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Processing test of an upgraded mechanical design for PERMCAT reactor
Fusion Engineering and Design 85 (2010) 2171–2175
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Processing test of an upgraded mechanical design for PERMCAT reactor Fabio Borgognoni a,∗ , David Demange b , Lothar Dörr b , Silvano Tosti a , Stefan Welte b a b
Associazione ENEA-Euratom sulla Fusione, C.R. ENEA Frascati, Via E. Fermi 45, Frascati, Roma I-00044, Italy Forschungszentrum Karlsruhe GmbH, Institute for Technical Physics, Tritium Laboratory Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany
a r t i c l e
i n f o
Article history: Available online 3 November 2010 Keywords: Pd membranes Isotope exchange reactions Plasma exhausts clean-up
a b s t r a c t The PERMCAT membrane reactor is a coaxial combination of a Pd/Ag permeator membrane and a catalyst bed. This device has been proposed for processing fusion reactor plasma exhaust gas. A stream containing tritium (up to 1% of tritium in different chemical forms such as water, methane or molecular hydrogen) is decontaminated in the PERMCAT by counter-current isotopic swamping with protium. Different mechanical designs of the membrane reactor have been proposed to improve robustness and lifetime. The ENEA membrane reactor uses a permeator tube with a length of about 500 mm produced via cold-rolling and diffusion welding of Pd/Ag thin foils: two stainless steel pre-tensioned bellows have been applied to the Pd/Ag tube in order to avoid any significant compressive and bending stresses due to the permeator tube elongation consequent to the hydrogen uptake. An experimental test campaign has been performed using this reactor in order to assess the influence of different operating parameters and to evaluate the overall performance (decontamination factor). Tests have been carried out on two reactor prototypes: a defect-free membrane with complete (infinite) hydrogen selectivity and not perm-selective membrane. In this last case, the study has been aimed at verifying the behaviour of the PERMCAT devices under non-normal (accidental) conditions in the view of providing information for future safety analysis. The paper will present the specific mechanical design and the experimental results of tests based on isotopic exchange between H2 O and D2 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction Tritium has to be recovered from contaminated gases for both safety and fuel economy reasons. Particularly, tritium is to be recovered from gaseous mixtures coming from the Tokamak Exhaust Processing (TEP) system of fusion reactor machines which use DT mixture. This policy is in accordance to the As Low As Reasonably Achievable (ALARA) principle in order to minimise the tritium in effluents and releases [1–3]. At Tritium Laboratory Karlsruhe (TLK) a three stage process, socalled CAPER, has been developed in order to support the design of the TEP system of ITER providing a semi-technical scale experimental results [4]. The impurity separation is the first step of CAPER, where a Pd/Ag permeator battery recovers more than 95% of the un-burnt DT fuel from the impurities (helium, hydrocarbons and water). During the second step, the impurity processing, the tritium is separated from the impurities (tritiated water or tritiated hydrocarbons) in a close loop where catalyzed cracking or conversion reactions take place.
∗ Corresponding author. Tel.: +39 0694005560; fax: +39 0694005147. E-mail address: [email protected] (F. Borgognoni). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.08.022
Especially the tritium is recovered by permeation using Pd/Ag permeators. The third step, the final clean-up, provides to recover almost all of the residual tritium by the so-called permeator catalyst reactor (PERMCAT) [5]. This special device is a direct combination of a Pd/Ag membrane with a catalyst bed and has been developed for the final clean-up of gases containing up to about 1% of tritium in different chemical forms such as water, hydrocarbons or molecular hydrogen isotopes. In particular the PERMCAT module has demonstrated to have very higher performance when compared to a system consisting of a traditional reactor plus a permeator. Particularly, the membrane reactor permits to optimise the tritium removal process, to keep the load to the Isotope Separation System of ITER reasonably low and to minimise at the same time memory effects by segregating contaminated parts from non-contaminated parts of the unit. In the PERMCAT reactor the tritium is removed from contaminated gases by isotopic swamping which operates in counter-current flow mode: in such a way, it is possible to obtain a very low tritium activity at the outlet of the component to be maintained [6,7]. Due to the thermal expansion and the hydrogen uptake in the Pd alloy lattice, the Pd/Ag membrane can elongate up to 1.5–2% of his length [8,9]. Therefore, it is essential that the mechanical design
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F. Borgognoni et al. / Fusion Engineering and Design 85 (2010) 2171–2175
Table 1 ENEA PERMCAT reactors mechanical design: geometrical features. Metal bellows supported Pd/ Ag thin membranes Thickness (m) Length (mm) Outer Diameter (mm) Permeation area (cm2 ) Mass of catalyst (g) Permeation area/catalyst (cm2 /g)
50 516 6.00 97.3 18.0 5.4
