Membrane reactors using metallic membranes

10 Membrane reactors using metallic membranes Fausto Gallucci,3 D.A. Pacheco Tanaka,2 J.A. Medrano,1 J.L. Viviente Sole2 1

Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands; 2TECNALIA, Energy and Environment Division, San Sebastian-Donostia, Spain; 3Inorganic Membranes and Membrane Reactors, Sustainable Process Engineering, Department of Chemical Engineering and Chemistry, Eindhoven, University of Technology (TU/e), Eindhoven, The Netherlands

Introduction There is general consensus on the contribution that process intensification (PI) can give to the chemical industry in terms of improved energy efficiency. In general, PI, is defined as “any chemical engineering development that leads to a substantially smaller, cleaner, safer and more energy efficient technology” [1] and is always referred to as the next revolution of the chemical industry. The need for more efficient processes, including further flexible engineering designs and, at the same time, increasing the safety and environmental impact of these processes, is pushing the industry to novel research in this field. The chemistry and related sectors have already recognized the benefits of PI and estimate a potential for energy saving of about 1000 ktoe/y using these processes. PI is, however, a very broad field and, in many cases, it is just a new and nicer name for practices that were already carried out in chemical industries. PI is not just about debottlenecking processes already working at industrial level, but rather strategies that can open new process windows not available with conventional systems. Several authors have reported reviews and books on PI, and an interested reader is referred to these works for more information [2e4]. The most interesting concepts can be summarized in Fig. 10.1. The strategies adopted are divided into four categories, where the PI will be achieved either in one or more of the domains. Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-818332-8.00010-7 © 2020 Elsevier Inc. All rights reserved.

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Process Intensificaon

Energy Domain

Spaal Domain

Plasma

Micro reactor

Microwaves

Flowchemistry

Ultrasounds

Synergec Domain

Temporal Domain

Membrane Reactors

Dynamic operaons

Reacve separaons

Chemical looping/Sorp on enhanced systems

Structured Centrifugal 3D prinng

Figure 10.1 Summary of the process intensification strategies.

One interesting strategy is achieved in the synergy domains, where functions are integrated in single units. Generally, the functions integrated are reaction and separation or reaction and heat management. The integration of functions promises to decrease the capital costs and operating costs compared to typical systems where these functions are separated. One of these “novel” concepts is the membrane reactor concept, in which membrane separation is integrated with reaction. The next section reports in more detail the concept of membrane reactors.

The membrane reactor concept As discussed in the previous section, a membrane reactor is a PI strategy in the synergetic domain, where the synergy is achieved between membrane separation and chemical reaction. The definition of membrane reactor is thus a multifunctional reactor in which reaction (generally catalyzed) and separation through a membrane are integrated, with the aim of achieving higher conversions at milder conditions compared with conventional systems. A typical scheme of a membrane reactor is reported in Fig. 10.2. There are several studies showing that membrane reactors have several advantages compared to conventional reactors. If one looks at reactors with metallic membranes, a good example of advantages of membrane reactors has been provided by Brunetti et al. [5] who reported the study on water gas shift (WGS) reactors.

Chapter 10 Membrane reactors using metallic membranes

Figure 10.2 Scheme of a typical (fluidized bed) membrane reactor for metallic membranes.

The WGS is a gas phase, catalyzed exothermic and equilibrium reaction as reported in Eq. (10.1). CO þ H2 O5CO2 þ H2 DH ¼ 41 kJ mol1

(10.1)

As the reaction is exothermic and equilibrium limited, it is conventionally carried out in two reactors, the first working at high temperature to make use of high reaction rates, and the other (much bigger) at low temperature to achieve higher conversions. If a membrane reactor that is used for hydrogen separation is used, the equilibrium is circumvented, so that one can work with a single reactor, working at high temperatures, as reported in Fig. 10.3. The authors have reported different advantages compared to conventional system, the most interesting being the reduction of reactor volume that can go down to 20% or more compared to conventional systems by using high pressures (or more permeable membranes) as reported in Fig. 10.4 [6]. This is achieved because the removal of hydrogen (which increases by increasing the reactor pressure) allows working at high temperatures, which also results in faster kinetics and thus the reactor volume decreases for two reasons. Additionally, if the membrane is very selective, downstream separations are also not required, and thus the complete hydrogen production

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(A)

(B)

Figure 10.3 Comparison between conventional reactor system and membrane reactors for water gas shift reaction system. Reprinted from G. Barbieri, A. Brunetti, A. Caravella, E. Drioli, Pd-based membrane reactors for one-stage process of water gas shift, RSC Adv. 1 (2011) 651e661. http://www.scopus.com/inward/record.url? eid¼2-s2.0-84859320558&partnerID¼40&md5¼7826c71ca85b38d09fabc102309a1824 with permission of RSC.

