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Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery
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Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery Yoshinori Kawamura a,∗ , Takumi Hayashi b , Toshihiko Yamanishi c a b c
Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, Ibaraki 311-0193, Japan Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai, Ibaraki 319-1195, Japan Japan Atomic Energy Agency, 2-166 Omotedate Obuchi, Rokkasho, Aomori 039-3212, Japan
h i g h l i g h t s • • • •
Precious metal catalyst based on cation-exchanged mordenite was prepared. Isotope exchange reaction between H2 and HTO on the catalyst was investigated. The order of entire reaction is not clear, but it is the first-order reaction as for HTO. Effect of exchanged cation may appear as the difference of the surface area of catalyst.
a r t i c l e
i n f o
Article history: Received 29 August 2015 Received in revised form 31 October 2015 Accepted 8 November 2015 Available online xxx Keywords: Ceramic breeder blanket Tritium Precious metal catalyst Cation-exchanged mordenite Isotope exchange reaction
a b s t r a c t It is known that the chemical forms of tritium released from a ceramic breeder blanket are hydrogen form and water form. To recover tritiated water vapor, adoption of dryer that is packed column of synthetic zeolite has been proposed. On the other hand, synthetic zeolite is often used as a support of precious metal catalyst. Such catalysts usually have a capability of hydrogen isotope exchange between gas and water vapor. If this catalyst is used to dryer, the dryer may obtain a preferable function for tritium recovery by isotopic exchange reaction. To assess such functions, reaction rate should be estimated. The results of water adsorption experiment on cation-exchanged mordenite-type zeolite suggested the possibility that state of adsorbed water varied by exchanged cation. So, in this work, precious metal catalyst based on cation-exchanged mordenite was prepared, and the reaction rate of chemical exchange between hydrogen and tritiated water was investigated under temperature range between 30 ◦ C and 80 ◦ C by the steady-state approximation. In the case of platinum on Na-mordenite, the reaction between gaseous hydrogen and tritiated water vapor was almost expressed as first-order reaction concerning tritiated water vapor concentration. © 2015 Elsevier B.V. All rights reserved.
1. Introduction To realize a fusion reactor, establishment of fuel cycle system is important. Especially, bred tritium extraction system having a good efficiency is important from the viewpoint of tritium economy in a fusion reactor. Chemical form of tritium released from a ceramic breeder blanket is known to be hydrogen form and water form [1]. For example, to recover tritiated water vapor, adoption of dryer column that is packed bed of synthetic zeolite has been proposed [2]. On the other hand, synthetic zeolite is often used as a support material of precious metal catalyst. Such catalysts usually have a
∗ Corresponding author. Tel.: +81 292707581; fax: +81 292707499. E-mail address: [email protected] (Y. Kawamura).
capability of hydrogen isotope exchange between gas and water vapor [3,4]. If this catalyst is used as packed material of dryer, the dryer may obtain a preferable function for tritium recovery. For example, tritium in hydrogen form may be captured, or tritium in water may be transferred to hydrogen. It depends on the gaseous contents whether tritium in adsorbed water moves to the hydrogen gas or not. However, to assess such functions, reaction rate should be estimated. The present authors have investigated adsorption property of water on cation-exchanged mordenite-type zeolite, and the result suggested the possibility that state of adsorbed water varied by exchanged cation [5]. So, in this work, precious metal catalyst based on cation-exchanged mordenite (MOR) was prepared. Practically, the reaction rate for assessment should be obtained under non-steady state. However, in this work, it is difficult to obtain
http://dx.doi.org/10.1016/j.fusengdes.2015.11.009 0920-3796/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Kawamura, et al., Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.11.009
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Nomenclatures CH2 CH2 ,in CH2 O CH2 O,in CHT CHTO CHTO,in Cw DL K kF
kR
L R Rate T t u ε
H2 concentration [mol/cm3 ] inlet H2 concentration [mol/cm3 ] H2 O concentration [mol/cm3 ] inlet H2 O concentration [mol/cm3 ] HT concentration [mol/cm3 ] HTO concentration [mol/cm3 ] inlet HTO concentration [mol/cm3 ] water vapor concentration [mol/cm3 ] axial dispersion coefficient [cm2 /s] equilibrium constant [dimensionless] reaction rate constant of forward reaction, 1storder reaction [1/s] or [s−1 ], 2nd-order reaction [cm3 /mol/s] reaction rate constant of reverse reaction, 1storder reaction [1/s] or [s−1 ], 2nd-order reaction [cm3 /mol/s] axial distance from the inlet of the bed [cm] gas constant [J/mol/K] reaction rate based on the gas phase [mol/cm3 /s] absolute temperature [K] time [s] superficial gas velocity [cm/s] void fraction [dimensionless] space-time [s]
experimentally. So, the reaction rate of chemical exchange between hydrogen and tritiated water on precious metal catalyst based on cation-exchanged MOR was investigated by the steady state approximation. 2. Experimentals 2.1. Sample preparation Start material was HSZ-642NAA, made in TOSO Co., which is classified to synthetic mordenite (MOR) having sodium (Na) ion which is exchangeable cation (Na-MOR). To replace the exchangeable cation with designed ions, Na-MOR has been immersed into nitrate aqueous solution (or hydrochloride aqueous solution or acetate aqueous solution) including designed cation. In this work, lithium (Li) ion and potassium (K) ion were selected as the designed cations. When the designed ion was K, Na in Na-MOR was almost replaced with K. When designed cation was Li. Na still remained [6]. However, these support materials shall be described as K-MOR and Li-MOR hereafter. Specifications of the materials were summarized in our previous work [6]. Platinum (Pt) was selected as precious metal of catalyst. Pt was put on the support material by impregnation method using hexachloro platinum (IV) acid HCl solution. After impregnation, the samples were heated by water bath. Then, the sample was baked at 150 ◦ C for 1 h and 300 ◦ C for 3 h under H2 (10%)/He gas atmosphere. Fractions of Pt were adjusted 0.5 wt% and/or 1.0 wt%. These fractions are not observation, but estimation from the raw materials. 2.2. Apparatus and experimental procedure Fig. 1 shows a schematic diagram of the experimental apparatus. The sample was packed into a stainless steel pipe with 6.35 mm in outer diameter with 1 mm in thickness. Packed weight was in the range between 0.17 g and 0.20 g, and the bed height was in the range between 1.95 cm and 2.05 cm. Prior to experiment, the
Fig. 1. A schematic diagram of the experimental apparatus.