of the PERMCAT module should consider this behaviour in order to prevent improper mechanical constraints that could cause the failure of the membrane reactor. TLK has tested different designs of PERMCAT reactors (finger like, metal bellow and corrugated membrane configurations) in order to compensate the length variation of the membrane due to the uploading of hydrogen isotopes into the metal lattice [8]. The finger like configuration of the membrane tube has been adopted as first generation of PERMCAT units. In this design, only one end of the membrane is tightly fixed at the shell module: the other end, closed, is free to elongate during the operative conditions. A capillary inner stainless steel tube is required inside the membrane to remove the retentate gas stream [8]. An additional PERMCAT module, adopting a further new mechanical design has been developed by ENEA Frascati Laboratories. This PERMCAT module contains permeator tubes produced by a special procedure of cold-rolling and diffusion welding of Pd/Ag foils [10,11]. These Pd/Ag tubes have a wall thickness (0.050 mm) thinner compared with the commercial ones (0.100–0.150 mm). Especially, as a consequence of the reduced thickness of the permeator tubes, a particular configuration of the membrane module has to be designed in order to ensure a long life of the thin wall membrane tube. As a matter of fact, the thermal and hydrogenation cycling of the thin wall and long (over 500 mm) membrane tube involves elongations/contractions which may produce the failure of the membrane. In order to avoid the rising of combined compressive and bending stress of these very thin permeators, between the permeator and the shell module two pre-tensioned metal bellows have been inserted. These pre-tensioned bellows maintains the membrane tube always tensioned or at least unstressed (never compressed) during operation [12]. This paper describes the experimental test campaign aimed to assess, in terms of decontamination factor, the performances of the PERMCAT module developed by ENEA. Especially, the experiments have tested two membrane tubes: both a defect-free membrane tube and a not completely selective to hydrogen (i.e. simulating the presence of membrane defects or module leakages). Particularly, the study of not perm-selective Pd/Ag membrane could be relevant from the safety point of view: in fact, under such an accidental condition traces of water could pass into the gas stream which has to be sent to the cryo-distillation. 2. Experimental The PERMCAT process is aimed to remove tritium chemically bound in molecules. In this experimental work, deuterium has been used instead of tritium: in particular, water (vapour) has been fuelled into the membrane lumen while deuterium has been fed in the shell side of the membrane reactor in counter-current mode. The isotope exchange reaction has been evaluated by measuring through an infra-red spectrometer the deuterated water collected at the membrane lumen outlet. The purpose of this experimental test campaign is intended to perform isotope exchange reaction tests of the ENEA PERMCAT prototype whose characteristics are reported in Table 1. The per-
formance of this membrane reactor has been evaluated in terms of decontamination factor according to the TLK PERMCAT experience [8]. Further, the behaviour of a not complete hydrogen isotope selectivity of the Pd/Ag thin wall permeator tube has been experimentally evaluated. A large part of the experimental work has been dedicated to prepare the PERMCAT devices to the campaign test. In particular the surfaces of the Pd/Ag membrane has been initially oxidised in order to remove eventually impurities. Afterwards the membrane surfaces have reduced at high temperature under a hydrogen flux. Finally, the catalyst (NIKKI 111) located in the inner part of the membrane reactor has been activated. A preliminary test campaign of the membrane reactor has been aimed to assess the hydrogen (both H2 and D2 ) isotopes permeability. Afterwards two series of isotope exchange reaction tests have been performed: tests where a defect-free membrane with complete hydrogen selectivity (ENEA PERMCAT prototype no. 1) and tests with membrane not completely selective to hydrogen (ENEA PERMCAT prototype no. 2). 2.1. The experimental apparatus The experimental apparatus is shown in Fig. 1. It consists of a series of devices and instruments to measure and control the flow, the pressure and the temperature of the gases. The mass flow controllers (RF001/RF004) are used to regulate the flow rates of the gaseous streams feeding the shell side with H2 (during the permeation tests) or D2 (during the permeation and the isotopic exchange reaction tests) and the membrane lumen with He (gas carrier). The flow rate of water vapour from the vapouriser (VD101) is controlled by the (RF005). The heating of the different devices is provided by electrical heaters (EH101/EH105) controlled and monitored by measuring their temperatures through thermocouples (RT101/RT109). The pressure of the gas in the experimental apparatus is measured by pressure sensors at membrane inlet and outlet (RP03 and RP02) and at the shell (RP01 and RP102). The outlet of the membrane is connected to a cold trap (CT01) operating at liquid nitrogen temperature where the water is condensed while the gas phase is discharged through the vacuum pump (VA001). A second cold trap (not displayed in Fig. 1) has been connected to the shell outlet for condensating the water vapour during the second part of the experimental test campaign (when not completely hydrogen selective membranes were used). A Helium Leak Detector Mass Spectrometer (Pfeiffer HLT 572) LD01 has been connected to the shell outlet and used as a sensor to monitor the hydrogen isotopes concentration during the tests. 3. Results and discussion Both the experimental campaigns, the permeation tests and the isotopic exchange reaction ones, have been performed at a temperature of 350 ◦ C while the reference tests [8] have been carried out at 400 ◦ C. The lower temperature operated in these tests permitted to characterize the Pd-based membranes under slightly more critical conditions: as a matter of fact, the hydrogen uploading into the metal lattice is larger at lower temperature. 3.1. The permeation tests Protium and deuterium permeability tests for the prototypes nos. 1 and 2 have been carried out at 350 ◦ C by feeding the gas into the membrane lumen with the outlet completely closed and maintaining the pressure of the shell at a fixed value using the throttle valve RV102. The hydrogen permeability through the metal
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Fig. 1. Flow diagram of the experimental set-up for isotope exchange reactions used to study the performances of upgraded PERMCAT reactors.
