Numerical Simulation of Flow Disturbance and Heat Transfer Effects

Article pubs.acs.org/EF

Numerical Simulation of Flow Disturbance and Heat Transfer Effects on the Methanol-Steam Reforming in Miniature Annulus Type Reformers Rei-Yu Chein,† Yen-Cho Chen,‡ Hung-Jang Zhu,† and J. N. Chung*,§ †

Department of Mechanical Engineering, National Chung-Hsing University, Taichung City, Taiwan 402 Department of Energy Engineering, National United University, Miaoli City, Taiwan 360 § Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611-6300, United States ‡

ABSTRACT: This study numerically investigates methanol-steam reforming (MSR) in miniature annulus type reactors with reactant flow disturbance and hot gas flow heat supply. The reactant flow disturbance is created by placing baffle plates into an inner tube packed with catalyst particles and heated using hot gas flow in the gap between the inner and outer annulus tubes. It was found that the baffle plate increases the heat transfer area and creates flow fluctuations inside the catalyst bed, which lead to enhanced heat transfer between the hot gas and reactant flows and also leads to higher mass transfer inside the catalyst bed. The reactant flow temperature can be increased closer to the hot gas flow temperature, and the enhanced mass transfer between the catalyst particles and reactant flow stream leads to improved methanol conversion in the reformer. The pressure drop across the reactor was found to be not significantly influenced by the baffle plates in miniature scale reactors. It was also found that reducing the thermal resistance between the hot gas flow and reactant flow can further improve the methanol conversion.

1. INTRODUCTION Fuel cells (FCs) have been recognized as an alternative power source owing to their high-energy efficiency and eco-friendly nature.1,2 One of the most promising fuel cell applications is as a power source for portable electronic devices.3 Fuel cells need hydrogen or a hydrogen-rich feed gas as the fuel. Several proven technologies, such as steam reforming,4 autothermal reforming,5 and partial oxidation6 can be used to extract hydrogen from hydrocarbon fuels. Among them, methanol-steam reforming (MSR) has the advantages of low catalytic reaction temperature (200−300 °C),7 liquid phase nature during the fuel distribution process, and potential production from biomass and coal. These features make methanol an attractive fuel for small-scale fuel processors in on-site and/or on-board applications. In MSR, the most widely used catalyst used is CuO/ZnO/ Al2O3 particles.8,9 The catalyst can be either packed in a tube to form a packed-bed reactor10,11 or coated onto the surface to form a plate reactor.12,13 Reactor geometry, steam-to-carbon ratio, reaction temperature, and flow pattern inside the reactor are all factors that affect a reformer’s performance.14,15 Because the MSR is an endothermic reaction, heat must be supplied from an external heat source to activate the reaction. Heat transfer in the reformer plays an important role in the reforming process.16−18 In a packed-bed reactor, effective heat transfer is required to prevent the non-isothermal reaction, which results in lowering the fuel conversion and catalyst degradation.19,20 The thermal resistance between the external heat source and catalyst bed must be reduced as much as possible. In addition to heat transfer, mass transfer also plays an important role in the fuel conversion efficiency. Based on the textbooks by Fogler21 and Mill,22 the steps required for a reforming process in a packed-bed reactor can be classified into three categories that include external diffusion, internal diffusion, and surface reaction. © 2012 American Chemical Society

All of these steps depend greatly on the temperature. As mentioned above, raising the reaction temperature by effective heat transfer is important in enhancing the fuel conversion. It was also shown that increasing the local reactant flow velocity can enhance the external diffusion and, consequently, the overall fuel conversion. Based on the above discussion, enhanced heat and mass transfers are the key issues in reformer design. A plug flow through the catalyst occurs because of the porous media in a packed-bed reformer. The flow velocity is approximately uniform throughout the catalyst bed and the convective heat transfer is poor. This limits reformer performance. Several techniques have been reported in the literature to enhance the heat and mass transfers in packed-bed reactors. Erickson and Liao23,24 introduced an externally applied acoustic field and internally distributed passive disturbers to enhance the heat and mass transfers inside a catalyst bed. Suh et al.25 carried out an experimental and theoretical study of MSR in a packed-bed with internal heating. They found that the methanol conversion and carbon monoxide concentration in an internally heated reformer increase, compared with that from an externally heated reformer under the same conditions. For on-site applications, it would be desired that the reformer be integrated with a fuel cell and a heat supply unit to form a complete portable power system.26 In such a design, the heat required for the reforming process may be provided from combustion in a combustor integrated with the reformer or from other sources such as waste heat. Several integrated reformer− combustor designs have been reported in the literature. Received: October 3, 2011 Revised: January 11, 2012 Published: January 12, 2012 1202

