284888 Comparison Between Microchannel and Packed-Bed Microreactor Performance for the Dry Reforming of CO2 to Syngas
The abundance of methane and carbon dioxide from various sources makes these greenhouse gases attractive feedstocks for the clean manufacture of fuels and chemicals if an efficient catalytic process can be identified for their conversion to synthesis gas (CO + H2). Synthesis gas is a key building block for the production of various chemicals, such as methanol, dimethyl ether, acetic acid and assorted hydrocarbons using the Fischer-Tropsch and related GTL processes (Havran et al., 2011). Microreactors and the associated microprocess technologies provide an attractive platform for studying new catalysts, catalytic kinetics, and sustainable process concepts owing to their various operational advantages when compared to larger, more conventional reactors and product-handling equipment. Both steam reforming and the partial oxidation of methane have been studied in parallel-plate microreactors (Mettler, 2011). However, CO2 reforming of CH4has not been studied extensively in microreactor systems. In terms of catalyst packages, Ni-based materials are known to be more efficient compared to Pt and Ru-based catalysts due to their higher turnover rates, stability and lesser cost (York, 2007).
The current study involves modeling of a microreactor for syngas production via the dry reforming of CO2 as a precursor to assessing the feasibility of using stored thermal energy to drive the endothermic reaction from a solar concentrator. The microreactor configuration is largely based upon the so-called DuPont-MIT T-type microreactor (Quiram et al., 2007a-d), which employs a high degree of semi-conductor type processing steps for its fabrication. To provide a reference for assessing the microreactor efficiency and performance, comparisons are made using an ideal packed-bed microreactor.
The microreactor geometry consists of 10 mm long channel whose height and width are 500 μm. The catalyst is assumed to be coated on the top and bottom surfaces as a thin film whose thickness is 10 μm. The reaction kinetics for the dry reforming reaction is based on a Langmuir-Hinshelwood kinetics-type model with competitive adsorption as proposed by Richardson and co-workers, which were derived from experiments using a Ni/CeO2-ZrO2 catalyst. The microreactor performance was evaluated at temperatures between 600 to 750°C and atmospheric pressure with N2 as the inert gas and using CH4:CO2:N2molar ratios of 0.4:0.4:0.2. The initial microchannel model is based upon 2-D axi-symmetric representation where the gas velocity, species transport, and energy transport are assumed to be a function of x and y components with negligible variation in the z-direction. The modeling equations include the Navier-Stokes for the gas hydrodynamics, the convection-diffusion equation for species transport, and both the convective and conduction forms of the energy balance equations for thermal energy transport in the gas and solid layers, respectively. Transport properties, such as the gas viscosity and diffusion coefficients for individual gas species were obtained from Chapman-Enskog theory, while the diffusion coefficients for species j in the gas mixture were obtained using the Wilke equation. The model was solved using by the finite element method using COMSOL Multiphysics 3.5a. The performance of the microchannel reactor was compared with a conventional packed-bed microreactor (PBR) having a diameter of 6.3 mm and an overall length of 30 mm.
In the ideal PBR, methane conversions of 47%, 75%, and 81% were obtained, respectively, at 600, 700, and 750 °C. The methane conversions were generally lower when compared to those for CO2 (50%, 78%, and 85%, respectively) due to the reverse water-gas shift reaction (RWGS) wherein CO2 and H2 combine to form CO and H2O. However, the thermodynamic Kp values of the RWGS were only the order of unity at high temperatures, which leads to only small changes in the observed CO2 concentrations. Increasing W/FA0 from 0.016 hr-1 to 0.08 hr-1 led to similar conversions, but at smaller reactor volume. For comparison to the microchannel at similar residence times, the catalyst weight was decreased from 53 mg to 5.3 mg and inlet velocities were varied from 0.5 mm/s 2 mm/s to give W/FA0 of 0.08 hr-1, 0.04 hr-1, and 0.018 hr-1, respectively. The methane conversions were found to be 35%, 55%, and 67% at temperatures of 600 °C, 700 °C, and 750 °C, which are lower than the PBR owing to the reduced gas-solid contacting efficiency and other non-ideal transport effects. Typical Reynolds numbers in the microchannel reactor were found to be in range of 0.03-0.3 corresponding to gas velocities of 1 mm/s – 10 mm/s, which is deep into the laminar flow region. Additional results and assessment of the microchannel reactor efficiency compared to a conventional packed-bed microreactor for this particular application will be discussed, along with the design that couples an energy storage system to the microreactor.
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