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Innovative structured catalytic systems for methane steam reforming intensification
Chemical Engineering & Processing: Process Intensification 120 (2017) 207–215
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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Innovative structured catalytic systems for methane steam reforming intensification Vincenzo Palma, Antonio Ricca, Marco Martino, Eugenio Meloni
MARK
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University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Structured catalysts Process intensification High conductive carriers Methane reforming Hydrogen production
The use of structured catalysts based on highly conductive carriers may allow a flattened radial temperature gradient along the catalytic bed due to a better heat transfer, so obtaining a consequently higher performance. The effect of thermal conductivity of structured carriers on highly endothermic Methane Steam Reforming (MSR) reaction is investigated. The performance of the structured catalysts, prepared starting by Silicon Carbide (SiC) monoliths, in the “wall flow” (WF) and “flow through” (FT) geometric configurations, demonstrates the direct correlation between the thermal conductivity of the carrier, the methane conversion and the hydrogen productivity. In particular the tests showed that the SiC “wall flow” (WF) monoliths guaranteed a better axial and radial thermal distribution, with respect to the SiC “flow through” (FT) ones, resulting in better catalytic activity up to a temperature reaction of 750 °C. Furthermore, the comparison among the performance of the structured catalysts and the commercial 57-4MQ, provided by Katalco-JM, highlights the choice of structured catalysts, which require a lower temperature outside of the reactor, so increasing the overall process efficiency.
1. Introduction Hydrogen is nowadays considered the most promising energy vector, thanks to its high capacity of storing energy from primary sources [1]. Despite considerable efforts to identify new green technologies and sources for hydrogen production, the use of fossil sources and the traditional production methods, are still widely affordable [2]. Among the production processes of hydrogen, the Steam Reforming of natural gas is still the less expensive and the most widespread industrial process, suitable for economies of scale and widely involved in the ammonia and methanol production processes. The Steam Reforming (SR) is a highly endothermic equilibrium reaction [3], the standard enthalpy reaction for methane is about 206 kJ/mol however, for more complex hydrocarbons, it can reach much high values, for example for n-C7H16, the ΔH°298 is about 1108 kJ/mol [4]. The Methane Steam Reforming (MSR) is normally described as the result of two main reactions, the reforming (Eq. (1)) and the water gas shift (WGS) reactions (Eq. (2)), CH4 + H2O ⇆ CO + 3H2 ΔH°298 = 206 kJ/mol
(1)
CO + H2O ⇆ CO2 + H2 ΔH°298 = − 41 kJ/mol
(2)
The thermodynamic of the system, coupled with the actually used catalysts, allow to reach acceptable conversions only for reaction
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Correspondence author. E-mail address: [email protected] (E. Meloni).
http://dx.doi.org/10.1016/j.cep.2017.07.012 Received 9 March 2017; Received in revised form 29 June 2017; Accepted 12 July 2017 Available online 26 July 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.
temperature exceeding the 700–750 °C [5]; to realize this hard conditions, over the catalytic bed, it required a large amount of heat, so the process temperatures must be forced above 1000 °C. The most widespread process configuration provides two stage, in the first stage methane and steam react at 700–800 °C, thus obtaining a conversion of the hydrocarbons up to 90%, in the second stage an amount of air is added to react with a part of the hydrogen, generating the necessary heat to reach a temperature of the reaction mixture of 1000–1200 °C, allowing an almost complete conversion [6]. So high process temperatures however present several problems, especially related to the resistance and durability of the materials, but also to the overall process efficiency in terms of produced hydrogen per supplied energy. On the basis of the above consideration it seems clear that the main problem of this kind of process, is related to the transfer of heat to the catalytic bed. The typical radial thermal profile of a catalytic bed, packed with pellets provides a considerable thermal gradient between the side wall and the center of the bed, part of the heat supplied, necessarily dissipates, with a decreasing from 1200 °C, at the outside, to 700 °C at the center of the catalytic bed (Fig. 1), making it necessary a huge expenditure of energy [7–10]. Much efforts were made in the direction of the preparation of new and more active catalytic formulations at low temperatures [11], or in the use of new technologies, such as non-thermal plasma [12]. However a process intensification [14] [13] seems to be a much more promising
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most common formulations are based on high surface area aluminas, a coating polymer and the desired support. The preparation procedure provides the dispersion of the support in a colloidal solution, obtained by dissolving pseudoboehmite in acidic solution, a vigorous stirring generates the homogeneous suspension, the sample is then dried and calcined. The washcoated monoliths are subsequently impregnated with the active metal, dried and calcined to obtain the desired catalyst. Among the supports of most interest, for sure, the ceria is one of the most interesting, especially for what regards the excellent oxygen store capacity [23], accepting oxygen in oxidizing and transferring under reducing conditions, and the promoter capacity against of several metals. Concerning the active components, many works were recently reported on the use of noble metals [23] and Cobalt [24,25], for the steam reforming process, however the less expensive Nickel-based catalysts are still widely spread [26]. Unfortunately the Ni-based catalysts suffer of deactivation due to coke formation [27] and metal sintering [28], however, recently interesting results were reported on the use of ceria as promoter of the oxidative removal of carbonaceous species [29]. On the basis of these reported results, the Ni/CeO2-Al2O3/ SiC catalytic system seems to be highly promising for methane steam reforming applications. In this work we report the preparation of structured catalysts, obtained by washcoating SiC monolith with a ceria/pseudobohemite slurry (95% wtCeO2/5% wtpseudobohemite) and subsequent impregnation with nickel salts, in order to reach a 5% wt load of nickel with respect to washcoat; these catalysts, identified as 5NiWSiC/FT and 5NiWSiC/WF depending on the flow configuration, were fully characterized and tested, and the results compared to the performance of commercial catalysts (KatalcoJM Quadralobe) for methane steam reforming.
Fig. 1. Schematic representation of the temperature profile in a reactor for steam reforming.
way, which involves also the design of new catalytic configurations. Structured catalysts, based on highly conductive carriers, due to the capability of flatten the thermal profile over the catalyst, are able to reduce the difference of temperature between the side wall of the reactor and the center of the bed and, therefore, a lower temperature of the furnace is required, for a given heat flux [15]. Among the most promising materials in the field of structured carriers, the silicon carbide (SiC) or carborundum, plays a major role; SiC is mainly a synthetic material, it exists in nature as extremely rare mineral called moissanite, it is a polymorph material but the most common crystalline structures are the α and the β forms. Silicon carbide, due to its characteristics, is used for various applications, as semiconductor for electrical applications, as abrasive for its hardness, as filter in clean-up of waste gases [16,17], as fuel in the steel production, and many other special applications [18]. Recently SiC has found wide applications also in the field of the heterogeneous catalysis [19], as highly conductive support, in the methane steam reforming process intensification [20]. The novelty in the use of these structures is that they allow to realize an excellent heat transfer, in a manner not achievable with the conventional beds; the high thermal conductivity of SiC allows, in fact, to overcome the heat transfer limitations, with a consequent improvement of the performance of the catalysts in the endothermic processes, such as steam reforming [21]. The SiC honeycomb monoliths can be realized in two configurations, flow-through and wall-flow, the former have the channels open on both sides, the latter are characterized by alternatively closed channels and porous side walls, as showed in Fig. 2. The wall-flow configuration is of particular interest in the field of catalysis, because it forces the reagents to flow through the walls, making available also the part of the catalyst embedded in the walls themselves. The common SiC monoliths are characterized by low surface area (S.S.A) therefore, the most widespread strategy to increase it, provides the impregnation by dip-coating procedure of the monolith with a high surface area washcoat slurry [22]. The washcoat composition depends on the requirements of the final catalysts, however the
2. Materials and methods 2.1. Carriers preparation SiC “flow-through” carriers were obtained by cutting a quasi-circular cross section from a commercial honeycomb monolith provided by Pirelli Ecotechnologies. The monoliths were properly shaped (diameter = 16 mm; length = 31 mm; total volume = 6.4 cm3) to be located in the reactor, and were entrapped in a thermo-expandable ceramic mat (3 M) in order to avoid bypass phenomena. In Table 1 the geometric characteristics of the monoliths are summarized. The “wall-flow” carriers were obtained from the corresponding “flow-through”, alternately plugging the inlet and outlet sections of each channel with a high temperature resistant ceramic glue, so forcing the gas to pass through the porous walls of the inner channels. The prepared carriers were calcined at 850 °C for 3 h, so allowing the coating of the SiC particles with SiO2 streaks, which can greatly help the washcoat adherence to the filter [30]. 2.2. Catalysts preparation The washcoat slurry was prepared by dispersing, under vigorous stirring, 19 wt% of ceria powder (Opaline®; Actalys HAS; Rhodia) in a colloidal solution obtained by dissolving 1 wt% of psudobohemite (Pural SB; Sasol) and 1 wt% of methyl cellulose (Viscosity 4000 cP; Table 1 Geometric characteristics of the employed monoliths. SiC Wall thickness [mm] Side channel [mm] Length [mm] Weight [g] Diameter [mm] Channels [number]
Fig. 2. schematic representation of Flow Through (a) and Wall Flow configuration (b).
