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Catalytic membrane reactors for tritium recovery from tritiated water in the ITER fuel cycle
Fusion Engineering and Design 49 – 50 (2000) 953 – 958 www.elsevier.com/locate/fusengdes
Catalytic membrane reactors for tritium recovery from tritiated water in the ITER fuel cycle S. Tosti a,*, V. Violante a, A. Basile b, G. Chiappetta b, S. Castelli c, M. De Francesco c, S. Scaglione c, F. Sarto c a
Associazione Euratom-ENEA Sulla Fusione, C.R. Frascati, Via E. Fermi 45, C.P. 65, 00044 Frascati, Rome, Italy b IRMERC-CNR, Uni6ersita` della Calabria, Arca6acata, CS, Italy c ENEA, C.R. Casaccia, Via Anguillarese, 301 -00060 S. Maria di Galeria, Rome, Italy
Abstract Palladium and palladium–silver permeators have been obtained by coating porous ceramic tubes with a thin metal layer. Three coating techniques have been studied and characterized: chemical electroless deposition (PdAg film thickness of 10 mm), ion sputtering (about 1 mm) and rolling of thin metal sheets (50 mm). The Pd-ceramic membranes have been used for manufacturing catalytic membrane reactors (CMR) for hydrogen and its isotopes recovering and purifying. These composite membranes and the CMR have been studied and developed for a closed-loop process with reference to the design requirements of the international thermonuclear experimental reactor (ITER) blanket tritium recovery system in the enhanced performance phase of operation. The membranes and CMR have been tested in a pilot plant equipped with temperature, pressure and flow-rate on-line measuring and controlling devices. The conversion value for the water gas shift reaction in the CMR has been measured close to 100% (always above the equilibrium one, 80% at 350°C): the effect of the membrane is very clear since the reaction is moved towards the products because of the continuous hydrogen separation. The rolled thin film membranes have separated the hydrogen from other gases with a complete selectivity and exhibited a slightly larger mass transfer resistance with respect to the electroless membranes. Preliminary tests on the sputtered membranes have also been carried out with a promising performance. Considerations on the use of different palladium alloy in order to improve the performances of the membranes in terms of permeation flux and mechanical strength, such as palladium/yttrium, are also reported. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Catalytic membrane reactors; International thermonuclear experimental reactor; Palladium
1. Introduction
* Corresponding author. Tel.: +39-6-94005160; fax: + 396-94005314. E-mail address: [email protected] (S. Tosti).
The use of the membrane technologies has been proposed for applications in the fusion reactor fuel cycle in order to reduce both the number of process units and operate in continuos mode [1– 4]. Therefore, several applications have been stud-
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 3 7 1 - 9
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S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
ied to apply the membrane technologies for process operations in the fusion fuel cycle in alternative to the traditional processes based on molecular sieves or cryogenics traps. For instance, the catalytic membrane reactors (CMR) technology, by coupling a selective membrane with a catalytic bed reactor, has been considered to be as a candidate technology to recover tritiated molecular hydrogen from tritiated water by the water gas shift reaction [5]: CO+ H2OUCO2 +H2 The effect of the membrane is very clear since the reaction is moved towards the products because of the continuous hydrogen separation; and values of the conversion close to 100%, well above the equilibrium, can be attained. A closed-loop process using a CMR for the water gas shift reaction has been studied for processing an amount of 1 mol/h of tritiated water collected as impurity in the cryotraps of the blanket tritium recovery system designed for the international thermonuclear experimental reactor (ITER) extended performance phase [6]. In this closed-loop process, the consumption of CO and the large production of CO2 wastes are avoided and the hydrogen is recovered through the shell side of the CMR by vacuum pumping or by means of a helium sweep stream.
