A CFD (computational fluid dynamics) model has been presented to simulate the steam reforming reaction of DME in this study. A three-dimensional numerical model introduced by the commercial software COMSOL was used to investigate the fluid flow of reformer bed and heating tubes, the energy transport of reformer bed and heating tubes, and the mass transport of reformer bed. The governing equations in the model consist of conservations of mass, momentum, energy and chemical species. In order to optimize the process of reaction, the steam to DME ratio, the inlet temperature, and porosity were investigated. The simulation results showed the changes of temperature distribution, DME conversion and hydrogen production of the steam reforming reaction with different operation conditions.
Energy is the indispensable material basis of human existence, and is also the driving force of social economy development. The rapid development of economy has brought double crisis of energy shortage and environmental pollution, so hydrogen as fuel for fuel cell vehicles gets rapid development [
Several reported studies of experimental and theoretical on steam reforming have been proposed. Chang-Feng Yan et al. investigated a micro-reactor with catalyst coated on nickel foam support and the effect of Cr promoter on performance of steam reforming of dimethyl ether in a metal foam micro-reactor [
The simulated microreactor consists of three domains: catalyst bed, heating tubes, and insulating jacket.
Assuming that the flow regime is stable, laminar and incompressible, the gas mixture is considered as ideal gas. The mixture consists of six species: DME, CH3OH, CO2, CO, H2O, and H2. In addition, the volumetric flow is in the axial direction, while the mass transfer occurs mainly in the lateral direction of diffusion to the reactor wall. Therefore, the three-dimension model is adequate to cope with the reforming reaction.
Based on the above assumptions, the mass, momentum, energy, and species equations of the porous region can be written as follows.
The average temperature distribution equation of the porous bed:
( ρ C p ) t ∂ T s r ∂ t + ∇ ⋅ ( − k s r ∇ T s r ) + ( ρ C p ) f ⋅ u ⋅ ∇ T s r = Q (1)
In the above equation, ρ denotes the gas density, Tsr is the temperature and ksr is the thermal dispersion of the reformer bed. Q represents a heat source, and u is the fluid velocity.
The energy transport equation in heating tubes:
∇ ⋅ ( − k h t ∇ T ) + ρ C p ⋅ u ⋅ ∇ T = 0 (2)
where kht is the thermal conductivity of the heating gas.
∇ ( ρ ω i u − ρ ω i ∑ j = 1 n D i j ( ∇ x j + ( x j − ω j ) ∇ p p ) − D i T ∇ T T ) = R i (3)
Here, ωi is the mass fraction of species i, xj is the molar fraction of species j, Dij is the ij component of the multicomponent Fick diffusivity. Di denotes the generalized thermal diffusion coefficient, T is the temperature, and Ri is the reaction rate.
The inlet and outlet boundary conditions describe a pressure drop across the bed. All other boundaries are impervious, corresponding to the condition:
− κ η ∇ p s r ⋅ n = 0 (4)
Convective heat transport at the outlet is assumed to be dominant:
n ⋅ ( k s r ∇ T s r ) = 0 (5)
For the flow of heating gas in the tubes, the boundary conditions are:
u ⋅ n = v 0 (6)
u = 0 (7)
p = p r e f (8)
Catalytic steam reforming of DME involves many reactions with different rates, the parameters in Equation (9) for DME hydrolysis into methanol were reported by Feng et al. [
r DME = r DMO + = k ′ F , DMO + C R 1 + − k ′ C , DMO + C DMO + (9)
The kinetic equations [
r R = ( 1 − ε ) ρ s k R C CH 3 OH (10)
r 3 = ( 1 − ε ) ρ s k D (11)
Here, ρ s is catalyst density, k R denotes the reforming rate constant, C CH 3 OH is the molar concentration of methanol, k D is the decomposition rate constant.
The kinetic model of WGS reactions is as following [
r WGS = C WGS k WGS ( p CO p H 2 O − p CO p H 2 / K e q ) (12)
where, k WGS and K e q are the rate constant and the equilibrium constant respectively.
A reliable model is crucial to simulate the physical phenomena. In order to verify the reliability and accuracy of the simulated model, numerical results and the experimental data under the same conditions were compared. In the experimental system, the steam and DME flow was controlled by the syringe pump, the mixed gas flowed into the reactor filled with a CuO/ZnO/Al2O3 + ZSM5 catalyst through a mass flow controller. Reactant and product concentrations were measured by gas chromatography.
The reactant gas was preheated to 200˚C, and the steam to DME ratio was 5, the DME conversion and the hydrogen yield are defined as follows:
DME conversion ( % ) = ( F DME , in − F DME , out ) / F DME , in × 100 % (13)
Hydrogen yield ( % ) = F H 2 , out / F DME , in × 1 / 6 × 100 % (14)
where FDME is the mass flow of DME, F H 2 is the mass flow of H2.
The simulation results for DME conversion and yield agree well with the experimental data. Therefore, the numerical simulation error is small and the result is reasonable, the current model can be used to study the reactor.
Effect of inlet temperature was shown in
shows the mole fraction of DME and H2 respectively with the change of gas inlet temperature. As seen in
As shown in
In this study, catalytic steam reforming of DME was simulated using a three-dimensional CFD model introduced in software COMSOL. On the basis of the model, the process of steam reforming reaction was investigated properly.
The simulated model can reveal the hydrogen production and DME conversion properly, and the conclusions are as follows.
1) The reaction with higher steam to DME ratio can produce more hydrogen and improve the DME conversion.
2) Higher inlet temperature can provide more heat needed for the reaction, so as to accelerate the reaction, and lead to more hydrogen.
3) Larger porosity can bring a higher DME conversion and more hydrogen because of a larger reaction surface and more catalyst. The pressure drop between the inlet and outlet of the reactor was smaller.
Wang, J. and Li, C. (2017) Kinetics Study and Simulation of DME Steam Reforming Reaction. Open Access Library Journal, 4: e4022. https://doi.org/10.4236/oalib.1104022