The iron sponge process uses the chemical reaction of ferric oxide with H2S to sweeten gas streams. This process is applied to gases with low H2S concentrations (300 ppm) operating at low to moderate pressures (50-500 psig). Carbon dioxide is not removed by this process.
The reaction of H2S and ferric oxide produces water and ferric sulfide as follows:
The reaction requires the presence of slightly alkaline water and a temperature below 110°F. If the gas does not contain sufficient water vapor, water may need to be injected into the inlet gas stream. Additionally, bed alkalinity should be checked daily. A pH level of 8-10 should be maintained through the injection of caustic soda with the water.
The ferric oxide is impregnated on wood chips, which produces a solid bed with a large ferric oxide surface area. Several grades of treated wood chips are available, based on iron oxide content. The most common grades are 6.5-, 9.0-, 15.0-, and 20-lb iron oxide/bushel. The chips are contained in a vessel, and sour gas flows through the bed and reacts with the ferric oxide. Figure 7-3 shows a typical vessel for the iron sponge process.
The ferric sulfide can be oxidized with air to produce sulfur and regenerate the ferric oxide. The reaction for ferric oxide regeneration is as follows:
The regeneration step must be performed with great care as the reaction with oxygen is exothermic (that is, gives off heat). Air must be introduced slowly so the heat of reaction can be dissipated. If air is introduced quickly the heat of reaction may ignite the bed.
Some of the elemental sulfur produced in the regeneration step remains in the bed. After several cycles this sulfur will cake over the ferric oxide, decreasing the reactivity of the bed. Typically, after 10 cycles
the bed must be removed from the vessel and replaced with a new bed.
In some designs the iron sponge may be operated with continuous regeneration by injecting a small amount of air into the sour gas feed. The air regenerates ferric sulfide while H2S is being removed by ferric oxide. This process is not as effective at regenerating the bed as the batch process. It requires a higher pressure air stream, and if not properly controlled may create an explosive mixture of air and gas.
Hydrocarbon liquids in the gas tend to coat the iron sponge media, inhibiting the reactions. The use of an adequately designed gas scrubber or filter separator upstream of the iron sponge unit will minimize the amount of liquids that condense on the bed. Sometimes the process can be arranged so that the scrubber operates at a lower temperature or higher pressure than the iron sponge unit, so that there is no possibility of hydrocarbon liquids condensing in the iron sponge unit.
Due to the difficulty of controlling the regeneration step, the eventual coating of the bed with elemental sulfur, the low cost of iron sponge material, and the possibility of hydrocarbon liquids coating the bed, iron sponge units are normally operated in the batch mode. The spent bed is removed from the unit and trucked to a disposal site. It is replaced with a new bed and the unit put back in service. The spent bed will react with the oxygen in air as shown in Equations 7-2 and 7-3 unless it is kept moist. In areas where iron sponge units are installed, service companies exist that can replace iron sponge beds and properly dispose of the waste material.