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Hydrogen Production
Any sulfur components are catalyst poisons for Ni catalyst of steam reforming. When organic sulfurs such as COS and mercaptans are contaminated, feed NG is hydrotreated with Co-Mo or Ni-Mo catalyst at first in order to convert such small amount of organic sulfurs to H2S as Reactions (H1) and (H2) below prior to desulfurization with Zn catalyst to convert H2S to ZnS as Reactions (H3) . H2 to Reaction (H1) is usually taken and recycled from a part of the product H2. The desulfurization temperature is about 350-380 ÌŠC. H2 concentration for hydrotreating may be about 2 mol% and, if considerablly higher carbon oxides are contained in the feed NG, Ni-Mo catalyst is used to avoid methanation Reaction (A1) and (A2) since Ni will promote the reverse reaction of methanation, i.e. steam reforming. ZnS is steady solid at ambient temperature and treated easily for disposal.
Hydrotreating, where R is alkyl group
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RSH + H2 = RH + H2S (H1)
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COS + H2 = CO + H2S (H2)
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Desulfurization
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H2S + ZnO = H2O + ZnS (H3)
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Desulfurized NG is conveyed to steam reformer for production of syngas in accordance with Reaction (1), (3) and (4). To promote the conversion of hydrocarbons to H2 as high as possible, the process temperature of reformer exit is kept very high because of endothermic behavior of the steam reforming reaction. Higher steam to carbon ratio (S/C) is used for the purpose avoiding carbon formation, but it also plays a role of providing with driving force toward higher conversion to H2 in the steam reforming reactions. The residue methane at the exit of steam reformer is further converted in shift converter(s) operated under the temperature range of 200-500 ÌŠC because of exothermic reaction of CO shift reaction. Finally, the H2 rich stream is purified by 8-10 stages of PSA for the desired product. The PSA off gas contains CO, CO2 and some loss of H2 and would be used as fuel of the steam reformer. See the figure below.
