Contact process

The contact process is a method for manufacturing 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 2 O 5 ) has since been preferred.

The contact process is a method for manufacturing 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 2 O 5 ) has since been preferred. ==History== This process was patented in 1831 by British vinegar merchant Peregrine Phillips. 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. In 1901 Eugen de Haën patented the basic process involving combining sulfur dioxide and oxygen in the presence of vanadium oxides, producing sulfur trioxide which was easily absorbed into water, producing sulfuric acid. 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. 2 SO 2(g) + O 2(g) ⇌ 2 SO 3(g) : ΔH = -197 kJ·mol −1 According to Le Chatelier's principle, lower temperatures favor formation of sulfur trioxide, but if the temperature is too low the reaction rate becomes uneconomical. In industrial practice, the oxidation is therefore carried out at about 420 to 620 °C over vanadium(V) oxide-based catalysts, typically around 450 °C, at near-atmospheric pressure. Below this range the catalyst is deactivated by formation of vanadium(IV) compounds, while at higher temperatures it begins to decompose. The active catalyst phase is not pure solid V 2 O 5 , but a molten salt formed from vanadium(V) oxide and added alkali metal sulfates. In this melt, a reactive complex of composition [(VO) 2 O(SO 4 ) 4 ] 4− is formed, which is regarded as the actual catalytic species. Oxygen and sulfur dioxide coordinate to this complex and react to form sulfur trioxide without a change in the oxidation state of vanadium. Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H 2 SO 4 in the absorption tower to form oleum. H 2 SO 4 + SO 3 → H 2 S 2 O 7 Note that directly dissolving SO 3 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 2 SO 4 . H 2 S 2 O 7 + H 2 O → 2 H 2 SO 4 The Repertory of Patent Inventions, no. 72 (April 1831), 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, pages 180-182. Ernest Cook (March 20, 1926) "Peregrine Phillips, the inventor of the contact process for sulphuric acid," Nature, 117 (2942) : 419–421. 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

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