2. Pd-ceramic composite membranes Pd-ceramic composite membranes are obtained by coating ceramic porous tubes with a thin palladium-based layer. Thin film membranes are more permeable to hydrogen because of the reduced thickness of their metallic layer and have a little geometrical constraints, as the structural strength is given by the support; therefore, they show a better behavior in comparison to conventional membrane permeators normally made by PdAg tubes having a thickness of more than 100 mm. For the purpose of applications in the fusion fuel cycle the membranes have to satisfy the requirements of both complete hydrogen selectivity and chemical stability in long term operations.
Our study has concerned with the study and the development of Pd-ceramic composite membranes obtained by three different techniques: electroless deposition, sputtering and rolling of thin foils. Composite membranes have been obtained by an electroless plating deposition of Pd and Ag layers on the asymmetrical alumina porous tubes with a diameter 7/10 mm and a pore size of 12 and 0.5 mm in bulk and external surface (top layer), respectively. A final heat treatment ensured the achievement of the uniform PdAg alloy (silver content 23–25 at.%) into the film, of a thickness about 10 mm. An improvement in the deposition technique is still required, since some microscopic defects (micro-holes) have been found on metallic surfaces, although the deposited film is sticking to the support [7]. Moreover, by using a dual ion beam sputtering the PdAg (silver content 23–25 at.%) coating of porous ceramic tubes has been carried out: thinner metal layer and a stronger adhesion of the metal over the ceramic support have been attained as compared with the electroless membranes. Samples of composite sputtered membranes have been produced with PdAg layers of 0.5–5 mm, supported by the above described asymmetrical alumina tubes. Finally, thin rolled Pd-based membranes (50– 70 mm) have also been produced in order to have a metallic layer thickness enough to avoid the defects of metallic layer and ensure a complete selectivity for hydrogen without giving large mass transfer resistance with respect to the electroless and sputtered membranes. For this, Pd and PdAg (silver content 23–25 at.%) foils have been coldrolled by a two-high laboratory mill [8]. Several steps of cold rolling and annealing of the palladium and palladium silver foils have been needed to relieve the mechanical stresses due to the work hardening upon rolling, particularly for the alloyed palladium. The annealing of the metal palladium and alloy foils have been carried out by heat treatment in a furnace at 1200°C, at atmospheric pressure for 2–3 h under Ar atmosphere to avoid the oxidation and tarnishing of the palladium surfaces. Then, the metallic foils of thickness up to 50 mm have been closed by TIG arc welding around a ceramic porous tube of 12 mm
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
pore size, without top layer, having the main function of separating the Pd membrane from the catalyst bed of the CMR; permeating tubes of 150 mm length and internal diameter of 10 mm have thus been obtained. Even though these rolled membranes have a reduced thickness with respect to the commercial ones, they are flawless: a complete selectivity for hydrogen and stability has been observed over many weeks of experimental operation.
3. Membranes testing apparatus A plant equipped with temperature, pressure and flow-rate on-line measuring and controlling devices have been arranged for both membrane and catalytic membrane reactor characterizing and testing. The composite membranes have been characterized by measuring the permeated flux (of hydrogen or inert gas) under a controlled transmembrane differential pressure. The mass transfer resistances through the gaseous film, the ceramic support and the metal layer control the gas flux through the composite membrane [9]. In the absence of an inert flux gas through the membrane the permeator is defined permselective for hydrogen [10]: the controlling mass transfer mechanism is the permeation through the metal layer (Sievert law, permeability measured in m3/m s Pa0.5). Conversely, for non-permselective membranes, a separation factor is defined as the ratio between the permeation fluxes of hydrogen and an inert gas (nitrogen) under the same differential pressure. In this case the controlling mass transfer mechanism is the permeation through the ceramic support and the metal pores (Knudsen diffusion, permeability measured in m3/m s Pa). The CMR have been realized by filling the composite membrane tube with a catalyst for the water gas shift reaction (obtained for research activity from Haldor Topsoe, DK). An o-ring system has been used to seal the membrane to the reactor [8]. Then the CMR have been characterized by feeding a CO/H2O mixture and recovering the hydrogen in the shell side by vacuum pumping or by means of a inert gas purge stream. A gas
955
chromatographic analysis has been carried out on the gas stream entering and leaving both the tubular membrane and the reactor shell in order to measure the composition of the gas stream and evaluate the reaction conversion in the catalytic membrane reactor. The experimental apparatus consists of two mass flow controllers at the inlet of the membrane tube (feed side) and shell side, three pressure gages at inlet and outlet of the membrane tube and at shell outlet, several thermocouples inside the reactor and in circuit line, two heating systems for controlling the membrane tube temperature and vaporizing the water fed inside the CMR, see Fig. 1.
