The iron sponge process (also called the dry box process), which is the oldest and still the most widely used batch process for sweetening of natural gas and natural gas liquids.
In the iron sponge for h2s removal process , the sour gas should pass down through the bed. In the case where continuous regeneration is to be utilized, a small concentration of air is added to the sour gas before it is processed.
This air serves to continuously regenerate the iron oxide, which has reacted with hydrogen sulfide, which serves to extend the on-stream life of a given tower but probably serves to decrease the total amount of sulfur that a given weight of bed will remove.
The number of vessels containing iron oxide can vary from one to four. In a two-vessel process, one of the vessels would be on stream removing hydrogen sulfide from the sour gas, while the second vessel would either be in the regeneration cycle or having the iron sponge bed replaced.
When periodic regeneration is used a tower is operated until the bed is saturated with sulfur and hydrogen sulfide begins to appear in the sweetened gas stream. At this point the vessel is removed from service and air is circulated through the bed to regenerate the iron oxide.