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Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7
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Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen Ozgur Yasar Caglar, C. Doga Demirhan, Ahmet K. Avci* Department of Chemical Engineering, Bogazici University, Bebek 34342, Istanbul, Turkey
article info
abstract
Article history:
The objectives of this study are to investigate steam reforming (SR) of glycerol, by-product
Received 16 August 2014
of biodiesel synthesis, in a wall-coated catalytic microchannel by a detailed mathematical
Received in revised form
model and design an adiabatic microchannel reformer that can be integrated into a bio-
12 October 2014
diesel production plant with an annual capacity of 4 103 m3/year. Modeling and simu-
Accepted 17 October 2014
lation studies are based on Finite Element Method (FEM) technique. Sizing and design of
Available online xxx
the multichannel reactor involving wall-separated arrays of parallel microchannels is based on converting a minimum of 85% of the glycerol obtained as a by-product of the
Keywords:
biodiesel production. The results show that the microchannel architecture enables fast and
Glycerol
uniform transfer of the sensible heat of the feed stream to the catalyst layer. This phe-
Steam reforming
nomenon, which is more pronounced by using thick and high thermal conductivity walls
Microchannel
between channels, allows SR to run without external heat supply and leads to glycerol
Modeling
conversions above 85%. It is demonstrated that a multichannel unit of 1 102 m3 volume,
Reactor design
operating without external energy supply, is sufficient to convert ca. 90% of the glycerol produced in a biodiesel plant with a capacity of 4 103 m3/year. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The importance of renewable fuels as alternatives to their existing, non-renewable counterparts is becoming more significant due to the increasing cost of obtaining high quality crude oil as a result of the depleting oil reserves [1]. One of the most attractive renewable fuels is biodiesel whose global use is expected to increase continuously. Biodiesel can be produced by catalytic trans-esterification of animal-based or vegetable oils with methanol (or ethanol) [2]. In this process, 1 mol of glycerol is produced for every 3 moles of biodiesel. This stoichiometry, however, causes undesirably high supply of glycerol. Due to the rising demand of biodiesel, the
amount of by-product glycerol that will be produced by 2020 is expected to increase up to 3 megatons, whereas the total glycerol demand will be less than 500 kilotons [2]. The significant gap between the demand and supply of glycerol will eventually increase the cost of biodiesel, as well as biodiesel blended diesel fuel due to the elevated cost of glycerol removal. Solution of this problem requires efficient conversion of glycerol into value-added products such as hydrogen, which is the feedstock for many petrochemical processes as well as for the fuel cells delivering efficient electricity generation. One of the most well known techniques for producing hydrogen from glycerol is by its reforming with steam, which is carried out at temperatures in excess of ca. 600 K over a supported catalyst:
* Corresponding author. Tel.: þ90 212 3597785; fax: þ90 212 2872460. E-mail address: [email protected] (A.K. Avci). http://dx.doi.org/10.1016/j.ijhydene.2014.10.070 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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C3 H8 O3 þ 3H2 O ¼ 3CO2 þ 7H2
DH0 ¼ 346:4 kJ mol
(1)
Catalysis of glycerol SR is well investigated. Most of the studies involve the use of Ni-based catalysts [3e6], which are demonstrated to deliver glycerol conversions above ca. 90% depending on the reaction temperature, molar steam-tocarbon ratio at the inlet and contact time. In addition to Nibased ones, the performance of noble metals such as Rh, Pt, Ru, Pd, Ir [7e9] are also investigated. Apart from the type of active metal, the nature of the catalyst support is found to have significant impact on glycerol conversion and hydrogen selectivity. Adhikari et al. [10] compared the use of CeO2, MgO and TiO2 as supports for Ni in glycerol SR and reported that hydrogen selectivity followed the order of Ni/CeO2 > Ni/ MgO > Ni/TiO2. Pompeo et al. [11] reported that glycerol SR over Pt/g-Al2O3 and Pt/ZrO2 led to carbon formation and the catalysts are deactivated rapidly due to the acidic nature of both supports. The same authors showed that the use of SiO2, a support with neutral properties, with Pt gave stable operation together with high activity and hydrogen selectivity [11]. An extensive review of catalysts and supports studied for glycerol-to-hydrogen conversion is reported elsewhere [2]. Hydrogen production from glycerol requires high temperatures and external energy demand, both of which depend strongly on the catalyst type and reactor geometry. Even though the effect of catalyst on glycerol SR is studied extensively, the reactor type is limited to the conventional packed bed configuration. Recent studies on non-oxidative and oxidative steam reforming of methane to synthesis gas showed that structuring the reactor and catalyst by using the microchannel technology can provide methane conversions and CO selectivities higher than those obtained in a packedbed reactor operated under equivalent conditions [12e14]. Similar results, demonstrating the superior properties of microchannel reactors in terms of improved conversions and high product selectivities are reported by other groups [15e17]. Microchannel reactors are defined as units involving parallel channels with hydraulic diameters below 1 103 m [18]. Inner walls of the channels are coated with porous catalyst layers with less than ca. 1 104 m thickness which offers minimum resistance to heat and mass transfer within the catalyst (Fig. 1a). Since the reactor block is mostly a metal and the surface area-to-volume ratio is very high (between ca. 1e5 104 m2/m3) due to tiny channels, heat distribution along the reactor and the coated catalyst layers, which are in direct contact with the reactor block, is much faster than observed in a typical packed-bed reactor [19]. These features allow efficient use of the catalyst and lead to higher reactant conversions. The objective of this study is to investigate steam reforming of glycerol to hydrogen in a microchannel reactor by a FEM-based mathematical model that can successfully describe detailed flow and catalytic reaction phenomena in the micro-scale, and to understand temperature distribution and glycerol conversion at different configurations of the reactor. The mathematical model is then used to estimate the size of a multichannel reformer that can convert glycerol produced in a plant producing 4 103 m3 of biodiesel annually. The studies are planned to provide insight into the
Fig. 1 e Description of the parallel microchannel reactor configuration (a) and the characteristic unit cell (b).
microchannel enabled heat distribution properties of the endothermic glycerol SR and to understand the degree of compactness that can be achieved by the microchannel architecture. Details of the microchannel reactor and the modeling technique are provided in the ‘System description and mathematical modeling’ section. The outcomes of the study and their discussion are given in the ‘Results and discussion’ section.
