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Hydrogen isotopes separation using frontal displacement chromatography with Pd–Al2O3 packed column
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen isotopes separation using frontal displacement chromatography with PdeAl2O3 packed column Xiaojun Deng*, Deli Luo, Cheng Qin, Xiaojin Qian, Wan Yang China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan, China
article info
abstract
Article history:
Separation or purification of tritium isotopes is one of the key technologies in ITER. A set of
Received 9 October 2011
frontal displacement chromatography (FDC) device was designed and constructed for
Received in revised form
hydrogen isotopes separation using palladium loading on/in alumina (PdeAl2O3) as the
29 March 2012
separation material. The hydrogen isotopes separation experiments were carried out. It
Accepted 8 April 2012
was found that deuterium abundance of the product was up to 98.5% and the average
Available online 5 May 2012
separation factor was as high as 64, under the condition of 273 K column temperature and 15 mL(NTP)/min flow rate, for a feed gas of 5%H2-5%D2-90%Ar. The deuterium recovery
Keywords:
ratio was 42% in this separation test. The results showed that the separation performance
Frontal displacement
of our FDC device was good by using PdeAl2O3 as separation materials, and it suggested
chromatography
considerable potential for the applicability of FDC in hydrogen isotopes separation.
Hydrogen isotopes
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Separation
reserved.
PdeAl2O3
1.
Introduction
Efficient recovery of tritium from exhaust gases of D-T fusion devices such as ITER is one of key issues for economical and safe recovery of tritium. Several kinds of methods for hydrogen isotopes separation have been proposed [1], in which displacement chromatography (DC) is promising and has received much attention [2e9]. DC has been developed into several techniques, for example, thermal diffusion (TD), thermal cycling absorption process (TCAP), hydrogen displacement chromatography (HDC) [2,3], etc. Separation performance of TD was not very satisfactory due to the production of large amount of medium abundance gas [1]. High tritium purity products were obtained by using the HDC facility of Joint European Tokomak [3] in 1980s, but a large amount of exhaust gas containing tritium was produced simultaneously. TCAP was well developed, and it had been on
stream since 1994 in Replacement Tritium Facility [10] of Savannah River Site in America. Stable distribution of isotopes in the TCAP column was just realized after series of complex manipulation. Improvements on engineering device and technical process optimization of TCAP are still in progress [10e14]. Materials containing palladium are always chosen as the separation material for hydrogen isotopes separation in DC because of the high hydrogen isotopes effect of Pd [15e22]. Pd is inclined to adsorb and/or absorb the lighter isotope first, since the adsorption/absorption rate for the lighter isotope is larger than that of the heavier one. As a result, the heavier isotope can be displaced by the lighter. On the other hand, frontal chromatography (FC) is a classic chromatography process with features of continuous feed gas input and the feed gas itself being the elution gas or carrier gas. On the basis of the advantages of DC and FC, a new method has been
* Corresponding author. Tel.: þ86 816 3626457; fax: þ86 816 3625900. E-mail address: [email protected] (X. Deng). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
proposed, which combines and applies the FC process and hydrogen isotopes effect of Pd in DC. This new method is frontal displacement chromatography. Feed gas in FDC, at the same time used as the displacement gas, continuously flows through the chromatography column packed with materials containing Pd, and the lighter isotope component later fed in then displaces the heavier isotope component earlier adsorbed/absorbed by Pd. Consequently, the heavier isotope component is enriched at end of the column. In this work, a set of FDC device using PdeAl2O3 as separation material for hydrogen isotopes separation was designed and constructed. Separation tests were carried out to evaluate the hydrogen isotopes separation properties of this device. The feasibility of hydrogen isotopes separation by FDC was then discussed. The properties of PdeAl2O3 were analyzed, too.
2.
