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Iron sponge process uses a single vessel to contain the hydrated ferric oxide wood shavings.
Inlet gas line should have taps for
Sampling for sulfide
Temperature measurement
Pressure gauge
Injection nozzle for methanol, water or inhibitors
Gas is carried into the top section of the vessel through an inlet nozzle approximately 12 in (0.3 m) above the sponge bed.
Provides uniform flow through the bed, thus minimizing the potential for channeling.
Iron sponge chips are supported by a perforated heavy metal support plate and a coarse support packing material.
This material may consist of scrap pipe thread protectors and 2–3 in (50–75 mm) sections of small diameter pipe.
This provides support for the bed, while offering some protection against detrimental pressure surges.
Gas exits the vessel at the bottom through the vessel side-wall.
This arrangement minimizes entrainment of fines.
Exit line should have a pressure tap and sample test tap.
Access to the vessel is provided through man ways in the top head and bottom head or side of a larger diameter vessel (diameters greater than 36 in (0.92 m) ID).
For small diameter vessels, flanged top and bottom heads offer a viable solution.
Vessel is generally constructed of carbon steel which has been heat treated.
Control of hardness is required because of the potential for sulfide stress cracking.
Vessel is either internally coated, lined, or clad with stainless steel.
Internal coating is normally used for vessels operating below 300 psig (2070 kPa).
Cladding is used for higher operating pressures.
In internally coated units, care must be exercised when setting the coarse packing in order to prevent damage to the coating.
Gas Sweetening
Maurice I. StewartJr. PhD, PE, in Surface Production Operations (Third Edition), Volume 2, 2014
9.4.2.1 Application
The iron sponge process is economically applied to gases containing small amounts of H2S (< 300 ppm) operating at low to moderate pressures in the range of 50-500 psig (344.7-3447 kPa). This process does not remove CO2.
The reaction requires the presence of slightly alkaline water (pH 8-10) and a temperature below 110 °F (47 °C). When temperatures exceed 110 °F (47 °C), careful control of pH must be maintained. If the gas does not contain sufficient water vapor, 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, lime, or ammonia with the water. pH control should be avoided whenever possible.
Although the presence of free alkalines enhances H2S removal, it also creates potential safety hazards and promotes the formation of undesirable salts, adding to capital costs.
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, based on iron oxide content. Ferric oxide wood chips are available in 6.5, 9.0, 15.0, and 20 lbs iron oxide/bushel. Chips are contained in a vessel, and sour gas flows downward through the bed and reacts with the ferric oxide.
Figure 9.2 shows a vertical vessel used in the iron sponge process.
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