System description and mathematical modeling The microchannel reactor is composed of parallel groups of identical channels separated by the reactor wall. Each channel, which is rectangular in shape, has dimensions of 6.5 104 m (Height) 1 102 m (Width) 1 101 m (Length), and 5 105 m thick layers of Ni/CeO2 catalyst are being coated onto opposite inner walls of each channel (Fig. 1). Channel dimensions are chosen to yield an aspect ratio (channel height/channel width) of 0.065, since this value ensures wide and shallow channels leading to reduced axial dispersion and uniform diffusion mixing [20]. Any given channel is part of a horizontal array of channels, each of which has the same reaction occurring within, as shown in the frontal view of the microchannel array on the y-z plane in Fig. 1a. Since the height of the channel is much smaller than its width, the share of reactor wall material in the y-direction becomes higher than that in the z-direction. Therefore, due to the chosen aspect ratio, any gradients in z-direction can be neglected and the reactor can be modeled as a 2D reactor in the x-y plane. This simplification allows the description of the whole reactor block by a repeating unit cell (Fig. 1b) that comprises halves of one channel and wall and a washcoated catalyst layer. The validity of employing a 2D unit cell to describe the whole microreactor block can also be found in the literature where the difference between 2D and 3D simulation of a similar microreactor configuration is reported to be negligible [21]. Moreover, the microchannel block is assumed
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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to be ideally insulated on the sides; therefore heat loss to surroundings is neglected. Scaling up strategy of the multichannel reactor is based on numbering up of the channels, whose details are explained in the ‘Scale-up and sizing of the multichannel reactor’ section. Simulations are carried out using COMSOL Multiphysics™ software, which uses the finite element method to solve the equations for conservation of mass, momentum and energy. These equations and corresponding boundary conditions involved in the simulations are given in Tables 1 and 2, respectively. The simulations are performed on an HP xw8600 workstation equipped with 2 3.00 GHz Xeon™ processors and 20 GB of memory. The modeling domain used in the simulations is shown in Fig. 1b. Triangular mesh elements are utilized for the analyses and the number of elements is taken to be between 4000 and 5000 for the discretization of the model equations. NaviereStokes equations are utilized to describe the conservation of momentum for an incompressible Newtonian fluid in two-dimensional Cartesian coordinates. Since the structure of washcoated catalyst layer is porous, Brinkmantype equations are used to model the flow in this section. Axial and lateral heat conduction effects in the metallic wall are also significant [22], so conjugate heat transfer between the wall and the fluids is accounted for by the equation of energy in the wall. During the simulations, steam reforming of glycerol is assumed to take place only within the washcoated catalyst layer and homogenous gas-phase reactions are
Table 1 e The 2D mathematical model used to simulate transport and reaction in the washcoated microchannels. Fluid phase Equation of continuity Equation of motion
vvx vx
þ
vvy vy
¼0
h2 i vvx v vx v 2 vx x rf vx vv ¼ vp vx þ m vx2 þ vy2 vx þ vy vy vv vv m rf vx vxy þ vy vyy ¼ vp vy þ
Equation of species continuity Equation of energy
vci i vx vc vx þ vy vy ¼ DAB
h
v2 ci vx2
2
þ vvyc2i
h
v2 vy vx2
þ
v 2 vy vy2
i
i
h2 i vT v T v2 T rf Cpf vx vT vx þ vy vy ¼ lf vx2 þ vy2
Washcoat phase Equation of continuity Equation of motion
vvx vx
þ
m k
m k
Equation of species continuity Equation of energy
vvy vy
¼0
vx ¼ vp vx þ vy ¼ vp vy þ
h m εp
h m εp
þ vvyv2x
v2 vy vx2
þ
vci i vx vc vx þ vy vy ¼ DAB;eff
h
2
v2 ci vx2
v2 vy vy2
i i
i 2 þ vvyc2i rs Ri
h2 i vT v T v2 T rs Cps vx vT vx þ vy vy ¼ leff vx2 þ vy2 þ rs ðDHÞðrÞ
Solid phase Equation of energy
v2 vx vx2
h lw
v2 Tw vx2w
i ¼ C3H8O3, H2O, CO2, H2
i 2 þ vvyT2w ¼ 0 w
Table 2 e Boundary conditions associated with the model equations given in Table 1. 1. Channel entrance: x ¼ 0; cy U ¼ Uin ci ¼ cin i T ¼ Tin 2. Symmetry at the centerline: y ¼ 0; cx n$v ¼ 0 n$ðDAB Vci þ vci Þ ¼ 0 n$ðlf VT þ vpf Cpf TÞ ¼ 0 3. Along the fluid-solid wall interface: y ¼ H/2 þ ds; cx n$v ¼ 0 n$ðDAB Vci þ vci Þ ¼ 0 n$ðlw VTw Þ ¼ n$ðlf VT þ vpf Cpf TÞ 4. Channel exit: x ¼ L; cy p ¼ pout n$ðDAB Vci Þ ¼ 0 n$ðlf VTÞ ¼ 0 5. Solid boundaries: x ¼ 0 and x ¼ L; cyw n$ðlw VTw Þ ¼ 0
neglected as the latter require longer times for initiation [23]. The concentrations of the species and inlet temperature are specified as the inlet conditions in simulations and the reactor is assumed to operate adiabatically. Axial symmetry about the channel centerlines is imposed for all types of transport phenomena. The inlet gas mixture is composed of glycerol and steam, whereas the outlet gas mixture consists of excess steam, unreacted glycerol and products, carbon dioxide and hydrogen. It is assumed that the glycerol is pure and does not contain any impurities that might block the microchannels. The flow regime is laminar with Reynolds numbers around 40. The rate law describing the kinetics of glycerol SR over Ni/ CeO2 catalyst is obtained from the literature [24] and is given as follows: n r ¼ k0 expð EA =RTÞ Fglycerol
(2)
The activation energy and the reaction order for the glycerol steam reforming reaction over Ni/CeO2 catalyst are reported as 103.4 kJ/mol and 0.233, respectively, based on a power model. In addition, the pre-exponential factor is given as 8135.5 kmol0.767/(s0.767 kgcat) [24]. The bulk density of the catalyst is taken as 1900 kg/m3 [25]. The feed rates of glycerol and steam are set equally for each channel as 2 105 mol/s and 2.4 104 mol/s, respectively. The feed temperature varies between 773 K and 873 K. All simulations are carried out at atmospheric pressure, so ideal gas assumption is made. The combination of the feed conditions, i.e. molar steam-tocarbon ratio and temperature at the inlet, and the catalyst type (Ni/CeO2) guarantee coke-fee operation [24]. Evaluation of the physical constants needed to solve the equations given in Table 1 is described elsewhere [26]. The simulations are focused on investigating the effect of feed temperature, thickness and material of the wall separating the channels on temperature distribution along the microchannel and on glycerol conversion. The studied inlet temperatures are 773, 798, 823, 848 and 873 K, while the tested wall thickness values vary between 3 104 m and 7 104 m with 1 104 m increments. Three different types of material
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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of construction, namely silicon carbide (SiC), 316 stainless steel (SS 316) and alumina (Al2O3), are simulated as wall materials. During the simulations, changes in values of density, specific heat capacity, viscosity and thermal conductivity are found to be below 10%. Thus, to increase computational power, those variables are selected to be constant at their average values.
