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2Dimensional Computational Fluid Dynamic Modeling on Comsol Multiphysics of Fischer Tropsch Fixed Bed Reactor Using a Novel Microfibrous Catalyst and Supercritical Reaction Media
Abstract
Fischer Tropsch synthesis (FT) is a highly exothermic catalyzed reaction to produce a variety of hydrocarbon products and valueadded 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 (SCFFT) as a reaction media with liquidlike 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 SCFFT as a reaction media and MFECC structures using a conventional cobaltbased catalyst in terms of thermal management, syngas conversion, and product selectivity. A 2D 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 gasphase conditions (GPFT) and nonconventional PB in SCFFT media. The potential of scalingup 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 2D model developed in this work was built on experimental data generated at a variety of FT operating conditions both in conventional GPFT operation and in SCFFT 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 SCFFT for a 4” ID bed was just 15 K compared to ~800 K in GPFT bed. The enhancement in thermal performance in the SCFFT 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.
Subject
α Chain growth probabilityα_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 Catoms
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 Preexponential 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 Preexponential factor
A_a Preexponential factor
A_M Preexponential 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 Preexponential 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 nparaffin formation according to detailed kinetics
〖R_(〖C_n H〗_2n )〗^Prod Rate of 1olefins 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
Citation
Abusrafa, Aya Emhemed (2019). 2Dimensional Computational Fluid Dynamic Modeling on Comsol Multiphysics of Fischer Tropsch Fixed Bed Reactor Using a Novel Microfibrous Catalyst and Supercritical Reaction Media. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /188958.
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