Analysis of catalytic partial oxidation of methane on rhodium-coated foam monolith using CFD with detailed chemistry
Introduction
The productions of hydrogen, liquid fuels and chemicals from methane are of growing economic importance to the petrochemical industry from natural gas (Wilhelm et al., 2001). Catalytic partial oxidation of methane (CPOM: ), which leads to nearly 100% methane conversion and more than 90% syngas yield via a millisecond-time exothermic reaction (Hickman and Schmidt, 1993; Reyes et al., 2003), has gained great interest as it provides a promising route for small-scale decentralized syngas production in a remote gas field (Leclerc et al., 2002; Neumann et al., 2004; Mitri et al., 2004).
Foam monoliths coated with noble metals (e.g., platinum and rhodium) have been widely used as the catalysts for CPOM (Bodke et al., 1998; Horn and Schmidt, 2001; Leclerc et al., 2002; Horn et al., 2007), as the high porosity of the monolith decreases the pressure drop and the good radial mixing inside improves the transport efficiency, respectively (Dalle Nogare et al., 2008). Extensive experiments have been done to analyze the effects of the operation variables, the foam geometry, and the nature of the catalyst by monitoring the species components at the exit (Bodke et al., 1998; Horn and Schmidt, 2001; Leclerc et al., 2002). Recently, Schmidt et al. (Horn et al., 2006, Horn et al., 2007) have found that a hot-spot existed in an autothermally operated foam catalyst by measuring the species and temperature profiles along the centerline of the foam with high resolution in space. The appearance of the hot-spot was caused by the strong exothermic oxidation reactions in the oxidation zone followed by the heat consumption of the strongly endothermic reforming reactions in the reforming zone. It has been reported that the high temperature was mainly responsible for the observed deactivation of the rhodium catalyst (Tavazzi et al., 2007). Therefore, reducing the temperature of the hot-spot by catalyst design would help to improve the stability of the catalyst. However, the extreme operating conditions (i.e., high flow rates at temperatures ranging from 700 to above 1000 °C) significantly limit the feasibility of doing measurements inside the reactor. Under this circumstance, CFD modeling with detailed chemistry is a powerful tool to obtain a better understanding of the transport phenomenon coupled with the reacting flows in the reactor (Tavazzi et al., 2006), and thus might provide some clues for the catalyst design (Stutz and Poulikakos, 2005; Stutz et al., 2006).
CFD with detailed reaction kinetics has been applied to investigate the complex interactions between the reacting flow and the catalytic surface in the CPOM process (Deutschmann and Schmidt, 1998; Mhadeshwar and Vlachos, 2005; Quiceno et al., 2006). A plug flow reactor (PFR) model has given good predictions of reaction conversions and selectivities (Goralski et al., 2000), but the axial profiles of the chemical species and the temperature were unsatisfactorily predicted because these models excluded the effects of transport phenomena (Williams et al., 2007). More satisfied results can be obtained by the two-dimensional (2D) channel model, which considers the effects of transport properties in the reacting flow (Deutschmann and Schmidt, 1998; Schwiedernoch et al., 2003; Williams et al., 2007). Some recent simulation works by the 2D channel model (Stutz and Poulikakos, 2005; Stutz et al., 2006) has showed that better catalytic performances and a lower maximum wall temperature of the catalytic bed could be achieved by adjusting the wall heat conduction, the channel diameter and the catalytic surface area. However, a deficiency of their works is that no explanations have been given for the observed non-conservations of elements and energy in their simulation results.
Our previous study (Ding et al., 2009) has found that the metal sintering caused by the highly exothermic reaction in the oxidation zone was the main reason for the catalyst deactivation, and the catalyst stability was different for the form monoliths with different pore sizes. Therefore, analyzing the temperature profiles of catalyst bed for the catalysts with different properties is of great importance for understanding how to improve the catalyst stability. In this work, the 2D channel model considering the complex reacting flow in the channel and the wall heat conduction was successfully established. The model proposed in the present work is advantageous over the other two reported models (Stutz and Poulikakos, 2005; Williams et al., 2007) as it firstly eliminated the non-conservations of elements and energy by incorporating the appropriate geometry and boundary conditions. Moreover, effects of the wall heat conduction, the channel diameter, and the catalytic surface area on the reactor performance of CPOM were investigated numerically by this model. Several possible routes for reducing the maximum wall temperature without a significant loss of CH4 conversion and syngas selectivity were proposed based on the simulation results, which would be beneficial for improving the catalyst stability.
Section snippets
Mathematical model
Catalytic partial oxidation of methane was usually simulated by a 2D axisymmetric channel model (Zerkle et al., 2000; Schwiedernoch et al., 2003). Transport phenomena in the channel can be described by the conservation equations of mass, momentum, chemical species and enthalpy. Gas-phase reactions are insignificant at atmospheric pressure for millisecond contact times (Slaa et al., 1997; Deutschmann and Schmidt, 1998), so that only the surface reactions are modeled by a detailed reaction
Comparison of model predictions under different geometries
The same model inputs were adopted to simulate the CPOM process on the Rh-coated catalyst (80-ppi foam monolith with a channel diameter of 0.32 mm, a wall thickness of 0.02 mm, and a catalytic surface area of 125 cm−1) using the three configurations in geometry as mentioned above. The simulated results are summarized in Table 1.
