Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation

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Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation Hong-Yue Tang, Jason Greenwood, Paul Erickson* Mechanical and Aerospace Engineering Department, UC, Davis, United States

article info

abstract

Article history:

Motivated by the need for small scale distributed hydrogen generation and lack of detailed

Received 22 January 2015

modeling tools to aid in reformation system design, two fully coupled models were

Received in revised form

developed to extend the current understanding of reformation processes as it relates to

17 April 2015

temperature and fuel conversion, two critical design criteria. This paper, describes the

Accepted 18 April 2015

construction and validation of a steam and autothermal reformation model which was

Available online xxx

then experimentally validated.

Keywords:

the model. Two reactor geometries were used to verify the model using methanol as a feed-

Hydrogen

stock on a copper-based catalyst. The model captured the important characteristics of the

Methanol

reformer with fuel flow rate, geometry, and Oxygen to carbon ratio. The model has several

Modeling

advantages including the ability to estimate the required reformer length for 100% fuel con-

Reformation

version as well as the effect that flow rate and geometry have on conversion efficiency.

Experiments were carried out to obtain the necessary parameters to construct and validate

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The low volumetric energy content of hydrogen and the lack of infrastructure for refueling present an obstacle for enabling distributed and mobile fuel cell systems for power generation. Producing hydrogen via reformation of a liquid fuel shows potential to bridge this gap by allowing the storage and transport of a much higher energy density liquid via the existing infrastructure [1,2]. Methanol has been considered as one possible feedstock for hydrogen production due to multiple renewable production pathways, ease of reforming, and low cost of production. Small-scale hydrogen production

plants may play an important role in the energy infrastructure by enabling decentralized hydrogen production [3,4]. These small reformers have different heat transfer properties and are expected to experience a larger range of dynamic load [5,6]. Detailed modeling is essential to gain insight for small scale reformer design, operation, and control. Lighter hydrocarbon fuels, such as methanol, can be reformed using a copper-based, medium temperature shift (MTS) catalyst [7]. Reformation is typically an endothermic reaction due to the hydrogen product having a higher energy state than the hydrocarbon fuel. Energy is required to volatilize the hydrocarbons and break the CeC and HeC bonds in the chain. In steam reformation (SR), an external heat source

* Corresponding author. MAE Department, One Shields Ave., Davis, CA 95616, United States. Tel.: þ1 530 754 5352, þ1 530 752 5360; fax: þ1 530 752 4158. E-mail address: [email protected] (P. Erickson). http://dx.doi.org/10.1016/j.ijhydene.2015.04.096 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Nomenclature A c Cp D E h H k kr n P q r R R T u u v

frequency factor for a species, s1 concentration of a species, mol/m3 heat capacity, J/(kg K) diffusivity coefficient, m2/s activation energy of a species, J/mol convective heat transfer constant, W/ K enthalpy, J/mol thermal conductivity, W/(m K) rate constant, s1 unit normal vector pressure, Pascal, N/m2 heat flux, W/m2 rate equation for a species reaction rate of a species, mol/(m3 s) universal gas constant, 8.314 J/mol K temperature,  K velocity, m/s velocity vector of the reactant, m/s volumetric flow rate at 101 kPa, m3/s

Subscripts 0 reactant properties at reactor inlet

is used to provide the energy required for the endothermic chemical reactions. In autothermal reformation (ATR), an oxidizer, i.e. air or pure oxygen, is introduced into the fuel stream allowing combustion to occur to provide an internal heating source. Part of the fuel is therefore consumed for heat generation, reducing the hydrogen yield. The rapid rate at which heat is transferred to the reformate sustains the reaction without sintering the catalyst. A catalyst matrix of copper (Cu) in the presence of zinc oxide (ZnO) and supported by alumina (Al2O3) is the most popular for MTS [8e13]. This catalyst can be used to reform methanol in both SR and ATR modes [14], a flexibility which offers distinct advantages in the design and implementation that are not currently well understood. While SR is more efficient in terms of hydrogen yield when compared to ATR, ATR can respond better to transient load demands present in small-scale reformers by internally producing the heat required for reaction [15e17]. In addition to load-following applications, ATR can realize faster start-up times [18,19] and has been studied for the potential use on fuel cell vehicles [20,21]. A multi-system model would enable better understanding of the similarities and differences between the two modes and possibly lead to the development of a control scheme to operate in a hybrid mode. This would allow rapid heat delivery into a steam reformer by temporarily running the reformer in ATR mode to handle transient load requirements. A reformer that can operate in both SR and ATR modes is attractive [22e25]; however, the heat and mass transfer properties are very different, which makes designing a hybrid mode reactor challenging [26,27]. A model is developed that can capture the behavior of the reformer running in both modes. Two models were developed and validated using two reactor geometries. The experimental setup and the details of