membrane is calculated with reference to the Sieverts’ law by the formula [13,14]: Fd Pe = √ √ ( P P shell)A
(1)
where Pe is the hydrogen permeability (mol m−1 s−1 Pa−0.5 ), F the hydrogen permeating flow rate (mol s−1 ), d the wall tube thickness (m), P the hydrogen isotopes partial pressure inside the membrane (Pa), Pshell the hydrogen isotopes partial pressure in the shell side (Pa), and A is the permeation surface area (m2 ) [9]. Table 2 shows the permeability range values calculated from the experimental measurements for these thin wall membranes: as expected, the protium permeability values are higher than those measured for the deuterium. Permeability values about 70% higher for the prototype no. 2 compared to the prototype no. 1 demonstrated the not complete selectivity to hydrogen of the considered membrane. The permeability values calculated in these experiments with the prototype no. 1 membrane are in agreement with the literature data reported for thin wall Pd/Ag tubes [9,11]. 3.2. The isotopic exchange tests The isotope exchange process is schematically shown in Fig. 2. The water vapour provided by the vaporiser is fed into the Pd/Ag membrane while D2 is sent in counter-current mode in the shell
Fig. 2. Scheme of the isotope exchange reaction process operated by the PERMCAT reactor.
side. The D2 permeate through the Pd/Ag tube into the membrane lumen where the isotope exchange reaction takes place over the catalyst bed. At the membrane outlet HDO is collected by a cold trap while the HD leaves the shell in the outlet side. The two membrane reactor prototypes (defect-free and not perm-selective membrane) have been characterized by isotope exchange reaction tests. The tests have been performed under these parametric conditions: H2 O and D2 inlet flow rate of 15, 30 and 45 mL/min; He (used as carrier gas of the water vapour into the membrane lumen) flow rate between 15 and 45 mL/min; D2 shell pressure of 50 mbar. The deuterated water collected into the cold trap at the outlet of the membrane after each 6–7 h operation test has been analysed by infra-red spectroscopy at 2510 cm−1 (D–O stretching mode) using
Table 2 Hydrogen and deuterium permeability range values calculated at temperature of 350 ◦ C for complete (prototype no. 1) and not complete hydrogen isotopes selective membranes (prototype no. 2).
Prototype no. 1 Prototype no. 2
H2 permeability (mol m−1 s−1 Pa−0.5 )
D2 permeability (mol m−1 s−1 Pa−0.5 )
4.0 × 10−9 –6.5 × 10−9 9.3 × 10−9 –1.1 × 10−8
3.2 × 10−9 –4.8 × 10−9 6.2 × 10−9 –6.8 × 10−9
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Fig. 3. The influence of the total inlet flow rates on the concentration of deuterium in the HDO collected at the end of the process (D2 /H2 O ratio = 1).
an analytical procedure developed and described by Demange et al. [8]. The results have been assessed with a ±2% of absolute accuracy. The values of deuterium concentration in the HDO collected at the lumen side outlet of PERMCAT prototypes nos. 1 (defectfree membrane) and 2 (not perm-selective membrane) have been experimentally measured. Especially, Figs. 3 and 4 report the deuterium concentration in the HDO vs. the water vapour feed flow rate and vs. D2 /H2 O sweeping ratio, respectively. As expected, the isotopic exchange increases when operating at lower water vapour flow rate (Fig. 3) and higher swamping flow rate (Fig. 4). In order to compare the results obtained in this experimental campaign to a PERMCAT reactors having different length and geometrical design, a decontamination factor for a membrane reactor of 1 m of length (DF(1 m) ) [8] has been calculated by this formula: DF(1 m) = e
(((ln 100/(100−Cp ))/L)∗100)
(2)
where Cp is the concentration of D in HDO and L is the length of the membrane (mm). This parameter is intended considering the amount of H2 O fuelled in the membrane per unit of permeation area and assuming a logarithmic trend of the concentration profile with the length of the membrane. The results are reported in Fig. 5. During the isotope exchange reaction tests an additional cold trap, placed at the shell outlet, has been used in order to trap the eventually HDO from the PERMCAT prototypes not completely selective to the hydrogen isotopes (He leak rate at room temperature of 10−3 mbar Ls−1 ). A very small amount of HDO, 0.1% and 0.2% (in weight, of the overall HDO produced in the run test) have been recovered in this cold trap during the test with the proto-
Fig. 4. Deuterium concentration in the water collected in the cold trap vs. the D2 /H2 O ratio for different PERMCAT reactors.
Fig. 5. Decontamination factor of the PERMCAT prototypes assessed considering these membranes of 1 m of length.