sample bed was heated up to 300 ◦ C under H2 /He gas flow until moisture monitor output becomes stably low; this is the process to remove residual water and to activate the catalyst. After that, the sample bed was by-passed, and its temperature was kept in the range between 30 and 80 ◦ C by water bath. Tritiated water between 0.588 and 0.650 M Bq/cm3 was put into gas-washing bottle, and its temperature was also kept at 30 ◦ C by water bath. The sample gas was H2 balanced with He, and its H2 concentration range was between 0.1 and 10%. The sample gas was passed through the gaswashing bottle to add tritiated water vapor. At first, the flow rate of the sample gas was adjusted between 0.2 and 0.8 l/min (std) by the mass flow controller (1). Water concentration was measured by moisture monitor and was adjusted by the mass flow controller (2) between −10 ◦ C and 8 ◦ C in the dew point. After water concentration became stable, the sample gas was passed through the sample bed. After the equilibrium state was attained, the effluent gas from the sample bed was introduced to HTO trap. The HTO trap is a couple of the gas-washing bottles, which are put the pure water (∼100 cm3 ) in, and were used one by one. The bubbling time for each bottle was 30 min. The HTO trap was removed after bubbling, and the water of 1 cm3 was sampled from it. After the sampling, the gas-washing bottle was re-attached for the next experimental condition. The water sampled was mixed with liquid scintillator, and its tritium activity was measured by liquid scintillation counter. After the sampling from these two gas-washing bottles, the experimental flow rate was changed, and the experiment was carried out in the similar procedure. In this work, all tritiated water was controlled by its weight. The experimental conditions are listed in Table 1.
3. Results and discussions Fig. 2 shows the comparison of the HTO concentration ratios of the outlet gas to the inlet gas among various samples. Spacetime, [s] is obtained from dividing bed height by superficial gas velocity. As shown in this figure, the stainless steel (blank) and NaMOR hardly contribute to the isotope exchange reaction. And, Pt contributes to progress of isotope exchange reaction. But, the reactivity between 0.5 wt% Pt on Na-MOR and 1.0 wt% Pt on Na-MOR was almost same. So, as for other support materials, Pt of 0.5 wt% was put on. Isotope exchange reaction is simply assumed as: kF → HT + H2 O, H2 + HTO − ← kR
(1)
and the equilibrium constant, K is defined as: K = kF /kR .
(2)
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Table 1 The experimental conditions in this work. Sample
1.0 wt% Pt Na-MOR
Particle size Packed weight (g) Bed height (cm) Bed temperature (◦ C) Sample gas H2 (Pa) H2 O (Pa) HTO/H2 O (dimensionless) Flow rate (l/min std)
16–32 mesh 0.1743 2.0 50.98 H2 (0.1-10%), H2 O, HTO balanced with He 8327 243.2 1.08 × 10−8 0.2–0.8
0.5 wt% Pt Na-MOR
0.5 wt% Pt Li-MOR
0.5 wt% Pt K-MOR
0.2001 2.0 29.6–80.6
0.1717 2.05 51.23
0.1743 1.95 51.34
91.0–8249 233.3–914.7 9.85 × 10−9 –1.08 × 10−8
8325 241.0 1.09 × 10−8
8244 270.8 1.08 × 10−8
the sample bed [–]. To assume that the axial dispersion is negligible, Eq. (8) is rewritten by use of Eq. (7) at the steady state as: u
CHTO,in Cw dCHTO Cw ). = −(1 − ε)kF (CH2 ,in + )(CHTO − K KCH2 ,in + Cw dL
(9)
As the boundary condition, CHTO is equal to CHTO,in , when L is 0. So, Eq. (9) is solved, and the solution is expressed using the spacetime, as; Cw + KCH2 ,in exp(−kF (1 − ε)(CH2 ,in + CHTO = CHTO,in KCH2 ,in + Cw
Fig. 2. Comparison of the HTO concentration ratios of the outlet gas to the inlet gas among various samples.
