Fusion Engineering and Design Molecular-sieving Effect of Zeolite

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Fusion Engineering and Design 84 (2009) 11081112

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Fusion Engineering and Design

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Molecular-sieving effect of zeolite 3A on adsorption of H 2, HD and D2K. Kotoh , S. Takashima, Y. NakamuraFaculty of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan

a r t i c l e i n f o

Article history:Available online 14 February 2009

Keywords:Hydrogen-isotope separationAdsorptionMolecular sieveSievingConnementTritium

a b s t r a c t

Synthetic zeolite 3A has the molecular-sieving windows of nominal diameter 0.3nm in its crystal latticeframework, which obstruct thecrystalline adsorption of moleculesof diameter largerthan 0.3nm, exceptwater, hydrogen and helium. The windows diameter slightly varies with temperature, however, that is

endorsed in experimental results that hydrogen cannot be adsorbed at the liquid-nitrogen temperature.Authorsmeasuredthe range of temperature where zeolite 3A permits hydrogenadsorption, andrevealedthe temperature difference of several degrees in appearance of molecular sieving for H 2 and D2. Thisdifference is important because from a H 2D2 mixture one isotope could be isolated by adsorption if operated at a temperature regulated between the molecular-sieving appearance temperatures. We havereported large values of D 2/H2 separation factor obtained from molecular-sieving experiments. In thisstudy, the effect of sieving for the hybrid-atomic isotope HD is examined using a H 2HDD2 mixture. Wehere report the experimental HD/H 2 separation factor evaluatedbetween the D 2/H2 factor and unity. Thisresult is signicant because where the effective molecular diameter concerning the sieving mechanismis suggested. From this knowledge, the isotopic effect of sieving for HT and DT can be predicted.

2009 Elsevier B.V. All rights reserved.

1. Introduction

Synthetic zeolites having the strong power of adsorptionfor water are conventionally used in such as tritiated watersafety-connement systems [14]. Since zeolites also exhibit theadsorptive power for hydrogen under cryogenic condition, zeo-lite packed-bed columns are applicable to the system of tritiumrecovery from DT fusion blanket [58]. In cryogenic adsorptionof hydrogen isotopes onto zeolites, isotope separation is observed[911] . Authors have claried the isotope separationfactors amongsix species H2, HD, D2, HT, DT and T2 adsorbed onto synthetic zeo-lites 5A and 13X at the liquid-nitrogen temperature 77.4K, throughvolumetric adsorption experiments and theoretic analyses [12].The separation factors suggest the reality of a hydrogen-isotopeseparation system using adsorption columns. Authors and collab-orators hence have been developing a system of pressure swingadsorptionorpressure-and-temperatureswing adsorption [1315] ,which may have advantages over the cryogenic distillation systemin operational convenience, cost performanceand reactant processinventory.

The isotope effecton adsorption is due to the difference in ther-modynamic interaction between adsorbentmaterialand adsorbatemolecules.Synthetic zeolites,however, have notonly theadsorptive

Corresponding author. Tel: +81 92 802 3507; fax: +81 92 802 3507.E-mail address: [email protected] (K. Kotoh).

power and selectivity but also the mechanical function of selec-tivity for molecular size or/and structure, molecular sieving, asthey are so-called molecular sieves. This sieving phenomenonis never observed in the adsorption of hydrogen isotopes ontozeolite-4A, -5A and -13X at least above 77.4K. In a previousstudy, however, we have shown the appearance of molecular-sieving effect on adsorption of H 2 and D2 onto zeolite-3A, throughpseudo-isobaric adsorption experiments, where the isotopic dif-ference is observed. And then, we have veried the isotope effectof molecular sieving with abnormally large values of D 2/H2 sepa-ration factor obtained by a sequential experiment of adsorption,connement, evacuation and desorption using a H 2D2 mixture[16].

An important interest is leaved in the previous work [16] howa hybrid-atomic isotope such as HD behaves on the sieves. If thelarger atomic diameter were the sieving determination factor, thehybrid HD would behave as its larger atomic twin H 2. Experimen-tal results shown here demonstrate that the sieving determinant isnot the atomic diameter but the larger molecular-diameter, that is,the kinetic diameterof a two-atomic molecule in rotational motion.The rotational motion would be required for hydrogen moleculesto pass through sieve meshes. Since the molecular sieving is of mechanical manner, this suggests the possibility of isolation of HTfrom a H2HT mixture or DT from a D2DT mixture. If the sievingis applicable to hydrogen-isotope separation, remarkable progresswould be expectable in development of effective tritium-handlingprocesses for fusion reactor fuel-cycle system.

0920-3796/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.fusengdes.2009.01.052
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K. Kotoh et al. / Fusion Engineering and Design 84 (2009) 11081112 1109

Fig. 1. Models of simplied molecular-sieving mechanism for hydrogen isotopes.

2. Theoretical [17,18]

Synthetic zeolite NaA (SZ-NaA) is the initiator of A-type zeo-lites, which is the crystal of alminosilicate containing sodiumcations,Na12Al12Si12O48 (dehydrated),formingacubiclatticestruc-ture. In the lattice framework, adsorptive cavities of 1.23nm innominal inner diameter are comprised, where adjacent cavitiesare connected together in three-dimension with a window of 8-oxygen-atomic alminosilicate ringhaving a nominal innerdiameterof 0.4 nm (4 ). Therefore, molecules of diameter larger than 4 are never adsorbed into SZ-NaA. Since synthetic zeolites exhibitthe molecular sieving, these are so-called molecular sieves andthe type NaA is also known as 4A: SZ-4A. The other zeolites of A-type are originated by exchanging the Na cations for anotherspecies.