cat cg eff gas heater i reactor

catalyst difference in property between catalyst and gas effective [property] reactant gas heat bands supplying thermal energy property of chemical species i reactor or reactor bed

Greek letters D change in property εp porosity [of reactor bed], Volumevoid/Volumereactor k permeability, m2/s l thermal conductivity, W/(m K) h viscosity, N s/m2 r density, kg/m3 Acronyms ATR autothermal reformation CPO catalytic partial oxidation MTS medium temperature shift [catalyst] oxygen to carbon ratio O2/C S/C steam to carbon ratio SR steam reformation

the model will be presented in the following sections. The same experimental setup is used for both the SR and the ATR experiments. The fully coupled multi-physics model uses the mass balance to describe the chemical reaction, the energy balance to describe the heat supplied and consumed by the reaction, and the momentum balance to describe the reactant flow. These simulation results were verified by experiments.

Experimental setup Reformer system A reconfigurable reformation system, shown in Fig. 1, is used for the experiments. The fuel, a methanol and water pre-mix with a steam-to-carbon ratio (S/C) of 1.6, is pumped into the vaporizers before entering the reactor. The S/C ratio is a key parameter in steam reformation systems and, in the case of methanol, is the molar ratio of H2O/CH3OH [28]. This parameter impacts fuel utilization, reformation efficiency, and operational life of the reactor. Sufficient steam is required to achieve full conversion and suppress CO formation while excessive steam will reduce efficiencies because of the vaporization energy required. Insufficient steam will result in carbon formation and degrade catalyst performance by means of fouling. Experimental work by many researchers has found that S/C ¼ 1.3e1.6 is optimum, resulting in a higher hydrogen yield at the reformer. To minimize the effect of fouling, an S/C ratio on the high end of the optimum range was used. Different reactor geometries can be tested. Nozzle band heaters are used to provide heating as needed for SR. Thermocouples are placed through side ports to monitor center line and side wall temperatures. Reformate is routed to a gas analyzer to evaluate conversion efficiency.

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 1 e Schematic of the experiment setup with interchangeable reactor for steam reforming mode. For ATR operation, a mass flow controller was added at the Air/CO2 valve to meter the oxidizer, air, into the reformer. The air is injected after the vaporizers, allowing ample time for mixing and heating of the oxidizer prior to entering the reactor. The fuel/water pre-mix with a steam-to-carbon ratio (S/C) of 1.6, was kept constant while the air flow rate was varied to obtain the desired oxygen-to-carbon ration (O2/C). In ATR, the O2/C is a critically important parameter. A typical reactor exhibits heat loss due to conduction, it is therefore necessary to have additional oxidizer to not only provide the energy for the endothermic SR reaction, but to also compensate for this heat loss. Researchers have found that the optimum O2/C ratio for Cu-based catalyst is between 0.2 and 0.3 [28,29]. Higher O2/C ratios will reduce the amount of hydrogen in the output stream and, depending on implementation, unnecessarily increase the reactor temperature or sintering of the catalyst. Lower O2/C ratios will result in low conversion due to insufficient heat [15]. Air is the most commonly used oxidant; when air is used to feed the autothermal reformer, nitrogen dilution will reduce the thermal efficiency and concentration of hydrogen in the reformate stream [30,31].

Reactor geometries The dominate limiting mechanisms in reformation are heat transfer, mass transfer, and chemical kinetics [14,26]. Two reactor geometries, shown in Fig. 2, were selected based on their volume and heat transfer properties to demonstrate the limiting mechanisms and to validate the proposed model. The larger diameter reactor, referred to as the low aspect ratio

reactor, is a schedule 40 stainless-steel pipe that is 25.4 cm long with a nominal diameter of 3.81 cm.The small diameter reactor, referred to as the high aspect ratio reactor, is a schedule 40 stainless-steel pipe that is 55.9 cm long with a nominal diameter of 1.9 cm (see Fig. 3).

