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Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition
international journal of hydrogen energy xxx (xxxx) xxx
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Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition Hongda Cheng a,b, Bo Meng a,**, Claudia Li c, Xiaobin Wang a, Xiuxia Meng a, Jaka Sunarso c,*, Xiaoyao Tan d, Shaomin Liu e a
School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China Key Laboratory of Biomedical Engineering and Technology in Universities of Shandong, Qilu Medical University, Zibo 255300, China c Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia d State Key Laboratory of Separation Membranes and Membrane Processes, Department of Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China e Department of Chemical Engineering, Curtin University, Perth, Western Australia 6102, Australia b
article info
abstract
Article history:
On-site hydrogen production via catalytic ammonia decomposition presents an attractive
Received 18 December 2018
pathway to realize H2 economy and to mitigate the risk associated with storing large
Received in revised form
amounts of H2. This work reports the synthesis and characterization of a dual-layer hollow
25 March 2019
fiber catalytic membrane reactor for simultaneous NH3 decomposition and H2 permeation
Accepted 10 April 2019
application. Such hollow fiber was synthesized via single-step co-extrusion and co-
Available online xxx
sintering method and constitutes of 26 mm-thick mixed protonic-electronic conducting Nd5.5Mo0.5W0.5O11.25-d (NMW) dense H2 separation layer and Nd5.5Mo0.5W0.5O11.25-d-Ni
Keywords:
(NMW-Ni) porous catalytic support. This dual-layer NMW/NMW-Ni hollow fiber exhibited
Ammonia decomposition
H2 permeation flux of 0.26 mL cm2 min1 at 900 C when 50 mL min1 of 50 vol% H2 in He
Dual-layer
was used as feed gas and 50 mL min1 N2 was used as sweep gas. Membrane reactor based
Hollow fiber
on dual-layer NMW/NMW-Ni hollow fiber achieved NH3 conversion of 99% at 750 C, which
Hydrogen production
was 24% higher relative to the packed-bed reactor with the same reactor volume. Such
Membrane reactor
higher conversion was enabled by concurrent H2 extraction out of the membrane reactor during the reaction. This membrane reactor also maintained stable NH3 conversion and H2 permeation flux as well as structure integrity over 75 h of reaction at 750 C. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Meng), [email protected], [email protected] (J. Sunarso). https://doi.org/10.1016/j.ijhydene.2019.04.101 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Introduction Hydrogen share in the energy market will increase rapidly in the near future given its role as a key chemical in catalytic hydrogenation, methanol production, and hydrogen fuel cell [1,2]. Specifically, hydrogen fuel cell vehicles present a prospective clean energy alternative to fossil fuel-powered vehicles. However, continuous production of affordable H2 is currently one of the principal issues that hinder large scale hydrogen fuel cell applications. The two main methods to supply H2 include large-scale H2 production combined with H2 transport and distribution, and on-site H2 production via H2containing carrier (or compound) at hydrogenation stations. Distributed, on-site portable H2 generation unit and supply facility represent a more attractive model to realize H2 economy relative to the centralized, off-site H2 production plant and storage unit [3,4]. This is since the risk associated with storing large amount of flammable and explosive H2 as well as the advanced facilities required to mitigate this risk are absent in the former model, which should translate to significantly lower capital and operation cost for the former [5e7]. An economic and efficient on-site H2 production unit is thus considered as an enabling technology for global scale implementation of H2. Although H2 has very large energy per unit of mass of 120 MJ kg1, its energy per unit of volume is low since it exists as a gaseous phase at room temperature [8]. This makes storing H2 becomes challenging especially when high fuel density in the storage is desirable. H2 can instead be stored as ammonia, which can accommodate up to 17.65 wt% of H2 per unit weight of ammonia. NH3 has a large volumetric H2 density of 0.107 kg H2 L1 and can be liquefied at 1 MPa and 25 C, which enables easy transportation and storage [9,10]. Unlike H2, NH3 is difficult to ignite and has a characteristic odor that alerts the surroundings when it disperses in air [11]. As the second most produced chemical globally, the transportation and storage infrastructure and technology for NH3 have been established [9]. Hence, massive investments and technical efforts in the development of a new system for H2 transport and storage can be minimized. Moreover, the combustion of NH3 does not lead to greenhouse gas emission as it produces only nitrogen and water vapor and NH3 can be used as a fuel directly in various engines, e.g., diesel engine, fuel cell, spark engine, and gas turbine [12e14]. These great advantages propel NH3 as a promising and attractive H2 energy carrier for on-site H2 production. NH3 can be converted to H2 on-site via the catalystassisted decomposition reaction. Different catalysts can be utilized to decompose NH3 to H2. Ruthenium-based catalysts have been extensively used for this purpose but given its high cost, research on the alternative non-noble metal catalysts (e.g., Fe, Co, Ni, and MoC) has gained momentum [15]. Torrente-Murciano et al. [16] studied NH3 decomposition over microporous carbon-supported Co catalysts. Between 25 and 550 C, the smaller (2 nm) Co crystallites displayed higher catalytic activities relative to the larger (3e5 nm) ruthenium nanoparticles. Gong et al. [17] reported MoS2 ultrathin nanosheet-coated CeO2 hollow sphere catalysts with three-dimensional hierarchical structure, which displayed enhanced catalytic activity and NH3 conversion of up to 80% at
700 C. Ni is another most promising catalyst candidate for decomposition of NH3 to H2 given its abundant availability, low cost, and high catalytic activity. Various groups have studied the catalytic activity of Ni on different supports such as Al2O3 (72.1% NH3 conversion at 550 C), Ce0.8Zr0.2O2 (95.7% NH3 conversion at 550 C), and mica (97.2% NH3 conversion at 650 C) [18e20]. Ni catalysts supported on perovskite-type oxides (ABO3) displayed close to complete NH3 conversion at 700 C [21]. Among the studied perovskite compositions, BaZrO3 support resulted in the highest Ni catalytic activity and produced stable NH3 conversion of about 62% at 550 C over 35 h of operation. Following NH3 decomposition reaction, H2 separation from the other gases is required to obtain high purity H2. Hollow fiber membranes have become the spotlight of numerous studies particularly for H2-related reaction and separation application given their large surface area to volume ratio and large H2 flux [22e25]. Song et al. [2] reported H2 permeation flux improvement from 0.164 mL cm2 min1 to 0.269 mL cm2 min1 for BaCe0.85Tb0.05Co0.10O3-d hollow fiber membrane when the fiber surface was deposited with Ni catalyst. Liu et al. [26] studied H2 separation through Nd5.5Mo0.5W0.5O11.25d (NMW) U-shaped hollow fiber membrane that produced H2 permeation flux of 1.29 mL cm2 min1 at 975 C, using 80 vol% H2 in He feed and humidified Ar sweep. Instead of applying two separate reactor and purification units, membrane reactor has the capability to carry out simultaneous reaction and separation in a single unit. Membrane reactor