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Study on oxidation of hydrogen over commercial catalyst for tritium recovery system
Fusion Engineering and Design 87 (2012) 1118–1122
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Study on oxidation of hydrogen over commercial catalyst for tritium recovery system K. Hara a,∗ , K. Munakata a , J. Nagane a , M. Fukuda a , K. Wada a , T. Sugiyama b , M. Tanaka c , T. Uda c a b c
Faculty of Engineering and Resource Science, Akita University, Tegata-gakuen-machi1-1, Akita 010-8502, Japan Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japan National Institute for Fusion Science, Oroshi-cho 322-6, Toki-city, Gifu 509-5292, Japan
a r t i c l e
i n f o
Article history: Available online 11 August 2012 Keywords: Catalyst Tritium Oxidation Adsorption Water vapor
a b s t r a c t For the establishment of the D-T fusion reactor technology, recovery of tritium released into the working area of fusion power plants is quite important. When tritium leaks to working areas, the last barrier is the wall of the building. Due to higher diffusion coefficient of tritium, it diffuses through the wall and would be readily liberated to the environment. Thus, the tritium recovery system is indispensable for the D-T fusion reactor. The objective of the present study is to develop the advanced technology of the tritium recovery system. In the near future, deuterium plasma discharge experiments scheduled be conducted with Large Helical Device (LHD) in National Institute for Fusion Science. A small amount of tritium is produced by D-D reaction in LHD. Tritium in plasma exhaust gases and process gas during discharge needs to be recovered, and thus the design and construction of the tritium recovery system used for that purpose is a matter of considerable urgency. The tritium recovery system usually consists of catalysts and adsorbents, which is the most conventional and reliable method for removing tritium that is accidentally released into the working area of these facilities. However, more recent and advanced type of catalysts on the market cannot be directly applied to the design of tritium recovery system, because of paucity of design data for tritium recovery system. In this study, the authors performed oxidation experiments of hydrogen over a catalyst. The experiments were performed by changing various experimental parameters. © 2012 Elsevier B.V. All rights reserved.
1. Introduction For the establishment of the D-T fusion reactor technology, recovery of tritium released is quite important. Thus, the tritium recovery system is indispensable to the D-T fusion reactor or experimental facilities such as LHD. LHD is known for its peculiar magnetic field confinement system (the world’s largest called ‘heliotron’), which has been uniquely developed in Japan. LHD is suitable for stable routine operation. Moreover, LHD is used for the elucidation of the research theme of physics, the engineering database for a nuclear fusion reactor. With LHD, NIFS places greater importance on the research in the generation and confinement of high-temperature high-density plasmas and confinement of LHD experiments as well as the extensive theoretical and simulation science research by using the supercomputer. In the near future, D-D plasma are scheduled in NIFS and thus tritium produced in the plasma needs to be recovered. For this reason, the authors carried
∗ Corresponding author. Tel.: +81 288892749. E-mail address: [email protected] (K. Hara). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.102
out extensive search in terms of feasible catalysts and adsorbents used for the design of the tritium recovery system. However, it was found that catalysts or adsorbents studied in the past have not been produced and are not available on the market at present. Thus, the authors selected a catalyst and an adsorbent that are used in general industries and are produced by credible and well-known manufacturers. Then, the authors have carried out intensive studies on the catalyst and adsorbent to obtain engineering database for supporting the design of detritiation systems. 2. Experimental 2.1. Catalysts The authors selected a Pt/alumina catalyst, DASH520, manufactured by N. E. Chemcat Co., which is widely used in industries for various purposes. The catalyst was deposited with 4.1 g/L of platinum. The average diameter of the catalyst is 3.25 mm. The Packing density was 770 g/L. Other physical characteristics are summarized in Table 1. The photographic view of the DASH520 catalyst pebbles is presented in Fig. 1. The catalyst heated up to 623 K under pure