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0.55 1.65 31.00 5.7 16.00 37
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The activity tests were carried out in a stainless steel tubular reactor (internal diameter = 17 mm), at atmospheric pressure, by supplying an S/C ratio of 3, at hourly space velocities GHSV of 25,000 and 6250 h−1 (calculated as the ratio between the volumetric flow rate and the total volume of the catalyst, fixed to 6.4 cm3 for all samples). The catalytic tests were performed by ranging furnace temperature from 900 to 500 °C, with a descending ramp of 2 °C/min. Before the tests the catalysts were reduced in-situ with a stream of 5% of hydrogen diluted in nitrogen at a flow rate of 1000 mL/min, from 20 to 850 °C with a ramp of 5 °C/min. The thermal reaction control was realized by means of an annular electrical furnace (nominal power 4 kW) supplied with three different heating zones. The reactant mixture was controlled by a massflow (Bronkhorst) controllers system, all the gases were supplied by SOL S.p.A. The water was previously vaporized in a coil at the entering part of the oven, before to be mixed with the gaseous feed. The main system parameters (pressure and temperatures) were continuously monitored by a pressure transducer, and two K-type thermocouples at the center input and output of the catalysts. The gas exiting from the reactor were on line monitored on dry bases, removing the vapor via a Julabo F12 refrigerator, by a Hiden Analytical mass spectrometer system, evaluating the concentration of the masses corresponding to hydrogen, methane, CO and CO2. The catalytic performances were evaluated in terms of hydrogen yield, methane conversion and yield to CO2 (to evaluate the SR and WGS contributions) the latter two defined in (3) and (4).
Sigma-Aldrich) in acidic solution (pH = 3-4 by nitric acid. The monoliths were impregnated by dip-coating procedure, the excess of slurry was removed by centrifuge (1500 rpm for 5 min), while the impregnated monoliths were dried at 120 °C for two hours and calcined at 850 °C for three hours, with a heating ramp of 10 °C/min. This procedure was repeated up to the desired loading of washcoat over the monoliths (2 g). The washcoated monoliths were impregnated with a nickel nitrate aqueous solution, obtained by dissolving 22 g of nickel nitrate hexahydrate (99.999% trace metals basis, Sigma-Aldrich) in 150 g of water, the resulting catalysts were dried at 120 °C for two hours and calcined at 850 °C for three hours, with a heating ramp of 10 °C/min, obtaining a nickel loading of 5%wt with respect the washcoat weight. 2.3. Catalysts characterization The catalysts were fully characterized by a series of physico-chemical analytical techniques. The chemical composition was checked by means of ARL QUANT’X ED-XRF spectrometer (Thermo Scientific). The mechanical resistance of the washcoat was evaluated by ultrasound adherence test [31]; the samples were immersed in a beaker containing 100 mL of petroleum ether (Carlo Erba reagenti), placed in a ultrasonic bath CP104 (EIA S.p.A.), and stressed by applying the 60% of rated power for six cycle of 5 min at 25 °C. After each cycle the monolith was dried at 120 °C for 1 h, cooled and the weight change was evaluated. The porosimetric distribution was evaluated, by Hg penetration technique, with a “PASCAL 140” and “PASCAL 240” (Thermo Finnigan Instruments), in particular to investigate on the variation of the samples porosimetric characteristics due to washcoat deposition. The prepared samples were also characterized by Scanning Electron Microscopy (SEM), using a Scanning Electron Microscope (SEM mod. LEO 420 V2.04, ASSING), and Energy Dispersive X-ray Spectroscopy (EDX), performed in an Energy Dispersive X-Ray analyzer (EDX mod. INCA Energy 350, Oxford Instruments).
XCH4 =
YCO2 =
IN OUT FCH − FCH 4 4 IN FCH 4
(3)
OUT FCO 2 IN FCH 4
(4)
where FIN and FOUT represent the molar rate on the indicated species at the inlet and outlet of the system. 3. Characterization and experimental results
2.4. Catalytic activity tests 3.1. Samples characterization The catalytic tests were carried out to investigate the effect of the structured catalyst geometric configuration and to compare their performances with a commercial KatalcoJM Quadralobe (57-4Q Series [35]) catalyst, provided in pellet, and composed of nickel on alumina. Other features of Katalco catalyst are not available, due to industrial patents. In the Fig. 3 were reported the pictures of the 3 catalytic samples employed in the tests.
3.1.1. Ultrasound adherence test The ultrasound adherence tests were performed in order to evaluate the resistance of the coating to a strong mechanical stress; the method consists in evaluating the weight loss caused by an ultrasonic cleaning. The weight loss percentage, reported in Fig. 4 for a single sample of Washcoat/SiC flow through (WSiC/FT) system, was calculated versus
Fig. 3. Monolithic samples in (a) Flow-Through configuration; (b)Wall-Flow configuration; (c) KataloJM pellets samples.
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monolith surface is covered by the washcoat and active species, the coating is also evident by comparing Fig. 5a and d (the last one is relevant to the bare support). Furthermore in Fig. 5c we showed a granule of monolith entirely covered by the catalyst, so evidencing that the catalytic layer covers all the SiC granules surface and deposits on the inner walls of the pores, decreasing their diameter but not plugging them. Fig. 5a and b, in particular, evidenced the good adherence and the tight contact between the catalyst layer and the SiC granules. The SEM-EDAX analysis results are reported in Fig. 6. Fig. 6 evidenced that the detected elements on the sample are related to the chemical structure of the SiC filter (C and O) and to the elements constituting the washcoat (Al, O and Ce) and the active species (Ni). These results highlighted that the preparation procedure is able to realize the deposition of active phases on the support surface, forming a uniform layer covering the entire surface of SiC monoliths. In particular, the very low signals of the structural elements (above all the signal related to C) confirmed the good coating of the SiC granules with the active species.
Fig. 4. Weight loss percentage of the prepared washcoated sample.
the lonely deposited washcoat, without considering the weight of the carriers (5).
