Iron sponge process is economically applied for gases containing small concentration of hydrogen sulfide (H₂S), usually less than 300 ppm, operating from low to moderate pressure in a range of 3.45-34.5 barg (50-500 psig). Iron sponge process cannot be used to remove carbon dioxide.

Iron sponge process is the oldest and still the most widely used batch process for sweetening of natural gas and natural gas liquids. Overall, iron sponge process has the following characteristics which make it still attractive to be applied: simple process, low capital cost, and relatively low chemical (iron oxide) cost. Furthermore, pressure has a relatively little effect on its adsorptive capacity of a gas sweetening agent.

Principle of Iron Sponge Process

Iron sponge process use the principle of adsorption of the acid gas on the surface of the solid sweetening agent. Reaction of iron sponge and hydrogen sulfide produces iron sulfide. The reactions are:

Fe₂O₃ + 3 H₂S -> Fe₂S₃ + 3 H₂O

FeO + H₂S -> FeS + H₂O

Ferric oxide is impregnated on wood chips, which produce a solid bed with a large ferric oxide surface area. Several grades of treated wood chips are available come up with an iron oxide content. Available in 6.5, 9.0 15.0, and 20.0 pounds iron oxide per bushel. The chips are contained in a vessel, and sour gas flows through the bed and reacts with the ferric oxide.

Figure below shows a typical vessel for the iron sponge process.

Operating Condition of Iron Sponge Process

The reaction requires the presence of slightly alkaline water (pH 8-10) and temperature below 47^oC (110^oF). When the temperatures above 47^oC (110^oF), careful control of pH must be maintained. If the gas does not contain sufficient water, water may need to be injected into the inlet gas stream. The pH level can be maintained through the injection of caustic soda, soda ash, are ammonia with the water. pH control should be avoided whenever possible.

Although the present of free alkaline enhances H₂S removal it creates several side effects. The side effects include creates potential safety hazards, promotes formation of undesirable salts, and add to capital cost.

Regeneration of Iron Sponge Vessel

The regeneration step must be performed with great care because oxygen reaction is exothermic which is give 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. For this reason, spent wood chips should be kept moist when removed from the vessels. Otherwise, the reaction with oxygen in the air may ignite the chips and cause them to smolder.

The reactions for oxygen regeneration are as follows.

2Fe₂S₃ + 3O₂ + 2H₂O -> 2Fe₂O₃(H₂O) + 6S + Heat

4FeS + 3O₂ + 2xH₂O -> 2Fe₂O₃(H₂O)x + 4S + Heat

S₂ + 2O₂ -> 2SO₂

Some of the elemental sulfur produced in the regeneration step remains in the bed. After several cycles, the sulfur will form a cake over the ferric oxide therefore decreasing the reactivity of the bed. Typically after approximately 10 cycles, depending upon the sulfur content of the gas stream, the bed must be removed, and the new bed introduced into the reactor.

Continuous Regeneration of Iron Sponge

Iron sponge process is possible to be operated with continuous regeneration by the introduction of small amounts of air into the sour gas feed. The oxygen in the air regenerates the iron sulfide and produce elemental sulfur. Although continuous regeneration decreases the amount of operating labor, it is not as effective as batch regeneration. It may create an explosive mixture of air and natural gas. Due to the added cost associated with the air compressor, continues regeneration does not proved to be economic choice for typically small quantities of gas involved.

Approximately 90% of hydrogen can be removed per bed, but clogging by elemental sulfur occurs and the bed must be discarded and the use of several beds in series is not usually economical. Removal of larger amounts of hydrogen sulfide from the gas streams requires a continuous process, such as the Ferrox process or the Stretford process.

Advantages and Disadvantages of Iron Sponge Process

Iron sponge process offers several advantages, such as:

1. Providing complete removal of small to moderate concentrations of hydrogen sulfide without removing carbon dioxide
2. Requiring relatively small investments, for small to moderate gas volumes, compared with other processes
3. Equally effective at any operating pressures
4. Being used to remove mercaptan derivatives or convert them into disulfides

However, there are several disadvantages of iron sponge process:

1. It is a batch process, requiring duplicate installation or flow interruption of processed gas
2. It is prone to hydrate formation when operated at higher pressures and at temperatures in the hydrate forming range
3. The process will effectually remove ethyl mercaptan that has been added for odorization
4. Coating off the iron spoon with entrained oil or distillate will require more frequent change out of the sponge bed

References:

1. Arnold, Ken and Maurice Stewart, “Surface Production Operations Volume 2”, Gulf Publishing Company, 1989
2. Speight, James G., “Natural Gas A Basic Handbook”, Gulf Publishing Company, 2019
3. Stewart, Maurice and Ken Arnold, “Gas Sweetening and Processing Field Manual”, Gulf Publishing Company, 2011