K. Hara et al. / Fusion Engineering and Design 87 (2012) 1118–1122 Table 1 Physical property of adsorbent.
Nomenclature Cin ,Cout b b0 E K KF n P P0 p Q q qs R RC SV T u V ε
concentrations of hydrogen at inlet and outlet of the reactor [mol/m3 ] Henry’s law constant [mol/(g Pa)] temperature-independent constant for Henry’s law constant [mol/g Pa] energy-related term in Henry’s law constant and constant in Freundlich equation [J/(mol K)] constant in Freundlich equation [arbitrary, depending on the value of u] overall mass transfer capacitance [s−1 ] maximum number of adsorption site in Langmuir or Langmuir–Fruedlich models pressure of water vapor [Pa] saturation vapor pressure of water vapor [Pa] partial pressure of adsorbate [Pa] volumetric velocity of the gas introduced to the reactor [m3 /s] number of moles in the surface phase [mol/g] maximum number of moles of i in the surface phase [mol/g] gas constant (=8.31) [J/mol K] conversion rate [%] space velocity [s−1 ] temperature [K] constant in Langnuir–Freundlich equation [–] apparent volume of the catalyst fractional coverage [–] adsorption potential [J/mol]
Subscript number of adsorption site i Langmuir–Freundlich models
in
Langmuir
or
Adsorption amount of N2 at 77 K BET surface area Pore volume Average pore diameter
hydrogen for reduction before use. After the reduction, the catalyst was heated up to 573 K under 20% O2 gas for the drying and activation. 2.2. Oxidation experiment Fig. 2 shows experiment apparatus used in this study. The experiments were performed under the steady state condition. The catalysts were packed in a reactor made of quartz. The temperature of the reactor was controlled with the constant temperature bath. Argon gas was used as a carrier gas. The Argon gas containing reactant gasses such as hydrogen and oxygen was pretreated using a molecular sieve 3A (MS3A) adsorption bed to remove residual water vapor contained in gas cylinders delivered from gas companies. The flow rate of the reactant gas was controlled with mass flow controllers (MODEL 2023) manufactured by KOFLOC Co. The argon gas containing hydrogen (about 300–800 ppm) and oxygen (20%) was introduced to the reactor. Experiments were also performed using wet process gases which contained water vapor (about 300–1000 Pa) in order to study the influence of coexistent water vapor on the catalytic activity for oxidation of hydrogen. This is because that it is known that the catalytic activity for oxidation of hydrogen over a Pt/alumina catalyst substantially decreases when water vapor coexists in process gases [1]. The experiments were performed under the condition of SV (space velocity) ≈ 10 000 [h−1 ] (=2.78 [s−1 ]). SV is defined by the following equation: Q V
(1)
39.4 cm3 (STP)/g 171 m2 /g 0.434 cm3 /g 10.1 nm
The concentrations of hydrogen at inlet and outlet stream of the reactor were measured with a gas-chromatograph (GC-8A) manufactured by SHIMAZU Co. 2.3. Experiment of water vapor adsorption The authors studied adsorption behavior of water vapor on the DASH520 catalyst, as well. The adsorption isotherms of water vapor on the DASH520 catalyst were measured using a volumetric gas adsorption instrument, BELSORP-max, manufactured by BEL Japan Inc. The instrument is designed for measurement of wide range of adsorption isotherms on surface area and pore size distribution analysis. The DASH520 (0.07 g) catalyst was preheated at 400 ◦ C in vacuum for 5 h. The adsorption isotherms are taken at the temperatures of 10, 30 and 50 ◦ C in the water vapor pressure range of 2 × 10−2 to 104 Pa. 3. Results and discussions 3.1. Oxidation performance Conversion rate RC and overall mass transfer capacitance KF were computed to assess catalytic performance. RC and KF are defined as RC = 100 × KF = SV ln
SV =
1119
Cin − Cout Cin
C in Cout
(2) (3)
The KF is the value which summarized the mass transfer on the surface of a catalyst. Fig. 3 shows results of the oxidation performance without coexistence of water vapor (so-called dry condition). The data of conversion of hydrogen over the DASH220 catalysts plotted in Fig. 3 were previously reported in our literature [2]. The DASH 220 catalyst was also produced by N.E. Chemcat Co. that manufactures the DASH520 catalyst as well. Fig. 3 indicates that the oxidation performance of the DASH520 catalyst is higher than that of the DASH220 catalyst. One of the major differences between DASH520 and DASH220 catalysts is the amounts of platinum deposited in the alumina substrate. The DASH220 catalyst was deposited with 1.8 g/L of platinum, while the Pt content of the DASH520 catalyst is 4.1 g/L. Hence, the difference in the amounts
Fig. 1. The photographs of DASH520 catalyst.