weight loss =
Final mass − Initial mass * 100 Initial washcoat mass
3.1.4. Activity tests Catalytic activity was investigated for the prepared samples, in particular to investigate the effect of the geometric configuration on the catalyst performance. Tests were carried out by assuring a reaction stream composed by 25 vol% of methane and 75 vol% of steam, and a S/ C ratio equal to 3, and were performed at various values of GHSV (Gas Hourly Space Velocity), calculated as the ratio between the volumetric rate and catalyst volume. In particular, the tests were performed at the values of 25,000 h−1 and 6250 h−1, and with the monoliths in the FT and WF geometric configurations. It was remarkable that in all tests, for all the investigated operating conditions, pressure drops were below 0.01 bar, therefore pressure effects on catalytic tests were negligible. The results are summarized in Figs. 7–9, in terms of methane conversion, CO2 yield and hydrogen yield. The comparison between the prepared monoliths reported in Figs. 7–9, evidenced the better catalytic performances of the WF monoliths, in particular at the highest value of GHSV, even if the system is far from equilibrium conditions: in these very stressfull conditions, the system geometry, characterized by the forced pass of the gases through the inner porous walls of the catalyst, allowed a deeper interaction between the reactants and the active species present in the porosities, so resulting in a full use of all the active species deposited on the entire surface of the carrier and, consequently, in a better axial and radial thermal profile distribution in the catalytic bed, as also demonstrated in an earlier paper [20]. Therefore, better exploiting of active phase and the minimization of mass transfer limitations resulted in appreciably higher performances of wall-flow configuration with respect to flow-through. On the contrary, at the lowest GHSV value, the better catalytic activity of the WF catalyst is evident only for temperatures lower than 700 °C, while for higher temperatures the performances of the two catalysts are very similar and approach to equilibrium conditions. The good results in terms of CO2 yield suggest that also water-gas shift reaction was promoted by the system, even if a strange behavior in carbon dioxide yield was observed at the lowest GHSV value at temperature above 650 °C, for which the monoliths in FT configuration showed better perfomances with respect to the WF ones. Such behavior could be addressed to the lower activity of the same monolith in steam reforming reaction: in this case less H2 is produced and the CO produced is converted to H2 through water gas shift reaction. It is finally worth to underline that the catalytic monoliths well approached thermodynamic equilibrium for temperature above 730 °C: the approach is more evident for hydrogen and CO2 yield with respect to methane conversion, so underlining that water-gas shift reaction was better promoted by the system. The excellent performances of such catalysts towards WGS may be ascribable to the high affinity of ceria supported catalysts towards CO-shift reaction [33], it is however relevant to remark that high activity of Ni/CeO2 based catalysts in such
(5)
As evident from Fig. 4, the greater loss was recorded after the first test cycle, with a weight loss equal to about the half of the total loss; in addition, after the first 20 min of test the weight loss stabilized at its maximum value, equal to about 8.72%; the two subsequent test cycles do not record further losses. Previous tests on the same type of monolith and with the same washcoat formulation but slightly less acid showed higher losses, reaching almost 20%. It is demonstrated, therefore, that a lower pH favors a greater adhesion of the washcoat, unlike as reported in some literature works for a different type of coating [22]. 3.1.2. Chemical composition and textural properties The chemical composition and the porosimetric characteristics of the prepared catalytic samples are summarized in Table 2, compared also with washcoat powder and bare SiC monoliths. The ED-XRF spectroscopic results substantially confirm the expected composition, highlighting the reproducibility of the coating techniques and confirming the homogeneity of the washcoat slurry. The porosimetric results show that the addition of the washcoat to the carriers, leads to a decrease of the average diameter and volume of the pores in the SiC monoliths. This expected result is related to porosimetric characteristics of starting SiC carrier, characterized by porous inner walls, with an average pore diameter of about 17 μm, and therefore, the deposition of washcoat and its adhesion to the inner walls of the pores reduces their section and diameter, as reported in previous works [32]. 3.1.3. SEM and SEM-EDAX results The SEM images related to the final prepared catalytic sample are reported in Fig. 5. By looking more deeply Fig. 5a, the reader can observe that all the Table 2 Chemical composition (wt%) and textural properties. Sample
Washcoat powder 5NiWSiC/FT 5NiWSiC/WF No calcined SiC bare monolith Calcined SiC bare monolith
XRF SiC
Ni
Al2O3
CeO2
– 73.60 73.50 100
– 1.32 1.30 –
3.91 0.88 0.89 –
96.09 24.20 24.31 –
100
V Pore [mm3/g]
Pore Dm [μm]
– 146.98 146.98 173.99
– 4.60 4.60 17.00
195.84
17.00
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Fig. 5. SEM images of the prepared catalytic sample at various magnitudes, (a) 584 X, (b) 5.00 KX, (c) 35.00 KX, and of the bare support (d).
Fig. 6. SEM images and distribution of elements, as obtained by EDAX element mapping, for the prepared catalytic sample.
commercial sample contains a substantially higher amount of nickel: in fact, in one hand a commercial nickel based catalyst contained an amount of active metal largely higher than 5 wt%, moreover, the mass of commercial catalyst (4.3 g) was more than twice with respect to the active washcoat deposed on the monoliths (about 2 g). Since in the lower operating temperature range reactions kinetics were relatively low, the high content of active phase on commercial sample promoted higher conversion of methane; by increasing operating temperature, reactions kinetics were thermally enhanced, improving the performances of monolithic samples. In particular, for high operating temperature, the expected high reaction kinetics required comparable mass and heat transfer rates between solid and gaseous phase. It is reasonable to suppose that for Katalco pellets heat (and mass) transfer mechanism resulted a limiting step in the highest temperature range, affecting the
reaction should require an optimal Ni morphology (particularly primary particle size and shape), thus enhancing redox properties of catalytic system (metal and support) [34]. The prepared catalytic samples were also compared in terms of methane conversion with a commercial Ni based catalyst, provided by KatalcoJM; with the aim to perform comparable tests, a volume of commercial catalyst in pellet equivalent to structured catalyst volume was used (about 6.4 cm3 corresponding to about 4.3 g). The results are reported in Figs. 10 and 11. Figs. 10 and 11 showed a very good catalytic activity of commercial catalyst in terms of methane conversion, evidencing conversion values comparable to the structured catalysts ones, in particular showing higher values at the highest GHSV value. Such behavior may be ascribable to Katalco catalyst features, and suggests that the loaded 211
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Fig. 7. Methane conversion − comparison of SiC FT and WF catalytic samples at (a) 25,000 h−1 and (b) 6250 h−1, for a S/C ratio equal to 3.
Fig. 9. Hydrogen yield − comparison of SiC FT and WF catalytic samples at (a) 25,000 h−1 and (b) 6250 h−1, for a S/C ratio equal to 3.
Fig. 10. Methane conversion − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 6250 h−1, ratio S/C=3).
overall catalyst performances. The heat transfer limitations of the pellet catalyst were confirmed by the lower temperature achievable for the commercial sample by fixing furnace temperature at 900 °C: at lower GHSV values, the gas temperature at the exit section was around 770 °C, at higher GHSV was around 660 °C. Such achievement, also considering that at 6250 h−1 the methane conversion was similar for the three samples, suggested that pellets catalyst suffered for heat transfer limitation from the heating medium to the reaction volume. The discussed phenomenon was also remarked by analyzing the comparison between the temperatures of gas exiting from the catalytic volume with respect to the furnace temperature (Fig. 12). As reported, in the pellets catalysts a higher difference between furnace temperature (and then reactor wall temperature) and products gas was obtained,
Fig. 8. CO2 yield − comparison of SiC FT and WF catalytic samples at (a) 25,000 h−1 and (b) 6250 h−1, for a S/C ratio equal to 3.
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Fig. 11. Methane conversion − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 25000 h−1, ratio S/C = 3).
Fig. 14. Methane conversion vs furnace temperature − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 25000 h−1, ratio S/C = 3).
to furnace temperature, at the two GHSV values used in the experimental tests. Experimental results evidenced that for higher furnace temperature, the SiC monoliths (both flow-through and wall-flow) had a better behavior with respect to the commercial catalyst, in particular at the lower GHSV value: in such conditions, all monolithic systems evidenced a very high methane conversion, therefore a high heat duty was required by catalytic volumes. The highly conductive SiC structures were able to minimize the heat transfer from external medium (furnace) and the catalyst, thus resulting in an overall higher temperature. In particular, the wall flow configuration allows to obtain the better performances for temperature close to 750 °C, while for higher temperatures the performances of wall flow and flow through configuration are similar. It is important to underline that for a furnace temperature of 850 °C, the SiC based catalysts showed a methane conversion of about 90%, about 12% higher than the methane conversion of Katalco commercial catalysts. In a different view, to achieve a methane conversion of 60%, a furnace temperature of about 730 °C is required for Katalco pellet catalyst, 660 °C for SiC monolith in flow-through configuration and 630 °C for SiC monolith in wall flow configuration. This important result is due to the higher thermal conductivity of SiC, that decreases the heat transfer resistances from the furnace to the reactor and catalyst, so allowing to reach the process temperature by using a lower external temperature: in an industrial case this condition results in an evident decrease in plant costs. A preliminary evaluation of the benefit linked to a highly conductive structured carrier was performed by investigating the overall heat transfer rate between heating medium (furnace) and reaction volume. In the estimation, 4 heat transfer resistances “in series” were considered: furnace-reactor wall (natural convection); reactor wall (conductivity); catalyst (conductivity); catalyst-process stream (forced convection) [36]. Since the thermocouples driving the furnace were in contact with the reactor external surface, it was considered that the temperature of the reactor wall corresponded to the furnace temperature; moreover, it was considered the absence of temperature radial gradient in the gas phase. Therefore, heat transfer from furnace to gas phase undergoes to Eq. 6, where Q(z) is the heat transfer along the reaction system, Din is the inner diameter of the tubular reactor, Tfurnace and Tgas were respectively the external temperature of the reactor and the gas temperature, and U is the overall heat exchange coefficient.
