Sulphurous refers to containing sulphur

First gas mixture is processed into a second and third mixture; the second goes into fourth; and the fourth goes through the final separation step. So these are ordinal numbers or just labels for the processing steps.

cccccc
Generating hydrogen gas from a hydrocarbon gas mixture by means of a converting device has drawbacks if the hydrocarbon gas mixture comprises sulphur or sulphur compounds, since many of the usual catalysts in a converting device, for instance a steam reformer, are contaminated by sulphur.

The invention relates to a method for generating hydrogen gas from a sulphurous first gas mixture comprising at least a hydrocarbon gas, which method comprises the steps of
(i) providing the sulphurous first gas mixture,
(ii) separating the sulphurous first gas mixture into a desulphurized second gas mixture and a sulphurous third gas mixture,
(iii) guiding the second gas mixture separated in the second step (ii) to a converting device for converting the second gas mixture into a fourth gas mixture of substantially hydrogen gas and residual gases, and
(iv) separating the fourth gas mixture into hydrogen gas and residual gases.

https://patents.google.com/patent/EP3085662A1/en