So, assuming the second-order reaction, the reaction rate is expressed as: Rate = kF CH2 CHTO = kR CHT CH2 O ,
(3)
where CH2 , CHT , CH2 O , CHTO mean concentration of H2 , HT, H2 O, and HTO in the gas phase [mol/cm3 ], respectively. Rate means the reaction rate based on the gas phase [cm3 /mol/s]. And, kF and kR mean reaction rate constant of forward reaction and reverse reaction, respectively. Their units depend on the order of reaction. When 2nd-order reaction is assumed, their units are [cm3 /mol/s]. The amount of tritium is extremely less than hydrogen and water (CH2,in CHT , Cw CHTO ). Therefore, following relations are applicable: CH2 = CH2 ,in − CHT ≈ CH2 ,in ,
(4)
Cw = CH2 O + CHTO = CH2 O,in + CHTO,in ≈ CH2 O ≈ CH2 O,in ,
(5)
CHT = CHTO,in − CHTO ,
(6)
where, “in” means the inlet of the sample bed and “w” means water vapor. Using Eqs. (2), (4)–(6), Eq. (3) is rewritten as; Rate = kF CH2 ,in CHTO − = kF (CH2 ,in +
kF − CHTO )Cw (C K HTO,in
CHTO,in Cw Cw ) )(CHTO − K KCH2 ,in + Cw
.
(7)
On the other hand, the mass balance of HTO in the sample bed is expressed as; 2
DL
∂ CHTO ∂CHTO ∂CHTO =u +ε + (1 − ε)Rate, ∂L 2 ∂L ∂t
(8)
where, DL means axial dispersion coefficient [cm2 /s], L means axial distance from the inlet of the sample bed [cm], u means superficial gas velocity [cm/s], t means time [s], and ε means void fraction of
Cw )) K
.
(10)
Izawa et al. have investigated isotope exchange reaction between deuterium and water on hydrophobic catalyst [7]. And, they have concluded the gaseous exchange reaction is expressed by the quasi first-order reaction. Because tritium is much less than hydrogen and water vapor, assuming first-order reaction about tritium might be preferable. In such case, Eq. (1) is rewritten as; kF → HT. HTO − ← kR
(11)
The reaction rate is expressed using Eq. (6) as; Rate = kF CHTO − kR CHT = kF CHTO −
kF (C − CHTO ) K HTO,in
CHTO,in 1 ) = kF (1 + )(CHTO − K K +1
,
(12)
where, kF and kR are reaction rate constant [1/s]. After incorporating Eq. (12) to Eq. (8), according to the similar procedure, the following equation is derived as solution;
1 + K exp −kF (1 − ε) 1 + CHTO = 1+K CHTO,in
1 K
,
(13)
HTO concentration ratio of the outlet to the inlet is calculated by using Eqs. (10) and (13). The forward reaction rate constant, kF and the equilibrium constant, K were obtained in comparison with experimental observations and the calculations. Fig. 3 shows a comparison with experimental observation and the calculation. As shown in this figure, the equilibrium constant in the case of the first-order reaction is larger than 1.0, and oppositely that of the second-order reaction is smaller than 1.0. Although the same phenomenon is being expressed, this means that the directions of reaction that advance easily are opposite. Fig. 4 shows temperature dependence of kF on 0.5 wt% Pt on Na-MOR. Temperature range was between 30 ◦ C and 80 ◦ C, and data except 50 ◦ C were few. Hence, discussion about temperature dependence might be difficult. Eq. (10) and Eq. (13) are similar each other in spite of the different order of the reaction. Therefore, temperature dependence is also almost same each other. Actually, obtained forward reaction rate constants, kF were expressed as;
10600
kF = 1.63 × 103 exp −
RT
(1st),
(14)
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Fig. 3. An example of the parameter fitting. Fig. 5. Dependence of kF against hydrogen partial pressure on 0.5 wt% Pt Na-MOR.
Fig. 4. Temperature dependence of kF assuming both the 1st-order reaction and the 2nd-order reaction.
10200
kF = 3.87 × 103 exp −
RT
(2nd),
(15)
where, R is gas constant [J/mol/K], and T is absolute temperature [K]. The equilibrium constants, K [–], were also obtained as follows: K = 0.66 exp K − 0.14 exp
2800 RT
4500 RT
(1st),
(16)
(2nd) .
(17)
These activation energies were lower. Obtained equilibrium constants seem to be almost constant in the experimental temperature range. The reverse reaction rate constant, kR , is obtained from dividing Eq. (14) or (15) by Eq. (16) or (17). As mentioned before, the equilibrium constant for the first-order reaction is larger than 1.0, and that for the second-order reaction is smaller than 1.0. This means the models were not reasonable to express the reaction. To validate the reaction model, other properties were also investigated. Fig. 5 shows the hydrogen partial pressure dependence of kF . Experimental temperature and the partial pressure of water vapor were adjusted at 50 ◦ C and about 240 Pa. Usually reaction rate constant does not depend on the concentration (partial pressure) of reactant, if the model is reasonable. However, as shown in Fig. 5, kF in both cases depended on the concentration (partial pressure) of hydrogen. This means the descriptions of hydrogen concentration in both models are wrong. And, the slope on the 1st-order reaction seems to be comparatively close to 0.5. Presumption from limited data is risky, but this may suggest that dissociation of hydrogen occurs. In the case of the 2nd-order reaction, the slope of kF went
Fig. 6. Dependence of kF against hydrogen partial pressure on 0.5 wt% Pt Na-MOR.
