SYNGAS PRODUCTION BY DRY REFORMING OF METHANE: THERMODYNAMIC AND MODELING STUDY

Abstract

Carbon dioxide reforming of methane produces synthesis gas with low hydrogen to carbon monoxide ratio, which is desirable for many industrial synthesis processes. This reaction also has very important environmental implications since both methane and carbon dioxide contribute to the greenhouse effect. Converting these gases into a valuable feedstock may significantly reduce the atmospheric emissions of CO2 and CH4.The dry reforming is carried out with excess CO2 to promote reverse water gas shift reaction (RWGS) which results in lower H2/CO ratio. Carbon deposition causing catalyst deactivation is the major problem inhibiting the industrial application of the dry reforming reaction. Thermodynamically, the most probable reactions leading to carbon formation during dry reforming are methane cracking, boudouard reaction, CO reduction and CO2 reduction reactions. All the four carbon forming reactions are employed to examine the thermodynamic possibility of carbon formation and thermodynamic equilibrium calculations have been performed with Aspen plus based on direct minimization of Gibbs free energy method. The effects of CO2/CH4 ratio (1– 3), reaction temperature (873–1500 K) and hydrogen addition (0-10%) at 1atm pressure on equilibrium conversions, product compositions and solid carbon were studied. On the basis of thermodynamic analysis the optimal working conditions for syngas having H2/CO ratio of 1 with negligible amount of carbon formation were obtained and were used to simulate the performance of four reactors, two fixed beds (FBR1 & FBR2) and two membrane reactors (MR1 &MR2) in terms of CH4 conversion, H2/CO ratio and yield of H2. For this purpose a comprehensive, steady state, one dimensional, isothermal mathematical model equations for dry reforming reactions in membrane and fixed bed reactor have been developed. Two types of catalysts have been used in the study: Rh/Al2O3 and Ni/Al2O3. The axial and the radial dispersion in the model are assumed to be negligible and the external mass transfer resistance in the catalyst bed is also neglected. The model equations developed have been solved by ODE solver tool in MATLAB. The reactors FBR1 and MR1 that were packed with Rh/Al2O3 catalyst gave higher CH4 conversion as compared to FBR2 and MR2 that were packed with Ni/Al2O3. Also, membrane reactors MR1 and MR2 have been found to provide better performance as compared to fixed bed reactor FBR1 and FBR2 respectively.