Results and discussion The effects of the parameters, i.e. inlet temperature, and thickness and material of the wall separating the channels on temperature distribution along the microchannel and on glycerol conversion are given in this section. Temperature profiles are taken from the centerline (i.e. line of symmetry) of the microchannels. Glycerol conversion is defined as the ratio of the number of moles of glycerol converted in Reaction 1 to the number of moles of glycerol in the feed. After the effects of parameters are discussed, numbering up of the microchannels is performed to calculate the volume of the microchannel unit and the amount of catalyst required for reaching the design criterion, which is set to be a minimum 85% conversion of the glycerol produced as a by-product of a plant producing 4 103 m3 of biodiesel annually.
Effect of wall thickness The effect of thickness of the reactor wall between adjacent channels is investigated and reported in terms of axial centerline temperature profiles and glycerol conversions in Figs. 2a and b, respectively. Wall thickness is changed from 3 104 m to 7 104 m with 1 104 m increments while other parameters, reactor material and inlet temperature are kept constant. Reactor material for this parametric study is selected as SiC and the inlet temperature is fixed at 873 K. The results show that both glycerol conversion and the exit temperature increase with wall thickness. This trend can be explained by the behavior of the reactor wall, which functions as a medium that can absorb and deliver the heat from the hot inlet reactant flow to the catalyst layer. This finding is in
alignment with the previous studies which reported that thicker walls favor axial component of heat conduction through the wall [21]. As a result, the difference between the inlet and exit temperatures becomes smaller and the degree of heat transfer to the catalyst layer, which is in direct contact with the wall (Fig. 1b), can be improved significantly. Since the catalyst layer is heated effectively, the reaction rate constant, with its Arrhenius temperature relation, is increased. This explains the reason of increased glycerol conversions with temperature. The results also demonstrate the capability of obtaining near complete glycerol conversions (98% at wall thickness of 7 104 m, Fig. 2b), with adiabatic operation, i.e. without external heat supply in a wall-coated microchannel reactor. In other words, the incoming sensible heat is sufficient to meet the endothermic heat requirement of glycerol SR.
Effect of reactor wall material The effect of material of the reactor wall between adjacent channels is investigated and reported in terms of axial centerline temperature profiles and glycerol conversions in Figs. 3a and b, respectively. The materials of construction investigated are SiC, Al2O3, and SS 316. During the simulations wall thickness and inlet temperature are kept constant at 5 104 m and 873 K, respectively. Figs. 3a and b show that using SiC material leads to the evolution of the highest glycerol conversion of 93%, whereas using SS 316 causes a notable decrease in conversion down to 73%. Using Al2O3 as the reactor material is found to give the lowest conversion (69%). This decrease in conversion is accompanied by the decrease in outlet stream temperature. These trends can be attributed to different thermal conductivities of the materials. As the thermal conductivity increases, the axial component of wall heat conduction becomes faster and delivers bigger portion of the incoming heat to the downstream of the reactor [21]. As a result, the catalyst layer becomes heated more uniformly, delivering improved glycerol conversions due to higher catalyst temperatures. SiC has the highest thermal conductivity among the three (106 W/m K), almost five times more conductive than
Fig. 2 e Effect of wall thickness between adjacent channels on centerline temperature profile (a) and glycerol conversion (b). Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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Fig. 3 e Effect of reactor wall material on centerline temperature profile (a) and glycerol conversion (b).
SS 316 (22 W/m K). Al2O3, on the other hand, has the lowest thermal conductivity (12 W/m K). The results also provide insight into the modularity of the microchannel reactors whose heat transfer properties can easily be modified by selecting a suitable material of construction.
Effect of inlet temperature The effect of inlet temperature of the reactive mixture is investigated and reported in terms of axial centerline temperature profiles and glycerol conversions in Figs. 4a and b, respectively. Simulations are carried out in the 773e873 K range, with increments of 25 K. The reactor material and wall thickness are set to be SiC and 5 104 m, respectively. It can be observed that 25 K incremental decreases in inlet temperature cause a gradual reduction in glycerol conversions. The rate constant of reaction is affected severely by the temperature changes, thus decreasing the inlet temperature slows down the reaction, hence reduces the conversions dramatically. It is worth noting that, near-complete conversions can be obtained at feed temperatures above 873 K.
The reactor configurations examined in this study are all adiabatic. Therefore, a continuous decrease in reaction temperatures (Fig. 4a) should be expected. It is worth noting that the rate of decrease of reaction temperature becomes faster at high inlet temperatures. This outcome is a characteristic output of the non-linear dependence of rate constant on temperature.
Scale-up and sizing of the multichannel reactor One of the objectives of this study is to calculate the reactor volume and the catalyst amount required to reach the design criterion, which is to obtain a minimum of 85% conversion of the glycerol produced in a plant with biodiesel production rate of 4 103 m3/year. A single microchannel is fed with 2 105 mol/s of glycerol and 2.4 104 mol/s of steam. These values are selected to provide sufficient contact of the reactive flow with the Ni-based catalyst to deliver glycerol conversion in excess of 85% in the base reactor configuration (wall type: SiC, wall thickness: 5 104 m, inlet temperature: 873 K). The total amount of glycerol to be processed in the multichannel
Fig. 4 e Effect of reactant feed temperature on centerline temperature profile (a) and glycerol conversion (b). Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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unit is back calculated from the stoichiometry of the transesterification reaction yielding a molar biodiesel-to-glycerol ratio of 3 and the annual biodiesel production rate expressed above. The results show that the number of channels (with the dimensions specified in the ‘System description and mathematical modeling’ section) required to reach the design capacity is 5400. The multichannel configuration is configured to consist of 20 microchannels stacked together horizontally, and 270 microchannels stacked together vertically. The wall thickness between channels in the horizontal, z-direction (Fig. 1a) is set to be 5 103 m. The resulting microchannel configuration is calculated to occupy a volume of only 1 102 m3. Using the bulk density of the Ni/CeO2 catalyst stated previously (1900 kg/m3) [25], the total amount of catalyst required for the washcoat layers is found to be 1 kg. These findings are in alignment with the compact nature of the microchannel units and show that high glycerol SR conversions can be obtained efficiently, i.e. without any external energy input and at small volumes.
Conclusions This study is focused on modeling and design of a multichannel microreactor to convert glycerol, side product of biodiesel synthesis, to hydrogen via steam reforming. FEMbased simulations are made on a 2D unit cell and a parametric study is conducted to observe effects of type and geometric properties of the reactor material, and the inlet temperature. SiC is shown to be a more suitable material compared to Al2O3 and SS 316 due to its high thermal conductivity, favoring fast transfer of the heat coming from hot inlet flow to the catalyst layer. A similar trend is also observed when the thickness of the wall separating the channels is increased. Improved heat transfer to the catalyst layer allowed its efficient use that delivered glycerol conversions up to 98% without any other external energy demand. Glycerol conversion is promoted also at higher feed temperatures. A multichannel reactor made of SiC consisting of 5400 microchannels with a total volume of 1 102 m3 and a total catalyst amount of 1 kg is estimated to convert a minimum of 90% of the glycerol supplied by a biodiesel synthesis plant with an annual capacity of 4 103 m3.