dx.doi.org/10.1021/ef201498t | Energy Fuels 2012, 26, 1202−1213

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Diisterwald et al.27 reported an experimental measurement of methanol conversion in a tubular packed-bed reactor with heat supplied from the steam flowing through a tube arranged concentrically with the reformer. Won et al.28 built a microchannel reactor with combustor for MSR, and they experimentally showed that the combustor performance could be controlled and maintained in the 210−290 °C temperature range. In the studies by Kim29 and Kim and Kwon,30 the performance of a microreformer consisting of a MSR reactor and a catalytic combustor was experimentally and numerically investigated. Good methanol conversion was found in their glass-made reformer− combustor unit. This study numerically investigates the performance of miniature scale reactors in which the tubular packed-bed reformer and combustor are integrated together. The integrated system forms an annulus with finite wall thicknesses, which is similar to the double-pipe heat exchanger.31 The heat supply to the reformer comes from a combustor, located in the shell side of the annulus. The inner tube of the annulus is the reformer filled with catalyst particles. Baffle plates are installed inside the catalyst bed for disturbing the reactant flow. To focus specifically on the flow disturbance in the reactor performance, the flow inside the shell is assumed to be high temperature hot gas flow produced from the combustion reaction. The hot gas temperature and flow disturbance arrangement effects on the methanol conversion efficiency, carbon dioxide production, and overall reactor pressure drop are examined numerically.

be practically implemented because the wall is assumed to be infinitely thin and the heat supply from the wall with a constant temperature can only be achieved in the laboratory. We therefore modified the reformer design to include a concentric cylinder outside the reformer, forming a double-pipe reactor, as shown in Figure 1b. Both the reformer wall and outer cylinder wall have finite wall thicknesses. The hot gas, with inlet temperature Tg,in and flow rate Fg,in, generated from a combustor or a waste heat source is introduced into the shell side for the heat supply. To enhance the heat and mass transfers inside the catalyst bed, baffle plates are introduced inside the reformer to disturb the flow.23,24,33 Figure 2 shows the long and short thin baffle plates placed

2. REACTOR DESIGN The miniature scale tubular reformer reported in the study by Suh et al.32 is used as the baseline case for the design in this study, as shown in Figure 1a. The reformer has a radius of

Figure 2. Integrated reformer−combustor with vertical baffle plates inside the reformer. d = 0.6 mm, s = 0.25 mm, baffle plate thickness = 0.02 mm. (a) 2p reformer. (b) 4p reformer.

perpendicularly to the reactant flow direction along the centerline and attached to the reformer wall at certain pitches. The outer baffle plates are attached to the wall and shaped as washers or annular fins. The inner baffle plates are vertical circular disks that can be held in place by the catalyst or, in practical applications, they can be held in place by a thin horizontal rod that goes through the center of each disk. Figure 3 shows that the baffle plates in Figure 2 are replaced by inclined and fishbone-shaped plates, respectively. These types of baffle plates are also expected to produce flow disturbances inside the catalyst bed. For discussion convenience the reformer design shown in Figure 1b is referred to as the 0p reformer. Considering one long and one short baffle plate as a pair, the reformer designs shown in parts a and b of Figure 2 are referred to as 2p and 4p reformers, respectively. Similarly, the reformer designs shown in parts a and b of Figure 3 are referred to as 2p-fishbone and 4p-fishbone reformers, respectively. Detailed geometric dimensions of the reactor are indicated in these figures. As shown in Figures 2 and 3, the reformer with symmetrically distributed baffle plates is adopted as the physical model because of its geometric simplicity. Although the fabrication of such baffle plates inside a small tube and the associated catalyst

Figure 1. Physical model of the methanol-steam reformer. (a) Thinwall reformer.32 (b) Integrated reformer−combustor with finitethickness walls. L = 10 mm, R1 = 0.5 mm, R2 = 1 mm, tr = tc = 0.2 mm.

R1 = 0.5 mm, and a length of L = 10 mm. The reformer wall is assumed to be infinitely thin and has an elevated temperature for the heat supply. The CuO/ZnO/Al2O3 catalyst particles are packed inside the tube, and the methanol−steam mixture, with an inlet temperature Tin, flow rate Fin, and steam−methanol molar ratio ϕ, enters the reformer. As mentioned in the Introduction, it would be desired that the reformer be integrated with the heat supply unit and the fuel cell to form a complete power supply system. The reformer reported in the study by Suh et al.32 cannot 1203


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kD = CDA2 exp( − E 2 /RT )

(8)

In eq 6, mcat is the catalyst weight and Vb is the catalyst bed volume in which the baffle plate volume is neglected. In eqs 7 and 8, A1, A2, and B1 are constants given in the study by Amphlett et al.35 obtained with temperature, pressure, and space time (ratio of methanol feed rate to catalyst weight) varied in the ranges 160−220 °C, 1−3 atm, and 0.5 × 10−2∼ 1.5 × 10−2 mol kg−1 s−1, respectively. E1 and E2 are the activation energies for the reforming and decomposition reactions, respectively. R and T are the universal gas constant and gas mixture temperature, respectively. CR and CD are the correction factors for the reforming and decomposition reactions accounting for the catalyst activity and effectiveness.32,36 From the reaction rates given in eqs 7 and 8, the generation rates for the species and energy required in MSR can be expressed as

Figure 3. Integrated reformer-combustor with incline and fishboneshaped baffle plates inside the reformer. c = 0.3 mm, s = 0.25 mm, baffle plate thickness = 0.02 mm. (a) 2p reformer. (b) 4p reformer.

loading may be difficult, it can serve as a conceptual model for examining the effect of baffle plates on the MSR performance. For practical applications, we may consider introducing the baffle plates in plate-type reformers.34 In such reformer design, the baffle plates can be easily fabricated with no catalyst loading problem.

