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Application of SAES and HWT gas purifiers forthe removal of impurities from helium-hydrogen gas mixtures
Journal of the Less-Common Metals, 172-174 (1991) 1157-1167
1157
Application of SAES and HWT gas purifiers for the removal of impurities from helium-hydrogen gas mixtures H. Albrecht, U. K u h n e s and W. Asel Kernforschungszentrum Karlsruhe, Institut fi2r Radiochemie, Projekt Kernfusion, Postfach 3640, W-7500 Karlsruhe (F.R.G.)
Abstract An experimental programme is described which is performed in the framework of a tritium technology task for the Next European Torus (NET). The aim is to investigate the applicability of commercial gas purifiers for the removal of gaseous impurities such as N2, CO, and CO2 from a helium-hydrogen or a helium-tritium gas stream. In addition, it is intended to recover chemically bound hydrogen isotopes by decomposition of hydrocarbons, ammonia and water. Experimental results obtained in the PEGASUS facility are reported for the sorption of CH 4 by three non-uranium getters as a function of temperature and gas flow rate, the enhancement of CH 4 sorption efficiency by continuous removal of hydrogen, and cosorption behaviour of CO, N2 and CH 4 in the presence of hydrogen. Gettering of CO and N2 was found to be much more effective than that of CH4. At temperatures above 200~ an increasing tendency for the reaction 2H2+ CO --*CH 4 + 89 was found to occur during the passage of the gas through the getter. This reaction can be suppressed by using two getters in series: the first is operated at about 250 ~ to remove 02, H20, CO and CO2, and the second at 400 ~ or more for the removal of residual amounts of these species as well as for the removal of N 2 and CH4.
1. I n t r o d u c t i o n It has been k n o w n for a long time t h a t storage and purification of h y d r o g e n isotopes can be achi eved by metals (titanium, zirconium, u r a n i u m etc.) as well as by bi nar y or t e r n a r y metal alloys (Zr-A1, L a - N i s , T i - V - M n ) . In addition, m u l t i c o m p o n e n t alloys h a v e been developed for which specific pr oper t i e s can be adjusted by alloying iron, chromium, cobalt, nickel, cerium or o t h e r elements with zirconium and/ or titanium. Examples of such substitution alloys are CeNi4.25Mno.7~ [1], Zr, _xTix Cr 1-3 Fel +y [2] and Tio.9sZro.02Vo.43 Feo.09Cro.05Mnl.5 [3, 4]. Pur i f i cat i on is obt ai ned as a resul t of the effect t h a t r e a c t i v e gases such as 02, N2, CO or CO2 can be irreversibly sorbed by the get t er alloy. In addition, molecules such as H2 O, NHa or CH4 can be decomposed at appropriate t e m p e r a t u r e s with s ubsequent sorpt i on of the oxygen, n i t r o g e n and c a r b o n por t i ons of these compounds. H y d r o g e n isotopes, on the c o n t r a r y , are
0022-5088/91/$3.50
/t Elsevier Sequoia]Printed in The Netherlands
1158
reversibly sorbed, i.e. they can be stored at low temperatures (20 ~ and desorbed at higher temperatures (e.g. above 300 ~ On the basis of these properties, commercial hydrogen storage getters and purifiers have been developed which are of interest also for the handling of tritium in nuclear fusion reactors. The applicability of such purifiers is being investigated, therefore, in the framework of a research task for NET. The gases to be processed contain 8 5 % - 9 5 % Q2 (Q = H, D, T), about 5% He and up to 10% of impurities (CO, CO2, N2, NQ3, Q20, CQ4 and other hydrocarbons [5-7]). The first step of a purification concept based on metal getters [8] includes a P d - A g diffuser, which allows a high percentage of the molecular hydrogen isotopes to be separated with a sufficient grade of purity [9]. NQ 3, if present, is catalytically decomposed at the surface of the permeator membrane [10]. Getter beds are used in the second purification step to decompose CQ4 and Q20 and to remove nitrogen, carbon and oxygen from the impurities. In the last step, a second P d - A g diffuser is applied to separate the liberated hydrogen isotopes from helium. The most important requirements with respect to the properties of the getter beds are as follows: (a) high cracking efficiency for hydrocarbons and other tritiated compounds such as Q20 to obtain maximum recovery of tritium; (b) operation at temperatures as low as possible to prevent major losses of tritium by permeation; (c) low residual content of tritium when the bed is exhausted and has to be disposed of. The principal objective of the tests performed so far has been the investigation of the CH4 cracking efficiency as a function of temperature, flow rate and getter type. Additional aims were to study the sorption behaviour of N 2 and CO, and to find out whether any cosorption effects might occur when a gas mixture of hydrogen and several impurities is sent through the purifier. The tests were carried out with purifiers containing getter materials of types SAES-ST-707, HWT-5804 (HTR-1) and HWT-5850 (HTR-2). Numerous publications are available on the properties of these materials [11-17]; however, most of these papers deal with the storage of hydrogen isotopes and sorption of N2, 02, CO and C02. Information on the removal of methane is very limited; in addition, the purifiers were mostly used for gases with initial impurity concentrations of 10 ppm or less which is quite different from the conditions to be discussed here.
2. Test p e r f o r m a n c e and evaluation A schematic diagram of the P E G A S U S facility is shown in Fig. 1. For simplicity, some components have been omitted in the figure, e.g. the ultrahigh vacuum pumping system, the manifold and all normal valves. The test gas containing at least one impurity component in helium as carrier gas is prepared in one of the buffer tanks B1 or B2. The circulation pump (Metal Bellows 151-DC) is used to transport the test gas to the purifiers G1 . . . . . G4,
1159
Oo e , o
I
_ _ I_
"/~ -- -1 --
--
R, N~
CO.,Ca, 7
(
"~
~th
U 1, U 2 , Uranium Beds G 1... G 4, Getter Beds B I...B 3, Gas Collection Tanks (~),(~) 9 Pressure, Flow Indicators
Glove Box
Fig. 1. Schematic diagram of the PEGASUS facility.
