A publication of
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 32, 2013
The Italian Association
of Chemical Engineering
www.aidic.it/cet
Chief Editors: Sauro Pierucci, Jiří J. Klemeš
Copyright © 2013, AIDIC Servizi S.r.l.,
ISBN 978-88-95608-23-5; ISSN 1974-9791
Performance analysis of Al2O3 and SiC catalysed foams in
methane auto-thermal reforming
Vincenzo Palma, Antonio Ricca*, Paolo Ciambelli
a
University of Salerno, Department of Industrial Engineering, via Ponte don Melillo - 84084 Fisciano (SA) ITALY
aricca@unisa.it
In this work the relevance of supports in methane auto-thermal reforming (ATR) was investigated. Catalytic
activity tests were carried out in a thermally integrated adiabatic reactor. Heat exchangers placed
downstream the catalytic volume were able to pre-heat water and air stream by exploiting the heat from
exhaust stream, allowing to feed reactants at room temperature as well as cool the reactor outlet stream to
a temperature suitable for further purification stages (Water Gas Shift, Preferential Oxidation). A series of
thermocouples and sampling lines placed along the catalytic bed allowed a continuous monitoring of the
catalytic process. Preliminary tests showed very promising catalytic system performances, in terms of
thermal efficiency and hydrocarbon conversion. Furthermore, the attention was focused on the support
material, and in particular on its heat transfer properties.
1. Introduction
In recent years, hydrogen was proposed as one of the most promising energy carriers (Urbaniec, 2013),
and a lot of interest was devoted to the production of hydrogen for high efficient electricity generation by
fuel cells. Despite the growing current interest towards renewable resources, the wide diffusion of fossil
fuels and their low cost low make hydrocarbons fuel processing the best solution for the start-up phase of
a hydrogen based economy (Palma et al., 2012b). Due to its high H:C ratio and its widespread existing
delivery pipelines, methane is the ideal fuel source for hydrogen production. Moreover, the debate on
hydrogen storage and transport pushes R&D interest toward distributed productions. However, the current
industrial hydrogen production technology does not seem able to meet the requirement of a small-scale
fuel processor. To this aim, Auto-Thermal Reforming (ATR) of hydrocarbons seems to be the most viable
solution for distributed H2 production, due to various characteristics such as simple, very compact and
versatile systems, fast start-up times and very quick response to feed changes. ATR process is often
considered the sequence of two main reactions: Partial Oxidation (POX) and Steam Reforming (SR)
(Scognamiglio et al., 2012, Zeman et al., 2011): the oxidative reaction occurs as a first step of the process,
while the steam reforming reaction starts when the process stream reaches a given temperature. ATR is a
self-sustained catalytic process, in which the heat generated by the hydrocarbon oxidation reactions
supplie to the system the heat needed for the SR reactions. Such reaction sequence generates a quick
temperature increase in the first zone of catalytic bed, followed by a slow temperature decreasing toward
the end zone (Simeone et al., 2008a). The amount of generated heat, hydrogen yield and hydrocarbon
conversion strictly depend on feed ratio values x (= O2/CH4) and y (= H2O/CH4) (Simeone et al., 2008b).
From this point of view, ATR reaction is a process able to work without any external heat sources
(Ciambelli et al., 2009, Lee et al., 2005).
Several crucial issues affect the performance of the ATR process. Since ATR reaction involves 3 different
reactants, both liquid and gaseous, the proper reactants mixing plays a fundamental role in the system
behavior. A non-uniform mixing of reactants may cause local lack of reagents, with a decrease of yield and
selectivity toward desired products, coke formation, and catalyst deactivation (Aasberg-Petersen et al.,
2011, Pasel et al., 2007). On the other hand, the reactants temperature affects the catalyst thermal profile,
the outlet temperature and then the fuel conversion and the H 2 yield (Ding and Chan, 2008, Zahedi nezhad
et al., 2009). In fact, according to thermodynamic predictions, higher temperature assures higher
hydrocarbons conversion.
