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Ind. Eng. Chem. Res. 2011, 50, 856–865
Experimental Study and Modeling Analysis of Catalytic Partial Oxidation of Methane with Addition of CO2 and H2O Using a Rh-Coated Foam Monolith Reactor Shi Ding,†,‡ Yinhong Cheng,† and Yi Cheng*,† Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China, and Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
The catalyst deactivation of rhodium-coated foam monolith with CO2 and/or H2O addition was investigated both experimentally and numerically to understand the means to improve the durability of the rhodium catalyst applied for catalytic partial oxidation of methane (CPOM). The results showed that the addition of He, CO2, and/or H2O could all improve the catalyst stability mainly due to the reduced hot-spot temperature in the oxidation zone for the reason of either the dilution effect or the simultaneously endothermic reaction of CO2/ steam reforming. In particular, the catalyst stability can be greatly enhanced even at a low C/O ratio (i.e., carbon/oxygen ratio in atom of 0.85) with the addition of CO2 or H2O. Under the same conditions, high CH4 conversion (e.g., 0.8-0.85) can be achieved. The H2 yield can be adjusted by the added quantities of CO2 and/or H2O, which would allow the conventional pure CPOM process to be flexible to tune the composition of its product gas. These experimental results improved the understanding of how to modulate the CPOM process to achieve different reactor performances with the corresponding catalyst stability. Furthermore, CFD simulation with detailed chemistry was carried out. The model predictions had good agreement with the experimental results using the modified kinetics of the CO2 adsorption reaction, which was mostly not addressed when simulating a pure CPOM process in the literature for the little effect of CO2. 1. Introduction The possibility of a hydrogen economy and the need for alternative clean fuels have renewed interest in hydrogen, especially in novel routes and/or sources for delocalized hydrogen production1 as an alternative to the conventional steam reforming (SR) on Ni catalysts. Steam reforming of methane shown in eq 1, generally regarded as the main process for current syngas production, is highly endothermic (almost one-half of natural gas is burnt to supply the necessary heat) and therefore usually performed in large furnaces to supply the necessary energy,2 which makes it not suitable for the use in decentralized, small-scale H2 production. In contrast to the steam reforming, catalytic partial oxidation of methane (i.e., CPOM in eq 2), which leads to nearly 100% methane conversion and more than 90% syngas yields via a millisecond-time exothermic reaction,3,4 has gained great interest as it makes the CPOM well suited for decentralized and small-scaled syngas production in a remote gas field.5-7 CH4 + H2O T CO + 3H2 ∆Hr0 ) +206 kJ mol-1
(1)
1 CH4 + O2 f CO + 2H2 ∆Hr0 ) -36 kJ mol-1 2
(2)
As compared to the traditional Ni catalyst, the supported group VIII noble metal catalysts with high activity and stability can accomplish the effective and fast conversion of CH4 to CO and H2 without carbon formation and external energy input, which is desired in the industrial process.5,7-10 Among them, Rh has been acknowledged as one of the best catalysts due to its high activity and selectivity, low tendency to carbon * To whom correspondence should be addressed. Tel.: 86-1062794468. Fax: 86-10-62772051. E-mail:
[email protected]. † Tsinghua University. ‡ SINOPEC.
