CIAMBELLI, Paolo

MALANGONE, LUCA

VACCARO, Salvatore

English

Archivio della Ricerca - Università di Salerno

 Modeling study of a Catalytic Microstructured Reactor L. Malangone, P. Ciambelli, S. Vaccaro Department of Chemical and Food Engineering, University of Salerno Via ponte don Melillo 84084 – Fisciano (SA) ITALY  1. Introduction This paper reports a modelling study for steam reforming of methane occurring in a microstructured catalytic reactor. The geometry adopted for modelling was tailored on a real lab-scale system and reflects a configuration commonly adopted in literature [1-2-3], where heat generated by methane combustion substains endotermic reactions (CH4 reforming). Mathematical simulations of this kind of system are important research tools for the investigation of the complex phenomena governing system performance and for suppling information about parameters that cannot be detected by real measuring devices. In developing microchannel reactors for the methane steam reforming, understanding the temperature profile within the reactor is important for designing and optimizing the structure of catalysts to achieve the maximum performance.  Numerous studies investigated the modelling of methane reforming and its related kinetics in micro-reactor especially on Ni catalysts [4-5-6]. Nevertheless, the topic was not widely investigated under a non-adiabatic restraint [7-8-9], where the need to adopt a 3D geometry becomes unavoidable. Furthermore, few comprehensive studies extended the energy and mass balance of the reactive flow within the thin catalytic layers (50 μ) [10-11]. On the contrary, simplified models are available in the literature where temperaure and concentration profiles are assumed constant along the catalyst thickness limiting, therefore, the possibility to use modelling as a design tool for the catalyst configuration. In this work, the modeling study was performed in a 3D geometry, solving the fully coupled problem of chemical reactions taking place inside catalytic layers with laminar flow occuring in an adiacent channel including mass diffusion due to thermal and concentration gradients. The contribution of the catalytic reforming reactions was modelled considering two different kinetic expressions suggested in the literature by Xu and Froment [4] and by Hou and Hughes [5]. Results of the simulations were compared with pertinent experimental results showing that a good agreement is achieved when applying the kinetic expressions suggested by Hou and Hughes. 2. Experimental setup Figure 1 reports a scheme of the plate reactor employed in the experiments with the flow pattern of the reactants while Figure 2 shows a functional view of the various reactor components. The entering reactants streams and the reactor were pre-heated at about 600°C by electrical heaters. The whole reactor and the related tubing were enveloped in a rock wool blanket to provide thermal insulation. The reactor exhaust products concentrations were monitored by ADVANCE OPTIMA, ABB analyzers. 1Processes and Technologies for a Sustainable Energy  The reactor channels are separated by aluchrom foils supplied by ECOCAT (0.5 mm tick) onto which different catalysts were deposited on the opposite faces (Figure 2). The structured catalysts consist of a washcoat containing 5 wt% Ni for the steam reforming reaction and a catalytic washcoat containing 3 wt% Pt for the catalytic combustion reaction. The gas temperatures were measured in each channel by three 1.0 mm diameter thermo-couples located at distances from one channel edge of 10, 30 and 50 mm, respectively, as shown in Figure 3B. Here only 6 of the 9 thermocouples are indicated, being the remaining 3 symmetrically located in the other combustion channel. Further details of the whole experimental setup are reported elsewhere [9]. 3. Modelling and kinetic expressions A three-dimensional heterogeneous steady-state model was developed by the authors [10-11] in order to describe velocity, concentration and temperature distributions inside the catalytic slabs reactor. The model consists of the material, energy and momentum balances equations for the system in the object and contains the constitutive equations for physical chemistry properties of the reactants species and the kinetic expressions for the reactions.  Three reactions are identified as the major reactions in the reforming process. They are the steam reforming reactions with CO (eq. 1) and CO2 (eq. 3) as products, and the water gas shift reaction reaction (eq. 2). molkJΔHHCOOHCH / 206                  3 298224                                                          (1)  molkJΔHHCOOHCO / 41                   298222                                                           (2) molkJΔHHCOOHCH / 165              42 2982224                                                           (3) For reaction 1, 2 and 3, two different intrinsic kinetic models are considered. One is that developed by Xu and Froment [4], which also is the oldest and widely used. The other is that developed by Hou and Hughes [5]. It is more recent and there are not numerous references about its use in a reactor model. Xu and Froment investigated methane steam reforming over Reforming cata-lyst  Combustion catalyst  Combustion channel Reforming channel Graphite  gasket Catalytic plates Combustion channel Reforming channel Combustion channel Insulation layer Metallic surface