offers several advantages such as spaceefficient, low energy consumption, and enhanced conversion and selectivity from the shifting of the equilibrium to the product side due to the simultaneous product removal, which makes it suitable for on-site portable H2 production [27e29]. Initial studies on NH3 decomposition using membrane reactors focused into supported palladium (Pd) membranes for H2 purification. Collins and Way [30] reported 94% NH3 conversion in Pd-alumina membrane packed with Ni/Al2O3 catalyst at 600 C and 1618 kPa. Identically high NH3 conversions were also found using other Pd-based membrane reactors packed with Na-promoted Ru catalysts that reach 100% conversion at 367 C [31] or Ni catalysts that provide 10% higher conversion than conventional packed bed reactor [32]. In departure from the supported Pd membrane direction, Li et al. [33] developed silica membranes for H2 separation. Their Rusoaked g-Al2O3/a-Al2O3 catalytic membranes produced 95.2% NH3 conversion over 50 h and had H2 permeance of 6.1$107 mol m2 s1 Pa1 with H2/N2 selectivity of 710. Lanthanide tungstates (Ln6WO12) have been viewed as one of the most promising membrane materials for H2 permeation given their simultaneously high mixed protonic-electronic conductivity and chemical stability, which lead to 100% H2 selectivity [34e39]. Notably, Nd5.5Mo0.5W0.5O11.25-d (NMW) membranes displayed the highest H2 fluxes (up to 0.3 mL cm2 min1) relative to the other mixed protonic-electronic conducting (MPEC) materials and operational stability in CO2 and H2S [35]. The high H2 fluxes in NMW membranes can be attributed to the predominant n-type electronic conduction above 700 C due to the partial substitution of W with Mo, which facilitates the formation of electrons [40,41]. This work features a dual-layer hollow fiber catalytic membrane reactor
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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for the on-site H2 production from NH3 decomposition that contains porous NMW-Ni layer and dense NMW layer. The mechanism of H2 permeation through the dual-layer NMW/NMW-Ni membrane is shown in Fig. 1. H2 initially dissociates into protons and electrons at the surface of the NMWNi support layer. Thereafter, the protons and the electrons pass through the membrane separately under the mixed conduction of NMW. Ni particles in the support layer accelerate the dissociation of H2 and the electron conduction. Finally, the protons and electrons recombine on the surface of NMW dense layer to form H2. NMW-Ni layer provides dual functions, i.e., as the catalytic reaction site for NH3 decomposition and as the membrane support layer while NMW layer serves as H2 selective membrane layer.
Table 1 e Synthesis parameter specifications for duallayer hollow fiber membrane precursor. Parameter
Composition of the starting solution (wt.%) NiO NMW PESf NMP PVP Spinning rate (mL min1) Air gap (cm) Internal and external coagulant
NMW (Dense outer NMW-NiO separation layer) (Porous inner catalytic layer)
0 75 5 20 0 1.5 10 De-ionized water
26.6 42.7 7.4 22.3 1 10
Experimental Preparation of the powders and the catalytic membranes Nd5.5Mo0.5W0.5O11.25-d (NMW) powder was synthesized using a conventional solid-state reaction method. Stoichiometric amounts of Nd2O3 (99.9%), WO3 (99.9%), and MoO3 (99.9%) were mixed and milled inside an agate container filled with zirconia balls using a planetary mill for 48 h. The resultant mixture was then calcined at 1100 C for 10 h to obtain the fluorite phase. To prepare the hollow fiber precursor for the subsequent phase inversion process, the calcined powder was ball milled again for 24 h and sieved through a 200-mesh sifter. The NMW powder was then mixed with NiO in a volume ratio of 1:1 to form NMW-NiO powder. The dual-layer NMW/NMW-NiO hollow fiber membranes were prepared by phase inversion and sintering technique, i.e., co-extrusion followed by co-sintering, as described elsewhere [42]. A homemade three-hole spinneret was used to spin the membrane precursor according to the synthesis parameters as specified in Table 1. This dual-layer hollow fiber was prepared via a single-step co-extrusion method using a spinneret that contains three concentric holes (Fig. 2). The internal coagulant, i.e., deionized water, flows out from the innermost hole while the porous support layer and the outermost dense layer are co-extruded by injection pump from the middle hole and the outer hole, respectively. Due to the presence of the porous support layer, the dense H2
separation layer can be made thinner to reduce the bulk diffusion resistance, which is important to obtain high H2 permeation flux through the membrane. This single-step method improves the membrane preparation efficiency relative to the tape casting or other deposition methods. The resultant hollow fiber membrane precursors were dried in air for more than 24 h at room temperature and then sintered at 1450 C for 5 h to ensure gas tightness. To reduce NiO to Ni, the 1450 C sintered dual-layer was heated in 40 vol% H2 in He at 700 C flowed at 50 mL min1 for 2 h.
Characterization The morphology and microstructure of the hollow fiber membrane were analyzed using scanning electron microscopy (SEM, FEISirion, Netherlands) and back scattered scanning electron microscopy (BSEM, NEON, 40EsB). The crystal structure of the hollow fiber membrane was determined using powder X-ray diffraction (XRD, Bruker D8 Advance) using Cu-Ka radiation with l of 0.15404 nm. The scan was performed from 2q of 20 e80 with a scanning step of 0.02 .
Membrane reactor test Fig. 3 shows the schematic diagram of the membrane reactor. Single 20 cm-length dual-layer hollow fiber membrane was positioned in the lumen of a quartz tube and tightened to the
Fig. 1 e Schematic illustration of NH3 decomposition and H2 permeation in dual-layer NMW/NMW-Ni hollow fiber membrane. Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 2 e Three-hole spinneret for hollow fiber spinning. (a) Side view of spinneret stack; (b) Top view of spinneret (Inset shows the three concentric holes that form the hollow fiber membrane); and (c) Inside view of spinneret.
Fig. 3 e Schematic diagram of membrane reactor setup for NH3 decomposition to H2.
quartz tube using two silicon rubber tubes at the opposite ends to allow flexible movement that offsets the thermal expansion under high temperature (above 650 C) permeation test condition. The reactor was set to operate inside the furnace with 5 cm effective heating length. H2eHe or NH3eHe mixture was fed to the membrane lumen side while N2 was introduced to the shell side as the sweep gas to collect the permeated H2. The retentate stream passed through water to remove unreacted NH3, followed by silica gel to remove excess water vapor. The composition of the permeate or retentate stream were analyzed at 25 C using a gas chromatograph (GC, Agilent 7890B) equipped with a carbon molecular sieve column (4 m 3 mm). The reaction is carried out at 1 atm and although higher pressure is beneficial for H2 separation, low pressure is more favorable for ammonia decomposition and incurs lower cost. As a comparison, reaction using a packedbed reactor version of Fig. 3 was also performed using the same amount of the same catalyst used in the dual-layer hollow fiber, i.e., 0.11 g of NMW-NiO (1:1 by volume) powder. Quartz tube with an identical inside diameter to the dual-layer hollow fiber was used to substitute the hollow fiber in packedbed reactor test. Two silica wools were fitted at the two opposing ends of quartz tube to restrain powder movement during the reaction.