type no. 2 and feeding the membrane with 30 and 45 mL/min of vapour, respectively. No water has been trapped at the end of the other tests. Such results are suggesting that the amount of water present in the stream leaving the shell side could be tolerated by the traps used before the cryo columns. However, further experiments should be carried out according to a detailed safety analysis which should consider the case of not perm-selective membrane or other leakages of the reactor module. 3.3. Helium Leak Detector Mass Spectrometer results The H2 /D2 isotopic exchange has been evaluated both by measuring the concentration of deuterium (or protium) at the shell outlet and the fraction of HDO into the water collected at the lumen outlet. In this experimental work a method (Demange et al. [15]) which uses a Helium Leak Detector Mass Spectrometer (HLDMS) has been applied in order to have a qualitative assessment of the hydrogen isotopes concentration at the shell outlet (LD01). Differently to the other analysers, this instrument gave us the possibility to monitor in real time the composition of the gas species at the shell outlet. For this purpose, the HLDMS has been calibrated for the D2 as well as the H2 signal (mass 4 and 2, respectively) at the shell operative pressure (0.050 bar) and used as a sensor during the isotope exchange reaction tests. The fraction of HDO into the water collected at the lumen outlet has been assessed by IR analysis which provided us an integral measurement of the deuterium transferred from shell into lumen side of the PERMCAT. The HLDMS has recorded, for the experimental tests with the PERMCAT prototype no. 1, the signal for atomic mass 4 and mass 2 for the other prototype. Fig. 6 shows the percentage of isotope percentage in the shell side during the isotopic exchange tests (performed with the same ratio D2 /H2 O = 2) for the different PERMCAT reactors prototypes. The steady state condition, for the isotope reaction tests operated with prototype no. 2 is reached by less than 1 h: the protium percentage remain practically constant until the end of the runs. Differently, the tests of the prototype no. 1 have shown a decrease of about 10% of the deuterium concentration along the run time. Further, it has been verified that the hydrogen percentage concentration in the shell side is inversely proportional when the D2 /H2 O ratio increase. Fig. 7 shows the PERMCAT prototype no.
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study has been aimed at verifying the behaviour of the PERMCAT devices under non-normal (accidental) conditions in the view of providing information for future safety analysis. The characterization of the PERMCAT module in hydrogen and deuterium permeation tests of the selective membrane has shown permeability values comparable to the literature data. The isotope exchange reaction tests performed with two prototypes show that the highest decontamination factors are reach when the residence time in the bed catalyst of the inlet species increase and, in particular, the absolute highest value is obtained using a prototype with a Pd/Ag membrane not completely selective to the hydrogen isotopes. The tests performed have shown that the deuterium concentration decrease slowly (about 10%) during the experiment using the first PERMCAT prototype as well as the steady state conditions for the hydrogen concentration in the system are reach in less of 1 h with the last reactor module prototype. Acknowledgement Fig. 6. Protium or deuterium percentage in the shell side during the isotopic exchange tests (D2 /H2 O = 2) for the two different PERMCAT reactors.
This work, supported by the European Communities under the contract of Association between EURATOM/ENEA, was carried out within the framework of the “Euratom Fusion Training Scheme” project (Contract no. 042862 – FU06). The views and opinions expressed herein do not necessarily reflect those of the European Commission. References
Fig. 7. PERMCAT prototype no. 2: protium concentration percentage during isotopic exchange reaction tests with different ratio D2 /H2 O.
2 during three isotopic exchange reaction tests with different ratio D2 /H2 O. The effect of the carrier gas (He) has been also evaluated: the percentage of D in HDO trapped at the end of the reactor during tests with flow rates of water vapour and deuterium of 15 mL/min decreases slightly (12% and 2% for the prototype nos. 1 and 2, respectively) when the flow rate of the gas carrier increases from 15 to 30 mL/min (i.e. the residence time decreases). 4. Conclusions A PERMCAT reactor using a thin wall Pd/Ag membrane tube has been tested. The influence of different operating parameters (H2 O and D2 flow rates, D2 /H2 O ratio) on the overall performance (decontamination factor) has been experimentally evaluated. Particularly, the tests have been carried out on two reactor prototypes: a defect-free membrane with complete (infinite) hydrogen selectivity and a not perm-selective membrane. In this last case, the