Figure 10.4 Volume index as a function of feed pressure at 280  C. Set CO conversion at 90%. Reproduced from A. Brunetti, A. Caravella, G. Barbieri, E. Drioli, Simulation study of water gas shift reaction in a membrane reactor, J. Membr. Sci. 306 (2007) 329e340. https://doi.org/10.1016/j.memsci.2007.09.009 with permission by Elsevier.

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system is much smaller (large CAPEX reduction) compared with a conventional system. There are two main considerations that should be highlighted here and are very important to be kept in mind. The first is about the effect of membrane reactors on equilibrium conversion (and it is more about terminology). Indeed, it is always reported that the membrane reactor is able to reach a conversion higher than the thermodynamic equilibrium. This statement is not true and should always be formulated as: a membrane reactor is able to reach a conversion higher than the thermodynamic equilibrium of a conventional system. This is because a membrane reactor is a different system and is subject itself to a new equilibrium that can be easily calculated by considering the operating conditions in the reactor and in the permeation side of a membrane [7]. The second one is more important because it is about limitations and applicability of membrane reactors. Indeed, having a reactor with a separation step downstream allows for optimizing (in terms of flows, pressure, and temperatures) the two steps separately; on the other hand, when the two are integrated, the reaction and separation should of course work at the same conditions that may be suboptimal for both. Thus, the integration of reaction and separation is not necessarily beneficial for the system. As a matter of fact, if the reaction step (or separation step) does not have any limitation/challenge, the integrated system will behave worse than the separated/conventional system. The systems where membrane reactors can give clear benefits (see Fig. 10.5) are reaction systems in which the conversion/yields are either limited by thermodynamic equilibrium or by consecutive/parallel reactions. In these cases, removal of the product (or feeding of a reactant) can give great benefits in terms of product yields. Most membrane reactors using metallic membranes are applied to equilibrium reactions, so that the removal of one of the products shifts the equilibrium toward higher yields. This will be discussed in more details in the following sections.

Types of membrane reactors As a membrane reactor is a combination of membrane separation and catalytic reaction, it is possible to have a classification of membrane reactors based on the type of catalytic system (or based on catalyst configuration) and based on the type of membranes used (both based on material and on membrane function) as reported in Fig. 10.6.

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conversion enhancement by selective permeation of a reactant product of an equilibrium limited reaction

conversion enhancement by coupling of reactions

selectivity enhancement by selective permeation of an intermediate product

selectivity enhancement by dosing a reactant through the membrane

Figure 10.5 Main application possibilities of (inorganic) membrane reactors.

Figure 10.6 Classification of membrane reactors.

An important classification or differentiation can be done based on the membrane material used. We can basically have polymeric/organic membrane reactors and inorganic membrane reactors. Typical polymeric membrane reactors are membrane bioreactors [8]. In these reactors, often used for removing pollutants from water or for wastewater treatment in general, hollow fibers or membrane cassettes are immersed in the reactor and are used for gas feeding or membrane separation. Generally, fouling of the membrane is a very severe challenge for this kind of membrane reactors. Recently, Maaz et al. [9] reported a nice overview

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of challenges and opportunities for membrane bioreactors as reported in Fig. 10.7. As shown in the figure, membrane fouling is only part of the challenges because dissolved gases, ammonia, and salinity are some challenges as well, but surely fouling reduces the lifetime of the membranes which are still a large part of the cost of the plant. An interested reader is referred to all these papers on bioreactors for more information. The second category is the inorganic membrane reactors, comprising all reactors using inorganic membranes. As a matter of fact, membrane bioreactors can still fall into this category, as for some aggressive solutions, inorganic membranes (either alumina based or SiC based) are used with good results [10]. Metallic membraneebased reactors clearly fall into this category, with Pd-based membrane reactors being the most studied. Other inorganic membrane reactors comprise reactors using ceramic (or mixed oxides) for oxygen separation, that can be used for oxidative coupling of methane [11e14], carbonate membranes for WGS with CO2 separation [15], methanol production [16,17], etc. Another classification is made on the way the membrane is used. In this case we can distinguish the first category (Extractor) comprising all membrane reactors where the membrane is used to separate from the reaction zone one or more products. Such membrane reactor type is the typical application of metallic

