Elsevier

Chemical Engineering Science

Volume 65, Issue 6, 15 March 2010, Pages 1989-1999
Chemical Engineering Science

Analysis of catalytic partial oxidation of methane on rhodium-coated foam monolith using CFD with detailed chemistry

https://doi.org/10.1016/j.ces.2009.11.033Get rights and content

Abstract

CFD simulation with detailed chemistry was conducted to understand the catalytic partial oxidation of methane (CPOM) on rhodium-coated foam monolith. For the underlying process occurred extremely fast with large gradients of temperature and species concentrations at the inlet, special attention must be paid to the appropriate treatment on computational geometry and corresponding boundary conditions for the simulation. Discussions were made carefully on this proposed issue in geometry modeling that the reliable predictions can be authentically obtained by adopting the same geometry as the experiments from the viewpoint of physics in order to fully consider the heat conduction/diffusion at the reactor inlet. The right model system was sufficiently validated by both the conceptual analysis and the experimental results. The reactor performance of CPOM process was thereafter studied by numerically revealing the effects of wall heat conduction, the channel diameter and the catalytic surface area on the profiles of temperature and species concentrations. The results showed that the maximum wall temperature, which was crucial for the catalyst stability, could be significantly reduced by increasing the thermal conductivity of the wall, and/or the channel diameter, and/or the catalytic surface area, but accompanied with a slight drop of the methane conversion. This deficiency can be retrieved by decreasing the atom feed ratio of C/O and/or elongating the catalytic bed. These results pointed out the necessity of facilitating the foam material, the channel diameter and the catalytic surface area with the operating conditions in order to achieve the best performance of the CPOM process in the millisecond reactor.

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: CH4+0.5O2CO+2H2ΔHrθ=35.6kJ/mol), 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

Apre-exponential constant, in mol, m, s, K
Cpspecific heat capacity, J kg−1 K−1
dchannel diameter, m
Di,mMaxwell-Stefan diffusivity of species i, m−2 s−1
Di,Tthermal diffusivity of species i, m−2 s−1
Eaactivation energy, J mol−1
JCconcentration-driven mass flux, kg m−2 s−1
Ji,rradial mass flux of species i, kg m−2 s−1
Ji,xaxial mass flux of species i, kg m−2 s−1
JTthermal-driven mass flux, kg m−2 s−1
kthermal conductivity, W m−1 K−1
kf,nforward reaction rate for reaction n, in mol, m, s
Kstotal 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)

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