4. Results The composite Pd-ceramic membranes have been tested to evaluate their selectivity to the hydrogen, permeation fluxes values and durability.
4.1. Electroless and sputtered membranes The electroless membranes have shown the absence of any significant defects in the metallic layer: however, the pinhole formation gives an incomplete selectivity and a limited durability. As a consequence, a separation factor between hydrogen and an inert gas (nitrogen and argon) has been observed, ranging between 10 and 100. At room temperature and in a pressure range 1× 105 – 1.5× 105 Pa the permeability of nitrogen through the asymmetrical ceramic support is 0.5×10 − 9 m3/m s Pa. At 300°C through the electroless composite membrane the permeability measured with nitrogen is 9.56× 10 − 12 m3/m s Pa. Specimens of composite sputtered membranes have been produced with PdAg layers up to 5 mm thickness. These metal coatings have shown a good adhesion to the support tube, but the pores of the ceramic have not been closed. In preliminary tests, these composite membranes have given a low separation H2/N2 ratio and permeability values comparable with one of the ceramic support tubes.
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
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Tests on CMR for the water gas shift have been also performed: in agreement with the theoretical assessments carried out by a computer code developed for modeling Pd-ceramic membranes and CMR [9], the membrane promotes the reaction conversion by continuously separating hydrogen. CMR with electroless membrane have exhibited conversion values above the equilibrium one, 80% at 350°C, also in the absence of any complete hydrogen selectivity of the electroless membrane.
was 3.05 × 10 − 5 and 7.13 ×10 − 4 mol/s with a CO/H2O stoichiometric composition. A sweep stream of nitrogen has been sent in the strip side at atmospheric pressure. The internal pressure was 1.1 × 105 Pa. For the tested CMR, Fig. 2 shows the reaction yield calculated as CO conversion versus the nitrogen strip flow-rate: the results exhibit the reaction conversion arising up to 100% by increasing the strip flow-rate. Due to the catalyst characteristics, the reaction yield optimum has been obtained at 330°C.
4.2. Rolled membranes 4.3. PdY alloy membranes Composite Pd rolled membranes of 55–60 mm of thickness at 350°C during several weeks of tests have shown a permeability value of the metal layer of 2.6 ×10 − 10 m3/m s Pa0.5. Seal tests with inert gas have shown the complete hydrogen selectivity of the rolled membrane and the gas-tight between the membrane and the reactor shell. The tests on a prototype of CMR with rolled membrane have been carried out at 331°C (cocurrent and counter-current mode) and 350°C (counter-current mode). The feed flow-rate range
Several studies have been reported the use of palladium yttrium alloy membranes for hydrogen separating and purifying [11–13]. As matter of fact, the use of yttrium as an alloying element (B 10 at%) in the palladium membranes is likely to increase both the hydrogen permeability and the mechanical strength. Pd–Y membranes of 100 mm thickness have shown at 600 K with 6.8 bar of differential pressure, permeability values higher than Pd-25%Ag by a factor 2 for Pd-6.6%Y and
Fig. 1. Scheme of the experimental apparatus for testing membranes and CMR.