Acknowledgments Financial support is provided by TUBITAK through project 113M962.
Nomenclature ci Cpf Cpi Cps DAB DAB,eff
concentration of species i, kmol/m3 specific heat of fluid, kJ/(kg K) specific heat of species i, kJ/(kg K) specific heat of washcoated catalyst layer, kJ/(kg K) species diffusivity (A into stagnant B), m2/s effective species diffusivity in the washcoat layer, m2/s
EA Fglycerol H DH k0 L Lw n n p pi r R Ri T Tw U vx,vy v x y
activation energy, kJ/kmol molar flow rate of glycerol, kmol/s height of microchannel, m enthalpy of reaction, kJ/kmol pre-exponential factor, kmol0.767/(s0.767 kgcat) length of microchannel, m wall thickness, m normal unit vector order of reaction with respect to glycerol total pressure in channel, atm partial pressure of species i, atm rate of reaction, kmol0.767/(s0.767 kgcat) universal gas constant, kJ/kmol K total rate of generation/depletion of species i in channel, kmol/(s kgcat) temperature in the channel, K temperature of the solid wall, K plug-flow velocity into channel, m/s x- and y-components of fluid velocity in channel (Cartesian coordinates), m/s fluid velocity in the channel axial coordinate in channel, m direction normal to the x-axis, m
Greek letters thickness of washcoated catalyst layer, m ds εp void fraction of washcoated catalyst layer k permeability of washcoated catalyst layer, m2 lf fluid thermal conductivity, W/(m K) effective thermal conductivity of the washcoated leff catalyst layer, W/(m K) thermal conductivity of microchannel wall, W/(m K) lw density of fluid density, kg/m3 rf rs density of the washcoated catalyst layer, kg/m3 m viscosity of fluid in the channel, kg/(m s) Subscripts and superscripts eff effective f fluid i species index in inlet o standard conditions out outlet s washcoat w wall
references
[1] Owen NA, Inderwildi OR, King DA. The status of conventional world oil reserves e hype or cause for concern? Energy Policy 2010;38:4743e9. [2] Lin Y. Catalytic valorization of glycerol to hydrogen and syngas. Int J Hydrogen Energy 2013;38:2678e700. [3] Adhikari S, Fernando DS, To SDF, Bricka RM, Steele PH, Haryanto A. Conversion of glycerol to hydrogen via a steam reforming process over nickel catalysts. Energy Fuels 2008;22:1220e6. [4] Buffoni IN, Pompeo F, Santori GF, Nichio NN. Nickel catalysts applied in steam reforming of glycerol for hydrogen production. Catal Commun 2009;10:1656e60.
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7
[5] Iriondo A, Barrio V, Cambra J, Arias PL, Gu¨emez MB, Navarro RM, et al. Hydrogen production from glycerol over Ni catalysts supported on Al2O3 modified by Mg, Zr, Ce or La. Top Catal 2008;49:46e58. [6] Valliyappan T, Ferdous D, Bakhshi NN, Dalai AK. Production of hydrogen and syngas via steam gasification of glycerol in a fixed-bed reactor. Top Catal 2008;49:59e67. [7] Byrd AJ, Pant KK, Gupta RB. Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. Fuel 2008;87:2956e60. [8] Chiodo V, Freni S, Galvagno A, Mondello N, Frusteri F. Catalytic features of Rh and Ni supported catalysts in the steam reforming of glycerol to produce hydrogen. Appl Catal A: Gen 2010;381:1e7. [9] Montini T, Singh R, Das P, Lorenzut B, Bertero N, Riello P, et al. Renewable H2 from glycerol steam reforming: effect of La2O3 and CeO2 addition to Pt/Al2O3 catalysts. ChemSusChem 2010;3:619e28. [10] Adhikari S, Fernando S, Haryanto A. A comparative thermodynamic and experimental analysis on hydrogen production by steam reforming of glycerin. Energy Fuels 2007;21:2306e10. [11] Pompeo F, Santori G, Nichio NN. Hydrogen and/or syngas from steam reforming of glycerol. Study of platinum catalysts. Int J Hydrogen Energy 2010;35:8912e20. [12] Karakaya M, Keskin S, Avci AK. Parametric study of methane steam reforming to syngas in a catalytic microchannel reactor. Appl Catal A: Gen 2012;411-412:114e22. [13] Karakaya M, Onsan ZI, Avci AK. Microchannel autothermal reforming of methane to synthesis gas. Top Catal 2013;56:1716e23. [14] Simsek E, Karakaya M, Avci AK, Onsan ZI. Oxidative steam reforming of methane to synthesis gas in microchannel reactors. Int J Hydrogen Energy 2013;38:870e8. [15] Karim A, Bravo J, Gorm D, Conant T, Datye A. Comparison of wall-coated and packed-bed reactors for steam reforming of methanol. Catal Today 2005;110:86e91.
7
[16] Ryu JH, Lee KL, La H, Kim HJ, Yang JI, Jung HJ. Ni catalyst wash-coated on metal monolith with enhanced heattransfer capability for steam reforming. J Power Sources 2007;171:499e505. [17] Wang Y, Chin YH, Rozmiarek RT, Johnson BR, Gao Y, Watson J, et al. Highly active and stable Rh/MgOeAl2O3 catalysts for methane steam reforming. Catal Today 2004;98:575e81. [18] Kolb G, Hessel V. Micro-structured reactors for gas phase reactions. Chem Eng J 2004;98:1e38. [19] Kiwi-Minsker L, Renken A. Microstructured reactors for catalytic reactions. Catal Today 2005;110:2e14. [20] Aubin J, Prat LE, Xuereb C, Gourdon C. Effect of microchannel aspect ratio on residence time distributions and the axial dispersion coefficient. Chem Eng Process 2009;48:554e9. [21] Karakaya M, Avci AK. Microchannel reactor modeling for combustion driven reforming of iso-octane. Int J Hydrogen Energy 2011;36:6569e77. [22] Hardt S, Ehrfeld W, Hessel V, Vanden Bussche KM. Strategies for size reduction of microreactors by heat transfer enhancement effects. Chem Eng Commun 2003;190:540e59. [23] Veser G, Frauhammer J. Modelling steady state and ignition during catalytic methane oxidation in a monolith reactor. Chem Eng Sci 2000;55:2271e86. [24] Adhikari S, Fernando SD, Haryanto A. Kinetics and reactor modeling of hydrogen production from glycerol via steam reforming process over Ni/CeO2 catalysts. Chem Eng Technol 2009;32:541e7. [25] Flytzani-Stephanopoulos M, Zhu T, Li Y. Ceria-based catalysts for the recovery of elemental sulfur from SO2-laden gas streams. Catal Today 2000;62:145e58. [26] Karakaya M, Avci AK. Comparison of compact reformer configurations for on-board fuel processing. Int J Hydrogen Energy 2010;35:2305e16.