Figure 10.7 Challenges and opportunities in anaerobic membrane bioreactor. Reproduced by M. Maaz, M. Yasin, M. Aslam, G. Kumar, A.E. Atabani, M. Idrees, F. Anjum, F. Jamil, R. Ahmad, A.L. Khan, G. Lesage, M. Heran, J. Kim, Anaerobic membrane bioreactors for wastewater treatment: novel configurations, fouling control and energy considerations, Bioresour. Technol. (2019). https://doi.org/10.1016/J.BIORTECH.2019.03.061 with permission by Elsevier.

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membranes for hydrogen production [18e20], but also for methanol production [21]. The second category of reactors (Distributor) comprises all the reactors where the membrane is used to distribute one of the reactants to the reaction system. One typical application is clearly the membrane bioreactor described earlier. Another application is based on oxygen selective membranes that can be used to feed oxygen to partial oxidation reaction [11,22]. Finally the third category Contactor comprises all concepts where the membrane is used for contacting two phases [23]. Another classification is made based on the type and configuration of the catalyst used. This is the typical classification used to distinguish metallic membrane reactors. The catalyst can be in fixed bed configuration; which means that the membranes are immersed in packed bed reactors. The most used packed bed configuration is the tubular one where the catalyst may be packed either in the membrane tube (Fig. 10.8A) or in the shell side (Figure 8b), while the permeation

Figure 10.8 Membrane reactor catalyst in tube (A) and catalyst in shell (B) configurations [24].

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stream is collected in the other side of the membrane (in case of hydrogen selective membranes) or one reactant is fed on the other side of the membrane. In real-life applications, one would always use a multitubular membrane reactor in order to achieve a high production capacity. An example of multitube membrane housing has been patented by Buxbaum [25] and reported in Fig. 10.9. In this case the catalyst is loaded in the shell side of the reactor while the membrane tubes are connected to a collector for the pure hydrogen. In particular, in the figure the possibility to use a catalyst in a separate chamber is shown. In case of reforming reactions, this chamber acts as a prereforming zone where the greatest temperature profiles are concentrated. In this way the membranes will work at an almost constant temperature. In packed bed membrane reactors, the catalyst particles are large, in order to avoid large pressure drops. As the membrane permeation is driven by pressure, a pressure drop due to the packed bed would also decrease the membrane flux. The large particle size in general results in internal mass transfer limitations (accounted for by considering low effectiveness factor of the catalyst). Other disadvantages of packed bed are (1) the bulk to wall mass transfer limitations (also called concentration polarization) which are detrimental for the membrane permeation and (2) heat transfer limitations which result in large temperature gradients that can be detrimental for the stability of the membranes (for exothermic reactions) or for the flux of the membrane (for endothermic reactions). Some of these disadvantages could be decreased by using different types of packed beds such as

Figure 10.9 Membrane housing (A), catalyst distribution (B), and membrane connectors (C) for a multitube membrane reactor [25].

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structured catalysts. The use of a catalytic foam in a membrane reactor was recently reported by Patrascu et al. [26]. A Pt(3)Ni(10)/CeO2 coated onto a SiC foam scaffold (high thermal conductivity) was used as a catalyst. Another possible configuration is the fluidized bed membrane reactor; in this configuration the membranes are immersed in a fluidized bed of catalyst particles [27]. To achieve fluidization, generally smaller particles are used, which means that the fluidized bed has generally higher effectiveness factors for the catalyst (no internal mass transfer limitations). The fluidization also allows higher heat and mass transfer coefficients. In general, fluidized bed reactors are operated in virtually isothermal conditions. The movement of the particles also allow avoiding concentration polarization, although latest results show that concentration polarization is still present in fluidized beds [28]; this will be discussed in more detail in the following sections. Finally, the membrane itself can act as a catalyst [29] in a catalytic membrane reactor (term that should be only used when the membrane is indeed catalytic).

