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Reaction kinetics of metal hydrides and their mixtures
119
Journal of the Less-Common Metals, 73 (1980) 119 - 126 0 ?%evier Sequoia &A., Lausanne -Printed in the Netherlands
REACTION KINETICS OF METAL HYDRIDES AND THEIR MIXTURES* S. SUDA and N. KOBAYASHI Department
of Chemical Engineering,
Kogakuin
University, Hachioji-shi, Tokyo
192
(Japan) K. YOSHIDA Central Research Laboratory,
Sekisui Chemical Industry Ltd., Misima-gun, Osaka 618
CJapaN
Summary Reaction rates in the hydriding and the dehydriding processes were studied experimentally for hydriding alloys such as LaNis, MmNis(Mm = mischmetall), aluminium-substituted mischmetall nickels, TiMnl.s and TiO.sZro.aCro,sMn,,. We also studied binary mixtures of these hydriding alloys to examine the effects of mixing on the kinetic properties. A term N was introduced for the evaluation of the pressure and temperature dependenees of the reaction rate.
1. Introduction Systematic surveys have been carried out to develop hydriding alloys that have high hydrogen storage capacity, high hydrogen storage density, low plateau pressure and low cost per unit of hydrogen storage capacity. These studies have been mainly concerned with developing ternary and quaternary alloys with low hysteresis and reasonable plateau pressures by adding third and fourth components to binary alloys such as TiFe hydrides [ 11, MmNi5 hydrides (Mm = mischmetall) [2 - 4] and TiCr,-type hydrides [ 5,6]. Apart from equilibrium aspects, greater emphasis should be placed on developing hydriding alloys with faster reaction rates, particularly in thermodynamic cycles such as heat pumping of low grade energies and power generation from industrial waste heat. Nevertheless, from the engineering point of view, it is quite inconvenient to apply a single hydride with fixed equilibrium and kinetic properties inherent in that hydride, A mixing method has been proposed by Suda and Uchida [7,8] and the effects of mixing on the equilibrium properties have been discussed in detail by Suda eCal. [ 91. *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11, 1980.
120
The purpose of this paper is to summarize a series of experimental studies of the reaction rates for several hyd~d~g alloys and to discuss the effects of mixing two different hydriding alloys on the kinetic properties. Rate studies were carried out for LaNis, MmNi,, aluminium-substituted mischmetall nickels, TiMnl.s, TiosZro&rosMnl_P and binary mixtures of their hydrides. The mixing of metal hydrides has two major advantages: it can improve the equilibrium behaviour of the original constituents; it permits ~exibility in the tempera~re range of application where the hyd~d~g and dehy~ding reactions are very fast.
2. Experimental procedures 2.1. Reaction rate measurements In most hydriding and dehydriding processes it takes appro~ma~ly 3 4 min to obtain a 100% reaction, but only a few tens of seconds to achieve a 70 - 80% reaction. Therefore special care must be taken to design a reaction cell in which thermal effects due to rapid reaction are eliminated. Figure 1 shows in some detail a double-walled reaction cell for rapid heat supply and rejection through the 0.5 mm space. Samples of 3 - 5 g were introduced into the cell and each experimental run was started after repeating the activation treatment with an automatic timer unit more than 50 times at room temperature and at a pressure of 40 - 80 atm. To prepare mixed hydriding alloys, non-activa~d samples were mixed to give a fixed composition and the mixed samples were then activated repeatedly in the same manner as the unmixed samples. As shown in Figs. 2 and 3, the hydriding and dehydriding reactions
24-
Fig, 1, Details of the double-walled reaction cell: 1, end cup; 2, filter of 2 pm sintered metal; 3, seal for thermocouple inlet; 4, inner tube for heating/cooling fluids; 6, reaction cell (2 ml); 6, O-ring seals; ‘7, C-C thermocouple 0.2 mm in diameter; 8, hydrogen gas inlet.
121
Pi
3, m E
$
Pf E a Pe
Hi
H
Hf
Conposition
I
115
mol-H2/mol-llI
Fig. 2. A simplified diagram for the isothermal hydriding reaction.