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For removing the adsorbed water and other impurities from the surfaces of PdeAl2O3 pellets, the separation material had to be activated before the separation experiment by the following steps: first, the system was evacuated below 10 Pa; then the column was slowly heated to 523 K at the same time of evacuation, and the heating would not be stopped until the pressure fell to 10 Pa below; then the system was purged with hydrogen gas at a certain flow rate and temperature for about 2 h; at last, the column was heated and evacuated below 10 Pa again. Prior to separation tests, PdeAl2O3 was characterized. The palladium content was determined by X-fluorescence spectrum (S4-X). The surface morphology and phase composition were observed with electron scanning microscope (SEM, Sirion-200) and X-ray diffraction (XRD, Y-2000), respectively. The BET specific surface area was measured by an N2 isothermal absorption method (Autosorb-1, AS1C-VP2).
Experimental
A 5%H2-5%D2-90%Ar mixture was utilized as the feed gas of the separation experiments. The purities of used H2, D2, and Ar gases were 99.999%, 99.7%, and 99.9995%, respectively. The chromatography column was prepared using a 5 m long coiled copper tube with a size of 410 1 mm. It was packed with 165 g of PdeAl2O3 (University of Beijing Chemical Engineering, 10e20meshes). Fig. 1 shows the scheme of the constructed experiment system for separation tests. The components and concentrations of the outflow gas were determined by GAM400 Gas Analysis system (In Process Instrument production) on line. Feed gas was continuously fed in and separated at certain column temperature and gas flow rate when the separation tests were conducted. The on line analysis instrument GAM400 was set to analyze sample gas every 2 min. One separation test was not finished until the column was broken through (it was broken through when the column reached the dynamic saturation, and the concentration of each component in the outflow was almost same with that of inflow.). The column was set in the cryogenic box to be maintained at a certain temperature, and feed gas was fed in after the temperature of the cryogenic box reached the setting value for 15minuts at least.
Fig. 1 e Scheme of separation experiment system. Graphics marked ①w⑨ indicate in turn the feed gas vessel, mass flow controller, standard vessel, chromatography column, pressure transmitter, GAM400 gas analysis system, vacuum gauge, vacuum pump and cryogenic box.
3.
Results and discussion
3.1.
Characterization of PdeAl2O3
The palladium content of PdeAl2O3 was 46.2wt%. The BET curve (Fig. 2) and the adsorption/desorption isothermal curve (Fig. 3) of PdeAl2O3 were obtained through the N2 isothermal absorption method. In these two figures, P0 denotes the saturated steam pressure, P is the instantaneous gas pressure, and Va denotes the adsorption amount of sample at the relative gas pressure of P/P0. The specific surface area and the open porosity were 59.8 m2/g and 69.5%, respectively, calculated from the adsorption/desorption data. The specific surface area was larger than that of spongy palladium by one magnitude at least, and the porosity was much larger [21], too. Both the specific surface area and porosity are important to chromatography separation. The larger specific surface area, the more active reaction points, the quicker separation rate. The gas penetrating performance is good when the porosity is large. As the separation column gets longer, the gas resistance of packed material becomes more crucial. If the gas resistance
Fig. 2 e BET curve of PdeAl2O3.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
Fig. 5 e XRD pattern of PdeAl2O3.
Fig. 3 e Adsorption/desorption isothermal curve of PdeAl2O3.
is too large in long column, the separation rate and efficiency will drop dramatically. Therefore, PdeAl2O3 used here with large specific surface area and open porosity is satisfactory in these two points. Fig. 4 shows the surface morphology of PdeAl2O3 pellets. It was seen that the pellets were normatively spherical, and there were no cracks on the surface, indicating that the surface situation was fine, and the Pd on the surface would not break down and clog the tube easily. The XRD pattern of PdeAl2O3 is shown in Fig. 5. The Pd was in metal phase, and no diffraction peak of Pd oxide was observed, as we had expected.
3.2.