Application of membrane reactors to different systems Membrane reactors have been used for a variety of applications. However, as far as metallic membranes are concerned, only hydrogen selective membranes have been employed in membrane reactors. Thus, in this section we will report only the applications of membrane reactors involving hydrogen permeation. Different reactions have been considered for hydrogen selective membranes, either for hydrogen production itself (from natural gas, biogas, methanol, and ethanol), for hydrogen carrier conversion (mostly ammonia decomposition), or for dehydrogenation reaction (i.e., cyclohexane, propane etc.). The reactions are reported in Table 11.1. Methane, the main constituent of natural gas, is the most used feedstock for hydrogen production. As shown in Table 10.1, the reaction is very endothermic, which means that the activation of methane requires very high temperature, which leads to a mixture of hydrogen and carbon monoxide. CO should then be converted in lower temperature WGS reactors (see the previous section for the effect of membrane reactors for WGS). A typical methane to hydrogen plant is as in Fig. 10.10, where several reactors and separator steps are required to achieve the hydrogen purity required.

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Table 10.1 Reactions used in hydrogen membrane reactors.

Reaction Water gas shift CO þ H2 O4CO2 þ H2 Decomposition (carbon production) CH4 4C þ 2H2 Steam reforming reactions CH4 þ H2 O 4CO þ 3H2 CH4 þ 2H2 O 4CO2 þ 4H2 CH3 OH þ H2 O4CO2 þ 3H2 C2 H5 OH þ H2 O 42CO þ 4H2 C2 H5 OH þ 3H2 O 42CO2 þ 6H2 Partial and full oxidation reactions CH4 þ 2O2 4CO2 þ 2H2 O CH4 þ O2 4CO2 þ 2H2 CH4 þ 12O2 4CO þ 2H2 CH3 OH þ 12O2 4CO2 þ 2H2 C2 H5 OH þ 12O2 42CO þ 3H2 Autothermal reforming reactions 4CH4 þ 2H2 O þ O2 410H2 þ 4CO 4CH3 OH þ 3H2 O þ 12O2 /4CO2 þ 11H2 C2 H5 OH þ 2H2 O þ 32O2 42CO2 þ 5H2 Dehydrogenation reaction C3 H8 4C3 H6 þ H2 Ammonia decomposition 2NH3 43H2 þ N2

Figure 10.10 Typical methane reforming plant.

DH298K kJ mol L 1 41.1 75 206.2 164.9 49 239.5 173 802 71 35.6 192.3 14.4 339 0 50 124




Chapter 10 Membrane reactors using metallic membranes

The application of hydrogen permeable metallic membranes for methane reforming is possibly the most studied application of membrane reactors. Methane reforming is generally a quite fast reaction at high temperatures, thus it is very simple to reach equilibrium conversions even at lab scale, and so it is also easy to show that a membrane reactor allows achieving higher conversions than the equilibrium limitation of the conventional system. This is even more evident at higher pressures, as a conventional reactor will have lower conversions at higher pressures, while a membrane reactor will achieve higher conversion at higher pressures (see Fig. 10.11). Larger conversion levels are achieved at lower temperatures (550e600 C) in the MR, when compared with conventional methane steam reforming (800e900 C). Additionally, the decrease in temperature allows carrying out both reforming and WGS reactions in the same unit [30]. Matsuka et al. [31] reported packed-bed membrane reactor tests with nickel catalyst using three different types of self-supported membranes: 25 and 50-mm-thick Pd77Ag23, 100-mm-thick V and V92Ni8 layers coated on both sides with Pd by magnetron sputtering (1 mm). The best performance was observed for the Pd-coated vanadium membrane, obtaining w45% methane conversion at 400 C while the permeation flux was 0.09 (mol m2 s1). Silva et al. [32] have also performed methane reforming in packed beds using a 76.2 mm thick PdeAg membrane at

1.0 0.9 0.8

CH4 conversion, -

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MR (1 bar) MR (10 bar) MR (20 bar) TR (1 bar) TR (10 bar) TR (20 bar)

1 bar

0.7 10 bar

0.6 0.5 0.4

20 bar

0.3 0.2 0.1 0.0 650

700

750

800

850

900

950

1000

Temperature, K

Figure 10.11 Typical equilibrium conversion in conventional system and comparison with membrane reactors at different pressures.

