Study on the effect of hydrogen purification with metal hydride
WHEC 16 / 13-16 June 2006 – Lyon France
Study on the effect of hydrogen purification with metal hydride Tae-Hwan Kima, Jung-Sik Choia, Ko-Yeon Chooa, Jae-Suk Sunga , Heondo Jeonga a
Hydrogen System Research Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-ku, Daejeon 305-343, Korea (e-mail : [email protected])
ABSTRACT: The effects of hydrogen purification with a AB5-type metal hydride were studied for the development of hydrogen purification system. The system set up two packed-beds, heat exchangers, data acquisition equipment and automatic control unit was used in the work and the compositions of two different gasmixtures have CO, CH4, CO2, O2 and N2. We investigated about its tolerance against impurities, pressurecompsition-isotherm and life cycle test, XRD and particle size analysis with a used metal hydride. Gas chromatograph was used for the analysis of feed and product gas. The used metal hydride is a La, Nd-rich Mm-based AB5 type which has the hydrogen storage capacity of 1.4 wt%. In life cycle test, there were no change of plateau pressure and hysteresis after 600 cycles but hydrogen storage capacity was decreased by about 6.8% and 10.7% after 220, 600 cycles, respectively. The used sample is high strong against CH4 and CO2 but very weak in CO atmosphere. The hydrogen purification performance with gas mixtures was decreased in the order of CH4 > CO > O2 > N2 > CO2. The reason CO investigated high purification effect in gas mixture is due to a strong chemisorption in metal hydride matrix that CO was not released out of the alloy. B
KEYWORDS : Metal hydride; Hydrogen purification; AB5-type; Hydrogen storage
1. Introduction Metal hydrides have been studied for various industrial applications. Selective adsorption of hydrogen by metal hydrides which can adsorb/desorb hydrogen reversibly offers the possibility of hydrogen separation from mixed gas and hydrogen purification to high level. Hydrogen can be basically be stored in gaseous or liquid form as well as chemically bonded in metal hydrides. By adsorbing hydrogen, the metals form a metal hydride release reaction heat simultaneously. The storage of hydrogen in a hydride-forming metal is generally expressed by the chemical reaction equation [1, 2]: Δr H ⎯−⎯ ⎯→ yMe + xH 2 Me y H 2 x ←⎯ ⎯⎯ +Δr H
(1)
At as definite temperature, a hydrogen storage alloy has a definite equilibrium pressure (or plateau pressure) Peq, which increases exponential with temperature. When an H2 containing gas flows through a hydrogen storage bed, and if the hydrogen partial pressure PH2 of the gas is higher than Peq, hydrogen reactd with the alloy to form a metal hydride. All the remaining gas contents which do not react with alloy flow out from the bed continuously (Fig. 1). As a rule, in the bed, along the direction of flow of the gas, the hydrogen contents decreases sharply in a narrow region called the reaction front, which advantage gradually in the direction. The entire bed is saturated with hydrogen as the reaction front moves to the end of the bed. In the meantime, hydrogen is stored in the bed in the form of hydride. After a blow-off operation, with the inlet valves closed, high purity hydrogen can be transported in the containers vary safely to the location for usage, as the hydrogen is now stored in the “solid stat instead of as a high pressure gas or liquefied hydrogen [3]. Several studied of hydrogen purification system by metal hydrides have been investigated [4-8]. Rundman et al. [9] proposed the use of thermal ballast in order to control temperature swings in the hydridingdehydriding cycle of the hydrogen purification system. Ron and coworkers [10, 11] and Tuscher et al. [12] proposed the use of porous metallic matrix hydrides. Also, some potentially promising techniques for hydrogen purification, e. g. the use of metal alloy slurries in organic solvent, have been development. 1/10
WHEC 16 / 13-16 June 2006 – Lyon France
However, commercial applications have not been achieved, because of the extreme intolerance metal alloy surface to impurities in hydrogen such as carbon monoxide, carbon dioxide, methane, water, oxygen, nitrogen, etc. Some researchers have investigated [13] that carbon monoxide seriously poisons AB5 metal alloy at low temperature. A monolayer of carbon monoxide chemisorbed strongly on the metal alloy surface can totally destroy the hydriding ability. Other impurities such as oxygen and water, even at low concentration level, can cause serious problems in applications involving cyclic system because oxygen reacts with metal alloy causing an irreversible decay. Therefore, the successful application of metal alloy for hydrogen purification from mixed gases included impurities will depend on the tolerance of the metal surface to various impurities in hydrogen uptake act as poisons metal alloy. It is believed that the practical applications of hydrogen purification using metal alloy might be impossible unless hydriding alloys can be made more tolerance to impurities. Reported in this work are characterization, hydrogen purification test in mixed gases containing hydrogen, methane, carbon monoxide, oxigen, nitrogen and carbon dioxide and tolerance abilities of metal hydrides composed by La, Nd-rich Mm-based AB5 type. The tolerance abilities on the metal hydrides for impurities was investigated in detail, with an aim of minimizing impurities deposition on the metal hydrides surface and improving cyclic hydriding/dehydriding performance of the metal hydrides.