Catalyst preparations A commercial Cu-based catalyst is used for both SR and ATR. The Sud-Chemie FCRM-2 is a Cu/Zn/Al2O3 based catalyst was designed for steam reformation, but has been shown to work in autothermal reformation [29]. The catalyst comes in pelletized form at 0.47 cm diameter and 0.25 cm height. The catalyst is first crushed and then sifted through two sizes of mesh to achieve an average 0.25 cm diameter particle. The crushed catalyst is then packed into the reactors. 450 g and 250 g of catalyst were used for the low- and high-aspect ratio reactors respectively. Reduction by hydrogen is required before use following manufacturer recommendations. To reduce the catalyst, a gas mixture of 5% hydrogen and 95% nitrogen by volume at 700 to 800 gas hourly space velocity was used based on Yoon et al. [27]. To minimize sintering of the catalyst, the catalyst bed temperature should be kept below 232  C as the reduction reaction can occur very rapidly.

Modeling of a fixed-bed reactor in steam reforming mode A fully coupled multi-physics approach is adopted to model the reactors. COMSOL Version 3.4, a commercially available

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 2 e Geometry of the reformer used in the experiments.

simulation package, was used to model the mass, momentum, and energy balance. First, the geometries of the reactors are modeled with an empty zone for the entry to the catalyst bed in which no chemical reactions will take place. The empty zone is followed by a porous region for the catalyst. Radial symmetry is assumed. Inside the actual reactors, there are free-flow regions before and after the catalyst bed. For simplification, the free-flow region after the catalyst bed was not included in the model. Next, the chemical kinetics are modeled using a mass balance. Then, to establish the energy and momentum balance, the heat produced and consumed by the chemical reaction is coupled. The details of the model are given in the following sections.

Mass balance A simplified chemical reaction model is obtained by removing slower rate and non-essential reactions from the chemical kinetics published in literature [32e38]. The

majority of the intermediate species and radicals were assumed to be consumed quickly. For SR, the primary reaction pathway is:

CH3OH þ H2O / CO2 þ 3H2 DH(298 K) ¼ 50 kJ/mol

(1)

An average, or bulk, diffusivity coefficient, Deff, was calculated based on the LennardeJones Parameters to model the molecular interactions in the reactor. The bulk mass transport within the reactor is described, in convection-diffusion mode, as: 
V$  Deff Vci þ ci u ¼ Ri

(2)

Taking rSR to be the rate equation for steam reformation, then RCH3 OH ¼ rSR is the rate of methanol consumption. The rate equation for steam reformation is also based on the partial pressure of the reactants, as shown below: rSR ðTÞ ¼ kr;SR ðTÞPCH3 OH PH2 O

(3)

where,  kr;SR ðTÞ ¼ ASR e

ESR RT

 (4)

The inlet boundary condition is specified by inward flux of the individual species and the outlet condition is an outward flux.

Energy balance

Fig. 3 e A finer mesh is placed on the boundary between the empty region and the catalyst bed.

Both mass and heat transfer limitations inside the catalyst bed influence the chemical kinetics. Previous work has shown that radiation and conduction were not the dominating heat transfer mode within the reactor [27,39]. In steam

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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reformation, the heat transfer limitation dominates the mass transfer. To capture these properties, the catalyst temperature is used. The catalyst temperature, which is related to pressure, governs chemical kinetics, as indicated by (3). The thermal capacity of the catalyst particle is small, but important in modeling the catalyst bed behavior [40,41]. In this reactor, the flow in the porous structure is laminar. For laminar flow, a convection model of a working fluid over a high surface area heat sink is a reasonable approach [42e52]. Heat from the external source is convectively transferred from the reactor wall to the catalyst using an adjusted convective heat transfer coefficient. At the same time, the endothermic chemical reaction will remove heat from the catalyst surface effectively cooling it. In the model, both the chemical reaction and the catalyst are modeled in the same domain. For the catalyst bed, the energy balance is:
 rcat Cp;cat V$Tcat  V$ leff VT ¼ heff ;cg DTcg