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temperatures up to 600  C resulting in 35% methane conversion, while 47% of the produced hydrogen was recovered. Gil et al. [33] used instead a catalytic hollow fiber membrane reactor where Ni (25 wt.%)/SBA-15 catalyst was coated on the inside of an Al2O3 hollow fiber. A methane conversion of 53% was reached at 560  C, and 43% of the produced hydrogen was recovered through a 3.3 mm-thick Pd layer deposited on top of the hollow fiber. Packed bed membrane reactor experimental data were reported by Gallucci et al. [34] using thick self-supported membranes which at temperatures higher than 350  C still gave a large benefit over conventional reactors (see Fig. 10.12). All these results, although interesting, were obtained with quite thick membranes which are not interesting for real industrial exploitation due to low flux and high costs. Other authors have also reported results with thinner membranes [24]. It has been already reported in literature that membranes thinner than 8 micron would give great benefits in terms of hydrogen permeation and final cost of the membrane reactors used. However, the thinner the membranes, the more the mass transfer limitations in the gas phase (or bulk-to-membrane, or concentration polarization) will be present and will limit the performance of the reactors [35]. The effect is more pronounced when thinner membranes are used and can be now easily modeled for hydrogen [36] separation and membrane reactors [37].

Figure 10.12 Methane conversion versus temperature for both conventional and membrane reactors [34].

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To avoid problems with concentration polarization and for a better heat transfer and temperature distribution, two types of membrane reactors have been proposed which follow two different PI strategies: the micro reactor concept (microchannel membrane reactor) and the fluidized bed membrane reactor concept. There are several studies on microchannel membrane reactor concepts for different applications [38,39]. The idea behind the use of micro reactor is that the microchannel system allows small gap between the catalyst (generally deposited or printed on the wall of the channels) and the membrane that seals the top of the channels. The membranes can be welded on the module. Of course a microchannel membrane reactor would produce very limited amount of hydrogen; however, the modularity of both microchannel reactors and membrane modules allows producing larger modules by stacking different units in larger modules; these can also integrate different reaction zones such that the heat required from the reaction is supplied by either heating fluids or combustion reactions (see Fig. 10.13). The integration of membranes in microchannel modules has been extensively studied in SINTEF as well (although generally for membrane separation only) and different strategies have been developed to improve the membrane stability at the high pressures required for membrane reactor operations [40]. The other family of reactors that promise to decrease the concentration polarization [41,42] and allows uniform temperature operations are the fluidized bed membrane reactors. These have been studied for reforming reactions, for WGS, and for complete systems like membrane-based micro-CHP systems. Helmi et al. [43] reported stable operation of fluidized bed membrane reactors for WGS for over 900 h reaction (see Fig. 10.14). Over the 900 h of continuous operation in the bubbling fluidization and WGS operating conditions, the catalyst and the membrane module have shown a very stable performance without any decrease in the performance of the catalyst and permeation properties of the membranes. The CO impurity of the permeate stream was 15 ppm in average during the complete operation (min: 10 ppm, max: 28 ppm). These tests were carried out feeding only CO and water as simulated WGS feed. However, when the system was operated with syngas composition coming from a reformer, the CO concentration in the permeate side was always below 10 ppm, thus producing in one step the hydrogen purity required by a low-temperature PEM fuel cell. Fluidized bed membrane reactors have also been used to produce hydrogen with CO2 capture. At least three different concepts have been reported in recent years.

Figure 10.13 Stack designs for combustion, reformer, and integrated module for the reforming of methane with integrated hydrogen separation. Reproduced by A. Wunsch, P. Kant, M. Mohr, K. Haas-Santo, P. Pfeifer, R. Dittmeyer, Recent developments in compact membrane reactors with hydrogen separation, Membrane 8 (2018). https://doi.org/10.3390/ membranes8040107.

Figure 10.14 Long-term performance of the membrane module during 900 h of continuous operation in the bubbling fluidization regime at high-temperature WGS conditions. Reproduced from A. Helmi, E. Fernandez, J. Melendez, D.A. Pacheco Tanaka, F. Gallucci, M. van Sint Annaland, Fluidized bed membrane reactors for ultra pure H₂ production-A step forward towards commercialization., Molecules. 21 (2016). https://doi.org/10.3390/molecules21030376.