Combustion catalyst Aluchrom foil Reforming catalyst Fig. 1 CAD-view of the catalytic plate reactor Fig. 2 Schematic rapresentation of reactor geometry 2Ischia, June, 27-30 - 2010 Ni/MgAl2O4 catalyst in a tubular reactor. The temperature was in the range of 500-575°C and the pressure was between 3 and 15 bar. The kinetic models from Hou and Hughes are based on experiments carried out at 120 kPa in the range of temperature 325-550°C for a catalyst containing 15-17% NiO supported on α-Al2O3. The rate expressions proposed by the mentioned authors can be found in the cited references. Both works consider a Langmuir-Hinshelwood mechanism for which reactants are assumed to adsorb on the surface of the catalyst, which is considered as a perfectly flat isotropic surface, consisting of fully equivalent adsorption sites. Chemical reaction on the catalytic surface is the rate-limiting step, which governs the overall rate of reaction: reaction occurs between adsorbed reactants, forming adsorbed products which, subsequently, desorb. According to this mechanism, a basic difference between Xu and Froment and Hou and Hughes kinetics subsists: the first one assume adsorption on catalyst surface of all the species involved but CO2, while in the second one methane and carbon dioxide are considered to volatilize immediately and does not adsorb on the catalyst.  In order to show the difference in reactivity between the two models, the total methane reaction rate eq.s 1 and 3) (r1+r2 [kmol·h-1·kgcat-1]), calculated according the two mechanisms is plotted in Figure 3, as a function of the methane partial pressure. The total rate of the reforming reaction is non-monotonic with respect to the methane partial pressure, meaning a positive as well as negative order dependence of the kinetic expressions on it. The two kinetics yield very different absolute values although the trend is very similar (Fig. 3). In any event, the rate of disappearance of methane is much higher for the Xu-Froment model with respect to that for the Hou-Hughes one (two order of magnitude).   Fig. 3: Reactivity comparison as a function of PCH4/PTOTAL ratio (PCO=PCO2=10-3bar; PH2=0.1 bar; T =600°C; PTOTAL=1 bar). The catalyst tends to be CH4 -poisoned for PCH4 /PTOTAL =0.9, and H2O-poisoned for PCH4 /PTOTAL = 0.  Methane combustion (eq. 4) was performed under fuel rich condition (feeding ratio O2/CH4=1.68) [13-14], inducing the formation of partial oxidation products; on the basis of the product experimental composition, the selectivities to H2 and CO was calculated and the following chemical expression was estimated. molkJΔH      CO.OH.CO.H.O.CH / 660            74070126030721 29822224            (4)    0.010.111010010000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1P_CH4/PR1+R3Hou - Hughes Xu - FromentPCH4 / PTOTAL r 1+r 3 [=] kmol·h-1·kgcat-1 3Processes and Technologies for a Sustainable Energy A power-law kinetic expression for the above reaction was assumed: a first and zero order with respect to methane and oxygen concentration respectively was considered [15]. For the mass balance in the combustion chamber only the species CH4, O2, N2, H2O and CO2 were taken in consideration. 4. Experimental results and models comparison Combustion and reforming reactants were fed in concurrent mode. The molar flow rates, expressed as mol/min (STP), employed in the experiments and in the simulations, were: CH4 (combustion channel) = 0.0045, CH4  (reforming channel) = 0.0089, Air (combustion channel) = 0.0357, steam (reforming channel) = 0.0271. The initial reactor temperature was 600°C. The comparison between experimental results and model predictions at steady state, for the test carried out feeding the molar flow rates, reported above, are shown in Table 1. Specifically, Table 1 reports the molar fractions on dry basis of the various species at the reforming channel outlet (yi), the methane conversion both in the reforming and in the combustion channels (XCH4-sr and XCH4-co, respectively) and the H2 yield (φ) defined as moles of H2 produced per mole of CH4 converted in the reforming channel. From data in Table 1 it is evident that when the Hou-Hughes kinetics is employed, the model fits experimental results with good accuracy while, when the kinetics suggested by Xu-Froment is used, the model deviates markedly from experimental findings, largely overestimating the methane conversion on the reforming side.  Table 1: Comparison between experimental performance of the non adiabatic reactor and those predicted by the model with the two different kinetics  φ XCH4-sr XCH4-co yH2 yCH4 yCO yCO2 Experimental values 3.45 67.34 88.25 65.49 9.25 10.33 8.76 Calculated with the Hou-Hughes kinetics 3.31 67.66 89.72 68.61 10.14 14.50 6.75 Calculated with the Xu-Froment model 3.28 89.60 89.45 74.22 2.68 15.76 7.33  The effect of the kinetics on the results of the model are evident also from the comparison between the calculated spatial profiles of the species along the axis of the reforming channel the reactor, shown in Figure 4. Here it can be observed that the methane molar fraction has an abrupt fall at the reactor beginning when the Xu and Froment kinetics is used, whereas the Hou-Hughes kinetics yields a less sudden composition change. The profiles along the reactor of the hydrogen molar fractions, obtained with the two different kinetics, show opposite trends with respect to those of the methane molar fractions