Catalyst beds after-H2SO4-making

The most efficient double contact plants have one catalyst bed after H2SO4 making, remainder before. 3 - 1 plants are more efficient than 2-2 plants. 4 - 1 plants are more efficient than 2-3 and 3-2 plants. 

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

Fig. 20.3. SO2 oxidation efficiency of acid plants with 1 catalyst bed after intermediate H2SO4 making. Oxidation efficiency increases with increasing number of before-intermediate-H2S04-making beds. However, the difference between 3 -1 and 4 - 1 plants is very small.

Fig. 20.5. 3 - 1 acid plant with one Cs catalyst bed (660 K gas input) and three K, Na catalyst beds (690 and 720 K). Maximum SO2 oxidation is obtained with the Cs catalyst in bed 4, i.e. after H2SO4 making. Bed 3 Oust before H2SO4 making) is nearly as good. The calculations are all based on Table 20.1 s values - except for gas input temperature. 

However, Cs catalyst is costly, so many acid plants use it in rmly one catalyst bed. Figure 20.5 indicates that the best location for the Cs catalyst bed is after H2SO4 making where it gives maximum total SO2 oxidation. 

Catalyst beds before intermediate H2SO4 making + beds after intermediate H2SO4 making. 

The latter causes S02+ /202 — SO3 oxidation to go almost to completion in the after intermediate H2SO4 making catalyst bed, Chapter 19. 

Cs catalyst is costly so many acid plants use it in only one catalyst bed. From the SO2 oxidation efficiency point of view, it is best used after intermediate H2SO4 making. 

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