down. It means assuming the first-order reaction concerning H2 is overestimation. Therefore, the order of reaction concerning H2 in the forward reaction is estimated to be between 0 and 1. Fig. 6 shows the water vapor pressure dependence of kF . It seems to have no dependence with the partial pressure of water vapor for both cases. Because tritium source (tritiated water) was not replaced, tritium contents were almost constant in a series of the experiments. Therefore, the HTO concentration in the gas is proportional to the partial pressure of water vapor. Fig. 6 suggests that kF does not depend on the HTO concentration, and it is assumed that the forward reaction is the 1st-order reaction concerning HTO. However, effect of H2 O concentration (partial pressure) is still not clear. To make clear the H2 O dependence, the experiment using tritiated water having different tritium contents is needed strictly. It might be reasonable that the half-order reaction and the 1storder reaction are assumed for H2 and HTO, respectively. However, the order of reaction could not make clear in this work. One of the causes is that the sample is hydrophilic. In such case, the amount of adsorbed water should be reflected into the reaction rate equation. It is assumed that investigation on water adsorption characteristic of these samples, especially, the non-steady state experiments are necessary. Only 0.5 wt% Pt on Na-MOR (hereafter Pt Na-MOR) has been discussed so far, because the data for other materials are much less than Pt Na-MOR. Hereafter, other materials are also compared. Fig. 7 shows the comparison of the HTO concentration ratio of the outlet to the inlet of the sample bed among various samples. As mentioned above, the order of reaction is not clear yet. However, assuming the 1st-order reaction, estimated forward reaction rate constants kF on Pt Li-MOR, Pt Na-MOR and
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4. Conclusion Catalysts supporting platinum with cation-exchanged mordenite were prepared, and isotope exchange reaction between hydrogen and tritiated water vapor was investigated. Assuming both the first-order reaction for tritium and the second-order reaction, the reaction rate constants and the equilibrium constant for 0.5 wt% Pt on Na-MOR were mainly estimated. The reaction rate is expressed as the first-order reaction for tritium. However, the reaction rate constants indicated dependency of hydrogen partial pressure. Therefore, assumption of other reaction model is necessary. Effect of the cation-exchanged carrier on the reaction rate constant seems to be caused by surface area of the carrier. Acknowledgments Fig. 7. Comparison of the HTO concentration ratio of the outlet to the inlet of the sample bed among various samples.
Pt K-MOR were 99.1, 81.2 and 49.3 s−1 , respectively. It is difficult to assume something from the comparison of estimated kF because of few data. However, the order of kF is similar to that of BET surface area for the support materials. The BET surface areas of their support materials have already been observed (LiMOR/Na-MOR/K-MOR: 352/317/283 m2 /g) [5]. This suggests the isotope exchange reaction is affected by the surface of the catalyst. In the previous work, the author has pointed out that chemisorbed water existed on Li-MOR surface [5]. Also, the effect of chemisorbed water on isotope exchange reaction is yet not clear in this work. In this work, the reaction based on the gas phase is assumed, because adsorption isotherms and isobars of water have not been clarified yet. These samples are based on synthetic mordenite and are hydrophilic. Therefore, it is assumed that the diffusion of adsorbed water into the pore of particle strongly affects the isotope exchange reaction actually. Other mass transfer process except surface reaction should be investigated hereafter. The discussion about effectiveness of addition of the new function for dryer column by applying of hydrophilic catalyst is difficult currently because of limited data.
In order to proceed to this work, the authors received funding support from Ministry of Education, Culture, Sports, Science and Technology in Japan (grant no. 26420859; Grant-in Aid for Scientific Research (c)). Authors really wish to acknowledge for this funding support:. References [1] Y. Kawamura, K. Ochiai, T. Hoshino, K. Kondo, Y. Iwai, et al., Effect of sweep gas species on tritium release behavior from lithium titanate packed bed during 14 MeV neutron irradiation, Fusion Eng. Des. 87 (2012) 1253–1257. [2] H. Tanigawa, T. Hoshino, Y. Kawamura, M. Nakamichi, K. Ochiai, et al., R&D of a Li2 TiO3 pebble bed for a test blanket module in JAEA, Nucl. Fusion 49 (2009) 055021, http://dx.doi.org/10.1088/0029-5515/49/5/055021 . [3] M. Enoeda, T. Higashijima, M. Nishikawa, N. Mitsuishi, Recovery of tritium in inert gas by precious metal catalyst supported by hydrophilic substrate, J. Nucl. Sci. Technol. 23 (1986) 1083–1093. [4] K. Munakata, M. Nishikawa, T. Takeishi, N. Mitsuishi, M. Enooeda, Recovery of tritium in room air by precious metal catalyst with hydrophilic substrate, J. Nucl. Sci. Technol. 25 (1988) 383–394. [5] Y. Kawamura, Y. Edao, Y. Iwai, T. Hayashi, T. Yamanishi, Hydrogen and water vapor adsorption properties on cation-exchanged mordenite for use to a tritium recovery system, Fusion Eng. Des. 89 (2014) 1539–1543. [6] Y. Kawamura, Y. Iwai, K. Munakata, T. Yamanishi, Effect of cation exchange on hydrogen adsorption property of mordenite for isotope separation, J. Nucl. Mater. 442 (2013) S455–S460. [7] H. Izawa, S. Isomura, R. Nakane, Gaseous exchange reaction of deuterium between hydrogen and water on hydrophobic catalyst supporting platinum, J. Nucl. Sci. Technol. 16 (1979) 741–749.