Composition
[ ml-ll2/,,!ol-M I
Fig. 3. A simplified diagram for the isothermal dehydriding reaction.
proceed linearly from Pi to P, or from Hi to Hf. Changes H - I& in hydride composition were easily converted into pressure terms using pressure-temperature-composition data which had previously been correlated as an analytical expression. The pressure changes were measured using a HewlettPackard 3052A data aquisition system which was connected directly to the experimental apparatus through electronic pressure detectors. The data were then stored in a tape cartridge for a period of 240 s with a scanning time of 0.2 s. The rate constants were computed using 600 data points in every experimental run. The reaction rate constants in both the hydriding and the dehydriding process were defined as functions of two determining factors, P-P, and H - I&, and are given in eqns. (1) and (3) (for eqns. (1) - (6), see Table 1). The value of S defined by the integral terms in eqns. (2) and (4) was evaluated from experimental pressure data and was then plotted against the time elapsed. The rate constant is given by the slope of the straight line in this type of plot. The term N was defined for evaluating the temperature and pressure dependences of the rate constant and is given in eqns. (5) and (6). A major
H
{(P -P,)/P~}m(Hf
dH
--~II)~ = Kabst
(4 -HI"
f
(31 &es =
CiH
reaction
J Hi ((P, - P)/Po}m’H”’
H
Desorption
pe PO 4
&es P
dH/dt K abs
N abs m (5)
amount of hydrogen absorbed in time dt reaction rate constant in absorption reaction (h-l ) reaction rate constant in desorption reaction (h-l ) hydrogen gas pressure exerted on metal hydride bed (MPa) equilibrium pressure (MPa) atmospheric pressure (MPa) maximum amount of hydrogen gas absorbed (mol Hz (mol metal)-’ )
PO
exp (A +23/T)
R T A B
PO
m’
P and H = H.)
amount of hydrogen gas absorbed at pressure P (mol Hz (mol metal)-’ ) frequency factor (h-l ) activation energy in absorption reaction (J (mol Hz)activation energy in desorption reaction (J (mol Hz)gas constant (J (mol Hz)-l K-l) absolute temperature (K) constant in the van’t Hoff equation InP, = A + B/T constant in the van% Hoff equation lnP, = A + B/T
%d
x
H
t = f, P =
= gdest
exp (A + B/T) -P
(The boundary conditions are as follows: at t 3: 0, P = Pi and H = Hi; at t = 00,P = Pf and H = Hf; at
Sabs =
Absorption reaction
Equations (1) - (6)
TABLE 1
(6)
(4)
(2)
123
advantage of N is that it facilitates the evaluation of properties and conditions such as the reactivity, the reaction temperature range, the reaction rate at a given temperature and pressure, the selection of a more reactive hyd~ding alloy and the optimum operating conditions in a given application.
3. Exper~en~l
results and discussion
Figure 4 shows S values computed for ~rn~i*.~Al~,~ hydride during hydriding at 30 “C. The extent of the reaction (%) is also shown. Rate constants in the hydriding and dehydrid~g reactions are shown in Figs. 5 and 6
Time
Esec3
4. S us. time for MmNi~.~Al*.~ hydride at 30 %. The extent of the reaction is also shown. Fig.
TsmpsraturoIdog.Cl 1QQ
Q0
‘xi3
68
Temperature
w
PQ
188 ) ,
1000 Baa
60 ,
,
60 /
j
Cdeg.CI
40 j
/
20 )
I I
/
Desorpt.ion
600
28 I --2.6
2.0
3.0
3.2 1080/T
3.4 CI.'Kl
3.6
3.8
101 2.6
’
1 ’ 2.8
’
’
3.8
’
f
3.2 1000/T
\ ’ 3.4
’
t 3.6
[i/T1
Fig. 5. F&ateconstant in the hydriding reaction us. reciprocal temperature for hydrides of T~~.~Zr~.~~~_aMn~,~~ LaNis and TiMnl.5. Fig. 6. Rate constant in the de~yd~~~ reaction vs. reciprocal temperature for hydrides of Tio.~Zro.~~o.~Mn~.z, LaNi5 and TiMnl.5.