(1)

decomposition: CH3OH → 2H2 + CO

(2)

water−gas shift: CO + H2O → H2 + CO2

(3)

As pointed out by Amphlett et al., the water−gas shift reaction can be neglected without loss in accuracy. For a packed-bed microscale reformer, Park et al.36 and Suh et al.32 proposed experimentally corrected semiempirical kinetic models based on the models of Amphlett et al.35 for the steam reforming and decomposition reactions as (4)

rD = (1 − ε)ρskD

(5)

where CCH3OH, ρs, ε, kR, and kD are the methanol molar concentration, catalyst density, catalyst bed porosity, and rate constants for reforming and decomposition reactions, respectively. ρs, kR, and kD are given as

ρs =

mcat m = cat Vb πR12L

kR = CR [A1 + B1 ln ϕ] exp( − E1/RT )

rH2O = − rR

(9b)

rCO2 = rR

(9c)

rCO = rD

(9d)

rH2 = 3rR + 2rD

(9e)

qc = − rR ΔHR − rDΔHD

(10)

4. GOVERNING EQUATIONS OF FLUID FLOW, HEAT TRANSFER, AND SPECIES TRANSPORT 4.1. Governing Equations. To simplify the analysis, the following assumptions are made: (1) The catalyst particles are assumed to be spherical with a diameter dp, and the catalyst bed is treated as a homogeneous porous medium with porosity ε and permeability K. (2) The flow is assumed to be steady. With the reactants entering the reformer in a flow parallel to the reformer axis at a small mass flow rate and then flowing through a homogeneous catalyst bed, the flow inside the reformer can be assumed to be laminar and axisymmetric. (3) The catalyst bed is in local thermal equilibrium with the surrounding gas mixture. (4) The species in the gas mixture are ideal gases. (5) The hot gas used for heat supply is treated as air. (6) There is no heat generation in the reformer and outer cylinder walls. (7) The reformer, baffle plate, and the outer cylinder are made of the same material. Based on the above assumptions, the governing equations for the mass conservation, fluid flow, energy transport, and mass transfer inside the reformer can be written as37−39

35

rR = (1 − ε)ρskR CCH3OH

(9a)

where ΔHR and ΔHD are the reaction heat during the reforming and decomposition reactions, respectively.

3. CHEMICAL REACTION MODEL Using CuO/ZnO/Al2O3 as the catalyst, the chemical reactions taking place during the MSR are8,9

steam reforming: CH3OH + H2O → 3H2 + CO2

rCH3OH = − rR − rD

∇· (ρV⃗ ) = 0

1 2

ε

(6)

(11)

⃗ ⃗ ) = −∇p + ∇· (ρVV −

(7) 1204

μ 1 ·∇(∇μV⃗ ) − V⃗ ε K

ρCF |V ⃗ |V ⃗ K

(12)


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⃗ ) = ∇· (λe∇T ) + ∇· (ερc pVT

ε2μ ⃗ |V | + qc K

⃗ i) = ∇· (Di∇Ci) + ri ∇· (εVC

λe, shown in eq 13, is the effective thermal conductivity of the catalyst bed, defined as

(13)

λe = ελ + (1 − ε)λ s

(14)

where λs is the thermal conductivity of the catalyst particle. Because there are no heat generations in the reformer wall, combustor wall, and the baffle plates, the temperature in these solid domains, Tc, is simply governed by the heat conduction equation:

In eqs 11−14, V⃗ , p, and Ci are the local velocity, pressure, and molar concentration of the ith species in the gas mixture, respectively. Assuming an ideal gas mixture, the mass-weighted density ρ and specific heat cp of the gas mixture can be expressed as

∇·(λc∇Tc) = 0

NG

NG

p ρ= RT

∑ xiMi

cp =

∑ mic p, i

where xi, mi, Mi, and cp,i are the molar fraction, mass fraction, molecular weight, and specific heat of the ith species, respectively, in the gas mixture, and NG is the number of species in the mixture. The thermal conductivity λ, dynamic viscosity μ of the gas mixture, and molecular diffusivity of the ith species Di can be evaluated based on the Chapman−Enskog theory for multicomponent gas mixtures at low density:22 NG