Fig. 1. The principle of hydrogen purification. 2. Experimental La, Nd-rich Mm-based AB5 type metal alloy was prepared by arc melting and subsequent grinding to powder of grain size less than 50 micro meters. In short, La and Nd rich Mn metal with desired mole ratio were mixed in the arc melter, and the sample port was in vacuum under 5.0 × 10-5 torr. Argon gas with ultra high purity was then injected into the unit cell with 10 torr pressure in a sample port to generate arc. After arc was generated between mixed metal and electrode, La, Nd-rich Mm-based AB5 type metal alloy was obtained. In order to be uniform composition of alloy, arc melting was processed for 3 – 4 times. The sample mass was 6 kg/unit cell and initial hydrogen pressure was 9 – 10 atm, temperature was set as 20oC at adsorption and 40 oC at desorption. The concentration of impurities in hydrogen was 196.1 – 197.8 ppm carbon monoxide, 196.4 – 196.8 ppm methane, 200.7 – 202.7 ppm carbon dioxide, 201.7 – 203.5 ppm oxygen and 201.1 – 203.9 ppm nitrogen. The hydrogen purification system is showed schematically in Fig. 2. There are two purification cells equipped with automatic temperature control system by circulate water around the unit cell. The feed gas was first introduced into AD 1 reactor on one side. During the adsorption was proceeded, other side reactor, AD 2, was released purified hydrogen in order to operate purification system continuously. The electromagnetic valves and air-operated valves in the system were adjusted so as to obtain purified hydrogen. The capacity of hydrogen purification system was 1Nm3/h and the dimension of unit cell was 1,066 mm in length, 700 mm in width, and 1,500 mm in height. Surface states of the metal alloys were investigated by XRD measurements (Rigaku, D/MAX-2000), and the sizes of metal ally particles were determined using particle size analyzer. The impurity concentration in the released hydrogen was measured by gas chromatograph (HP 6890). In the gas chromatography column 5A molecular sieve was used to detect impurities. Tolerance of metal alloy was obtained by pressure2/10
WHEC 16 / 13-16 June 2006 – Lyon France
composition-isotherm using a conventional Sieverts apparatus. Maximum cycle was 600 times and the concentration impurity was 961 ppm carbon monoxide, 1,010 ppm carbon dioxide and 1,020 ppm methane. Commercial grade hydrogen (purity 99.9 %) which had been passed through a pre-treatment unit to remove water vapour was used in this experiment.
Fig. 2. Schematic illustration of hydrogen purification system. 3. Results and discussion The pressure-composition isotherms for La, Nd-rich Mm-based AB5 type metal alloy are shown in Fig. 3. They show a well-defined plateau in the cycle range up to 600 cycles. The hysteresis is not significant and the obtained maximum equilibrium pressure differences are almost same as the function of cycle. For the pressure-composition isotherm of fresh alloy, an absorption equilibrium pressure of 13atm is observed with a hydrogen concentration of 1.4 wt.%. Absorbed hydrogen concentration on the alloy is slowly decreased with increasing adsorb/desorb cycle. After 220 and 600 cycles, the hydrogen concentration in the alloy is decreased about 6.8 % and 10.7 % respectably compared with fresh metal alloy. This result implies that La, Nd-rich Mm-based AB5 type metal alloy has a good hydrogen storage capacity and tolerance ability against the cycling rate degradation of hydrogen adsorb/desorb. The Van’t Hoff plots for the fresh and used alloy are shown in Fig. 4. Low pressure hydriding and small hysteresis can be achieved with the fresh alloy. The La, Nd-rich Mm-based AB5 type metal alloy exhibited about 30 kJ/mol H2 reaction enthalpy and 112J/mol H2 K reaction entropy (Table 1).