[1] B. Bornschein, M. Glugla, K. Gunther, R. Lasser, T.L. Le, K.H. Simon, S. Welte, Tritium tests with a technical PERMCAT for final clean-up of ITER exhaust gases, Fusion Eng. Des. 69 (2003) 51–56. [2] M. Glugla, A. Antipenkov, S. Beloglazov, C. Caldwell-Nichols, I.R. Cristescu, I. Cristescu, C. Day, L. Doerr, J.-P. Girard, E. Tada, The ITER tritium systems, Fusion Eng. Des. 82 (2007) 472–487. [3] M. Glugla, D.K. Murdoch, A. Antipenkov, S. Beloglazov, I. Cristescu, I.-R. Cristescu, C. Day, R. Laesser, A. Mack, ITER fuel cycle R&D: consequences for the design, Fusion Eng. Des. 81 (2006) 733–744. [4] M. Glugla, R. Lässer, T.L. Le, R.-D. Penzhorn, K.H. Simon, Experience gained during the modification of the Caprice system to Caper, Fusion Eng. Des. 49–50 (2000) 811–816. [5] B. Bornschein, D. Corneli, M. Glugla, K. Günther, T.L. Le, K.H. Simon, Experimental validation of a method for performance monitoring of the impurity processing stage in the TEP system of ITER, Fusion Eng. Des. 82 (2007) 2133–2139. [6] M. Glugla, A. Perevezentsev, D. Niyongabo, R.D. Penzhorn, A. Bell, P. Herrmann, A PERMCAT reactor for impurity processing in the JET active gas handling, Fusion Eng. Des. 49/50 (2000) 817–823. [7] R. Lässer, M. Glugla, K. Günther, T.L. Le, D. Niyongabo, R.-D. Penzhorn, K.H. Simon, Experimental validation of main components of the Tokamak exhaust process for ITER-FEAT, Fusion Eng. Des. 58–59 (2001) 371–375. [8] D. Demange, S. Welte, M. Glugla, Experimental validation of upgraded designs for PERMCAT reactors considering mechanical behaviour of Pd/Ag membranes under H2 atmosphere, Fusion Eng. Des. 82 (2007) 2383–2389. [9] S. Tosti, A. Basile, L. Bettinali, F. Borgognoni, F. Chiaravalloti, Gallucci Longterm tests of Pd–Ag thin wall permeator tube, J. Membr. Sci. 284 (2006) 393– 397. [10] S. Tosti, L. Bettinali, Diffusion bonding of Pd–Ag membranes, J. Mater. Sci. 39 (2004) 3041–3046. [11] S. Tosti, L. Bettinali, D. Lecci, F. Marini, V. Violante, Method of bonding thin foils made of metal alloys selectively permeable to hydrogen, particularly providing membrane devices, and apparatus for carrying out the same, European Patent EP 1184125 (2001). [12] S. Tosti, L. Bettinali, F. Borgognoni, D.K. Murdoch, Mechanical design of a PERMCAT reactor module, Fusion Eng. Des. 82 (2007) 153–161. [13] A. Basile, F. Gallucci, S. Tosti, Synthesis, Characterization, and applications of palladium membranes, Membr. Sci. Technol. 13 (2008) 255–323. [14] S.N Paglieri, J.D. Way, Innovations in palladium membrane research, Sep. Purif. Methods 31/1 (2002) 1–169. [15] D. Demange, M. Grivet, H. Pialot, A. Chambaudet, Indirect tritium determination by an original 3He ingrowth method using a standard helium leak detector mass spectrometer, Anal. Chem. 74 (2002) 3183–3189.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Processing test of an upgraded mechanical design for PERMCAT reactor Fabio Borgognoni a,∗ , David Demange b , Lothar Dörr b , Silvano Tosti a , Stefan Welte b a b
Associazione ENEA-Euratom sulla Fusione, C.R. ENEA Frascati, Via E. Fermi 45, Frascati, Roma I-00044, Italy Forschungszentrum Karlsruhe GmbH, Institute for Technical Physics, Tritium Laboratory Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany
a r t i c l e
i n f o
Article history: Available online 3 November 2010 Keywords: Pd membranes Isotope exchange reactions Plasma exhausts clean-up
a b s t r a c t The PERMCAT membrane reactor is a coaxial combination of a Pd/Ag permeator membrane and a catalyst bed. This device has been proposed for processing fusion reactor plasma exhaust gas. A stream containing tritium (up to 1% of tritium in different chemical forms such as water, methane or molecular hydrogen) is decontaminated in the PERMCAT by counter-current isotopic swamping with protium. Different mechanical designs of the membrane reactor have been proposed to improve robustness and lifetime. The ENEA membrane reactor uses a permeator tube with a length of about 500 mm produced via cold-rolling and diffusion welding of Pd/Ag thin foils: two stainless steel pre-tensioned bellows have been applied to the Pd/Ag tube in order to avoid any significant compressive and bending stresses due to the permeator tube elongation consequent to the hydrogen uptake. An experimental test campaign has been performed using this reactor in order to assess the influence of different operating parameters and to evaluate the overall performance (decontamination factor). Tests have been carried out on two reactor prototypes: a defect-free membrane with complete (infinite) hydrogen selectivity and not perm-selective membrane. In this last case, the study has been aimed at verifying the behaviour of the PERMCAT devices under non-normal (accidental) conditions in the view of providing information for future safety analysis. The paper will present the specific mechanical design and the experimental results of tests based on isotopic exchange between H2 O and D2 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction Tritium has to be recovered from contaminated gases for both safety and fuel economy reasons. Particularly, tritium is to be recovered from gaseous mixtures coming from the Tokamak Exhaust Processing (TEP) system of fusion reactor machines which use DT mixture. This policy is in accordance to the As Low As Reasonably Achievable (ALARA) principle in order to minimise the tritium in effluents and releases [1–3]. At Tritium Laboratory Karlsruhe (TLK) a three stage process, socalled CAPER, has been developed in order to support the design of the TEP system of ITER providing a semi-technical scale experimental results [4]. The impurity separation is the first step of CAPER, where a Pd/Ag permeator battery recovers more than 95% of the un-burnt DT fuel from the impurities (helium, hydrocarbons and water). During the second step, the impurity processing, the tritium is separated from the impurities (tritiated water or tritiated hydrocarbons) in a close loop where catalyzed cracking or conversion reactions take place.