λ=

∑

μ=

G ∑N j = 1 xj ϕij

i=1

Di =

NG

xi λ i

∑ i=1

xiμi

(16)

where λi and μi are the thermal conductivity and viscosity of the ith species, respectively, given as

λ i = (8.32 × 10−2)

T / Mi σi2Ωλ, i

μi = 2.67 × 10−6)

(

M iT 2 σ i Ω μ, i

(17)

Dij and ϕij are the binary diffusivity and Chapman−Enskog parameter, defined as

ϕij =

[1 + (μi /μj)

8 [1 + (Mj /Mi)]1/2

]

CF = 1.75/( 150 ε3/2)

∇· (ρgcp ,gVg⃗ Tg) = ∇· (λ g∇Tg)

(25)

C H2 = 0

CCO = 0

(26a)

(2) reformer outlet (z = L, 0 < r < R1)

∂Ci ∂V⃗ ∂T = = =0 (26b) ∂z ∂z ∂z (3) reformer walls at inlet and outlet (z = 0, z = L, R1 < r < R1 + tr)

(18)

where σi is the collision diameter of the ith species and σij = 1 /2(σi + σj). Ωλ,i, Ωμ,i, and ΩD,ij are the collision integrals for thermal conductivity, viscosity, and binary diffusion, respectively. Equation 12 is known as the Brinkman−Darcy−Forchheimer model for the fluid flow in a porous medium with a homogeneous porosity. Using the Carman−Kozeny model,37−39 the permeability K and the Forchheimer drag coefficient CF for a packed bed with spherical particles can be written as

K = d p2 ε3/(150(1 − ε)2 )

(24)

C H2 = 0

1/4 2

(M j / M i )

∇· (ρgVg⃗ Vg⃗ ) = − ∇pg + ∇· (∇μgVg⃗ )

C H2O = ϕCCH3OH,in

pσij2Ω D, ij 1/2

(23)

u = u in v = 0 T = Tin CCH3OH = CCH3OH,in

⎛1 1 ⎞ T 3⎜ + ⎟ Mj ⎠ ⎝ Mi

Dij = (1.86 × 10−7)

∇· (ρgVg⃗ ) = 0

where ρg, μg, λg, cp,g, V⃗ g, pg, and Tg are the density, viscosity, thermal conductivity, specific heat, velocity, pressure, and temperature of the hot gas, respectively. All governing equations were written in a cylindrical coordinate system (r, θ, z). Utilizing the symmetry in the θ coordinate, we can recast the problem as an axisymmetric twodimensional model (r, z). The fluid velocity components in the r- and z-directions for the reactant flow are u and v, respectively. For the hot gas flow, the velocity components are ug and vg, respectively. 4.2. Boundary Conditions. Boundary conditions must be specified to complete the mathematical model. Referring to Figures 1, 2, and 3, the boundary conditions are specified as follows: (1) reformer inlet (z = 0, 0 < r < R1)

G ∑N j = 1 xj ϕij

1 − xi xj NG ∑ j = 1, j ≠ i Dij

(22)

where λc is the thermal conductivity of the solid. The flow velocity and energy transport of the hot gas flowing through the annulus are governed by the following equations:31

(15)

i=1

i=1

(21)

∂Tc =0 ∂z (4) reformer inner wall (0 < z < L, r = R1) V⃗ = 0

(19)

T = Tc

∂Ci =0 ∂r

(20) 1205

λe

(26c)

∂T ∂T = λc c ∂r ∂r (26d)


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residuals are less than 10−8 for the energy and species distributions. Because the numerical solution accuracy strongly depends on the mesh size, a refined mesh is necessary in the region where the dependent variable gradients are pronounced. Finer meshes were used to capture the subtle changes in velocity, temperature, and species concentration in the nearwall region. In a catalytic reactor with endothermic reactions, the velocity and temperature are primarily the independent variables, while the methanol molar fraction is a highly dependent variable. Therefore, we believe that if the molar fraction is proved to be grid-independent then the velocity and temperature must also be grid-independent. Under the conditions of Fin = 1000 μL/min, mcat = 25 mg, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min for the 4p-fishbone reformer, the variation of methanol molar fraction along the reformer centerline is examined for grid sensitivity test, and the result is shown in Figure 4. As shown in Figure 4, the deviation

(5) reformer outer wall (0 < z < L, r = R1 + tr)

Vg⃗ = 0

Tg = Tc

λg

∂Tg ∂r

= λc

∂Tc ∂r

(26e)

(6) baffle plate surfaces

V⃗ = 0 λc

T = Tc

λc

∂Tc ∂T = λe ∂n ∂n

∂Tc ∂T = λe ∂n ∂n

n = normal outward direction (7) along reformer centerline (0 < z < L, r = 0)

∂Ci ∂V⃗ ∂T = = =0 ∂r ∂r ∂r (8) hot gas inlet (z = 0, R1 + tc < r < R2) ug = ucg,in ug = ug,in Tg = Tg,in

(26f)

