles and [[arsenous oxide]]. Therefore, it must be purified to avoid catalyst poisoning (i.e.: destroying catalytic activity and loss of efficiency). In this process, the gas is washed with water, and dried by sulfuric acid. In the dusting tower, the sulfur dioxide is exposed to a steam which removes the dust particles. After the gas is cooled, the sulfur dioxide enters the washing tower where it is sprayed by water to remove any soluble impurities. In the drying tower, sulfuric acid is sprayed on the gas to remove the moisture from it. Finally, the [[arsenic oxide]] is removed when the gas is exposed to [[ferric hydroxide]].

== Double contact double absorption ==
The next step to the contact process is ''double contact double absorption'' (DCDA). In this process the product gases (SO<sub>2</sub>) and (SO<sub>3</sub>) are passed through absorption towers twice to achieve further absorption and conversion of SO<sub>2</sub> to SO<sub>3</sub> and production of higher grade sulfuric acid.

SO<sub>2</sub>-rich gases enter the catalytic converter, usually a tower with multiple catalyst beds, and are converted to SO<sub>3</sub>, achieving the first stage of conversion. The exit gases from this stage contain both SO<sub>2</sub> and SO<sub>3</sub> which are passed through intermediate absorption towers where sulfuric acid is trickled down packed columns and SO<sub>3</sub> reacts with water increasing the sulfuric acid concentration. Though SO<sub>2</sub> too passes through the tower it is unreactive and comes out of the absorption tower.

This stream of gas containing SO<sub>2</sub>, after necessary cooling is passed through the catalytic converter bed column again achieving up to 99.8% conversion of SO<sub>2</sub> to SO<sub>3</sub> and the gases are again passed through the final absorption column thus  achieving not only high conversion efficiency for SO<sub>2,</sub> but also enabling production of a higher concentration of sulfuric acid.

The industrial production of sulfuric acid involves proper control of temperatures and flow rates of the gases as both the conversion efficiency and absorption are dependent on these.

== Notes ==

{{reflist}}

== References ==

{{refbegin}}
* ''The Repertory of Patent Inventions'', no. 72 (April 1831), [https://books.google.com/books?id=EAcLAQAAIAAJ&pg=PA248 page 248].
* (Anon.) (1832) "English patents:  Specification of the patent granted to Peregrine Phillips, Jr. of Bristol, in the county of Somersetshire, Vinegar Maker, for an improvement in manufacturing Sulphuric Acid.  Dated March 21, 1831." ''Journal of the Franklin Institute'', new series, vol. 9, [https://books.google.com/books?id=RL5IAAAAMAAJ&pg=PA180 pages 180-182.]
* Ernest Cook (March 20, 1926) "Peregrine Phillips, the inventor of the contact process for sulphuric acid," ''Nature'', '''117''' (2942) : 419–421.
* [https://books.google.com/books?id=SJIZAQAAIAAJ&pg=PA975 Lunge, ''Theoretical and Practical Treatise on the Manufacture of Sulphuric Acid and Alkali, with the Collateral Branches'', 3rd ed., vol. 1, part 2 (London, England:  Gurney and Jackson, 1903),  page 975]
{{refend}}

== External links ==

* {{cite web | author = Jim Clark | title = The Contact Process | url = http://www.chemguide.co.uk/physical/equilibria/contact.html | year = 2002 | website = Chemguide}}
* {{cite web | title = The Contact Process | url = http://www.citycollegiate.com/sulphuric_acid1.htm | year = 2009 | work = City Collegiate | access-date = 2009-05-01 | archive-date = 2015-03-02 | archive-url = https://web.archive.org/web/20150302044739/http://www.citycollegiate.com/sulphuric_acid1.htm | url-status = usurped }}
* {{cite web | title = Absorption Tower | url = http://www.mining-enc.ru/a/absorbcionnaya-kolonna | website = Mining Encyclopedia}}

[[Category:Chemical processes]]
[[Category:Vanadium]]
[[Category:Sulfur]]
[[Category:Catalysis]]