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen Ozgur Yasar Caglar, C. Doga Demirhan, Ahmet K. Avci* Department of Chemical Engineering, Bogazici University, Bebek 34342, Istanbul, Turkey
article info
abstract
Article history:
The objectives of this study are to investigate steam reforming (SR) of glycerol, by-product
Received 16 August 2014
of biodiesel synthesis, in a wall-coated catalytic microchannel by a detailed mathematical
Received in revised form
model and design an adiabatic microchannel reformer that can be integrated into a bio-
12 October 2014
diesel production plant with an annual capacity of 4 103 m3/year. Modeling and simu-
Accepted 17 October 2014
lation studies are based on Finite Element Method (FEM) technique. Sizing and design of
Available online xxx
the multichannel reactor involving wall-separated arrays of parallel microchannels is based on converting a minimum of 85% of the glycerol obtained as a by-product of the
Keywords:
biodiesel production. The results show that the microchannel architecture enables fast and
Glycerol
uniform transfer of the sensible heat of the feed stream to the catalyst layer. This phe-
Steam reforming
nomenon, which is more pronounced by using thick and high thermal conductivity walls
Microchannel
between channels, allows SR to run without external heat supply and leads to glycerol
Modeling
conversions above 85%. It is demonstrated that a multichannel unit of 1 102 m3 volume,
Reactor design
operating without external energy supply, is sufficient to convert ca. 90% of the glycerol produced in a biodiesel plant with a capacity of 4 103 m3/year. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The importance of renewable fuels as alternatives to their existing, non-renewable counterparts is becoming more significant due to the increasing cost of obtaining high quality crude oil as a result of the depleting oil reserves [1]. One of the most attractive renewable fuels is biodiesel whose global use is expected to increase continuously. Biodiesel can be produced by catalytic trans-esterification of animal-based or vegetable oils with methanol (or ethanol) [2]. In this process, 1 mol of glycerol is produced for every 3 moles of biodiesel. This stoichiometry, however, causes undesirably high supply of glycerol. Due to the rising demand of biodiesel, the
amount of by-product glycerol that will be produced by 2020 is expected to increase up to 3 megatons, whereas the total glycerol demand will be less than 500 kilotons [2]. The significant gap between the demand and supply of glycerol will eventually increase the cost of biodiesel, as well as biodiesel blended diesel fuel due to the elevated cost of glycerol removal. Solution of this problem requires efficient conversion of glycerol into value-added products such as hydrogen, which is the feedstock for many petrochemical processes as well as for the fuel cells delivering efficient electricity generation. One of the most well known techniques for producing hydrogen from glycerol is by its reforming with steam, which is carried out at temperatures in excess of ca. 600 K over a supported catalyst:
* Corresponding author. Tel.: þ90 212 3597785; fax: þ90 212 2872460. E-mail address: [email protected] (A.K. Avci). http://dx.doi.org/10.1016/j.ijhydene.2014.10.070 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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C3 H8 O3 þ 3H2 O ¼ 3CO2 þ 7H2
DH0 ¼ 346:4 kJ mol
(1)
Catalysis of glycerol SR is well investigated. Most of the studies involve the use of Ni-based catalysts [3e6], which are demonstrated to deliver glycerol conversions above ca. 90% depending on the reaction temperature, molar steam-tocarbon ratio at the inlet and contact time. In addition to Nibased ones, the performance of noble metals such as Rh, Pt, Ru, Pd, Ir [7e9] are also investigated. Apart from the type of active metal, the nature of the catalyst support is found to have significant impact on glycerol conversion and hydrogen selectivity. Adhikari et al. [10] compared the use of CeO2, MgO and TiO2 as supports for Ni in glycerol SR and reported that hydrogen selectivity followed the order of Ni/CeO2 > Ni/ MgO > Ni/TiO2. Pompeo et al. [11] reported that glycerol SR over Pt/g-Al2O3 and Pt/ZrO2 led to carbon formation and the catalysts are deactivated rapidly due to the acidic nature of both supports. The same authors showed that the use of SiO2, a support with neutral properties, with Pt gave stable operation together with high activity and hydrogen selectivity [11]. An extensive review of catalysts and supports studied for glycerol-to-hydrogen conversion is reported elsewhere [2]. Hydrogen production from glycerol requires high temperatures and external energy demand, both of which depend strongly on the catalyst type and reactor geometry. Even though the effect of catalyst on glycerol SR is studied extensively, the reactor type is limited to the conventional packed bed configuration. Recent studies on non-oxidative and oxidative steam reforming of methane to synthesis gas showed that structuring the reactor and catalyst by using the microchannel technology can provide methane conversions and CO selectivities higher than those obtained in a packedbed reactor operated under equivalent conditions [12e14]. Similar results, demonstrating the superior properties of microchannel reactors in terms of improved conversions and high product selectivities are reported by other groups [15e17]. Microchannel reactors are defined as units involving parallel channels with hydraulic diameters below 1 103 m [18]. Inner walls of the channels are coated with porous catalyst layers with less than ca. 1 104 m thickness which offers minimum resistance to heat and mass transfer within the catalyst (Fig. 1a). Since the reactor block is mostly a metal and the surface area-to-volume ratio is very high (between ca. 1e5 104 m2/m3) due to tiny channels, heat distribution along the reactor and the coated catalyst layers, which are in direct contact with the reactor block, is much faster than observed in a typical packed-bed reactor [19]. These features allow efficient use of the catalyst and lead to higher reactant conversions. The objective of this study is to investigate steam reforming of glycerol to hydrogen in a microchannel reactor by a FEM-based mathematical model that can successfully describe detailed flow and catalytic reaction phenomena in the micro-scale, and to understand temperature distribution and glycerol conversion at different configurations of the reactor. The mathematical model is then used to estimate the size of a multichannel reformer that can convert glycerol produced in a plant producing 4 103 m3 of biodiesel annually. The studies are planned to provide insight into the
Fig. 1 e Description of the parallel microchannel reactor configuration (a) and the characteristic unit cell (b).
microchannel enabled heat distribution properties of the endothermic glycerol SR and to understand the degree of compactness that can be achieved by the microchannel architecture. Details of the microchannel reactor and the modeling technique are provided in the ‘System description and mathematical modeling’ section. The outcomes of the study and their discussion are given in the ‘Results and discussion’ section.