NMW pattern exhibits characteristic peaks that can be indexed according to fluorite lattice with a F43m space group (#216) (PDF#00-022-0744) [43]. NMW-NiO pattern, on the other hand, contains characteristic peaks that can be attributed solely to the mixture of the characteristic peaks from NMW phase and the characteristics peaks from NiO phase (PDF#01078-0643) [43]. The absence of the new peaks other than the characteristic peaks of NMW and NiO following the
Results and discussion Hollow fiber phase structure and morphology Fig. 4 shows the powder X-ray diffraction (XRD) patterns of Nd5.5Mo0.5W0.5O11.2-d (NMW) and 1450 C sintered Nd5.5Mo0.5W0.5O11.2-d-NiO (1:1 by volume e NMW-NiO) hollow fibers.
Fig. 4 e Powder X-ray diffraction (XRD) patterns of Nd5.5Mo0.5W0.5O11.2-d (NMW) and 1450 C sintered Nd5.5Mo0.5W0.5O11.2-d-NiO (1:1 by volume e NMW-NiO) hollow fibers.
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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calcination of NMW-NiO powder mixture at 1450 C indicates the phase compatibility between NMW and NiO phases. The 1450 C sintered dual-layer membrane was further heated in 40 vol% H2 in He at 700 C flowed at 50 mL min1 for 2 h to reduce its NiO content to Ni. The reduction of NiO to Ni created additional porosity in the NMW-Ni layer with respect to the original NMW-NiO layer. Fig. 5 shows the morphology and microstructure of the resultant NMW/NMW-Ni hollow fiber with outer diameter of 1.54 ± 0.027 mm and inner diameter of 1.10 ± 0.045 mm. The asymmetric structure consists of dense NMW separation layer in the outer region and porous NMW-Ni catalytic support in the inner region of the hollow fiber (Fig. 5(a) and (b)). Porous Ni particles appear to be deposited on the cavities of the inner circumference surface of the fiber (indicated by red circles in Fig. 5(c)). The 26 mm-thick dense layer (Fig. 5(d)) of the dual-layer NMW/NMW-Ni hollow fiber across its cross-section is comparable to a 20-30 mm-thick dense La5.4WO12-d membrane layer supported by a 250350 mm-thick porous disk La5.4WO12-d made using sequential tape casting method [38]. Similar results were also reported for a 25 mm-thick dense La28-xW4þxO54þ3x/2 (x ¼ 1.22) membrane layer supported by a 1 mm-thick porous layer made using powder pressing method and subsequent dip coating [44]. For large-scale production, single spinning step carried
Fig. 5 e Scanning electron microscopy (SEM) images of 1450 C calcined dual-layer NMW/NMW-NiO hollow fiber membrane. (a) and (b) Cross-section of the hollow fiber at different magnifications; (c) Inner circumference surface of the hollow fiber; (d) Finger-like pores in porous layer shown in cross-section of the hollow fiber; (e) Outer circumference surface of the hollow fiber; and (f) Back scanning electron microscopy (BSEM) image on the inner circumference surface of the hollow fiber.
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Fig. 6 e H2 permeation fluxes of dual-layer NMW/NMW-Ni between 650 and 900 C (Feed gas was 50 mL min¡1 of 50 vol % H2 in He and sweep gas was 50 mL min¡1 of N2).
out using a three-hole spinneret in this work is more desirable over the lengthy preparation route that involves several deposition and sintering processes. The formation of fingerlike pores on the central region of the hollow fiber support section is attributed to the different diffusion and coagulation rates at different solvent/non-solvent/polymer interfaces during phase inversion process (Fig. 5(d)). Ni catalysts were dispersed on these finger-like pores, which provide the fiber with numerous micro reactor channels for NH3 decomposition (indicated by red arrows in Fig. 5(d)). Fig. 5(e) shows that the outer circumference surface of the fiber formed dense structure with varying grain size in the order of 2e10 mm. Backscattered scanning electron microscopy (BSEM) images on the inner surface of the fiber reveal the presence of two regions with different brightness where the brighter region represents NMW while the darker spot-like region represents
Fig. 7 e NH3 conversion of dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor and conventional packed-bed reactor between 600 and 750 C (Feed gas was 50 mL min¡1 of 20 vol % NH3 in He and sweep gas was 50 mL min¡1 of N2).
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 8 e NH3 conversion and H2 separation efficiency of dual-layer NMW/NMW-Ni hollow fiber membrane between 600 and 750 C for three different NH3 contents in the feed gas. Solid symbols correspond to NH3 conversion; hollow symbols correspond to H2 separation efficiency.
Ni (Fig. 5(f)). This is since the heavier NMW backscatter electrons more strongly than the lighter Ni.
Hydrogen permeation fluxes H2 permeation flux through dual-layer NMW/NMW-Ni hollow fiber increases with temperature rise between 650 and 900 C (Fig. 6). This is due to the enhancements of surface exchange kinetics and proton bulk diffusion rate at higher temperature. H2 flux reaches up to 0.26 mL cm2 min1 at the highest temperature tested of 900 C. The flux is significantly higher than 0.15 mL cm2 min1 reported for 0.9 mm-thick NMW discs, tested using 50 vol% H2 in He feed with both sides of the membrane humidified [35,40]. However, the flux is considerably lower than 0.6 mL cm2 min1 reported for U-shaped NMW hollow fiber at 900 C when 80 mL min1 of 50 vol% H2 in He was used as feed gas and 100 mL min1 of Ar was used as sweep gas [26]. The large discrepancy could be attributed to the difference in the feed and sweep flow rates, which produced larger H2 partial pressure difference driving force by efficient H2 permeate removal.
Membrane reactor performance Membrane reactor based on dual-layer NMW/NMW-Ni hollow fiber displays higher NH3 conversion relative to the packedbed reactor with the same volume (Fig. 7). At 750 C, for example, 99% conversion was attained for the membrane
reactor while only 75% conversion was achieved for the packed-bed reactor. Such higher conversion for membrane reactor occurs due to the simultaneous H2 removal from the reactor chamber, i.e., H2 permeates from the lumen side to the shell side. The simultaneous removal of H2 retains the hydrogen partial pressure difference on the feed and permeate sides and ensures the H2 permeation process persists along with the NH3 decomposition reaction. Additionally, while the amount of catalyst used in the packed-bed reactor is similar to that used in the catalytic membrane reactor, i.e., 0.11 g of NMW-NiO powder, the packed-bed reactor experiences uniform heating while the catalytic membrane reactor is limited by an effective heating length. Outside of this 5-cm effective heating length, other sections of the 20 cm-long hollow fiber may also contribute to the NH3 conversion of the catalytic membrane reactor, and thus increase the difference in NH3 conversion between the two reactors. The porous support of the dual-layer hollow fiber not only provides the mechanical support role to the dense selective layer but also serves as reaction sites given the presence of Ni particles on its surface. The homogeneous dispersion of Ni active sites along the gas channels facilitates the contact between NH3 and Ni, the subsequent decomposition of NH3 to H2, as well as the dissociation of H2 to protons, which then permeate through the dense NMW film to the other side of the fiber. The fast permeation of H2 through the hollow fiber reduces the H2 residence time on the Ni particles, and results in more available active catalytic sites for NH3 decomposition, which increases the NH3 conversion for the catalytic membrane reactor. Fig. 8 shows the NH3 conversion and H2 separation efficiency of dual-layer NMW/NMW-Ni hollow fiber membrane, the operating conditions of which are listed in Table 2. H2 separation efficiency (h) was calculated following Eq. (1). h¼
Qp ðCH2 bCHe Þ 100% Qp CH2 þ Qr C'H2
(1)
where Q p is the flow rate of the permeate stream (mL min1), Q r is the flow rate of the retentate stream excluding NH3 (mL min1), CH2 and C'H2 are the H2 content (%) in the permeate stream and the retentate stream, respectively, CHe is the leaked He content (%) from the sealing imperfections in permeate stream, and b is the ratio of H2 to He in the feed stream. Small amounts of He was detected in permeate stream (<0.33%), indicating minor leakage occurred during the permeation experiments. The amount of leaked H2 was deducted proportionally according to the analyzed amount of He leakage.
