Innovative joining of Pd-Ag permeator tubes

Fusion Engineering and Design xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Innovative joining of Pd-Ag permeator tubes Andrea Moriania, Giacomo Brunib, Marco Incellic, Alessia Santuccia, Karine Ligerd, ⁎ Michele Troulayd, Silvano Tostia, a ENEA, Dip. Fusione e Tecnologie per la Sicurezza Nucleare, via Fermi 45, 00044, Frascati, Italy CIRDER, Università della Tuscia, via San Camillo de Lellis snc, 01100, Viterbo, Italy DEIM, Università della Tuscia, via del Paradiso 47, 01100, Viterbo, Italy d CEA, DEN, DTN\SMTA\LIPC Cadarache, F-13108, Saint Paul-lez-Durance, France b c A R T I C L E I N F O A B S T R A C T Keywords: Pd-membrane Tritium separation Thin-walled membrane tube Pd-alloy membrane tubes are used for hydrogen isotopes separation in the fusion fuel cycle. The efficiency of these separation processes has been significantly increased by the adoption of thin-walled self-supported membrane tubes that exhibit high permeance and infinite selectivity to hydrogen. A critical aspect in the realization of Pd-based membrane devices (both separators and membrane reactors) is related to the joining of the thin-walled tubes to the membrane module. In this work, an innovative fitting of thin-walled tubes has been obtained by coupling two special stainless steel joints (a weld adapter and a sealing insert) that tighten the flared edge of the tube. In particular, the design of the compression fitting relies on the elastic deformation reserve of the conical edge of the sealing insert that is capable: i) to ensure the sealing of the joining also in presence of the expansion/contraction of the Pd-Ag alloy during the hydrogenation cycling, and ii) to compensate small irregularities of the thin-walled tube (thickness, circularity, etc.). The effectiveness of the new joining technique in terms of both hydrogen perm-selectivity and stability has been verified in preliminary permeations tests. 1. Introduction Several membrane processes are proposed for the purification of hydrogen and its isotopes. In the fusion fuel cycle the membranes are applied in the plasma exhaust treatment, the extraction of tritium from the breeding blanket and the water detritiation [1]. The rising interest for the membrane processes is due to their reduced energy consumption, continuous operation, modularity and easy integration with traditional separation processes. These characteristics permit to increase both the system availability and the safety by reducing the tritium inventory in the fusion applications. In particular, only the hydrogen isotopes can permeate dense metal lattice and, therefore, when used in the fusion fuel cycle as separators (also called purifiers or diffusers) and membrane reactors, the selfsupported Pd-alloy tubes exhibit the complete selectivity to the hydrogen isotopes thus permitting to attain high detritiation factors [2,3]. Many applications of the self-supported Pd-Ag tubes have been also studied for producing ultra-pure hydrogen via reforming and other dehydrogenation reactions of hydrocarbons and alcohols [4]. In these reactors, the membrane continuously removes one of the reaction products, the hydrogen, and then high reaction conversion values, also ⁎ beyond the thermodynamic equilibrium, can be achieved. According to the Sieverts’ law, the hydrogen permeation flux is inversely proportional to the membrane thickness and, generally, the efficiency of both the permeators and the membrane reactors consisting of Pd-alloy tubes can be dramatically increased by the adoption of thinwalled tubes. However, in order to satisfy the requirements of durability and reliability of the nuclear fusion applications, the design of the membrane modules has to take into consideration special configurations (i.e. finger-like assembling of the Pd-alloy tubes, use of pretensioned springs and bellows, etc.) avoiding any compressive mechanical stress of the thin-walled tubes [1]. For the same reasons, the joining of thin-walled tubes to the membrane module cannot be realized by common procedures. In fact, the small thickness of the Pd-Ag tubes (0.100–0.250 mm) does not allow the application of the standard welding techniques, while the commercial compression tube fittings cannot be applied because the application of the tightening load could involve the collapse of the thin-walled tubes. This work describes the development of an innovative compression fitting carried out by ENEA and CEA in the framework of a joint project aimed to realize Pd-membrane reactors for recovering tritium from tritiated water. The results of preliminary permeation and leakage tests are also reported. Corresponding author. E-mail address: silvano.tosti@enea.it (S. Tosti). https://doi.org/10.1016/j.fusengdes.2018.02.075 Received 26 July 2017; Received in revised form 20 February 2018; Accepted 20 February 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved. Please cite this article as: Moriani, A., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.02.075 Fusion Engineering and Design xxx (xxxx) xxx–xxx A. Moriani et al. Fig. 1. Brazing of a thin-walled Pd-Ag tube to a stainless tube. Fig. 3. Exploded-view drawing of the new compression fitting. 