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
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Fig. 2. CMR with rolled thin membrane, CO conversion vs. the N2 strip flow-rate, (1) equilibrium value at 350°C; (2) equilibrium value at 331°C; (3) co-current with feed flow-rate 7.13 × 10 − 4 mol/s at 350°C; (4) counter-current with feed flow-rate 3.05 × 10 − 5 mol/s at 331°C; (5) co-current with feed flow-rate 3.05 × 10 − 5 mol/s at 331. Table 1 Evaluated hydrogen permeation flux through the PdAg and PdY composite membranes Composite membrane
Permeation flux (cm3/s)
Specific permeation flux (cm3/(cm2 s))
Electroless: PdAg film 10 mm thick, ceramic tube pore size 12 mm (bulk) and 0.5 mm (top layer) Sputtered: PdAg film 1 mm thick, ceramic tube pore size 12 mm (bulk) and 0.5 mm (top layer) Rolled: PdAg 50 mm thick, ceramic tube pore size 12 mm Rolled: PdY 6.6 at.%, 50 mm thick, ceramic tube pore size 12 mm
59.97
6.363×10−1
74.36
7.890×10−1
36.16 51.76
3.837×10−1 5.492×10−1
2.3 for Pd-10%Y [11]. Thus, the Pd – Y membrane permeators could still be resulted in thinner and more permeable to hydrogen than the palladium– silver ones. By using the above quoted computer code, the hydrogen permeation fluxes through electroless, sputtered and rolled permselective tubes, length 300 mm, internal diameter 10 mm, operating with a DP (differential hydrogen pressure through the membrane) of 0.2 MPa at 300°C have been comparatively evaluated. In Table 1, the hydrogen flux through the permeation surface (94.25 cm2) is reported. The rolled PdY tube of 50 mm thickness shows a hydrogen permeation flux comparable with the electroless one. Moreover, the complete selectivity of the rolled membrane with respect to the electroless has to be considered.
5. Conclusions Pd-ceramic composite membranes obtained by electroless, sputtering and rolling techniques have been studied and developed for the hydrogen and its isotopes separation and purification. Catalytic membrane reactors have been realized by using these composite membranes for the purpose of the tritium recovery from tritiated water by the water gas shift reaction. The electroless membranes have shown high hydrogen permeation flux and a significant separation factor between hydrogen and other gases. However, the actual quality of these membranes is not suitable for nuclear applications: because of the presence of surface defects (micro-holes) and not good durability, these membranes are not
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S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
permselective for hydrogen. Future study on this coating technique foresees the improvements in the quality of the thin metal layer on ceramic support. Conversely, the rolled membranes have separated the hydrogen from other gases with complete selectivity, showing a slightly larger mass transfer resistance with respect to the electroless membranes. Thus, the rolled membranes are adequate for applications in the fusion fuel cycle as well as in the industrial processes where high pure hydrogen is required (i.e. hydrocarbon reforming for fuel cells). The CMR have shown conversion values for the water gas shift reaction close to 100% (always above the equilibrium one at 350°C, 80%): the effect of the membrane is very clear, since the reaction is moved towards the products because of the continuous hydrogen separation. Moreover, an analysis of the use of membranes of PdY instead of the PdAg has demonstrated the possibility to increase the performances of the
membranes in terms of higher hydrogen permeation flux and mechanical strength.