Please cite this article in press as: Y. Kawamura, et al., Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.11.009
ARTICLE IN PRESS
FUSION-8325; No. of Pages 5
Fusion Engineering and Design xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery Yoshinori Kawamura a,∗ , Takumi Hayashi b , Toshihiko Yamanishi c a b c
Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, Ibaraki 311-0193, Japan Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai, Ibaraki 319-1195, Japan Japan Atomic Energy Agency, 2-166 Omotedate Obuchi, Rokkasho, Aomori 039-3212, Japan
h i g h l i g h t s • • • •
Precious metal catalyst based on cation-exchanged mordenite was prepared. Isotope exchange reaction between H2 and HTO on the catalyst was investigated. The order of entire reaction is not clear, but it is the first-order reaction as for HTO. Effect of exchanged cation may appear as the difference of the surface area of catalyst.
a r t i c l e
i n f o
Article history: Received 29 August 2015 Received in revised form 31 October 2015 Accepted 8 November 2015 Available online xxx Keywords: Ceramic breeder blanket Tritium Precious metal catalyst Cation-exchanged mordenite Isotope exchange reaction
a b s t r a c t It is known that the chemical forms of tritium released from a ceramic breeder blanket are hydrogen form and water form. To recover tritiated water vapor, adoption of dryer that is packed column of synthetic zeolite has been proposed. On the other hand, synthetic zeolite is often used as a support of precious metal catalyst. Such catalysts usually have a capability of hydrogen isotope exchange between gas and water vapor. If this catalyst is used to dryer, the dryer may obtain a preferable function for tritium recovery by isotopic exchange reaction. To assess such functions, reaction rate should be estimated. The results of water adsorption experiment on cation-exchanged mordenite-type zeolite suggested the possibility that state of adsorbed water varied by exchanged cation. So, in this work, precious metal catalyst based on cation-exchanged mordenite was prepared, and the reaction rate of chemical exchange between hydrogen and tritiated water was investigated under temperature range between 30 ◦ C and 80 ◦ C by the steady-state approximation. In the case of platinum on Na-mordenite, the reaction between gaseous hydrogen and tritiated water vapor was almost expressed as first-order reaction concerning tritiated water vapor concentration. © 2015 Elsevier B.V. All rights reserved.
1. Introduction To realize a fusion reactor, establishment of fuel cycle system is important. Especially, bred tritium extraction system having a good efficiency is important from the viewpoint of tritium economy in a fusion reactor. Chemical form of tritium released from a ceramic breeder blanket is known to be hydrogen form and water form [1]. For example, to recover tritiated water vapor, adoption of dryer column that is packed bed of synthetic zeolite has been proposed [2]. On the other hand, synthetic zeolite is often used as a support material of precious metal catalyst. Such catalysts usually have a
∗ Corresponding author. Tel.: +81 292707581; fax: +81 292707499. E-mail address: [email protected] (Y. Kawamura).
capability of hydrogen isotope exchange between gas and water vapor [3,4]. If this catalyst is used as packed material of dryer, the dryer may obtain a preferable function for tritium recovery. For example, tritium in hydrogen form may be captured, or tritium in water may be transferred to hydrogen. It depends on the gaseous contents whether tritium in adsorbed water moves to the hydrogen gas or not. However, to assess such functions, reaction rate should be estimated. The present authors have investigated adsorption property of water on cation-exchanged mordenite-type zeolite, and the result suggested the possibility that state of adsorbed water varied by exchanged cation [5]. So, in this work, precious metal catalyst based on cation-exchanged mordenite (MOR) was prepared. Practically, the reaction rate for assessment should be obtained under non-steady state. However, in this work, it is difficult to obtain
http://dx.doi.org/10.1016/j.fusengdes.2015.11.009 0920-3796/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Kawamura, et al., Isotope exchange reaction of tritium on precious metal catalyst based on cation-exchanged mordenite for blanket tritium recovery, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.11.009
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Nomenclatures CH2 CH2 ,in CH2 O CH2 O,in CHT CHTO CHTO,in Cw DL K kF
kR
L R Rate T t u ε
H2 concentration [mol/cm3 ] inlet H2 concentration [mol/cm3 ] H2 O concentration [mol/cm3 ] inlet H2 O concentration [mol/cm3 ] HT concentration [mol/cm3 ] HTO concentration [mol/cm3 ] inlet HTO concentration [mol/cm3 ] water vapor concentration [mol/cm3 ] axial dispersion coefficient [cm2 /s] equilibrium constant [dimensionless] reaction rate constant of forward reaction, 1storder reaction [1/s] or [s−1 ], 2nd-order reaction [cm3 /mol/s] reaction rate constant of reverse reaction, 1storder reaction [1/s] or [s−1 ], 2nd-order reaction [cm3 /mol/s] axial distance from the inlet of the bed [cm] gas constant [J/mol/K] reaction rate based on the gas phase [mol/cm3 /s] absolute temperature [K] time [s] superficial gas velocity [cm/s] void fraction [dimensionless] space-time [s]
experimentally. So, the reaction rate of chemical exchange between hydrogen and tritiated water on precious metal catalyst based on cation-exchanged MOR was investigated by the steady state approximation. 2. Experimentals 2.1. Sample preparation Start material was HSZ-642NAA, made in TOSO Co., which is classified to synthetic mordenite (MOR) having sodium (Na) ion which is exchangeable cation (Na-MOR). To replace the exchangeable cation with designed ions, Na-MOR has been immersed into nitrate aqueous solution (or hydrochloride aqueous solution or acetate aqueous solution) including designed cation. In this work, lithium (Li) ion and potassium (K) ion were selected as the designed cations. When the designed ion was K, Na in Na-MOR was almost replaced with K. When designed cation was Li. Na still remained [6]. However, these support materials shall be described as K-MOR and Li-MOR hereafter. Specifications of the materials were summarized in our previous work [6]. Platinum (Pt) was selected as precious metal of catalyst. Pt was put on the support material by impregnation method using hexachloro platinum (IV) acid HCl solution. After impregnation, the samples were heated by water bath. Then, the sample was baked at 150 ◦ C for 1 h and 300 ◦ C for 3 h under H2 (10%)/He gas atmosphere. Fractions of Pt were adjusted 0.5 wt% and/or 1.0 wt%. These fractions are not observation, but estimation from the raw materials. 2.2. Apparatus and experimental procedure Fig. 1 shows a schematic diagram of the experimental apparatus. The sample was packed into a stainless steel pipe with 6.35 mm in outer diameter with 1 mm in thickness. Packed weight was in the range between 0.17 g and 0.20 g, and the bed height was in the range between 1.95 cm and 2.05 cm. Prior to experiment, the
Fig. 1. A schematic diagram of the experimental apparatus.