Fig. 12. Dependence of products gas temperature to the furnace temperature (S/C = 3, GHSV = 6250 h-1).
suggesting that along the reactor, higher radial thermal gradient occurred. As well, since by fixing furnace conditions the same reactants temperature should be obtained at the catalyst entrance section, the higher temperature of monolithic samples leads to suppose a flatter thermal profiles in the axial direction with respect to the pellets catalyst. In order to better highlight the potential advantages in the use of highly conductive structured carrier, in Figs. 13 and 14 methane conversion for each catalytic system investigated was reported with respect
dQ (z ) = U ·Din ·dz·[Tfurnace − Tgas (z )]
(6)
The enthalpic contribution of the SR and WGS reactions (Eqs. (1) and (2) was also considered, for which kinetic expressions, proposed Habermann and Young [37], were considered. The model parameters were achieved by dedicated tests on a 5%Ni/Washcoat powder sample, obtained results were summarized in Table 3.
Fig. 13. Methane conversion vs furnace temperature − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 6250 h−1, ratio S/C = 3).
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with nickel nitrate solution. The as prepared catalysts were tested for the methane steam reforming reaction, the traditional flow-through configuration was compared to the alternative wall-flow geometry and to the commercial catalyst KatalcoJM Quadralobe. The activity tests showed the benefits in using the structured catalysts, highlighting the better distribution of the reaction heat through the high thermal conductive SiC carrier, both in axial and in radial directions. The results showed in particular a higher catalytic activity with the wall-flow configuration in the temperature range 500–750 °C, which results in a higher methane conversion and hydrogen yield. This effect was attributed to the better interaction between the reacting gas and all the catalyst deposited on the carrier surface and in its porosities: by forcing the pass through the walls is possible to exploit the entire available catalytic surface and improve the distribution of the heat. The performance of the structured catalysts were also compared with a conventional commercial catalyst; the activity tests showed, in particular, that, even if the commercial formulation was highly active, fixing the furnace temperature the structured catalysts provided much higher values of methane conversion, so allowing to reach the process temperature by using a lower external temperature and so resulting in a feasible process intensification.
Table 3 Kinetic parameters for the catalytic formulation.
k0 Ea
Steam Reforming
Water Gas Shift
5.53*10−2 mol*m−3*s−1*Pa−2 106.2 kJ/mol
7.82*10−7 mol*m−3*s−1 8.42 kJ/mol
Table 4 Estimated overall heat exchange coefficient in the proposed reaction system [Tfurnace = 750 °C). U[W *m−2*K−1] KatalcoJM FT WF
21.4 85.6 121.8
References
Fig. 15. Theoretical methane conversion profiles for (Tfurnace = 750 °C; S/C = 3; GHSV = 6250 h−1, ratio S/C = 3).
considered
[1] G. Krajačić, R. Martins, A. Busuttil, N. Duić, M. da Graça Carvalho, Hydrogen as an energy vector in the islands’ energy supply, Int. J. Hydr. Energ. 33 (2008) 1091–1103. [2] J.R. Bartels, M.B. Pate, N.K. Olson, An economic survey of hydrogen production from conventional and alternative energy sources, Int. J. Hydr. Energ. 35 (2010) 8371–8384. [3] B. Bej, N.C. Pradhan, S. Neogi, Production of hydrogen by steam reforming of methane over alumina supported nano-NiO/SiO2 catalyst, Catal. Today 207 (2013) 28–35. [4] M.A. Rakib, J.R. Grace, C. Jim Lim, S.S.E.H. Elnashaie, Steam reforming of heptane in a fluidized bed membrane reactor, J. Power Sources 195 (2010) 5749–5760. [5] M.S. Nobandegani, M.R. Sardashti Birjandi, T. Darbandi, M.M. Khalilipour, F. Shahraki, D. Mohebbi-Kalhori, An industrial Steam Methane Reformer optimization using response surface methodology, J. Nat. Gas Sci. Eng. 36 (2016) 540–549. [6] H. Harold Gunardson, Industrial Gases in Petrochemical Processing: Chemical Industries, CRC Press, 1997 19. [7] M. Nijemeisland, A.G. Dixon, E. Hugh Stitt, Catalyst design by CFD for heat transfer and reaction in steam reforming, Chem. Eng. Sci. 59 (2004) 5185–5191. [8] J. Shayegan, M.M.Y.M. Hashemi, K. Vakhshouri, Operation of an industrial steam reformer under severe condition: a simulation study, Can. J. Chem. Eng. 86 (2008) 747–755. [9] G.B. Hawkins, Steam reforming: practical operation, https://www.slideshare.net/ GerardBHawkins/steam-reforming-practical-operations (2013). [10] C.V.S. Murty, M.V.K. Murthy, Modeling and simulation of a top-fired reformer, Ind. Eng. Chem. Res. 27 (1988) 1832–1840. [11] C. Jimenez-González, Z. Boukha, B. de Rivas, J.R. González-Velasco, J.I. Gutierreź Ortiz, R. López-Fonseca, Behavior of coprecipitated NiAl2O4/Al2O3 catalysts for low temperature methane steam reforming, Energ. Fuel 28 (2014) 7109–7121. [12] Qi Wang, Berta Spasova, Volker Hessel, Gunther Kolb, Methane reforming in a small-scaled plasma reactor −industrial application of a plasma process from the viewpoint of the environmental profile, Chem. Eng. J. 262 (2015) 766–774. [13] M. Mbodji, J.M. Commenge, L. Falk, D. Di Marco, F. Rossignol, L. Prost, S. Valentin, R. Joly, P. Del-Gallo, Steam methane reforming reaction process intensification by using a millistructured reactor: experimental setup and model validation for global kinetic reaction rate estimation, Chem. Eng. J. 207–208 (2012) 871–884. [14] D. Reay, C. Ramshaw, A. Harvey, Process Intensification Engineering for Efficiency: Sustainability and Flexibility, Elsevier, 2008. [15] H.-S. Roh, D.K. Lee, K.Y. Koo, U.H. Jung, W.L. Yoon, Natural gas steam reforming for hydrogen production over metal monolith catalyst with efficient heat-transfer, Int. J. Hydrogen Energ. 35 (2010) 1613–1619. [16] V. Palma, P. Ciambelli, E. Meloni, A. Sin, Catalytic DPF microwave assisted active regeneration, Fuel 140 (2015) 50–61. [17] V. Palma, P. Ciambelli, E. Meloni, A. Sin, Study of the catalyst load for a microwave susceptible catalytic DPF, Catal. Today 216 (2013) 185–193. [18] T. Kimoto, J.A. Cooper, Fundamentals of Silicon Carbide Technology: Growth, Characterization, Devices, and Applications, John Wiley & Sons Singapore Pte. Ltd., 2014. [19] R. Moene, M. Makkee, J.A. Moulijn, High surface area silicon carbide as catalyst support characterization and stability, Appl. Cat. A-Gen. 167 (1998) 321–330. [20] V. Palma, A. Ricca, E. Meloni, M. Martino, M. Miccio, P. Ciambelli, Experimental and numerical investigations on structured catalysts for methane steam reforming intensification, J. Clean. Prod. 111 (2016) 217–230. [21] V. Palma, A. Ricca, M. Martino, E. Meloni, Innovative catalytic systems for methane
samples