Table 2 e Operating conditions of the dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor. Feed stream
20 vol% NH3 in He 30 vol% NH3 in He 40 vol% NH3 in He
Feed stream flow rate (mL min1)
Sweep stream
Sweep stream flow rate (mL min1)
Temperature ( C)
Pressure (bar)
50
N2
50
600 650 700 750
1
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 9 e NH3 conversion as a function of feed gas flow rate between 50 and 125 mL min¡1 in dual-layer NMW/NMWNi hollow fiber membrane reactor at 650 C (Feed gas composition was kept constant at 20 vol% of NH3 in He and sweep gas was 50 mL min¡1 of N2).
Since NH3 decomposition is an endothermic reaction, in all three cases, NH3 conversion increases with temperature rise due to increase in the reaction rate. Likewise, in all three cases, H2 separation efficiency also increases with temperature rise due to enhanced bulk-diffusion and surface exchange reaction at higher temperature. NH3 conversion is
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higher when lower amounts of NH3 are present in the feed stream since there are limited reaction sites in the membrane reactor. Larger amounts of unconverted NH3 flows out of the reactor when feed with higher NH3 content is introduced, resulting in lower conversion. H2 separation efficiency also decreases with increasing NH3 content, which likely indicates the lower H2 permeation rate relative to the rate of NH3 decomposition to H2. More H2 is produced when larger NH3 content is introduced into the feed. However, although more H2 permeated through the membrane, the amount of H2 that remained in the membrane also increased. As the NH3 content in the feed increased, the ratio of H2 produced by NH3 decomposition to the permeated H2 increased and the H2 separation efficiency decreased. Although the reported H2 separation efficiency was noticeably small, our report here aims mainly to improve the NH3 conversion through the use of the membrane reactor. At a constant NH3 content of 20 vol% in feed gas with the remaining made up of He, the NH3 conversion reduces with the increase of the feed flow rate (Fig. 9). For the hollow fiber membrane reactor, the factors affecting NH3 conversion are radial diffusion and residence time. Due to the small inner diameter of the hollow fiber (1.10 mm), the effect of radial diffusion on the NH3 conversion becomes negligible. Therefore, the NH3 conversion of the membrane reactor mainly depends on the residence time of NH3 in reactor. Hence, the results can be rationalized in terms of the reduced residence time of NH3 in the reactor since higher flow rate translates to higher space velocity. The higher the feed flow rate, the lower the NH3 residence time in the reactor and the smaller the NH3
Fig. 10 e NH3 conversion and H2 permeation flux of dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor obtained from the 75-h continuous test at 750 C (Feed gas was 50 mL min¡1 of 40 vol% NH3 in He and sweep gas was 50 mL min¡1 of N2). Red line corresponds to NH3 conversion; black line corresponds to H2 permeation flux. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 11 e SEM images of dual-layer NMW/NMW-Ni hollow fiber membrane after the 75-h continuous test at 750 C. (a) and (b) Cross-section of the hollow fiber at different magnifications; and (c) Outer circumference surface of the hollow fiber.
conversion. This principle may also be applicable to conventional systems such as packed bed reactors whereby the feed gas flow rate is inversely proportional to the NH3 conversion. If a packed bed reactor with a smaller feed flow rate, or with a greater length is used, the NH3 conversion may end up higher or be on par with that of the catalytic membrane reactor at the same reaction conditions. Based on Fig. 9, the degree of NH3 conversion increases with the decrease in the feed flow rate. However, the H2 production efficiency of the reactor at lower feed flow rate is expected to be small, due to slower NH3 decomposition rate. Although it may be interesting to investigate the actual permeation behavior at low feed flow rates, it is recommended to improve the NH3 conversion at high feed flow rate in the future works to ensure the overall process efficiency.
homogeneously in the porous support. The reduction of NiO to Ni resulted into a highly porous support structure that provides high surface area for catalytic reaction. The fingerlike pores in the support that were filled with porous Ni particles serve as micro reactor channels for NH3 decomposition. Dual-layer NMW/NMW-Ni hollow fiber displayed H2 permeation flux of up to 0.26 mL cm2 min1 at 900 C when 50 mL min1 of 50 vol% H2 in He was used as feed gas and 50 mL min1 N2 was used as sweep gas. Membrane reactor based on this dual-layer hollow fiber achieved 99% NH3 conversion at 750 C, which was 24% higher relative to the packed-bed reactor with the same reactor volume. The membrane reactor also maintained very stable NH3 conversion and H2 permeation flux over 75-h continuous test at 750 C.
Membrane reactor operational stability Fig. 10 displays NH3 conversion and H2 permeation flux profiles of dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor during the 75-h continuous test at 750 C when 50 mL min1 of 40 vol % NH3 in He and 50 mL min1 of N2 were used as feed gas and sweep gas, respectively. NH3 conversion and H2 permeation flux exhibit marginal fluctuations along median values of 91% and 0.12 mL cm2 min1; which indicates the long term structure stability of the dual-layer hollow fiber in NH3-containing gas atmosphere. Fig. 11 depicts SEM images of dual-layer NMW/NMW-Ni hollow fiber membrane after the 75-h test. The hollow fiber retains its original cross-sectional structure with no evidence of mechanical disintegration or cracks (Fig. 11(a) and (b)). The outer circumference surface of the fiber retains its original dense structure (Fig. 11(c)). This observation is consistent with the long term structure stability implied by membrane reactor performance over 75-h test.
Conclusions A dual-layer NMW/NMW-Ni hollow fiber catalytic membrane reactor was synthesized in single spinning step using a homemade three-hole spinneret for on-site H2 production via Ni catalyst-assisted NH3 decomposition. The 26 mm-thick NMW dense H2 separation layer integrated well with the NMW-Ni porous catalytic support. Ni catalyst was distributed
Acknowledgments The authors gratefully acknowledge the research funding provided by the National Natural Science Foundation of China (NSFC, No. 21476131, 21776166), the Natural Science Foundation of Shandong Province (ZR2014BM010, ZR2016JL011), Key Research and Development Program of Shandong Province (2017GGX70105), SDUT & Zibo City Integration Development Project (2017ZBXC107, 2017ZBXC137) and the Research Fund for the Doctoral Program of Higher Education of China (RFDP 20131201110007).