References [1] R.D. Penzhorn, et al., Fusion Tech. 14 (1988) 450 –455. [2] A. Iasonna, et al., Proceedings of the 16th SOFT, 1 (1990) 723 – 727. [3] V. Violante, et al., Fusion Eng. Des. 22 (1993) 257–263. [4] H. Yoshida, et al., Nucl. Tech./Fusion 3 (1983) 471–484. [5] V. Violante, et al., Fusion Eng. Des. 30 (1995) 217–223. [6] S. Tosti, et al., 20th SOFT, September 7 – 11, 1998, Fusion Tech. 2, 1033 – 1036. [7] S. Castelli, et al., 20th SOFT, September 7 – 11, 1998, Fusion Tech. 2, 993 – 996. [8] S. Tosti, et al., Rolled thin Pd and PdAg membranes for hydrogen separation and production, Int. J. Hydrogen Energy, in press. [9] S. Tosti, et al., 19th SOFT, September 16 – 20, Lisbon, Fusion Tech. (1996) 1209 – 1212. [10] J.N. Armor, J. Membrane Sci. 147 (1998) 217 – 233. [11] D. Fort, et al., J. Less-Common Metals 39 (1975) 293–308. [12] D. Fort, I.R. Harris, J. Less-Common Metals 41 (1975) 313 – 327. [13] J. Shu, et al., Can. J. Chem. Eng. 69 (1991) 1036 –1060.
Catalytic membrane reactors for tritium recovery from tritiated water in the ITER fuel cycle S. Tosti a,*, V. Violante a, A. Basile b, G. Chiappetta b, S. Castelli c, M. De Francesco c, S. Scaglione c, F. Sarto c a
Associazione Euratom-ENEA Sulla Fusione, C.R. Frascati, Via E. Fermi 45, C.P. 65, 00044 Frascati, Rome, Italy b IRMERC-CNR, Uni6ersita` della Calabria, Arca6acata, CS, Italy c ENEA, C.R. Casaccia, Via Anguillarese, 301 -00060 S. Maria di Galeria, Rome, Italy
Abstract Palladium and palladium–silver permeators have been obtained by coating porous ceramic tubes with a thin metal layer. Three coating techniques have been studied and characterized: chemical electroless deposition (PdAg film thickness of 10 mm), ion sputtering (about 1 mm) and rolling of thin metal sheets (50 mm). The Pd-ceramic membranes have been used for manufacturing catalytic membrane reactors (CMR) for hydrogen and its isotopes recovering and purifying. These composite membranes and the CMR have been studied and developed for a closed-loop process with reference to the design requirements of the international thermonuclear experimental reactor (ITER) blanket tritium recovery system in the enhanced performance phase of operation. The membranes and CMR have been tested in a pilot plant equipped with temperature, pressure and flow-rate on-line measuring and controlling devices. The conversion value for the water gas shift reaction in the CMR has been measured close to 100% (always above the equilibrium one, 80% at 350°C): the effect of the membrane is very clear since the reaction is moved towards the products because of the continuous hydrogen separation. The rolled thin film membranes have separated the hydrogen from other gases with a complete selectivity and exhibited a slightly larger mass transfer resistance with respect to the electroless membranes. Preliminary tests on the sputtered membranes have also been carried out with a promising performance. Considerations on the use of different palladium alloy in order to improve the performances of the membranes in terms of permeation flux and mechanical strength, such as palladium/yttrium, are also reported. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Catalytic membrane reactors; International thermonuclear experimental reactor; Palladium
1. Introduction
* Corresponding author. Tel.: +39-6-94005160; fax: + 396-94005314. E-mail address: [email protected] (S. Tosti).
The use of the membrane technologies has been proposed for applications in the fusion reactor fuel cycle in order to reduce both the number of process units and operate in continuos mode [1– 4]. Therefore, several applications have been stud-
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 3 7 1 - 9
954
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
ied to apply the membrane technologies for process operations in the fusion fuel cycle in alternative to the traditional processes based on molecular sieves or cryogenics traps. For instance, the catalytic membrane reactors (CMR) technology, by coupling a selective membrane with a catalytic bed reactor, has been considered to be as a candidate technology to recover tritiated molecular hydrogen from tritiated water by the water gas shift reaction [5]: CO+ H2OUCO2 +H2 The effect of the membrane is very clear since the reaction is moved towards the products because of the continuous hydrogen separation; and values of the conversion close to 100%, well above the equilibrium, can be attained. A closed-loop process using a CMR for the water gas shift reaction has been studied for processing an amount of 1 mol/h of tritiated water collected as impurity in the cryotraps of the blanket tritium recovery system designed for the international thermonuclear experimental reactor (ITER) extended performance phase [6]. In this closed-loop process, the consumption of CO and the large production of CO2 wastes are avoided and the hydrogen is recovered through the shell side of the CMR by vacuum pumping or by means of a helium sweep stream.