Separation performance
Fig. 6 shows the breakthrough curve for a 5%H2-5%D2-90%Ar mixture, at the column temperature of 273 K and the gas flow rate of 15 mL(NTP)/min. The breakthrough curve can be understood as three stages (denoted by the two dashed lines in Fig. 6). Stage (a): both protium and deuterium were adsorbed and absorbed, and the concentrations were at the level of baseline. This stage is called as “adsorption/absorption stage”. Stage (b): all the three components (H2, D2 and hybrid-atomic molecule HD) were determined in the outflow gas, and separation effect was
observed. It is called as “separation stage”. It is seen that there are two obviously different parts in Stage (b), namely Part (b1) and (b2) (Fig. 6). In Part (b2), the concentrations of the three components were stable, and we collected the product in this period. Thus, Part (b2) is expressed as “stable separation stage”. Part (b1) is called as “transitional stage”, in which the concentration of D2 was increasing, while the concentrations of H2 and HD were decreasing. Accordingly, the separation factor (SF, defined later on.) rose gradually in Part (b1). It suggests that the better separation efficiency was, the shorter time Part (b1) sustained. The last stage, Stage (c): the concentrations of each component in the outflow gas were almost the same with that of the inflow gas, there was no separation effect in this period. This stage is called as “break through stage”. SF is one of the most significant references to evaluate the separation performance for chromatography. SF can be defined as Formula (1), c denotes concentration, and the subscripts “out” and “in” mean the outflow gas and the gas fed in, respectively. aDH ¼
ðcD =cH Þout ðcD =cH Þn
(1)
Fig. 7 shows the SFs of the separation test. The Y-axis presents the instantaneous SFs. It is seen that the figure does not include Stage (a), which is reasonable because the component concentrations of Stage (a) were at the baseline
Fig. 4 e SEM images of PdeAl2O3.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
Table 1 e Calculation values of deuterium abundance, deuterium recovery ratio, etc. Experiment condition
xD Maximum h(%) V(L/NTP) (%) of xD (%)
Column temperature: 273 K; 98.5 gas flow rate: 15 mL(NTP)/min
98.6
42
78.2
gas, V were defined in Formula (2) w (4). where tefficient is the time of Part (b2); ttotal includes the time of Stage (a) and Stage (b); xD2，xHD，xH2 are in turn the concentrations of D2, HD and H2 in outflow gas during the time of tefficient; xin is the deuterium concentration of feed gas; v is the volume flow rate of feed gas; xD is the average value of xD. The calculation results were listed in Table 1. Fig. 6 e Breakthrough curve at the column temperature of 273 K and gas flow rate of 15 mL(NTP)/min. The dashed lines indicate divisions of “the stages”.
xD ¼
h¼ level and almost undetermined, and according to Formula (1) a little fluctuation of low concentration values could cause fierce changes of SF. It means the SFs of Stage (a) are not so meaningful, even minus value appearing occasionally. As a result, the SF curve started from Stage (b). Corresponding to the breakthrough curve, the SF curve can also be regarded as the Stages: Part (b1) “transitional stage”, Part (b2) “stable separation stage” and Stage (c) “break through stage”. SFs of Part (b1) increased gradually. SFs of Part (b2) were almost invariable with little swing, and the values were all above 60 with the maximum 67. SF of Stage (c) was unity, which was consistent with the meaning of this stage. We could thus conclude that a long time of Part (b2) and at the same time a short Part (b1) are required on the premise of obtaining good separation efficiency. The deuterium abundance of the product gas, xD; the deuterium recovery ratio, h; and the volume of separated feed
v tefficient xD 100% v ttotal xin
V ¼ v ttotal
(2)
(3)
(4)
We found the deuterium abundance of the product gas was high though the deuterium recovery ratio was not so satisfactory, due to the excessively long time of Part (b1) and inadequate time of Part (b2) as discussed above. According to the relation between the hydrogen isotopes effect of Pd and temperature [13], it is pointed out that the lower temperature, the larger SF, and then the longer “stable separation stage” and shorter “transitional stage”. Therefore, better separation efficiency would be obtained if the column temperature decreased. It suggests better separation performance could be expected below 273 K. We could conclude that the separation efficiency was fine despite the application of only single column. The exhaust gas one separation produced was almost all stored in the column. If it was treated by a second column, the deuterium recovery ratio would be higher.