TABLE 1 Main technical data of the tested purifiers Type
ST-707
HTR-1
HTR-2
Supplier Alloy Content T~na~ Pmax dm,x
SAES Zr-Fe-V 700 g 800 ~ a 10 b a r 5 1min-'
HWT Ti-V-Fe-Mn 3000 g 500 ~ 10 bar 16 1 min -~
HWT Ti-V-Fe-Ni-Mn 3000 g 500 ~ 10 bar 16 1 min - '
Tin,x, maximum working temperature; Pmax, maximum i n p u t pressure; dma x, maximum gas flow rate. "Purifier w i t h o u t water cooling: 400 ~ or less.
which can be exposed to the gas either separately or in series. The main technical data of the purifiers are summarized in Table 1. C o n s t a n t gas flow rates up to 15 1 min -1 are obtained by a flow control system (FIC, Tylan). Selective removal of hydrogen can be achieved with a P d - A g diffuser (Leybold PA 150 with 290 cm 2 surface area). A gas chromatograph (Carlo Erba Fractovap 2700) with a helium ionization detector is used for quantitative analysis of the gas in the buffer t a n k s B1 and B2 (i.e. at the getter input) or at the getter output. The gas c h r o m a t o g r a p h is placed in a bypass to the main loop.
1160
No tests with tritium have been performed so far, but most of the installations needed for such tests have been integrated into the facility: two uranium beds for tritium storage, an additional buffer tank B3, a large glove-box (3.2 m • 1.2 m x 2.0 m) and a glove-box detritiation system. The facility is normally operated in the closed loop mode, i.e. the gas is circulated several times through the getter(s) until a given level of purity is attained. The impurity concentration in the buffer tank decreases exponentially with time t:
c(t) = Coe-~t, ~ =
fd
(1)
pV
where Co is the initial impurity concentration, f is the purification factor (0 ~
f=~\
lnc'~
d-/ }
(2)
A characteristic variable of each test is the half-period tl/2, of the concentration decrease. Its value can be determined from the c(t) curve or according to
tl/2
pVln 2 d f
(3)
In the case of the maximum purification efficiency ( f = 1), the half-period attains a minimum
train
t~/2- f
(4)
which is equivalent to an upper limit of the purification velocity for a given set of experimental parameters. If more than one impurity component is present in the carrier gas, characteristic c(t) curves and results for f and t~/2 are obtained for each impurity, but only one value for tmi,-
3. S o r p t i o n o f CH 4 a s a f u n c t i o n
of temperature
To investigate the sorption efficiencies for methane on the purifiers mentioned above, several tests have been carried out with getter bed temperatures ranging from 300 to 600 ~ (Table 2). The gas flow rate was 1.0 1 min -1 in all cases. The results of the tests with getter ST-707 are shown in Fig. 2. The semilogarithmic c(t) plots of the inlet and of the outlet concentrations to
1161 TABLE 2 CH 4 sorption tests with various purifiers Test
Purifier
T (~
Co (%)
ce (%)
tr (rain)
PV-2a PV-2b PV-2c PV-4 PV-9 PV-20 PV-21 PV-22 PV-23
ST-707 ST-707 ST-707 HTR-1 HTR-1 HTR-2 HTR-2 HTR-2 HTR-1
300 400 600 400 500 400 400 400 400
0.45 0.80 0.50 4.81 0.75 0.30 0.30 0.33 0.27
0.065 0.012 0.008 0.110 0.011 0.017 0.018 0.020 0.021
130 130 100 200 120 45 53 68 80
co, c~, concentration at getter inlet at start and end of the test; Q, duration of gas circulation through the getter.
1.0
%CH 4
TG - 300~C
""-..~~_'~
\X•. \'.\
TG = 400~
9
0.1
TG= 6000C
~
CoI,',X. "\.
",,,. \, \.
Cout = O. 68- ci. ]
0.01
20 40 60 80 100 120 0 20 40 60 80 1000 20 40 60 80 rain Fig. 2. Sorption of CH4 by SAES getter ST-707.
the getter bed were found to be straight .lines, an indication that the purification factor was constant during the tests. An increase in the temperature of the getter bed caused an increasing speed of CH4 removal and a decreasing ratio of the outlet:inlet concentration. At TG = 600 ~ the half-period of the concentration decrease was 17.0 min. As this value is already close to the theoretical minimum (tmin ----14.5 min), no significant acceleration of the CH4 removal can be expected at getter bed temperatures above 600 ~ but probably a further decrease in the ratio
Cout/Cin. The corresponding property of the two HWT purifiers HTR-1/2 was similar to that of the SAES getter (Fig. 3). The higher retention efficiency of
1162 1.0
i
J
i
i
PV4/9/23
c/c o
O.S
\ \
k ~
\ ~
,T~-I
~
~
,o.r,t
0.2 0.1
\
",,,
"~x
HTR-1 ~ T. 500~ X t,/z. 19.5mln ~
0.05
,i,-V---~-M;--
HTR-2 , Ti-V-Fe-Ni-M. Carriergas, 1.2 bar He
"\
~
,,on
HTR-1
~
T.400"C
",, HTR-2 . " ~ tl/2 937.4 min T" 400_'C . . z(.1 mm
0.02 I
50
I
I
100
150
I
200
min
Fig. 3. Sorption of CH4 by two different HWT getters.
the purifier HTR-2 at 400~ compared with that of HTR-1 can be explained by an increased cracking capability for C H 4 which is obtained by the catalytic property of the additional content of 7% nickel in the getter alloy.