∗ Corresponding author. Tel.: +39 0694005560; fax: +39 0694005147. E-mail address: [email protected] (F. Borgognoni). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.08.022
Especially the tritium is recovered by permeation using Pd/Ag permeators. The third step, the final clean-up, provides to recover almost all of the residual tritium by the so-called permeator catalyst reactor (PERMCAT) [5]. This special device is a direct combination of a Pd/Ag membrane with a catalyst bed and has been developed for the final clean-up of gases containing up to about 1% of tritium in different chemical forms such as water, hydrocarbons or molecular hydrogen isotopes. In particular the PERMCAT module has demonstrated to have very higher performance when compared to a system consisting of a traditional reactor plus a permeator. Particularly, the membrane reactor permits to optimise the tritium removal process, to keep the load to the Isotope Separation System of ITER reasonably low and to minimise at the same time memory effects by segregating contaminated parts from non-contaminated parts of the unit. In the PERMCAT reactor the tritium is removed from contaminated gases by isotopic swamping which operates in counter-current flow mode: in such a way, it is possible to obtain a very low tritium activity at the outlet of the component to be maintained [6,7]. Due to the thermal expansion and the hydrogen uptake in the Pd alloy lattice, the Pd/Ag membrane can elongate up to 1.5–2% of his length [8,9]. Therefore, it is essential that the mechanical design
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F. Borgognoni et al. / Fusion Engineering and Design 85 (2010) 2171–2175
Table 1 ENEA PERMCAT reactors mechanical design: geometrical features. Metal bellows supported Pd/ Ag thin membranes Thickness (m) Length (mm) Outer Diameter (mm) Permeation area (cm2 ) Mass of catalyst (g) Permeation area/catalyst (cm2 /g)
50 516 6.00 97.3 18.0 5.4
of the PERMCAT module should consider this behaviour in order to prevent improper mechanical constraints that could cause the failure of the membrane reactor. TLK has tested different designs of PERMCAT reactors (finger like, metal bellow and corrugated membrane configurations) in order to compensate the length variation of the membrane due to the uploading of hydrogen isotopes into the metal lattice [8]. The finger like configuration of the membrane tube has been adopted as first generation of PERMCAT units. In this design, only one end of the membrane is tightly fixed at the shell module: the other end, closed, is free to elongate during the operative conditions. A capillary inner stainless steel tube is required inside the membrane to remove the retentate gas stream [8]. An additional PERMCAT module, adopting a further new mechanical design has been developed by ENEA Frascati Laboratories. This PERMCAT module contains permeator tubes produced by a special procedure of cold-rolling and diffusion welding of Pd/Ag foils [10,11]. These Pd/Ag tubes have a wall thickness (0.050 mm) thinner compared with the commercial ones (0.100–0.150 mm). Especially, as a consequence of the reduced thickness of the permeator tubes, a particular configuration of the membrane module has to be designed in order to ensure a long life of the thin wall membrane tube. As a matter of fact, the thermal and hydrogenation cycling of the thin wall and long (over 500 mm) membrane tube involves elongations/contractions which may produce the failure of the membrane. In order to avoid the rising of combined compressive and bending stress of these very thin permeators, between the permeator and the shell module two pre-tensioned metal bellows have been inserted. These pre-tensioned bellows maintains the membrane tube always tensioned or at least unstressed (never compressed) during operation [12]. This paper describes the experimental test campaign aimed to assess, in terms of decontamination factor, the performances of the PERMCAT module developed by ENEA. Especially, the experiments have tested two membrane tubes: both a defect-free membrane tube and a not completely selective to hydrogen (i.e. simulating the presence of membrane defects or module leakages). Particularly, the study of not perm-selective Pd/Ag membrane could be relevant from the safety point of view: in fact, under such an accidental condition traces of water could pass into the gas stream which has to be sent to the cryo-distillation. 2. Experimental The PERMCAT process is aimed to remove tritium chemically bound in molecules. In this experimental work, deuterium has been used instead of tritium: in particular, water (vapour) has been fuelled into the membrane lumen while deuterium has been fed in the shell side of the membrane reactor in counter-current mode. The isotope exchange reaction has been evaluated by measuring through an infra-red spectrometer the deuterated water collected at the membrane lumen outlet. The purpose of this experimental test campaign is intended to perform isotope exchange reaction tests of the ENEA PERMCAT prototype whose characteristics are reported in Table 1. The per-