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Gallucci et al. [44,45] have reported two reactor configurations using fluidized membrane reactor to produce pure hydrogen with integrated CO2 capture. One concept was based on a combination of oxygen permeating membranes and hydrogen permeating membranes. The second (and more technically feasible) is based on a combination of Pd-based membranes (in part to produce the pure hydrogen and in part to combust hydrogen and supply the heat to the reforming reaction). The concept is reported in Fig. 10.15. The concept is very interesting as it allows complete conversion of the fuel in pure hydrogen and a stream of CO2 and steam, which upon condensation releases pure CO2. However, to achieve this, a very large membrane area is required, and 1/3 of it is just used for heat supply to the system.

Figure 10.15 Membrane-assisted reforming for hydrogen production with integrated CO2 capture. Reproduced from F. Gallucci, M. Van Sint Annaland, J.A.M. Kuipers, Autothermal reforming of methane with integrated CO2 capture in a novel fluidized bed membrane reactor. Part 1: experimental demonstration, Top. Catal. 51 (2008) 133e145. http://www.scopus.com/inward/record.url?eid¼2-s2.057049146926&partnerID¼40&md5¼6d9327901e48018606a202a5ed9f7093.

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To circumvent this disadvantage and make the hydrogen production with CO2 capture more efficient, Medrano et al. [46] have developed a new concept in which the heat supply is attained by using a chemical looping system, while the pure hydrogen is recovered through Pd-based membranes immersed in the reforming section. The concept called membrane-assisted chemical looping reforming (MA-CLR) is reported in Fig. 10.16. Thermodynamic calculations have shown that the system is indeed more efficient than the previous membrane reactor concept, as less membranes are used, and the heat generation is obtained with the very efficient chemical looping (gasesolid redox system). The results have shown that MA-CLR can be continuously operated at lab scale [47] while pure hydrogen is recovered by Pd-based membranes and Ni-based particles are used as both oxygen carried for the chemical looping system and as catalyst for the reforming reaction. If the system is scaled up and operated at high pressure, the hydrogen production will be even cheaper than the actual steam methane reforming while also avoiding a large part of the CO2 emissions associated with hydrogen production [48]. MA-CLR requires two reactors connected to each other, which is a solids circulation between the two reactors. This is a common

Figure 10.16 Schematic of the membrane-assisted chemical looping reforming system reported by Ref. [47].

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configuration that has, however, only been used (at industrial level) at lower pressures. The high-pressure operation is thus one of the challenges to be solved for larger-scale implementation. One other concept proposed that combines chemical looping and membrane reactor for operation at high pressure is the membrane-assisted gas switching reforming. This concept foresees dynamically operated fluidized bed reactors where the solids are periodically oxidized and reduced (reforming stage). Thus, a series of reactors operates in parallel to achieve steady-state production of pure hydrogen. The concept has been demonstrated experimentally at lab scale [49] and a full techno-economic analysis has been carried out [50]. The results have shown that, although an interesting concept, the economics are worst than the MA-CLR proposed before and the stability of the membranes are compromised because of the continue oxidation/reduction cycles they will experience. Another interesting concept for the production of pure hydrogen with CO2 capture has been reported by Silva et al.[51] that combines sorption (of carbon dioxide) with membraneassisted reforming of glycerol (see Fig. 10.17). By combining the sorption of carbon dioxide on a solid sorbent (exothermic reaction) and the hydrogen recovery through membranes, a double shift effect is obtained because both products are removed from the reaction system. The reactor should obviously be dynamically operated to allow regeneration of the sorbent, or other concepts should be used for such scope. Membrane reactors with metallic membranes have also been successfully used for reforming of a variety of feedstock such as methanol [52,53], ethanol [54,55], and other gases [56]. Most of these works have investigated the use of Pd-based membranes, in binary and ternary alloys, as metallic membranes. However, other interesting works have used different membranes for hydrogen separation in membrane reactors. Wang et al. [57] have used nickel-based hollow fiber membranes for hydrogen permeation and WGS reaction (see Fig. 10.18). The thin wall nickel membranes have shown very good stability in cyclic operation as well as very good membrane selectivity (at the expenses of course of the fluxes). Interestingly, the stability of the membrane in different gas atmospheres including presence of H2S is in general a killer for Pd-based membranes [58]. Other interesting membranes used in membrane reactors have been produced from Tantalum. Bhushan et al. [59] have studied Ta-based membranes in membrane reactors for enhancing the HI decomposition reaction. By a combination of experiments over a very thin (2.5 micron) supported Ta membrane and a

