Modeling Development of a Combined Methane Fixed Bed Reactor Reformer
Authors: Elbashir Nimir , Challiwala Mohamed Sufiyan , Ghouri Mohammed Minhaj , Linke Patrik , El-Halwagi Mahmoud 
Published: July 2019

The growing concern of many countries globally about the greenhouse gas emissions have emphasized interest towards dry reforming of methane (DRM). For an oil and gas based economy such as the state of Qatar, CO2 emission is a big challenge, as it has rendered Qatar as the highest CO2 emitting country per capita in the world. The potential of DRM process for integration in the existing infrastructure of Qatar is a key aspect of this research as a part of exceptional proposal granted to Dr. Nimir Elbashir by QNRF aimed at CO2 fixation. DRM is a heterogeneous chemical reaction in which the two greenhouse gases; CH4 and CO2 are converted to synthesis gas. Synthesis gas or ‘syngas’ is a precursor to a large variety of value added chemicals including hydrocarbons via Fischer-Tropsch Synthesis (FTS). In addition to CO2, steam can also be used to reform methane into syngas in a process known as steam reforming of methane (SRM). Steam Reforming of Methane (SRM) ΔH298 =  206 kJ/mol (1) Dry Reforming of Methane (DRM) ΔH298 =  247 kJ/mol (2) In addition to these two processes, there is also an exothermic reforming process, known as the partial oxidation of methane (POX), where methane is combusted to yield syngas. DRM process is beset by numerous major process limitations including its high endothermicity, high rate of catalyst deactivation (due to carbon formation) and low-quality syngas yield ratio (H2:CO⇐1:1). These challenges have posed severe obstruction towards widespread commercialization of this technique. A synergistic amalgamation of the reforming of methane as DRM+SRM, DRM+POX and DRM+SRM+POX have been recommended in the literature as a way to tackle the intrinsic limitations of the DRM process. In the current work, such combinations of methane reforming processes have been simulated thermodynamically using direct Gibbs free energy (GFE) minimization method. Energy valuations of various case scenarios have been carried out under varying operating conditions (temperature, presssure and feed mole ratios) assuming both ideal gas conditions and non ideal regimes using cubic equations of state (Peng Robinson (PR), Redlich Kwong (RK) and Soave Redlich Kwong). The main objective of the thermodynamics aspect of this study is to find optimized condition of reduced energy requirement and reduced carbon deposition while maintaining considerable CO2 fixation in a combined reforming process. In order to completely understand the system, a one-dimensional pseudo-homogeneous fixed bed reactor model which incorporates all the transport limitations (heat, mass and momentum) for combined SRM/DRM processes is developed. Reaction kinetics utilizing Langmuir-Hinshelwood Hougen-Watson (LHHW) type rate expressions published in the literature for SRM and DRM under analogous operating conditions have been used in the reactor bed model. These model results will be further validated against the experimental data published in literature. The kinetic conversion profiles are then compared with the thermodynamic results to systematically determine the regimes of kinetic deviation (from equilibrium) for the combined SRM/DRM system. This approach of carrying out both thermodynamic and reaction engineering analysis is advantageous in understanding the reforming process in a broader view and will also help in setting base for experimental investigations. These modeling results will be used as pre-experimental initial findings for the NPRP exceptional project aimed towards development of highly effective and coke resistant catalysts.

References

Pakhare, D. and J. Spivey, A review of dry (CO2) reforming of methane over noble metal catalysts. Chemical Society Reviews, 2014. 43(22): p. 7813-7837.

Song, C., Tri-reforming: a new process for reducing CO2 emissions. Chemical Innovation, 2001. 31: p. 21-26.

Jiang H, Li H, Zhang Y., Tri-reforming of methane to syngas over Ni/Al2O3—thermal distribution in the catalyst bed. Journal of Fuel Chemistry and Technology 2007. 35: p. 72-78.

Noureldin, M.M.B., N.O. Elbashir, and M.M. El-Halwagi, Optimization and Selection of Reforming Approaches for Syngas Generation from Natural/Shale Gas. Industrial & Engineering Chemistry Research, 2014. 53(5): p. 1841-1855.