For either a conductive or an insulated wall, the simulated mass fraction of Ar and the mole ratios of H/C and O/C at the exit under Model 1 are different from those given
Conclusions
CFD simulations considering the fluid flow, wall heat conduction and the detailed elementary surface reaction mechanism on the channel wall were performed to better understand the CPOM process using Rh-coated foam monolith as the reactor. It was specially emphasized that the appropriate geometry and boundary conditions played dominant roles to achieve the physically reasonable predictions of the underlying millisecond reacting flows. Simplified treatments on the geometry and boundary conditions
Notation
A pre-exponential constant, in mol, m, s, K Cp specific heat capacity, J kg−1 K−1 d channel diameter, m Di,m Maxwell-Stefan diffusivity of species i, m−2 s−1 Di,T thermal diffusivity of species i, m−2 s−1 Ea activation energy, J mol−1 JC concentration-driven mass flux, kg m−2 s−1 Ji,r radial mass flux of species i, kg m−2 s−1 Ji,x axial mass flux of species i, kg m−2 s−1 JT thermal-driven mass flux, kg m−2 s−1 k thermal conductivity, W m−1 K−1 kf,n forward reaction rate for reaction n, in mol, m, s Ks total number of elementary
Acknowledgments
This work is financially supported by CNPC Innovation Foundation and A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (No. 200245). Continuous support by PetroChina is highly acknowledged.
References (30)
- et al.
The effect of ceramic supports on partial oxidation of hydrocarbons over noble metal coated monoliths
Journal of Catalysis
(1998) - et al.
Modeling spatially resolved profiles of methane partial oxidation on a Rh foam catalyst with detailed chemistry
Journal of Catalysis
(2008) - et al.
Natural gas conversion in monolithic catalysts: interaction of chemical reactions and transport phenomena
Studies in Surface Science and Catalysis
(2001) - et al.
Modeling homogeneous and heterogeneous chemistry in the production of syngas from methane
Chemical Engineering Science
(2000) - et al.
Syngas by catalytic partial oxidation of methane on rhodium: mechanistic conclusions from spatially resolved measurements and numerical simulations
Journal of Catalysis
(2006) - et al.
Methane catalytic partial oxidation on autothermal Rh and Pt foam catalysts: oxidation and reforming zones, transport effects, and approach to thermodynamic equilibrium
Journal of Catalysis
(2007) - et al.
Reverse-flow reactor operation and catalyst deactivation during high-temperature catalytic partial oxidation
Chemical Engineering Science
(2004) - et al.
Towards an efficient process for small-scale, decentralized conversion of methane to synthesis gas: combined reactor engineering and catalyst synthesis
Catalysis Today
(2004) - et al.
Modeling the high-temperature catalytic partial oxidation of methane over platinum gauze: detailed gas-phase and surface chemistries coupled with 3D flow field simulation
Applied Catalysis A
(2006) - et al.
Experimental and numerical study on the transient behavior of partial oxidation of methane in a catalytic monolith
Chemical Engineering Science
(2003)
Effects of microreactor wall heat conduction on the reforming process of methane
Chemical Engineering Science
Optimization of methane reforming in a microreactor—effects of catalyst loading and geometry
Chemical Engineering Science
Experimental and modeling analysis of the effect of catalyst aging on the performance of a short contact time adiabatic CH4-CPO reactor
Catalysis Today
Syngas production for gas-to-liquids applications: technologies, issues and outlook
Fuel Processing Technology
Understanding homogeneous and heterogeneous contributions to the platinum-catalyzed partial oxidation of ethane in a short-contact-time reactor
Journal of Catalysis
Cited by (24)
High-performance catalyst of methanol steam reformer based on Cu foam with nanofiber architectures
2024, International Journal of Hydrogen EnergyDiffusion and reaction in foam-based catalysts: Identifying the shape factor
2022, Chemical Engineering ScienceA theoretical study on high pressure partial oxidation of methane in Rh-washcoated monoliths
2017, Chemical Engineering Research and DesignNumerical analysis on steam methane reforming in a plate microchannel reactor: Effect of washcoat properties
2016, International Journal of Hydrogen EnergyCitation Excerpt :It can capture behaviors of the reacting system over a wide range of operating conditions. Such reliable detailed chemistry models have been successfully applied in our previous work [35–37]. The objective of this work is to explore the effect of washcoat properties on the performance of a plate microchannel reactor for SMR theoretically.
Simulation of exhaust gas reforming of natural gas in a microchannel reactor
2016, International Journal of Hydrogen EnergyCitation Excerpt :Therefore, compared to those of conventional reactors, microchannel units offer very high heat transfer rates which become crucial in the presence of highly exothermic and endothermic reactions [8]. In the case of OSR, it is important that heat produced by oxidation is distributed throughout the reactor without forming a hot-spot, since high temperatures can lead to the deactivation of the catalyst [9]. For an efficient EGR process, the heat of the exhaust gas and the heat raised by TOX should be transferred to the endothermic SR to increase the rate of reforming and to produce higher amounts of H2.