formance of this membrane reactor has been evaluated in terms of decontamination factor according to the TLK PERMCAT experience [8]. Further, the behaviour of a not complete hydrogen isotope selectivity of the Pd/Ag thin wall permeator tube has been experimentally evaluated. A large part of the experimental work has been dedicated to prepare the PERMCAT devices to the campaign test. In particular the surfaces of the Pd/Ag membrane has been initially oxidised in order to remove eventually impurities. Afterwards the membrane surfaces have reduced at high temperature under a hydrogen flux. Finally, the catalyst (NIKKI 111) located in the inner part of the membrane reactor has been activated. A preliminary test campaign of the membrane reactor has been aimed to assess the hydrogen (both H2 and D2 ) isotopes permeability. Afterwards two series of isotope exchange reaction tests have been performed: tests where a defect-free membrane with complete hydrogen selectivity (ENEA PERMCAT prototype no. 1) and tests with membrane not completely selective to hydrogen (ENEA PERMCAT prototype no. 2). 2.1. The experimental apparatus The experimental apparatus is shown in Fig. 1. It consists of a series of devices and instruments to measure and control the flow, the pressure and the temperature of the gases. The mass flow controllers (RF001/RF004) are used to regulate the flow rates of the gaseous streams feeding the shell side with H2 (during the permeation tests) or D2 (during the permeation and the isotopic exchange reaction tests) and the membrane lumen with He (gas carrier). The flow rate of water vapour from the vapouriser (VD101) is controlled by the (RF005). The heating of the different devices is provided by electrical heaters (EH101/EH105) controlled and monitored by measuring their temperatures through thermocouples (RT101/RT109). The pressure of the gas in the experimental apparatus is measured by pressure sensors at membrane inlet and outlet (RP03 and RP02) and at the shell (RP01 and RP102). The outlet of the membrane is connected to a cold trap (CT01) operating at liquid nitrogen temperature where the water is condensed while the gas phase is discharged through the vacuum pump (VA001). A second cold trap (not displayed in Fig. 1) has been connected to the shell outlet for condensating the water vapour during the second part of the experimental test campaign (when not completely hydrogen selective membranes were used). A Helium Leak Detector Mass Spectrometer (Pfeiffer HLT 572) LD01 has been connected to the shell outlet and used as a sensor to monitor the hydrogen isotopes concentration during the tests. 3. Results and discussion Both the experimental campaigns, the permeation tests and the isotopic exchange reaction ones, have been performed at a temperature of 350 ◦ C while the reference tests [8] have been carried out at 400 ◦ C. The lower temperature operated in these tests permitted to characterize the Pd-based membranes under slightly more critical conditions: as a matter of fact, the hydrogen uploading into the metal lattice is larger at lower temperature. 3.1. The permeation tests Protium and deuterium permeability tests for the prototypes nos. 1 and 2 have been carried out at 350 ◦ C by feeding the gas into the membrane lumen with the outlet completely closed and maintaining the pressure of the shell at a fixed value using the throttle valve RV102. The hydrogen permeability through the metal
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Fig. 1. Flow diagram of the experimental set-up for isotope exchange reactions used to study the performances of upgraded PERMCAT reactors.
membrane is calculated with reference to the Sieverts’ law by the formula [13,14]: Fd Pe = √ √ ( P P shell)A
(1)
where Pe is the hydrogen permeability (mol m−1 s−1 Pa−0.5 ), F the hydrogen permeating flow rate (mol s−1 ), d the wall tube thickness (m), P the hydrogen isotopes partial pressure inside the membrane (Pa), Pshell the hydrogen isotopes partial pressure in the shell side (Pa), and A is the permeation surface area (m2 ) [9]. Table 2 shows the permeability range values calculated from the experimental measurements for these thin wall membranes: as expected, the protium permeability values are higher than those measured for the deuterium. Permeability values about 70% higher for the prototype no. 2 compared to the prototype no. 1 demonstrated the not complete selectivity to hydrogen of the considered membrane. The permeability values calculated in these experiments with the prototype no. 1 membrane are in agreement with the literature data reported for thin wall Pd/Ag tubes [9,11]. 3.2. The isotopic exchange tests The isotope exchange process is schematically shown in Fig. 2. The water vapour provided by the vaporiser is fed into the Pd/Ag membrane while D2 is sent in counter-current mode in the shell
Fig. 2. Scheme of the isotope exchange reaction process operated by the PERMCAT reactor.
side. The D2 permeate through the Pd/Ag tube into the membrane lumen where the isotope exchange reaction takes place over the catalyst bed. At the membrane outlet HDO is collected by a cold trap while the HD leaves the shell in the outlet side. The two membrane reactor prototypes (defect-free and not perm-selective membrane) have been characterized by isotope exchange reaction tests. The tests have been performed under these parametric conditions: H2 O and D2 inlet flow rate of 15, 30 and 45 mL/min; He (used as carrier gas of the water vapour into the membrane lumen) flow rate between 15 and 45 mL/min; D2 shell pressure of 50 mbar. The deuterated water collected into the cold trap at the outlet of the membrane after each 6–7 h operation test has been analysed by infra-red spectroscopy at 2510 cm−1 (D–O stretching mode) using
Table 2 Hydrogen and deuterium permeability range values calculated at temperature of 350 ◦ C for complete (prototype no. 1) and not complete hydrogen isotopes selective membranes (prototype no. 2).
Prototype no. 1 Prototype no. 2
H2 permeability (mol m−1 s−1 Pa−0.5 )
D2 permeability (mol m−1 s−1 Pa−0.5 )
4.0 × 10−9 –6.5 × 10−9 9.3 × 10−9 –1.1 × 10−8
3.2 × 10−9 –4.8 × 10−9 6.2 × 10−9 –6.8 × 10−9
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Fig. 3. The influence of the total inlet flow rates on the concentration of deuterium in the HDO collected at the end of the process (D2 /H2 O ratio = 1).