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Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition Hongda Cheng a,b, Bo Meng a,**, Claudia Li c, Xiaobin Wang a, Xiuxia Meng a, Jaka Sunarso c,*, Xiaoyao Tan d, Shaomin Liu e a
School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China Key Laboratory of Biomedical Engineering and Technology in Universities of Shandong, Qilu Medical University, Zibo 255300, China c Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia d State Key Laboratory of Separation Membranes and Membrane Processes, Department of Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China e Department of Chemical Engineering, Curtin University, Perth, Western Australia 6102, Australia b
article info
abstract
Article history:
On-site hydrogen production via catalytic ammonia decomposition presents an attractive
Received 18 December 2018
pathway to realize H2 economy and to mitigate the risk associated with storing large
Received in revised form
amounts of H2. This work reports the synthesis and characterization of a dual-layer hollow
25 March 2019
fiber catalytic membrane reactor for simultaneous NH3 decomposition and H2 permeation
Accepted 10 April 2019
application. Such hollow fiber was synthesized via single-step co-extrusion and co-
Available online xxx
sintering method and constitutes of 26 mm-thick mixed protonic-electronic conducting Nd5.5Mo0.5W0.5O11.25-d (NMW) dense H2 separation layer and Nd5.5Mo0.5W0.5O11.25-d-Ni
Keywords:
(NMW-Ni) porous catalytic support. This dual-layer NMW/NMW-Ni hollow fiber exhibited
Ammonia decomposition
H2 permeation flux of 0.26 mL cm2 min1 at 900 C when 50 mL min1 of 50 vol% H2 in He
Dual-layer
was used as feed gas and 50 mL min1 N2 was used as sweep gas. Membrane reactor based
Hollow fiber
on dual-layer NMW/NMW-Ni hollow fiber achieved NH3 conversion of 99% at 750 C, which
Hydrogen production
was 24% higher relative to the packed-bed reactor with the same reactor volume. Such
Membrane reactor
higher conversion was enabled by concurrent H2 extraction out of the membrane reactor during the reaction. This membrane reactor also maintained stable NH3 conversion and H2 permeation flux as well as structure integrity over 75 h of reaction at 750 C. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Meng), [email protected], [email protected] (J. Sunarso). https://doi.org/10.1016/j.ijhydene.2019.04.101 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Introduction Hydrogen share in the energy market will increase rapidly in the near future given its role as a key chemical in catalytic hydrogenation, methanol production, and hydrogen fuel cell [1,2]. Specifically, hydrogen fuel cell vehicles present a prospective clean energy alternative to fossil fuel-powered vehicles. However, continuous production of affordable H2 is currently one of the principal issues that hinder large scale hydrogen fuel cell applications. The two main methods to supply H2 include large-scale H2 production combined with H2 transport and distribution, and on-site H2 production via H2containing carrier (or compound) at hydrogenation stations. Distributed, on-site portable H2 generation unit and supply facility represent a more attractive model to realize H2 economy relative to the centralized, off-site H2 production plant and storage unit [3,4]. This is since the risk associated with storing large amount of flammable and explosive H2 as well as the advanced facilities required to mitigate this risk are absent in the former model, which should translate to significantly lower capital and operation cost for the former [5e7]. An economic and efficient on-site H2 production unit is thus considered as an enabling technology for global scale implementation of H2. Although H2 has very large energy per unit of mass of 120 MJ kg1, its energy per unit of volume is low since it exists as a gaseous phase at room temperature [8]. This makes storing H2 becomes challenging especially when high fuel density in the storage is desirable. H2 can instead be stored as ammonia, which can accommodate up to 17.65 wt% of H2 per unit weight of ammonia. NH3 has a large volumetric H2 density of 0.107 kg H2 L1 and can be liquefied at 1 MPa and 25 C, which enables easy transportation and storage [9,10]. Unlike H2, NH3 is difficult to ignite and has a characteristic odor that alerts the surroundings when it disperses in air [11]. As the second most produced chemical globally, the transportation and storage infrastructure and technology for NH3 have been established [9]. Hence, massive investments and technical efforts in the development of a new system for H2 transport and storage can be minimized. Moreover, the combustion of NH3 does not lead to greenhouse gas emission as it produces only nitrogen and water vapor and NH3 can be used as a fuel directly in various engines, e.g., diesel engine, fuel cell, spark engine, and gas turbine [12e14]. These great advantages propel NH3 as a promising and attractive H2 energy carrier for on-site H2 production. NH3 can be converted to H2 on-site via the catalystassisted decomposition reaction. Different catalysts can be utilized to decompose NH3 to H2. Ruthenium-based catalysts have been extensively used for this purpose but given its high cost, research on the alternative non-noble metal catalysts (e.g., Fe, Co, Ni, and MoC) has gained momentum [15]. Torrente-Murciano et al. [16] studied NH3 decomposition over microporous carbon-supported Co catalysts. Between 25 and 550 C, the smaller (2 nm) Co crystallites displayed higher catalytic activities relative to the larger (3e5 nm) ruthenium nanoparticles. Gong et al. [17] reported MoS2 ultrathin nanosheet-coated CeO2 hollow sphere catalysts with three-dimensional hierarchical structure, which displayed enhanced catalytic activity and NH3 conversion of up to 80% at
700 C. Ni is another most promising catalyst candidate for decomposition of NH3 to H2 given its abundant availability, low cost, and high catalytic activity. Various groups have studied the catalytic activity of Ni on different supports such as Al2O3 (72.1% NH3 conversion at 550 C), Ce0.8Zr0.2O2 (95.7% NH3 conversion at 550 C), and mica (97.2% NH3 conversion at 650 C) [18e20]. Ni catalysts supported on perovskite-type oxides (ABO3) displayed close to complete NH3 conversion at 700 C [21]. Among the studied perovskite compositions, BaZrO3 support resulted in the highest Ni catalytic activity and produced stable NH3 conversion of about 62% at 550 C over 35 h of operation. Following NH3 decomposition reaction, H2 separation from the other gases is required to obtain high purity H2. Hollow fiber membranes have become the spotlight of numerous studies particularly for H2-related reaction and separation application given their large surface area to volume ratio and large H2 flux [22e25]. Song et al. [2] reported H2 permeation flux improvement from 0.164 mL cm2 min1 to 0.269 mL cm2 min1 for BaCe0.85Tb0.05Co0.10O3-d hollow fiber membrane when the fiber surface was deposited with Ni catalyst. Liu et al. [26] studied H2 separation through Nd5.5Mo0.5W0.5O11.25d (NMW) U-shaped hollow fiber membrane that produced H2 permeation flux of 1.29 mL cm2 min1 at 975 C, using 80 vol% H2 in He feed and humidified Ar sweep. Instead of applying two separate reactor and purification units, membrane reactor has the capability to carry out simultaneous reaction and separation in a single unit. Membrane reactor offers several advantages such as spaceefficient, low energy consumption, and enhanced