4. E n h a n c e m e n t of C H the released h y d r o g e n
4
sorption efficiency by c o n t i n u o u s r e m o v a l of
The half-period of 27.1 min measured in test PV-23 for CH4 removal at 400 ~ can be further decreased by additional provisions as applied in test PV-22. Before this test the getter was dehydrided at 450~ to a hydrogen equilibrium pressure of 0.36 mbar. During the test, the hydrogen resulting from the remaining equilibrium pressure as well as from the cracking of CH4 was pumped off with the P d - A g diffuser. In this way, the decomposition CH4 --* C + 2H2 was accelerated by continuously shifting the equilibrium to the right. The result is shown in Fig. 4. The output:input ratio decreased by almost a factor of 2, and the half-period of CH4 removal was now 18.3 min, which is even less than the corresponding value in test PV-9, where the getter HTR-1 was operated at 500 ~ (cf. Fig. 3). For the application of this method it is essential that the dehydrogenation of the getter is continued to keep the equilibrium pressure of the getter hydride lower than the hydrogen pressure established from the cracking of CH4.
1163
PV- 22 / 23
Y, CH 4
0.5 Getter , HWT-HTR 2 Temperature, 400~ , 1.2 bar He
Carrier gas
0.2
0.1
0.05
0.02
0.01
O.005
Continuous H2 removal
Cout C~
(~
no
O.50
27.1
(~
yes
0.26
18.3
J
J
i
20
tl/2
i
(min)
i
40
i
i
60
min
80
Fig. 4, Sorption of CH 4 with and without continuous H 2 removal.
5. S o r p t i o n o f
CH 4 as
a f u n c t i o n o f flow r a t e
First results on the influence of the flow rate on the sorption speed can be derived from tests PV-2b and PV-2d (Fig. 5): At a flow rate of 0.5 1 min -1 a higher purification effect, i.e. a smaller value for the ratio Cout/cin, was found. This is due to the longer contact time of a given gas volume in the getter than at a flow rate of 1.0 1 min -1. On the contrary, the concentration in the buffer tank decreased more slowly, because the gas was less frequently pumped through the getter. In other words, the longer residence time in the purifier is overcompensated by the higher frequency of the gas circulation. In analogue tests carried out with the H W T getter HTR-2 at flow rates of 1.0, 2.0, and 4.0 1 min -1, a faster decrease in concentration was again found with increasing flow rate (Fig. 6). It is apparent, however, that the effect becomes smaller with further flow rate increase. Although these results have been obtained for CH4 only, it is assumed that they are also valid for other impurities such as CO or N2.
1164
1.0l L
o,=
~ Ii/~2
PV2
ET,,. 400"C ~ it,/2%.. 9
I~" 400"C [
\
- -
i \ \c,~ --
0.01[ 0
20
J
40
60
-,,,\
80 100 120
0
20
40
60
80 100
120rain
F i g . 5. I n f l u e n c e o f g a s flow r a t e o n CH.~ s o r p t i o n ( g e t t e r ST-707).
1.0 c/c~
~
PV-20/21/22'
/min~
0.1
4 I/rain
~
" ~ ' . 2 I/min
0.05
0.02
O.01
"~
"~ Getter , HWT-HTR 2 Temperature, 400 =C Carrier gas , 1.2 bar He
,
,
,
20
, 40
,
~ 60
, min
Fig. 6. Sorption of CH 4 at different gas flow rates.
6. C o s o r p t i o n t e s t s To investigate the question of mutual interactions between different impurities during the sorption process two tests were carried out with H2, CH4, N2 and CO as components of the carrier gas (Fig. 7). Before the first of these tests the purifier HTR-1 was dehydrided during the activation at 400 ~ At a getter temperature of 200 ~ hydrogen was gettered to a large extent at the beginning; however, the change in the slope of the curve at t = 40 min indicates an approach to the equilibrium condition where the rates of uptake become smaller. At 300 ~ the hydrogen desorption pressure of the
1165
,0\
............
0.2
~o.
9 0'
50
Lplo
~'~o.
100 ....
1~0 ....
. . . . ,Pv3
N~Z
50 ....
100' nlin' '150
Fig. 7. Simultaneous sorption of various gas components (getter HTR-1).
getter was already higher than the partial pressure in the loop, so its concentration increased. For CO and N2 a higher sorption speed was observed for 300 ~ than for 200 ~ as expected. For CH4 the getter temperature of 200 ~ was obviously too low to cause any measurable sorption effect. At 300 ~ however, a surprising effect was found: an increase in concentration rather than the expected decrease. This can be explained by the formation of additional methane caused by the interaction of carbon monoxide and hydrogen during the passage through the getter: 2H2 + CO --* CH4 + 89 The corresponding amount of oxygen was directly sorbed by the getter. As there was a strong decrease in the CO concentration in the first 20 min of the test, the rate of CH4 formation decreased also. The carbon sorbed in the preceding test at 200 ~ was not involved in the reaction, otherwise the amount of the additional methane would have been much higher. The formation of additional methane is very undesirable from the point of view of gas purification. To suppress this formation two getter beds can be applied in series. The first bed is operated in the range 200-250 ~ to reduce the concentration of CO and H2 and the second at a temperature of at least 400 ~ to remove effectively all the CH4 and Ne. In Fig. 8 the concentration of three impurity components is shown as measured in test PV-15 at 200 and 400 ~ with an initial gas volume of 105 1. The purification efficiency of the getter bed combination is further'increased by applying higher temperatures. For 250 and 450 ~ as an example, the Cout/ci, ratios decrease by almost a factor of 3 for each of the impurity components mentioned above.
