1995-压块净化装置

Journal of AllOYS AND COMPOUND5 ELSEVIER Journal of Alloys and Compounds 231 (1995) 865-870

Hydrogen purification with metal hydride sintered pellets using pressure swing adsorption method Taichi Saitou a, Kazuhiro Sugiyama b aProce3s Technology Research Laboratories, Nippon Steel Corporation, Oita Works, Oita, Japan bTechnical Development, Oita Works, Nippon Steel Chemical Corporation, Oita Works, Oita, Japan

Abstract The process of hydrogen purification and the purification ability of FeTio.95Mmo.08 sintered pellets pressed with copper have been studied in the temperature range 25-90 °C by continuous measurements of the impurity concentration of released hydrogen using quadrupole mass spectrometry with gas chromatography. A production rate of purified hydrogen (purity greater than 99.99999%, excluding H20 ) of 0.55 N m3h ] with 6 kg of pellets (containing 4.2 kg of metal hydride) in the hydrogen pressure range 0.3-10.8 MPa and a product yield of 90% have been achieved in a purification system consisting of two series of reactors and a tank which stores relatively purified hydrogen. The rapid absorption and desorption of hydrogen in this system is due to the use of an absorber with high thermal conductivity and resistance to disintegration and heat variation as thermal ballast. The hydrogen purification system with pellets using the pressure swing absorption method is advantageous from the standpoint of the conservation of energy during system operation. On the other hand, it is suggested that the product yield of purified hydrogen decreases with increasing impurity concentration in the feed gas because of the mixing of impurities and rapidly released hydrogen from the pellets. An investigation was conducted to evaluate the distribution of carbon in the pellets. The results indicate that the CH 4 concentration increase can be explained via the synthesis of cn 4 by reaction between hydrogen and carbon in the pellets followed by desorption of adsorbed CH 4 from the pellet surface. Keywords: Purification; PSA; Pellet; FeTi; Ch 4

1. Introduction 2. Experimental details Metal hydrides have been studied for various in- 2.1. Pellet preparation dustrial applications [1]. Hydrogen separation studies have been performed most effectively using FeTi0.95Mm0.08 alloys were prepared by arc melting their intrinsic property of removing impurity gases and subsequent grinding to powders of grain size less from hydrogen. Several studies of hydrogen purifica- than 150 txm. Each of the alloy powders was mixed tion systems have been made [2-6]. Rundman et al. with electrolytic Cu powder of grain size less than [7] proposed the use of thermal ballast in order to 10 Ixm. The mixture was compacted to pellets 10 mm control temperature swings in the hydriding-dehy- thick and 18 mm in diameter. The pellets have a hole driding cycle of the hydrogen purification system. 6 mm in diameter at the centre to allow hydrogen flow. Ron and coworkers [8,9] and Tuscher et al. [10] The pellets were sintered in vacuum. The thermo- proposed the use of porous metallic matrix hy- dynamic properties of FeTi0.95Mm0.08 sintered pellets drides, have been studied in relation to the metal hydride We have investigated the hydrogen absorption and structure and a suitable reactor design for the pellets desorption properties of FeTi and FeTi0.95Mm0.08 [11]. A Cu content of 30 wt.%, a compacting pressure (mm, misch metal) pellets sintered with Cu powder of 4400kgcm -z and a sintering temperature of 850°C [11]. The operating conditions and pellet properties were employed for this experiment. The operating are discussed with respect to the hydrogen purification pressure range of 0.3-10.8 MPa was selected for ability, practical use. The operating temperature was varied in Elsevier Science S.A. SSDI 0925-8388(95)01774-7

866 T. Saitou, K. Sugiyama / Journal of Alloys and Compounds 2_31 (1995) 865-870

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II II ~B .... ' -- P.T.D aeuum Pump Purging Gas Feed 6as Product H~ Fig. 1. Schematic illustration of hydrogen purification system. the range 25-90°C. The pressure-composition iso- 8 a h J//"f therms were determined using a conventional Sieverts 4 ,,/~f /~ apparatus. ~,(~//,, / "~ 2 ii 2.2. Hydrogen purification system ~: 11 The hydrogen purification system is illustrated ~ 0t-8 schematically in Fig. 1. There are six reactors in the ~ o.~ /(, water bath, the temperature of which was controlled ~ /-" automatically. The feed gas was first introduced into ~" 0.2 / /' / three reactors on one side. After the absorption was ,- complete, about 10% of the hydrogen was blown off ~ 0.1 O. the reactors so that the purity of the exit gas was -o" 0.0a Pellet (80"C) raised to 99.99999%. The next half-cycle was per- ~ 0.06 formed with the three reactors on the other side. The "- 0.04 - b P e 1 l e t (2 0"C ) electromagnetic valves and air-operated valves in the c P o ~d e r (2 5"C ) system were adjusted so as to obtain purified hydrogen 0.02 t I l I I J l continuously. The hydriding-dehydriding cycle time 0 o.l 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 was varied to obtain suitable operating conditions. The A t o r. i e tt a t i o ( H / ~i ) mass of the pellets was 6 kg in this system. Fig. 2. Pressure-composition isotherms for FeTiog~Mmoo ~ pellets The impurity concentration in the released hydro- sintered with copper at 850 °C. " " gen was measured by quadrupole mass spectrometry with gas chromatography. In the gas chromatography shown in Fig. 2. The adsorption pressure of the column 5a a molecular sieve was used to detect 02, powder specimen at 25 °C is almost the same as that of N 2, Ch 4 and Ar. Commercial grade hydrogen (purity the pellet specimen at 80 °C. The van't Hoff plots for 99.99%) which had been passed through a pretreat- the powder and pellet specimens are shown in Fig. 3. ment unit to remove water vapour was used in this Low pressure hydriding and small hysteresis can be experiment, achieved with the sintered pellets. During the sintering at high temperature, substitution of Cu for Ti is considered to take place in the Fe-Ti region. The 3. Results and discussion reaction results in lattice expansion, leading to a low dissociation pressure [11]. The dissociation pressure 3.1. Pellet properties decreased with increasing sintering temperature and diffusion time. The pressure-composition isotherms for FeTi0.95- Pellet specimens can be used in practice for hydro- Mmo.o8 pellets sintered with copper at 850 °C are gen purification in the range of hydrogen pressure