(5)

For the reactants, the energy balance is in the form shown below: m
 X ðDHi Ri Þ  heff ;cg DTcg ugas rgas Cp;gas V$Tgas  V$ lgas VTgas ¼

(6)

n¼1

with, DTcg ¼ Tgas  Tcat

(7)

The term heff,cgDTcg accounts for the heat transfer between the reactant and the catalyst, which is the difference between Tgas and Tcat. Both the enthalpy change, DHi, and heat capacity, Cp,gas, are temperature dependent, thus it is necessary to calculate the enthalpy change, DHi, in the domain. To simplify the computation, the NASA Polynomials [53e55] were used to compute Hi and Cp,gas. Similar to the mass balance, the inlet and outlet conditions are specified by temperature and outward flux. The reactant inlet temperature is defined as: Tgas ¼ T0

(8)

where T0 ¼ reformer inlet temperature, which was kept between 210 and 240  C. The outlet condition is assumed to be dominated by convection, which is modeled as:




n$ kVTgas ¼ 0

(9)

At the wall, a heat flux boundary condition is applied as:   n$  kVTgas þ rgas Cp;gas Tgas u ¼ qheater

(10)

In comparing the thermal efficiencies of the high and low aspect ratio reformers, qheater is also changed to emulate imperfect insulation. For all catalyst boundary conditions, except the axis of symmetry, the heat transfer in the normal outward direction is insulated, which is defined as: 
n$  kVTcat þ rcat Cp;cat Tcat u ¼ 0

(11)

By accounting for the thermal capacity of the catalyst bed, much better results were obtained. Moreover, in an effort to consistently model the reactor for steam and autothermal

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reformation, modeling the thermal properties of the catalyst and the reactant separately gives insight to the catalyst operating condition, which is important to understand issues like sintering and heat distribution. By operating at high temperature, or at elevated temperature, for extended time, the catalyst will degrade over time. This degradation changes the reactor temperature distribution, fuel conversion, and other performance characteristics. It is possible, but beyond the scope of this paper, to correlate the model to a degraded reactor to study the condition of the catalyst.

Momentum balance In a packed bed, the catalyst is effectively a porous media, thus the flow characteristics and reactant distribution can influence chemical reactions, especially at the wall where flow is faster due to lower flow resistance [49,56,57]. Furthermore, a simple approach to flow through porous media, such as Darcy's law, is inadequate when the boundary effect is significant, thus a more sophisticated formulation, the Brinkman-extended Darcy model, is used [50,58e60]. In addition, the convective heat transfer is influenced by the flow properties [61,62], which will impact the activity of the catalyst affecting the chemical kinetics. In the empty free-flow domain, the NaviereStokes equation is used: k j  V$  h Vu þ ðVuÞT þ pI ¼ rðu$VÞu

(12)

V$u ¼ 0

(13)

In the reactor bed domain, the Brinkman equation is used:  V$ 

 h h Vu þ ðVuÞT þ pI ¼  u εp k

V$u ¼ 0

(14)

(15)

The boundary conditions are similar to those applied to the mass balance, where inlet and outlet conditions are specified by velocity and outward flux. For the inlet condition, the velocity is defined as: u ¼ ngas

(16)

Note that vgas is the pre-mix flow rate. For the outlet condition, atmospheric pressure and an exit flow direction parallel to the axis of the reactor are applied. At the wall, the noslip condition is applied.

Modeling of a fixed-bed reactor in autothermal reforming mode The autothermal reformation process was modeled, in a simplified form, as two simultaneous processes: steam reformation and methanol combustion. The steam reformation process, developed above, was extended to include the methanol combustion process required to properly model autothermal reformation. When extending the SR model, special attention was given to the interface boundary between the empty region and the catalyst region. Chemical reactions occur rapidly at this

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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boundary resulting in a chemical species discontinuity. Next the energy balance is then modified to account for heat generated by methanol oxidation. Finally, the momentum balance is adapted to include the effect of air dilution on the reactant velocity. For methanol reformation on a Cu-based catalyst, the rate of exothermic oxidation reaction is at least two orders of magnitude faster than the endothermic steam reforming reaction [19,63e68]. Experiments with CPO of various liquid hydrocarbon fuels have been reported by Cheekatamarla [69].

