Liquid phase processes have been applied to cleaning gas streams other than coal gas. But in cases such as, gas streams from petroleum treatment, certain natural gas streams and coal gas from high sulphur coals it has become more usual to use either physical or chemical absorption processes followed firstly by desorption of the hydrogen sulphide and then by the gas phase Claus reaction.

Overall this reaction is:

2 H₂S + O₂==2S + H₂O

This reaction, in the presence of a catalyst was the original Claus reaction.

Later modifications used two stage reactions to allow operation under appropriate temperature conditions.

H₂S + 2O₂== SO₂ + 2H₂O and 2 H₂S + SO₂== 3S + 2 H₂O.

This can be achieved either as a two stage divided process as in the 1932 IG Claus Process or a two or three stage sequential process as in the 1936 Modified Claus Process.

When the Claus reaction is used in these circumstances the liquid phase purification processes used in coal gas sweetening are often used as a final polish to the gaseous effluents from Claus plants.

So what can we learn from this;

1. The first thing we need to consider is why we might use a chemical process rather than adsorption.

The main reason is that Henry's law limits adsorptive processes. The pollutant either dissolves in the scrubbing liquid or is sorbed onto the solid.

Eventually a concentration is reached at which the pollutant in solution is in equilibrium with the pollutant in the gas phase.

But- if we use a chemical reaction we can remove the pollutant by converting it to something else. Thus in the Stretford process we dissolve the hydrogen sulphide as the bisulphide. This would rapidly saturate the solution unless we can remove it. In our case we remove it sulphur, a valuable product.

2. We should also consider the economic and environmental consequences of the process. An iron oxide process will remove hydrogen sulphide from a gas stream very well but it cannot be used to recover sulphur in its most valuable form and it produces a solid waste, which is hard to get rid of. The Manchester process failed, not because it did not work or was not economic but because of the local environmental concerns arising from the odour problem.

3. The final lesson is that the choice of method depends on both the nature of the effluent or gas stream and the concentration of the pollutant. Each situation will require a different solution. As environmental managers we may not need to understand all the chemistry but we do need to have an appropriate conversation with the specialists.