Great attention is obviously devoted to the catalytic system with respect to hydrocarbon conversion and
reactions selectivity. Since the ATR process includes both SR and POX reactions (Chen et al., 2010), an
optimal catalytic system should ensure a high selectivity towards these two reactions, as well as inhibit
hydrocarbon cracking reactions, that lead to the catalyst deactivation. In several studies the good catalytic
activity of nickel (Hoang and Chan, 2007, Mosayebi et al., 2012, Rezaei et al., 2011) as well as noble
metals (Pt, Rh, Ru) (Li et al., 2011, Qi et al., 2007b) supported on Al2O3, CeO2 or ZrO2 has been reported
(Cai et al., 2008, Lisboa et al., 2011, Souza and Schmal, 2005, Yuan et al., 2009). Furthermore, significant
improvement of stability and selectivity have been achieved with bimetallic catalytic systems (Cimino et al.,
2010, Palma et al., 2012a). Finally, catalyst structure (e.g. powder, pellets, honeycomb, foams, etc.) plays
an important role for the system behavior (Liu et al., 2004). Recent literature highlights that a proper
thermal management in the catalytic volume may improve catalyst performance (Palma et al., 2009,
Halabi et al., 2011) . In this way, high conductive metal based honeycomb, foam and monoliths are pointed
as very promising supports for ATR catalytic systems (Ciambelli et al., 2010).
The overall thermal management was also an issue to be taken into account for the system design.
Typically, reaction stream leaves ATR catalytic volume at about 700÷800°C, while further purification
stages, like Water Gas Shift (WGS) and CO Preferential Oxidation (PROX) need lower temperature,
respectively about 300°C and about 100°C (Ding and Chan, 2009, Hwang et al., 2011, Kim et al., 2010,
Laguna et al., 2011, Liguori et al., 2012, Sekhavatjou et al., 2011, Sirichaiprasert et al., 2008). Therefore,
effective heat transfer from products stream to fed streams, may result in several advantages, both in plant
size and operating costs (Qi et al., 2007a, Schildhauer and Geissler, 2007).
In this work, the behavior of foam catalysts for a thermally integrated ATR reformer was studied, in order to
investigate the role of catalyst support on the system performance.
2. Experimental
Activity tests were carried out in an adiabatic catalytic methane of up to 10 Nm3/h of H2 capacity. The
system was equipped by a thermal recovery module, aimed to preheat reactants by exploiting sensible
heat from exhaust products stream: so a self-sustained system was obtained, and no external heat
sources are needed (Palma et al., 2011).
Air (as oxidant gas), bi-distilled water and methane (as surrogate of natural gas) were selected as
reactants. Methane (purity of 99.95%) and air (ultrapure, 99.998%), both supplied from SOL spa, were
delivered to the system by means of thermal mass flow controllers (Brooks Instruments); bi-distilled water
was stocked in a 7 atm pressurized vessel, and delivered to the system by means of a Coriolis based
mass flow controller (Quantim, Brooks Instruments). Air and liquid water were delivered to the heat
exchange module for pre-heating procedure; the obtained warm air and super-heated steam were sent in
mixing module, together with methane at room temperature.
A thermal recovery module was placed just downstream the catalytic zone in order to pre-heat reactants
by recovering sensible heat by exhaust stream. It is a tube-shell type, and basically consists of three heat
exchangers, two of which devoted to water vaporization and overheating, and one devoted to the air
heating. Each heat exchanger is composed by a series of rectangular coils, realized with stainless steel
tubes (o.d. 1/8”, thickness 0,74mm), mounted in parallel on two manifolds (in and out) (Figure 1Error!
Reference source not found.). Such configuration assured a uniform distribution of reactants stream in
all coils, and minimized pressure drop in tube-side. Heat exchangers were disposed in a rectangular
module, crosswise the exhaust gas flow. Further details are available in a previous work (Palma et al.,
2011).
Figure 1 - Heat exchanger: disassembled and assembled views
The pre-heated air and steam and the cold hydrocarbon were fed to a mixing module (Figure 2aError!
Reference source not found.) to obtain a homogeneous mixture.
Figure 2 - Mixing module (a) and Reaction module (b)
The mixing module is made as a series of truncated cones placed in series: the particular shape module,
showed in Figure 2aError! Reference source not found.)Error! Reference source not found., causes a
subsequent expanding and shrinking that allow the formation of eddies and whirlpools, resulting in a very
effective reactant mixing.
