(83a) Computational Fluid Dynamics Study of Heat Transfer in a Microchannel Reactor for Fischer-Tropsch Synthesis

One of the outstanding characteristics of microreactors or micro-structured reactors is the heat transfer enhancement that results from the increase of the heat transfer coefficients as well as of the heat exchange surface area per unit volume with decreasing the channel hydraulic diameter. This means an intrinsic gain in operational safety by preventing of hot spots and high thermal performance of these micro devices. As a result, higher reactants concentrations and higher catalyst loadings are possible for highly exothermic reactions thus improving productivity. Moreover, it is possible to achieve the virtually isothermal operation of microreactors when very fast reactions with large reaction heat are carried out [1, 2]. There is a renewed interest in the Fischer-Tropsch (FT) synthesis that is largely due to its role in the gas-to-liquid (GTL) processes and the need to monetize the huge natural gas reserves existing in remote areas, and the flaring constraints imposed on associated gases (CH₄ and CO₂ emissions) [3]. As it is well-known, the FT synthesis consists in the conversion of syngas (CO + H₂) over Fe- or Co-based catalysts into a complex multicomponent mixture consisting of linear hydrocarbons (paraffins, olefins and light hydrocarbons) and oxygenates (alcohols, aldehydes, ketones, and acids). The FT synthesis is very exothermic (ΔH_R = - 165 kJ/mol_CO) and heat removal is a critical issue in the design and safe operation of FT reactors; moreover, this reaction is usually carried out under significant mass-transfer limitations. Therefore, microreactor technology provides an interesting opportunity for the intensification of the FT synthesis [4]. Microchannel reactors are being commercialized for this reaction. The microchannel technology developed by Velocys Inc. and Pacific Northwest National Laboratory (Richland, WA) is based on a reactor coupling the exothermic FT synthesis with the partial boiling of water for heat removal [5]. In this work, a Computational Fluid Dynamics (CFD) study with ANSYS CFX software of heat transfer in a microchannel reactor for FT synthesis is presented. The simulated microreactor is a SS316 block 17 mm high, 21 mm width, and 21 mm long containing 80 microchannels arranged in a crossed flow configuration with 40 microchannels in each direction for the transport of syngas and cooling water, respectively. This block can be viewed as the stacking of 8 stainless steel sheets 2 mm thick, 21 mm width, and 21 mm long; in practice, the two outer sheets are 2.5 mm thick. Every sheet is stacked in an alternate position with regard to the one that is below to provide the crossed flow configuration. The sheets have 10 parallel square microchannels 1 mm high, 1 mm width, and 21 mm long. The distance between contiguous channels is 1 mm in both directions whereas from every end of a sheet to the first one of the channels the distance is 0.5 mm. The access to and the exit from the channels is through rectangular chambers 17 mm high, 21 mm width, and 10 mm long that are connected to circular pipes (7.2 mm internal diameter) by means of pyramidal-shaped prolongations 10 mm long. Syngas and water are fed to the microreactor through the pipes. Simulations have been conducted under operating conditions relevant for the intensification of the FT synthesis. It has been assumed that a very thin layer of an active catalyst has been uniformly deposited onto the walls of the microchannels. Gas hourly space velocities (GHSV) in the 5,000-10,000 h^-1 range at standard temperature and pressure (STP) have been considered for the syngas stream (H₂/CO molar ratio = 2). These GHSV values are above the ones typical of the FT synthesis conducted in commercial slurry and tubular fixed bed reactors [6]. Taking into account that the available volume for the syngas stream in the microchannels is 40 × 0.1 × 0.1 × 2.1 = 0.84 cm³, the adopted GHSVs imply feeding the microreactor with 70-140 cm³/min (STP) of syngas. This results in a low contact time of 0.36-0.72 s that obviously require a very active FT catalyst such as the Co-Re/Al₂O₃ or Co-Ru/Al₂O₃ ones prepared by Jarosch et al. [5] and Wang et al. [7], respectively. The aim of our study was to analyze the conditions under which the FT synthesis could be conducted in the above-described microreactor at a reasonably high CO conversion and under close to isothermal conditions at any desired reaction temperature between 200 and 250ºC. This means that 1.43-2.86 W of heat have to be quickly removed. In our study we have simulated heat generation due to the synthesis reaction as a source term associated to the catalyst layer on the walls so that heat flux through the microchannels walls surface is 425-850 W/m². It has been found that isothermicity can be achieved by setting the operating pressure at a value at which the boiling point of water is slightly below the temperature at which the FT synthesis has to be carried out and adjusting the cooling water flow rate. This allows to efficiently removing heat by steam generation. Thus for the 200-250ºC range, total pressure should be approximately between 10 and 39 atm. Interestingly, this pressure range is also of interest for the FT synthesis reaction in order to increase the reaction rate and improve the selectivity for high molecular weight hydrocarbons. Temperature inside the metallic block is extremely uniform and near to that of the boiling water. Gas temperature and velocity in the syngas microchannels are also very uniform. Figure 1 includes a representative example illustrating the isothermicity of the system. It is shown the temperature in a plane parallel to the direction of the syngas flow; as can be seen the gas temperature at the inlet is 241ºC and 239ºC at the outlet. Acknowledgements We gratefully acknowledge financial support of this work by Petróleo Brasileiro S.A. ? PETROBRAS and Ministry of Education and Science of the Spanish Government (MAT2006-12386-C05-04). References [1] Hessel, V., Hardt, S., Löwe, H. Chemical Micro Process Engineering; Wiley-VCH, Weinheim, 2004, pp. 48-49. [2] Celata, G.P., Zummo, G. In Heat Transfer and Fluid Flow in Microhannels (G.P. Celata, Ed.); Begell House, Inc., New York, 2004, pp. 1-2. [3] Iliuta, I., Larachi, F., Anfray, J., Dromard, N., Schweich, D. AIChE J., 2007, 53, 2062-2083. [4] Guillou, L., Balloy, D., Supiot, Ph., Le Courtois, V. Appl. Catal. A, 2007, 324, 42-51. [5] Jarosch, K.T., Tonkovich, A.L.Y., Perry, S.T., Kuhlmann, D., Wang, Y. Microchannel Reactors for Intensifying Gas-to-Liquid Technology. In Microreactor Technology and Process Intensification (Y. Wang and J.D. Holladay, Eds.); ACS Symposium Series 914, American Chemical society, Washington, 2005, Chapter 16, pp. 258-272. [6] Cao, C., Wang, Y., Jones, S.B., Hu, J., Li, X.S., Elliott, D.C., Stevens, D.J. Microchannel Catalytic Processes for Converting Biomass-Derived Syngas to transportation Fuels. In Microreactor Technology and Process Intensification (Y. Wang and J.D. Holladay, Eds.); ACS Symposium Series 914, American Chemical society, Washington, 2005, Chapter 17, pp. 273-284. [7] Wang, Y., Vanderwiel, D.P., Tonkovich, A.L.Y., Gao, Y., Baker, E.G. Catalyst Structure and method of Fischer-Tropsch Synthesis. US Patent 6,451,864 B1 to Battelle Memorial Institute.