Radial Microchannel Reactors (RMRs) for Efficient and Compact

May 10, 2013 - This paper provides a first report of a novel radial microchannel reactor (RMR) architecture for catalytic steam reforming of methane t...
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Radial Microchannel Reactors (RMRs) for Efficient and Compact Steam Reforming of Methane: Experimental Demonstration and Design Simulations Benjamin A. Wilhite,† Luis Breziner,‡ Jacques Mettes,‡ and Peter Bossard*,‡ †

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77845, United States Power & Energy, Incorporated, Ivyland, Pennsylvania 18974, United States

‡

ABSTRACT: This paper provides a first report of a novel radial microchannel reactor (RMR) architecture for catalytic steam reforming of methane to hydrogen and/or synthesis gas. A bench-scale RMR prototype with a channel width of 0.700 mm and wash-coated with 0.030 mm of a Ni-based catalyst is operated at 750 °C and 11 bar with feed composition of 25 mol % CH4, balance H2O. Results indicate that volumetric heat fluxes in excess of 150 W cm−3 are achievable at residence times of 6.8 ms, while volumetric heat fluxes in excess of 100 W cm−3 are observed at overall thermodynamic efficiencies >70%. Computational fluid dynamic (CFD) simulations of the RMR design are demonstrated for accurately predicting methane steam reformer performance and, subsequently, for rapidly investigating the influence of microchannel dimensions upon RMR performance. Design simulations predict that 20−100% improvements in volumetric heat-transfer rates may be achieved by reducing channel widths from 0.700 to 0.300 mm.

1. INTRODUCTION The endothermic steam reforming of hydrocarbons into hydrogen and carbon oxides plays a central role in the production of H2 for upgrading of petroleum distillates1,2 and bio-derived fuels,2,3 as well as the conversion of natural gas to syngas (a mixture of H2 and CO) for producing synthetic fuels and/or petrochemicals via Fischer−Tropsch processing.4,5 Process intensification via heat-exchanger reactor designs promises breakthroughs in thermal efficiency by coupling endothermic steam reforming with exothermic combustion in separate reaction volumes, thus avoiding the product separation requirements of autothermal reformers.6−8 However, high heattransfer rates between separate steam reforming and combustion volumes are required to achieve competitive overall thermal efficiencies. Microchannel reactor technology provides order-of-magnitude improvements in heat-transport rates, alongside benefits in system portability and achievable energy densities.9,10 To date, several microchannel reactor designs have been reported for integrating catalytic combustion with endothermic steam reforming of hydrocarbons6−8,11−17 using a heat-exchanger reactor configuration. Current microchannel heat-exchanger reactors are manufactured by machining two-dimensional (2D) patterns into individual silicon,11,12 metal,13−16 or ceramic17 plates via chemical etching or laser cutting, followed by sealing and packaging of a “stack” of such plates to achieve the desired total reactor volume.10,12,13 This approach results in a near-linear cost of scale-up, which, until recently, has limited the market penetration of microchannel reactors to the research and nichechemical markets. Recently, the extension of mass-production techniques, including roll embossing and laser cutting, to the production of individual metal plates has enabled the production of industrial-scale microreactors.18,19 However, while these techniques address scale-up needs, the challenge of maintaining gastight sealing between individual plates over © XXXX American Chemical Society

practical operating times remains a significant limitation on both operating pressure and stack size. The latter limitation arises from the accumulation of individual plate “bow” or deformation20−22 during both initial assembly of the microchannel reactor “stack” and, more importantly, high-temperature operation; the formation of significant thermal gradients within the planar microchannel reactor results in different magnitudes of individual plate thermal expansions, placing significant additional strain upon bonding surfaces. The planar microchannel reactor design also suffers inherent drawbacks in terms of thermal efficiency for heat-exchanger applications. In the planar design, the multitude of individual reaction channels is embedded in a single monolithic substrate, allowing for significant thermal “cross-talk” between individual microchannels; more importantly, this means that each microchannel is provided with a direct conductive pathway to the outer surface of the microchannel reactor. These drawbacks have been shown to play a significant role in reducing the effectiveness of microchannel reactors for heat-exchanger reactor applications aimed at high thermal efficiencies and/or autothermal operation.8,23−25 Current solutions to this challenge involve the combustion of additional hydrogen and/or fuel to compensate for heat losses at the external surface of the microchannel reactor system.8 Recently, a theoretical study by Besser26 illustrated the potential of radial microchannel reactor (RMR) designs to mitigate the limitations of planar microchannel reactors. Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: December 25, 2012 Revised: May 2, 2013

