Design Procedures For Iron Sponge Unit
The iron-sponge process generally uses a single vessel to contain the hydrated ferric oxide wood shavings. A drawing of an iron-sponge unit showing typical provisions for internal and external design requirements was presented in Figure 7-3.
The inlet gas line should have taps for gas sampling, temperature measurement, pressure measurement, and for an injection nozzle for methanol, water, or inhibitors. The gas is carried into the top section of the vessel in a distributor and discharged upward. This causes the gas to reverse flow downward and provides for more uniform flow through the bed, minimizing the potential for channeling.
Supporting the hydrated ferric oxide chips is a combination of 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. 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 sidewall. This arrangement minimizes entrainment of fines. Additionally, a cone strainer should be included in the exit line. This line should also have a pressure tap and sample test tap.
The vessel is generally constructed of carbon steel that has been heat treated. Control of metal hardness is required because of the potential of sulfide-stress cracking. The iron-sponge vessel is either internally coated or clad with stainless steel.
The superficial gas velocity (that is, gas flow rate divided by vessel cross-sectional area) through the iron-sponge bed is normally limited to a maximum of 10 ft/min at actual flow conditions to promote proper contact with the bed and to guard against excessive pressure drop. Thus, the vessel minimum diameter is given by:
A maximum rate of deposition of 15 grains of H2S/min/ft2 of bed cross-sectional area is also recommended to allow for the dissipation of the heat of reaction. This requirement also establishes a minimum required diameter, which is given by:
The larger of the diameters calculated by Equation 7-18 or 7-19 will set the minimum vessel diameter. Any choice of diameter equal to or larger than this diameter will be an acceptable choice.
At very low superficial gas velocities (less than 2 ft/min) channeling of the gas through the bed may occur. Thus, it is preferred to limit the vessel diameter to:
where dmax = maximum recommended vessel diameter, in.
A contact time of 60 seconds is considered a minimum in choosing a bed volume. A larger volume may be considered, as it will extend the bed life and thus extend the cycle time between bed change outs. Assuming a minimum contact time of 60 seconds, any combination of vessel diameter and bed height that satisfies the following is acceptable:
In selecting acceptable combination, the bed height should be at least 10 ft for H2S removal and 20 ft for mercaptan removal. This height will produce sufficient pressure drop to assure proper flow distribution over the entire cross-section. Thus, the correct vessel size will be one that has a bed height of at least 10 ft (20 ft if mercaptans must be removed) and a vessel diameter between dmin and dmax.
Iron sponge is normally sold in the U.S. by the bushel. The volume in bushels can be determined from the following equation once the bed dimensions of diameter and height are known:
where Bu = volume, bushels
The amount of iron oxide that is impregnated on the wood chips is normally specified in units of pounds of iron oxide (Fe2O3) per bushel. Common grades are 9, 15 or 20 Ib Fe2O3/bushel.
Bed life for the iron sponge between change outs is determined from:
The iron-sponge material is normally specified to have a size distribution with 0% retained on 16 mesh, 80% between 30 and 60 mesh, and 100% retained on 325 mesh. It is purchased with a moisture content of 20% by weight and buffering to meet a flood pH of 10. Because it is necessary to maintain a moist alkaline condition, provisions should be included in the design to add water and caustic.
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