1166 1000
'
'
'
I
PV-15
ppm
9
I::o:%':
9
CO ~'~;
I Carrier gas, He. O. 5% H z
100
10 5
'
50
100
,
150
,
200
250 min
Fig. 8. Simultaneous sorption of impurities by two getters in series.
7. C o n c l u s i o n s Metal getter beds can be used to remove gaseous impurities from helium-hydrogen gas mixtures. In tritium technology, the removal of methane is of special interest because of its high concentration in the plasma exhaust and the necessity to recover tritium from tritiated methane. As low operation temperatures of the getter beds are desirable to avoid tritium permeation losses, the concentration of methane cannot be reduced by 3-5 orders of magnitude unless the gas is circulated several times through the same getter bed in a closed loop. To optimize the purification efficiency for operation temperatures of 400-450 ~ the following procedure is recommended: (a) separation of hydrogen isotopes in molecular form by using a P d - A g diffuser in a first purification step; (b) use of a getter alloy containing a component with a high cracking capability for methane, e.g. nickel; (c) application of a P d - A g diffuser in the purification loop to shift the equilibrium of the reaction CH4 --*C -t- 2H2 to the right by continuous removal of the liberated hydrogen isotopes; (d) prevention of additional CH4 formation by sorption of CO on a second getter bed and by continuous H2 removal; (e) increase in the gas flow rate. Good results were obtained with the HWT gas purifier HTR-2 because the nickel component of its getter alloy leads to an increased cracking efficiency and because higher gas flow rates can be used than for the tested SAES purifier. In addition, the equilibrium pressure of the hydrogen isotopes is higher by several orders of magnitude for both HWT getters; it is much easier, therefore, to keep the tritium losses low when the exhausted getter beds must be disposed of.
1167 T h e e x p e r i m e n t a l p r o g r a m m e is b e i n g c o n t i n u e d w i t h i n v e s t i g a t i o n s o f t h e s o r p t i o n o f w a t e r v a p o u r a n d o f t h e d e p e n d e n c e of s o r p t i o n e f f i c i e n c y o n impurity uptake.
References 1 F. Pourarian and W. E. Wallace, The effect of substitution of Mn or A1 on the hydrogen sorption characteristics of CeNi s, Int. J. Hydrogen Energy, 10 (1) (1985) 49. 2 J.-M. Park and J.-Y. Lee, Hydrogenation characteristics of the Zr~ _xTixCr~ _yFe~ § Laves phase alloys, J. Less-Common Met., 160 (1990) 259. 3 O. Bernauer and C. Halene, Properties of metal hydrides for use in industrial applications, J. Less-Common Met., 131 (1987) 213. 4 O. Bernauer, J. Toepler, D. Nor~us, R. Hempelmann and Richter, Fundamentals and properties of some Ti/Mn based Laves phase hydrides, Int. J. Hydrogen Energy, 14 (3) (1989) 187. 5 S. K. Sood and O. K. Kveton, Tritium systems concepts for the Next European Torus (NET), CFFTP Rep. G-86020, 1986. 6 G. Bourque, B. Terreault, B. C. Gregory, G. W. Pacher et al., Chemical impurity production in the Tokamak de Varennes, Fusion Technol., 17 (1990) 588. 7 G. M. McCracken, S. J. Fielding, G. F. Matthews and C. S. Pichter, Impurity production due to wall interactions in tokamaks, J. Nucl. Mater., 162-164 (1989) 392. 8 H. Albrecht, R.-D. Penzhorn, Th. Kastner and M. Sirch, Plasma exhaust purification with metal getters, Fusion Eng. Des., 10 (1989) 349. 9 R. V. Carlson, K. E. Binning, S. Konishi, H. Yoshida and Y. Naruse, Results on tritium experiments on ceramic electrolysis cells and palladium diffusors for application to fusion reactor fuel clean-up systems, 12th Syrup. on Fusion Energy, Monterey, CA, October 1987. 10 R.-D. Penzhorn and M. Glugla, Process to recover tritium from fusion cycle impurities, Fusion Technol., 10 (1986) 1345. 11 B. Ferrario, C. Boffito, F. Doni and L. Rosai, Zr based gettering alloys for hydrogen isotope handling, 13th Syrup. on Fusion Technology (SOFT), Varese, September 24-28, 1984. 12 B. Bonizzoni, A. Conte, G. Gatto, G. Gervasini, F. Ghezzi and M. Rigamonti, Tritium storage plant based on a combination of ST-707 and ST-737 getter alloy beds for high field fusion machines, 11th Int. Vacuum Congr. Cologne, September 25-29, 1989. 13 J. L. Cecchi and R. J. Knize, Gettering in fusion devices, J. Vac. Sci. Technol. A2 (2) (1984) 1214. 14 H. Heimbach, H. R. Ihle and C. H. Wu, Removal of nitrogen and methane from hydrogen by metal getters, 13th Syrup. on Fusion Technology (SOFT), Varese, September 24-28, 1984. 15 W. J. Holtslander, R. E. Johnson, F. B. Gravelle and C. M. Shulz, An experimental evaluation of a small fuel cleanup system, Fusion Technol., 10 (1986) 1340. 16 O. Bernauer, Metal hydride storage, Z. Phys. Chem., N.F., 164 (1989) 1381. 17 O. Bernauer, Metal hydride technology, J. Hydrogen Energy, 13 (3) (1988) 181.