2. Compression fitting of thin-walled tubes In previous applications, the Pd-Ag permeator tubes have been joined to the stainless steel fittings via brazing as shown in Fig. 1. Nevertheless, in some cases the metal parts surrounding the brazing were thermally stressed and then became inadequate to withstand the mechanical stresses coming from the differential expansion of the hydrogenated Pd-Ag with respect to the stainless steel. Commercial coupling systems are also available for joining tubes made up of ductile metals or plastics [5]. However, when applied to the thin-walled tubes (i.e. to the Pd-alloy permeators), the large flare angle (i.e. 37°) of these commercial fittings might cause sizable deformations and induce the failure of the tubes. In particular, for joining thin-walled tube, it is suggested the use of commercial compression fittings coupled with a support sleeve to be inserted inside the thin-walled tube, see Fig. 2. Such a solution has been experimentally tested in our laboratory, but after few hydrogenation cycles, the presence of leaks has been revealed [6]. In fact, the interaction of the hydrogen with the metals can affect significantly their thermal expansion behavior. At 400 °C the elongation of non-hydrogenated Pd-alloy is about 0.05%, i.e. not far from that of the stainless steel (0.07% for type 316 SS) [7], therefore the coupling of parts made up of these two metals should be stable by increasing the temperature. On the other hand, when hydrogenated, the metal lattice of the Pd and its alloys expands significantly thus involving the expansion of the permeator tubes. In particular, at 100 kPa and room temperature the Pg-Ag alloy commercially used for manufacturing the hydrogen separators (silver content of 20–25 wt.%) solubilizes more than 200 mg of hydrogen per 100 g of alloy [8]: under these conditions, the strain of the hydrogenated Pd-Ag achieves the value of about 1.5% [9]. Such an expansion could involve the loss of the tightening when Pd-Ag components are coupled to stainless steel ones. Consequently, the Pd-based permeators are recommended to be operated in presence of hydrogen only at high temperature (namely over 250–300 °C) where the hydrogen uploading into the Pd-alloy is negligible as well as the expansion of the metal lattice. Nevertheless, the reliable and safe operation of Pd-membranes in nuclear facilities has to be verified under the most conservative conditions that correspond to operate at low temperature in presence of hydrogen (i.e. when the elongation of the Pd-based membrane is about 1.5%). For these reasons, a new joining technique has been developed: it is Fig. 4. Particular of the compression fitting joining the Pd-Ag membrane to the stainless steel tube. based on the concept of compression tube fittings without using any support inside the Pd-based thin-walled tubes [10]. The fitting is realized by applying a compression force on two joint inserts closing the flared edge of the tube. In particular, the compression fitting has been developed for Pd-Ag membrane tubes characterized by a ratio thickness/diameter below 0.03. The innovative compression fitting is described in the exploded-view drawing of Fig. 3 and shown in the picture of Fig. 4. It comprises in combination: – the Pd-Ag thin-walled tube (T), – a bottom locking ring nut (BRN) with a thread, – a weld adapter (WA) provided with a conical edge that narrows towards its end and with a contact axial ring upon which is pushed the bottom locking ring nut (BRN), – a top locking ring nut (TRN) with a thread designed to mate with the thread of the bottom locking ring nut (BRN), – a sealing insert (SI) provided with a conical edge that widens towards its end and provided with a contact axial ring upon which is pushed the top locking ring nut (TRN). The new sealed fitting of the Pd-tube with the membrane module is obtained by using a stainless steel weld adapter (WA) with a conical edge (i.e. 11°) larger than the angle of inclination (i.e. 10°) of the tube outwards flare and of the sealing insert (SI). In this way, the elastic deformation reserve of the conical edge of the insert (SI) is capable: i) to ensure the sealing of the joining also in presence of the expansion/ contraction of the Pd-Ag alloy during the hydrogenation cycling, and ii) to compensate small irregularities (i.e. in thickness and/or circularity) of the thin-walled tube. 2.1. Stress analysis Generally, the proper mechanical design of the compression fitting has to define the axial force for tightening the top (TRN) and bottom (BRN) locking nuts in order to obtain a firm connection. For the compression fitting of thin-walled tubes, such a condition is achieved when Fig. 2. A commercial compression tube fitting adapted to a thin-walled Pd-Ag tube with a support sleeve inside. 