The comparison between proposed model and experimental achievements allowed to estimate the overall heat exchange coefficient U for the different catalytic samples, the results were summarized in Table 4. As reported, the optimization of catalyst carrier is expected to produce a consistent decreasing in heat transfer resistance ( = 1/U), thus maximizing the heat transfer rate from the heating medium to catalytic volume. The wall-flow configuration also contributes to increase heat transfer rate to the process gas, since overall heat exchange coefficient increased of about 43% with respect to traditional flow through monolith. To better understand the benefits of catalyst carrier optimization, in Fig. 15 theoretical methane conversion profiles along a reaction system were proposed for the 3 investigated samples, supposing to fix furnace temperature to 750 °C. The reaction system length was normalized with respect to the length required for achieving a methane conversion of 95% with commercial pellets catalysts. As reported, by employing SiC based structured catalyst, the same methane conversion value could be achieved in a very minor length, confirming that highly conductive structured carrier enabled a conspicuous plant size reduction (around 61% for the FT configuration, around 68% for the WF configuration), thus resulting in a clear process intensification. Moreover, the weak difference of conversion profiles estimated for FT and WF geometries indicated that, for the considered experimental conditions, by employing highly conductive silicon carbide monoliths the heat transfer limitation were markedly reduced, while the chemical reaction kinetics became relevant in the system performances. 4. Conclusions A set of structured catalysts were prepared, by washcoating SiC monoliths with Ceria/Alumina slurry and subsequent impregnation 214
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catalysts for reforming reactions, Chem. Soc. Rev. 43 (2014) 7245–7256. [30] V. Palma, E. Meloni, Microwave assisted regeneration of a catalytic diesel soot trap, Fuel 181 (2016) 412–429. [31] M. Valentini, G. Groppi, C. Cristiani, M. Levi, E. Tronconi, P. Forzatti, The deposition of γ-Al2O3 layers on ceramic and metallic supports for the preparation of structured catalysts, Catal. Today 69 (2001) 307–314. [32] V. Palma, P. Ciambelli, E. Meloni, A. Sin, Catalytic DPF microwave assisted active regeneration, Fuel 140 (2015) 50–61. [33] D.-W. Jeong, H.S. Potdar, J.-O. Shim, W.-J. Jang, H.-S. Roh, H2 production from a single stage water–gas shift reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1-x)Zr(x)O2 catalysts, Int. J. Hydr. Energ. 38 (2013) 4502–4507. [34] A. Kubacka, M. Fernández-García, A. Martínez-Arias, Catalytic hydrogen production through WGS or steam reforming of alcohols over Cu, Ni and Co catalysts, Appl. Cat. A-Gen. 518 (2016) 2–17. [35] http://www.jmprotech.com/images-ploaded/files/JM%20Hydrogen%20Brochure. pdf, pag 10. [36] E. Tronconi, G. Groppi, T. Boger, A. Heibel, Monolithic catalysts with ‘high conductivity’ honeycomb supports for gas/solid exothermic reactions: characterization of the heat-transfer properties, Chem. Eng. Sci. 59 (2004) 4941–4949. [37] B.A. Haberman, J.B. Young, Three-dimensional simulation of chemically reacting gas flows in the porous support structure of an integrated-planar solid oxide fuel cell, Int. J. Heat Mass Transfer 47 (2004) 3617–3629.
steam reforming intensification, Chem. Eng. Trans. 52 (2016) 301–306. [22] M. Adamowska, P. Da Costa, Structured Pd/γ-Al2O3 prepared by washcoated deposition on a ceramic honeycomb for compressed natural gas applications, J. Nanopart. 2015 (2015) 601941(9 pages). [23] E. Mamontov, T. Egami, R. Brezny, M. Koranne, S. Tyagi, Lattice defects and oxygen storage capacity of nanocrystalline ceria and ceria-zirconia, J. Phys. Chem. B 104 (2000) 11110–11116. [24] Um-E-Salma Amjad, A. Vita, C. Galletti, L. Pino, S. Specchia, Comparative study on steam and oxidative steam reforming of methane with noble metal catalysts, Ind. Eng. Chem. Res. 52 (2013) 15428–15436. [25] A. Fonseca Lucredio, E. Moreira Assaf, Cobalt catalysts prepared from hydrotalcite precursors and tested in methane steam reforming, J. Power Sources 159 (2006) 667–672. [26] Y. Chen, P. Cui, G. Xiong, H. Xu, Novel nickel-based catalyst for low temperature hydrogen production from methane steam reforming in membrane reformer, AsiaPac. J. Chem. Eng. 5 (2010) 93–100. [27] C.-J. Liu, J. Ye, J. Jiang, Y. Pan, Progress in the preparation of coke resistant Nibased catalyst for steam and CO2 reforming of methane, ChemCatChem 3 (2011) 529–541. [28] A.J. Majewski, J. Wood, W. Bujalski, Nickel-silica core@shell catalyst for methane reforming, Int. J. Hydr. Energ. 38 (2013) 14531–14541. [29] S. Li, J. Gong, Strategies for improving the performance and stability of Ni-based
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Contents lists available at ScienceDirect
Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Innovative structured catalytic systems for methane steam reforming intensification Vincenzo Palma, Antonio Ricca, Marco Martino, Eugenio Meloni
MARK
⁎
University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Structured catalysts Process intensification High conductive carriers Methane reforming Hydrogen production
The use of structured catalysts based on highly conductive carriers may allow a flattened radial temperature gradient along the catalytic bed due to a better heat transfer, so obtaining a consequently higher performance. The effect of thermal conductivity of structured carriers on highly endothermic Methane Steam Reforming (MSR) reaction is investigated. The performance of the structured catalysts, prepared starting by Silicon Carbide (SiC) monoliths, in the “wall flow” (WF) and “flow through” (FT) geometric configurations, demonstrates the direct correlation between the thermal conductivity of the carrier, the methane conversion and the hydrogen productivity. In particular the tests showed that the SiC “wall flow” (WF) monoliths guaranteed a better axial and radial thermal distribution, with respect to the SiC “flow through” (FT) ones, resulting in better catalytic activity up to a temperature reaction of 750 °C. Furthermore, the comparison among the performance of the structured catalysts and the commercial 57-4MQ, provided by Katalco-JM, highlights the choice of structured catalysts, which require a lower temperature outside of the reactor, so increasing the overall process efficiency.
1. Introduction Hydrogen is nowadays considered the most promising energy vector, thanks to its high capacity of storing energy from primary sources [1]. Despite considerable efforts to identify new green technologies and sources for hydrogen production, the use of fossil sources and the traditional production methods, are still widely affordable [2]. Among the production processes of hydrogen, the Steam Reforming of natural gas is still the less expensive and the most widespread industrial process, suitable for economies of scale and widely involved in the ammonia and methanol production processes. The Steam Reforming (SR) is a highly endothermic equilibrium reaction [3], the standard enthalpy reaction for methane is about 206 kJ/mol however, for more complex hydrocarbons, it can reach much high values, for example for n-C7H16, the ΔH°298 is about 1108 kJ/mol [4]. The Methane Steam Reforming (MSR) is normally described as the result of two main reactions, the reforming (Eq. (1)) and the water gas shift (WGS) reactions (Eq. (2)), CH4 + H2O ⇆ CO + 3H2 ΔH°298 = 206 kJ/mol
(1)
CO + H2O ⇆ CO2 + H2 ΔH°298 = − 41 kJ/mol
(2)
The thermodynamic of the system, coupled with the actually used catalysts, allow to reach acceptable conversions only for reaction
⁎
Correspondence author. E-mail address: [email protected] (E. Meloni).
http://dx.doi.org/10.1016/j.cep.2017.07.012 Received 9 March 2017; Received in revised form 29 June 2017; Accepted 12 July 2017 Available online 26 July 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.