A

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Potential advantages of the RMR design, as compared to planar microchannel reactors, include (i) reduced manufacturing costs [significant reduction in the cost of scale-up may be achieved using mass-produced tubular substrates and conventional (automated laser welding) with short sealing requirements], (ii) increased durability and scalability (by minimizing the packaging and sealing contact area, device durability is increased by reducing the occurrence of gas leaks and seal failure; placement of seals is such that each can be inspected and repaired if necessary prior to final reactor assembly), and (iii) minimized thermal stress [by anchoring (small circular welds) the tubes that make up the RMR at one end and allowing the other end to “float”, individual RMRs are capable of absorbing significant thermal gradients without the risk of failure because of thermal strain; likewise, because individual RMR channels are not in contact with each other, differential thermal expansion between the tubes does not compromise seals]. The amount of catalyst surface area per unit of required weld length highlights a significant advantage of the RMR design. For a typical catalyst length of 20−40 cm, this ratio is 10−20 cm, given by eq 1.

Power & Energy, Inc. has developed manufacturing processes for producing RMRs capable of addressing the limitations discussed above. The heart of this technology is the use of tubein-tube assemblies to form individual, thermally isolated RMR subsystems (Figure 1) integrated in a single overall system in

πd i,1LC π (do,1 + do,2)

Figure 1. Schematic of a prototype radial microreactor (RMR) subsystem for syngas and hydrogen production from methane.

≈

LC 2

(1)

This manufacturing technology has been demonstrated for creating massively scaled arrays of tube-in-tube inorganic membranes for hydrogen purification, with over 250 purifiers sold by Power & Energy, Inc. since 2007, all using the RMR design. A commercial system comprised of 1400 parallel RMRs packaged into a 6 in. outer diameter housing is shown in Figure 2c. In light of the growing demand for efficient, low-cost hydrogen and syngas production in the energy and fuels sector, the RMR system is currently being investigated for use in methane or natural gas reforming. In this paper, the authors present the first experimental and theoretical investigations of the commercial RMR technology for the specific application of methane steam reforming (MSR). A bench-scale prototype RMR with channel width of 0.700 mm is employed to measure reactor power requirements over a range of constant reactor wall temperatures (as maintained by variable external heating) and gas flow rates. The power measured is used to determine the overall hydrogen productivity and thermal efficiency of the single-channel RMR prototype. In addition, 2D computational fluid dynamic (CFD) simulations of the RMR prototype are also presented. A comparison of simulation results to experimental data provides validation of the CFD model, which is subsequently demonstrated as a valuable design tool for investigating the impact of RMR geometry upon reformer performance.

parallel or series to achieve scale-up. Each RMR channel is formed by the annular gap (0.3−0.7 mm) between a pair of coaxially aligned tubes; one end of the outer tube is welded closed such that the open end of the inner tube supplies the inlet or exit path for the fluid near the closed end of the outer tube. Manufacturing of an array of identical, parallel RMRs is achieved by laser-welding inner and outer tubes to separate inlet and outlet manifolds (panels a and b of Figure 2), such

Figure 2. Manufacturing techniques developed by Power & Energy, Inc. for creating large arrays of RMRs: (a) laser welding of individual microtubes to the distributor plate, (b) alignment of the inner microtube to the outer microtube to form a radial microchannel, and (c) tube-in-tube system for hydrogen purification, consisting of 1400 individual radial microchannels capable of purifying 76 N m3 h−1 of H2.