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Fig. 2. Experimental apparatus.
of platinum would be one of the main reasons of the difference in catalytic activities of DASH520 and DASH220 catalysts. The mass transfer capacitances for oxidation of hydrogen of DASH520 and DASH220 catalysts are expressed as [DASH 220] KF = 1.81 × 104 exp [DASH 520] KF = 1.16 × 105 exp
−22300 RT
−25400 RT
(4) (5)
The values of the standard deviation of Eqs. (4) and (5) are 0.029 and 0.052, respectively. As shown in the above equations and in Fig. 3, the activation energy of the mass transfer capacitance of the DASH520 catalyst is slightly greater than that of the DASH220 catalyst. 3.2. Effect of water vapor Fig. 4 shows conversion rate of hydrogen over the DASH520 catalyst under the coexistence of water vapor in the process gas. The figure suggests that the conversion rate of hydrogen over the DASH520 catalyst was greatly decreased with increasing concentrations of water vapor. Addition of water vapor as low as 303 Pa
Fig. 3. Oxidation performance in dry condition.
greatly decreased the conversion rate. If there is no coexistent water vapor, hydrogen can be completely oxidized at the temperature of 310 K. In the case where 1013 Pa of water vapor was added to the process gas, less than 20% of hydrogen was oxidized over the DASH520 catalyst. Therefore, it could be quite important to consider the effect in designing detritiation systems. Next, the authors examined the relation between oxidization performance and adsorption potential. The adsorption potential ε is defined as follows: ε = RT ln
P 0
P
(6)
The relation between the values of mass transfer capacitance and those of adsorption potential is shown in Fig. 5. The vertical axis in the figure represents the values of KF /KF0 , where KF and KF0 are overall transfer capacitance in wet conditions and that in the dry condition, respectively. As seen in Fig. 5, a strong correlation between the values of KF /KF0 and adsorption potential is not observed. The adsorption potential theory is widely used for correlation of adsorption isotherms [3,4]. Nevertheless, a clear correlation between catalytic activity and adsorption amount (postulated from the adsorption potential) was not found. Thus, the authors investigated the adsorption isotherm of water vapor on the catalyst, in order to further investigate relations between catalytic activity and amount of adsorbed water.
Fig. 4. Conversion rate of hydrogen that added water vapor.
K. Hara et al. / Fusion Engineering and Design 87 (2012) 1118–1122
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Table 2 Parameters in the Langmuir and 2-site Langmuir models determined by the leastsquares analysis.
qs,i [mol/g] b0,i [mol/(g·Pa)] Ei [J/(mol·K)] qs,i [mol/g] b0,i [mol/(g·Pa)] Ei [J/(mol·K)] Standard deviation
i=1
i=2
Langmuir
2-site L.
7.57 × 10−3 8.58 × 10−6 17 000 –
1.25 × 10−3 3.30 × 10−2 2410 8.19 × 102 5.90 × 10−17 44 700 0.41
0.67
Fig. 5. Relation between the value of KF /KF0 and the values of adsorption potential.
3.3. Adsorption isotherm of water vapor Fig. 6(a) shows adsorption isotherms of water vapor on the DASH520 catalyst. The adsorption isotherms can be categorized in so-called S-curve types. A point expresses an experimental value and a line expresses an approximated curve. The author performed fitting model equations of adsorption equilibrium to the experimental isotherms, in order to investigate the effect of an adsorption behavior on the catalytic activity. The parameters in the equation were optimized by a least squares analysis. The
Fig. 6. Adsorption isotherm of water vapor and result of correlation with the (a) Langmuir model and (b) 2-site Langmuir model.
Fig. 7. Adsorption isotherm of water vapor and result of correlation with the (a) Langmuir–Freundlich model (1-site) and (b) Langmuir–Freundlich model (2-site) and (c) Langmuir–Freundlich model (3-site).
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Table 3 Parameters in Langmuir–Freudlich models determined by the least-squares analysis.
