Industrial SO2 Oxidation

SO2 oxidation requires O2. Sulfur burning furnace offgas already contains O2, Table 7.1. None needs to be added. Metallurgical and waste acid decomposition furnace offgases often contain little O2, so some must be added before catalytic SO2 oxidation. It is added in air or ventilation gas before gas dehydration. 

Volume% 02/volume% SO2 ratios in industrial catalytic oxidation feed gas are in the range of 1 to 2, Table 7.2. This is 2 to 4 times the stoichiometric S02+ /202- SO3 requirement of 0.5 moles O2 per mole of SO2. The excess O2 promotes rapid SO2 oxidation. 

Gas source and composition, volume% Sulfur burning furnace Metallurgical furnaces Waste acid decomposition furnace 

Prior treatment usually none, O2 gas cleaning and gas cleaning and 

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O2 does not oxidize SO2 to SO3 without a catalyst. All industrial SO2 oxidation is done by sending SO2 bearing gas down through beds of catalyst. Fig. 1-2. The reaction is ... 

Fig. 7.1. Bed of catalyst pieces for oxidizing SO2 to SO3. It is circular, 7 to 17 m diameter. Industrial SO2 oxidation is done in a converter of 3 to 5 such beds, Figs. 7.6 and 7.7. Downward gas flows are 25 NmVminute per m of top surface. Active catalyst consists of a molten V, K, Na, Cs, S, O phase supported on a solid porous silica substrate. Chapter 8. A top layer of silica rock holds the catalyst in place. A bottom layer prevents the catalyst from sticking to the stainless steel support grid.

All industrial SO2 oxidation is done in beds of catalyst. Sander et al (1984) explain the need for catalyst as follows ... 

Industrial SO2 oxidation is done in a sequence of 3 to 5 catalyst beds. Figs. 7.6 and 7.7. This section and Fig. 7.8 describe passage of warm feed gas through three catalyst beds with gas cooling between. The sequence is ... 

The chapter concludes that maximum industrial SO2 oxidation is achieved when Reaction (1.1) ... 

All industrial SO2 oxidation is done in contact with V, alkali metal, S, O, Si02 catalyst. Chapters 7 and 8. 

In fact, the only truly adjustable parameters in industrial SO2 oxidation are catalyst bed... 

Industrial SO2 oxidation efficiencies are slightly lower than those in this chapter -because equilibrium is not quite attained in industrial processes. However, the trends in the chapter are instructive as to best double contact practice. 

Figure 29.1 shows an example of industrial SO2 oxidation for a three catalyst bed acid plant. It shows heatup paths that do not intersect the equilibrium curve as in Fig. 18.2. Slightly higher SO2 emissions from the acid plant occur because equilibrium SO2 oxidation is not reached in each catalyst bed.

Table 29.1 compares equilibrium SO2 oxidation with industrial SO2 oxidation for a three pass contact type acid plant. In this example, the industrial acid plant s overall SO2 oxidation is 1 % less than equilibrium, but the tail gas SO2 concentration is 50% higher. This is important because regulatory tail gas SO2 emission limits are often concentration based. 

Section 19.8 showed that removing SO3 and sending the gas through one additional catalyst bed increase SO2 oxidation. Maximum industrial SO2 oxidation for a four bed single contact acid plant is 99%. Greater than 99% SO2 oxidation often requires double contact or scrubbing of single contact acid plant tail gas (Fig. 29.2). 

These five equations assume ideal gas behavior (based on the low pressure, 1 bar, of industrial SO2 oxidation). 

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