4.
Fig. 7 e SF curve of experiment at the column temperature of 273 K and gas flow rate of 15 mL(NTP)/min. The dashed lines indicate divisions of “the stages”.
xD2 2 þ xHD 100% 2ðxD2 þ xH2 þ xHD Þ
Conclusions
Hydrogen isotopes separation by the FDC system here proposed by using PdeAl2O3 as separation material has been investigated. It was shown that not only the deuterium abundance of product gas reached 98.5%, but also the deuterium recovery ratio was as high as 42% after a 5%H2-5%D2-90% Ar mixture was separated at 273 K column temperature and 15 mL(NTP)/min flow rate. It suggests that a prominent separation performance was obtained at 273 K. Furthermore, the SFs of the “stable separation stage” were all above 60. Therefore, it was concluded that FDC with the PdeAl2O3 packed column is feasible and can be regarded as an effective way to hydrogen isotopes separation, with obvious advantages of extra displacement gas avoided, potential decrease of exhaust gas containing tritium and simple operation procedure. Further experiments are needed to evaluate the separation performance of FDC at different conditions.
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Acknowledgment This work was supported by the National Magnetic Confinement Fusion Science Program of China “Design of the tritium systems for China TBMs and R&D on the key components technologies (2010GB112000)” “and Conceptual design and key technology research of magnetic confinement fusion reactor tritium plant (2011GB111000)”. The authors were grateful to Jie Li, Xiaojing Song, Huogen Huang, Chunli Jiang, Jie Du for the analyses of separation material.
references
[1] Glueckauf E, Kitt P. Hydrogen isotope separation by chromatography. Proceedings of the international symposium on isotope separation. North-Holland Amsterdam, 1957. New York: Wiley-Interscience; 1958. pp. 210e226. [2] Lu G. Exchange and displacement of hydrogen isotopes. J Isotopes 2003;16:231e5. [3] Lasser R, Bell A, Bainbridge N, Brennan D, Do¨rr L, Hemmerich JL, et al. The preparative gas chromatographic system for the JET active gas handling system-tritium commissioning and use during and after DTE1. Fusion Eng Des 1999;47:301e19. [4] Zhu X, Zhu H, Li J, Zhao P. Hydrogen isotopes separation using twin combined columns packed with Pd and La Ni4.7Al0.3. J Nucl Radi 2007;29:96e101. [5] Fujiwara H, Fukada S. Hydrogen isotope separation by selfdisplacement chromatography using palladium particles. J Nucl Sci Technol 2000;37:724e6. [6] Fujiwara H, Fukada S, Yamaguchi Y. Hydrogenating rates of twin columns packed with Pd and molecular sieve with an alternately counter-current flow for hydrogen isotope separation. Int J Hydrogen Energy 2000;25:127e32. [7] Fukada S, Mohamad SB, Fujiwara H, Nishikaw M. Hydrogen isotope ecrichment using multi-column palladium bed. Fusion Sci Technol 2002;41:1082e6.