System description and mathematical modeling The microchannel reactor is composed of parallel groups of identical channels separated by the reactor wall. Each channel, which is rectangular in shape, has dimensions of 6.5 104 m (Height) 1 102 m (Width) 1 101 m (Length), and 5 105 m thick layers of Ni/CeO2 catalyst are being coated onto opposite inner walls of each channel (Fig. 1). Channel dimensions are chosen to yield an aspect ratio (channel height/channel width) of 0.065, since this value ensures wide and shallow channels leading to reduced axial dispersion and uniform diffusion mixing [20]. Any given channel is part of a horizontal array of channels, each of which has the same reaction occurring within, as shown in the frontal view of the microchannel array on the y-z plane in Fig. 1a. Since the height of the channel is much smaller than its width, the share of reactor wall material in the y-direction becomes higher than that in the z-direction. Therefore, due to the chosen aspect ratio, any gradients in z-direction can be neglected and the reactor can be modeled as a 2D reactor in the x-y plane. This simplification allows the description of the whole reactor block by a repeating unit cell (Fig. 1b) that comprises halves of one channel and wall and a washcoated catalyst layer. The validity of employing a 2D unit cell to describe the whole microreactor block can also be found in the literature where the difference between 2D and 3D simulation of a similar microreactor configuration is reported to be negligible [21]. Moreover, the microchannel block is assumed
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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to be ideally insulated on the sides; therefore heat loss to surroundings is neglected. Scaling up strategy of the multichannel reactor is based on numbering up of the channels, whose details are explained in the ‘Scale-up and sizing of the multichannel reactor’ section. Simulations are carried out using COMSOL Multiphysics™ software, which uses the finite element method to solve the equations for conservation of mass, momentum and energy. These equations and corresponding boundary conditions involved in the simulations are given in Tables 1 and 2, respectively. The simulations are performed on an HP xw8600 workstation equipped with 2 3.00 GHz Xeon™ processors and 20 GB of memory. The modeling domain used in the simulations is shown in Fig. 1b. Triangular mesh elements are utilized for the analyses and the number of elements is taken to be between 4000 and 5000 for the discretization of the model equations. NaviereStokes equations are utilized to describe the conservation of momentum for an incompressible Newtonian fluid in two-dimensional Cartesian coordinates. Since the structure of washcoated catalyst layer is porous, Brinkmantype equations are used to model the flow in this section. Axial and lateral heat conduction effects in the metallic wall are also significant [22], so conjugate heat transfer between the wall and the fluids is accounted for by the equation of energy in the wall. During the simulations, steam reforming of glycerol is assumed to take place only within the washcoated catalyst layer and homogenous gas-phase reactions are
Table 1 e The 2D mathematical model used to simulate transport and reaction in the washcoated microchannels. Fluid phase Equation of continuity Equation of motion
vvx vx
þ
vvy vy
¼0
h2 i vvx v vx v 2 vx x rf vx vv ¼ vp vx þ m vx2 þ vy2 vx þ vy vy vv vv m rf vx vxy þ vy vyy ¼ vp vy þ
Equation of species continuity Equation of energy
vci i vx vc vx þ vy vy ¼ DAB
h
v2 ci vx2
2
þ vvyc2i
h
v2 vy vx2
þ
v 2 vy vy2
i
i
h2 i vT v T v2 T rf Cpf vx vT vx þ vy vy ¼ lf vx2 þ vy2
Washcoat phase Equation of continuity Equation of motion
vvx vx
þ
m k
m k
Equation of species continuity Equation of energy
vvy vy
¼0
vx ¼ vp vx þ vy ¼ vp vy þ
h m εp
h m εp
þ vvyv2x
v2 vy vx2
þ
vci i vx vc vx þ vy vy ¼ DAB;eff
h
2
v2 ci vx2
v2 vy vy2
i i
i 2 þ vvyc2i rs Ri
h2 i vT v T v2 T rs Cps vx vT vx þ vy vy ¼ leff vx2 þ vy2 þ rs ðDHÞðrÞ
Solid phase Equation of energy
v2 vx vx2
h lw
v2 Tw vx2w
i ¼ C3H8O3, H2O, CO2, H2
i 2 þ vvyT2w ¼ 0 w
Table 2 e Boundary conditions associated with the model equations given in Table 1. 1. Channel entrance: x ¼ 0; cy U ¼ Uin ci ¼ cin i T ¼ Tin 2. Symmetry at the centerline: y ¼ 0; cx n$v ¼ 0 n$ðDAB Vci þ vci Þ ¼ 0 n$ðlf VT þ vpf Cpf TÞ ¼ 0 3. Along the fluid-solid wall interface: y ¼ H/2 þ ds; cx n$v ¼ 0 n$ðDAB Vci þ vci Þ ¼ 0 n$ðlw VTw Þ ¼ n$ðlf VT þ vpf Cpf TÞ 4. Channel exit: x ¼ L; cy p ¼ pout n$ðDAB Vci Þ ¼ 0 n$ðlf VTÞ ¼ 0 5. Solid boundaries: x ¼ 0 and x ¼ L; cyw n$ðlw VTw Þ ¼ 0
neglected as the latter require longer times for initiation [23]. The concentrations of the species and inlet temperature are specified as the inlet conditions in simulations and the reactor is assumed to operate adiabatically. Axial symmetry about the channel centerlines is imposed for all types of transport phenomena. The inlet gas mixture is composed of glycerol and steam, whereas the outlet gas mixture consists of excess steam, unreacted glycerol and products, carbon dioxide and hydrogen. It is assumed that the glycerol is pure and does not contain any impurities that might block the microchannels. The flow regime is laminar with Reynolds numbers around 40. The rate law describing the kinetics of glycerol SR over Ni/ CeO2 catalyst is obtained from the literature [24] and is given as follows: n r ¼ k0 expð EA =RTÞ Fglycerol
(2)
The activation energy and the reaction order for the glycerol steam reforming reaction over Ni/CeO2 catalyst are reported as 103.4 kJ/mol and 0.233, respectively, based on a power model. In addition, the pre-exponential factor is given as 8135.5 kmol0.767/(s0.767 kgcat) [24]. The bulk density of the catalyst is taken as 1900 kg/m3 [25]. The feed rates of glycerol and steam are set equally for each channel as 2 105 mol/s and 2.4 104 mol/s, respectively. The feed temperature varies between 773 K and 873 K. All simulations are carried out at atmospheric pressure, so ideal gas assumption is made. The combination of the feed conditions, i.e. molar steam-tocarbon ratio and temperature at the inlet, and the catalyst type (Ni/CeO2) guarantee coke-fee operation [24]. Evaluation of the physical constants needed to solve the equations given in Table 1 is described elsewhere [26]. The simulations are focused on investigating the effect of feed temperature, thickness and material of the wall separating the channels on temperature distribution along the microchannel and on glycerol conversion. The studied inlet temperatures are 773, 798, 823, 848 and 873 K, while the tested wall thickness values vary between 3 104 m and 7 104 m with 1 104 m increments. Three different types of material
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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of construction, namely silicon carbide (SiC), 316 stainless steel (SS 316) and alumina (Al2O3), are simulated as wall materials. During the simulations, changes in values of density, specific heat capacity, viscosity and thermal conductivity are found to be below 10%. Thus, to increase computational power, those variables are selected to be constant at their average values.