formation, and low volatility.11,12 On the other hand, foam monoliths coated with noble metals (e.g., platinum and rhodium) have been widely used as the catalysts for CPOM,5,6,9,13 as the high porosity of the monolith decreases the pressure drop and the good radial mixing inside improves the transport efficiency, respectively.14 However, the stability of Rh-based catalyst must be taken into account for its practical application in CPOM due to the high cost of Rh, especially under extreme reaction conditions such as around 1000 °C and millisecond residence time. Tavazzi et al.15 pointed out that 0.5 wt % Rh/Al2O3 catalyst suffered from significant deactivation over the course of a 150 h timeon-stream test under quasi-adiabatic conditions. Schmidt and co-workers 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.13,16 The appearance of the hot-spot is caused by the strongly exothermic oxidation reactions in the oxidation zone followed by the heat consumption of the strongly endothermic reforming reactions in the reforming zone. In agreement with these results, our previous study had found that the metal sintering caused by the highly exothermic reaction in the oxidation zone was the main reason for the catalyst deactivation.17 Therefore, it can be concluded that reducing the temperature of the hot-spot should help to improve the stability of the catalyst. Considering that CO2 and/or H2O addition would lower the hot-spot temperature,18 we thus anticipate that the additional CO2 and/or H2O should help to improve the catalyst stability. Generally, understanding the effect of CO2 and H2O on CPOM requires the analysis of the roles of steam reforming, CO2 reforming, and water gas shift (WGS) in terms of the reaction path. Moreover, interpretation of reactor performance demands consideration of a number of factors, including the coupling of heat and mass transfer to the overall kinetics and chemical
10.1021/ie1018996 2011 American Chemical Society Published on Web 12/09/2010
Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011
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Figure 2. Schematic of the computational domains in geometry using the 2D channel model.
inside a ceramic heater maintained at 400 °C to reduce the radial and axial heat loss through the quartz tube. Figure 1. Schematic of the millisecond reactor.
mechanism. As an alternative solution, CFD with detailed reaction kinetics has been acknowledged as a powerful tool to investigate the complex interactions between the reacting flow and the catalytic surface in CPOM process.19,20 Our previous study has established a 2D CFD model with detail chemistry to understand CPOM on rhodium-coated foam monolith.22 However, the role of CO2 reforming in the scheme is still an open question.13,21 In this work, the impact of the introduction of He, H2O, and CO2 to the feed stream on the H2 and CO selectivities, CH4 conversion, and catalyst stability was evaluated in the experiments. Meanwhile, the CFD model established in our former work was applied to simulate the CPOM reaction with H2O and CO2 addition by adopting the appropriate kinetics. The reactor performance of CPOM process with CO2 and H2O was studied by numerically revealing complex flow patterns and the detailed profiles of temperature and concentration inside the reactor. 2. Experimental Section 2.1. Catalyst Preparation. Catalysts were prepared by impregnating monolithic alumina supports with aqueous RhCl3, followed by drying in air at 110 °C for 3 h and calcining at 600 °C in air for 6 h. The foam monolith, which was used as the catalyst support and the structured reactor as well, had the features of 45 ppi (pores per linear inch), 15 mm in diameter, and 10 mm in length (99% Al2O3, Hi-Tech Ceramics). The geometric surface area per unit volume was 6.21 × 104 m2/m3. 