qs,i [mol/g] K0,i [(mol/(g Pa))1/ui ] Ei [J/(mol K)] ui [–] qs,i [mol/g] K0,i [(mol/(g Pa))1/ui ] Ei [J/(mol K)] ui [–] qs,i [mol/g] K0,i [(mol/(g Pa))1/ui ] Ei [J/(mol K)] ui [–] Standard deviation
i=1
i=2
i=3
L–F
2-site L–F
3-site L–F
1.85 × 104 7.16 × 10−13 22 300 0.56 – – – – – – – – 0.57
4.64 × 10−1 2.25 × 10−19 66 800 1.54 1.29 × 10−3 4.26 × 10−8 36 800 2.48 – – – – 0.28
2.00 1.32 × 10−21 72 900 1.69 1.40 × 10−3 2.97 × 10−8 34 600 1.84 1.40 × 10−4 3.82 × 10−3 39 200 19.9 0.24
Levenberg–Marquardt algorithm was embedded into a FORTRAN code, which was used for optimization of parameters in model equations used in this work. Fig. 6 also shows the result of correlation with the Langmuir model. The Langmuir model is defined as following equation: q=
qs bp 1 + bp
b = b0 exp
(7)
E RT
(8)
The results indicate that Langmuir model cannot reproduce the experimental isotherms. The values of the parameters optimized in the Langmuir model were presented in Table 2. Therefore, thus authors tested 2-site Langmuir model [5] for correlation of the experimental isotherm. The 2-site Langmuir model is expressed as q=
qs,1 b1 p qs,2 b2 p + 1 + b1 p 1 + b2 p
bi = b0,i exp
E i
RT
(9) (10)
Fig. 6(b) shows the result of correlation with the 2-site Langmuir model. The values of the parameters optimized in the model were presented in Table 2. It appears that the model can correlate the experimental isotherms in the higher partial pressure range. However, correlation of the experimental isotherms in the lower partial pressure range is not satisfactory. Fig. 6(a) and (b) indicates that the Langmuir model and 2-site Langmuir model cannot well correlate the experimental isotherms. Thus, authors tested the Langmuir–Freudlich equation [6]. The equation is defined as the following equation: q=
qs Kpu 1 + Kpu
K = K0 exp
(11)
E RT
(13)
1 + Ki pui
Ki = K0,i exp
The performance for catalytic oxidation of hydrogen over the DASH520 catalyst was experimentally investigated. The amount of the platinum deposited in catalyst appears to influence its performance. It was found that the oxidation performance of the catalyst decreases with increasing water vapor content and its influence varies depending on temperature. The adsorption potential theory was applied to correlate relationship between catalytic activity and adsorption amount, however it was not successful. Thus, adsorption isotherms of water on the catalyst were investigated using a volumetric gas adsorption instrument. The experimental isotherms were satisfactory correlated using the multi-site Langmuir–Freundlich model. Further experimental studies are necessary to understand the detailed affect of water vapor on catalyst oxidation rate for the accurate design of the tritium recovery system of fusion reactors or fusion machines such as LHD. References
n qs,i Ki pui i=1
4. Conclusion
(12)
Fig. 7(a) shows the result of correlation with the Langmuir–Freundlich model, which suggests that the model can better correlate the experimental isotherms compared with the cases of the Langmuir and 2-site Langmuir models. The values of the parameters optimized in the model were presented in Table 3. Thus, authors further tested multi-site Langmuir–Freundlich model [6], which is expressed as q=
Fig. 7(b) shows the result of correlation with the multi-site Langmuir–Freundlich model when the two sites were taken into consideration (n = 2). It is suggested in the figure that correlation of the experimental isotherms was considerably improved. Then, the authors tested the model in the case of n = 3. Fig. 7(c) shows the result of correlation with the multi-site Langmuir–Freundlich model (n = 3). When three adsorption sites were taken into account, the adsorption isotherms were more properly reproduced using the model. The values of the parameters optimized in the model were presented in Table 3. These results could indicate that the adsorption of water vapor on the alumina substrate of the DASH520 catalyst is considerably heterogeneous. We are now in the process of testing other adsorption models and of investigating the relation between catalytic activity of the DASH520 catalyst and the amount of water vapor adsorbed on the substrate of the catalyst, alumina.
E i
RT
(14)
[1] K. Munakata, M. Nishikawa, T. Takeishi, N. Mitsuishi, M. Enoeda, Recovery of tritium in room air by precious metal catalysts with hydrophilic substrate, Journal of Nuclear Science and Technology 25 (1988) 383. [2] K. Munakata, T. Wajima, K. Hara, K. Wada, Y. Shinozaki, K. Katekari, K. Mochizuki, M. Tanaka, T. Uda, Oxidation of hydrogen isotopes over honeycomb catalysts, Fusion Engineering and Design, in press. [3] M. Polanyi, Theories of the adsorption of gases. A general survey and some additional remarks, Introductory paper to section III, Transactions of the Faraday Society 28 (1932) 316. [4] M.M. Dubinin, Adsorption and Desorption Phenomena, Academic Press, NY, 1972. [5] E.C. Markham, A.F. Benton, Adsorption of gas mixtures by silica, Journal of the American Chemical Society 53 (1931) 497. [6] (a) H.C. Thomas, Journal of the American Chemical Society 66 (1944) 1664; (b) H.C. Thomas, Annals of the New York Academy of Sciences 49 (1948) 161.