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Fig. 3. Pressure-composition isotherms for La, Nd-rich Mm-based AB5 type metal alloy.
Fig. 4. Van’t Hoff plot for La, Nd-rich Mm-based AB5 type metal alloy. Table 1 Mean reaction enthalpies and mean reaction entropies
Metal hydride alloy
Reaction entalpy, ΔrH (kJ/mol H2)
Reaction entropy, ΔrS (J/mol H2 K)
Fresh metal alloy
30 ± 2
112 ± 4
After 200 cycles
35 ± 3
138 ± 7
After 600 cycles
36 ± 3
158 ± 7
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Fig. 5 shows the XRD patterns of fresh and used La, Nd-rich Mm-based AB5 type metal alloy. In the fresh and used alloy, La crystalline phases were clearly observed at 2θ = 30, 42, and 43o. Each sample have no change the intensity of crystalline La peaks. This indicates that phase of metal alloy is not changed after cycling adsorb/desorb hydrogen test, leading to strong trolerance ability. La particle sizes in the fresh and used alloy determined from X-ray line broadening of La peak (2θ = 43o) are almost same, indicating that La particles are not sintered during the cycling test.
Fig. 5. XRD patterns of La, Nd-rich Mm-based AB5 type metal alloy. Particle sizes of metal alloy for fresh and used alloy are determined by particle size analyzer. Fig. 6 shows particle size distribution of fresh and used 600 cycles alloy. Particle sizes of metal alloy are decreased with increasing cycling test. At fresh alloy, particle size distribution shows a maximum peak about 25 micro meters, however particle size distribution for used alloy shows a maximum peak about 10 micro meters. Also, distribution profile alloy obtained after cycling test are narrow and almost symmetrical. This indicates that reaction heat during the adsorb/desorb of hydrogen can be acted as a grinding effect. The metal grinding effect by reaction heat should be minimized because this can be acted as reducing factor for hydrogen storage capacity of metal alloy.
(a)
(b)
Fig. 6. Particle size distribution of fresh (a) and after 600 cycle test metal alloy (b).
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The tolerance of impurity gases such as methane (1,020 ppm), carbon dioxide (1,010 ppm) and carbon dioxide (961 ppm) are examined at 20oC of adsorption and 40oC of desorption. The changes of hydrogen concentration in the metal alloy as a function of time are shown in Fig. 7, 8 and 9. It is observed that the hydrogen concentration in the metal alloy containing methane or carbon dioxide is almost the same as that using pure hydrogen. In the case using hydrogen + carbon monoxide, a significantly decease of hydrogen storage capacity with time is observed. The hydrogen storage capacity of metal alloy rapidly decrease from the 1st cycle and after the 2nd cycle, hydrogen storage ability of metal alloy is almost disappeared. These results indicated that a significant deactivation by carbon monoxide poisoning has been obtained for the La, Nd-rich Mm-based AB5 type metal alloy. Sandrock and Goodell [13-15] have reported similar phenomena that carbon monoxide seriously poisons the AB5-type allow at lo temperature (< 373.2 K) and leads to complete loss of hydrogen transfer capability.
Fig. 7. Tolerance ability as a function of time in hydrogen containing 1,020 ppm methane at 20oC of adsorption and 40oC of desorption.