1157
Application of SAES and HWT gas purifiers for the removal of impurities from helium-hydrogen gas mixtures H. Albrecht, U. K u h n e s and W. Asel Kernforschungszentrum Karlsruhe, Institut fi2r Radiochemie, Projekt Kernfusion, Postfach 3640, W-7500 Karlsruhe (F.R.G.)
Abstract An experimental programme is described which is performed in the framework of a tritium technology task for the Next European Torus (NET). The aim is to investigate the applicability of commercial gas purifiers for the removal of gaseous impurities such as N2, CO, and CO2 from a helium-hydrogen or a helium-tritium gas stream. In addition, it is intended to recover chemically bound hydrogen isotopes by decomposition of hydrocarbons, ammonia and water. Experimental results obtained in the PEGASUS facility are reported for the sorption of CH 4 by three non-uranium getters as a function of temperature and gas flow rate, the enhancement of CH 4 sorption efficiency by continuous removal of hydrogen, and cosorption behaviour of CO, N2 and CH 4 in the presence of hydrogen. Gettering of CO and N2 was found to be much more effective than that of CH4. At temperatures above 200~ an increasing tendency for the reaction 2H2+ CO --*CH 4 + 89 was found to occur during the passage of the gas through the getter. This reaction can be suppressed by using two getters in series: the first is operated at about 250 ~ to remove 02, H20, CO and CO2, and the second at 400 ~ or more for the removal of residual amounts of these species as well as for the removal of N 2 and CH4.
1. I n t r o d u c t i o n It has been k n o w n for a long time t h a t storage and purification of h y d r o g e n isotopes can be achi eved by metals (titanium, zirconium, u r a n i u m etc.) as well as by bi nar y or t e r n a r y metal alloys (Zr-A1, L a - N i s , T i - V - M n ) . In addition, m u l t i c o m p o n e n t alloys h a v e been developed for which specific pr oper t i e s can be adjusted by alloying iron, chromium, cobalt, nickel, cerium or o t h e r elements with zirconium and/ or titanium. Examples of such substitution alloys are CeNi4.25Mno.7~ [1], Zr, _xTix Cr 1-3 Fel +y [2] and Tio.9sZro.02Vo.43 Feo.09Cro.05Mnl.5 [3, 4]. Pur i f i cat i on is obt ai ned as a resul t of the effect t h a t r e a c t i v e gases such as 02, N2, CO or CO2 can be irreversibly sorbed by the get t er alloy. In addition, molecules such as H2 O, NHa or CH4 can be decomposed at appropriate t e m p e r a t u r e s with s ubsequent sorpt i on of the oxygen, n i t r o g e n and c a r b o n por t i ons of these compounds. H y d r o g e n isotopes, on the c o n t r a r y , are
0022-5088/91/$3.50
/t Elsevier Sequoia]Printed in The Netherlands
1158
reversibly sorbed, i.e. they can be stored at low temperatures (20 ~ and desorbed at higher temperatures (e.g. above 300 ~ On the basis of these properties, commercial hydrogen storage getters and purifiers have been developed which are of interest also for the handling of tritium in nuclear fusion reactors. The applicability of such purifiers is being investigated, therefore, in the framework of a research task for NET. The gases to be processed contain 8 5 % - 9 5 % Q2 (Q = H, D, T), about 5% He and up to 10% of impurities (CO, CO2, N2, NQ3, Q20, CQ4 and other hydrocarbons [5-7]). The first step of a purification concept based on metal getters [8] includes a P d - A g diffuser, which allows a high percentage of the molecular hydrogen isotopes to be separated with a sufficient grade of purity [9]. NQ 3, if present, is catalytically decomposed at the surface of the permeator membrane [10]. Getter beds are used in the second purification step to decompose CQ4 and Q20 and to remove nitrogen, carbon and oxygen from the impurities. In the last step, a second P d - A g diffuser is applied to separate the liberated hydrogen isotopes from helium. The most important requirements with respect to the properties of the getter beds are as follows: (a) high cracking efficiency for hydrocarbons and other tritiated compounds such as Q20 to obtain maximum recovery of tritium; (b) operation at temperatures as low as possible to prevent major losses of tritium by permeation; (c) low residual content of tritium when the bed is exhausted and has to be disposed of. The principal objective of the tests performed so far has been the investigation of the CH4 cracking efficiency as a function of temperature, flow rate and getter type. Additional aims were to study the sorption behaviour of N 2 and CO, and to find out whether any cosorption effects might occur when a gas mixture of hydrogen and several impurities is sent through the purifier. The tests were carried out with purifiers containing getter materials of types SAES-ST-707, HWT-5804 (HTR-1) and HWT-5850 (HTR-2). Numerous publications are available on the properties of these materials [11-17]; however, most of these papers deal with the storage of hydrogen isotopes and sorption of N2, 02, CO and C02. Information on the removal of methane is very limited; in addition, the purifiers were mostly used for gases with initial impurity concentrations of 10 ppm or less which is quite different from the conditions to be discussed here.
2. Test p e r f o r m a n c e and evaluation A schematic diagram of the P E G A S U S facility is shown in Fig. 1. For simplicity, some components have been omitted in the figure, e.g. the ultrahigh vacuum pumping system, the manifold and all normal valves. The test gas containing at least one impurity component in helium as carrier gas is prepared in one of the buffer tanks B1 or B2. The circulation pump (Metal Bellows 151-DC) is used to transport the test gas to the purifiers G1 . . . . . G4,
1159
Oo e , o
I
_ _ I_
"/~ -- -1 --
--
R, N~
CO.,Ca, 7
(
"~
~th
U 1, U 2 , Uranium Beds G 1... G 4, Getter Beds B I...B 3, Gas Collection Tanks (~),(~) 9 Pressure, Flow Indicators
Glove Box
Fig. 1. Schematic diagram of the PEGASUS facility.
