Reaction-transport simulations of non-oxidative methane conversion with continuous hydrogen removal - homogeneous-heterogeneous reaction pathways

Detailed kinetic-transport models were used to explore thermodynamic and ki netic barriers in the non-oxidative conversion of CH4 via homogeneous and h omogeneous-heterogeneous pathways and the effects of continuous hydrogen re moval and of catalytic sites on attainable yields of useful C-2-C-10 produc ts. The homogeneous kinetic model combines separately developed models for low-conversion pyrolysis and for chain growth to form large aromatics and c arbon. The H-2 formed in the reaction decreases CH4 pyrolysis rates and equ ilibrium conversions and it favors the formation of lighter products. The r emoval of H-2 along tubular reactors with permeable walls increases reactio n rates and equilibrium CH4 conversions. C-2-C-10 yields reach values great er than 90% at intermediate values of dimensionless transport rates (delta = 1-10), defined as the ratio hydrogen transport and methane conversion rat es. Homogeneous reactions require impractical residence times, even with H- 2 removal, because of slow initiation and chain transfer rates. The introdu ction of heterogeneous chain initiation pathways using surface sites that F orm methyl radicals eliminates the induction period without influencing the homogeneous product distribution. methane conversion, however, occurs pred ominately in the chain transfer regime, within which individual transfer st eps and the formation of C-2 intermediates become limited by thermodynamic constraints. Catalytic sites alone cannot overcome these constraints. Catal ytic membrane reactors with continuous H, removal remove these thermodynami c obstacles and decrease the required residence time. Reaction rates become limited by homoeeneous reactions of C-2 products to form C6+ aromatics. Hi gher delta values lead to subuequznt conversion oi. the desired C-2-C-10 pr oducts to larger polynuclear aromatics. We conclude that catalytic methane pyrolysis at the low temperatures required for restricted chain growth and the elimination of thermodynamics constraints via continuous hydrogen remov al provide a practical path for the direct conversion of methane to higher hydrocarbons. The rigorous design criteria developed are being implemented using shape-selective bifunctional pyrolysis catalysts and perovskite membr ane films in a parallel experimental effort. (C) 2001 Elsevier Science Ltd, All rights reserved.