124
253
273
Tempsratura 293 313
CK3 333
Temperature 353
373
253
273
293
CKI 333
313
353
373
"'7
Temperature
tdog.C3
Temperature
Cdcg.Cl
Fig. 7. Isobaric rate constants for TiMn,~S hydride in the hpdriding process. Fig. 8, Isobaric rate constants for LaNis hydride in the hydriding process,
for hydrides of TiMnt,&, L,aNi5and Ti0.sZro.zCr0.eMnl_2.Figures 7 - 9 show N values computed from egn, (5) for the hydriding of hydrides of TiMnle5, IJaNi, -d Ti~.szr~.~~~.s~~l_2 respectively. The N values of these three hydrides at 20 atm are compared in Fig. 10. It can be seen that N has a maximum in each isobar and that at a given isobaric condition the reaction temperature is restricted to a comp~tively narrow range. This means that the
253 253
273
Temparature 293 313
CK3 333
353
373
Tamparaturs 293 313
CKI 333
353
373
1B4
10'
-20
Tsmpsraturo
273
Edeg.Cl
Fig. 3, Isobaric rate constants for Tio.~Zr~.~~~,~M~~=~
0
20 Tsmporaturo
40
60
(90
108
Cdsg.C3
hydride in the hydriding wocesc;.
Fig. 10.1vvs. temperature for the hydriding reaction of hydrides of Ti~.~~ro,*~o.~M~~.~, LaNi5 and TiMnl.5 at a pressure of 20 atm.
12s
absorption reaction is fastest at the maximum value of N and that almost no reaction occurs at any temperature outside each loop covering an isobaric reaction. The reaction rates for the hydrides can be evaluated easily by comparing N values at a given pressure and temperature. As stated earlier, N is particularly useful for evaluating the pressure and temperature dependences of a reaction; e.g. 7iiMnr.s absorbs hydrogen very slowly whereas Tie.sZrO.aCrO.sMnl.s hydride is very reactive at comparatively low pressure and low temperature. Because N is significantly affected by temperature and pressure, it is considered to be a good measure of the reaction rate and the reactivity of a hydride. In addition N is valuable as a criterion for selecting hyd~ding alloys or mixtures for use at a given temperature and pressure. Of the three hydrides considered, LaNi, hydride is the best hydriding alloy because of its rate constant and available temperature range. Since the hydriding reaction is very dependent on the system pressure, it is preferable from a kinetic standpoint to use high pressure conditions in any given technological development. The dependence of N on pressure in a dehydriding reaction is illustrated in Fig. 11 for LaNi5 hydride. It can be seen that the dehydrogenation is strongly dependent on temperature. The rate at which hydrogen gas is released increases as the temperature increases. The effect of mixing on the absorption reaction rate was also studied for several mixtures of LaNi,, MmNis, aluminium-substituted mischmetall nickels, TiMn,., and Ti0.sZr0.eCr0.sMn1~2hydrides. N values for LaNisTie.sZrO.&%.sMnr.s mixed hydrides of various compositions are shown in Temperature 253 10’1
273
293
a 1 !
I
CKI
313
I
I
Temporatore
333
I
I
I
353
373
I
1 I
253
273
293
313
20
40
WI 333
353
373
60
00
li30
10’-
10” 8 -20
’
111
0
20
f
Tempcrnturo
I
’ 40
’
I, 60
Cdeg.CJ
,
, 80
,
’ 100
-20
0
Temperature
Cdsg.Cl
Fig. 11. Isobaric rate constants of LaNi5 hydride in the dehydriding process. Fig. 12. Isobaric rate constants of hydrides of LaNic, and Tio.sZro,&ko.+nl.2 (Ti-4) and their mixtures in the dehydriding process: curve A, 3: 7 LaNib :Ti-4; curve B, 1: 1 LaNiS :Ti-4; curve C, 7:3 LaNiS :Ti-4.
126
Fig. 12. A significant improvement in the reaction temperature range is obtained. The reaction temperature range is considerably increased by mixing two metal hydrides with different properties. Mixing of hydriding materials offers several technical advantages: (1) it can improve the equilibrium behaviour of the original constituents; (2) it makes it possible to control the reaction rate and the kinetic properties by varying the mixing ratio; (3) it allows flexibility in the temperature range of application where the hydriding and dehydriding reactions are very fast.
