Thermodynamic Analysis And Process Technol-Economic Evaluation Of Direct CO₂ Methylation To PX

The concentration of carbon dioxide（CO₂）in the atmosphere is increasing year by year,leading to more serious problems such as global warming and ocean acidification.It is urgent to develop efficient CO₂ capture and utilization（CCUS）technologies,and one of the most popular CCUS technologies is CO₂ conversion and utilization using hydrogen generated from renewable energy sources.The direct synthesis of high-value chemicals such as paraxylene（PX）based on a bifunctional catalyst coupling CO₂ hydrogenation and toluene methylation processes has also shown great potential.This route combines the atomic economy effects of CO₂ hydrogenation and toluene methylation for PX preparation,green hydrogen storage and CO₂utilization,and also has important socio-economic significance for cross-fertilization and comprehensive resource allocation in petrochemical,coal chemical and new energy development industries.To address the issues of conversion potential and optimal conversion conditions for direct CO₂ methylation,this paper calculates and analyzes the thermodynamic trends and product distributions of various reactions based on the reaction network and based on the reaction thermodynamic equilibrium and Gibbs free energy minimization methods.The results show that the feedstock conversion and product selectivity of direct CO₂ methylation are higher at thermodynamic conditions of 360～420℃,3MPa,CO₂:C₇H₈=1:1～1:4 and CO₂:H₂=1:3～1:6.Further,around both direct and indirect CO₂methylation routes,the thermodynamic and catalytic experimental results were compared and analyzed,and it was found that the toluene co-feed would break the thermodynamic limit of pure CO₂ hydrogenation conversion and have a higher potential of>60%CO₂ conversion;the direct CO₂ methylation route also has an advantage over the methanol methylation route with a shortened reaction path and PX selectivity in xylene isomers>90%.Aiming at the techno-economic potential of direct CO₂ methylation,a production process for direct CO₂ methylation to PX was constructed based on the catalytic experimental results of three Zn Zr OX/HZSM-5 series bifunctional catalysts with high feedstock conversion,high methylation rate and high PX selectivity,and the process was simulated and techno-economically evaluated.Among them,the production process based on 70.8%PX selective catalyst has lower separation demand,and has better economics with less raw material consumption and lower energy consumption than the two production processes with 25%toluene conversion and 92.4%xylene selective catalyst,and the total utility load is reduced by 37.56%and 49.57%,respectively;and the total cost is saved by 29.73%and 36.15%,respectively.In order to further evaluate and enhance the techno-economic potential of direct CO₂ methylation,the production process of direct CO₂ methylation to PX was optimized based on the thermodynamic equilibrium conversion results.Based on the thermodynamic equilibrium process,the feedstock such as CO₂ was converted to a greater degree of reaction,and the toluene conversion rate reached 75.78%,and the methylation utilization rate of C element in CO₂ reached 95.76%,which was much higher than the catalytic experimental results of 25%toluene conversion rate.However,comparing the process results of the highly selective catalytic experiments,it was found that the selectivity of xylene in the methylation product decreased to 68.62%and the yield of heavy aromatics increased,which increased the separation energy consumption and reduced the overall process technical economy.Therefore,enhancing the feedstock conversion potential and maintaining the high selectivity of PX is the key to the technology development of direct CO₂ methylation process.