Sulphuric acid dewpoint

Figure 1 illustrates the overall block diagram. To raise S03/air with a volumetric content between 4 and 7% SO3, the process air should be dried to prevent the formation of sulphuric acid mist. Filtered ambient air is compressed (about 0.6 bar gauge), chilled (about +5 C) to remove the major part of water by condensation and subsequently dried with a desiccant (silica gel. Alumina) to arrive at a process air dewpoint of about -60 C. 

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 importance of the air dewpoint on sulphuric acid/oleum formation in the gas raising plant can be seen from the following table ... 

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. 

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