4. Conclusions The addition of a small amount of a more reactive hydride to a less reactive hydride is quite effective in improving the hydriding properties of the less reactive metal hydride. From the kinetic standpoint, the use of high pressure conditions is effective in closed-cycle applications, e.g. heat pump systems, chemical engines, thermal compressors and power generating plants, which require particularly rapid reaction rates with a comparatively limited amount of hydriding alloy. Mixing is known to increase the number of available metal hydrides and also offers valuable applications in the utilization of sources of low grade energy, e.g. industrial waste energy, solar energy, geothermal energy, oceanic thermal energy and atmospheric energy.
References 1 J. J. Reilly and J. R. Johnson, in Proc. 1st World Hydrogen Energy Conf., Miami Beach, Florida, March 1 - 3, 1976, Vol. 2, Pergamon, Oxford, 1976, Paper 8B, pp. 3 - 26. 2 G. D. Sandrock, in Proc. p2th Int. Energy Conversion Engineering Conf., Washington, D.C., August 28 - September 2, 1977, Vol. 1, pp. 951 - 958. 3 Y. Ohsumi, A. Kato, H. Suzuki, M. Nakane and Y. Miyake, Chem. Ind. Chem., 11 (1978) 1472 - 1477. 4 Y. Ohsumi, A. Kato, H. Suzuki, M. Nakane and Y. Miyake, J. Less-Common Met., 66 (1979) 67 - 75. 5 T. Gamo, Y. Moriwaki, N. Yanagihara, T. Yamashita and T. Iwaki, Nat1 Tech. Rep. (Matsushita Electr. Ind. Co., Osaka), 25 (5) (1979) 941 - 952. 6 Y. Machida, T. Yamadaya and M. Asanuma, Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August 14 - 19, 1977, Pergamon, Oxford, 1978, pp. 329 - 336. 7 S. Suda and M. Uchida, Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August 14 - 19, 1977, Pergamon, Oxford, 1978, pp. 515 - 525. 8 S. Suda and M. Uchida, in Hydrogen Energy System: Proc. 2nd World Hydrogen Energy Conf., Zurich, 1977, Vol. 3, Pergamon, Oxford, 1977, pp. 1561 - 1574. 9 S. Suda, Y. Komazaki and N. Kobayashi, in Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 1980, Pergamon, Oxford, to be published.
Journal of the Less-Common Metals, 73 (1980) 119 - 126 0 ?%evier Sequoia &A., Lausanne -Printed in the Netherlands
REACTION KINETICS OF METAL HYDRIDES AND THEIR MIXTURES* S. SUDA and N. KOBAYASHI Department
of Chemical Engineering,
Kogakuin
University, Hachioji-shi, Tokyo
192
(Japan) K. YOSHIDA Central Research Laboratory,
Sekisui Chemical Industry Ltd., Misima-gun, Osaka 618
CJapaN
Summary Reaction rates in the hydriding and the dehydriding processes were studied experimentally for hydriding alloys such as LaNis, MmNis(Mm = mischmetall), aluminium-substituted mischmetall nickels, TiMnl.s and TiO.sZro.aCro,sMn,,. We also studied binary mixtures of these hydriding alloys to examine the effects of mixing on the kinetic properties. A term N was introduced for the evaluation of the pressure and temperature dependenees of the reaction rate.
1. Introduction Systematic surveys have been carried out to develop hydriding alloys that have high hydrogen storage capacity, high hydrogen storage density, low plateau pressure and low cost per unit of hydrogen storage capacity. These studies have been mainly concerned with developing ternary and quaternary alloys with low hysteresis and reasonable plateau pressures by adding third and fourth components to binary alloys such as TiFe hydrides [ 11, MmNi5 hydrides (Mm = mischmetall) [2 - 4] and TiCr,-type hydrides [ 5,6]. Apart from equilibrium aspects, greater emphasis should be placed on developing hydriding alloys with faster reaction rates, particularly in thermodynamic cycles such as heat pumping of low grade energies and power generation from industrial waste heat. Nevertheless, from the engineering point of view, it is quite inconvenient to apply a single hydride with fixed equilibrium and kinetic properties inherent in that hydride, A mixing method has been proposed by Suda and Uchida [7,8] and the effects of mixing on the equilibrium properties have been discussed in detail by Suda eCal. [ 91. *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11, 1980.