sample bed was heated up to 300 ◦ C under H2 /He gas flow until moisture monitor output becomes stably low; this is the process to remove residual water and to activate the catalyst. After that, the sample bed was by-passed, and its temperature was kept in the range between 30 and 80 ◦ C by water bath. Tritiated water between 0.588 and 0.650 M Bq/cm3 was put into gas-washing bottle, and its temperature was also kept at 30 ◦ C by water bath. The sample gas was H2 balanced with He, and its H2 concentration range was between 0.1 and 10%. The sample gas was passed through the gaswashing bottle to add tritiated water vapor. At first, the flow rate of the sample gas was adjusted between 0.2 and 0.8 l/min (std) by the mass flow controller (1). Water concentration was measured by moisture monitor and was adjusted by the mass flow controller (2) between −10 ◦ C and 8 ◦ C in the dew point. After water concentration became stable, the sample gas was passed through the sample bed. After the equilibrium state was attained, the effluent gas from the sample bed was introduced to HTO trap. The HTO trap is a couple of the gas-washing bottles, which are put the pure water (∼100 cm3 ) in, and were used one by one. The bubbling time for each bottle was 30 min. The HTO trap was removed after bubbling, and the water of 1 cm3 was sampled from it. After the sampling, the gas-washing bottle was re-attached for the next experimental condition. The water sampled was mixed with liquid scintillator, and its tritium activity was measured by liquid scintillation counter. After the sampling from these two gas-washing bottles, the experimental flow rate was changed, and the experiment was carried out in the similar procedure. In this work, all tritiated water was controlled by its weight. The experimental conditions are listed in Table 1.
3. Results and discussions Fig. 2 shows the comparison of the HTO concentration ratios of the outlet gas to the inlet gas among various samples. Spacetime, [s] is obtained from dividing bed height by superficial gas velocity. As shown in this figure, the stainless steel (blank) and NaMOR hardly contribute to the isotope exchange reaction. And, Pt contributes to progress of isotope exchange reaction. But, the reactivity between 0.5 wt% Pt on Na-MOR and 1.0 wt% Pt on Na-MOR was almost same. So, as for other support materials, Pt of 0.5 wt% was put on. Isotope exchange reaction is simply assumed as: kF → HT + H2 O, H2 + HTO − ← kR
(1)
and the equilibrium constant, K is defined as: K = kF /kR .
(2)
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Table 1 The experimental conditions in this work. Sample
1.0 wt% Pt Na-MOR
Particle size Packed weight (g) Bed height (cm) Bed temperature (◦ C) Sample gas H2 (Pa) H2 O (Pa) HTO/H2 O (dimensionless) Flow rate (l/min std)
16–32 mesh 0.1743 2.0 50.98 H2 (0.1-10%), H2 O, HTO balanced with He 8327 243.2 1.08 × 10−8 0.2–0.8
0.5 wt% Pt Na-MOR
0.5 wt% Pt Li-MOR
0.5 wt% Pt K-MOR
0.2001 2.0 29.6–80.6
0.1717 2.05 51.23
0.1743 1.95 51.34
91.0–8249 233.3–914.7 9.85 × 10−9 –1.08 × 10−8
8325 241.0 1.09 × 10−8
8244 270.8 1.08 × 10−8
the sample bed [–]. To assume that the axial dispersion is negligible, Eq. (8) is rewritten by use of Eq. (7) at the steady state as: u
CHTO,in Cw dCHTO Cw ). = −(1 − ε)kF (CH2 ,in + )(CHTO − K KCH2 ,in + Cw dL
(9)
As the boundary condition, CHTO is equal to CHTO,in , when L is 0. So, Eq. (9) is solved, and the solution is expressed using the spacetime, as; Cw + KCH2 ,in exp(−kF (1 − ε)(CH2 ,in + CHTO = CHTO,in KCH2 ,in + Cw
Fig. 2. Comparison of the HTO concentration ratios of the outlet gas to the inlet gas among various samples.