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Study on oxidation of hydrogen over commercial catalyst for tritium recovery system K. Hara a,∗ , K. Munakata a , J. Nagane a , M. Fukuda a , K. Wada a , T. Sugiyama b , M. Tanaka c , T. Uda c a b c
Faculty of Engineering and Resource Science, Akita University, Tegata-gakuen-machi1-1, Akita 010-8502, Japan Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japan National Institute for Fusion Science, Oroshi-cho 322-6, Toki-city, Gifu 509-5292, Japan
a r t i c l e
i n f o
Article history: Available online 11 August 2012 Keywords: Catalyst Tritium Oxidation Adsorption Water vapor
a b s t r a c t For the establishment of the D-T fusion reactor technology, recovery of tritium released into the working area of fusion power plants is quite important. When tritium leaks to working areas, the last barrier is the wall of the building. Due to higher diffusion coefficient of tritium, it diffuses through the wall and would be readily liberated to the environment. Thus, the tritium recovery system is indispensable for the D-T fusion reactor. The objective of the present study is to develop the advanced technology of the tritium recovery system. In the near future, deuterium plasma discharge experiments scheduled be conducted with Large Helical Device (LHD) in National Institute for Fusion Science. A small amount of tritium is produced by D-D reaction in LHD. Tritium in plasma exhaust gases and process gas during discharge needs to be recovered, and thus the design and construction of the tritium recovery system used for that purpose is a matter of considerable urgency. The tritium recovery system usually consists of catalysts and adsorbents, which is the most conventional and reliable method for removing tritium that is accidentally released into the working area of these facilities. However, more recent and advanced type of catalysts on the market cannot be directly applied to the design of tritium recovery system, because of paucity of design data for tritium recovery system. In this study, the authors performed oxidation experiments of hydrogen over a catalyst. The experiments were performed by changing various experimental parameters. © 2012 Elsevier B.V. All rights reserved.
1. Introduction For the establishment of the D-T fusion reactor technology, recovery of tritium released is quite important. Thus, the tritium recovery system is indispensable to the D-T fusion reactor or experimental facilities such as LHD. LHD is known for its peculiar magnetic field confinement system (the world’s largest called ‘heliotron’), which has been uniquely developed in Japan. LHD is suitable for stable routine operation. Moreover, LHD is used for the elucidation of the research theme of physics, the engineering database for a nuclear fusion reactor. With LHD, NIFS places greater importance on the research in the generation and confinement of high-temperature high-density plasmas and confinement of LHD experiments as well as the extensive theoretical and simulation science research by using the supercomputer. In the near future, D-D plasma are scheduled in NIFS and thus tritium produced in the plasma needs to be recovered. For this reason, the authors carried
∗ Corresponding author. Tel.: +81 288892749. E-mail address: [email protected] (K. Hara). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.102
out extensive search in terms of feasible catalysts and adsorbents used for the design of the tritium recovery system. However, it was found that catalysts or adsorbents studied in the past have not been produced and are not available on the market at present. Thus, the authors selected a catalyst and an adsorbent that are used in general industries and are produced by credible and well-known manufacturers. Then, the authors have carried out intensive studies on the catalyst and adsorbent to obtain engineering database for supporting the design of detritiation systems. 2. Experimental 2.1. Catalysts The authors selected a Pt/alumina catalyst, DASH520, manufactured by N. E. Chemcat Co., which is widely used in industries for various purposes. The catalyst was deposited with 4.1 g/L of platinum. The average diameter of the catalyst is 3.25 mm. The Packing density was 770 g/L. Other physical characteristics are summarized in Table 1. The photographic view of the DASH520 catalyst pebbles is presented in Fig. 1. The catalyst heated up to 623 K under pure
