The Contact Process

Sulfuric acid is used in many important manufacturing processes. Some examples are  

In 1990, the total world-wide output of sulfuric acid, per day, was about 200000 tonnes. 

Production begins with the burning of molten sulfur in dry air to produce sulfur dioxide  

An initial input of heat is needed to get the reaction started, but then it is so exothermic that the reaction is self-sustaining, and some of the excess energy released can be used to produce steam at high pressure that can in turn power pumps and provide heat elsewhere in the plant. 

The second and key part of this process is also the most problematic  

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Since the catalyst is in the gaseous state, it is being continually removed from the mixing chambers. Its recovery, and the necessity of continual charging of the incoming gases with it, make the lead chamber plant complicated by comparison with that of the Contact process. 

The conversion of sulphur trioxide to sulphuric acid arises as a separate reaction only in the Contact process. 

In the finely divided state platinum is an excellent catalyst, having long been used in the contact process for producing sulfuric acid. It is also used as a catalyst in cracking petroleum products. Much interest exists in using platinum as a catalyst in fuel cells and in antipollution devices for automobiles. 

In 1875, a paper by Winkler awakened interest in the contact process, first patented in 1831. Winkler claimed that successfiil conversion of SO2 to SO could only be achieved with stoichiometric, undiluted ratios of SO2 and O2. Although erroneous, this beUef was widely accepted for more than 20 years and was employed by a number of firms. Meanwhile, other German firms expended a tremendous amount of time and money on research. This culminated in 1901 with Knietsch s lecture before the German Chemical Society (3) revealing some of the investigations carried out by the Badische Anilin-iind-Soda-Fahrik. This revealed the abandonment of Winkler s theory and further described principles necessary for successfiil appHcation of the contact process. 

Sulfuric acid may be produced by the contact process from a wide range of sulfur-bearing raw materials by several different process variants, depending largely on the raw material used. In some cases sulfuric acid is made as a by-product of other operations, primarily as an economical or convenient means of minimising air pollution (qv) or disposing of unwanted by-products. 

In early years the contact process frequentiy employed only two or three catalyst stages (passes) to obtain overall SO2 conversions of approximately 95—96%. Later, four pass converters were used to obtain conversions of from 97% to slightiy better than 98%. For sulfur-burning plants, this typically resulted in sulfur dioxide stack emissions of 1500—2000 ppm. 

Process Details and Flow Sheets. The stoichiometric relation between reactants and products for the contact process may be represented as foUows ... 

Particulate emissions from zinc processing are collected in baghouses or ESPs. SO2 in high concentrations is passed directly to an acid plant for production of sulfuric acid by the contact process. Low-concentration SO2 streams are scrubbed with an aqueous ammonia solution. The resulting ammonium sulfate is processed to the crystalline form and marketed as fertilizer. 

The contact process which replaced the chamber process reacts the products using a platinum or a vanadium catalyst. 

The newer process uses a solid catalyst for reaction (19). Either finely divided platinum or vanadium pentoxide, V205, is effective. Because catalysis occurs where the gas contacts the surface of the catalyst, this process is called the contact process. 

Reaction (19) is carried out at a high temperature (about 500°C in the contact process). How does temperature affect equilibrium, according to Le Chatelier s Principle In view of your answer, propose an explanation of why the temperature is kept high. 

Haber process for ammonia. Another is finely divided vanadium pentoxide, V205, which is used in the contact process for the production of sulfuric acid ... 

Sulfuric acid, H2S04, is produced commercially in the contact process, in which sulfur is first burned in oxygen and the S02 produced is oxidized to S03 over a V205 catalyst ... 

The physical processes that occur during indentation are schematically illustrated in Fig. 31. As the indenter is driven into the material, both elastic and plastic deformation occurs, which results in the formation of a hardness impression conforming to the shape of the indenter to some contact depth, h. During indenter withdrawal, only the elastic portion of the displacement is recovered, which facilitates the use of elastic solutions in modeling the contact process. 

Today, sulfuric acid is produced by a method known as the contact process in which S02 is oxidized to S03, which then reacts with water to give the acid. Oxidation of S02 requires a suitable catalyst such as spongy platinum or sodium vanadate. In many cases, S03 is dissolved in 98% sulfuric acid to produce disulfuric acid oleum], which can be shipped, diluted, and still give 100% sulfuric acid. The concentration of S03 varies from 10 to 70% in commercial oleum. 

In the second stage, a mixture of sulphur dioxide and air is passed over a catalyst of vanadium (V) oxide, V205, at a temperature of about 430°C. This is called the Contact process. 

CAT-OX [Catalytic oxidation] An adaptation of the Contact process for making sulfuric acid, using the dilute sulfur dioxide in flue-gases. A conventional vanadium pentoxide catalyst is used. Developed by Monsanto Enviro-Chemical Systems, and operated in Pennsylvania and Illinois in the early 1970s. 

Miles, F. D., The Manufacture of Sulfuric Acid by the Contact Process, Gurney Jackson, London, 1925. 

IPA [Interpass absorption] Also called Double absorption, and Double catalysis. An improved version of the Contact process for making sulfuric acid, by which the efficiency of the conversion of sulfur to sulfuric acid is increased from 98 percent to over 99.5 percent. 

Knietsch An early version of the Contact process for making sulfuric acid. Developed by R. Knietsch at BASF, Ludwigshaven. 

Schroder-Grillo Also known as Grillo-Schroder. An early version of the contact process for making sulfuric acid. The catalyst was magnesium sulfate impregnated with platinum. The process was invented in 1899 by A. Hecksher at the New Jersey Zinc Company and first used at its plant in Mineral Point, WI, in 1901 this was the first use of the contact process in the United States. In the United Kingdom it was first used in Widnes in 1917. See also Mannheim (2). 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

N2(g) + 3H2(g) 2NH3(g) AH° = —92 kj/mol Because the reaction is exothermic, heat is released as the reaction proceeds. Le Chatelier s principle predicts that the yield of ammonia is greater at lower temperatures. Just as in the contact process for manufacturing sulfuric acid, however, high yield is not the only important factor. The rate of reaction for ammonia synthesis is too slow at low temperatures. 

Some of its compounds, particularly the oxides, are used in chemical industries as catalysts to speed up organic chemical reactions. The yellow-brown vanadium pentoxide (V O ) is used as a catalyst to facilitate the production of sulfuric acid by the contact process. Vanadium pent-oxide is also used as a photographic developer, to dye textiles, and in the production of artificial rubber. When combined with glass, it acts as a filter against ultraviolet rays from sunlight. 

Sulfuric Acid. Essentially all sulfuric acid manufactured in this industry is produced by the contact process, in which SO2 and oxygen contact each other on the surface of a catalyst (vanadium pentaoxide) to form SO3 gas. Sulfur trioxide gas is added to water to form sulfuric acid. The sulfur dioxide used in the process is produced by burning elemental sulfur in a furnace. 

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