2 Fusion Engineering and Design xxx (xxxx) xxx–xxx A. Moriani et al. of the compression fitting. 3. Preliminary testing The new compression fitting has been applied to a Pd-Ag (23 wt.% silver) membrane tube of diameter 10 mm, length 445 mm and wall thickness 0.155 mm. The Pd-alloy tube has been assembled in a stainless steel module and aimed to recover tritium from tritiated water for applications at the Cadarache laboratories of the CEA accordingly to the process scheme reported in Fig. 7 [3]. Tritiated water (HTOin) enters the membrane lumen, while protium (H2) is fed in the shell side of the reactor. Only the hydrogen isotopes (H2, HT) can permeates through the Pd-tube: in particular, over the catalyst located into the membrane lumen, the protium exchanges with the tritium. In such a way, the tritiated water is converted into water (H2O) while tritium is recovered in the shell side through the permeate stream. Theoretically, the effectiveness of the detritiation process is determined by the correct sizing of the reactor (membrane wall thickness, length) and the optimization of the operating conditions (P, T, feed and sweeping flowrates). In practice, only the complete selectivity of the Pd-membrane (i.e. absence of defects of the permeator tube) and the gas tightness of the fittings can ensure the achievement of the expected detritiation capability. In fact, in case of leaks the tritiated water can pass into the permeate side of the reactor by reducing its partial pressure in the membrane lumen and then affecting negatively the isotopic exchange reaction. Furthermore, the presence of tritiated water in the permeate stream might make more difficult the processes for recovering the tritium downstream the membrane reactor. The membrane reactor adopts the finger-like configuration in which the Pd-Ag tube is closed at one end, while the retentate stream is recovered through a small stainless steel tube inserted inside the membrane lumen. Especially, two compression fittings have been used: i) the first one joining the membrane tube to a stainless steel tube, and ii) the second one acting as a plug for the membrane tube that has to be closed at one end according to the finger-like configuration adopted. At ENEA Frascati laboratories leak tests of the membrane tube have been performed at room temperature before and after an experimental campaign consisting of hydrogenation cycles performed by flowing into the membrane module hydrogen and He at 400 °C for 15 h. Once completed the hydrogenation cycles, preliminary permeation tests have been executed in the temperature range 300–400 °C by feeding pure hydrogen into the membrane lumen at 250 kPa while the permeated hydrogen has been recovered in the shell side at atmospheric pressure. According to the Arrhenius’ law, the permeability data have been collected vs. the inverse of the absolute temperature: Fig. 5. The thin-walled tube with the new fittings after the hydraulic test. the axial tightening force is capable of determining the elastic backdown of the conical edge of the weld adapter (WA) and the sealing insert (SI) thus ensuring their good annular contact around the flared edge of the tube (T). The stress analysis of the fitting has been carried out via the code Ansys Workbench 15.0 under the hypothesis of “bilinear isotropic hardening” of the materials. This analysis has been performed on a stainless steel tube (internal diameter 9.6 mm and wall thickness 0.25 mm) used later for a hydraulic rupture test. In this test, two compression fittings have been applied to the stainless steel tube that has been pressurized with water. The rupture of the tube took place at 30 MPa without exhibiting any leaks in the compression fitting, see Fig. 5. The model assessment has been performed for a value of the axial tightening force of 5.7 kN. Considering that both the top (TRN) and bottom (BRN) locking nuts use a M14 thread (pitch 1.5 mm), the torque required for generating the designed axial force is 12.5 Nm that can be easily hand-applied and, therefore, such a compression fitting is of practical use in laboratory applications. This coupling requires high geometric accuracy to