Double absorption contact process

Efficiency of conversion of sulphur dioxide to sulphuric acid can be increased to 99.6% by using two absorption stages. The effect of removing sulphur trioxide product at the first absorption stage is to push the reaction equilibrium of the remaining sulphur dioxide and oxygen in the direction of more product.

diagram: double absorption of sulphur trioxide in the contact process

The first absorption occurs between the third and fourth bed, and second absorption after the final bed. The final bed may include a catalyst promoter, usually caesium, which makes the catalyst more effective. For more information on the role of promoters, see the Catalysis-ed website.

Reduced emissions

One of the important advantages of using two stages of absorption is that the remaining sulphur dioxide needing to be removed from the tail gas is much reduced.


The conversion of sulphur dioxide to sulphur trioxide is exothermic, releasing heat to the surroundings, so the temperature increases after each pass over a catalyst bed. However, after each successive bed there is less SO2 available for conversion in the reaction gas mixture, and so less heat released after each successive catalyst bed.

The temperature must not be allowed to rise, as this will push the equilibrium in the wrong direction.



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