(26g)

(26h)

(9) hot gas outlet (z = L, R1 + tc < r < R2)

∂Vg⃗

=

∂Tg

=0 (26i) ∂z ∂z (10) combustor walls at inlet and outlet (z = 0, z = L, R2 < r < R2 + tc)

∂Tc =0 ∂z (11) combustor outer wall (0 < z < L, r = R2 + tc) −λc

∂Tc = h∞(Tc − T∞) ∂z

(26j)

(26k)

In eqs 26a, uin = Fin/πR12 is the reformer inlet velocity, inlet temperature, molar concentration of CH3OH, and the molar ratio of steam to methanol, respectively. For the reformer outlet, Neumann boundary conditions are specified for the flow velocity, temperature, and species concentrations, as indicated in eq 26b. In eq 26h, ug,in = Fg,in/[π(R22 − R12)] and Tg,in are the hot gas inlet velocity and temperature, respectively. For the hot gas outlet, Neumann boundary conditions are specified for the flow velocity and temperature, as indicated in eq 26i. At the interfaces, as indicated in eqs 26d, 26e, and 26f, there is no species deposition and no-slip boundary conditions are specified for both the fluid flow and the temperature distributions. The reformer and combustor walls at the inlet and outlet are assumed to be insulated as shown in eqs 26c and 26j, respectively. In eq 26k, the outer wall of the combustor is exposed to an environment with heat transfer coefficient of h∞ and temperature of T∞. Finally, eq 26g indicated that the flow velocity, temperature, and species molar concentration are all symmetrical with respect to the reactor centerline. 4.3. Numerical Methods. All of the governing equations along with the boundary conditions were solved simultaneously using the commercial CFD code FLUENT version 6.2.40 The governing equations were discretized by the finite volume method. The second order upwind scheme was used for the discretized convective terms, and the SIMPLE algorithm was used for the coupling the pressure and velocity. The convergence criterion was that the mass residual should be less than 10−6 for the flow field, and the energy and species

Figure 4. Variation of methanol molar fraction along the reactor centerline for various grid numbers. Fin = 1000 μL/min, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min.

of the methanol molar fraction variation becomes insignificant as the grid number is greater than 120 000. In the current study, we went even further by using 200 000 grid points, so we are confident that our results are grid-independent.

5. RESULTS AND DISCUSSION The numerical model used in this study is verified by comparing the methanol conversion efficiency results computed from the present numerical model with those reported by Suh et al.32 under the same reformer geometry and catalyst bed parameters. Under the conditions of Fin = 10 μL/min and Tin is equal to the wall temperature Tw, Figure 5 shows that the agreement between our predictions and those by Suh et al.32 is good for the methanol conversion efficiency as a function of the reformer wall temperature and catalyst weight. On the basis of this comparison, the present numerical model can then be extended to the methanol conversion simulation in the integrated reformer−combustor unit. To specifically focus on the flow disturbance and hot gas temperature effects on MSR, Table 1 lists the fixed parameters used for the results reported in the following, while the reformer geometric dimensions have 1206


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flow at the inlet is 0.1965 and the flow is in the laminar and creeping regime. Because the Reynolds number is very low, there is no vortex shedding as the flow impinges on and passes over the baffle plates. The sizes of the recirculation zones behind the baffle plates are also small. From the flow streamlines, the flow disturbance is clearly observed by the presence of baffle plates. For the temperature distributions, it is seen that a significant temperature variation can be found at the entrance region due to a large temperature difference and endothermic MSR reaction. Greater understanding of the flow and temperature distributions inside the catalyst bed can be addressed by examining the velocity and temperature variations along the reformer centerline and cross-sectional profiles at various reformer lengths. In Figure 7, the flow velocity characteristics in the reformers are shown. Compared with the 0p reformer shown in Figure 7a, the flow velocities for both 4p and 4p-fishbone reformers fluctuate with slightly increasing amplitude because of the presence of baffle plates, while the velocity of the 0p reformer increases slightly along the reformer length without fluctuation. The slight increase in flow velocity is due to the decrease in gas mixture density during the reforming process. The flow fluctuations and increased local velocity magnitude are believed to enhance the heat transfer between the reactant flow and hot gas flow and also to increase the mass transfer from the catalyst particle surface to the free reactant flow stream by the increase in the effective reactant flow path.23,24 On the basis of the results shown in Figure 7a, it is found that the inclined and fishbone-like baffle plates can result in slightly higher velocity fluctuation, as compared with the vertical plate case. In Figure 7b and c, the velocity profiles at three reformer axial locations are shown for a more detailed understanding on the effect of baffle plates on the flow characteristics. With the introduction of baffle plates, the local velocity is increased, as compared with 0p reformer, which has a plug-like velocity profile. The results shown in Figure 7b and c indicate that more severe velocity disturbance can be generated in the 4p-fishbone reformer. In Figure 8a, temperature distributions along the reformer and combustor centerlines are shown. By the introduction of