So, assuming the second-order reaction, the reaction rate is expressed as: Rate = kF CH2 CHTO = kR CHT CH2 O ,
(3)
where CH2 , CHT , CH2 O , CHTO mean concentration of H2 , HT, H2 O, and HTO in the gas phase [mol/cm3 ], respectively. Rate means the reaction rate based on the gas phase [cm3 /mol/s]. And, kF and kR mean reaction rate constant of forward reaction and reverse reaction, respectively. Their units depend on the order of reaction. When 2nd-order reaction is assumed, their units are [cm3 /mol/s]. The amount of tritium is extremely less than hydrogen and water (CH2,in CHT , Cw CHTO ). Therefore, following relations are applicable: CH2 = CH2 ,in − CHT ≈ CH2 ,in ,
(4)
Cw = CH2 O + CHTO = CH2 O,in + CHTO,in ≈ CH2 O ≈ CH2 O,in ,
(5)
CHT = CHTO,in − CHTO ,
(6)
where, “in” means the inlet of the sample bed and “w” means water vapor. Using Eqs. (2), (4)–(6), Eq. (3) is rewritten as; Rate = kF CH2 ,in CHTO − = kF (CH2 ,in +
kF − CHTO )Cw (C K HTO,in
CHTO,in Cw Cw ) )(CHTO − K KCH2 ,in + Cw
.
(7)
On the other hand, the mass balance of HTO in the sample bed is expressed as; 2
DL
∂ CHTO ∂CHTO ∂CHTO =u +ε + (1 − ε)Rate, ∂L 2 ∂L ∂t
(8)
where, DL means axial dispersion coefficient [cm2 /s], L means axial distance from the inlet of the sample bed [cm], u means superficial gas velocity [cm/s], t means time [s], and ε means void fraction of
Cw )) K
.
(10)
Izawa et al. have investigated isotope exchange reaction between deuterium and water on hydrophobic catalyst [7]. And, they have concluded the gaseous exchange reaction is expressed by the quasi first-order reaction. Because tritium is much less than hydrogen and water vapor, assuming first-order reaction about tritium might be preferable. In such case, Eq. (1) is rewritten as; kF → HT. HTO − ← kR
(11)
The reaction rate is expressed using Eq. (6) as; Rate = kF CHTO − kR CHT = kF CHTO −
kF (C − CHTO ) K HTO,in
CHTO,in 1 ) = kF (1 + )(CHTO − K K +1
,
(12)
where, kF and kR are reaction rate constant [1/s]. After incorporating Eq. (12) to Eq. (8), according to the similar procedure, the following equation is derived as solution;
1 + K exp −kF (1 − ε) 1 + CHTO = 1+K CHTO,in
1 K
,
(13)
HTO concentration ratio of the outlet to the inlet is calculated by using Eqs. (10) and (13). The forward reaction rate constant, kF and the equilibrium constant, K were obtained in comparison with experimental observations and the calculations. Fig. 3 shows a comparison with experimental observation and the calculation. As shown in this figure, the equilibrium constant in the case of the first-order reaction is larger than 1.0, and oppositely that of the second-order reaction is smaller than 1.0. Although the same phenomenon is being expressed, this means that the directions of reaction that advance easily are opposite. Fig. 4 shows temperature dependence of kF on 0.5 wt% Pt on Na-MOR. Temperature range was between 30 ◦ C and 80 ◦ C, and data except 50 ◦ C were few. Hence, discussion about temperature dependence might be difficult. Eq. (10) and Eq. (13) are similar each other in spite of the different order of the reaction. Therefore, temperature dependence is also almost same each other. Actually, obtained forward reaction rate constants, kF were expressed as;
10600
kF = 1.63 × 103 exp −
RT
(1st),
(14)
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Fig. 3. An example of the parameter fitting. Fig. 5. Dependence of kF against hydrogen partial pressure on 0.5 wt% Pt Na-MOR.
Fig. 4. Temperature dependence of kF assuming both the 1st-order reaction and the 2nd-order reaction.
10200
kF = 3.87 × 103 exp −
RT
(2nd),
(15)
where, R is gas constant [J/mol/K], and T is absolute temperature [K]. The equilibrium constants, K [–], were also obtained as follows: K = 0.66 exp K − 0.14 exp
2800 RT
4500 RT
(1st),
(16)
(2nd) .
(17)
These activation energies were lower. Obtained equilibrium constants seem to be almost constant in the experimental temperature range. The reverse reaction rate constant, kR , is obtained from dividing Eq. (14) or (15) by Eq. (16) or (17). As mentioned before, the equilibrium constant for the first-order reaction is larger than 1.0, and that for the second-order reaction is smaller than 1.0. This means the models were not reasonable to express the reaction. To validate the reaction model, other properties were also investigated. Fig. 5 shows the hydrogen partial pressure dependence of kF . Experimental temperature and the partial pressure of water vapor were adjusted at 50 ◦ C and about 240 Pa. Usually reaction rate constant does not depend on the concentration (partial pressure) of reactant, if the model is reasonable. However, as shown in Fig. 5, kF in both cases depended on the concentration (partial pressure) of hydrogen. This means the descriptions of hydrogen concentration in both models are wrong. And, the slope on the 1st-order reaction seems to be comparatively close to 0.5. Presumption from limited data is risky, but this may suggest that dissociation of hydrogen occurs. In the case of the 2nd-order reaction, the slope of kF went
Fig. 6. Dependence of kF against hydrogen partial pressure on 0.5 wt% Pt Na-MOR.