2. EXPERIMENTAL SECTION A bench-scale RMR prototype was designed for assessing reactor performance in terms of heat-transfer rates to catalytic MSR over a range of gas flow rates and reactor temperatures. To facilitate heat distribution and monitoring, the prototype RMR was divided into three separate catalytic segments, as shown in Figure 3a. Each segment was constructed from a 38.1 × 30 mm outer diameter cylindrical block of FeCrAl alloy (Kanthal, Sandvik AB). A 7.75 mm hole was machined in each block, and two evenly spaced wells were machined perpendicular to the outer surface at a depth of 7.95 mm to allow for placement of thermocouples at 3.18 mm from the catalyst film. Introduction of proprietary 1 wt % Rh and 15 wt % Ni on an alumina

that individual tubes may freely expand to relieve thermal stresses independently of each other. This is accomplished using an in-house automated 200 W continuous laser-welding system capable of achieving high-quality seals (more than 300 000 failure-free laser welds in the field since 2006) at a rate of ∼3 s/tube. Mechanical stabilization is provided at the sealed end of the RMR via insertion through a “slip plate”, which minimizes transverse deflection of individual tubes while allowing for axial expansion with the changing temperature. B

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Figure 3. Prototype RMR system employed in the present study for investigating the performance of a 700 × 10−6 m width radial microchannel coated with 30 × 10−6 m of steam reforming catalyst for hydrogen production from methane: (a) three-segment assembly and (b) schematic of a single RMR segment. catalyst (PN 1170) at a uniform coating thickness of 0.030 mm and 28.6 mm in length was provided by Catacel Corp. Individual segments were connected to each other using standard 316L stainless-steel tubes and fittings, which allowed for incorporation of standard weld fittings for gas sampling between each segment and inlet−outlet manifolding. The inner tube has a 6.35 mm outer diameter (inner diameter of 4.57 mm) and was placed axially inside the larger bore (Figure 3b). The inlet of the inner tube and the outlet of the outer tube were welded to separate gas manifolds to complete the assembly. The resulting annular microchannel has a channel width of 0.70 mm and is divided into three separate catalytic regions of 28.6 mm in length each. An automated test bench (Figure 4) was constructed to operate and analyze the performance of the RMR prototype. Gas composition

supplying the inner tube of the RMR assembly alleviates any internal preheating of the feed gas, thus isolating steam reforming heat duty, as measured by techniques detailed below. The RMR temperature was maintained using a combination of variable and fixed-power heating bands (Tempco MPP02533) placed around the three catalytic segments and the four adjacent non-catalytic regions. The reactor wall temperature for each segment of the RMR was measured at the interface of the catalyst film and the supporting microchannel wall via K-thermocouples (Omega CAIN-116U-12) connected to a National Instruments touch screen (TPC-2515), equipped with a programmable logic controller (cRIO-9075). Baseline heating rates necessary to maintain each target RMR wall temperature are determined in the absence of methane using Ar and steam flow rates identical to those employed for subsequent reaction experiments. This baseline power supply was maintained constant during MSR experiments for the four non-catalytic regions, to compensate for heat losses to ambient between each catalytic segment. The power supply to the catalytic RMR segments was varied to maintain target reactor wall temperatures under steam reforming conditions. The incremental power required to maintain the RMR wall temperature of each segment is therefore an accurate measurement of heat consumption by the catalytic reforming of methane, which is used to measure the efficiency and performance of the RMR prototype in the present study, as well as providing validation for a CFD model of the RMR system, discussed below.