Fig. 8. Tolerance ability as a function of time in hydrogen containing 1,010 ppm carbon dioxide at 20oC of adsorption and 40oC of desorption. 6/10
WHEC 16 / 13-16 June 2006 – Lyon France
Fig. 9. Tolerance ability as a function of time in hydrogen containing 961 ppm carbon monoxide at 20oC of adsorption and 40oC of desorption. In order to regenerate metal alloy by poisoning carbon monoxide, used metal alloy are treated by increasing the temperature from 100oC to 160oC with vacuum condition. Fig. 10 shows effect of temperature on the regeneration step. Without heating, only vacuum condition, there is no hydrogen storage capacity similar to behaviour of untreated sample. With heating, however, hydrogen storage capacity is increasing with increasing treatment temperature. At the treatment temperature of 160oC, metal alloy is obtained where almost no degradation of hydrogen storage capacity is observed with time. From these observations, it was found that the damage caused by carbon monoxide was reduced to a great extent by the heat treatment of alloy. High activity was maintained after many adsorb/desorb of hydrogen cycles for regenerating alloy even at low temperature. However, the untreated alloy was severely deactivated under the same conditions. The regenerative nature of La, Nd-rich Mm-based AB5 type metal alloy encouraged the application of hydrogen purification using metal hydrides.
Fig. 10. The regeneration ability of La, Nd-rich Mm-based AB5 type metal alloy by heat treatment. 7/10
WHEC 16 / 13-16 June 2006 – Lyon France
The hydrogen purification behaviour of La, Nd-rich Mm-based AB5 type metal alloy has been investigated at hydriding and dehydriding temperature of 20oC and 40oC respectively under a pressure of 9 – 10 atm. The impurity concentration in the hydrogen released from the unit cell is shown in Fig. 11 and 12. As for feed gas impurities, gas mixture I, the concentration of carbon monoxide is 196.1 ppm, that of methane is 196.8 ppm, that of carbon dioxide is 200.7 ppm that of oxygen is 201.7ppm and that of nitrogen is 201.1 ppm. Similarly, the composition of gas mixture II is 197.8 ppm of carbon monoxide, 196.4 ppm of methane, 202.4 ppm of carbon dioxide, 203.5 ppm of oxygen and 203.9 ppm of nitrogen. It is clear that nitrogen and carbon dioxide are remained high concentration in the desorbed gas, while the concentration of methane remains about 0.3 ppm during dehydriding. Similarly excellent purification efficiency is obtained against carbon monoxide. It is observed that the concentration of carbon monoxide was reduced to a level about 7 – 8 ppm. It can also be seen that there is no degradation of purification efficiency after regeneration by heat treatment.
feed product 200
196.1
200.7
196.8
202.1
201.7
180
Conc. of impurity, ppm
160 140 120 100 80 60 40
31.80
25.30
20
8.30
12.20
0.28
0 CO
CH4
CO2
O2
N2
Gas mixture
Fig. 11 Concentration of impurities in hydrogen released from unit cell at gas mixture I.
feed product 200
197.8
202.4
198.4
203.9
203.5
180
Conc. of impurity, ppm
160 140 120 100 80 60 40 20
13.50
7.96
1.00
0.30
4.70
0 CO
CH4
CO2
O2
N2
Gas mixture
Fig. 12 Concentration of impurities in hydrogen released from unit cell at gas mixture II.
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The product yield obtained using this purification system with hydrogen storage tank was 90% and without the storage tank 85%. Purified hydrogen was obtained constantly at a rate of 1Nm3/h with 6 kg of metal alloy in this purification system. 4. Conclusions Hydrogen purification with mixed gas was investigated over a Metal hydride such as AB5-type containing La and Nd. La, Nd-rich Mm-based AB5 type metal alloy showed high hydrogen storage capacity and hydrogen purification ability. The principle problem with practical application of hydrogen purification was the extreme sensitivity of the alloy surface to carbon monoxide. By means of heat treatment with vacuum, metal alloy poisoning by carbon are regenerated. Gas purification using this metal alloy can provide ultra high pure hydrogen after a few percent release of hydrogen at low temperature. The hydrogen purification performance with gas mixtures was decreased in the order of CH4 > CO > O2 > N2 > CO2.
