Effect of Process Parameters on Methanol to Olefins Reactions over SAPO Catalysts
Type of DegreeThesis
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Acidic catalysts such as silicoaluminophosphate oxides (SAPO) have been known to play an essential role in olefins synthesis reactions. Because of their unique selectivity towards light olefins, these SAPO catalysts offer a promising alternative as reaction media in catalytic reactions based on the abundant supply of natural gas. In this thesis several studies were undertaken in order to examine the effects of process parameters on the methanol to olefins reactions (MTO) with particular emphasis on catalytic activity, lifetime, and product selectivity. All of the MTO reactions were performed under atmospheric pressure with catalyst bed temperature maintained at 400 oC in a continuous fixed bed reactor by employing small pore SAPO catalysts: SAPO-34, SAPO-44, SAPO-47, and SAPO-56 (0.3 ~0.9 Si). In order to judiciously make good use of the limited catalyst stocks, a scale-down analysis was performed by reducing all reaction parameters by 40%, i.e. catalyst load, methanol feed, and nitrogen flow. Product distribution profiles between before and after parameter reductions indicated almost identical catalytic activities with minor variations. A significant portion of this thesis was devoted to the particle size effects on the reaction behavior. The results from the ground particles demonstrated all catalysts maintained their stability. With the exception of SAPO-56 which showed no apparent activity difference for reduced particles, all SAPO’s tested showed enhanced catalytic activity and lifetime. Improvements in C2 to C4 olefins selectivity and methanol conversion were observed as well. These same effects were also apparent upon temperature variation (300 oC to 500 oC). Effect of silicon content over SAPO-56 prepared with three different silicon amounts (0.3, 0.6, and 0.9 Si) indicated optimum olefins selectivity at 0.6 Si. Also, a spent catalyst was ground and tested for activity. From that reaction, only dimethyl ether (DME) and methanol (MeOH) were present in the product stream, indicating that all of the particle active sites have been covered with coke deposits. A thorough examination of all reactions over ground particles pointed to increased DME yields after catalyst deactivation began, as compared to before grinding. Reactions over catalysts modified with Ru only showed slight improvements when compared with the unmodified catalysts. Modification with metals such as palladium (Pd) and platinum (Pt) would be interesting.