2. Pd-ceramic composite membranes Pd-ceramic composite membranes are obtained by coating ceramic porous tubes with a thin palladium-based layer. Thin film membranes are more permeable to hydrogen because of the reduced thickness of their metallic layer and have a little geometrical constraints, as the structural strength is given by the support; therefore, they show a better behavior in comparison to conventional membrane permeators normally made by PdAg tubes having a thickness of more than 100 mm. For the purpose of applications in the fusion fuel cycle the membranes have to satisfy the requirements of both complete hydrogen selectivity and chemical stability in long term operations.
Our study has concerned with the study and the development of Pd-ceramic composite membranes obtained by three different techniques: electroless deposition, sputtering and rolling of thin foils. Composite membranes have been obtained by an electroless plating deposition of Pd and Ag layers on the asymmetrical alumina porous tubes with a diameter 7/10 mm and a pore size of 12 and 0.5 mm in bulk and external surface (top layer), respectively. A final heat treatment ensured the achievement of the uniform PdAg alloy (silver content 23–25 at.%) into the film, of a thickness about 10 mm. An improvement in the deposition technique is still required, since some microscopic defects (micro-holes) have been found on metallic surfaces, although the deposited film is sticking to the support [7]. Moreover, by using a dual ion beam sputtering the PdAg (silver content 23–25 at.%) coating of porous ceramic tubes has been carried out: thinner metal layer and a stronger adhesion of the metal over the ceramic support have been attained as compared with the electroless membranes. Samples of composite sputtered membranes have been produced with PdAg layers of 0.5–5 mm, supported by the above described asymmetrical alumina tubes. Finally, thin rolled Pd-based membranes (50– 70 mm) have also been produced in order to have a metallic layer thickness enough to avoid the defects of metallic layer and ensure a complete selectivity for hydrogen without giving large mass transfer resistance with respect to the electroless and sputtered membranes. For this, Pd and PdAg (silver content 23–25 at.%) foils have been coldrolled by a two-high laboratory mill [8]. Several steps of cold rolling and annealing of the palladium and palladium silver foils have been needed to relieve the mechanical stresses due to the work hardening upon rolling, particularly for the alloyed palladium. The annealing of the metal palladium and alloy foils have been carried out by heat treatment in a furnace at 1200°C, at atmospheric pressure for 2–3 h under Ar atmosphere to avoid the oxidation and tarnishing of the palladium surfaces. Then, the metallic foils of thickness up to 50 mm have been closed by TIG arc welding around a ceramic porous tube of 12 mm
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
pore size, without top layer, having the main function of separating the Pd membrane from the catalyst bed of the CMR; permeating tubes of 150 mm length and internal diameter of 10 mm have thus been obtained. Even though these rolled membranes have a reduced thickness with respect to the commercial ones, they are flawless: a complete selectivity for hydrogen and stability has been observed over many weeks of experimental operation.