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References [1] G. Sandrock, Metal hydride technology, in: Energieträger Wasserstoff, VDI, Düsseldorf, (1991) 143. [2] H. Buchner, Energiespeicherung in Metallhydriden, Springer, New York, (1982). [3] Ming Au et al., Int. J. Hydrogen Energy, 21, 1 (1996) 33. [4] J.J. Sheriden et al., J. Less-Commom Met., 89 (1983) 447. [5] T. Gamo et al., J. Less-Commom Met., 89 (1983) 495. [6] C. Chen et al., Phys. Chem., 183 (1994) 251. [7] O. Benauer et al., J. Less-Commom Met., 131 (1987) 213. [8] F.R. Block et al., J. Less-Commom Met., 131 (1987) 329. [9] P.S. Rundman et al., J. Less-Commom Met., 89 (1983) 437. [10] M. Ron et al., J. Less-Commom Met., 74 (1980) 445. [11] Y. Josephy et al., J. Less-Commom Met., 90 (1984) 297. [12] E. Tuscher et al., Int. J. Hydrogen Energy, 8, 3 (1983) 199. [13] G.D. Sandrock et al., J. Less-Commom Met., 104 (1984) 159. [14] F.G. Eisenberg et al., J. Less-Commom Met., 89 (1983) 55. [15] P.D. Goodell et al., J. Less-Commom Met., 89 (1983) 45.
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Study on the effect of hydrogen purification with metal hydride Tae-Hwan Kima, Jung-Sik Choia, Ko-Yeon Chooa, Jae-Suk Sunga , Heondo Jeonga a
Hydrogen System Research Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-ku, Daejeon 305-343, Korea (e-mail : [email protected])
ABSTRACT: The effects of hydrogen purification with a AB5-type metal hydride were studied for the development of hydrogen purification system. The system set up two packed-beds, heat exchangers, data acquisition equipment and automatic control unit was used in the work and the compositions of two different gasmixtures have CO, CH4, CO2, O2 and N2. We investigated about its tolerance against impurities, pressurecompsition-isotherm and life cycle test, XRD and particle size analysis with a used metal hydride. Gas chromatograph was used for the analysis of feed and product gas. The used metal hydride is a La, Nd-rich Mm-based AB5 type which has the hydrogen storage capacity of 1.4 wt%. In life cycle test, there were no change of plateau pressure and hysteresis after 600 cycles but hydrogen storage capacity was decreased by about 6.8% and 10.7% after 220, 600 cycles, respectively. The used sample is high strong against CH4 and CO2 but very weak in CO atmosphere. The hydrogen purification performance with gas mixtures was decreased in the order of CH4 > CO > O2 > N2 > CO2. The reason CO investigated high purification effect in gas mixture is due to a strong chemisorption in metal hydride matrix that CO was not released out of the alloy. B
KEYWORDS : Metal hydride; Hydrogen purification; AB5-type; Hydrogen storage
1. Introduction Metal hydrides have been studied for various industrial applications. Selective adsorption of hydrogen by metal hydrides which can adsorb/desorb hydrogen reversibly offers the possibility of hydrogen separation from mixed gas and hydrogen purification to high level. Hydrogen can be basically be stored in gaseous or liquid form as well as chemically bonded in metal hydrides. By adsorbing hydrogen, the metals form a metal hydride release reaction heat simultaneously. The storage of hydrogen in a hydride-forming metal is generally expressed by the chemical reaction equation [1, 2]: Δr H ⎯−⎯ ⎯→ yMe + xH 2 Me y H 2 x ←⎯ ⎯⎯ +Δr H
(1)
At as definite temperature, a hydrogen storage alloy has a definite equilibrium pressure (or plateau pressure) Peq, which increases exponential with temperature. When an H2 containing gas flows through a hydrogen storage bed, and if the hydrogen partial pressure PH2 of the gas is higher than Peq, hydrogen reactd with the alloy to form a metal hydride. All the remaining gas contents which do not react with alloy flow out from the bed continuously (Fig. 1). As a rule, in the bed, along the direction of flow of the gas, the hydrogen contents decreases sharply in a narrow region called the reaction front, which advantage gradually in the direction. The entire bed is saturated with hydrogen as the reaction front moves to the end of the bed. In the meantime, hydrogen is stored in the bed in the form of hydride. After a blow-off operation, with the inlet valves closed, high purity hydrogen can be transported in the containers vary safely to the location for usage, as the hydrogen is now stored in the “solid stat instead of as a high pressure gas or liquefied hydrogen [3]. Several studied of hydrogen purification system by metal hydrides have been investigated [4-8]. Rundman et al. [9] proposed the use of thermal ballast in order to control temperature swings in the hydridingdehydriding cycle of the hydrogen purification system. Ron and coworkers [10, 11] and Tuscher et al. [12] proposed the use of porous metallic matrix hydrides. Also, some potentially promising techniques for hydrogen purification, e. g. the use of metal alloy slurries in organic solvent, have been development. 1/10
