Preheat temperatures for heavy feedstocks are limited by the potential for cracking. The heavier the hydrocarbon, the lower the temperature at which thermal cracking will take place, and carbon deposits may be formed. As pre-reforming converts heavier hydrocarbons to methane, a higher level of preheat may be reached without the risk of cracking. When feeds containing significant quantities of higher hydrocarbons, such as liquefied petroleum gas (LPG) and naphtha, are pre-reformed, the overall reaction is exothermic, and the temperature of the process stream rises overall. With the heaviest feeds, such as naphtha, there can be an initial drop in temperature near the start of the bed. This is due to the initial dominance of the endothermic steam reforming reactions. “ The lifetime of pre-reforming catalysts is normally defined by when higher hydrocarbon slip starts to occur or when the pressure drop builds to a point where it is unsatisfactory ” The temperature profile also changes with operating rates and catalyst age. At low operating rates, the reaction profile will be sharp and complete within a small fraction of the bed, whereas at higher operating rates, the reaction profile is extended deeper into the bed. As the catalyst ages, an inactive section at the top of the bed grows due to the effects of catalyst poisoning, and loss of activity through sintering also lengthens the reaction profile. Pre-reforming catalysts Nickel is almost always the metal of choice for pre-reforming because of its relative cost, high activity, and stability in the metallic state at typical steam-to-hydrogen partial pressure ratios. The active metals are typically dispersed on alumina or alkaline earth metal aluminate- based supports, which offer sufficient chemical and thermal stability under reaction conditions. The support provides a structure to assist with the dispersion of the active metal to give stable metal crystallites, resulting in stable metal crystallites and a large active metal surface area.
While the intrinsic kinetics of the catalyst are important, the observed rate of reaction per unit volume in pre-reforming is mass transfer-limited, as the rate of pore diffusion is slow compared to the rates of the reactions. This means that much of the conversion takes place near the surface of the pellet, and the catalyst’s pore structure has a significant influence on observed activity. A higher rate of reaction per unit volume of catalysts bed can therefore often be achieved by using smaller pellets. Pellet size is thus a trade-off between activity and the increased pressure drop over the catalyst bed, which results from a decrease in pellet size. The lifetime of pre-reforming catalysts is normally defined by when higher hydrocarbon slip starts to occur or when the pressure drop builds to a point where it is unsatisfactory. Poisoning and thermal sintering are more commonly life-limiting when natural gas is the chosen plant feedstock, whereas for feedstocks containing notable quantities of C 3 +, such as LPG and naphtha, carbon laydown can also be a limiting factor. Catalyst poisoning Poisoning of the catalyst is the most common deactivation mechanism for pre-reformers and generally defines the usable life of the catalyst. As the top of the bed is deactivated through poisoning, the reaction profile is pushed further down the bed and eventually heavier hydrocarbons slip through. There are many chemical species that are capable of poisoning pre-reforming catalysts. These include compounds of sulphur, halides, alkali metals, heavy metals, and silica. The species that cause the biggest problem are sulphur compounds, which are present in all fossil fuel feedstocks in varying quantities. Hydrodesulphurisation (HDS) followed by zinc oxide for hydrogen sulphide removal is the process normally used upstream of the pre-reformer to remove the vast majority of these sulphur species in order to protect the pre-reformer and other catalysts further downstream. For a higher level of purification, an active metal-promoted ultra-purification absorbent may be used.
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