3. Membranes testing apparatus A plant equipped with temperature, pressure and flow-rate on-line measuring and controlling devices have been arranged for both membrane and catalytic membrane reactor characterizing and testing. The composite membranes have been characterized by measuring the permeated flux (of hydrogen or inert gas) under a controlled transmembrane differential pressure. The mass transfer resistances through the gaseous film, the ceramic support and the metal layer control the gas flux through the composite membrane [9]. In the absence of an inert flux gas through the membrane the permeator is defined permselective for hydrogen [10]: the controlling mass transfer mechanism is the permeation through the metal layer (Sievert law, permeability measured in m3/m s Pa0.5). Conversely, for non-permselective membranes, a separation factor is defined as the ratio between the permeation fluxes of hydrogen and an inert gas (nitrogen) under the same differential pressure. In this case the controlling mass transfer mechanism is the permeation through the ceramic support and the metal pores (Knudsen diffusion, permeability measured in m3/m s Pa). The CMR have been realized by filling the composite membrane tube with a catalyst for the water gas shift reaction (obtained for research activity from Haldor Topsoe, DK). An o-ring system has been used to seal the membrane to the reactor [8]. Then the CMR have been characterized by feeding a CO/H2O mixture and recovering the hydrogen in the shell side by vacuum pumping or by means of a inert gas purge stream. A gas
955
chromatographic analysis has been carried out on the gas stream entering and leaving both the tubular membrane and the reactor shell in order to measure the composition of the gas stream and evaluate the reaction conversion in the catalytic membrane reactor. The experimental apparatus consists of two mass flow controllers at the inlet of the membrane tube (feed side) and shell side, three pressure gages at inlet and outlet of the membrane tube and at shell outlet, several thermocouples inside the reactor and in circuit line, two heating systems for controlling the membrane tube temperature and vaporizing the water fed inside the CMR, see Fig. 1.
4. Results The composite Pd-ceramic membranes have been tested to evaluate their selectivity to the hydrogen, permeation fluxes values and durability.
4.1. Electroless and sputtered membranes The electroless membranes have shown the absence of any significant defects in the metallic layer: however, the pinhole formation gives an incomplete selectivity and a limited durability. As a consequence, a separation factor between hydrogen and an inert gas (nitrogen and argon) has been observed, ranging between 10 and 100. At room temperature and in a pressure range 1× 105 – 1.5× 105 Pa the permeability of nitrogen through the asymmetrical ceramic support is 0.5×10 − 9 m3/m s Pa. At 300°C through the electroless composite membrane the permeability measured with nitrogen is 9.56× 10 − 12 m3/m s Pa. Specimens of composite sputtered membranes have been produced with PdAg layers up to 5 mm thickness. These metal coatings have shown a good adhesion to the support tube, but the pores of the ceramic have not been closed. In preliminary tests, these composite membranes have given a low separation H2/N2 ratio and permeability values comparable with one of the ceramic support tubes.
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
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Tests on CMR for the water gas shift have been also performed: in agreement with the theoretical assessments carried out by a computer code developed for modeling Pd-ceramic membranes and CMR [9], the membrane promotes the reaction conversion by continuously separating hydrogen. CMR with electroless membrane have exhibited conversion values above the equilibrium one, 80% at 350°C, also in the absence of any complete hydrogen selectivity of the electroless membrane.
was 3.05 × 10 − 5 and 7.13 ×10 − 4 mol/s with a CO/H2O stoichiometric composition. A sweep stream of nitrogen has been sent in the strip side at atmospheric pressure. The internal pressure was 1.1 × 105 Pa. For the tested CMR, Fig. 2 shows the reaction yield calculated as CO conversion versus the nitrogen strip flow-rate: the results exhibit the reaction conversion arising up to 100% by increasing the strip flow-rate. Due to the catalyst characteristics, the reaction yield optimum has been obtained at 330°C.
4.2. Rolled membranes 4.3. PdY alloy membranes Composite Pd rolled membranes of 55–60 mm of thickness at 350°C during several weeks of tests have shown a permeability value of the metal layer of 2.6 ×10 − 10 m3/m s Pa0.5. Seal tests with inert gas have shown the complete hydrogen selectivity of the rolled membrane and the gas-tight between the membrane and the reactor shell. The tests on a prototype of CMR with rolled membrane have been carried out at 331°C (cocurrent and counter-current mode) and 350°C (counter-current mode). The feed flow-rate range
Several studies have been reported the use of palladium yttrium alloy membranes for hydrogen separating and purifying [11–13]. As matter of fact, the use of yttrium as an alloying element (B 10 at%) in the palladium membranes is likely to increase both the hydrogen permeability and the mechanical strength. Pd–Y membranes of 100 mm thickness have shown at 600 K with 6.8 bar of differential pressure, permeability values higher than Pd-25%Ag by a factor 2 for Pd-6.6%Y and
Fig. 1. Scheme of the experimental apparatus for testing membranes and CMR.
