Issues related to abnormal conditions in hydrogen production
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Experts: There are some issues that require your clarification. 1. Regarding the reduction of the conversion catalyst in hydrogen production, can natural gas be fed in once steam is supplied? Then, a large cycle is carried out: hydrogen is introduced after PSA, and an wait is done until the hydrogen purity reaches 70% before proceeding with the reduction process? Do I need to bring both the hydrogenation and desulfurization reactors? 2. During the shutdown process, it is said that this should be done in a reducing atmosphere; if there is no hydrogen source available, hydrogen from the PSA adsorption tower can be used. Personally, I think this applies to installations with a hydrogen production capacity of 10,000 standard cubic feet or less If there is not enough PSA hydrogen, what impact will failing to shut down under a reducing atmosphere have on the next start-up? Will it be fine to maintain the reduction reaction for 12 hours during the next operation? If there is a hydrogen source, at what temperature should the hydrogenation reactor be shut down?
1. Regarding the catalyst reduction method for natural gas reforming hydrogen production plants, larger-scale plants typically use a nitrogen-hydrogen circulation reduction method; for example, in plants with a capacity of over 10,000 Nm³, a large amount of catalyst is used, and nitrogen-hydrogen reduction helps to extend the catalyst’s active life while reducing the operating costs of the plant. Therefore, externally supplied hydrogen is used. Due to the operating conditions imposed by the hydrodesulfurization catalyst, the hydrogen content in the reduced recycle gas will not be very high (specific parameters should be obtained from the operating conditions of the hydrodesulfurization catalyst); otherwise, the fluidized hydrodesulfurization catalyst will be reduced to elemental cobalt and molybdenum, losing its activity. In normal circumstances, system restoration involves operating the desulfurization system together while heating up, so that once the catalyst has been reduced, normal production can commence immediately. For smaller scales, direct reduction with natural gas is used. 2. During the shutdown of the hydrogen production plant, the conversion catalyst must be kept in a hydrogen atmosphere; there is a high-temperature phase during which a hydrogen-reducing atmosphere is required, while in the low-temperature phase an oxidizing steam atmosphere can be used. Regarding shutdown operations, these include cooling down, reducing the load, and increasing the water-to-carbon ratio; the cooling rate should follow the parameters specified by the catalyst manufacturer, with the water-to-carbon ratio ranging from 15 to 20. During shutdown, the gas must be vented (through pre-PSA venting or recovery), and the temperature at which the hydrogen atmosphere is removed is typically around 550 degrees.