Mass balance To compensate for the species discontinuity between the empty and catalyst regions, a finer mesh is used at the transition boundary and a Heaviside function, with second degree continuity, is applied to the mass balance. In autothermal reformation, the steam reformation process uses a lower frequency factor, ASR ¼ 6675, as opposed to ASR ¼ 33375 without oxygen addition and is primarily caused by nitrogen dilution. Although air is used to supply the oxidizer, only oxygen and nitrogen are accounted for in the model with nitrogen being a non-reactive species. The methanol combustion reaction, shown in (17), is significantly faster than the steam reformation process and has been observed to take place rapidly at the top of the catalyst bed [66,70,71]. The catalytic combustion rate, rATR, is described in (18) and (19); Isotropic diffusivity is assumed.

above. The boundary condition for the reactant changes because external heaters are not required in ATR; this changes the boundary condition to:   n$  kVTgas þ rgas Cp;gas Tgas u ¼ 0

(20)

This boundary condition can be modified to emulate imperfect insulation, which is extremely important to reactor design. Although important in ATR reactor design, the effect of imperfect insulation is more prominent with SR. With SR, both mass and heat transfer limitations influence chemical kinetics [26,39], while ATR is limited by chemical kinetics via reactant concentrations as indicated by (3). Results presented in the Results section demonstrate the effect of imperfect insulation in an ATR reactor.

Momentum balance Reactant flow is modeled by the NaviereStokes equation in the empty region and the Brinkman-extended Darcy model in the catalyst bed. This is similar to the SR model, except the bulk gas volumetric flow rate is significantly higher for the autothermal reformer after accounting for the oxygen and nitrogen. Liu et al. show that the catalyst temperature is influenced by reactant distribution in an autothermal reactor [72]. Therefore, it is important to capture the effect of nitrogen dilution on reactant flow velocities. These parameters were coupled with the mass and energy balance equations.

CH3OH þ 1.5O2 / 2CO2 þ 3H2 DH(298 K) ¼ 509 kJ/mol

(17)

Experimental results: steam reformation mode


2
3 rATR ðTÞ ¼ kr;ATR ðTÞ cCH3 OH cO2

(18)

In this section, the SR simulation and experimental results are presented and compared. The low aspect ratio reactor will be discussed first, followed by the high aspect, and finally a comparison of the two reactor geometries. The pre-mix flow rate (ml/min) is used for comparing the absolute reforming capacity of different reactors in this section. Typically the steam-to-carbon ratio is also required; however, this ratio was kept constant for all experiments and simulations. The parameters used in the simulations of the two reactors are given below in Table 1. The density of the catalyst, rcat, was found to vary dramatically. This was primarily caused by variation in catalyst particle size, settling of catalyst, and a

 kr;ATR ðTÞ ¼ AATR e

EATR RT

 (19)

Note that the EATR (1245 J/mol) is much lower than the ESR (83000 J/mol), thus the reaction will first deplete oxygen before a portion of methanol undergoes steam reformation. The amount of oxidizer to methanol ratio, represented by the oxygen-to-carbon ratio (stoichiometric O2/C), can be varied; e.g. to compensate for heat loss in the reactor to ensure complete fuel conversion. Experiments on reactor geometry and O2/C have been reported by Tang et al. [14].

Energy balance

Table 1 e Reactor modeling parameters for steam reforming mode.

In autothermal reformation, a portion of the heat generated from the catalytic combustion of methanol will be consumed by the steam reforming step on the same catalyst surface. Due to the differences in the reaction rates, excess thermal energy will be convectively transferred from the catalyst surface to the reactants and propagate to the lower portion of the reactor. This process provides the energy required for the slower endothermic SR reaction. The energy balance of the catalyst bed and reactant are modeled separately because the catalyst particle and the reactant will not reach an equilibrium temperature. This renders the modeling approach the same as SR, described

Reactor Parameter rcat (kg/m3) heff,cg (W/ K) εp k (D) Deff (m2/s) ASR (s1)a ESR (J/mol) Cp,cat (J/(kg  K)) leff,cat (W/(m  K)) a

High aspect ratio

Low aspect ratio

Value 1274.38 15000 0.2 1e9 6.8e5 33375 83000 900 27

Value 1815.56 20000 0.11 1e10 6.8e5 33375 83000 900 27

Calculated from experimental data.