WHEC 16 / 13-16 June 2006 – Lyon France
However, commercial applications have not been achieved, because of the extreme intolerance metal alloy surface to impurities in hydrogen such as carbon monoxide, carbon dioxide, methane, water, oxygen, nitrogen, etc. Some researchers have investigated [13] that carbon monoxide seriously poisons AB5 metal alloy at low temperature. A monolayer of carbon monoxide chemisorbed strongly on the metal alloy surface can totally destroy the hydriding ability. Other impurities such as oxygen and water, even at low concentration level, can cause serious problems in applications involving cyclic system because oxygen reacts with metal alloy causing an irreversible decay. Therefore, the successful application of metal alloy for hydrogen purification from mixed gases included impurities will depend on the tolerance of the metal surface to various impurities in hydrogen uptake act as poisons metal alloy. It is believed that the practical applications of hydrogen purification using metal alloy might be impossible unless hydriding alloys can be made more tolerance to impurities. Reported in this work are characterization, hydrogen purification test in mixed gases containing hydrogen, methane, carbon monoxide, oxigen, nitrogen and carbon dioxide and tolerance abilities of metal hydrides composed by La, Nd-rich Mm-based AB5 type. The tolerance abilities on the metal hydrides for impurities was investigated in detail, with an aim of minimizing impurities deposition on the metal hydrides surface and improving cyclic hydriding/dehydriding performance of the metal hydrides.
Fig. 1. The principle of hydrogen purification. 2. Experimental La, Nd-rich Mm-based AB5 type metal alloy was prepared by arc melting and subsequent grinding to powder of grain size less than 50 micro meters. In short, La and Nd rich Mn metal with desired mole ratio were mixed in the arc melter, and the sample port was in vacuum under 5.0 × 10-5 torr. Argon gas with ultra high purity was then injected into the unit cell with 10 torr pressure in a sample port to generate arc. After arc was generated between mixed metal and electrode, La, Nd-rich Mm-based AB5 type metal alloy was obtained. In order to be uniform composition of alloy, arc melting was processed for 3 – 4 times. The sample mass was 6 kg/unit cell and initial hydrogen pressure was 9 – 10 atm, temperature was set as 20oC at adsorption and 40 oC at desorption. The concentration of impurities in hydrogen was 196.1 – 197.8 ppm carbon monoxide, 196.4 – 196.8 ppm methane, 200.7 – 202.7 ppm carbon dioxide, 201.7 – 203.5 ppm oxygen and 201.1 – 203.9 ppm nitrogen. The hydrogen purification system is showed schematically in Fig. 2. There are two purification cells equipped with automatic temperature control system by circulate water around the unit cell. The feed gas was first introduced into AD 1 reactor on one side. During the adsorption was proceeded, other side reactor, AD 2, was released purified hydrogen in order to operate purification system continuously. The electromagnetic valves and air-operated valves in the system were adjusted so as to obtain purified hydrogen. The capacity of hydrogen purification system was 1Nm3/h and the dimension of unit cell was 1,066 mm in length, 700 mm in width, and 1,500 mm in height. Surface states of the metal alloys were investigated by XRD measurements (Rigaku, D/MAX-2000), and the sizes of metal ally particles were determined using particle size analyzer. The impurity concentration in the released hydrogen was measured by gas chromatograph (HP 6890). In the gas chromatography column 5A molecular sieve was used to detect impurities. Tolerance of metal alloy was obtained by pressure2/10
WHEC 16 / 13-16 June 2006 – Lyon France
composition-isotherm using a conventional Sieverts apparatus. Maximum cycle was 600 times and the concentration impurity was 961 ppm carbon monoxide, 1,010 ppm carbon dioxide and 1,020 ppm methane. Commercial grade hydrogen (purity 99.9 %) which had been passed through a pre-treatment unit to remove water vapour was used in this experiment.