TABLE 1 Main technical data of the tested purifiers Type
ST-707
HTR-1
HTR-2
Supplier Alloy Content T~na~ Pmax dm,x
SAES Zr-Fe-V 700 g 800 ~ a 10 b a r 5 1min-'
HWT Ti-V-Fe-Mn 3000 g 500 ~ 10 bar 16 1 min -~
HWT Ti-V-Fe-Ni-Mn 3000 g 500 ~ 10 bar 16 1 min - '
Tin,x, maximum working temperature; Pmax, maximum i n p u t pressure; dma x, maximum gas flow rate. "Purifier w i t h o u t water cooling: 400 ~ or less.
which can be exposed to the gas either separately or in series. The main technical data of the purifiers are summarized in Table 1. C o n s t a n t gas flow rates up to 15 1 min -1 are obtained by a flow control system (FIC, Tylan). Selective removal of hydrogen can be achieved with a P d - A g diffuser (Leybold PA 150 with 290 cm 2 surface area). A gas chromatograph (Carlo Erba Fractovap 2700) with a helium ionization detector is used for quantitative analysis of the gas in the buffer t a n k s B1 and B2 (i.e. at the getter input) or at the getter output. The gas c h r o m a t o g r a p h is placed in a bypass to the main loop.
1160
No tests with tritium have been performed so far, but most of the installations needed for such tests have been integrated into the facility: two uranium beds for tritium storage, an additional buffer tank B3, a large glove-box (3.2 m • 1.2 m x 2.0 m) and a glove-box detritiation system. The facility is normally operated in the closed loop mode, i.e. the gas is circulated several times through the getter(s) until a given level of purity is attained. The impurity concentration in the buffer tank decreases exponentially with time t:
c(t) = Coe-~t, ~ =
fd
(1)
pV
where Co is the initial impurity concentration, f is the purification factor (0 ~
f=~\
lnc'~
d-/ }
(2)
A characteristic variable of each test is the half-period tl/2, of the concentration decrease. Its value can be determined from the c(t) curve or according to
tl/2
pVln 2 d f
(3)
In the case of the maximum purification efficiency ( f = 1), the half-period attains a minimum
train
t~/2- f
(4)
which is equivalent to an upper limit of the purification velocity for a given set of experimental parameters. If more than one impurity component is present in the carrier gas, characteristic c(t) curves and results for f and t~/2 are obtained for each impurity, but only one value for tmi,-
3. S o r p t i o n o f CH 4 a s a f u n c t i o n
of temperature
To investigate the sorption efficiencies for methane on the purifiers mentioned above, several tests have been carried out with getter bed temperatures ranging from 300 to 600 ~ (Table 2). The gas flow rate was 1.0 1 min -1 in all cases. The results of the tests with getter ST-707 are shown in Fig. 2. The semilogarithmic c(t) plots of the inlet and of the outlet concentrations to
1161 TABLE 2 CH 4 sorption tests with various purifiers Test
Purifier
T (~
Co (%)
ce (%)
tr (rain)
PV-2a PV-2b PV-2c PV-4 PV-9 PV-20 PV-21 PV-22 PV-23
ST-707 ST-707 ST-707 HTR-1 HTR-1 HTR-2 HTR-2 HTR-2 HTR-1
300 400 600 400 500 400 400 400 400
0.45 0.80 0.50 4.81 0.75 0.30 0.30 0.33 0.27
0.065 0.012 0.008 0.110 0.011 0.017 0.018 0.020 0.021
130 130 100 200 120 45 53 68 80
co, c~, concentration at getter inlet at start and end of the test; Q, duration of gas circulation through the getter.
1.0
%CH 4
TG - 300~C
""-..~~_'~
\X•. \'.\
TG = 400~
9
0.1
TG= 6000C
~
CoI,',X. "\.
",,,. \, \.
Cout = O. 68- ci. ]
0.01
20 40 60 80 100 120 0 20 40 60 80 1000 20 40 60 80 rain Fig. 2. Sorption of CH4 by SAES getter ST-707.
the getter bed were found to be straight .lines, an indication that the purification factor was constant during the tests. An increase in the temperature of the getter bed caused an increasing speed of CH4 removal and a decreasing ratio of the outlet:inlet concentration. At TG = 600 ~ the half-period of the concentration decrease was 17.0 min. As this value is already close to the theoretical minimum (tmin ----14.5 min), no significant acceleration of the CH4 removal can be expected at getter bed temperatures above 600 ~ but probably a further decrease in the ratio
Cout/Cin. The corresponding property of the two HWT purifiers HTR-1/2 was similar to that of the SAES getter (Fig. 3). The higher retention efficiency of
1162 1.0
i
J
i
i
PV4/9/23
c/c o
O.S
\ \
k ~
\ ~
,T~-I
~
~
,o.r,t
0.2 0.1
\
",,,
"~x
HTR-1 ~ T. 500~ X t,/z. 19.5mln ~
0.05
,i,-V---~-M;--
HTR-2 , Ti-V-Fe-Ni-M. Carriergas, 1.2 bar He
"\
~
,,on
HTR-1
~
T.400"C
",, HTR-2 . " ~ tl/2 937.4 min T" 400_'C . . z(.1 mm
0.02 I
50
I
I
100
150
I
200
min
Fig. 3. Sorption of CH4 by two different HWT getters.
the purifier HTR-2 at 400~ compared with that of HTR-1 can be explained by an increased cracking capability for C H 4 which is obtained by the catalytic property of the additional content of 7% nickel in the getter alloy.