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Table 2 e Reactor modeling parameters. Reactor Parameter 3

rcat (kg/m ) heff,cg (W/ K) εp k (D) Deff (m2/s) AATR (s1) EATR (J/mol) Cp,cat (J/(kg  K)) leff,cat (W/(m  K))

High aspect ratio

Low aspect ratio

Value

Value

1274.38 15000 0.2 1e9 6.8e5 3.69e6 1245 900 27

1815.56 20000 0.11 1e10 6.8e5 3.69e6 1245 900 27

difference in the packing factor. This has important implication on the power density of the reactor. The model takes into account the real-world loading of the catalyst through the energy balance. The measured temperature data were interpolation assuming a 2nd order spline to fit the side wall and center line temperatures. Fig. 4 shows the interpolated profile and simulated reactant temperature profile of the low aspect ratio reformer at 6 ml/min pre-mix fuel flow rate. The thermocouple readings were influenced by both the catalyst and reactant temperatures; therefore, the interpolated temperature profile is for visualization only. Conversely, the simulated reactant temperature closely correlated to the temperature at the point of measurement as shown in Fig. 5 for the pre-mix flow rate from 3 ml/min to 9 ml/min, with an appropriate inlet and external heater temperature to ensure 100% fuel conversion. The trends are very

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similar; there is a noticeable temperature drop at the beginning of the catalyst bed which extends down the length of the bed as the fuel flow rate increases. The outlet temperatures also have good agreement, indicating the assumption of the convective heat transfer from the reactor wall into the catalyst bed was reasonable. Temperature variations in the experiments were likely caused by oscillations in the chemical reactions which were not fully captured by the model. The results from the experiments and simulations of the high aspect ratio reactor are shown in Figs. 6 and 7. Both the model and the actual reactor exhibit similar behavior. This reactor could reform a maximum of 8 ml/min with 100% fuel conversion. There was noticeable heat loss in the end of the actual reactor due to insufficient insulation, which was not completely captured in the model. With smaller reactor radius, the convective heat transfer improves, allowing the use of a fixed inlet temperature. The reactant temperature also does not drop as low as in the low aspect ratio reactor, possibly due to higher reactant velocities moving the reactant downstream. With the low aspect ratio reactors, due to high thermal resistance in heat transfer insufficient heat is available to fully carry the reaction to completion, thus fuel conversion is poor. This conclusion can also be drawn by comparing the simulation reactor length required to reach 100% conversion for the two reactor geometries. In the case of a 5 ml/min premix flow rate, it will require 0.18 m reactor length to fully convert all the fuel. If we pack a low aspect ratio reactor with the same amount of catalyst, as in Fig. 8, it is not possible to achieve complete fuel conversion using the partially packed low aspect ratio reactor at the partially packed length of 0.11 m.

Fig. 4 e Actual (left) and simulated (right) reactor temperature profile at 6 ml/min pre-mix flow rate in the low aspect ratio reactor in steam reforming mode. Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 5 e Actual and simulated centerline temperatures at various pre-mix flow rate in steam reformation mode. Lines: Simulation results. Circles: Measured temperatures.

However, with the same amount of catalyst, the high aspect ratio reactor at the length of 0.6 m can achieve complete fuel conversion as shown in Fig. 9. This indicates in steam reforming, sufficient temperature is crucial to effective catalyst utilization. In other words, poor heat transfer reactor design will reduce the catalyst effectiveness, or decrease the reforming capacity of the reformer.

Experimental results: autothermal reformation mode In this section, the ATR simulation and experimental results are presented and compared. The low aspect ratio reactor will be discussed first, followed by the high aspect, and then a comparison of the two reactor geometries. The pre-mix flow rate (ml/min) and O2/C ratio are used for comparing the two geometries. The pre-mix flow rate, as described above, is used for comparing the absolute reforming capacity of different reactors in this section. The O2/C ratio describes the amount of oxidizer added to the pre-mix; this is useful for comparing the percentage of fuel converted to heat. Typically the steam-tocarbon ratio is also required; however, this ratio was kept constant for all experiments and simulations. The parameters used in the simulations of the two reactors are given below (see Table 2)..