Fig. 2. Schematic illustration of hydrogen purification system. 3. Results and discussion The pressure-composition isotherms for La, Nd-rich Mm-based AB5 type metal alloy are shown in Fig. 3. They show a well-defined plateau in the cycle range up to 600 cycles. The hysteresis is not significant and the obtained maximum equilibrium pressure differences are almost same as the function of cycle. For the pressure-composition isotherm of fresh alloy, an absorption equilibrium pressure of 13atm is observed with a hydrogen concentration of 1.4 wt.%. Absorbed hydrogen concentration on the alloy is slowly decreased with increasing adsorb/desorb cycle. After 220 and 600 cycles, the hydrogen concentration in the alloy is decreased about 6.8 % and 10.7 % respectably compared with fresh metal alloy. This result implies that La, Nd-rich Mm-based AB5 type metal alloy has a good hydrogen storage capacity and tolerance ability against the cycling rate degradation of hydrogen adsorb/desorb. The Van’t Hoff plots for the fresh and used alloy are shown in Fig. 4. Low pressure hydriding and small hysteresis can be achieved with the fresh alloy. The La, Nd-rich Mm-based AB5 type metal alloy exhibited about 30 kJ/mol H2 reaction enthalpy and 112J/mol H2 K reaction entropy (Table 1).
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Fig. 3. Pressure-composition isotherms for La, Nd-rich Mm-based AB5 type metal alloy.
Fig. 4. Van’t Hoff plot for La, Nd-rich Mm-based AB5 type metal alloy. Table 1 Mean reaction enthalpies and mean reaction entropies
Metal hydride alloy
Reaction entalpy, ΔrH (kJ/mol H2)
Reaction entropy, ΔrS (J/mol H2 K)
Fresh metal alloy
30 ± 2
112 ± 4
After 200 cycles
35 ± 3
138 ± 7
After 600 cycles
36 ± 3
158 ± 7
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Fig. 5 shows the XRD patterns of fresh and used La, Nd-rich Mm-based AB5 type metal alloy. In the fresh and used alloy, La crystalline phases were clearly observed at 2θ = 30, 42, and 43o. Each sample have no change the intensity of crystalline La peaks. This indicates that phase of metal alloy is not changed after cycling adsorb/desorb hydrogen test, leading to strong trolerance ability. La particle sizes in the fresh and used alloy determined from X-ray line broadening of La peak (2θ = 43o) are almost same, indicating that La particles are not sintered during the cycling test.
Fig. 5. XRD patterns of La, Nd-rich Mm-based AB5 type metal alloy. Particle sizes of metal alloy for fresh and used alloy are determined by particle size analyzer. Fig. 6 shows particle size distribution of fresh and used 600 cycles alloy. Particle sizes of metal alloy are decreased with increasing cycling test. At fresh alloy, particle size distribution shows a maximum peak about 25 micro meters, however particle size distribution for used alloy shows a maximum peak about 10 micro meters. Also, distribution profile alloy obtained after cycling test are narrow and almost symmetrical. This indicates that reaction heat during the adsorb/desorb of hydrogen can be acted as a grinding effect. The metal grinding effect by reaction heat should be minimized because this can be acted as reducing factor for hydrogen storage capacity of metal alloy.
(a)
(b)
Fig. 6. Particle size distribution of fresh (a) and after 600 cycle test metal alloy (b).
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The tolerance of impurity gases such as methane (1,020 ppm), carbon dioxide (1,010 ppm) and carbon dioxide (961 ppm) are examined at 20oC of adsorption and 40oC of desorption. The changes of hydrogen concentration in the metal alloy as a function of time are shown in Fig. 7, 8 and 9. It is observed that the hydrogen concentration in the metal alloy containing methane or carbon dioxide is almost the same as that using pure hydrogen. In the case using hydrogen + carbon monoxide, a significantly decease of hydrogen storage capacity with time is observed. The hydrogen storage capacity of metal alloy rapidly decrease from the 1st cycle and after the 2nd cycle, hydrogen storage ability of metal alloy is almost disappeared. These results indicated that a significant deactivation by carbon monoxide poisoning has been obtained for the La, Nd-rich Mm-based AB5 type metal alloy. Sandrock and Goodell [13-15] have reported similar phenomena that carbon monoxide seriously poisons the AB5-type allow at lo temperature (< 373.2 K) and leads to complete loss of hydrogen transfer capability.
Fig. 7. Tolerance ability as a function of time in hydrogen containing 1,020 ppm methane at 20oC of adsorption and 40oC of desorption.