2.2. Experimental Apparatus. The reactor was comprised of a quartz tube (i.d. ) 18 mm) containing a cylindrical R-Al2O3 foam monolith coated with rhodium (see Figure 1). The foam monolith coated with Rh catalyst acted as the structured reactor, which was flanked with two blank foams in tandem as the front and the back heat shields. These monoliths were wrapped tightly with aluminosilicate cloth to prevent gas bypassing. The calibrated mass flow controllers (MFCs) were used to control the flow rates of the reactant gases, and the gases were premixed before flowing into the quartz tube of the reactor. The product gas mixture was analyzed by a gas chromatograph (Techcomp 7890 II) with a TDX01 column and a thermal conductivity detector (TCD), except that the quantity of H2O was calculated by closing the mole balance of O2. The exit temperature was measured at the point between the catalyst and the back heat shield using thermocouples. For all data presented, the reactor feed rate was 3 SLPM (∼1.0 × 105 h-1 GHSV) at room temperature and 1 atm. The C/O ratio was defined with respect to only the CH4 and O2 feeds. To make the process approach the adiabatic operation for all feed compositions studied, the quartz tube was accommodated
3. Mathematical Model and Numerical Simulation 3.1. Mathematical Model. In the experiments, the short contact-time CPOM process was performed over Rh catalyst foam, which was sandwiched between the front and the back heat shields. Because the inlet and outlet conditions were uniform, indicating that the condition in each pore was the same, the foam monolith can be approximated as radially symmetric, especially if the number of pores be large. Therefore, an axisymmetric-channel model system was used, which had a conductive channel wall and a flow channel, representing the solid and flow parts of the foam, respectively. Moreover, the model system contained not only a catalyst zone and a blank development zone but also an insulated blank development zone at the entrance (see Figure 2). The model system used here was discussed in detail in our previous work.22 Transport phenomena in the channel can be described by the conservation equations of mass, momentum, chemical species, and enthalpy, leading to a set of nonlinear partial differential equations. Gas-phase reactions were assumed insignificant at atmospheric pressure for millisecond contact times,23 so that only the surface reactions were modeled by a detailed reaction mechanism. The flow inside the microchannel was considered laminar due to the low Reynolds number less than 400 at the inlet. The fluid was assumed to be an ideal gas mixture. Conservation Equations. Table 1 shows the conservation equations of mass, momentum, chemical species, and enthalpy and the governing equations of surface chemistry. Surface Chemistry Model. A detailed reaction mechanism proposed by Deutschmann (see Table a1 in the Appendix) with 6 gas-phase species, 12 surface-absorbed site species, and 38 elementary surface reactions was used to describe the surface reactions.27 The state of the catalytic surface was described by its temperature and coverage of the adsorbed species. When the concentration of CO2 was very low during the CPO process, the CO2 adsorption reaction (CO2 + Rh(s) f CO2(s)) had little influence on the whole process. However, when a large amount of CO2 was added, the influence of CO2 adsorption reaction became significant and should be taken into account seriously. The kinetics data of CO2 adsorption reaction rate suggested by Deutschmann may be insufficient to reflect the influence of CO2 addition on the whole CPO process, which underestimated the reforming reaction between CH4 and CO2. Thus, a modified CO2 adsorption kinetics should be used to better describe the absorption rate of CO2. Mhadeshwar et al.20 suggested that the CO2 adsorption reaction was relevant to the H2O adsorption reaction (H2O + Rh(s) f H2O(s)) and temperature. The reaction rate of H2O adsorption was about 57-130 fold of that of CO2 adsorption when temperature was between 1073 and 1373 K, and we fixed it to 100 fold in our model. Table 2 gives the comparison among the experimental measurement, numerical prediction without modifying the CO2 adsorption reaction rate,
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Table 1. Governing Equations for CFD Coupled with Detailed Elementary Kinetics