conversion and selectivity from the shifting of the equilibrium to the product side due to the simultaneous product removal, which makes it suitable for on-site portable H2 production [27e29]. Initial studies on NH3 decomposition using membrane reactors focused into supported palladium (Pd) membranes for H2 purification. Collins and Way [30] reported 94% NH3 conversion in Pd-alumina membrane packed with Ni/Al2O3 catalyst at 600 C and 1618 kPa. Identically high NH3 conversions were also found using other Pd-based membrane reactors packed with Na-promoted Ru catalysts that reach 100% conversion at 367 C [31] or Ni catalysts that provide 10% higher conversion than conventional packed bed reactor [32]. In departure from the supported Pd membrane direction, Li et al. [33] developed silica membranes for H2 separation. Their Rusoaked g-Al2O3/a-Al2O3 catalytic membranes produced 95.2% NH3 conversion over 50 h and had H2 permeance of 6.1$107 mol m2 s1 Pa1 with H2/N2 selectivity of 710. Lanthanide tungstates (Ln6WO12) have been viewed as one of the most promising membrane materials for H2 permeation given their simultaneously high mixed protonic-electronic conductivity and chemical stability, which lead to 100% H2 selectivity [34e39]. Notably, Nd5.5Mo0.5W0.5O11.25-d (NMW) membranes displayed the highest H2 fluxes (up to 0.3 mL cm2 min1) relative to the other mixed protonic-electronic conducting (MPEC) materials and operational stability in CO2 and H2S [35]. The high H2 fluxes in NMW membranes can be attributed to the predominant n-type electronic conduction above 700 C due to the partial substitution of W with Mo, which facilitates the formation of electrons [40,41]. This work features a dual-layer hollow fiber catalytic membrane reactor
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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for the on-site H2 production from NH3 decomposition that contains porous NMW-Ni layer and dense NMW layer. The mechanism of H2 permeation through the dual-layer NMW/NMW-Ni membrane is shown in Fig. 1. H2 initially dissociates into protons and electrons at the surface of the NMWNi support layer. Thereafter, the protons and the electrons pass through the membrane separately under the mixed conduction of NMW. Ni particles in the support layer accelerate the dissociation of H2 and the electron conduction. Finally, the protons and electrons recombine on the surface of NMW dense layer to form H2. NMW-Ni layer provides dual functions, i.e., as the catalytic reaction site for NH3 decomposition and as the membrane support layer while NMW layer serves as H2 selective membrane layer.
Table 1 e Synthesis parameter specifications for duallayer hollow fiber membrane precursor. Parameter
Composition of the starting solution (wt.%) NiO NMW PESf NMP PVP Spinning rate (mL min1) Air gap (cm) Internal and external coagulant
NMW (Dense outer NMW-NiO separation layer) (Porous inner catalytic layer)
0 75 5 20 0 1.5 10 De-ionized water
26.6 42.7 7.4 22.3 1 10
Experimental Preparation of the powders and the catalytic membranes Nd5.5Mo0.5W0.5O11.25-d (NMW) powder was synthesized using a conventional solid-state reaction method. Stoichiometric amounts of Nd2O3 (99.9%), WO3 (99.9%), and MoO3 (99.9%) were mixed and milled inside an agate container filled with zirconia balls using a planetary mill for 48 h. The resultant mixture was then calcined at 1100 C for 10 h to obtain the fluorite phase. To prepare the hollow fiber precursor for the subsequent phase inversion process, the calcined powder was ball milled again for 24 h and sieved through a 200-mesh sifter. The NMW powder was then mixed with NiO in a volume ratio of 1:1 to form NMW-NiO powder. The dual-layer NMW/NMW-NiO hollow fiber membranes were prepared by phase inversion and sintering technique, i.e., co-extrusion followed by co-sintering, as described elsewhere [42]. A homemade three-hole spinneret was used to spin the membrane precursor according to the synthesis parameters as specified in Table 1. This dual-layer hollow fiber was prepared via a single-step co-extrusion method using a spinneret that contains three concentric holes (Fig. 2). The internal coagulant, i.e., deionized water, flows out from the innermost hole while the porous support layer and the outermost dense layer are co-extruded by injection pump from the middle hole and the outer hole, respectively. Due to the presence of the porous support layer, the dense H2
separation layer can be made thinner to reduce the bulk diffusion resistance, which is important to obtain high H2 permeation flux through the membrane. This single-step method improves the membrane preparation efficiency relative to the tape casting or other deposition methods. The resultant hollow fiber membrane precursors were dried in air for more than 24 h at room temperature and then sintered at 1450 C for 5 h to ensure gas tightness. To reduce NiO to Ni, the 1450 C sintered dual-layer was heated in 40 vol% H2 in He at 700 C flowed at 50 mL min1 for 2 h.
Characterization The morphology and microstructure of the hollow fiber membrane were analyzed using scanning electron microscopy (SEM, FEISirion, Netherlands) and back scattered scanning electron microscopy (BSEM, NEON, 40EsB). The crystal structure of the hollow fiber membrane was determined using powder X-ray diffraction (XRD, Bruker D8 Advance) using Cu-Ka radiation with l of 0.15404 nm. The scan was performed from 2q of 20 e80 with a scanning step of 0.02 .
Membrane reactor test Fig. 3 shows the schematic diagram of the membrane reactor. Single 20 cm-length dual-layer hollow fiber membrane was positioned in the lumen of a quartz tube and tightened to the
Fig. 1 e Schematic illustration of NH3 decomposition and H2 permeation in dual-layer NMW/NMW-Ni hollow fiber membrane. Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 2 e Three-hole spinneret for hollow fiber spinning. (a) Side view of spinneret stack; (b) Top view of spinneret (Inset shows the three concentric holes that form the hollow fiber membrane); and (c) Inside view of spinneret.
Fig. 3 e Schematic diagram of membrane reactor setup for NH3 decomposition to H2.
quartz tube using two silicon rubber tubes at the opposite ends to allow flexible movement that offsets the thermal expansion under high temperature (above 650 C) permeation test condition. The reactor was set to operate inside the furnace with 5 cm effective heating length. H2eHe or NH3eHe mixture was fed to the membrane lumen side while N2 was introduced to the shell side as the sweep gas to collect the permeated H2. The retentate stream passed through water to remove unreacted NH3, followed by silica gel to remove excess water vapor. The composition of the permeate or retentate stream were analyzed at 25 C using a gas chromatograph (GC, Agilent 7890B) equipped with a carbon molecular sieve column (4 m 3 mm). The reaction is carried out at 1 atm and although higher pressure is beneficial for H2 separation, low pressure is more favorable for ammonia decomposition and incurs lower cost. As a comparison, reaction using a packedbed reactor version of Fig. 3 was also performed using the same amount of the same catalyst used in the dual-layer hollow fiber, i.e., 0.11 g of NMW-NiO (1:1 by volume) powder. Quartz tube with an identical inside diameter to the dual-layer hollow fiber was used to substitute the hollow fiber in packedbed reactor test. Two silica wools were fitted at the two opposing ends of quartz tube to restrain powder movement during the reaction.