an analytical procedure developed and described by Demange et al. [8]. The results have been assessed with a ±2% of absolute accuracy. The values of deuterium concentration in the HDO collected at the lumen side outlet of PERMCAT prototypes nos. 1 (defectfree membrane) and 2 (not perm-selective membrane) have been experimentally measured. Especially, Figs. 3 and 4 report the deuterium concentration in the HDO vs. the water vapour feed flow rate and vs. D2 /H2 O sweeping ratio, respectively. As expected, the isotopic exchange increases when operating at lower water vapour flow rate (Fig. 3) and higher swamping flow rate (Fig. 4). In order to compare the results obtained in this experimental campaign to a PERMCAT reactors having different length and geometrical design, a decontamination factor for a membrane reactor of 1 m of length (DF(1 m) ) [8] has been calculated by this formula: DF(1 m) = e
(((ln 100/(100−Cp ))/L)∗100)
(2)
where Cp is the concentration of D in HDO and L is the length of the membrane (mm). This parameter is intended considering the amount of H2 O fuelled in the membrane per unit of permeation area and assuming a logarithmic trend of the concentration profile with the length of the membrane. The results are reported in Fig. 5. During the isotope exchange reaction tests an additional cold trap, placed at the shell outlet, has been used in order to trap the eventually HDO from the PERMCAT prototypes not completely selective to the hydrogen isotopes (He leak rate at room temperature of 10−3 mbar Ls−1 ). A very small amount of HDO, 0.1% and 0.2% (in weight, of the overall HDO produced in the run test) have been recovered in this cold trap during the test with the proto-
Fig. 4. Deuterium concentration in the water collected in the cold trap vs. the D2 /H2 O ratio for different PERMCAT reactors.
Fig. 5. Decontamination factor of the PERMCAT prototypes assessed considering these membranes of 1 m of length.
type no. 2 and feeding the membrane with 30 and 45 mL/min of vapour, respectively. No water has been trapped at the end of the other tests. Such results are suggesting that the amount of water present in the stream leaving the shell side could be tolerated by the traps used before the cryo columns. However, further experiments should be carried out according to a detailed safety analysis which should consider the case of not perm-selective membrane or other leakages of the reactor module. 3.3. Helium Leak Detector Mass Spectrometer results The H2 /D2 isotopic exchange has been evaluated both by measuring the concentration of deuterium (or protium) at the shell outlet and the fraction of HDO into the water collected at the lumen outlet. In this experimental work a method (Demange et al. [15]) which uses a Helium Leak Detector Mass Spectrometer (HLDMS) has been applied in order to have a qualitative assessment of the hydrogen isotopes concentration at the shell outlet (LD01). Differently to the other analysers, this instrument gave us the possibility to monitor in real time the composition of the gas species at the shell outlet. For this purpose, the HLDMS has been calibrated for the D2 as well as the H2 signal (mass 4 and 2, respectively) at the shell operative pressure (0.050 bar) and used as a sensor during the isotope exchange reaction tests. The fraction of HDO into the water collected at the lumen outlet has been assessed by IR analysis which provided us an integral measurement of the deuterium transferred from shell into lumen side of the PERMCAT. The HLDMS has recorded, for the experimental tests with the PERMCAT prototype no. 1, the signal for atomic mass 4 and mass 2 for the other prototype. Fig. 6 shows the percentage of isotope percentage in the shell side during the isotopic exchange tests (performed with the same ratio D2 /H2 O = 2) for the different PERMCAT reactors prototypes. The steady state condition, for the isotope reaction tests operated with prototype no. 2 is reached by less than 1 h: the protium percentage remain practically constant until the end of the runs. Differently, the tests of the prototype no. 1 have shown a decrease of about 10% of the deuterium concentration along the run time. Further, it has been verified that the hydrogen percentage concentration in the shell side is inversely proportional when the D2 /H2 O ratio increase. Fig. 7 shows the PERMCAT prototype no.
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study has been aimed at verifying the behaviour of the PERMCAT devices under non-normal (accidental) conditions in the view of providing information for future safety analysis. The characterization of the PERMCAT module in hydrogen and deuterium permeation tests of the selective membrane has shown permeability values comparable to the literature data. The isotope exchange reaction tests performed with two prototypes show that the highest decontamination factors are reach when the residence time in the bed catalyst of the inlet species increase and, in particular, the absolute highest value is obtained using a prototype with a Pd/Ag membrane not completely selective to the hydrogen isotopes. The tests performed have shown that the deuterium concentration decrease slowly (about 10%) during the experiment using the first PERMCAT prototype as well as the steady state conditions for the hydrogen concentration in the system are reach in less of 1 h with the last reactor module prototype. Acknowledgement Fig. 6. Protium or deuterium percentage in the shell side during the isotopic exchange tests (D2 /H2 O = 2) for the two different PERMCAT reactors.
This work, supported by the European Communities under the contract of Association between EURATOM/ENEA, was carried out within the framework of the “Euratom Fusion Training Scheme” project (Contract no. 042862 – FU06). The views and opinions expressed herein do not necessarily reflect those of the European Commission. References
Fig. 7. PERMCAT prototype no. 2: protium concentration percentage during isotopic exchange reaction tests with different ratio D2 /H2 O.
2 during three isotopic exchange reaction tests with different ratio D2 /H2 O. The effect of the carrier gas (He) has been also evaluated: the percentage of D in HDO trapped at the end of the reactor during tests with flow rates of water vapour and deuterium of 15 mL/min decreases slightly (12% and 2% for the prototype nos. 1 and 2, respectively) when the flow rate of the gas carrier increases from 15 to 30 mL/min (i.e. the residence time decreases). 4. Conclusions A PERMCAT reactor using a thin wall Pd/Ag membrane tube has been tested. The influence of different operating parameters (H2 O and D2 flow rates, D2 /H2 O ratio) on the overall performance (decontamination factor) has been experimentally evaluated. Particularly, the tests have been carried out on two reactor prototypes: a defect-free membrane with complete (infinite) hydrogen selectivity and a not perm-selective membrane. In this last case, the
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