Chapter 10 Membrane reactors using metallic membranes

Figure 10.17 Sorption-enhanced membrane reforming of glycerol for hydrogen production with CO2 capture reproduced from Ref. [51].

modeling study the authors have reported that the HI conversion can be increased up to 95% in conditions in which the thermodynamic equilibrium of a conventional system is limited to 22% conversion. Again, the membrane was effectively used in an environment very difficult for Pd-based membranes. Another Ta-based membrane reactor has been proposed by Suk et al. [60] for hydrogen production from ammonia decomposition. The authors have used a Ta tube and coated it with a very thin Pd layer (to enhance hydrogen dissociation) and have used it for ammonia decomposition reaction. The membrane has proven to be very stable in conditions in which Pd-based membranes

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Figure 10.18 Photo of a bundle Ni hollow fibers (A); SEM images of the nickel hollow fiber membrane (B-E). From M. Wang, Y. Zhou, X. Tan, J. Gao, S. Liu, Nickel hollow fiber membranes for hydrogen separation from reformate gases and water gas shift reactions operated at high temperatures, J. Membr. Sci. 575 (2019) 89e97. https://doi.org/10.1016/J.MEMSCI. 2019.01.009.

could suffer ammonia poisoning. The authors have shown very good permeation properties and stable ultrapure hydrogen production (with ammonia impurities below 0.1 ppm). A similar reaction has been carried out by Lamb et al. [61] by using a V-based membrane. However, in this case a cascade of catalytic bed and membrane separation has been used rather than a membrane reactor. Another interesting application of metallic membranes is the propane dehydrogenation reaction. In this case, the product is not hydrogen but propylene. This reaction system is generally a dynamic process because the catalyst should be periodically regenerated to remove the carbon deposited during the reaction. By removing hydrogen from the system, the equilibrium is shifted toward the product and similar yields can be achieved at lower temperatures, which in turn will reduce the carbon deposition and reduce the regeneration frequency. This concept has been studied in literature by several authors [62e64]. These studies have shown that conventional Pd membranes are quickly deactivated when used for propane dehydrogenation, probably due to carbonaceous species formed on the surface due to the interaction with

Chapter 10 Membrane reactors using metallic membranes

propylene. On the other hand, membranes that are more resistant are protected with an intermediate porous layer that prevents the direct interaction of Pd with propylene, such as in the doubleskin membranes recently proposed by Arratibel et al. [65]. More data on membranes and membrane reactor applications have been published in recent reviews, and an interested reader is referred to Refs. [24,66,67].

Conclusions and future trends This chapter reports the description of membrane reactors and the latest results on metallic membrane reactors for hydrogen production and other systems. The membrane reactors have been tested so far only at lab scale, although the chapter on EU project in this book will show that scale-up is being achieved in larger consortia. The next steps for a large-scale implementation of this concept (that for too long has only been promising) will be the scale-up to 10e100 m3 h1 unit and its operation in the field for at least 10,000 h this will allow achieving the credibility required for moving larger investments in this area and will bring the concept directly to the market of distributed hydrogen production.

List of acronyms ATR CAPEX ktoe/y MA-CLR MR OPEX PEM PI SR TR WGS

Autothermal Reforming Capital Expenditures Kiloton oil equivalent per year Membrane-assisted chemical looping reforming Membrane reactor Operational Expenditures Proton exchange membrane Process Intensification Steam Reforming Traditional/conventional reactor Water Gas Shift reaction

References [1] D. Reay, C. Ramshaw, A. Harvey, D. Reay, C. Ramshaw, A. Harvey, A brief history of process intensification, Process Intensif (2013) 1e25. https:// doi.org/10.1016/B978-0-08-098304-2.00001-8. [2] F. Gallucci, J. Zuniga, Catalytic Reactors with Membrane Separation, 2015. https://doi.org/10.1002/9783527686605.ch33. [3] Y. Tian, S.E. Demirel, M.M.F. Hasan, E.N. Pistikopoulos, An overview of process systems engineering approaches for process intensification: state of the art, Chem. Eng. Process. - Process Intensif. 133 (2018) 160e210. https:// doi.org/10.1016/J.CEP.2018.07.014.

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