temperature exceeding the 700–750 °C [5]; to realize this hard conditions, over the catalytic bed, it required a large amount of heat, so the process temperatures must be forced above 1000 °C. The most widespread process configuration provides two stage, in the first stage methane and steam react at 700–800 °C, thus obtaining a conversion of the hydrocarbons up to 90%, in the second stage an amount of air is added to react with a part of the hydrogen, generating the necessary heat to reach a temperature of the reaction mixture of 1000–1200 °C, allowing an almost complete conversion [6]. So high process temperatures however present several problems, especially related to the resistance and durability of the materials, but also to the overall process efficiency in terms of produced hydrogen per supplied energy. On the basis of the above consideration it seems clear that the main problem of this kind of process, is related to the transfer of heat to the catalytic bed. The typical radial thermal profile of a catalytic bed, packed with pellets provides a considerable thermal gradient between the side wall and the center of the bed, part of the heat supplied, necessarily dissipates, with a decreasing from 1200 °C, at the outside, to 700 °C at the center of the catalytic bed (Fig. 1), making it necessary a huge expenditure of energy [7–10]. Much efforts were made in the direction of the preparation of new and more active catalytic formulations at low temperatures [11], or in the use of new technologies, such as non-thermal plasma [12]. However a process intensification [14] [13] seems to be a much more promising
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most common formulations are based on high surface area aluminas, a coating polymer and the desired support. The preparation procedure provides the dispersion of the support in a colloidal solution, obtained by dissolving pseudoboehmite in acidic solution, a vigorous stirring generates the homogeneous suspension, the sample is then dried and calcined. The washcoated monoliths are subsequently impregnated with the active metal, dried and calcined to obtain the desired catalyst. Among the supports of most interest, for sure, the ceria is one of the most interesting, especially for what regards the excellent oxygen store capacity [23], accepting oxygen in oxidizing and transferring under reducing conditions, and the promoter capacity against of several metals. Concerning the active components, many works were recently reported on the use of noble metals [23] and Cobalt [24,25], for the steam reforming process, however the less expensive Nickel-based catalysts are still widely spread [26]. Unfortunately the Ni-based catalysts suffer of deactivation due to coke formation [27] and metal sintering [28], however, recently interesting results were reported on the use of ceria as promoter of the oxidative removal of carbonaceous species [29]. On the basis of these reported results, the Ni/CeO2-Al2O3/ SiC catalytic system seems to be highly promising for methane steam reforming applications. In this work we report the preparation of structured catalysts, obtained by washcoating SiC monolith with a ceria/pseudobohemite slurry (95% wtCeO2/5% wtpseudobohemite) and subsequent impregnation with nickel salts, in order to reach a 5% wt load of nickel with respect to washcoat; these catalysts, identified as 5NiWSiC/FT and 5NiWSiC/WF depending on the flow configuration, were fully characterized and tested, and the results compared to the performance of commercial catalysts (KatalcoJM Quadralobe) for methane steam reforming.
Fig. 1. Schematic representation of the temperature profile in a reactor for steam reforming.
way, which involves also the design of new catalytic configurations. Structured catalysts, based on highly conductive carriers, due to the capability of flatten the thermal profile over the catalyst, are able to reduce the difference of temperature between the side wall of the reactor and the center of the bed and, therefore, a lower temperature of the furnace is required, for a given heat flux [15]. Among the most promising materials in the field of structured carriers, the silicon carbide (SiC) or carborundum, plays a major role; SiC is mainly a synthetic material, it exists in nature as extremely rare mineral called moissanite, it is a polymorph material but the most common crystalline structures are the α and the β forms. Silicon carbide, due to its characteristics, is used for various applications, as semiconductor for electrical applications, as abrasive for its hardness, as filter in clean-up of waste gases [16,17], as fuel in the steel production, and many other special applications [18]. Recently SiC has found wide applications also in the field of the heterogeneous catalysis [19], as highly conductive support, in the methane steam reforming process intensification [20]. The novelty in the use of these structures is that they allow to realize an excellent heat transfer, in a manner not achievable with the conventional beds; the high thermal conductivity of SiC allows, in fact, to overcome the heat transfer limitations, with a consequent improvement of the performance of the catalysts in the endothermic processes, such as steam reforming [21]. The SiC honeycomb monoliths can be realized in two configurations, flow-through and wall-flow, the former have the channels open on both sides, the latter are characterized by alternatively closed channels and porous side walls, as showed in Fig. 2. The wall-flow configuration is of particular interest in the field of catalysis, because it forces the reagents to flow through the walls, making available also the part of the catalyst embedded in the walls themselves. The common SiC monoliths are characterized by low surface area (S.S.A) therefore, the most widespread strategy to increase it, provides the impregnation by dip-coating procedure of the monolith with a high surface area washcoat slurry [22]. The washcoat composition depends on the requirements of the final catalysts, however the
2. Materials and methods 2.1. Carriers preparation SiC “flow-through” carriers were obtained by cutting a quasi-circular cross section from a commercial honeycomb monolith provided by Pirelli Ecotechnologies. The monoliths were properly shaped (diameter = 16 mm; length = 31 mm; total volume = 6.4 cm3) to be located in the reactor, and were entrapped in a thermo-expandable ceramic mat (3 M) in order to avoid bypass phenomena. In Table 1 the geometric characteristics of the monoliths are summarized. The “wall-flow” carriers were obtained from the corresponding “flow-through”, alternately plugging the inlet and outlet sections of each channel with a high temperature resistant ceramic glue, so forcing the gas to pass through the porous walls of the inner channels. The prepared carriers were calcined at 850 °C for 3 h, so allowing the coating of the SiC particles with SiO2 streaks, which can greatly help the washcoat adherence to the filter [30]. 2.2. Catalysts preparation The washcoat slurry was prepared by dispersing, under vigorous stirring, 19 wt% of ceria powder (Opaline®; Actalys HAS; Rhodia) in a colloidal solution obtained by dissolving 1 wt% of psudobohemite (Pural SB; Sasol) and 1 wt% of methyl cellulose (Viscosity 4000 cP; Table 1 Geometric characteristics of the employed monoliths. SiC Wall thickness [mm] Side channel [mm] Length [mm] Weight [g] Diameter [mm] Channels [number]
Fig. 2. schematic representation of Flow Through (a) and Wall Flow configuration (b).
208
0.55 1.65 31.00 5.7 16.00 37
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The activity tests were carried out in a stainless steel tubular reactor (internal diameter = 17 mm), at atmospheric pressure, by supplying an S/C ratio of 3, at hourly space velocities GHSV of 25,000 and 6250 h−1 (calculated as the ratio between the volumetric flow rate and the total volume of the catalyst, fixed to 6.4 cm3 for all samples). The catalytic tests were performed by ranging furnace temperature from 900 to 500 °C, with a descending ramp of 2 °C/min. Before the tests the catalysts were reduced in-situ with a stream of 5% of hydrogen diluted in nitrogen at a flow rate of 1000 mL/min, from 20 to 850 °C with a ramp of 5 °C/min. The thermal reaction control was realized by means of an annular electrical furnace (nominal power 4 kW) supplied with three different heating zones. The reactant mixture was controlled by a massflow (Bronkhorst) controllers system, all the gases were supplied by SOL S.p.A. The water was previously vaporized in a coil at the entering part of the oven, before to be mixed with the gaseous feed. The main system parameters (pressure and temperatures) were continuously monitored by a pressure transducer, and two K-type thermocouples at the center input and output of the catalysts. The gas exiting from the reactor were on line monitored on dry bases, removing the vapor via a Julabo F12 refrigerator, by a Hiden Analytical mass spectrometer system, evaluating the concentration of the masses corresponding to hydrogen, methane, CO and CO2. The catalytic performances were evaluated in terms of hydrogen yield, methane conversion and yield to CO2 (to evaluate the SR and WGS contributions) the latter two defined in (3) and (4).