guarantee tightness and uniform compression of the flared tube edge: therefore, its components has to be realized by computer numerical control machine tools. Since the Pd-membrane reactor working temperature is in the range 350–400 °C, the stress analysis has considered a limit value of the yield strength of the stainless steel parts of 200 MPa. Such a value can be reasonable by considering that for the annealed type 316 stainless steel at 427 °C the yield strength is reported in the range 114–174 MPa [11], while for the ferritic-martensitic steels (EUROFER97) at 400 °C the values of yield strength are above 400 MPa [12]. The results of the analysis reported in Fig. 6 show that the stress values in the steels components are generally below 200 MPa. In particular, only about half of the section of the upper stainless steel conical insert near the edge of the Pd-Ag tube exhibits stresses higher than 200 MPa and, therefore, this conical insert is expected to be to some extent plastically deformed. Nevertheless, because of the action of the lower steel conical insert that keeps a complete elastic behavior, the overall fitting system should exhibit an elastic reserve enough to seal the joint in a tight way. The mechanical properties of the alloy Pd-Ag with silver 20–25 wt. % are similar to those of mild stainless steels: for instance, this Pd-alloy exhibits an ultimate tensile strength of about 380 and 680 MPa in the annealed and worked state, respectively [13]. Anyway, because of the small contribution of the Pd-Ag thin-walled tube to the stiffness of the system, its behavior has been neglected in the stress analysis. In fact, although the Pd-tube results plastically deformed between the two stainless steel joints, its deformations can be compensated by the elastic behavior of the conical steel parts. In case of different materials or tube geometry this stress analysis model can be used to assess the value of the torque to be applied for correctly tightening the joint and to verify the elastic reserve of the conical edge of the sealing insert capable to guarantee the effectiveness Ea P (T ) = P0 e− RT (1) where: P(T) is the hydrogen permeability, mol s−1 m−1 Pa−0.5, P0 is the pre-exponential factor, mol s−1 m−1 Pa−0.5, Ea the activation energy, J mol−1, R the gas constant, 8.341 J mol−1 K−1, and T the absolute temperature, K. The values measured of the pre-exponential factor and the activation energy of 2.50 10−8 mol s−1 m−1 Pa−0.5 and 4280.88 J K−1, respectively, are in agreement with the literature [14,15]. The leak tests have been carried out by a mass spectrometer leak detector (mod. Adixen ASM 310) used in sniffer mode. The Pd-Ag tube has been filled with He at 240–300 kPa and the sniffer has controlled the absence of He releases outside the membrane tube. These tests performed at ambient temperature before and after the activation procedure and the preliminary permeation tests have exhibited values of He leaks of 5.0 10−5 and 9.5 10−6 mbar L s−1, respectively. 3 Fusion Engineering and Design xxx (xxxx) xxx–xxx A. Moriani et al. Fig. 6. Results of the stress analysis for the thin-walled tube tightened between the conical edge and the sealing insert. Fig. 7. Scheme of the membrane reactor. 4. Conclusions Important spin-off of this technology is expected in the manufacturing of Pd-membrane devices for the production of ultra-pure hydrogen. In this field, relevant applications concern the membrane systems for feeding hydrogen to polymeric fuel cells and for electrochemical hydrogen generators for laboratory uses. A new compression fitting of thin-walled tubes has been designed and realized for joining Pd-alloy permeators used in the separation processes of the fusion fuel cycle. The new fitting has been checked through preliminary leaks and permeation tests at ENEA Frascati, while long–term tests will be performed at the CEA Cadarache laboratories with the aim to verify the effectiveness of the proposed technique. 4 Fusion Engineering and Design xxx (xxxx) xxx–xxx A. Moriani et al. Acknowledgment [5] http://ph.parker.com/us/en/triple-lok-37-flare-jic-tube-fittings-and-adapters. [6] http://www.parker.com/parkerimages/Parker.com/Literature/Tube Fittings Division Europe/New/BUL-4015-1-UK.pdf. [7] I.B. Fieldhouse, et al., Measurements of Thermal Properties, Wright Air Development Center (WADC) Technical Report 58–274, U.S. Air Force, 1958, pp. 55–79 (November). [8] A.G. Knapton, Palladium alloys for hydrogen diffusion membranes, Platinum Met. Rev. 21 (1977) 44–50. [9] D. Fort, I.R. Harris, The physical properties of some palladium alloy hydrogen diffusion membrane materials, J. Less-Common Met. 41 (2016) 313–327. [10] F. 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