120
The purpose of this paper is to summarize a series of experimental studies of the reaction rates for several hyd~d~g alloys and to discuss the effects of mixing two different hydriding alloys on the kinetic properties. Rate studies were carried out for LaNis, MmNi,, aluminium-substituted mischmetall nickels, TiMnl.s, TiosZro&rosMnl_P and binary mixtures of their hydrides. The mixing of metal hydrides has two major advantages: it can improve the equilibrium behaviour of the original constituents; it permits ~exibility in the tempera~re range of application where the hyd~d~g and dehy~ding reactions are very fast.
2. Experimental procedures 2.1. Reaction rate measurements In most hydriding and dehydriding processes it takes appro~ma~ly 3 4 min to obtain a 100% reaction, but only a few tens of seconds to achieve a 70 - 80% reaction. Therefore special care must be taken to design a reaction cell in which thermal effects due to rapid reaction are eliminated. Figure 1 shows in some detail a double-walled reaction cell for rapid heat supply and rejection through the 0.5 mm space. Samples of 3 - 5 g were introduced into the cell and each experimental run was started after repeating the activation treatment with an automatic timer unit more than 50 times at room temperature and at a pressure of 40 - 80 atm. To prepare mixed hydriding alloys, non-activa~d samples were mixed to give a fixed composition and the mixed samples were then activated repeatedly in the same manner as the unmixed samples. As shown in Figs. 2 and 3, the hydriding and dehydriding reactions
24-
Fig, 1, Details of the double-walled reaction cell: 1, end cup; 2, filter of 2 pm sintered metal; 3, seal for thermocouple inlet; 4, inner tube for heating/cooling fluids; 6, reaction cell (2 ml); 6, O-ring seals; ‘7, C-C thermocouple 0.2 mm in diameter; 8, hydrogen gas inlet.
121
Pi
3, m E
$
Pf E a Pe
Hi
H
Hf
Conposition
I
115
mol-H2/mol-llI
Fig. 2. A simplified diagram for the isothermal hydriding reaction.
Composition
[ ml-ll2/,,!ol-M I
Fig. 3. A simplified diagram for the isothermal dehydriding reaction.
proceed linearly from Pi to P, or from Hi to Hf. Changes H - I& in hydride composition were easily converted into pressure terms using pressure-temperature-composition data which had previously been correlated as an analytical expression. The pressure changes were measured using a HewlettPackard 3052A data aquisition system which was connected directly to the experimental apparatus through electronic pressure detectors. The data were then stored in a tape cartridge for a period of 240 s with a scanning time of 0.2 s. The rate constants were computed using 600 data points in every experimental run. The reaction rate constants in both the hydriding and the dehydriding process were defined as functions of two determining factors, P-P, and H - I&, and are given in eqns. (1) and (3) (for eqns. (1) - (6), see Table 1). The value of S defined by the integral terms in eqns. (2) and (4) was evaluated from experimental pressure data and was then plotted against the time elapsed. The rate constant is given by the slope of the straight line in this type of plot. The term N was defined for evaluating the temperature and pressure dependences of the rate constant and is given in eqns. (5) and (6). A major
H
{(P -P,)/P~}m(Hf
dH
--~II)~ = Kabst
(4 -HI"
f
(31 &es =
CiH
reaction
J Hi ((P, - P)/Po}m’H”’
H
Desorption
pe PO 4
&es P
dH/dt K abs
N abs m (5)
amount of hydrogen absorbed in time dt reaction rate constant in absorption reaction (h-l ) reaction rate constant in desorption reaction (h-l ) hydrogen gas pressure exerted on metal hydride bed (MPa) equilibrium pressure (MPa) atmospheric pressure (MPa) maximum amount of hydrogen gas absorbed (mol Hz (mol metal)-’ )
PO
exp (A +23/T)
R T A B
PO
m’
P and H = H.)
amount of hydrogen gas absorbed at pressure P (mol Hz (mol metal)-’ ) frequency factor (h-l ) activation energy in absorption reaction (J (mol Hz)activation energy in desorption reaction (J (mol Hz)gas constant (J (mol Hz)-l K-l) absolute temperature (K) constant in the van’t Hoff equation InP, = A + B/T constant in the van% Hoff equation lnP, = A + B/T
%d
x
H
t = f, P =
= gdest
exp (A + B/T) -P
(The boundary conditions are as follows: at t 3: 0, P = Pi and H = Hi; at t = 00,P = Pf and H = Hf; at
Sabs =
Absorption reaction
Equations (1) - (6)
TABLE 1
(6)
(4)
(2)
123
advantage of N is that it facilitates the evaluation of properties and conditions such as the reactivity, the reaction temperature range, the reaction rate at a given temperature and pressure, the selection of a more reactive hyd~ding alloy and the optimum operating conditions in a given application.
