Study On The Chemical Attrition Of Fe-based Fischer-Tropsch Catalyst

Fisher-Tropsch synthesis,by which CO and H2 can be transformed into long chain hydrocarbon and value-added chemicals,was considered as an important way for coal,natural gas and biomass utilization.Iron-based Fischer-Tropsch catalysts have been found widely industry application due to their lower cost,higher activity,moderate operating temperature and flexible products spectrum compared to cobalt catalysts.The slurry-bed reactor has long been used in industry owing to its efficient reaction heat removal,low pressure drop and convenient online catalyst replacement.However,the catalysts used in the Slurry-bed Reactor should be highly attrition resistive since the attrition of catalyst would lead to the loss of catalyst,reduce the catalyst activity and stability,and enhance the selectivity of low carbon hydrocarbon.Besides,the attrition of catalyst would lead to the blockage of filter,which would make the online separation of catalyst and wax more difficult.Therefore,research and development of high attrition resistance iron-based catalysts are vital for efficient Fischer-Tropsch synthesis process.The attrition behavior of iron-based catalysts during reaction was mainly determined by the chemical attrition which was generally referred to the strength variation caused by structural evolution.The effect of different pretreatment protocols on the attrition resistance of the catalyst was extensively studied in present thesis.Specifically,the effect of the pretreatment conditions on the phase structure and texture properties of the catalysts was examined by a combination of characterization methods.Then,the intrinsic cause of chemical attrition of catalysts was proposed,which would help the development of new generation iron-based catalysts with higher attrition resistance and also optimized catalytic performance.The main work of this thesis and related conclusions were summarized below.In the case of fresh model samples with high attrition resistance,the carburization rate and carbon deposition rate increased with the increase of carburization time when low temperature and low space velocity were employed,while the macro size of catalysts decreased,the specific surface area,pore volume,and pore size reduced.One can find the attrition resistance of catalysts increased.Similarly,the increase of carburization temperature led to the increased carbide content and carbon deposition content,meanwhile,the macro size of catalysts decreased and attrition resistance increased.In the case of fresh model samples with low attrition resistance,the increase of space velocity and reaction time made the carbide content increased to a certain level and then kept constant,while the degree of carbon deposition and catalyst particle size kept increasing.One can also find the attrition resistance of catalyst increased first and then decreased.Based on the results obtained in present study,the picture of attrition resistance variation caused by pretreatment could be tentatively drawn.In the initial stage of pretreatment,the carburization process of catalysts lead to the volume shrinkage,catalyst particle size decreasing and finally the attrition resistance increasing.While in the late stage of pretreatment,where carbon deposition process dominated,the catalyst volume expansion,particle size increasing could be observed,which caused the attrition resistance decreasing.