[8] Lee W. Tritium separation using metal hydrides. DP-MS86e11. Oxnard: California SRL; 1986. [9] Fukada S, Fuichinoue K, Nishikawa M. Hydrogen isotope separation based on isotopic exchange in palladium bed. Fusion Technol 1995;28:608e13. [10] Qian X. Sdudy on separation of protium-deuterium by thermal cycling absorption process. Dissertation of Master. China academy of Engineering Physics; Mianyang, China, 2006. [11] Ducret D, Ballanger A, Steimetza J, Laquerbea C, Baudouinb O, Peyrigain P. Hydrogen isotopes separation by thermal cycling absorption process. Fusion Eng Des 2001; 58e59:417e21. [12] Lu G, Jiang G, Shen C. An experiment investigation for hydrogen and deuterium separation by thermal cycling absorption process. Fusion Sci Technol 1995;28:672e5. [13] Jiang G, Luo D, Lu G, Sun L. Tritium and industry techniques of tritium. Beijing: National Defense Industry Press; 2007. [14] Mosley WC. Pd/K for RTF and 232-H TCAP unit. DOE Conference on electron microscope; 1991. California, US. [15] Li G, Lu G, Jiang G. Study on hydrogen2deuterium displacement effects in particle bed of Pd hydride. J Nucl Radi 2000;22(4):200e6. [16] Lu G, Jiang G, Li G, Wang X, Fu Y. Studies on quick displacement of hydrogen isotopes in metal hydride column. At Energy Sci Technol 2003;37:176e80. [17] Lu G, Li G, Jiang G. Studies on displacement behavior between hydrogen and deuterium in hydride column. Chin J Nucl Sci Eng 2001;21:356e62. [18] Li G, Lu G, Jiang G. Displacement of H-D at high flow rate in palladium particle bed. J Nucl Radi 2003;25:18e21. [19] Li G, Lu G, Jiang G. Study on the performance of deuterium displacement recovery and loading by using Pd particle bed. Rare Metal Mater Eng 2003;32:383e6. [20] Sazonov AB, Magomedbekov P. Estimate of the efficiency of hydrogen-metal hydride (intermetallic compound) systems in the separation of the three isotope mixtures H-D-T. At Energy 2000;89:736e44. [21] Tao T. Study on the thermodynamic properties of hydrogen absorption/desorption of Pd. Dissertation of Master. China academy of Engineering Physics; Mianyang, China, 2003. [22] Sicking G. Isotope effects in metal-hydrogen system. J LessCommon Metals 1984;101:169e90.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen isotopes separation using frontal displacement chromatography with PdeAl2O3 packed column Xiaojun Deng*, Deli Luo, Cheng Qin, Xiaojin Qian, Wan Yang China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan, China
article info
abstract
Article history:
Separation or purification of tritium isotopes is one of the key technologies in ITER. A set of
Received 9 October 2011
frontal displacement chromatography (FDC) device was designed and constructed for
Received in revised form
hydrogen isotopes separation using palladium loading on/in alumina (PdeAl2O3) as the
29 March 2012
separation material. The hydrogen isotopes separation experiments were carried out. It
Accepted 8 April 2012
was found that deuterium abundance of the product was up to 98.5% and the average
Available online 5 May 2012
separation factor was as high as 64, under the condition of 273 K column temperature and 15 mL(NTP)/min flow rate, for a feed gas of 5%H2-5%D2-90%Ar. The deuterium recovery
Keywords:
ratio was 42% in this separation test. The results showed that the separation performance
Frontal displacement
of our FDC device was good by using PdeAl2O3 as separation materials, and it suggested
chromatography
considerable potential for the applicability of FDC in hydrogen isotopes separation.
Hydrogen isotopes
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Separation
reserved.
PdeAl2O3
1.