Table 2. Comparison between Experimental Measurement and Numerical Prediction with Different Kinetics of CO2 Adsorption (45 ppi Foam Monolith, 1.8 × 105 h-1 GHSV, Inlet Temperature 298 K, Alumina Wall)
continuity equation ∂ 1∂ ∂F + (Fu) + (rFV) ∂t ∂x r ∂r momentum equationa ∂ ∂P ∂ 2 ∂ ∂u ∂ (Fu) + (Fu2) + (FuV) ) + - µ∇ · δ + 2µ ∂t ∂x ∂r ∂x ∂x ∂x 3 ∂u 1∂ ∂V + µr r ∂r ∂x ∂r
(
) [ (
conversion and selectivity
)]
)]
(
)
species equationb ∂Ji,x 1 ∂(rJi,r) ∂ ∂ ∂ (FY ) + (FuYi) + (FVYi) ) + (i ) 1, ..., Ng) ∂t i ∂x ∂r ∂x r ∂r
[
]
C T Ji,x ) Ji,x + Ji,x ) -FDi,j
C T + ji,r ) -FDi,j ji,r ) ji,r
∂Yi ∂T - Di,T ∂x ∂x
∂Yi ∂T - Di,T ∂r ∂r
energy equationc ∂(FuCpT) ∂(FVCpT) ∂(FCpT) + + ∂t ∂x ∂r Ng
)
∂ ∂T ∂ ∂T ∂ k + rk - ( ∂x ∂x ∂r ∂r ∂x
( )
( )
∑
Ng
CpTji,x) -
i
∂ ( ∂r
∑ C Tj ) + p
i,r
i
∂P ∂P ∂P +u +V ∂t ∂x ∂r reactions rate Ng+Ns
Ks
Si )
∑V
i,nkf,n
n)1
∏ [X ]
Vj,n
j
( )∏
kf,n ) ATβ,n exp
-Ean RT
(i ) 1, ..., Ng + Ns)
j)1
reactions rate constant
Ns
i)1
( )
Θiφ,n exp
εi,nΘi γ ) τ RT Γ
XCH4 (%) XCO2 (%) SH2 (%)
70.07 9.86 71.61
69.39 -6.59 82.51
69.78 6.20 72.45
CH4:O2:CO2 ) 1.7:1:2
)
[(
numerical prediction (modified)
CH4:O2:CO2 ) 2:1:2
∂ ∂ ∂ 2 ∂P ∂V ∂ (Fu) + (FuV) + (FV2) ) + 2µ - µ∇ · δ + ∂t ∂x ∂r ∂r ∂r ∂r 3 ∂u V 2µ ∂V ∂ ∂V + µ + ∂x ∂x ∂r r ∂r r
(
measurement
numerical prediction
RT 2πMi
species flux balance at the reacting surface for the gas-phase species SiMi ) Ji,r + FVYi(i ) 1, ..., Ng) auxiliary equations for steady-state solution18 Si ∂Θ ) ) 0 (i ) 1, ...Ns) Γ ) 2.7 × 10-5 mol m-2 ∂t Γ a The local dynamic viscosity is obtained on the basis of the Chapman-Enskog theory for multicomponent ideal gas, and the dynamic viscosity of each species is assumed as monatomic gases.24 b The concentration-driven and thermal-driven diffusion coefficients of each species are expressed by rigorous Maxwell-Stefan formulation.25,26 c Thermal conductivity of the ideal gas mixture is computed using the mixture thermal conductivity based on kinetic theory.24
and the numerical prediction with the modified CO2 adsorption reaction rate. It is easy to see that when CO2 was added, the experimental measurement showed a CO2 net consumption, but the numerical prediction without modifying the CO2 adsorption reaction rate showed a CO2 net production. When the CO2 adsorption reaction rate was modified, it can well predict the CO2 net consumption and showed a good match to the experimental measurement.
XCH4 (%) XCO2 (%) SH2 (%)
80.32 14.39 66.23
81.03 -7.62 82.76
80.20 11.83 67.06
3.2. Boundary Conditions. In this work, the boundary condition of the channel model at the outer diameter of the wall was set adiabatic, because the foam monolith was approximated as radically symmetric. The channel diameter was set to 0.576 mm corresponding to the 45 ppi foam monolith, and the channel length was 10 mm, while the wall thickness was set to 0.035 mm. The catalytic surface area was regarded to be equal to the geometric surface area. The solid phase was modeled using the thermal properties of alumina as a polynomial function of temperature.28 The flow entered the channel with a uniform inlet velocity of 0.5 m s-1 (∼1.8 × 105 h-1 GHSV) and a temperature of 298 K at 1 atm. 3.3. Solution Scheme. The solution to the established reacting flow model with the detailed reaction mechanism has been successfully implemented using the commercial CFD package of FLUENT 6.3. The equations of gas flow were solved using FLUENT solver, while the surface reactions were coded in C language as the user defined functions (UDFs) to be connected with FLUENT. The grid was finer at the inlet region and near the catalytic surface due to the large gradients of temperature and species concentration. The number of the computational cells was varied to ensure the solution being grid independent. A second-order discretization scheme was utilized with a convergence criterion of 10-6 for each scaled residual component (i.e., continuity, x-velocity, r-velocity, energy, and species). The solutions showed no significant difference in the fields of velocity, temperature, or concentrations (