K. Hara et al. / Fusion Engineering and Design 87 (2012) 1118–1122 Table 1 Physical property of adsorbent.
Nomenclature Cin ,Cout b b0 E K KF n P P0 p Q q qs R RC SV T u V ε
concentrations of hydrogen at inlet and outlet of the reactor [mol/m3 ] Henry’s law constant [mol/(g Pa)] temperature-independent constant for Henry’s law constant [mol/g Pa] energy-related term in Henry’s law constant and constant in Freundlich equation [J/(mol K)] constant in Freundlich equation [arbitrary, depending on the value of u] overall mass transfer capacitance [s−1 ] maximum number of adsorption site in Langmuir or Langmuir–Fruedlich models pressure of water vapor [Pa] saturation vapor pressure of water vapor [Pa] partial pressure of adsorbate [Pa] volumetric velocity of the gas introduced to the reactor [m3 /s] number of moles in the surface phase [mol/g] maximum number of moles of i in the surface phase [mol/g] gas constant (=8.31) [J/mol K] conversion rate [%] space velocity [s−1 ] temperature [K] constant in Langnuir–Freundlich equation [–] apparent volume of the catalyst fractional coverage [–] adsorption potential [J/mol]
Subscript number of adsorption site i Langmuir–Freundlich models
in
Langmuir
or
Adsorption amount of N2 at 77 K BET surface area Pore volume Average pore diameter
hydrogen for reduction before use. After the reduction, the catalyst was heated up to 573 K under 20% O2 gas for the drying and activation. 2.2. Oxidation experiment Fig. 2 shows experiment apparatus used in this study. The experiments were performed under the steady state condition. The catalysts were packed in a reactor made of quartz. The temperature of the reactor was controlled with the constant temperature bath. Argon gas was used as a carrier gas. The Argon gas containing reactant gasses such as hydrogen and oxygen was pretreated using a molecular sieve 3A (MS3A) adsorption bed to remove residual water vapor contained in gas cylinders delivered from gas companies. The flow rate of the reactant gas was controlled with mass flow controllers (MODEL 2023) manufactured by KOFLOC Co. The argon gas containing hydrogen (about 300–800 ppm) and oxygen (20%) was introduced to the reactor. Experiments were also performed using wet process gases which contained water vapor (about 300–1000 Pa) in order to study the influence of coexistent water vapor on the catalytic activity for oxidation of hydrogen. This is because that it is known that the catalytic activity for oxidation of hydrogen over a Pt/alumina catalyst substantially decreases when water vapor coexists in process gases [1]. The experiments were performed under the condition of SV (space velocity) ≈ 10 000 [h−1 ] (=2.78 [s−1 ]). SV is defined by the following equation: Q V
(1)
39.4 cm3 (STP)/g 171 m2 /g 0.434 cm3 /g 10.1 nm
The concentrations of hydrogen at inlet and outlet stream of the reactor were measured with a gas-chromatograph (GC-8A) manufactured by SHIMAZU Co. 2.3. Experiment of water vapor adsorption The authors studied adsorption behavior of water vapor on the DASH520 catalyst, as well. The adsorption isotherms of water vapor on the DASH520 catalyst were measured using a volumetric gas adsorption instrument, BELSORP-max, manufactured by BEL Japan Inc. The instrument is designed for measurement of wide range of adsorption isotherms on surface area and pore size distribution analysis. The DASH520 (0.07 g) catalyst was preheated at 400 ◦ C in vacuum for 5 h. The adsorption isotherms are taken at the temperatures of 10, 30 and 50 ◦ C in the water vapor pressure range of 2 × 10−2 to 104 Pa. 3. Results and discussions 3.1. Oxidation performance Conversion rate RC and overall mass transfer capacitance KF were computed to assess catalytic performance. RC and KF are defined as RC = 100 × KF = SV ln
SV =
1119
Cin − Cout Cin
C in Cout
(2) (3)
The KF is the value which summarized the mass transfer on the surface of a catalyst. Fig. 3 shows results of the oxidation performance without coexistence of water vapor (so-called dry condition). The data of conversion of hydrogen over the DASH220 catalysts plotted in Fig. 3 were previously reported in our literature [2]. The DASH 220 catalyst was also produced by N.E. Chemcat Co. that manufactures the DASH520 catalyst as well. Fig. 3 indicates that the oxidation performance of the DASH520 catalyst is higher than that of the DASH220 catalyst. One of the major differences between DASH520 and DASH220 catalysts is the amounts of platinum deposited in the alumina substrate. The DASH220 catalyst was deposited with 1.8 g/L of platinum, while the Pt content of the DASH520 catalyst is 4.1 g/L. Hence, the difference in the amounts
Fig. 1. The photographs of DASH520 catalyst.