Introduction
Efficient recovery of tritium from exhaust gases of D-T fusion devices such as ITER is one of key issues for economical and safe recovery of tritium. Several kinds of methods for hydrogen isotopes separation have been proposed [1], in which displacement chromatography (DC) is promising and has received much attention [2e9]. DC has been developed into several techniques, for example, thermal diffusion (TD), thermal cycling absorption process (TCAP), hydrogen displacement chromatography (HDC) [2,3], etc. Separation performance of TD was not very satisfactory due to the production of large amount of medium abundance gas [1]. High tritium purity products were obtained by using the HDC facility of Joint European Tokomak [3] in 1980s, but a large amount of exhaust gas containing tritium was produced simultaneously. TCAP was well developed, and it had been on
stream since 1994 in Replacement Tritium Facility [10] of Savannah River Site in America. Stable distribution of isotopes in the TCAP column was just realized after series of complex manipulation. Improvements on engineering device and technical process optimization of TCAP are still in progress [10e14]. Materials containing palladium are always chosen as the separation material for hydrogen isotopes separation in DC because of the high hydrogen isotopes effect of Pd [15e22]. Pd is inclined to adsorb and/or absorb the lighter isotope first, since the adsorption/absorption rate for the lighter isotope is larger than that of the heavier one. As a result, the heavier isotope can be displaced by the lighter. On the other hand, frontal chromatography (FC) is a classic chromatography process with features of continuous feed gas input and the feed gas itself being the elution gas or carrier gas. On the basis of the advantages of DC and FC, a new method has been
* Corresponding author. Tel.: þ86 816 3626457; fax: þ86 816 3625900. E-mail address: [email protected] (X. Deng). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
proposed, which combines and applies the FC process and hydrogen isotopes effect of Pd in DC. This new method is frontal displacement chromatography. Feed gas in FDC, at the same time used as the displacement gas, continuously flows through the chromatography column packed with materials containing Pd, and the lighter isotope component later fed in then displaces the heavier isotope component earlier adsorbed/absorbed by Pd. Consequently, the heavier isotope component is enriched at end of the column. In this work, a set of FDC device using PdeAl2O3 as separation material for hydrogen isotopes separation was designed and constructed. Separation tests were carried out to evaluate the hydrogen isotopes separation properties of this device. The feasibility of hydrogen isotopes separation by FDC was then discussed. The properties of PdeAl2O3 were analyzed, too.
2.
10775
For removing the adsorbed water and other impurities from the surfaces of PdeAl2O3 pellets, the separation material had to be activated before the separation experiment by the following steps: first, the system was evacuated below 10 Pa; then the column was slowly heated to 523 K at the same time of evacuation, and the heating would not be stopped until the pressure fell to 10 Pa below; then the system was purged with hydrogen gas at a certain flow rate and temperature for about 2 h; at last, the column was heated and evacuated below 10 Pa again. Prior to separation tests, PdeAl2O3 was characterized. The palladium content was determined by X-fluorescence spectrum (S4-X). The surface morphology and phase composition were observed with electron scanning microscope (SEM, Sirion-200) and X-ray diffraction (XRD, Y-2000), respectively. The BET specific surface area was measured by an N2 isothermal absorption method (Autosorb-1, AS1C-VP2).
Experimental
A 5%H2-5%D2-90%Ar mixture was utilized as the feed gas of the separation experiments. The purities of used H2, D2, and Ar gases were 99.999%, 99.7%, and 99.9995%, respectively. The chromatography column was prepared using a 5 m long coiled copper tube with a size of 410 1 mm. It was packed with 165 g of PdeAl2O3 (University of Beijing Chemical Engineering, 10e20meshes). Fig. 1 shows the scheme of the constructed experiment system for separation tests. The components and concentrations of the outflow gas were determined by GAM400 Gas Analysis system (In Process Instrument production) on line. Feed gas was continuously fed in and separated at certain column temperature and gas flow rate when the separation tests were conducted. The on line analysis instrument GAM400 was set to analyze sample gas every 2 min. One separation test was not finished until the column was broken through (it was broken through when the column reached the dynamic saturation, and the concentration of each component in the outflow was almost same with that of inflow.). The column was set in the cryogenic box to be maintained at a certain temperature, and feed gas was fed in after the temperature of the cryogenic box reached the setting value for 15minuts at least.
Fig. 1 e Scheme of separation experiment system. Graphics marked ①w⑨ indicate in turn the feed gas vessel, mass flow controller, standard vessel, chromatography column, pressure transmitter, GAM400 gas analysis system, vacuum gauge, vacuum pump and cryogenic box.