Figure 5. Comparison of methanol conversion predicted from the numerical model used in this study with the results reported in the study of Suh et al.32 Fin = 10 μL/min; Tin = Tw.

been shown in Figures 1−3. The inlet volumetric flow rate of the water−methanol mixture Fin is chosen in the range 10−1000 μL/min, and the inlet temperatures for all cases studied are fixed at Tin = 473 K. On the basis of these conditions, the Reynolds number at the reformer inlet ranges from 0.00393 to 0.393. The gas hourly space velocity (GHSV), defined as the ratio of Fin to the catalyst bed volume Vb defined in eq 6, ranges from 0.19 to 1909. As listed in Table 1, the steam−methanol molar ratio ϕ is fixed at 1.1. This corresponds to inlet molar fractions of methanol and steam of 0.476 and 0.524, respectively. The combustor outer wall is insulated. Figure 6 shows typical flow patterns and temperature distributions in the 4p and 4p-fishbone reformers under the conditions: Fin = 500 μL/min, Tin = 473 K, and Tg,in = 600 K. Under these conditions, the Reynolds number of the reactant Table 1. Fixed Parameters Used in the Computation param

value

steam−methanol molar ratio, ϕ reformer and combustor wall thermal conductivity, λc catalyst thermal conductivity, λs catalyst particle diameter, dp catalyst layer porosity, ε catalyst wt, mcat pre-exp factor, A1 pre-exp factor, B1 pre-exp factor, A2 correction factor for reforming reaction, CR correction factor for decomp. reaction, CD activation energy of reforming reaction, E1 activation energy of decomp. reaction, E2 hot gas flow rate, Fg,in hot gas density, ρg hot gas specific heat, cp,g hot gas viscosity, μg hot gas thermal conductivity, λg environ. heat transfer coefficient, h∞ environ. temp., T∞

1.1 18 20 150 0.35 25 1.15 × 106 9.41 × 105 7.09 × 107 5.5 3.5 84100 111200 8.5 0.706−0.548 (500K∼650K) 1024−1063 (500K∼650K) 26.33−31.77 × 10−6 (500K∼650K) 0.0389−0.0475 (500−650K) 0 298 1207

unit

ref

W m−1 K−1 W m−1 K−1 μm

AISI316 stainless steel22 Suh et al.32 Suh et al.32 Suh et al.32 Suh et al.32 Amphlett et al.35 Amphlett et al.35 Amphlett et al.35 Suh et al.32 Suh et al.32 Amphlett et al.35 Amphlett et al.35

mg m3 s−1 kg−1 m3 s−1 kg−1 mol/kg s

J mol−1 J mol−1 μL min−1 kg m−3 J g−1 K−1 kg m−1 s−1 W m−1 K−1 W m−2 K−1 K

air31 air31 air31 air31


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Figure 6. Flow patterns and temperature distributions in the integrated reformer-combustors under the conditions : Fin = 500 μL/min, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min.

in the catalyst bed can be raised closer to the hot gas temperature for the reformer with baffle plates because of the reduced thermal resistance, as shown in Figure 8a. From Figure 8a, it is also found that there is not much difference in the temperature distributions for both 4p and 4p-fishbone reformers. This indicates that the heat transfer enhancement is mainly due to the heat conduction mechanism. The temperature distributions shown in Figure 8a also indicate that there is no significant temperature drop occurring at the reformer entrance, as found in the larger scale reformers.15,23 The reason for this observation may be attributed to the reduced thermal resistance between the hot gas flow and reactant flow in a miniature scale reactor. Based on the studies of Karim et al.20 and Mears,41 the temperature difference between

baffle plates, the heat transfer between the hot gas flow and reactant flow can be enhanced by two mechanisms. Because the baffle plates attached on the reformer wall can be regarded as fins, heat conduction transfer from the hot gas flow to the catalyst bed is enhanced due to the increased heat conduction area. Inside the catalyst bed, local reactant flow velocity is increased due to the baffle plates, as shown in Figure 7b and c. The increased velocity enhances the convective heat transfer inside the catalyst bed. Both heat transfer enhancement mechanisms lead to the reduced thermal resistance between the hot gas flow and reactant flow that results in a higher temperature inside the 4p and 4p-fishbone reformers, as compared with the 0p reformer. The reactant flow temperature 1208


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Figure 7. Comparison of velocity variations between the reformers with and without the baffle plates. (a) Velocity along the reformer centerline. Velocity profiles of (b) 4p reformers and (c) 4p-fishbone reformers, at various reformer locations. Fin = 500 μL/min, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min.