Sigma-Aldrich) in acidic solution (pH = 3-4 by nitric acid. The monoliths were impregnated by dip-coating procedure, the excess of slurry was removed by centrifuge (1500 rpm for 5 min), while the impregnated monoliths were dried at 120 °C for two hours and calcined at 850 °C for three hours, with a heating ramp of 10 °C/min. This procedure was repeated up to the desired loading of washcoat over the monoliths (2 g). The washcoated monoliths were impregnated with a nickel nitrate aqueous solution, obtained by dissolving 22 g of nickel nitrate hexahydrate (99.999% trace metals basis, Sigma-Aldrich) in 150 g of water, the resulting catalysts were dried at 120 °C for two hours and calcined at 850 °C for three hours, with a heating ramp of 10 °C/min, obtaining a nickel loading of 5%wt with respect the washcoat weight. 2.3. Catalysts characterization The catalysts were fully characterized by a series of physico-chemical analytical techniques. The chemical composition was checked by means of ARL QUANT’X ED-XRF spectrometer (Thermo Scientific). The mechanical resistance of the washcoat was evaluated by ultrasound adherence test [31]; the samples were immersed in a beaker containing 100 mL of petroleum ether (Carlo Erba reagenti), placed in a ultrasonic bath CP104 (EIA S.p.A.), and stressed by applying the 60% of rated power for six cycle of 5 min at 25 °C. After each cycle the monolith was dried at 120 °C for 1 h, cooled and the weight change was evaluated. The porosimetric distribution was evaluated, by Hg penetration technique, with a “PASCAL 140” and “PASCAL 240” (Thermo Finnigan Instruments), in particular to investigate on the variation of the samples porosimetric characteristics due to washcoat deposition. The prepared samples were also characterized by Scanning Electron Microscopy (SEM), using a Scanning Electron Microscope (SEM mod. LEO 420 V2.04, ASSING), and Energy Dispersive X-ray Spectroscopy (EDX), performed in an Energy Dispersive X-Ray analyzer (EDX mod. INCA Energy 350, Oxford Instruments).
XCH4 =
YCO2 =
IN OUT FCH − FCH 4 4 IN FCH 4
(3)
OUT FCO 2 IN FCH 4
(4)
where FIN and FOUT represent the molar rate on the indicated species at the inlet and outlet of the system. 3. Characterization and experimental results
2.4. Catalytic activity tests 3.1. Samples characterization The catalytic tests were carried out to investigate the effect of the structured catalyst geometric configuration and to compare their performances with a commercial KatalcoJM Quadralobe (57-4Q Series [35]) catalyst, provided in pellet, and composed of nickel on alumina. Other features of Katalco catalyst are not available, due to industrial patents. In the Fig. 3 were reported the pictures of the 3 catalytic samples employed in the tests.
3.1.1. Ultrasound adherence test The ultrasound adherence tests were performed in order to evaluate the resistance of the coating to a strong mechanical stress; the method consists in evaluating the weight loss caused by an ultrasonic cleaning. The weight loss percentage, reported in Fig. 4 for a single sample of Washcoat/SiC flow through (WSiC/FT) system, was calculated versus
Fig. 3. Monolithic samples in (a) Flow-Through configuration; (b)Wall-Flow configuration; (c) KataloJM pellets samples.
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monolith surface is covered by the washcoat and active species, the coating is also evident by comparing Fig. 5a and d (the last one is relevant to the bare support). Furthermore in Fig. 5c we showed a granule of monolith entirely covered by the catalyst, so evidencing that the catalytic layer covers all the SiC granules surface and deposits on the inner walls of the pores, decreasing their diameter but not plugging them. Fig. 5a and b, in particular, evidenced the good adherence and the tight contact between the catalyst layer and the SiC granules. The SEM-EDAX analysis results are reported in Fig. 6. Fig. 6 evidenced that the detected elements on the sample are related to the chemical structure of the SiC filter (C and O) and to the elements constituting the washcoat (Al, O and Ce) and the active species (Ni). These results highlighted that the preparation procedure is able to realize the deposition of active phases on the support surface, forming a uniform layer covering the entire surface of SiC monoliths. In particular, the very low signals of the structural elements (above all the signal related to C) confirmed the good coating of the SiC granules with the active species.
Fig. 4. Weight loss percentage of the prepared washcoated sample.
the lonely deposited washcoat, without considering the weight of the carriers (5).
weight loss =
Final mass − Initial mass * 100 Initial washcoat mass
3.1.4. Activity tests Catalytic activity was investigated for the prepared samples, in particular to investigate the effect of the geometric configuration on the catalyst performance. Tests were carried out by assuring a reaction stream composed by 25 vol% of methane and 75 vol% of steam, and a S/ C ratio equal to 3, and were performed at various values of GHSV (Gas Hourly Space Velocity), calculated as the ratio between the volumetric rate and catalyst volume. In particular, the tests were performed at the values of 25,000 h−1 and 6250 h−1, and with the monoliths in the FT and WF geometric configurations. It was remarkable that in all tests, for all the investigated operating conditions, pressure drops were below 0.01 bar, therefore pressure effects on catalytic tests were negligible. The results are summarized in Figs. 7–9, in terms of methane conversion, CO2 yield and hydrogen yield. The comparison between the prepared monoliths reported in Figs. 7–9, evidenced the better catalytic performances of the WF monoliths, in particular at the highest value of GHSV, even if the system is far from equilibrium conditions: in these very stressfull conditions, the system geometry, characterized by the forced pass of the gases through the inner porous walls of the catalyst, allowed a deeper interaction between the reactants and the active species present in the porosities, so resulting in a full use of all the active species deposited on the entire surface of the carrier and, consequently, in a better axial and radial thermal profile distribution in the catalytic bed, as also demonstrated in an earlier paper [20]. Therefore, better exploiting of active phase and the minimization of mass transfer limitations resulted in appreciably higher performances of wall-flow configuration with respect to flow-through. On the contrary, at the lowest GHSV value, the better catalytic activity of the WF catalyst is evident only for temperatures lower than 700 °C, while for higher temperatures the performances of the two catalysts are very similar and approach to equilibrium conditions. The good results in terms of CO2 yield suggest that also water-gas shift reaction was promoted by the system, even if a strange behavior in carbon dioxide yield was observed at the lowest GHSV value at temperature above 650 °C, for which the monoliths in FT configuration showed better perfomances with respect to the WF ones. Such behavior could be addressed to the lower activity of the same monolith in steam reforming reaction: in this case less H2 is produced and the CO produced is converted to H2 through water gas shift reaction. It is finally worth to underline that the catalytic monoliths well approached thermodynamic equilibrium for temperature above 730 °C: the approach is more evident for hydrogen and CO2 yield with respect to methane conversion, so underlining that water-gas shift reaction was better promoted by the system. The excellent performances of such catalysts towards WGS may be ascribable to the high affinity of ceria supported catalysts towards CO-shift reaction [33], it is however relevant to remark that high activity of Ni/CeO2 based catalysts in such
(5)
As evident from Fig. 4, the greater loss was recorded after the first test cycle, with a weight loss equal to about the half of the total loss; in addition, after the first 20 min of test the weight loss stabilized at its maximum value, equal to about 8.72%; the two subsequent test cycles do not record further losses. Previous tests on the same type of monolith and with the same washcoat formulation but slightly less acid showed higher losses, reaching almost 20%. It is demonstrated, therefore, that a lower pH favors a greater adhesion of the washcoat, unlike as reported in some literature works for a different type of coating [22]. 3.1.2. Chemical composition and textural properties The chemical composition and the porosimetric characteristics of the prepared catalytic samples are summarized in Table 2, compared also with washcoat powder and bare SiC monoliths. The ED-XRF spectroscopic results substantially confirm the expected composition, highlighting the reproducibility of the coating techniques and confirming the homogeneity of the washcoat slurry. The porosimetric results show that the addition of the washcoat to the carriers, leads to a decrease of the average diameter and volume of the pores in the SiC monoliths. This expected result is related to porosimetric characteristics of starting SiC carrier, characterized by porous inner walls, with an average pore diameter of about 17 μm, and therefore, the deposition of washcoat and its adhesion to the inner walls of the pores reduces their section and diameter, as reported in previous works [32]. 3.1.3. SEM and SEM-EDAX results The SEM images related to the final prepared catalytic sample are reported in Fig. 5. By looking more deeply Fig. 5a, the reader can observe that all the Table 2 Chemical composition (wt%) and textural properties. Sample
Washcoat powder 5NiWSiC/FT 5NiWSiC/WF No calcined SiC bare monolith Calcined SiC bare monolith
XRF SiC
Ni
Al2O3
CeO2
– 73.60 73.50 100
– 1.32 1.30 –
3.91 0.88 0.89 –
96.09 24.20 24.31 –
100
V Pore [mm3/g]
Pore Dm [μm]
– 146.98 146.98 173.99
– 4.60 4.60 17.00
195.84
17.00
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Fig. 5. SEM images of the prepared catalytic sample at various magnitudes, (a) 584 X, (b) 5.00 KX, (c) 35.00 KX, and of the bare support (d).