3.
Results and discussion
3.1.
Characterization of PdeAl2O3
The palladium content of PdeAl2O3 was 46.2wt%. The BET curve (Fig. 2) and the adsorption/desorption isothermal curve (Fig. 3) of PdeAl2O3 were obtained through the N2 isothermal absorption method. In these two figures, P0 denotes the saturated steam pressure, P is the instantaneous gas pressure, and Va denotes the adsorption amount of sample at the relative gas pressure of P/P0. The specific surface area and the open porosity were 59.8 m2/g and 69.5%, respectively, calculated from the adsorption/desorption data. The specific surface area was larger than that of spongy palladium by one magnitude at least, and the porosity was much larger [21], too. Both the specific surface area and porosity are important to chromatography separation. The larger specific surface area, the more active reaction points, the quicker separation rate. The gas penetrating performance is good when the porosity is large. As the separation column gets longer, the gas resistance of packed material becomes more crucial. If the gas resistance
Fig. 2 e BET curve of PdeAl2O3.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
Fig. 5 e XRD pattern of PdeAl2O3.
Fig. 3 e Adsorption/desorption isothermal curve of PdeAl2O3.
is too large in long column, the separation rate and efficiency will drop dramatically. Therefore, PdeAl2O3 used here with large specific surface area and open porosity is satisfactory in these two points. Fig. 4 shows the surface morphology of PdeAl2O3 pellets. It was seen that the pellets were normatively spherical, and there were no cracks on the surface, indicating that the surface situation was fine, and the Pd on the surface would not break down and clog the tube easily. The XRD pattern of PdeAl2O3 is shown in Fig. 5. The Pd was in metal phase, and no diffraction peak of Pd oxide was observed, as we had expected.
3.2.
Separation performance
Fig. 6 shows the breakthrough curve for a 5%H2-5%D2-90%Ar mixture, at the column temperature of 273 K and the gas flow rate of 15 mL(NTP)/min. The breakthrough curve can be understood as three stages (denoted by the two dashed lines in Fig. 6). Stage (a): both protium and deuterium were adsorbed and absorbed, and the concentrations were at the level of baseline. This stage is called as “adsorption/absorption stage”. Stage (b): all the three components (H2, D2 and hybrid-atomic molecule HD) were determined in the outflow gas, and separation effect was
observed. It is called as “separation stage”. It is seen that there are two obviously different parts in Stage (b), namely Part (b1) and (b2) (Fig. 6). In Part (b2), the concentrations of the three components were stable, and we collected the product in this period. Thus, Part (b2) is expressed as “stable separation stage”. Part (b1) is called as “transitional stage”, in which the concentration of D2 was increasing, while the concentrations of H2 and HD were decreasing. Accordingly, the separation factor (SF, defined later on.) rose gradually in Part (b1). It suggests that the better separation efficiency was, the shorter time Part (b1) sustained. The last stage, Stage (c): the concentrations of each component in the outflow gas were almost the same with that of the inflow gas, there was no separation effect in this period. This stage is called as “break through stage”. SF is one of the most significant references to evaluate the separation performance for chromatography. SF can be defined as Formula (1), c denotes concentration, and the subscripts “out” and “in” mean the outflow gas and the gas fed in, respectively. aDH ¼
ðcD =cH Þout ðcD =cH Þn
(1)
Fig. 7 shows the SFs of the separation test. The Y-axis presents the instantaneous SFs. It is seen that the figure does not include Stage (a), which is reasonable because the component concentrations of Stage (a) were at the baseline
Fig. 4 e SEM images of PdeAl2O3.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
Table 1 e Calculation values of deuterium abundance, deuterium recovery ratio, etc. Experiment condition
xD Maximum h(%) V(L/NTP) (%) of xD (%)
Column temperature: 273 K; 98.5 gas flow rate: 15 mL(NTP)/min
98.6
42
78.2
gas, V were defined in Formula (2) w (4). where tefficient is the time of Part (b2); ttotal includes the time of Stage (a) and Stage (b); xD2，xHD，xH2 are in turn the concentrations of D2, HD and H2 in outflow gas during the time of tefficient; xin is the deuterium concentration of feed gas; v is the volume flow rate of feed gas; xD is the average value of xD. The calculation results were listed in Table 1. Fig. 6 e Breakthrough curve at the column temperature of 273 K and gas flow rate of 15 mL(NTP)/min. The dashed lines indicate divisions of “the stages”.