3. THEORETICAL SECTION A 2D simulation of the radial microreactor (RMR) system was developed and validated through comparison to experimental results and, subsequently, employed to predict RMR performance over a range of reactor space velocities for multiple microchannel widths. A schematic of the 2D reactor model is presented in Figure 5. Dimensions were selected to match those of the experimental system, with additional non-catalytic inlet and outlet flow regions of 10 mm in length each, to provide additional stabilization to the system of differential equations by providing a buffer region between the catalytic zone and inlet/outlet boundary conditions. The radial microchannel width was varied by decreasing the inner diameter of the outer tube at a constant inner tube outer diameter and catalyst film thickness. Details of model development and solution are provided below. 3.1. Fluid-Phase Model Expressions. The fluid phase is described using a combination of continuity and Navier−Stokes equations for weakly compressible flow

Figure 4. Experimental apparatus for testing the RMR performance: (a) schematic and (b) image of the final assembly during operation. employed in all reaction studies was a 3:1 molar supply of H2O and CH4. Steam was provided by pressure-displacement pumping of deionized (DI) water using Ar (99.999%, Airgas) at 14.6 bar from a 4000 mL reservoir. Uniform flow of DI water into an evaporator was maintained using a liquid mass flow controller (Bronkhorst LiquiFLOW L23 V12). Methane (Airgas CP Gr 2.5) was supplied by a gas mass flow controller (Bronkhorst EL-FLOW F201CV) and preheated to the evaporator outlet temperature (400 °C) prior to mixing with steam. The resulting CH4−H2O mixture was then brought to the desired RMR operating temperature using a second preheater and supplied to the inner tube of the RMR assembly at the desired operating temperature of the RMR. Preheating of the feed gas prior to

∇ (ρ u ) = 0

(2)

⎤ ⎡ 2 ρ(u∇)u = ∇⎢ −P I + η(∇u + (∇u)T ) − η(∇u)I⎥ ⎦ ⎣ 3 (3) C

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assuming molecular diffusion volumes (vi) for CH4, CO, CO2, H2O, H2, and N2 of 24.9, 18.9, 26.9, 12.7, 7.07, and 17.9, respectively. 3.2. Catalyst-Phase Model Expressions. Reaction and transport within the catalyst layer is described by Maxwell− Stefan multi-component diffusion assuming a meso−macroporous (∼100−1000 nm) catalyst film such that viscous and Knudsen diffusion contributions are negligible. ⎛ ⎛ ∇P ⎟⎞⎞ ⎟= ∇⎜⎜ρwi ∑ Di , k ⎜∇xk + (xk − wk) ⎝ P ⎠⎟⎠ ⎝ k

∑ R i ,j + ρu∇wi j=1

(9)

The above equation is coupled with the Brinkman equation describing flow in porous media ⎡ ⎛ 1 ⎞⎧ ⎫⎤ ⎛2 ⎞ η u = ∇⎢ −pI + ⎜⎜ ⎟⎟⎨η(∇u + (∇u)T ) − ⎜ η⎟(∇u)I⎬⎥ ⎝3 ⎠ ⎢⎣ ⎭⎥⎦ κ ⎝ εp ⎠⎩ (10)

where the permeability of the catalyst film and the effective Ficks diffusivity of each binary pair are calculated assuming a pore diameter of 150 nm, porosity of 45%, and tortuosity of 1.5.

Figure 5. Schematic of 2D COMSOL simulation, showing system dimensions and boundary conditions: (I) no slip and no heat or mass flux, (II) no slip, no mass flux, and fixed temperature, (III) uniform velocity and species mass flux, and (IV) open boundary for convective heat and mass transport and uniform pressure.

κ=

assuming a constant gas-phase viscosity of 3.5 × 10−5 Pa s and variable densities calculated from individual species mass fractions (wi) via the ideal gas law. ρ=

P RT

i=1

(4)

Heat transport is described by convection and conduction model expression ∇( −k∇T ) = −ρCp,mix u∇T

(5)

assuming a constant fluid-phase thermal conductivity of 0.025 W m−1 K−1. A mass-averaged fluid-phase heat capacity is calculated from individual species mass fractions and heat capacities

∑ wC i p, i i=1

(6)

(12)