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Fig. 2. Experimental apparatus.
of platinum would be one of the main reasons of the difference in catalytic activities of DASH520 and DASH220 catalysts. The mass transfer capacitances for oxidation of hydrogen of DASH520 and DASH220 catalysts are expressed as [DASH 220] KF = 1.81 × 104 exp [DASH 520] KF = 1.16 × 105 exp
−22300 RT
−25400 RT
(4) (5)
The values of the standard deviation of Eqs. (4) and (5) are 0.029 and 0.052, respectively. As shown in the above equations and in Fig. 3, the activation energy of the mass transfer capacitance of the DASH520 catalyst is slightly greater than that of the DASH220 catalyst. 3.2. Effect of water vapor Fig. 4 shows conversion rate of hydrogen over the DASH520 catalyst under the coexistence of water vapor in the process gas. The figure suggests that the conversion rate of hydrogen over the DASH520 catalyst was greatly decreased with increasing concentrations of water vapor. Addition of water vapor as low as 303 Pa
Fig. 3. Oxidation performance in dry condition.
greatly decreased the conversion rate. If there is no coexistent water vapor, hydrogen can be completely oxidized at the temperature of 310 K. In the case where 1013 Pa of water vapor was added to the process gas, less than 20% of hydrogen was oxidized over the DASH520 catalyst. Therefore, it could be quite important to consider the effect in designing detritiation systems. Next, the authors examined the relation between oxidization performance and adsorption potential. The adsorption potential ε is defined as follows: ε = RT ln
P 0
P
(6)
The relation between the values of mass transfer capacitance and those of adsorption potential is shown in Fig. 5. The vertical axis in the figure represents the values of KF /KF0 , where KF and KF0 are overall transfer capacitance in wet conditions and that in the dry condition, respectively. As seen in Fig. 5, a strong correlation between the values of KF /KF0 and adsorption potential is not observed. The adsorption potential theory is widely used for correlation of adsorption isotherms [3,4]. Nevertheless, a clear correlation between catalytic activity and adsorption amount (postulated from the adsorption potential) was not found. Thus, the authors investigated the adsorption isotherm of water vapor on the catalyst, in order to further investigate relations between catalytic activity and amount of adsorbed water.
Fig. 4. Conversion rate of hydrogen that added water vapor.
K. Hara et al. / Fusion Engineering and Design 87 (2012) 1118–1122
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Table 2 Parameters in the Langmuir and 2-site Langmuir models determined by the leastsquares analysis.
qs,i [mol/g] b0,i [mol/(g·Pa)] Ei [J/(mol·K)] qs,i [mol/g] b0,i [mol/(g·Pa)] Ei [J/(mol·K)] Standard deviation
i=1
i=2
Langmuir
2-site L.
7.57 × 10−3 8.58 × 10−6 17 000 –
1.25 × 10−3 3.30 × 10−2 2410 8.19 × 102 5.90 × 10−17 44 700 0.41
0.67
Fig. 5. Relation between the value of KF /KF0 and the values of adsorption potential.
3.3. Adsorption isotherm of water vapor Fig. 6(a) shows adsorption isotherms of water vapor on the DASH520 catalyst. The adsorption isotherms can be categorized in so-called S-curve types. A point expresses an experimental value and a line expresses an approximated curve. The author performed fitting model equations of adsorption equilibrium to the experimental isotherms, in order to investigate the effect of an adsorption behavior on the catalytic activity. The parameters in the equation were optimized by a least squares analysis. The
Fig. 6. Adsorption isotherm of water vapor and result of correlation with the (a) Langmuir model and (b) 2-site Langmuir model.
Fig. 7. Adsorption isotherm of water vapor and result of correlation with the (a) Langmuir–Freundlich model (1-site) and (b) Langmuir–Freundlich model (2-site) and (c) Langmuir–Freundlich model (3-site).
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K. Hara et al. / Fusion Engineering and Design 87 (2012) 1118–1122
Table 3 Parameters in Langmuir–Freudlich models determined by the least-squares analysis.