As described above, a second-order spline was used to interpolate the measured temperatures along the length of the reactor to fit both the side wall and centerline temperatures. The thermocouple readings may have been influenced by both the catalyst and reactant temperatures; therefore, the interpolated temperature profile is for visualization only. Fig. 10 shows the interpolated and simulated reactant temperature profiles of the low aspect ratio reformer operating in ATR mode at 20 ml/min pre-mix fuel flow rate and an O2/C ratio of 0.16. The simulation results demonstrated similar temperature characteristics as the experimental data; i.e. a hot reaction zone occurred at the top of the catalyst bed and then gradually decreased as the reactants traveled down the length of the reactor. The noticeable difference is a more pronounced hot zone in the middle of the experimental reactor. The pronounced hot zone could be due to several factors. First, in the actual reactor a tube supplied the reactants to the reactor inlet, potentially directing the reactant flow to the center. Increased reactant velocity, caused by air dilution, would have intensified the jet effect at the inlet reaction zone. Second, the stainless-steel reactor wall may have experienced higher heat loss than expected. Third, the 2nd order spline interpolation may have obscured the actual profile by minimizing the overall error, but increasing the local error.

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 6 e Actual (left) and simulated (right) reactor temperature profile at 4 ml/min in the high aspect ratio reactor in steam reforming mode.

Using centerline temperatures yields a more representative comparison between simulation and experimental results. This comparison, shown in Fig. 11, gives a comparison between simulated and actual results at an O2/C ratio of 0.14 for fuel flow rates between 10 ml/min to 30 ml/min. These data show that the measured temperatures are in good agreement with the measured data for an O2/C ratio of 0.14 at the three flow rates tested. The model also shows good agreement with the experimental data for constant fuel flow rate while varying O2/C ratios, and is shown in Fig. 12. The pre-mix fuel flow rate was fixed at 20 ml/min while the O2/C ratios were varied between 0.14 and 0.20. The results from the experiments and simulations of the high aspect ratio reactor are shown in Fig. 13e15. The temperature profile in Fig. 13 suggests methanol combustion occurs at both the catalyst boundary and along the reactor walls. The observed temperature profile is likely caused by channeling and reactor housing conduction. Channeling is caused by catalyst packing, which creates more void space near the wall than at the center. This allows reactants to travel further down the reactor before the reaction takes place. Reactor housing conduction likely transferred heat down the length of the reactor, artificially increasing the reactor wall temperatures. Comparing the centerline temperatures at flow rate between 10 ml/min and 30 ml/min, shown in Fig. 14, demonstrate that the simulations capture the general behavior of the actual reactor. Measured reactor inlet temperatures were noticeably higher than the simulation predicted for high O2/C ratios. This is likely the result of additional heat generation near the top of the catalyst bed.

Temperature drop off at end of the actual reactor was likely caused by convective heat loss from insufficient reactor insulation. The same temperature drop at the end of the actual reactor was also observed when varying the O2/C ratio with a fixed pre-mix flow rate, as shown in Fig. 15. Although a few discrepancies were identified, the simulation made a reasonable estimation of the centerline temperature. A comparison of the simulated methanol concentration in a partially packed low aspect ratio and fully packed high aspect ratio reactors, shown in Fig. 16, found that the reforming process finishes near the top of the catalyst bed. This suggests that a much shorter reactor can be used, despite higher fuel flow rates than running the same reactors in steam reforming (SR). Also, the reformer's energy density is significantly higher when running in ATR mode than in SR using the same reactor. For steam reformation, the reactors were shown to have low thermal conductivity within the catalyst bed. Theoretical methanol concentrations, presented in Fig. 16, suggest that this property enables the low aspect ratio reactor to retain more heat, thus the reactants are reformed much closer to the top of the catalyst bed. This effect is coupled with a reduction of bulk fluid velocity. Consider the partially packed low aspect ratio reactor running in SR mode. Using the same amount of catalyst as in the high aspect ratio reactor, the larger reactor radius can enhance the reaction rate and reduce the amount of oxidizer required, increasing the hydrogen yield. The reader is referred to Tang et al. [14] for further discussion about reformer performance, geometries, and operating mode.

Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 7 e Actual and simulated centerline temperature of the high aspect ratio fully packed reactor at various premix flow rate in steam reformation mode. Line: Simulation results. Circle: Measured temperatures.

Both simulation and experimental data suggest the reactant temperatures can reach well above the recommended maximum operating temperature, 350  C, due to rapid methanol oxidation at the catalyst boundary. Although these

temperatures would normally sinter the copper-based catalyst, the catalyst not only survives, but remains below the recommended temperature limit. One explanation is that the methanol oxidation reaction occurs rapidly and thus does not

Fig. 8 e Volume comparison of the low and high aspect ratio reactors (approximately 250 g of catalyst). Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 9 e Simulated methanol concentration in the high aspect ratio (Blue) and low aspect ratio (Red) reformer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

have sufficient time to transfer heat to the catalyst, rather, most of the thermal energy is transferred to the gas. What energy is transferred to the catalyst through a temperature gradient is removed by the endothermic SR reaction, occurring on the surface of the catalyst Fig. 17.

Summary With the objective to produce a model that can describe the temperature distribution and fuel conversion efficiency of

Fig. 10 e Reactor temperature profiles in degrees Celsius. Left: experimental interpolated temperature profile. Right: simulated temperature profile. Profiles generated at O2/C ¼ 0.16, 20 ml/min premix feed rate, low aspect ratio reactor, autothermal mode. Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 11 e Fuel flow rate and centerline temperatures. Line: simulation results. Circle: measured temperatures. Generated at O2/C ¼ 0.14, low aspect ratio reactor, autothermal mode.

both steam and autothermal reformation, experiments were carried out to obtain the necessary parameters for constructing the multi-physics models. Two reactor geometries were used to verify the model, and the results were presented for both SR and ATR. Although there was some discrepancy between the models and experiments, the models captured

the important characteristics of the reformer and were shown to be capable of estimating the required reformer length for 100% fuel conversion. There were several factors contributing to the discrepancy between the measured temperature and the simulation. The insulation used in the actual reactors is not perfect, thereby

Fig. 12 e O2/C Ratio and centerline temperatures. Line: Simulation results. Circle: Measured temperatures. Generated at premix flow rate of 20 ml/min, low aspect ratio reactor, autothermal mode. Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 13 e Reactor temperature profiles in degrees Celsius. Left: experimental interpolated temperature profile. Right: simulated temperature profile. Profiles generated at O2/C ¼ 0.16, 20 ml/min premix feed rate, high aspect ratio reactor, autothermal mode.

reducing wall temperature and increasing heat transfer out of the reactor. Since the reactions are highly sensitive to temperature through the reaction rate constant, the resulting temperature profiles are obscured by the accuracy of the boundary conditions. The electrical heater elements were

assumed to provide heat uniformly. The modeling of the flow profile and the porous catalyst bed are based on general assumptions used in the literature. The various assumptions that contributed to differences between the measured temperatures and simulation were

Fig. 14 e Fuel flow rate and centerline temperatures. Line: simulation results. Circle: measured temperatures. Generated at O2/C ¼ 0.14, high aspect ratio reactor, autothermal mode. Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 15 e O2/C Ratio and centerline temperatures. Line: Simulation results. Circle: Measured temperatures. Generated at premix flow rate of 20 ml/min, high aspect ratio reactor, autothermal mode.

Fig. 16 e Methanol concentration and reactor length. Blue: Simulated methanol concentration, high aspect ratio reformer. Red: Simulated methanol concentration, low aspect ratio reformer. Generated at premix flow rate of 20 ml/min, O2/C ¼ 0.16, autothermal mode. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Tang H-Y, et al., Modeling of a fixed-bed copper-based catalyst for reforming methanol: Steam and autothermal reformation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.04.096

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Fig. 17 e Reactor catalyst and gas temperatures in degrees Celsius. Left: Simulated catalyst temperature. Right: Simulated reactant gas temperature.

simplifications that give ways to understanding the overall behavior of the reactor. As a result, the ‘effective’ parameters are used in the simulation, capturing the bulk behavior with reasonable trends and accuracies.

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