Fig. 6. SEM images and distribution of elements, as obtained by EDAX element mapping, for the prepared catalytic sample.
commercial sample contains a substantially higher amount of nickel: in fact, in one hand a commercial nickel based catalyst contained an amount of active metal largely higher than 5 wt%, moreover, the mass of commercial catalyst (4.3 g) was more than twice with respect to the active washcoat deposed on the monoliths (about 2 g). Since in the lower operating temperature range reactions kinetics were relatively low, the high content of active phase on commercial sample promoted higher conversion of methane; by increasing operating temperature, reactions kinetics were thermally enhanced, improving the performances of monolithic samples. In particular, for high operating temperature, the expected high reaction kinetics required comparable mass and heat transfer rates between solid and gaseous phase. It is reasonable to suppose that for Katalco pellets heat (and mass) transfer mechanism resulted a limiting step in the highest temperature range, affecting the
reaction should require an optimal Ni morphology (particularly primary particle size and shape), thus enhancing redox properties of catalytic system (metal and support) [34]. The prepared catalytic samples were also compared in terms of methane conversion with a commercial Ni based catalyst, provided by KatalcoJM; with the aim to perform comparable tests, a volume of commercial catalyst in pellet equivalent to structured catalyst volume was used (about 6.4 cm3 corresponding to about 4.3 g). The results are reported in Figs. 10 and 11. Figs. 10 and 11 showed a very good catalytic activity of commercial catalyst in terms of methane conversion, evidencing conversion values comparable to the structured catalysts ones, in particular showing higher values at the highest GHSV value. Such behavior may be ascribable to Katalco catalyst features, and suggests that the loaded 211
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Fig. 7. Methane conversion − comparison of SiC FT and WF catalytic samples at (a) 25,000 h−1 and (b) 6250 h−1, for a S/C ratio equal to 3.
Fig. 9. Hydrogen yield − comparison of SiC FT and WF catalytic samples at (a) 25,000 h−1 and (b) 6250 h−1, for a S/C ratio equal to 3.
Fig. 10. Methane conversion − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 6250 h−1, ratio S/C=3).
overall catalyst performances. The heat transfer limitations of the pellet catalyst were confirmed by the lower temperature achievable for the commercial sample by fixing furnace temperature at 900 °C: at lower GHSV values, the gas temperature at the exit section was around 770 °C, at higher GHSV was around 660 °C. Such achievement, also considering that at 6250 h−1 the methane conversion was similar for the three samples, suggested that pellets catalyst suffered for heat transfer limitation from the heating medium to the reaction volume. The discussed phenomenon was also remarked by analyzing the comparison between the temperatures of gas exiting from the catalytic volume with respect to the furnace temperature (Fig. 12). As reported, in the pellets catalysts a higher difference between furnace temperature (and then reactor wall temperature) and products gas was obtained,
Fig. 8. CO2 yield − comparison of SiC FT and WF catalytic samples at (a) 25,000 h−1 and (b) 6250 h−1, for a S/C ratio equal to 3.
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Fig. 11. Methane conversion − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 25000 h−1, ratio S/C = 3).
Fig. 14. Methane conversion vs furnace temperature − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 25000 h−1, ratio S/C = 3).
to furnace temperature, at the two GHSV values used in the experimental tests. Experimental results evidenced that for higher furnace temperature, the SiC monoliths (both flow-through and wall-flow) had a better behavior with respect to the commercial catalyst, in particular at the lower GHSV value: in such conditions, all monolithic systems evidenced a very high methane conversion, therefore a high heat duty was required by catalytic volumes. The highly conductive SiC structures were able to minimize the heat transfer from external medium (furnace) and the catalyst, thus resulting in an overall higher temperature. In particular, the wall flow configuration allows to obtain the better performances for temperature close to 750 °C, while for higher temperatures the performances of wall flow and flow through configuration are similar. It is important to underline that for a furnace temperature of 850 °C, the SiC based catalysts showed a methane conversion of about 90%, about 12% higher than the methane conversion of Katalco commercial catalysts. In a different view, to achieve a methane conversion of 60%, a furnace temperature of about 730 °C is required for Katalco pellet catalyst, 660 °C for SiC monolith in flow-through configuration and 630 °C for SiC monolith in wall flow configuration. This important result is due to the higher thermal conductivity of SiC, that decreases the heat transfer resistances from the furnace to the reactor and catalyst, so allowing to reach the process temperature by using a lower external temperature: in an industrial case this condition results in an evident decrease in plant costs. A preliminary evaluation of the benefit linked to a highly conductive structured carrier was performed by investigating the overall heat transfer rate between heating medium (furnace) and reaction volume. In the estimation, 4 heat transfer resistances “in series” were considered: furnace-reactor wall (natural convection); reactor wall (conductivity); catalyst (conductivity); catalyst-process stream (forced convection) [36]. Since the thermocouples driving the furnace were in contact with the reactor external surface, it was considered that the temperature of the reactor wall corresponded to the furnace temperature; moreover, it was considered the absence of temperature radial gradient in the gas phase. Therefore, heat transfer from furnace to gas phase undergoes to Eq. 6, where Q(z) is the heat transfer along the reaction system, Din is the inner diameter of the tubular reactor, Tfurnace and Tgas were respectively the external temperature of the reactor and the gas temperature, and U is the overall heat exchange coefficient.
Fig. 12. Dependence of products gas temperature to the furnace temperature (S/C = 3, GHSV = 6250 h-1).
suggesting that along the reactor, higher radial thermal gradient occurred. As well, since by fixing furnace conditions the same reactants temperature should be obtained at the catalyst entrance section, the higher temperature of monolithic samples leads to suppose a flatter thermal profiles in the axial direction with respect to the pellets catalyst. In order to better highlight the potential advantages in the use of highly conductive structured carrier, in Figs. 13 and 14 methane conversion for each catalytic system investigated was reported with respect
dQ (z ) = U ·Din ·dz·[Tfurnace − Tgas (z )]
(6)
The enthalpic contribution of the SR and WGS reactions (Eqs. (1) and (2) was also considered, for which kinetic expressions, proposed Habermann and Young [37], were considered. The model parameters were achieved by dedicated tests on a 5%Ni/Washcoat powder sample, obtained results were summarized in Table 3.
Fig. 13. Methane conversion vs furnace temperature − comparison between structured catalysts in Ft and WF configurations, and commercial catalyst (GHSV = 6250 h−1, ratio S/C = 3).
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with nickel nitrate solution. The as prepared catalysts were tested for the methane steam reforming reaction, the traditional flow-through configuration was compared to the alternative wall-flow geometry and to the commercial catalyst KatalcoJM Quadralobe. The activity tests showed the benefits in using the structured catalysts, highlighting the better distribution of the reaction heat through the high thermal conductive SiC carrier, both in axial and in radial directions. The results showed in particular a higher catalytic activity with the wall-flow configuration in the temperature range 500–750 °C, which results in a higher methane conversion and hydrogen yield. This effect was attributed to the better interaction between the reacting gas and all the catalyst deposited on the carrier surface and in its porosities: by forcing the pass through the walls is possible to exploit the entire available catalytic surface and improve the distribution of the heat. The performance of the structured catalysts were also compared with a conventional commercial catalyst; the activity tests showed, in particular, that, even if the commercial formulation was highly active, fixing the furnace temperature the structured catalysts provided much higher values of methane conversion, so allowing to reach the process temperature by using a lower external temperature and so resulting in a feasible process intensification.
Table 3 Kinetic parameters for the catalytic formulation.
k0 Ea
Steam Reforming
Water Gas Shift
5.53*10−2 mol*m−3*s−1*Pa−2 106.2 kJ/mol
7.82*10−7 mol*m−3*s−1 8.42 kJ/mol
Table 4 Estimated overall heat exchange coefficient in the proposed reaction system [Tfurnace = 750 °C). U[W *m−2*K−1] KatalcoJM FT WF
21.4 85.6 121.8
References
Fig. 15. Theoretical methane conversion profiles for (Tfurnace = 750 °C; S/C = 3; GHSV = 6250 h−1, ratio S/C = 3).
considered
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