profiles although the differences between the two kinetics persists. Comparing the experimental and the calculated temperature profiles along the channels axes of the reactor (Figure 5), it can be noticed that the model with both kinetics simulates with acceptable accuracy the measured temperatures in both channels except the value in the reforming channel at 1 cm from the reactor inlet. Both models are also able to predict the inversion of the temperature gradient, which occurs between the two channels when considering the temperatures values at 3 and 5 cm from the reactor inlet (Figure 5): this circumstance is due to the non adiabatic conditions of the system. Indeed, it reflects the possibility that in the combustion channel in proximity of the external wall the gas temperature reaches a value lower than that measured at the center of the reforming channel, because of the thermal dissipations through the external wall. In fact, due to the steep 4Ischia, June, 27-30 - 2010 6206406606807007207407607800 1 2 3 4 5 6Reactor length (cm)T (°C)Reforming Hou-Hughes Reforming Xu-Froment T(exp. ref)Combustion Hue-Hughes Combustion Xu-Froment T(exp.comb)transverse thermal gradient establishing across the channels, the detection of the inversion in temperature gradient between the channels becomes dependent only by the positions where the temperatures are monitored. Discrepancies in temperature values could be related to the difficulties in matching standard thermocouples with such a small device. In this case, in fact, the correct temperature measurement becomes difficult, due to the potential interference with local hydrodynamics and to the presence of radiative and conductive thermal fluxes from the catalyst layers and to the external walls, as pointed out recently by the authors [9]. In developing microchannel reactors, the knowledge of the temperature profiles within the reactor is important for designing and optimizing the structured catalysts and to achieve the maximum performance. Therefore the need for a model able to accurately predict system performance appears evident. Figure 4: Methane and hydrogen molar fraction profiles along the reactor length on the centre-line of the reforming channel (at -0.03 ≤ x ≤ 0.03; y=z=0 as indicated in figure 2. 5. Conclusions Results obtained comparing model simulations performed using kinetics suggested by Xu-Froment and by Hou-Hughes showed that both are able to simulate satisfactory temperature profiles, but only the second model successfully predict reactor performance in terms of methane conversion on reforming side. Xu-Froment kinetic model predicted a higher methane conversion affecting significantly the composition (yi) of the reforming products. Due to the thermal coupling between endo- and exo-thermic channels, system performance becomes directly related to the temperature profiles inside the reactor; however, due to the limited dimension of the system, direct temperature measurements using standard thermocouples is not really reliable and, therefore, information deducible from a mathematical model can be very useful. 5Processes and Technologies for a Sustainable Energy 00,10,20,30,40,50,60,70,80,90 1 2 3 4 5 6Reactor length (cm)yiy_CH4 Hou-Hughes y_CH4 Xu-Fromenty_H2 Hou-Hughes y_H2 Xu-FromentFigure 5: Comparison between the experimental temperature values in the reforming and combustion channels and the corresponding profiles simu-lated with the two kinetic models (in figure 2: -0.03 ≤ x ≤ 0.03; y = z =0 for reforming profiles; -0.03 ≤ x ≤ 0.03; y=0; z=2.85·10-3 for combustion profiles) 6. References 1. Venkataraman, K., Wanat, E.C., Schmidt L.D.: AIChE Journal, 49(5): 1127 (2003)  2. Cao, C., Wang, Y., Rozmiarek, R.T.: Catal. Today, 110: 92 (2005) 3. Kirillov, V.A., Kuzin, N.A., Kulikov, A.V., Fadeev, S.I., Shigarov, A. B., Sobyanin, V. A.: Theoretical Foundations of Chemical Engineering, 37(3): 276 (2003) 4. Xu, J., Froment, G.F. : AIChE Journal, 35(1): 88 (1989) 5. Hou, K., Hughes, R. Chem. Eng. Journal, 82: 311 (2001). 6. Luna, A.E.C., Becerra, A.M., Dimitrijewits, M.I.: React. Kinet. Catal. Lett., 67(2): 247 (1999). 7. Ciambelli, P., Malangone, L., Vaccaro, S.: Chem. Eng. Trans., 17:1245 (2009). 8. Vaccaro, S., Ciambelli, P., Malangone L.: Proceedings of XXXII Combustion Meeting of The Italian Section , IV-5, April 26-28 (2009). 9. Vaccaro S., Malangone L., Ciambelli P.: Accepted for publication in Ind.&Eng.Chem. Res. (2010). 10. Vaccaro S., Malangone L., Ciambelli P.: Int. J. Chem. React. Eng., 8(A10) (2010). 11. Vaccaro, S. Ciambelli, P. in Recent Advances in Modelling and Simulations, G. Petrone and G. Cammarata ed.s, Ch. 30, I-Tech Education and publishing, Vienna, Austria, 580-606 (2008) 12. Ugues, D., Specchia S., Saracco, G.: Ind. Eng. Chem. Res. 43(9):1990 (2004). 13. Ciambelli P., Malangone L., Manna M.A., Vaccaro S.: Chem. Eng Trans.,17: 157 (2009). 14 Lyubovsky M., Smith L.L., Castaldi M., Karim H., Nentwick B., Etemad S.,  LaPierre R., Pfefferle W.C.: Catalysis Today, 83: 71 (2003). 15. Zanfir, M., Gavriilidis, A.: Chem. Eng. Sci., 58(17):3947 (2003). 6

Modeling study of a Catalytic Microstructured Reactor

https://www.iris.unisa.it/bitstream/11386/3014951/1/Modeling%20study%20of%20a%20Catalytic%20Microstructured%20Reactor.pdf

Modeling study of a Catalytic Microstructured Reactor

Abstract

Similar works

Full text

Available Versions

Archivio della Ricerca - Università di Salerno