down. It means assuming the first-order reaction concerning H2 is overestimation. Therefore, the order of reaction concerning H2 in the forward reaction is estimated to be between 0 and 1. Fig. 6 shows the water vapor pressure dependence of kF . It seems to have no dependence with the partial pressure of water vapor for both cases. Because tritium source (tritiated water) was not replaced, tritium contents were almost constant in a series of the experiments. Therefore, the HTO concentration in the gas is proportional to the partial pressure of water vapor. Fig. 6 suggests that kF does not depend on the HTO concentration, and it is assumed that the forward reaction is the 1st-order reaction concerning HTO. However, effect of H2 O concentration (partial pressure) is still not clear. To make clear the H2 O dependence, the experiment using tritiated water having different tritium contents is needed strictly. It might be reasonable that the half-order reaction and the 1storder reaction are assumed for H2 and HTO, respectively. However, the order of reaction could not make clear in this work. One of the causes is that the sample is hydrophilic. In such case, the amount of adsorbed water should be reflected into the reaction rate equation. It is assumed that investigation on water adsorption characteristic of these samples, especially, the non-steady state experiments are necessary. Only 0.5 wt% Pt on Na-MOR (hereafter Pt Na-MOR) has been discussed so far, because the data for other materials are much less than Pt Na-MOR. Hereafter, other materials are also compared. Fig. 7 shows the comparison of the HTO concentration ratio of the outlet to the inlet of the sample bed among various samples. As mentioned above, the order of reaction is not clear yet. However, assuming the 1st-order reaction, estimated forward reaction rate constants kF on Pt Li-MOR, Pt Na-MOR and
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4. Conclusion Catalysts supporting platinum with cation-exchanged mordenite were prepared, and isotope exchange reaction between hydrogen and tritiated water vapor was investigated. Assuming both the first-order reaction for tritium and the second-order reaction, the reaction rate constants and the equilibrium constant for 0.5 wt% Pt on Na-MOR were mainly estimated. The reaction rate is expressed as the first-order reaction for tritium. However, the reaction rate constants indicated dependency of hydrogen partial pressure. Therefore, assumption of other reaction model is necessary. Effect of the cation-exchanged carrier on the reaction rate constant seems to be caused by surface area of the carrier. Acknowledgments Fig. 7. Comparison of the HTO concentration ratio of the outlet to the inlet of the sample bed among various samples.
Pt K-MOR were 99.1, 81.2 and 49.3 s−1 , respectively. It is difficult to assume something from the comparison of estimated kF because of few data. However, the order of kF is similar to that of BET surface area for the support materials. The BET surface areas of their support materials have already been observed (LiMOR/Na-MOR/K-MOR: 352/317/283 m2 /g) [5]. This suggests the isotope exchange reaction is affected by the surface of the catalyst. In the previous work, the author has pointed out that chemisorbed water existed on Li-MOR surface [5]. Also, the effect of chemisorbed water on isotope exchange reaction is yet not clear in this work. In this work, the reaction based on the gas phase is assumed, because adsorption isotherms and isobars of water have not been clarified yet. These samples are based on synthetic mordenite and are hydrophilic. Therefore, it is assumed that the diffusion of adsorbed water into the pore of particle strongly affects the isotope exchange reaction actually. Other mass transfer process except surface reaction should be investigated hereafter. The discussion about effectiveness of addition of the new function for dryer column by applying of hydrophilic catalyst is difficult currently because of limited data.
In order to proceed to this work, the authors received funding support from Ministry of Education, Culture, Sports, Science and Technology in Japan (grant no. 26420859; Grant-in Aid for Scientific Research (c)). Authors really wish to acknowledge for this funding support:. References [1] Y. Kawamura, K. Ochiai, T. Hoshino, K. Kondo, Y. Iwai, et al., Effect of sweep gas species on tritium release behavior from lithium titanate packed bed during 14 MeV neutron irradiation, Fusion Eng. Des. 87 (2012) 1253–1257. [2] H. Tanigawa, T. Hoshino, Y. Kawamura, M. Nakamichi, K. Ochiai, et al., R&D of a Li2 TiO3 pebble bed for a test blanket module in JAEA, Nucl. Fusion 49 (2009) 055021, http://dx.doi.org/10.1088/0029-5515/49/5/055021 . [3] M. Enoeda, T. Higashijima, M. Nishikawa, N. Mitsuishi, Recovery of tritium in inert gas by precious metal catalyst supported by hydrophilic substrate, J. Nucl. Sci. Technol. 23 (1986) 1083–1093. [4] K. Munakata, M. Nishikawa, T. Takeishi, N. Mitsuishi, M. Enooeda, Recovery of tritium in room air by precious metal catalyst with hydrophilic substrate, J. Nucl. Sci. Technol. 25 (1988) 383–394. [5] Y. Kawamura, Y. Edao, Y. Iwai, T. Hayashi, T. Yamanishi, Hydrogen and water vapor adsorption properties on cation-exchanged mordenite for use to a tritium recovery system, Fusion Eng. Des. 89 (2014) 1539–1543. [6] Y. Kawamura, Y. Iwai, K. Munakata, T. Yamanishi, Effect of cation exchange on hydrogen adsorption property of mordenite for isotope separation, J. Nucl. Mater. 442 (2013) S455–S460. [7] H. Izawa, S. Isomura, R. Nakane, Gaseous exchange reaction of deuterium between hydrogen and water on hydrophobic catalyst supporting platinum, J. Nucl. Sci. Technol. 16 (1979) 741–749.
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