Deactivation of Catalytic Membrane Reactors

The nature of many high-temperature hydrocarbon reactions which potentially can benefit from inorganic membrane reactors (particularly catalytic membrane reactors) is such that the catalysts or the catalytic membranes are subject to poisoning over time. Deactivation and regeneration of many catalysts in the form of pellets are well known, but the same issues related to either catalyst-impregnated membranes or inherently catalytic membranes are new to industrial practitioners. They are addressed in this section. 

Poisoning. Both the membrane and the catalyst in a membrane reactor may become deactivated over time in the application environment. This poisoning arises from some species present in the feed stream or from some product(s) of the reaction. When the poison is present in the feed stream at a relatively high concentration and is weakly adsorbed onto the catalyst or membrane surface or when the poison is formed by reaction, it is uniformly distributed throughout the catalyst. On the other hand, if the poison is present in the feed stream in a relatively low concentration and is strongly adsorbed, the outer pore surfaces can completely lose catalytic activity before the inside pore surfaces do. When significant deactivation of either or both occurs, effective 

A catalyst can lose its activity or selectivity in many ways poisoning, fouling, loss of some active area or species [Satterfield, 1980]. For metal catalysts, poisoning and reduction of the active area are predominant Most of poisoning takes place when some impurity in the process stream adsorbs on the active sites of the catalyst and causes the catalyst activity to be lowered. Different compounds, sometimes even in trace quantity, can become poisons to different catalysts. 

Coke deposits. A special type of poisoning in many hydrocarbon reactions (e.g., catalytic cracking, dehydrogenations and reforming) is the formation of coke or carbonaceous deposits as a result of reaction on catalyst particles or membranes or both, depending on the reactor configuration. Carbonaceous or coke deposits block active sites of a catalyst or catalytic membrane and ultimately leads to deactivation of the catalyst or the membrane. 

The nature of the these deposits depends on the reactions and conditions under which they are formed. When formed at relatively low temperatures, say below 300 or 400 C, the deposits appear to be some form of high molecular weight polymers while a substantial amount of those formed above 400 SOO C show a pseudographite form and may be represented empirically as CH where x lies between 0.5 and 1.0 [Satterfield, 1980]. In several hydrogenation reactions, hydrogen used comes from carbonaceous sources and the resultant carbon monoxide may be decomposed to form coke. Coke or carbonaceous deposits can also form on catalysts or membranes in a reducing chemical environment as a result of the decomposition of methane or other reactions. 

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