Production of SO3 in the Contact process

Production of sulfuric acid, ammonia and phosphate rock (see Section 15.2) heads the inorganic chemical and mineral industries in the US. The oxidation of SO2 to SO3 (equ. 25.37) is the first step in the Contact process, and in Section 16.8 we discussed how the yield of SO3 depends on temperature and pressure. At ordinary temperatures, the reaction is too slow to be commercially viable, while at very high temperatures, equilibrium 25.37 shifts to the left, decreasing the yield of SO3. 

A catalytic converter consists of a honeycomb ceramic stmcture coated in finely divided AI2O3 (the washcoat). Fine particles of catalytically active Pt, Pd and Rh are dispersed within the cavities of the washcoat and the whole unit is contained in a stainless steel vessel placed in sequence in the vehicle s exhaust pipe. As the exhaust gases pass through the converter at high temperatures, redox reactions 25.38-25.42 occur (C3Hg is a representative hydrocarbon). Under legislation, the oidy acceptable emission products are CO2, N2 and H2O. 

Whereas CO and hydrocarbons are oxidized, the destruction of NO c involves its reduction. Modem catalytic converters have a three-way system which promotes both oxidation and reduction. Pd and Pt catalyse reactions 25.38 and 25.39, while Rh catalyses reactions 25.40 and 25.41, and Pt catalyses reaction 25.42. 

A catalytic converter cannot function immediately after the cold start of an engine. At its light-off temperature (typically 620 K), the catalyst operates at 50% efficiency but during the 90-120 s lead time, exhaust emissions are not controlled. Several methods have been developed to counter this problem, e.g. electrical heating of the catalyst using power from the vehicle s battery. 

The development of catalytic converters has recently encompassed the use of zeolites, e.g. Cu-ZSM-5 (a copper-modified ZSM-5 system), but at the present time, and despite some advantages such as low light-off temperatures, zeolite-based catalysts have not shown themselves to be sufficiently durable for their use in catalytic converters to be commercially viable. 

A catalytic converter consists of a honeycomb ceramic structure coated in finely divided AI2O3 (the washcoat). Fine particles of catalytically active Pt, Pd and Rh are 

38 occur (C3H8 is a representative hydrocarbon). Under legislation, the only acceptable emission products are CO2, N2 and H2O.  

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Reaction yields in the contact process were increased in several ways—by selecting an optimum temperature, using an efficient catalyst, removing a product from a reaction that does not go to completion, and by controlling the rate of reaction of SO3 with water. Keeping operating pressures at the correct values also increases yield. 

The equilibrium constant for this reaction is 1.7 X 10 , which indicates that for all practical purposes the reaction should go almost completely to products. Yet when sulfur is burned in air or oxygen, it forms predominantly SO2 and very little SO3. Oxidation of SO2 to SO3 is simply too slow to give a significant amount of product. However, the rate of the reaction is appreciable in the presence of a platinum or divanadium pentoxide catalyst. The oxidation of SO2 in the presence of a catalyst is the main step in the contact process for the industrial production of sulfuric acid, H2SO4. Sulfur trioxide reacts with water to form sulfuric acid. (In the industrial process, SO3 is dissolved in concentrated H2SO4, which is then diluted.) ... 

Removal of sulfur dioxide from a gas stream. If a fuel that contains sulfur is burned, the product gas contains sulfur dioxide. If the gas is released directly into the atmosphere, the SO2 combines with atmospheric oxygen to form sulfur trioxide. The SO3 in turn combines with water vapor in the atmosphere to form sulfuric acid (H7SO4), which eventually precipitates as acid rain. To prevent this occurrence, the combustion product gas is contacted with a liquid solution in an absorption or scrubbing process. The SO2 dissolves in the solvent and the clean gas that remains is released to the atmosphere. 

Continuing the line of catalytic oxidation on platinum. Peregrine Philips (1831, British Patent No. 6096) patented the oxidation of SO2 to SO3 on platinum, but he must have died before the first contact process plant for the production of sulfuric acid went on stream. And finally, along this line of work, Schweigger in the same year discovered that hydrogen sulfide poisoned platinum. 

A good example where process intensification has the potential to transform a complete plant operation centres on the manufacture of sulphuric add. This can be seen from a study of the SO3/H2SO4 contact process. In order to appreciate the potential impact this approach could have on a well-estabhshed process, it is worth discussing the flow sheet for the manufacture of sulphuric acid/oleum. A sulphur burner produces SO2 which is then reacted over vanadium pentoxide catalyst at 1 bar to produce gaseous SO3. This is then absorbed in recycling sulphuric acid to give product oleum. As is often the case, the reactor is the heart of the process. 

Mkenyhulfonates, fatty alcohol sulfates and fatty alcohol ether sulfates are produced today by contacting alkenes, fatty alcohols, or fatty alcohol ethers with gaseous SO3 in multi-tubular falling film reactors. Typical reactor productivities reach up to 10 t of product per hour. An even temperature distribution throughout the reactor is an important prerequisite for optimum product quality in these processes. 

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