xD ¼
h¼ level and almost undetermined, and according to Formula (1) a little fluctuation of low concentration values could cause fierce changes of SF. It means the SFs of Stage (a) are not so meaningful, even minus value appearing occasionally. As a result, the SF curve started from Stage (b). Corresponding to the breakthrough curve, the SF curve can also be regarded as the Stages: Part (b1) “transitional stage”, Part (b2) “stable separation stage” and Stage (c) “break through stage”. SFs of Part (b1) increased gradually. SFs of Part (b2) were almost invariable with little swing, and the values were all above 60 with the maximum 67. SF of Stage (c) was unity, which was consistent with the meaning of this stage. We could thus conclude that a long time of Part (b2) and at the same time a short Part (b1) are required on the premise of obtaining good separation efficiency. The deuterium abundance of the product gas, xD; the deuterium recovery ratio, h; and the volume of separated feed
v tefficient xD 100% v ttotal xin
V ¼ v ttotal
(2)
(3)
(4)
We found the deuterium abundance of the product gas was high though the deuterium recovery ratio was not so satisfactory, due to the excessively long time of Part (b1) and inadequate time of Part (b2) as discussed above. According to the relation between the hydrogen isotopes effect of Pd and temperature [13], it is pointed out that the lower temperature, the larger SF, and then the longer “stable separation stage” and shorter “transitional stage”. Therefore, better separation efficiency would be obtained if the column temperature decreased. It suggests better separation performance could be expected below 273 K. We could conclude that the separation efficiency was fine despite the application of only single column. The exhaust gas one separation produced was almost all stored in the column. If it was treated by a second column, the deuterium recovery ratio would be higher.
4.
Fig. 7 e SF curve of experiment at the column temperature of 273 K and gas flow rate of 15 mL(NTP)/min. The dashed lines indicate divisions of “the stages”.
xD2 2 þ xHD 100% 2ðxD2 þ xH2 þ xHD Þ
Conclusions
Hydrogen isotopes separation by the FDC system here proposed by using PdeAl2O3 as separation material has been investigated. It was shown that not only the deuterium abundance of product gas reached 98.5%, but also the deuterium recovery ratio was as high as 42% after a 5%H2-5%D2-90% Ar mixture was separated at 273 K column temperature and 15 mL(NTP)/min flow rate. It suggests that a prominent separation performance was obtained at 273 K. Furthermore, the SFs of the “stable separation stage” were all above 60. Therefore, it was concluded that FDC with the PdeAl2O3 packed column is feasible and can be regarded as an effective way to hydrogen isotopes separation, with obvious advantages of extra displacement gas avoided, potential decrease of exhaust gas containing tritium and simple operation procedure. Further experiments are needed to evaluate the separation performance of FDC at different conditions.
10778
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 7 7 4 e1 0 7 7 8
Acknowledgment This work was supported by the National Magnetic Confinement Fusion Science Program of China “Design of the tritium systems for China TBMs and R&D on the key components technologies (2010GB112000)” “and Conceptual design and key technology research of magnetic confinement fusion reactor tritium plant (2011GB111000)”. The authors were grateful to Jie Li, Xiaojing Song, Huogen Huang, Chunli Jiang, Jie Du for the analyses of separation material.
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