3. Exper~en~l
results and discussion
Figure 4 shows S values computed for ~rn~i*.~Al~,~ hydride during hydriding at 30 “C. The extent of the reaction (%) is also shown. Rate constants in the hydriding and dehydrid~g reactions are shown in Figs. 5 and 6
Time
Esec3
4. S us. time for MmNi~.~Al*.~ hydride at 30 %. The extent of the reaction is also shown. Fig.
TsmpsraturoIdog.Cl 1QQ
Q0
‘xi3
68
Temperature
w
PQ
188 ) ,
1000 Baa
60 ,
,
60 /
j
Cdeg.CI
40 j
/
20 )
I I
/
Desorpt.ion
600
28 I --2.6
2.0
3.0
3.2 1080/T
3.4 CI.'Kl
3.6
3.8
101 2.6
’
1 ’ 2.8
’
’
3.8
’
f
3.2 1000/T
\ ’ 3.4
’
t 3.6
[i/T1
Fig. 5. F&ateconstant in the hydriding reaction us. reciprocal temperature for hydrides of T~~.~Zr~.~~~_aMn~,~~ LaNis and TiMnl.5. Fig. 6. Rate constant in the de~yd~~~ reaction vs. reciprocal temperature for hydrides of Tio.~Zro.~~o.~Mn~.z, LaNi5 and TiMnl.5.
124
253
273
Tempsratura 293 313
CK3 333
Temperature 353
373
253
273
293
CKI 333
313
353
373
"'7
Temperature
tdog.C3
Temperature
Cdcg.Cl
Fig. 7. Isobaric rate constants for TiMn,~S hydride in the hpdriding process. Fig. 8, Isobaric rate constants for LaNis hydride in the hydriding process,
for hydrides of TiMnt,&, L,aNi5and Ti0.sZro.zCr0.eMnl_2.Figures 7 - 9 show N values computed from egn, (5) for the hydriding of hydrides of TiMnle5, IJaNi, -d Ti~.szr~.~~~.s~~l_2 respectively. The N values of these three hydrides at 20 atm are compared in Fig. 10. It can be seen that N has a maximum in each isobar and that at a given isobaric condition the reaction temperature is restricted to a comp~tively narrow range. This means that the
253 253
273
Temparature 293 313
CK3 333
353
373
Tamparaturs 293 313
CKI 333
353
373
1B4
10'
-20
Tsmpsraturo
273
Edeg.Cl
Fig. 3, Isobaric rate constants for Tio.~Zr~.~~~,~M~~=~
0
20 Tsmporaturo
40
60
(90
108
Cdsg.C3
hydride in the hydriding wocesc;.
Fig. 10.1vvs. temperature for the hydriding reaction of hydrides of Ti~.~~ro,*~o.~M~~.~, LaNi5 and TiMnl.5 at a pressure of 20 atm.
12s
absorption reaction is fastest at the maximum value of N and that almost no reaction occurs at any temperature outside each loop covering an isobaric reaction. The reaction rates for the hydrides can be evaluated easily by comparing N values at a given pressure and temperature. As stated earlier, N is particularly useful for evaluating the pressure and temperature dependences of a reaction; e.g. 7iiMnr.s absorbs hydrogen very slowly whereas Tie.sZrO.aCrO.sMnl.s hydride is very reactive at comparatively low pressure and low temperature. Because N is significantly affected by temperature and pressure, it is considered to be a good measure of the reaction rate and the reactivity of a hydride. In addition N is valuable as a criterion for selecting hyd~ding alloys or mixtures for use at a given temperature and pressure. Of the three hydrides considered, LaNi, hydride is the best hydriding alloy because of its rate constant and available temperature range. Since the hydriding reaction is very dependent on the system pressure, it is preferable from a kinetic standpoint to use high pressure conditions in any given technological development. The dependence of N on pressure in a dehydriding reaction is illustrated in Fig. 11 for LaNi5 hydride. It can be seen that the dehydrogenation is strongly dependent on temperature. The rate at which hydrogen gas is released increases as the temperature increases. The effect of mixing on the absorption reaction rate was also studied for several mixtures of LaNi,, MmNis, aluminium-substituted mischmetall nickels, TiMn,., and Ti0.sZr0.eCr0.sMn1~2hydrides. N values for LaNisTie.sZrO.&%.sMnr.s mixed hydrides of various compositions are shown in Temperature 253 10’1
273
293
a 1 !
