Sulphuric acid/oleum mist

Notwithstanding the low process air dewpoint, some sulphuric acid/oleum mist condenses in the coolers following the converter tower at temperatures of about 45-50 C. This highly reactive mist can affect the quality of the subsequent sulphonation reaction and therefore a Wgh-efficiency demister is installed before the actual sulphonation step. 

The other main gaseous pollutant which may be discharged to the atmosphere is sulphuric acid/oleum mist. This is formed mainly from a reaction between non-reacted SO3 gas and moisture still present in the dried process air and/or derived from the combustion of hydrocarbons commonly present as impurities in the sulphur. This means that the amount of acid formed depends on the efficiency of the air drying system (dewpoint) and the hydrocarbon content of sulphur. 

The amount of sulphuric acid (oleum 20%) collected in the bottoms of the SOs/air cooler system and in the SO3 mist eliminator, is directly related to the dewpoint of the dried process air (see 5.2.2.). With an air dewpoint of -60 C, only 2 kg oleum per 24 hours will be formed for a 1 ton AD/h sulphonation plant. With increasing dewpoint the amount of oleum collected will rise sharply. For sulphonation plants working with an SO3 absorber (see section 5.2.6) it is recommended that the collected amounts of sulphuric acid/oleum be discharged into the vessel at the SO3 absorbing system. The acid coming from the various parts of the gas raising plant should be collected via gravity flow in an intermediate tank with level control from where the acid is pumped off with a membrane pump to the SO3 absorber. The use of compressed air is not recommended for safety reasons. 

Griffiths, R. (ed.) (1995) Sulphur Trioxide, Oleum and Sulphuric Acid Mist, Institution of Chemical Engineers, Rugby. Hamngton, J.M. and Gardiner, K. (1995) Occupational Hygiene, 2nd edn, Blackwell Science, Oxford. 

The failure of a filling or unloading line attached to a road tanker of, say, 18 ton, capacity has been identified as an event which is representative of the foreseeable sizes of release from a typical installation. Severance of the line, e.g. by tanker moving while it is being filled or unloaded, would cause the release of sulphur trioxide. The SO3 released will spread in a pool, reacting violently with any water present, either in or on the ground, until all the water is used up. The result will be the production of a large cloud of dense sulphuric acid mist. Once the readily available water has been used up, the pool of sulphur trioxide will continue to emit sulphuric acid mist fumes from reaction with moisture in the air. This will continue until all the sulphur trioxide is used up or remedial measures have been taken. Trials by ICI with 65% oleum... 

Sulphur trioxide frimes form oleum with atmospheric moisture. The exposure to sulphuric acid mist has been described to some extent in section 3.3.1. Hazards of sulphur trioxide. 

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