Figure 8. Comparison of temperature variations between the reformers with and without the baffle plates. (a) Temperature along the reformer and combustor centerlines. (b) Temperature profiles of 4p reformers at various reformer locations. (c) Ttemperature profiles of 4p-fishbone reformers at various reformer locations. Fin = 500 μL/min, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min.

the reformer wall and catalyst bed in an annulus packed bed reactor can be evaluated as

1 |qc|rCH3OH ⎛ 2 4λe ⎞ Tmin = Tw − R1⎟ ⎜R1 + hw ⎠ 4 λe ⎝

where Tmin is the minimum temperature in the catalyst bed, Tw is the reformer wall temperature, and hw is the heat transfer coefficient at the wall. From eq 27, it is clearly seen that the temperature drop depends on the reformer size. Therefore, the

(27) 1209


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Figure 9. (a) Molar fraction distributions of CH3OH, H2, and CO in the reformer and (b) molar fraction variations of all the species along the reformer centerline for 4p-fishbone reformer under the operation conditions: Fin = 500 μL/min, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min.

temperature difference between the wall and the catalyst bed would be reduced as the reformer size is reduced. In our study, the reformer has a radius of 0.5 mm. For such a miniature scale reactor, there is almost no temperature drop in the reformer entrance zone, as shown in large scale reactors.15,23 In Figure 8b and c, the temperature profiles at three locations are shown, and there is no significant difference in the temperature profiles between the 4p and 4p- reformers. As mentioned above, the heat conduction is the main mechanism in enhancing the heat transfer between the hot gas and reactant flows. In Figure 9, typical species molar fraction distributions in the reformer under the operation conditions of Fin = 500 μL/min, Tin = 473 K, Tg,in = 600 K, and Fg,in = 8.5 μL/min are shown. In Figure 9a, the methanol molar fraction decreases along the reformer length as a result of the reforming reaction. Because of the reforming reaction, both molar fractions of H2 and CO increase along the reformer length, as shown in the molar fraction distributions of H2 and CO, respectively. In Figure 9b, molar fraction variations of all species involved in the reforming reaction along the reformer centerline are shown for a better understanding on the methanol consumption and hydrogen generation in the reformer. These variations are discontinuous due to the presence of baffle plates. The overall methanol conversion efficiency and carbon monoxide (CO) production efficiency are defined as

ηconv =

CCH3OH,in − C̅CH3OH,out CCH3OH,in

Figure 10. Methanol conversion efficiency as functions of hot gas inlet temeprature and GHSV for various types of reformers: (a) Tg,in = 500 K, (b) Tg,in = 600 K, and (c) Tg,in = 650 K.

ηCO =

× 100 (28) 1210

C̅CO,out CCH3OH,in

× 100 (29)


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where C̅ CH3OH,out and C̅ CO,out are the averaged methanol and CO concentrations at the reformer outlet calculated as

Ci̅ ,out =

1 πR12

∫0

R1

Ci ,out 2πr dr

i = CH3OH, CO

(30)

Figure 10 shows the methanol conversion efficiencies as a function of GHSV for various types of reformers and for different inlet hot gas temperatures. It is seen that the methanol conversion can be enhanced by introducing the flow disturbances. With more baffle plates placed in the catalyst bed, higher conversion can be obtained but not significantly. Because of the low Reynolds number flow, the results shown in Figure 9 indicate that the type of baffle plate would produce insignificant effect in the chemical reaction. That is, the MSR is a temperature dominant reaction in the miniature scale reformer. The methanol conversion is also enhanced using higher hot gas temperatures. For a small flow rate and high inlet hot gas temperature with Tg,in = 650 K, almost 100% methanol conversion efficiency can be obtained and it is about 70% increase as compared with the 0p reformer. Figure 11 shows

Figure 11. CO generation as functions of hot gas inlet temeprature and GHSV for various types of reformers: (a) Tg,in = 500 K, (b) Tg,in = 600 K, and (c) Tg,in = 650 K.

the CO production corresponding to the results in Figure 8. With enhanced methanol conversion, the CO production is also increased. Because the fuel cell can be severely poisoned by an extremely small amount of carbon monoxide, an additional CO cleanup unit should be integrated into the reformer to reduce the CO concentration in the reformed gas. Preferential oxidation (PROX) is one of the chemical reactions that can be used to eliminate CO in the reformed gas.42 Figure 12 shows the pressure drop across the reformer for various GHSV under the Tg,in = 600 K condition. The pressure drop increases with the increase in baffle plate numbers and flow rate. Reformers with an inclined and fishbone-shaped baffle plates have higher pressure drops compared with vertical baffle plate reformers. Although the pressure drop is increased by the introduction of baffle plates, it is not significant compared with a reformer without any baffle plates when a mini- or microscale reformer is considered.

Figure 12. Pressure drop across the reformers as a function of GHSV for various types of reformers.