Results and discussion The effects of the parameters, i.e. inlet temperature, and thickness and material of the wall separating the channels on temperature distribution along the microchannel and on glycerol conversion are given in this section. Temperature profiles are taken from the centerline (i.e. line of symmetry) of the microchannels. Glycerol conversion is defined as the ratio of the number of moles of glycerol converted in Reaction 1 to the number of moles of glycerol in the feed. After the effects of parameters are discussed, numbering up of the microchannels is performed to calculate the volume of the microchannel unit and the amount of catalyst required for reaching the design criterion, which is set to be a minimum 85% conversion of the glycerol produced as a by-product of a plant producing 4 103 m3 of biodiesel annually.
Effect of wall thickness The effect of thickness of the reactor wall between adjacent channels is investigated and reported in terms of axial centerline temperature profiles and glycerol conversions in Figs. 2a and b, respectively. Wall thickness is changed from 3 104 m to 7 104 m with 1 104 m increments while other parameters, reactor material and inlet temperature are kept constant. Reactor material for this parametric study is selected as SiC and the inlet temperature is fixed at 873 K. The results show that both glycerol conversion and the exit temperature increase with wall thickness. This trend can be explained by the behavior of the reactor wall, which functions as a medium that can absorb and deliver the heat from the hot inlet reactant flow to the catalyst layer. This finding is in
alignment with the previous studies which reported that thicker walls favor axial component of heat conduction through the wall [21]. As a result, the difference between the inlet and exit temperatures becomes smaller and the degree of heat transfer to the catalyst layer, which is in direct contact with the wall (Fig. 1b), can be improved significantly. Since the catalyst layer is heated effectively, the reaction rate constant, with its Arrhenius temperature relation, is increased. This explains the reason of increased glycerol conversions with temperature. The results also demonstrate the capability of obtaining near complete glycerol conversions (98% at wall thickness of 7 104 m, Fig. 2b), with adiabatic operation, i.e. without external heat supply in a wall-coated microchannel reactor. In other words, the incoming sensible heat is sufficient to meet the endothermic heat requirement of glycerol SR.
Effect of reactor wall material The effect of material of the reactor wall between adjacent channels is investigated and reported in terms of axial centerline temperature profiles and glycerol conversions in Figs. 3a and b, respectively. The materials of construction investigated are SiC, Al2O3, and SS 316. During the simulations wall thickness and inlet temperature are kept constant at 5 104 m and 873 K, respectively. Figs. 3a and b show that using SiC material leads to the evolution of the highest glycerol conversion of 93%, whereas using SS 316 causes a notable decrease in conversion down to 73%. Using Al2O3 as the reactor material is found to give the lowest conversion (69%). This decrease in conversion is accompanied by the decrease in outlet stream temperature. These trends can be attributed to different thermal conductivities of the materials. As the thermal conductivity increases, the axial component of wall heat conduction becomes faster and delivers bigger portion of the incoming heat to the downstream of the reactor [21]. As a result, the catalyst layer becomes heated more uniformly, delivering improved glycerol conversions due to higher catalyst temperatures. SiC has the highest thermal conductivity among the three (106 W/m K), almost five times more conductive than
Fig. 2 e Effect of wall thickness between adjacent channels on centerline temperature profile (a) and glycerol conversion (b). Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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Fig. 3 e Effect of reactor wall material on centerline temperature profile (a) and glycerol conversion (b).
SS 316 (22 W/m K). Al2O3, on the other hand, has the lowest thermal conductivity (12 W/m K). The results also provide insight into the modularity of the microchannel reactors whose heat transfer properties can easily be modified by selecting a suitable material of construction.
Effect of inlet temperature The effect of inlet temperature of the reactive mixture is investigated and reported in terms of axial centerline temperature profiles and glycerol conversions in Figs. 4a and b, respectively. Simulations are carried out in the 773e873 K range, with increments of 25 K. The reactor material and wall thickness are set to be SiC and 5 104 m, respectively. It can be observed that 25 K incremental decreases in inlet temperature cause a gradual reduction in glycerol conversions. The rate constant of reaction is affected severely by the temperature changes, thus decreasing the inlet temperature slows down the reaction, hence reduces the conversions dramatically. It is worth noting that, near-complete conversions can be obtained at feed temperatures above 873 K.
The reactor configurations examined in this study are all adiabatic. Therefore, a continuous decrease in reaction temperatures (Fig. 4a) should be expected. It is worth noting that the rate of decrease of reaction temperature becomes faster at high inlet temperatures. This outcome is a characteristic output of the non-linear dependence of rate constant on temperature.
Scale-up and sizing of the multichannel reactor One of the objectives of this study is to calculate the reactor volume and the catalyst amount required to reach the design criterion, which is to obtain a minimum of 85% conversion of the glycerol produced in a plant with biodiesel production rate of 4 103 m3/year. A single microchannel is fed with 2 105 mol/s of glycerol and 2.4 104 mol/s of steam. These values are selected to provide sufficient contact of the reactive flow with the Ni-based catalyst to deliver glycerol conversion in excess of 85% in the base reactor configuration (wall type: SiC, wall thickness: 5 104 m, inlet temperature: 873 K). The total amount of glycerol to be processed in the multichannel
Fig. 4 e Effect of reactant feed temperature on centerline temperature profile (a) and glycerol conversion (b). Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
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unit is back calculated from the stoichiometry of the transesterification reaction yielding a molar biodiesel-to-glycerol ratio of 3 and the annual biodiesel production rate expressed above. The results show that the number of channels (with the dimensions specified in the ‘System description and mathematical modeling’ section) required to reach the design capacity is 5400. The multichannel configuration is configured to consist of 20 microchannels stacked together horizontally, and 270 microchannels stacked together vertically. The wall thickness between channels in the horizontal, z-direction (Fig. 1a) is set to be 5 103 m. The resulting microchannel configuration is calculated to occupy a volume of only 1 102 m3. Using the bulk density of the Ni/CeO2 catalyst stated previously (1900 kg/m3) [25], the total amount of catalyst required for the washcoat layers is found to be 1 kg. These findings are in alignment with the compact nature of the microchannel units and show that high glycerol SR conversions can be obtained efficiently, i.e. without any external energy input and at small volumes.
Conclusions This study is focused on modeling and design of a multichannel microreactor to convert glycerol, side product of biodiesel synthesis, to hydrogen via steam reforming. FEMbased simulations are made on a 2D unit cell and a parametric study is conducted to observe effects of type and geometric properties of the reactor material, and the inlet temperature. SiC is shown to be a more suitable material compared to Al2O3 and SS 316 due to its high thermal conductivity, favoring fast transfer of the heat coming from hot inlet flow to the catalyst layer. A similar trend is also observed when the thickness of the wall separating the channels is increased. Improved heat transfer to the catalyst layer allowed its efficient use that delivered glycerol conversions up to 98% without any other external energy demand. Glycerol conversion is promoted also at higher feed temperatures. A multichannel reactor made of SiC consisting of 5400 microchannels with a total volume of 1 102 m3 and a total catalyst amount of 1 kg is estimated to convert a minimum of 90% of the glycerol supplied by a biodiesel synthesis plant with an annual capacity of 4 103 m3.
Acknowledgments Financial support is provided by TUBITAK through project 113M962.