NMW pattern exhibits characteristic peaks that can be indexed according to fluorite lattice with a F43m space group (#216) (PDF#00-022-0744) [43]. NMW-NiO pattern, on the other hand, contains characteristic peaks that can be attributed solely to the mixture of the characteristic peaks from NMW phase and the characteristics peaks from NiO phase (PDF#01078-0643) [43]. The absence of the new peaks other than the characteristic peaks of NMW and NiO following the
Results and discussion Hollow fiber phase structure and morphology Fig. 4 shows the powder X-ray diffraction (XRD) patterns of Nd5.5Mo0.5W0.5O11.2-d (NMW) and 1450 C sintered Nd5.5Mo0.5W0.5O11.2-d-NiO (1:1 by volume e NMW-NiO) hollow fibers.
Fig. 4 e Powder X-ray diffraction (XRD) patterns of Nd5.5Mo0.5W0.5O11.2-d (NMW) and 1450 C sintered Nd5.5Mo0.5W0.5O11.2-d-NiO (1:1 by volume e NMW-NiO) hollow fibers.
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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calcination of NMW-NiO powder mixture at 1450 C indicates the phase compatibility between NMW and NiO phases. The 1450 C sintered dual-layer membrane was further heated in 40 vol% H2 in He at 700 C flowed at 50 mL min1 for 2 h to reduce its NiO content to Ni. The reduction of NiO to Ni created additional porosity in the NMW-Ni layer with respect to the original NMW-NiO layer. Fig. 5 shows the morphology and microstructure of the resultant NMW/NMW-Ni hollow fiber with outer diameter of 1.54 ± 0.027 mm and inner diameter of 1.10 ± 0.045 mm. The asymmetric structure consists of dense NMW separation layer in the outer region and porous NMW-Ni catalytic support in the inner region of the hollow fiber (Fig. 5(a) and (b)). Porous Ni particles appear to be deposited on the cavities of the inner circumference surface of the fiber (indicated by red circles in Fig. 5(c)). The 26 mm-thick dense layer (Fig. 5(d)) of the dual-layer NMW/NMW-Ni hollow fiber across its cross-section is comparable to a 20-30 mm-thick dense La5.4WO12-d membrane layer supported by a 250350 mm-thick porous disk La5.4WO12-d made using sequential tape casting method [38]. Similar results were also reported for a 25 mm-thick dense La28-xW4þxO54þ3x/2 (x ¼ 1.22) membrane layer supported by a 1 mm-thick porous layer made using powder pressing method and subsequent dip coating [44]. For large-scale production, single spinning step carried
Fig. 5 e Scanning electron microscopy (SEM) images of 1450 C calcined dual-layer NMW/NMW-NiO hollow fiber membrane. (a) and (b) Cross-section of the hollow fiber at different magnifications; (c) Inner circumference surface of the hollow fiber; (d) Finger-like pores in porous layer shown in cross-section of the hollow fiber; (e) Outer circumference surface of the hollow fiber; and (f) Back scanning electron microscopy (BSEM) image on the inner circumference surface of the hollow fiber.
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Fig. 6 e H2 permeation fluxes of dual-layer NMW/NMW-Ni between 650 and 900 C (Feed gas was 50 mL min¡1 of 50 vol % H2 in He and sweep gas was 50 mL min¡1 of N2).
out using a three-hole spinneret in this work is more desirable over the lengthy preparation route that involves several deposition and sintering processes. The formation of fingerlike pores on the central region of the hollow fiber support section is attributed to the different diffusion and coagulation rates at different solvent/non-solvent/polymer interfaces during phase inversion process (Fig. 5(d)). Ni catalysts were dispersed on these finger-like pores, which provide the fiber with numerous micro reactor channels for NH3 decomposition (indicated by red arrows in Fig. 5(d)). Fig. 5(e) shows that the outer circumference surface of the fiber formed dense structure with varying grain size in the order of 2e10 mm. Backscattered scanning electron microscopy (BSEM) images on the inner surface of the fiber reveal the presence of two regions with different brightness where the brighter region represents NMW while the darker spot-like region represents
Fig. 7 e NH3 conversion of dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor and conventional packed-bed reactor between 600 and 750 C (Feed gas was 50 mL min¡1 of 20 vol % NH3 in He and sweep gas was 50 mL min¡1 of N2).
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 8 e NH3 conversion and H2 separation efficiency of dual-layer NMW/NMW-Ni hollow fiber membrane between 600 and 750 C for three different NH3 contents in the feed gas. Solid symbols correspond to NH3 conversion; hollow symbols correspond to H2 separation efficiency.
Ni (Fig. 5(f)). This is since the heavier NMW backscatter electrons more strongly than the lighter Ni.
Hydrogen permeation fluxes H2 permeation flux through dual-layer NMW/NMW-Ni hollow fiber increases with temperature rise between 650 and 900 C (Fig. 6). This is due to the enhancements of surface exchange kinetics and proton bulk diffusion rate at higher temperature. H2 flux reaches up to 0.26 mL cm2 min1 at the highest temperature tested of 900 C. The flux is significantly higher than 0.15 mL cm2 min1 reported for 0.9 mm-thick NMW discs, tested using 50 vol% H2 in He feed with both sides of the membrane humidified [35,40]. However, the flux is considerably lower than 0.6 mL cm2 min1 reported for U-shaped NMW hollow fiber at 900 C when 80 mL min1 of 50 vol% H2 in He was used as feed gas and 100 mL min1 of Ar was used as sweep gas [26]. The large discrepancy could be attributed to the difference in the feed and sweep flow rates, which produced larger H2 partial pressure difference driving force by efficient H2 permeate removal.
Membrane reactor performance Membrane reactor based on dual-layer NMW/NMW-Ni hollow fiber displays higher NH3 conversion relative to the packedbed reactor with the same volume (Fig. 7). At 750 C, for example, 99% conversion was attained for the membrane
reactor while only 75% conversion was achieved for the packed-bed reactor. Such higher conversion for membrane reactor occurs due to the simultaneous H2 removal from the reactor chamber, i.e., H2 permeates from the lumen side to the shell side. The simultaneous removal of H2 retains the hydrogen partial pressure difference on the feed and permeate sides and ensures the H2 permeation process persists along with the NH3 decomposition reaction. Additionally, while the amount of catalyst used in the packed-bed reactor is similar to that used in the catalytic membrane reactor, i.e., 0.11 g of NMW-NiO powder, the packed-bed reactor experiences uniform heating while the catalytic membrane reactor is limited by an effective heating length. Outside of this 5-cm effective heating length, other sections of the 20 cm-long hollow fiber may also contribute to the NH3 conversion of the catalytic membrane reactor, and thus increase the difference in NH3 conversion between the two reactors. The porous support of the dual-layer hollow fiber not only provides the mechanical support role to the dense selective layer but also serves as reaction sites given the presence of Ni particles on its surface. The homogeneous dispersion of Ni active sites along the gas channels facilitates the contact between NH3 and Ni, the subsequent decomposition of NH3 to H2, as well as the dissociation of H2 to protons, which then permeate through the dense NMW film to the other side of the fiber. The fast permeation of H2 through the hollow fiber reduces the H2 residence time on the Ni particles, and results in more available active catalytic sites for NH3 decomposition, which increases the NH3 conversion for the catalytic membrane reactor. Fig. 8 shows the NH3 conversion and H2 separation efficiency of dual-layer NMW/NMW-Ni hollow fiber membrane, the operating conditions of which are listed in Table 2. H2 separation efficiency (h) was calculated following Eq. (1). h¼
Qp ðCH2 bCHe Þ 100% Qp CH2 þ Qr C'H2
(1)
where Q p is the flow rate of the permeate stream (mL min1), Q r is the flow rate of the retentate stream excluding NH3 (mL min1), CH2 and C'H2 are the H2 content (%) in the permeate stream and the retentate stream, respectively, CHe is the leaked He content (%) from the sealing imperfections in permeate stream, and b is the ratio of H2 to He in the feed stream. Small amounts of He was detected in permeate stream (<0.33%), indicating minor leakage occurred during the permeation experiments. The amount of leaked H2 was deducted proportionally according to the analyzed amount of He leakage.