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
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Fig. 2. CMR with rolled thin membrane, CO conversion vs. the N2 strip flow-rate, (1) equilibrium value at 350°C; (2) equilibrium value at 331°C; (3) co-current with feed flow-rate 7.13 × 10 − 4 mol/s at 350°C; (4) counter-current with feed flow-rate 3.05 × 10 − 5 mol/s at 331°C; (5) co-current with feed flow-rate 3.05 × 10 − 5 mol/s at 331. Table 1 Evaluated hydrogen permeation flux through the PdAg and PdY composite membranes Composite membrane
Permeation flux (cm3/s)
Specific permeation flux (cm3/(cm2 s))
Electroless: PdAg film 10 mm thick, ceramic tube pore size 12 mm (bulk) and 0.5 mm (top layer) Sputtered: PdAg film 1 mm thick, ceramic tube pore size 12 mm (bulk) and 0.5 mm (top layer) Rolled: PdAg 50 mm thick, ceramic tube pore size 12 mm Rolled: PdY 6.6 at.%, 50 mm thick, ceramic tube pore size 12 mm
59.97
6.363×10−1
74.36
7.890×10−1
36.16 51.76
3.837×10−1 5.492×10−1
2.3 for Pd-10%Y [11]. Thus, the Pd – Y membrane permeators could still be resulted in thinner and more permeable to hydrogen than the palladium– silver ones. By using the above quoted computer code, the hydrogen permeation fluxes through electroless, sputtered and rolled permselective tubes, length 300 mm, internal diameter 10 mm, operating with a DP (differential hydrogen pressure through the membrane) of 0.2 MPa at 300°C have been comparatively evaluated. In Table 1, the hydrogen flux through the permeation surface (94.25 cm2) is reported. The rolled PdY tube of 50 mm thickness shows a hydrogen permeation flux comparable with the electroless one. Moreover, the complete selectivity of the rolled membrane with respect to the electroless has to be considered.
5. Conclusions Pd-ceramic composite membranes obtained by electroless, sputtering and rolling techniques have been studied and developed for the hydrogen and its isotopes separation and purification. Catalytic membrane reactors have been realized by using these composite membranes for the purpose of the tritium recovery from tritiated water by the water gas shift reaction. The electroless membranes have shown high hydrogen permeation flux and a significant separation factor between hydrogen and other gases. However, the actual quality of these membranes is not suitable for nuclear applications: because of the presence of surface defects (micro-holes) and not good durability, these membranes are not
958
S. Tosti et al. / Fusion Engineering and Design 49–50 (2000) 953–958
permselective for hydrogen. Future study on this coating technique foresees the improvements in the quality of the thin metal layer on ceramic support. Conversely, the rolled membranes have separated the hydrogen from other gases with complete selectivity, showing a slightly larger mass transfer resistance with respect to the electroless membranes. Thus, the rolled membranes are adequate for applications in the fusion fuel cycle as well as in the industrial processes where high pure hydrogen is required (i.e. hydrocarbon reforming for fuel cells). The CMR have shown conversion values for the water gas shift reaction close to 100% (always above the equilibrium one at 350°C, 80%): the effect of the membrane is very clear, since the reaction is moved towards the products because of the continuous hydrogen separation. Moreover, an analysis of the use of membranes of PdY instead of the PdAg has demonstrated the possibility to increase the performances of the
membranes in terms of higher hydrogen permeation flux and mechanical strength.
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