耐磨球形Ni/Al_(2)O_(3)催化剂催化CO甲烷化反应的本征动力学

Intrinsic kinetics of CO methanation over spherical Ni/Al_(2)O_(3) catalyst with high attrition resistance

一氧化碳和氢气在催化剂作用下合成甲烷,常应用于煤制合成天然气、焦炉煤气制液化天然气和生物质合成气等领域,有利于我国能源结构优化。针对CO甲烷化的快速表面反应、强放热特性,相比固定床,采用微球型催化剂的流化床甲烷化技术在移热和催化性能方面具有明显的技术优势。在前期开发的耐磨球形Ni/Al_(2)O_(3)催化剂的基础上,采用常压微分固定床反应器对其催化CO甲烷化反应本征动力学进行了研究,以揭示反应机理和路径。在消除内外扩散的前提下,通过控制催化剂装填量和反应气体的量,将出口CO转化率控制在15%以下,从而获得不同CO/H_(2)比和不同反应温度下的CH_(4)生成速率,采用幂指数型动力学模型和双曲线型动力学模型分别进行数据拟合。基于幂指数型动力学模型计算的动力学参数结果表明,随着反应温度从260℃升高至350℃,CO甲烷化反应活化能从145.99 kJ/mol逐渐降至123.54 kJ/mol,CO的反应级数由-1.22增加至0.34,H_(2)的反应级数由0.31增加至2.28。为了进一步分析反应机理,根据不同温度下CO和H2浓度对反应速率影响程度不同,基于双曲线型动力学模型假设不同温度区间内的速控步骤并根据实验结果推导出相应的双曲线型反应速率方程,发现260~280℃下H_(2)的解离、280~310℃下CO的加氢解离、310~350℃下碳中间体的进一步加氢分别为速控步骤时,R2大于0.99,表明甲烷化反应速度控制步骤随温度变化而发生改变。

Carbon monoxide and hydrogen can be used to synthesize methane with catalyst application and this reaction is used in the field of coal to substitute natural gas,coke oven gas to liquid natural gas,and biomass synthesis gas to optimize the energy structure in China.Compared with fixed bed,the fluidized bed methanation technology adopting spherical catalyst with small diameters shows obvious superiority in activity and heat transfer efficiency due to the fast surface reaction characteristics of highly exothermic CO methanation.Based on the developed spherical Ni/Al_(2)O_(3) catalyst with high attrition resistance,the intrinsic kinetics of CO methanation were tested in a differential fixed bed reactor under atmospheric pressure to reveal the reaction mechanism and route.The formation rates of CH_(4) at different ratios of CO/H_(2)and reaction temperatures were calculated on the premise of eliminating internal and external diffusion and controlling the CO conversion to less than 15% by decreasing the catalyst amount or increasing the gas feed rate.Then data fitting was conducted based on the power kinetic model and hyperbolic kinetic model,respectively.The results based on the power dynamics model showed that with the increase of reaction temperature from 260℃to 350C,the activation energy gradually decreased from 145.99 kJ/mol to 123.54 kJ/mol,the reaction order of CO changed from-1.22 to 0.34,and the reaction order of H,increased from 0.31 to 2.28.To further analyze the methanation mechanisms,the rate-determining steps were assumed based on the hyperbolic dynamics model according to the effect of CO and H,concentration on the reaction rate at different temperature ranges.As the rate determining step at 260~280℃ was assumed to be H,dissociation,at 280~310℃ was the hydrogenation of CO and at 310~350℃ was the hydrogenation of carbon intermediate.The R^(2)of the hyperbolic rate equations obtained were all greater than 0.99,which indicated that the rate determining step of methanation would change with the temperature.