I
CKI
313
I
I
Temporatore
333
I
I
I
353
373
I
1 I
253
273
293
313
20
40
WI 333
353
373
60
00
li30
10’-
10” 8 -20
’
111
0
20
f
Tempcrnturo
I
’ 40
’
I, 60
Cdeg.CJ
,
, 80
,
’ 100
-20
0
Temperature
Cdsg.Cl
Fig. 11. Isobaric rate constants of LaNi5 hydride in the dehydriding process. Fig. 12. Isobaric rate constants of hydrides of LaNic, and Tio.sZro,&ko.+nl.2 (Ti-4) and their mixtures in the dehydriding process: curve A, 3: 7 LaNib :Ti-4; curve B, 1: 1 LaNiS :Ti-4; curve C, 7:3 LaNiS :Ti-4.
126
Fig. 12. A significant improvement in the reaction temperature range is obtained. The reaction temperature range is considerably increased by mixing two metal hydrides with different properties. Mixing of hydriding materials offers several technical advantages: (1) it can improve the equilibrium behaviour of the original constituents; (2) it makes it possible to control the reaction rate and the kinetic properties by varying the mixing ratio; (3) it allows flexibility in the temperature range of application where the hydriding and dehydriding reactions are very fast.
4. Conclusions The addition of a small amount of a more reactive hydride to a less reactive hydride is quite effective in improving the hydriding properties of the less reactive metal hydride. From the kinetic standpoint, the use of high pressure conditions is effective in closed-cycle applications, e.g. heat pump systems, chemical engines, thermal compressors and power generating plants, which require particularly rapid reaction rates with a comparatively limited amount of hydriding alloy. Mixing is known to increase the number of available metal hydrides and also offers valuable applications in the utilization of sources of low grade energy, e.g. industrial waste energy, solar energy, geothermal energy, oceanic thermal energy and atmospheric energy.
References 1 J. J. Reilly and J. R. Johnson, in Proc. 1st World Hydrogen Energy Conf., Miami Beach, Florida, March 1 - 3, 1976, Vol. 2, Pergamon, Oxford, 1976, Paper 8B, pp. 3 - 26. 2 G. D. Sandrock, in Proc. p2th Int. Energy Conversion Engineering Conf., Washington, D.C., August 28 - September 2, 1977, Vol. 1, pp. 951 - 958. 3 Y. Ohsumi, A. Kato, H. Suzuki, M. Nakane and Y. Miyake, Chem. Ind. Chem., 11 (1978) 1472 - 1477. 4 Y. Ohsumi, A. Kato, H. Suzuki, M. Nakane and Y. Miyake, J. Less-Common Met., 66 (1979) 67 - 75. 5 T. Gamo, Y. Moriwaki, N. Yanagihara, T. Yamashita and T. Iwaki, Nat1 Tech. Rep. (Matsushita Electr. Ind. Co., Osaka), 25 (5) (1979) 941 - 952. 6 Y. Machida, T. Yamadaya and M. Asanuma, Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August 14 - 19, 1977, Pergamon, Oxford, 1978, pp. 329 - 336. 7 S. Suda and M. Uchida, Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August 14 - 19, 1977, Pergamon, Oxford, 1978, pp. 515 - 525. 8 S. Suda and M. Uchida, in Hydrogen Energy System: Proc. 2nd World Hydrogen Energy Conf., Zurich, 1977, Vol. 3, Pergamon, Oxford, 1977, pp. 1561 - 1574. 9 S. Suda, Y. Komazaki and N. Kobayashi, in Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 1980, Pergamon, Oxford, to be published.