The rectangular section (60x80 mm) catalytic module (Figure 2bError! Reference source not found.)
was placed downstream the mixing module. The catalytic volume was able to contain up to 240 cm3
catalyst, and was closed by means of a rectangular flange that facilitated the maintenance procedures. An
electrical trigger was placed just before the catalytic bed to assist the start-up procedure.
The overall reaction system (total length within 50 cm), was obtained by assembling the three modules by
means of special flanges (Figure 3Error! Reference source not found.), that assured a quick system
disassembly.
Figure 3 - Assembled reactor
The setup of the reactor was completed with a complex monitoring system, constituted by 6 k-type
thermocouples and 6 sampling lines placed along the catalytic bet between the inlet and outlet sections, at
a distance of 14 mm each other, to monitor temperature and composition profiles along the catalyst. The
gas analysis was performed by means of a Hiden Analytical mass spectrometer, equipped with a Proteus
multivalve able to switch between sampling lines. The mass spectrometer calibration was performed
before, during and after each catalytic test: in this way it is possible to evaluate an experimental error
within 3% of the measured values.
Performance analysis was carried out for two different commercial foam monoliths (noble metals based,
deposited on Al2O3 and OBSiC (oxide-bonded silicon carbide, 50% Al2O3; 40% SiC; 10% SiO2) 65 ppi
foam, provided by Johnson Matthey). Each monolith was composed by 5 foam disks sized D 29 x W 15
mm, attached between them to obtain a cylindrical shape sized D 29 x W 75 mm, for a total catalytic
volume of 50 cm3. The foam cylinders were fixed in an insulating brick sized 60x80x75 mm and
surrounded by insulating materials, in order to reduce heat loss in catalytic volume.
The experimental tests were carried out at 2.5 bar, by feeding to the system air, water and methane: the
feed ratios (H2O:O2:C = 0.49.0.56:1) and the space velocity (GHSV = 120,000h -1) were fixed.
The gas composition obtained in each point of catalytic bed was compared to thermodynamic equilibrium
values, calculated by GasEq software (a Windows freeware software based on the minimization of Gibbs
free energy) (Morley) as an isotherm reaction at the corresponding temperature and pressure.
3. Results
In the Figure 4 the hydrogen and carbon dioxide concentrations and methane conversion are reported for
the two catalytic systems.
Figure 4 - Reaction stream composition profiles along the Al2O3 foam catalytic bed (a) and the SiC foam
catalytic
As a first result, for both catalytic systems the main part of the reaction process seems to occur just in the
first 14mm of catalyst bed. As a result, the gas composition and hydrocarbon conversion appeared very
close to thermodynamic equilibrium values in each point of catalytic bed. However, a flatter composition
profile was found for the catalyzed SiC foam.
Figure 5 - Performances key features (a) and temperature profiles (b) comparison between foam
monolithic catalysts
By comparing the performances of Al2O3 and SiC foam catalysts (Figure 5aError! Reference source not
found.) no relevant differences were found. The analysis of thermal profiles shows very flat profiles for
both catalytic foams (Figure 5bError! Reference source not found.). Moreover, just after the 2rd foam
disk no relevant temperature changes were recorded, and according to the composition behavior along the
catalytic bed, the ATR reaction mainly occurs in the second disk. Finally, a flatter temperature profile was
observed for SiC foam catalyst, that results in flatter composition profiles, and leads to higher hydrogen
production.
4. Conclusions
Activity tests of methane ATR were carried out on two catalytic foams (based on Al 2O3 and SiC) and the
influence of material support was investigated. The results evidenced the very high activity of both catalytic
systems that quickly reached the equilibrium values. In each case, the highest part of the process seems
to take place in the first part of the catalytic volume: this behavior suggests that the catalyst could be able
to operate at very higher GHSV values, and so it could produce a very higher hydrogen amount. The
influence of the stream mixing improves reactions kinetics, leading to a very flat thermal profile, The
recorded thermal jump was within 130°C for the Al 2O3 foam, and 40°C for the SiC foam: the flatter thermal
profile in the catalyzed SiC foam could be likely due to an higher thermal conductivity. Specific studies of
this properties are in progress.
5. Acknowledgements
The research leading to these results has received funding from the European Union Seventh Framework
Programme FP7-NMP-2010-Large-4, under Grant Agreement n° 263007 (acronym CARENA).
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