Nomenclature ci Cpf Cpi Cps DAB DAB,eff
concentration of species i, kmol/m3 specific heat of fluid, kJ/(kg K) specific heat of species i, kJ/(kg K) specific heat of washcoated catalyst layer, kJ/(kg K) species diffusivity (A into stagnant B), m2/s effective species diffusivity in the washcoat layer, m2/s
EA Fglycerol H DH k0 L Lw n n p pi r R Ri T Tw U vx,vy v x y
activation energy, kJ/kmol molar flow rate of glycerol, kmol/s height of microchannel, m enthalpy of reaction, kJ/kmol pre-exponential factor, kmol0.767/(s0.767 kgcat) length of microchannel, m wall thickness, m normal unit vector order of reaction with respect to glycerol total pressure in channel, atm partial pressure of species i, atm rate of reaction, kmol0.767/(s0.767 kgcat) universal gas constant, kJ/kmol K total rate of generation/depletion of species i in channel, kmol/(s kgcat) temperature in the channel, K temperature of the solid wall, K plug-flow velocity into channel, m/s x- and y-components of fluid velocity in channel (Cartesian coordinates), m/s fluid velocity in the channel axial coordinate in channel, m direction normal to the x-axis, m
Greek letters thickness of washcoated catalyst layer, m ds εp void fraction of washcoated catalyst layer k permeability of washcoated catalyst layer, m2 lf fluid thermal conductivity, W/(m K) effective thermal conductivity of the washcoated leff catalyst layer, W/(m K) thermal conductivity of microchannel wall, W/(m K) lw density of fluid density, kg/m3 rf rs density of the washcoated catalyst layer, kg/m3 m viscosity of fluid in the channel, kg/(m s) Subscripts and superscripts eff effective f fluid i species index in inlet o standard conditions out outlet s washcoat w wall
references
[1] Owen NA, Inderwildi OR, King DA. The status of conventional world oil reserves e hype or cause for concern? Energy Policy 2010;38:4743e9. [2] Lin Y. Catalytic valorization of glycerol to hydrogen and syngas. Int J Hydrogen Energy 2013;38:2678e700. [3] Adhikari S, Fernando DS, To SDF, Bricka RM, Steele PH, Haryanto A. Conversion of glycerol to hydrogen via a steam reforming process over nickel catalysts. Energy Fuels 2008;22:1220e6. [4] Buffoni IN, Pompeo F, Santori GF, Nichio NN. Nickel catalysts applied in steam reforming of glycerol for hydrogen production. Catal Commun 2009;10:1656e60.
Please cite this article in press as: Caglar OY, et al., Modeling and design of a microchannel reformer for efficient conversion of glycerol to hydrogen, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.070
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7
[5] Iriondo A, Barrio V, Cambra J, Arias PL, Gu¨emez MB, Navarro RM, et al. Hydrogen production from glycerol over Ni catalysts supported on Al2O3 modified by Mg, Zr, Ce or La. Top Catal 2008;49:46e58. [6] Valliyappan T, Ferdous D, Bakhshi NN, Dalai AK. Production of hydrogen and syngas via steam gasification of glycerol in a fixed-bed reactor. Top Catal 2008;49:59e67. [7] Byrd AJ, Pant KK, Gupta RB. Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. Fuel 2008;87:2956e60. [8] Chiodo V, Freni S, Galvagno A, Mondello N, Frusteri F. Catalytic features of Rh and Ni supported catalysts in the steam reforming of glycerol to produce hydrogen. Appl Catal A: Gen 2010;381:1e7. [9] Montini T, Singh R, Das P, Lorenzut B, Bertero N, Riello P, et al. Renewable H2 from glycerol steam reforming: effect of La2O3 and CeO2 addition to Pt/Al2O3 catalysts. ChemSusChem 2010;3:619e28. [10] Adhikari S, Fernando S, Haryanto A. A comparative thermodynamic and experimental analysis on hydrogen production by steam reforming of glycerin. Energy Fuels 2007;21:2306e10. [11] Pompeo F, Santori G, Nichio NN. Hydrogen and/or syngas from steam reforming of glycerol. Study of platinum catalysts. Int J Hydrogen Energy 2010;35:8912e20. [12] Karakaya M, Keskin S, Avci AK. Parametric study of methane steam reforming to syngas in a catalytic microchannel reactor. Appl Catal A: Gen 2012;411-412:114e22. [13] Karakaya M, Onsan ZI, Avci AK. Microchannel autothermal reforming of methane to synthesis gas. Top Catal 2013;56:1716e23. [14] Simsek E, Karakaya M, Avci AK, Onsan ZI. Oxidative steam reforming of methane to synthesis gas in microchannel reactors. Int J Hydrogen Energy 2013;38:870e8. [15] Karim A, Bravo J, Gorm D, Conant T, Datye A. Comparison of wall-coated and packed-bed reactors for steam reforming of methanol. Catal Today 2005;110:86e91.
7
[16] Ryu JH, Lee KL, La H, Kim HJ, Yang JI, Jung HJ. Ni catalyst wash-coated on metal monolith with enhanced heattransfer capability for steam reforming. J Power Sources 2007;171:499e505. [17] Wang Y, Chin YH, Rozmiarek RT, Johnson BR, Gao Y, Watson J, et al. Highly active and stable Rh/MgOeAl2O3 catalysts for methane steam reforming. Catal Today 2004;98:575e81. [18] Kolb G, Hessel V. Micro-structured reactors for gas phase reactions. Chem Eng J 2004;98:1e38. [19] Kiwi-Minsker L, Renken A. Microstructured reactors for catalytic reactions. Catal Today 2005;110:2e14. [20] Aubin J, Prat LE, Xuereb C, Gourdon C. Effect of microchannel aspect ratio on residence time distributions and the axial dispersion coefficient. Chem Eng Process 2009;48:554e9. [21] Karakaya M, Avci AK. Microchannel reactor modeling for combustion driven reforming of iso-octane. Int J Hydrogen Energy 2011;36:6569e77. [22] Hardt S, Ehrfeld W, Hessel V, Vanden Bussche KM. Strategies for size reduction of microreactors by heat transfer enhancement effects. Chem Eng Commun 2003;190:540e59. [23] Veser G, Frauhammer J. Modelling steady state and ignition during catalytic methane oxidation in a monolith reactor. Chem Eng Sci 2000;55:2271e86. [24] Adhikari S, Fernando SD, Haryanto A. Kinetics and reactor modeling of hydrogen production from glycerol via steam reforming process over Ni/CeO2 catalysts. Chem Eng Technol 2009;32:541e7. [25] Flytzani-Stephanopoulos M, Zhu T, Li Y. Ceria-based catalysts for the recovery of elemental sulfur from SO2-laden gas streams. Catal Today 2000;62:145e58. [26] Karakaya M, Avci AK. Comparison of compact reformer configurations for on-board fuel processing. Int J Hydrogen Energy 2010;35:2305e16.
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