RMSR = ((kMSR /pH 2.5 )(pCH pH O − (pH 3 pCO /Ke,MSR ))) 2

4

2

2

/((1 + K COpCO + K H2pH + K CH4pCH + (K H2OpH O /pH ))2 ) 2

4

2

2

(13) R WGS = ((k WGS/pH 2.5 )(pCO pH O − (pH pCO /Ke,WGS))) 2

(7)

2

2

2

/((1 + K COpCO + K H2pH + K CH4pCH + (K H2OpH O /pH ))2 ) 2

4

2

2

(14)

with temperature dependencies for rate coefficients (kj), individual species adsorption coefficients (Ki), and reaction equilibrium coefficients (Kj) described by

10−7T1.75(Mi−1 + Mj−1)1/2 P(υi1/3 + υj1/3)2

(11)

MSR and water−gas shift (WGS) reactions are described using rate expressions presented by Xu and Froment29 for a Ni/ MgAl2O4 catalyst

Individual multi-component Fick diffusivities are calculated from the method by Fuller et al.27 Di , j =

ε Di , j τ

− ρCp,mix u∇T

where individual species heat capacities are calculated using a five-parameter Shomate equation as a function of the fluid temperature. Individual species mass transport is described using the Maxwell−Stefan equation for multi-component diffusion assuming negligible thermal diffusion contributions. ⎛ ⎛ ∇P ⎟⎞⎞ ⎟ = ρ u∇wi ∇⎜⎜ρwi ∑ Di , k ⎜∇xk + (xk − wk) ⎝ P ⎠⎟⎠ ⎝ k

Dieff ,j =

and

∇( −keff ∇T ) = ( −ΔHMSR )RMSR + ( −ΔHWGS)R WGS

N

Cp,mix =

32τ

Heat transport within the catalyst film is described using a convection and conduction model, assuming heat transport via a combination of fluid- and solid-phase thermal conduction and fluid-phase convection. For the former, an effective thermal conductivity of 0.15 J s−1 m−1 K−1 is employed, as calculated from correlations by Butt28 for porous catalysts. The latter term employs fluid-phase density and heat capacity as determined by eqs 5 and 6 in conjunction with the velocity field obtained from the solution of eq 10.

N

∑ ωiMi

d p2εp

(8) D

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Table 1. Kinetic Parameters Employed To Describe Rates of MSR and WGS Reactions Based on the Study by Xu and Froment29 pre-exponential

exponential

kMSR,o kWGS,o KCO KCH4

2.7 × 1019 mol bar0.5 m−3 cat s−1 1.25 × 1013 mol bar−1 m−3 cat s−1 8.23 × 10−5 bar−1 6.65 × 10−4 bar−1

EA,MSR EA,WGS ΔHCO ΔHCH4

240.1 kJ mol−1 67.13 kJ mol−1 −70.7 kJ mol−1 −38.3 kJ mol−1

KH2O

1.77 × 105 bar−1

ΔHH2O

+88.7 kJ mol−1

KH2

6.12 × 10−9 bar−1

ΔHH2

−82.9 kJ mol−1

⎡ EA, j ⎤ kj = kj ,o exp⎢ − ⎥, ⎣ RT ⎦

⎡ ΔHi ⎤ K i = K i ,o exp⎢ , ⎣ RT ⎥⎦

⎡ 26830 ⎤ Ke,MSR = exp⎢ + 30.114⎥ , ⎣ T ⎦ ⎤ ⎡ 4400 Ke,WGS = exp⎢ − 4.036⎥ ⎦ ⎣ T

3.4. Numerical Methods. The above 2D model was implemented using the commercial COMSOL Multiphysics v3.5 programming environment equipped with the chemical engineering module. Both subdomains are described using a combination of weakly compressible flow, convection and conduction, and Maxwell−Stefan multi-component diffusion physics packages, solved in 2D, radial symmetry (r, z) space. Typical finite-element meshes employed in the present work consisted of ∼29 000 individual elements corresponding to ∼480 000 degree of freedom. Error associated with numerical solutions was calculated from a combination of total and individual atomic mass balances and was consistently