4. E n h a n c e m e n t of C H the released h y d r o g e n
4
sorption efficiency by c o n t i n u o u s r e m o v a l of
The half-period of 27.1 min measured in test PV-23 for CH4 removal at 400 ~ can be further decreased by additional provisions as applied in test PV-22. Before this test the getter was dehydrided at 450~ to a hydrogen equilibrium pressure of 0.36 mbar. During the test, the hydrogen resulting from the remaining equilibrium pressure as well as from the cracking of CH4 was pumped off with the P d - A g diffuser. In this way, the decomposition CH4 --* C + 2H2 was accelerated by continuously shifting the equilibrium to the right. The result is shown in Fig. 4. The output:input ratio decreased by almost a factor of 2, and the half-period of CH4 removal was now 18.3 min, which is even less than the corresponding value in test PV-9, where the getter HTR-1 was operated at 500 ~ (cf. Fig. 3). For the application of this method it is essential that the dehydrogenation of the getter is continued to keep the equilibrium pressure of the getter hydride lower than the hydrogen pressure established from the cracking of CH4.
1163
PV- 22 / 23
Y, CH 4
0.5 Getter , HWT-HTR 2 Temperature, 400~ , 1.2 bar He
Carrier gas
0.2
0.1
0.05
0.02
0.01
O.005
Continuous H2 removal
Cout C~
(~
no
O.50
27.1
(~
yes
0.26
18.3
J
J
i
20
tl/2
i
(min)
i
40
i
i
60
min
80
Fig. 4, Sorption of CH 4 with and without continuous H 2 removal.
5. S o r p t i o n o f
CH 4 as
a f u n c t i o n o f flow r a t e
First results on the influence of the flow rate on the sorption speed can be derived from tests PV-2b and PV-2d (Fig. 5): At a flow rate of 0.5 1 min -1 a higher purification effect, i.e. a smaller value for the ratio Cout/cin, was found. This is due to the longer contact time of a given gas volume in the getter than at a flow rate of 1.0 1 min -1. On the contrary, the concentration in the buffer tank decreased more slowly, because the gas was less frequently pumped through the getter. In other words, the longer residence time in the purifier is overcompensated by the higher frequency of the gas circulation. In analogue tests carried out with the H W T getter HTR-2 at flow rates of 1.0, 2.0, and 4.0 1 min -1, a faster decrease in concentration was again found with increasing flow rate (Fig. 6). It is apparent, however, that the effect becomes smaller with further flow rate increase. Although these results have been obtained for CH4 only, it is assumed that they are also valid for other impurities such as CO or N2.
1164
1.0l L
o,=
~ Ii/~2
PV2
ET,,. 400"C ~ it,/2%.. 9
I~" 400"C [
\
- -
i \ \c,~ --
0.01[ 0
20
J
40
60
-,,,\
80 100 120
0
20
40
60
80 100
120rain
F i g . 5. I n f l u e n c e o f g a s flow r a t e o n CH.~ s o r p t i o n ( g e t t e r ST-707).
1.0 c/c~
~
PV-20/21/22'
/min~
0.1
4 I/rain
~
" ~ ' . 2 I/min
0.05
0.02
O.01
"~
"~ Getter , HWT-HTR 2 Temperature, 400 =C Carrier gas , 1.2 bar He
,
,
,
20
, 40
,
~ 60
, min
Fig. 6. Sorption of CH 4 at different gas flow rates.
6. C o s o r p t i o n t e s t s To investigate the question of mutual interactions between different impurities during the sorption process two tests were carried out with H2, CH4, N2 and CO as components of the carrier gas (Fig. 7). Before the first of these tests the purifier HTR-1 was dehydrided during the activation at 400 ~ At a getter temperature of 200 ~ hydrogen was gettered to a large extent at the beginning; however, the change in the slope of the curve at t = 40 min indicates an approach to the equilibrium condition where the rates of uptake become smaller. At 300 ~ the hydrogen desorption pressure of the
1165
,0\
............
0.2
~o.
9 0'
50
Lplo
~'~o.
100 ....
1~0 ....
. . . . ,Pv3
N~Z
50 ....
100' nlin' '150
Fig. 7. Simultaneous sorption of various gas components (getter HTR-1).
getter was already higher than the partial pressure in the loop, so its concentration increased. For CO and N2 a higher sorption speed was observed for 300 ~ than for 200 ~ as expected. For CH4 the getter temperature of 200 ~ was obviously too low to cause any measurable sorption effect. At 300 ~ however, a surprising effect was found: an increase in concentration rather than the expected decrease. This can be explained by the formation of additional methane caused by the interaction of carbon monoxide and hydrogen during the passage through the getter: 2H2 + CO --* CH4 + 89 The corresponding amount of oxygen was directly sorbed by the getter. As there was a strong decrease in the CO concentration in the first 20 min of the test, the rate of CH4 formation decreased also. The carbon sorbed in the preceding test at 200 ~ was not involved in the reaction, otherwise the amount of the additional methane would have been much higher. The formation of additional methane is very undesirable from the point of view of gas purification. To suppress this formation two getter beds can be applied in series. The first bed is operated in the range 200-250 ~ to reduce the concentration of CO and H2 and the second at a temperature of at least 400 ~ to remove effectively all the CH4 and Ne. In Fig. 8 the concentration of three impurity components is shown as measured in test PV-15 at 200 and 400 ~ with an initial gas volume of 105 1. The purification efficiency of the getter bed combination is further'increased by applying higher temperatures. For 250 and 450 ~ as an example, the Cout/ci, ratios decrease by almost a factor of 3 for each of the impurity components mentioned above.
