Industrial H2SO4 Making

Industrial H2SO4 making is similar to the air/gas dehydration process described in Chapter 6. In both cases, gas is passed upwards through descending strong sulfuric acid. Figs. 6.3 and 9.1. The difference is that  

H2SO4 making reacts SOsfg) with the H20(f) in descending acid, i.e. 

Gas dehydration dilutes the input acid while H2SO4 making strengthens it. Both reactions are exothermic. Both heat up their circulating acid so that it must be cooled before being recycled or sent to storage. 

These residence times give 99+% transformation of 803(g) to H2S04(f). 

Air/gas dehydration (Chapter 6) produces fine spray of 10-250 pm diameter acid droplets. The droplets are removed from the dehydrated gas to prevent downstream corrosion. They are usually captured in a knitted Teflon /stainless steel pad at the gas exit. Fig. 6.3. 

Gas residence time in packing (s) Acid residence time in packing (s) 

These residence times give 99-h% transformation of SO3 to H2S04( ). 

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Industrial H2SO4 making input and output acid temperatures (Table 9.3) are typically ... 

Gas enters industrial H2SO4 making towers at 450-500 K. This is hot enough to avoid H2S04(f) condensation in the flues between catalytic SO2 oxidation and H2SO4 making. It is cool enough to avoid excessive acid mist formation. 

Fig. 5.1. Spent sulfuric acid regeneration flowsheet. H2SO4(0 the contaminated spent acid is decomposed to S02(g), 02(g) and H20(g) in a mildly oxidizing, 1300 K fuel fired furnace. The fiunace offgas (6-14 volume% SO2,2 volume% O2, remainder N2, H2O, CO2) is cooled, cleaned and dried. It is then sent to catalytic SO2 + AO2 SO3 oxidation and H2SO4 making, Eqn. (1.2). Air is added just before dehydration (top right) to provide O2 for catalytic SO2 oxidation. Molten sulfur is often burnt as fuel in the decomposition fiimace. It provides heat for H2SO4 decomposition and SO2 for additional H2SO4 production. Tables 5.2 and 5.3 give details of industrial operations.

See Table 19.3 (end of this chapter) for industrial after H2SO4 making catalyst bed data. 

This is somewhat above industrial total SO2 oxidation (99.5-99.9% Hansen, 2004), but it confirms the high SO2 oxidation and H2SO4 making efficiencies of double contact acid plants. 

Fig. 21.1. Heat transfer flowsheet for single contact, suliiir burning sulfuric acid plant. It is simpler than industrial plants, which nearly always have 4 catalyst beds rather than 3. The gaseous product is cool, SO3 rich gas, ready for H2SO4 making. The heat transfer product is superheated steam. All calculations in this chapter are based on this figure s feed gas composition and catalyst bed input gas temperatures. All bed pressures are 1.2 bar. The catalyst bed output gas temperatures are the intercept temperatures calculated in Sections 12.2, 15.2 and 16.3.

Of course, output acid temperature will increase if mass% H2SO4 in output acid is allowed to increase with increasing volume% SO3 in H2SO4 making tower input gas. Fig. 24.4. This is the case when input acid flowrate and mass% H2SO4 are kept constant while volume% SO3 in input gas increases. This is usual industrial practice. 

Fig. 24.7 Schematic of acid heat to steam energy recovery system, after Puricelli et al., 1998. It is for intermediate H2SO4 making, Fig. 9.6. Note (i) the double packed bed H2SO4 making tower and (ii) boiler. Industrial acid heat recovery H2SO4 making towers are 25m high and 10 m diameter. They produce 2000 to 4000 tonnes of H2SO4 per day. For photographs see Sulfur, 2004.- large flows. small flows.

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