Fig. 8. Tolerance ability as a function of time in hydrogen containing 1,010 ppm carbon dioxide at 20oC of adsorption and 40oC of desorption. 6/10
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Fig. 9. Tolerance ability as a function of time in hydrogen containing 961 ppm carbon monoxide at 20oC of adsorption and 40oC of desorption. In order to regenerate metal alloy by poisoning carbon monoxide, used metal alloy are treated by increasing the temperature from 100oC to 160oC with vacuum condition. Fig. 10 shows effect of temperature on the regeneration step. Without heating, only vacuum condition, there is no hydrogen storage capacity similar to behaviour of untreated sample. With heating, however, hydrogen storage capacity is increasing with increasing treatment temperature. At the treatment temperature of 160oC, metal alloy is obtained where almost no degradation of hydrogen storage capacity is observed with time. From these observations, it was found that the damage caused by carbon monoxide was reduced to a great extent by the heat treatment of alloy. High activity was maintained after many adsorb/desorb of hydrogen cycles for regenerating alloy even at low temperature. However, the untreated alloy was severely deactivated under the same conditions. The regenerative nature of La, Nd-rich Mm-based AB5 type metal alloy encouraged the application of hydrogen purification using metal hydrides.
Fig. 10. The regeneration ability of La, Nd-rich Mm-based AB5 type metal alloy by heat treatment. 7/10
WHEC 16 / 13-16 June 2006 – Lyon France
The hydrogen purification behaviour of La, Nd-rich Mm-based AB5 type metal alloy has been investigated at hydriding and dehydriding temperature of 20oC and 40oC respectively under a pressure of 9 – 10 atm. The impurity concentration in the hydrogen released from the unit cell is shown in Fig. 11 and 12. As for feed gas impurities, gas mixture I, the concentration of carbon monoxide is 196.1 ppm, that of methane is 196.8 ppm, that of carbon dioxide is 200.7 ppm that of oxygen is 201.7ppm and that of nitrogen is 201.1 ppm. Similarly, the composition of gas mixture II is 197.8 ppm of carbon monoxide, 196.4 ppm of methane, 202.4 ppm of carbon dioxide, 203.5 ppm of oxygen and 203.9 ppm of nitrogen. It is clear that nitrogen and carbon dioxide are remained high concentration in the desorbed gas, while the concentration of methane remains about 0.3 ppm during dehydriding. Similarly excellent purification efficiency is obtained against carbon monoxide. It is observed that the concentration of carbon monoxide was reduced to a level about 7 – 8 ppm. It can also be seen that there is no degradation of purification efficiency after regeneration by heat treatment.
feed product 200
196.1
200.7
196.8
202.1
201.7
180
Conc. of impurity, ppm
160 140 120 100 80 60 40
31.80
25.30
20
8.30
12.20
0.28
0 CO
CH4
CO2
O2
N2
Gas mixture
Fig. 11 Concentration of impurities in hydrogen released from unit cell at gas mixture I.
feed product 200
197.8
202.4
198.4
203.9
203.5
180
Conc. of impurity, ppm
160 140 120 100 80 60 40 20
13.50
7.96
1.00
0.30
4.70
0 CO
CH4
CO2
O2
N2
Gas mixture
Fig. 12 Concentration of impurities in hydrogen released from unit cell at gas mixture II.
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The product yield obtained using this purification system with hydrogen storage tank was 90% and without the storage tank 85%. Purified hydrogen was obtained constantly at a rate of 1Nm3/h with 6 kg of metal alloy in this purification system. 4. Conclusions Hydrogen purification with mixed gas was investigated over a Metal hydride such as AB5-type containing La and Nd. La, Nd-rich Mm-based AB5 type metal alloy showed high hydrogen storage capacity and hydrogen purification ability. The principle problem with practical application of hydrogen purification was the extreme sensitivity of the alloy surface to carbon monoxide. By means of heat treatment with vacuum, metal alloy poisoning by carbon are regenerated. Gas purification using this metal alloy can provide ultra high pure hydrogen after a few percent release of hydrogen at low temperature. The hydrogen purification performance with gas mixtures was decreased in the order of CH4 > CO > O2 > N2 > CO2.
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