qs,i [mol/g] K0,i [(mol/(g Pa))1/ui ] Ei [J/(mol K)] ui [–] qs,i [mol/g] K0,i [(mol/(g Pa))1/ui ] Ei [J/(mol K)] ui [–] qs,i [mol/g] K0,i [(mol/(g Pa))1/ui ] Ei [J/(mol K)] ui [–] Standard deviation
i=1
i=2
i=3
L–F
2-site L–F
3-site L–F
1.85 × 104 7.16 × 10−13 22 300 0.56 – – – – – – – – 0.57
4.64 × 10−1 2.25 × 10−19 66 800 1.54 1.29 × 10−3 4.26 × 10−8 36 800 2.48 – – – – 0.28
2.00 1.32 × 10−21 72 900 1.69 1.40 × 10−3 2.97 × 10−8 34 600 1.84 1.40 × 10−4 3.82 × 10−3 39 200 19.9 0.24
Levenberg–Marquardt algorithm was embedded into a FORTRAN code, which was used for optimization of parameters in model equations used in this work. Fig. 6 also shows the result of correlation with the Langmuir model. The Langmuir model is defined as following equation: q=
qs bp 1 + bp
b = b0 exp
(7)
E RT
(8)
The results indicate that Langmuir model cannot reproduce the experimental isotherms. The values of the parameters optimized in the Langmuir model were presented in Table 2. Therefore, thus authors tested 2-site Langmuir model [5] for correlation of the experimental isotherm. The 2-site Langmuir model is expressed as q=
qs,1 b1 p qs,2 b2 p + 1 + b1 p 1 + b2 p
bi = b0,i exp
E i
RT
(9) (10)
Fig. 6(b) shows the result of correlation with the 2-site Langmuir model. The values of the parameters optimized in the model were presented in Table 2. It appears that the model can correlate the experimental isotherms in the higher partial pressure range. However, correlation of the experimental isotherms in the lower partial pressure range is not satisfactory. Fig. 6(a) and (b) indicates that the Langmuir model and 2-site Langmuir model cannot well correlate the experimental isotherms. Thus, authors tested the Langmuir–Freudlich equation [6]. The equation is defined as the following equation: q=
qs Kpu 1 + Kpu
K = K0 exp
(11)
E RT
(13)
1 + Ki pui
Ki = K0,i exp
The performance for catalytic oxidation of hydrogen over the DASH520 catalyst was experimentally investigated. The amount of the platinum deposited in catalyst appears to influence its performance. It was found that the oxidation performance of the catalyst decreases with increasing water vapor content and its influence varies depending on temperature. The adsorption potential theory was applied to correlate relationship between catalytic activity and adsorption amount, however it was not successful. Thus, adsorption isotherms of water on the catalyst were investigated using a volumetric gas adsorption instrument. The experimental isotherms were satisfactory correlated using the multi-site Langmuir–Freundlich model. Further experimental studies are necessary to understand the detailed affect of water vapor on catalyst oxidation rate for the accurate design of the tritium recovery system of fusion reactors or fusion machines such as LHD. References
n qs,i Ki pui i=1
4. Conclusion
(12)
Fig. 7(a) shows the result of correlation with the Langmuir–Freundlich model, which suggests that the model can better correlate the experimental isotherms compared with the cases of the Langmuir and 2-site Langmuir models. The values of the parameters optimized in the model were presented in Table 3. Thus, authors further tested multi-site Langmuir–Freundlich model [6], which is expressed as q=
Fig. 7(b) shows the result of correlation with the multi-site Langmuir–Freundlich model when the two sites were taken into consideration (n = 2). It is suggested in the figure that correlation of the experimental isotherms was considerably improved. Then, the authors tested the model in the case of n = 3. Fig. 7(c) shows the result of correlation with the multi-site Langmuir–Freundlich model (n = 3). When three adsorption sites were taken into account, the adsorption isotherms were more properly reproduced using the model. The values of the parameters optimized in the model were presented in Table 3. These results could indicate that the adsorption of water vapor on the alumina substrate of the DASH520 catalyst is considerably heterogeneous. We are now in the process of testing other adsorption models and of investigating the relation between catalytic activity of the DASH520 catalyst and the amount of water vapor adsorbed on the substrate of the catalyst, alumina.
E i
RT
(14)
[1] K. Munakata, M. Nishikawa, T. Takeishi, N. Mitsuishi, M. Enoeda, Recovery of tritium in room air by precious metal catalysts with hydrophilic substrate, Journal of Nuclear Science and Technology 25 (1988) 383. [2] K. Munakata, T. Wajima, K. Hara, K. Wada, Y. Shinozaki, K. Katekari, K. Mochizuki, M. Tanaka, T. Uda, Oxidation of hydrogen isotopes over honeycomb catalysts, Fusion Engineering and Design, in press. [3] M. Polanyi, Theories of the adsorption of gases. A general survey and some additional remarks, Introductory paper to section III, Transactions of the Faraday Society 28 (1932) 316. [4] M.M. Dubinin, Adsorption and Desorption Phenomena, Academic Press, NY, 1972. [5] E.C. Markham, A.F. Benton, Adsorption of gas mixtures by silica, Journal of the American Chemical Society 53 (1931) 497. [6] (a) H.C. Thomas, Journal of the American Chemical Society 66 (1944) 1664; (b) H.C. Thomas, Annals of the New York Academy of Sciences 49 (1948) 161.