Table 2 e Operating conditions of the dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor. Feed stream
20 vol% NH3 in He 30 vol% NH3 in He 40 vol% NH3 in He
Feed stream flow rate (mL min1)
Sweep stream
Sweep stream flow rate (mL min1)
Temperature ( C)
Pressure (bar)
50
N2
50
600 650 700 750
1
Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 9 e NH3 conversion as a function of feed gas flow rate between 50 and 125 mL min¡1 in dual-layer NMW/NMWNi hollow fiber membrane reactor at 650 C (Feed gas composition was kept constant at 20 vol% of NH3 in He and sweep gas was 50 mL min¡1 of N2).
Since NH3 decomposition is an endothermic reaction, in all three cases, NH3 conversion increases with temperature rise due to increase in the reaction rate. Likewise, in all three cases, H2 separation efficiency also increases with temperature rise due to enhanced bulk-diffusion and surface exchange reaction at higher temperature. NH3 conversion is
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higher when lower amounts of NH3 are present in the feed stream since there are limited reaction sites in the membrane reactor. Larger amounts of unconverted NH3 flows out of the reactor when feed with higher NH3 content is introduced, resulting in lower conversion. H2 separation efficiency also decreases with increasing NH3 content, which likely indicates the lower H2 permeation rate relative to the rate of NH3 decomposition to H2. More H2 is produced when larger NH3 content is introduced into the feed. However, although more H2 permeated through the membrane, the amount of H2 that remained in the membrane also increased. As the NH3 content in the feed increased, the ratio of H2 produced by NH3 decomposition to the permeated H2 increased and the H2 separation efficiency decreased. Although the reported H2 separation efficiency was noticeably small, our report here aims mainly to improve the NH3 conversion through the use of the membrane reactor. At a constant NH3 content of 20 vol% in feed gas with the remaining made up of He, the NH3 conversion reduces with the increase of the feed flow rate (Fig. 9). For the hollow fiber membrane reactor, the factors affecting NH3 conversion are radial diffusion and residence time. Due to the small inner diameter of the hollow fiber (1.10 mm), the effect of radial diffusion on the NH3 conversion becomes negligible. Therefore, the NH3 conversion of the membrane reactor mainly depends on the residence time of NH3 in reactor. Hence, the results can be rationalized in terms of the reduced residence time of NH3 in the reactor since higher flow rate translates to higher space velocity. The higher the feed flow rate, the lower the NH3 residence time in the reactor and the smaller the NH3
Fig. 10 e NH3 conversion and H2 permeation flux of dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor obtained from the 75-h continuous test at 750 C (Feed gas was 50 mL min¡1 of 40 vol% NH3 in He and sweep gas was 50 mL min¡1 of N2). Red line corresponds to NH3 conversion; black line corresponds to H2 permeation flux. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Cheng H et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.101
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Fig. 11 e SEM images of dual-layer NMW/NMW-Ni hollow fiber membrane after the 75-h continuous test at 750 C. (a) and (b) Cross-section of the hollow fiber at different magnifications; and (c) Outer circumference surface of the hollow fiber.
conversion. This principle may also be applicable to conventional systems such as packed bed reactors whereby the feed gas flow rate is inversely proportional to the NH3 conversion. If a packed bed reactor with a smaller feed flow rate, or with a greater length is used, the NH3 conversion may end up higher or be on par with that of the catalytic membrane reactor at the same reaction conditions. Based on Fig. 9, the degree of NH3 conversion increases with the decrease in the feed flow rate. However, the H2 production efficiency of the reactor at lower feed flow rate is expected to be small, due to slower NH3 decomposition rate. Although it may be interesting to investigate the actual permeation behavior at low feed flow rates, it is recommended to improve the NH3 conversion at high feed flow rate in the future works to ensure the overall process efficiency.
homogeneously in the porous support. The reduction of NiO to Ni resulted into a highly porous support structure that provides high surface area for catalytic reaction. The fingerlike pores in the support that were filled with porous Ni particles serve as micro reactor channels for NH3 decomposition. Dual-layer NMW/NMW-Ni hollow fiber displayed H2 permeation flux of up to 0.26 mL cm2 min1 at 900 C when 50 mL min1 of 50 vol% H2 in He was used as feed gas and 50 mL min1 N2 was used as sweep gas. Membrane reactor based on this dual-layer hollow fiber achieved 99% NH3 conversion at 750 C, which was 24% higher relative to the packed-bed reactor with the same reactor volume. The membrane reactor also maintained very stable NH3 conversion and H2 permeation flux over 75-h continuous test at 750 C.
Membrane reactor operational stability Fig. 10 displays NH3 conversion and H2 permeation flux profiles of dual-layer NMW/NMW-Ni hollow fiber-based membrane reactor during the 75-h continuous test at 750 C when 50 mL min1 of 40 vol % NH3 in He and 50 mL min1 of N2 were used as feed gas and sweep gas, respectively. NH3 conversion and H2 permeation flux exhibit marginal fluctuations along median values of 91% and 0.12 mL cm2 min1; which indicates the long term structure stability of the dual-layer hollow fiber in NH3-containing gas atmosphere. Fig. 11 depicts SEM images of dual-layer NMW/NMW-Ni hollow fiber membrane after the 75-h test. The hollow fiber retains its original cross-sectional structure with no evidence of mechanical disintegration or cracks (Fig. 11(a) and (b)). The outer circumference surface of the fiber retains its original dense structure (Fig. 11(c)). This observation is consistent with the long term structure stability implied by membrane reactor performance over 75-h test.
Conclusions A dual-layer NMW/NMW-Ni hollow fiber catalytic membrane reactor was synthesized in single spinning step using a homemade three-hole spinneret for on-site H2 production via Ni catalyst-assisted NH3 decomposition. The 26 mm-thick NMW dense H2 separation layer integrated well with the NMW-Ni porous catalytic support. Ni catalyst was distributed
Acknowledgments The authors gratefully acknowledge the research funding provided by the National Natural Science Foundation of China (NSFC, No. 21476131, 21776166), the Natural Science Foundation of Shandong Province (ZR2014BM010, ZR2016JL011), Key Research and Development Program of Shandong Province (2017GGX70105), SDUT & Zibo City Integration Development Project (2017ZBXC107, 2017ZBXC137) and the Research Fund for the Doctoral Program of Higher Education of China (RFDP 20131201110007).
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