1166 1000
'
'
'
I
PV-15
ppm
9
I::o:%':
9
CO ~'~;
I Carrier gas, He. O. 5% H z
100
10 5
'
50
100
,
150
,
200
250 min
Fig. 8. Simultaneous sorption of impurities by two getters in series.
7. C o n c l u s i o n s Metal getter beds can be used to remove gaseous impurities from helium-hydrogen gas mixtures. In tritium technology, the removal of methane is of special interest because of its high concentration in the plasma exhaust and the necessity to recover tritium from tritiated methane. As low operation temperatures of the getter beds are desirable to avoid tritium permeation losses, the concentration of methane cannot be reduced by 3-5 orders of magnitude unless the gas is circulated several times through the same getter bed in a closed loop. To optimize the purification efficiency for operation temperatures of 400-450 ~ the following procedure is recommended: (a) separation of hydrogen isotopes in molecular form by using a P d - A g diffuser in a first purification step; (b) use of a getter alloy containing a component with a high cracking capability for methane, e.g. nickel; (c) application of a P d - A g diffuser in the purification loop to shift the equilibrium of the reaction CH4 --*C -t- 2H2 to the right by continuous removal of the liberated hydrogen isotopes; (d) prevention of additional CH4 formation by sorption of CO on a second getter bed and by continuous H2 removal; (e) increase in the gas flow rate. Good results were obtained with the HWT gas purifier HTR-2 because the nickel component of its getter alloy leads to an increased cracking efficiency and because higher gas flow rates can be used than for the tested SAES purifier. In addition, the equilibrium pressure of the hydrogen isotopes is higher by several orders of magnitude for both HWT getters; it is much easier, therefore, to keep the tritium losses low when the exhausted getter beds must be disposed of.
1167 T h e e x p e r i m e n t a l p r o g r a m m e is b e i n g c o n t i n u e d w i t h i n v e s t i g a t i o n s o f t h e s o r p t i o n o f w a t e r v a p o u r a n d o f t h e d e p e n d e n c e of s o r p t i o n e f f i c i e n c y o n impurity uptake.
References 1 F. Pourarian and W. E. Wallace, The effect of substitution of Mn or A1 on the hydrogen sorption characteristics of CeNi s, Int. J. Hydrogen Energy, 10 (1) (1985) 49. 2 J.-M. Park and J.-Y. Lee, Hydrogenation characteristics of the Zr~ _xTixCr~ _yFe~ § Laves phase alloys, J. Less-Common Met., 160 (1990) 259. 3 O. Bernauer and C. Halene, Properties of metal hydrides for use in industrial applications, J. Less-Common Met., 131 (1987) 213. 4 O. Bernauer, J. Toepler, D. Nor~us, R. Hempelmann and Richter, Fundamentals and properties of some Ti/Mn based Laves phase hydrides, Int. J. Hydrogen Energy, 14 (3) (1989) 187. 5 S. K. Sood and O. K. Kveton, Tritium systems concepts for the Next European Torus (NET), CFFTP Rep. G-86020, 1986. 6 G. Bourque, B. Terreault, B. C. Gregory, G. W. Pacher et al., Chemical impurity production in the Tokamak de Varennes, Fusion Technol., 17 (1990) 588. 7 G. M. McCracken, S. J. Fielding, G. F. Matthews and C. S. Pichter, Impurity production due to wall interactions in tokamaks, J. Nucl. Mater., 162-164 (1989) 392. 8 H. Albrecht, R.-D. Penzhorn, Th. Kastner and M. Sirch, Plasma exhaust purification with metal getters, Fusion Eng. Des., 10 (1989) 349. 9 R. V. Carlson, K. E. Binning, S. Konishi, H. Yoshida and Y. Naruse, Results on tritium experiments on ceramic electrolysis cells and palladium diffusors for application to fusion reactor fuel clean-up systems, 12th Syrup. on Fusion Energy, Monterey, CA, October 1987. 10 R.-D. Penzhorn and M. Glugla, Process to recover tritium from fusion cycle impurities, Fusion Technol., 10 (1986) 1345. 11 B. Ferrario, C. Boffito, F. Doni and L. Rosai, Zr based gettering alloys for hydrogen isotope handling, 13th Syrup. on Fusion Technology (SOFT), Varese, September 24-28, 1984. 12 B. Bonizzoni, A. Conte, G. Gatto, G. Gervasini, F. Ghezzi and M. Rigamonti, Tritium storage plant based on a combination of ST-707 and ST-737 getter alloy beds for high field fusion machines, 11th Int. Vacuum Congr. Cologne, September 25-29, 1989. 13 J. L. Cecchi and R. J. Knize, Gettering in fusion devices, J. Vac. Sci. Technol. A2 (2) (1984) 1214. 14 H. Heimbach, H. R. Ihle and C. H. Wu, Removal of nitrogen and methane from hydrogen by metal getters, 13th Syrup. on Fusion Technology (SOFT), Varese, September 24-28, 1984. 15 W. J. Holtslander, R. E. Johnson, F. B. Gravelle and C. M. Shulz, An experimental evaluation of a small fuel cleanup system, Fusion Technol., 10 (1986) 1340. 16 O. Bernauer, Metal hydride storage, Z. Phys. Chem., N.F., 164 (1989) 1381. 17 O. Bernauer, Metal hydride technology, J. Hydrogen Energy, 13 (3) (1988) 181.