Acid Gas Removal (Part 2: Iron Sponge)

Below is explanation about Iron Sponge:

• Use chemical reaction: 2 Fe2O3 (ferric oxide) + 6 H2S ---> 2 Fe2S3 + 6 H2O
• Applied to gas with low H2S concentration (300 ppm). Carbon dioxide is not removed by this process Operating at low to moderate pressure (50-500 psig) 
• Temperature below 110 F 
• Need alkaline water with pH level 8-10. Injection of caustic soda with water into inlet gas stream is needed if the water is not sufficient 
• Ferric oxide is impregnated on wood chips to produce large surface area. Common grades based on iron oxide content are: 6.5, 9.0, 15.0, 20 lb iron oxide/bushel 
• Gas scrubber or filter separator on upstream of iron sponge unit is needed to minimize amount of hydrocarbon liquids condense to bed. Hydrocarbon liquids can coat iron sponge and inhibiting the reaction. The scrubber operates at lower temperature or higher pressure than iron sponge unit, to prevent possibility hydrocarbon liquids condensing in iron sponge unit. 
• Normally operated in batch mode without regeneration due to difficulty of controlling regeneration step, the eventual coating of the bed with elemental sulfur, the low cost of iron material, and the possibility of hydrocarbon liquids coating the bed. 
• The spent bed is removed from the unit and trucked to disposal site. It is replaced with a new bed and the unit put back in service.

Amine Treating Unit will be explained in next post :)

Reference:
DMAN-TPE-ENGPRO-008 Acid Gas Treating Design Manual