According to the study of Won et al.,28 the maximum exhaust gas temperature from Pt-catalytic combustion using methanol/ air mixture as the reactant is about 270 °C. Therefore, the exhaust gas from the combustor can be used as the hot gas flow for supplying the heat for reforming. With the introduction of baffle plates, the methanol conversion can be enhanced because of the enhanced heat transfer between the hot gas flow and reactant flow. As described above, the enhanced heat transfer 1211


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of the annulus. The reactant flow inside the catalyst bed was disturbed by introducing baffle plates. On the basis of the results presented in this study, the following conclusions can be drawn: (1) The flow disturbance inside the catalyst bed created by the baffle plate enhances the heat transfer between the hot gas flow and the reactant flow. The reactant flow temperature can be raised closer to the hot gas flow temperature to improve the chemical reaction in the catalyst bed. Due to the flow fluctuation, the mass transfer between the catalyst particle and reactant flow stream is also enhanced by the increased local velocity. (2) The pressure drop across the reformer increases when the baffle plate is introduced. However, it was found that the pressure drop increase is not significant compared with the reformer without baffle plates when a mini- or microscale reformer is considered. (3) The thermal resistance between the hot gas flow and the reactant flow plays an important role in the integrated reformer−combustor unit. Reducing the thermal resistance by either enhancing convective or conductive heat transfer can further improve the methanol conversion. A CO cleanup unit must be incorporated to reduce the amount of CO in the reformed gas because the CO generation is also enhanced. (4) The performance enhancement for the methanol conversion demonstrated in a miniature annulus reactor with baffle plates may be implemented by using the plate type reactor to avoid the difficulties in baffle plate fabrication and catalyst loading in a small tube.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 352-3921071. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 13. Effect of reformer wall thickness on the methanol conversion efficiency (a) and the reactant temperature (b).

implies that the thermal resistance between the hot gas flow and the reactant flow is reduced. That is, reducing the thermal resistance between the reactant flow and the hot gas flow results in an enhanced reforming efficiency for the integrated reformer-combustor design. Besides the introduction of baffle plates, using a high thermal conductivity material for the reformer and reducing the reformer wall thickness may also lead to reduced thermal resistance. Figure 13a shows that the methanol conversion efficiency can be increased by reducing the reformer wall thickness from 0.2 to 0.1 mm for the 0p reformers. The increase in methanol conversion efficiency is clearly due to the thermal resistance reduction that produces about a 5 K temperature increase for the reactant flow, as shown in Figure 13b.

6. CONCLUSION The performance of miniature scale annulus type reformers was numerically studied. A hot gas flowing in the shell side of the annulus is used as the heat source for the reforming process taking place inside the reformer, which is the inner tube 1212

NOMENCLATURE A1, B1 = constants in reforming reaction rate, m3 s−1 kg−1. A2 = constant in decomposition reaction rate, mol s−1 kg−1. CD = correction factor decomposition reaction rate. CF = Forchheimer drag coefficient. Ci = molar concentration of species i, mol m−3. cp = gas mixture specific heat, J kg−1 K−1. c = height of baffle plate, m. CR = correction factor for reforming reaction rate. D = mass diffusivity, m2 s−1. dp = catalyst particle diameter, m. E1 = activation energy for reforming reaction, J mol−1. E2 = activation energy for decomposition reaction, J mol−1. F = volumetric flow rate, m3 s−1. GHSV = gas hourly space velocity, h−1. h = heat transfer coefficient, W m−2 K−1. K = catalyst layer permeability, m−2. kR = rate constant of MSR reaction, m3 s−1 kg−1. kD = rate constant of methanol decomposition reaction, mol s−1 kg−1. L = reformer length, m. mcat = catalyst weight, kg. M = molecular weight, g mol−1. m = mass fraction. MSR = methanol-steam reforming. dx.doi.org/10.1021/ef201498t | Energy Fuels 2012, 26, 1202−1213

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NG = number of species in the gas mixture. p = pressure, Pa. qc = energy source term due to the chemical reaction, J m−3. R1 = reformer radius, m. R2 = combustor radius, m. ri = molar generation rate of species i, mol m−3 s−1. rR = MSR reaction rate, mol m−3 s−1. rD = methanol decomposition reaction rate, mol m−3 s−1. s = height of baffle plate, m. T = temperature, K. tr = reformer wall thickness, m. tc = combustor wall thickness, m. u = axial velocity component, m s−1. V⃗ = velocity vector, m s−1. v = radial velocity component, m s−1. Vb = catalyst bed volume, m3. x = mole fraction. Greek Symbols

ΔHD = heat of decomposition reaction, J mol−1. ΔHR = heat of reforming reaction, J mol−1. ε = catalyst bed porosity. λ = thermal conductivity, W m−1 K−1. μ = viscosity, kg m−1 s−1. ϕ = steam−methanol molar ratio. ρ = gas mixture density, kg m−3. σ = collision diameter, Å. ΩD = collision integral for diffusion. Ωλ = collision integral for thermal conductivity. Ωμ = collision integral for viscosity.

Subscripts

c = solid wall i = ith species of the gas mixture in = inlet out = outlet g = hot gas flow s = catalyst particle w = wall ∞ = surrounding

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Numerical Simulation of Flow Disturbance and Heat Transfer Effects

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