2-Dimensional Computational Fluid Dynamic Modeling on Comsol Multiphysics of Fischer Tropsch Fixed Bed Reactor Using a Novel Microfibrous Catalyst and Supercritical Reaction Media
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Date
2019-10-17
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Abstract
Fischer Tropsch synthesis (FT) is a highly exothermic catalyzed reaction to produce a variety of hydrocarbon products and value-added chemicals. To overcome the limitations associated with conventional FT reactors, utilizing high conductivity catalytic structures consisting of microfibrous entrapped cobalt catalyst (MFECC) has been proposed to enhance heat removal from the reactor bed. Additionally, utilization of supercritical fluids (SCF-FT) as a reaction media with liquid-like heat capacity and gaslike diffusivity have been employed to mitigate hot spot formation in FT reactors. The objective of the present study is to investigate the performance of FT Fixed bed/PB reactors operating using SCF-FT as a reaction media and MFECC structures using a conventional cobalt-based catalyst in terms of thermal management, syngas conversion, and product selectivity. A 2-D Computational Fluid Dynamics (CFD) model of an FT reactor was developed in COMSOL® Multiphysics v5.3a for three systems; nonconventional MFECC bed and conventional PB under gas-phase conditions (GP-FT) and non-conventional PB in SCF-FT media. The potential of scaling-up a typical industrial 1.5'' diameter reactor bed to a larger tube diameter (up to 4” ID) was studied as a first step towards process intensification of the FT technology. An advantage of increasing the tube diameter is that it allows for the use of higher gas flow rates, thus enabling higher reactor productivity and a reduction in the number of tubes required to achieve a targeted capacity. The high fidelity 2-D model developed in this work was built on experimental data generated at a variety of FT operating conditions both in conventional GP-FT operation and in SCF-FT reactor bed.
Results showed that the MFECC bed provided excellent temperature control and low selectivity toward undesired methane (CHv4) and high selectivity toward the desired hydrocarbon cuts (C5+). For the 4'' diameter, the maximum temperature rise in the MFECC bed was always 2% below the inlet operational temperature. However, in PB the temperature can go up to 53% higher than the inlet temperature. This resulted in 100% selectivity toward methane and 0% selectivity toward the higher hydrocarbon cuts (C5+).
On the other hand, the CH4 selectivity in the MFECC case was maintained below 24%, while the Cv5+ selectivity was higher than 70%. Similarly, the maximum temperature rise in SCF-FT for a 4” ID bed was just 15 K compared to ~800 K in GP-FT bed. The enhancement in thermal performance in the SCF-FT reactor bed is attributed to the high thermal capacity of SCF media (~2500 J/kg/K) compared to the GP media (~1300 J/kg/K), which resulted in the elimination of hotspot formation.
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α Chain growth probability, α_i Parameter in MSRK Eos, α_(w,int) Heat transfer coefficient from the bed to the inner wall of the tube, [W/m2/K], α_(w,ext) Heat transfer coefficient from the tube wall to the cooling liquid, [W/m2/K], ϵ_bed Bed porosity, κ_bed Bed permeability, [m2], μ_f Fluid viscosity, [Pa. s], ∅_p Sphericityl
λ_er Effective radial heat coefficient [W/m/K], λ_w Thermal conductivity of reactor wall, [W/m/K], γ_i Parameter in MSRK Eos, ρ_f Density of the fluid mixture [kg/m3], α_n Chain growth probability n C-atoms, α_i Parameter in MSRK Eos, α_(w,int) Heat transfer coefficient from the bed to the inner wall of the tube, α_(w,ext) Heat transfer coefficient from the tube wall to the cooling liquid, ϵ_bed Bed porosity, κ_bed Bed permeability, μ_f Fluid viscosity, μ_i Pure component viscosity, ∅_p Sphericity, ∅_ij Dimensionless energy parameter, λ_er Effective radial heat coefficient, λ_w Thermal conductivity of reactor wall, γ_i Parameter in MSRK Eos, ρ_f Density of the fluid mixture, ρ_i Pure component density, a_0 Pre-exponential kinetic parameter, a_M Reaction order of CO, a_ii Binary interaction parameter between species (i) in a mixture, a_ij Binary interaction parameter between species (i) and (j) in a mixture, a_m Parameter in MSRK Eos, A_k Pre-exponential factor, A_a Pre-exponential factor, A_M Pre-exponential factor, b_m Parameter in MSRK Eo, s
b_M Reaction order of H2, β_f Forchheimer drag coefficient, b_ii Binary interaction parameter between species (i) in a mixture, b_ij Binary interaction parameter between species (i) and (j) in a mixture, b_0 Pre-exponential kinetic parameter, C_(p,f) Fluid heat capacity, 〖C_p〗_s Solid heat capacity, C_p Heat capacity within the reactor bed, 〖Cp〗_i Pure component molar heat capacity
c_ij Binary interaction parameter between species (i) and (j) in a mixturel
d_k Diffusional driving force of species, d_p Average particle diameter, d_t Tube diameter, d_w Wall thickness, D_ik Binary pair Maxwell Stefan diffusivities, E_k Activation energy factor in kinetic expression, E_a Activation energy factor in kinetic expression, E_M Activation energy factor in kinetic expressionl
f_co Fugacity of CO, f_(H_2 ) Fugacity of H2, j_i Diffusive flux vector, k Kinetic parameter, k_ij Binary interaction parameter between species (i) and (j) in a mixture, K_1,K_2,K_3 Kinetic parameters, k_eff Effective bed thermal conductivity, k_s Thermal conductivity of solid phase, k_M Kinetic parameter, k_bed Thermal conductivity of the bed, k_f Thermal conductivity of fluid phase, K_i Equilibrium constants, k_i Kinetic rate constants, k_i Pure component thermal conductivity, 〖MW〗_i Molecular weight of species (i), m ̇ Mass flow rate, m_i Parameter in MSRK Eos, m_M Water effect coefficient, n Carbon number, N_i Total flux of species i, p Local reactor pressure, P_co Partial pressure of CO, P_(H_2 ) Partial pressure of H2, P_(c,i) Critical pressure of species (i), Pr Prandtl number, Q Heat source or sink, q Conductive heat flux, r Radial dimension, r_bed Bed radius, 〖〖-R〗_CO〗^YS Rate of carbon monoxide consumption (Yates and Satterfield model), 〖R_(〖CH〗_4 )〗^Ma Rate of formation of methane (Ma model), 〖-R〗_(H_2 ) Rate of hydrogen consumption, R_(H_2O ) Rate of water formation, 〖R_(〖C_2 H〗_4 )〗^Prod Rate of ethene formation according to detailed kinetics, 〖R_(〖C_n H〗_(2n+2) )〗^Prod Rate of n-paraffin formation according to detailed kinetics, 〖R_(〖C_n H〗_2n )〗^Prod Rate of 1-olefins formation according to detailed kinetics, R_i Rate of consumption or production of species i, 〖Re〗_pa Reynolds number, R Universal gas constant, [S] Fraction of vacant sites, T,T_c Local temperature/ Coolant Temperature, T_(c,i) Critical temperature of species (i), u Local velocity vector, U_overall Overall heat transfer coefficient, V_(c,i) Molar volume of species (i), ν_i Stoichiometry coefficient of species (i), w_i Weight fraction of each species (i), ω_i Acentric factor, x_i Mole fraction of species (i), z Axial dimension, ∆H_rxn Enthalpy of FT reaction, Z Compressibility factor