Synthetic zeolite KA (SZ-KA) including potassium makes the8-oxygen rings to be 3 in nominal inner diameter, also knownas SZ-3A, where water, helium and hydrogen of diameter lessthan 0.3 nm are permitted crystalline adsorption. Hydrogen hav-ing nominal diameters of 0.24nm 0.31nm, however, cannot beadsorbed at a cryogenic temperature approaching to the liquid-nitrogentemperature,becausethe 8-oxygen ringstructure dependson temperature, that is, the sieve meshes reduce with decreasingtemperature [17].

In the previous study [16], authors observed the temperature

effective to the crystalline adsorption of H 2 or D2, and revealed thetemperature difference of a few degrees in appearance of molec-ular sieving for H2 and D2. This difference is important; since themolecularsieving is of a mechanical separation manner, it is ideallyexpectablethatthesmallerisotopemoleculescanbeisolatedfromaH2D2 mixtureifoperatedbetweenthe sieving appearancetemper-atures. We have veried the sieving isotope separation with valuesof D2/H2 separation factor, abnormally large in comparison withthosedue tothe differencein thermodynamicadsorptioncharacter-istics. A problem is there, however, that the zeolite sieves regulatewhich diameter of a twin-atomic molecule, larger or smaller one,connecting to the distinction of hybrid-atomic molecules fromtwin-atomic ones.

Fig. 1 shows schematic diagrams of simplied models of

molecular-sieving mechanism for hydrogen isotopes. If the smaller

diameter of a two-atomic molecule corresponding to its largeratomic diameter is the effective geometry, as shown as Case(a), the hybrid HD would behave together with the twin H 2larger than D 2. In this case, the sieves never isolate HD from aH2HD mixture. On the other hand, if the larger molecular diam-eter is of determination for the sieving effect as illustrated asCase (b), the hybrid HD could be isolated from a H2HD mix-ture.

3. Experimental

In order to examine the effect of sieving for hybrid-atomicmolecules, using a volumetric adsorption apparatus shown inFig. 2, we carried out the following sequential operations inan adsorptiondesorption experiment with a packed-bed of SZ-3A:

Fig. 2. Schematic diagram of volumetric adsorption apparatus.


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Fig. 3. Normalized build-up pressure curvesof D 2 and H2 released frommolecular-sieving connement with respect to temperature.

(1) Preparation: loading a gasmixture of H 2HDD2 at a totalpres-sure around 3.6 103 Pa onto the activated packed-bed cooledat the liquid-nitrogen temperature 77.4 K.

(2) Sieving adsorption: increasingtemperatureup toa pointwithina specic range indicated in pseudo-thermostatic desorptionexperiments for D 2 and H2, where the sieves govern the crys-talline adsorption.

(3) Connement: re-cooling down to 77.4 K, and then, evacuatingthe gas holder including the packed-bed until arriving at lowerthan 10 5 Pa.

(4) Desorption: recovering the crystalline-adsorbed sample byheating up to room temperature.

(5) Analysis: measuring fractions of H 2, HD and D2 in gas samplesby a mass-spectrometric system.

SZ-3A pellets of 2 mm spherical (Merck Ltd.) were packed atabout 24g in a test cell set at the end of a nger. The initial sampleof H2HDD2 was prepared from a mixture of 40% H2 and 60% D2at a total pressure of 5.8 103 Pa by operating an electric dischargetube producing the reaction, H 2 + D2 2HD. The three-componentmixtures produced were composed of about 25% H 2, 30% HD and45%D2. In the second step, 130K, 140K or 160K was selected as thepoint of temperature.

In advance, preliminary experiments were made for examiningthe temperature ranges of appearance of sieving for H 2 and D2, asfollows:

(1) Adsorption/connement: loadinga volumeofH 2 or D2 atapres-sure of 8.5 103 Pa onto the packed-bed at room temperaturewhich hasbeen activated beforehand by heating at 573 K underthe evacuation arriving at 5 10 5 Pa, and then, cooling down

to 77.4 K.(2) Isolation: evacuating the gas holder including the packed-beduntil reaching lower than 10 5 Pa, after arrival at the lowesttemperature of 77.4K.

(3) Recovery: releasing a volume of H2 or D2 from connement byslowly heating up to room temperature.

In therststep, theconnementwouldtakeplace from thewall-side to the central part of the packed-bed in the order of arrival atthe critical temperature. In the second step, the volume of samplegas except the amount conned in crystalline pores was evalu-ated. In the last step, the heating has to be operated as slow aspossible, because of pseudo-thermostatic experiment for examin-ing the temperature range of sieving function appearance. So, we

employed a bulking agent of shaved ice as coolant in the cryogenicvacuum pot, in alternating combination with liquid nitrogen, inorder to moderate a rising rate of temperature in the test section.The pressure build-up in the volumetric closed system indicates atemperaturerange wheremoleculesare releasedfromconnementby sieves.

4. Results and discussion

Fig. 3 shows results from the preliminary experiments, whereeach curve indicates the build-up pressure of H 2 or D2 in the vol-umetric closed system. The pressure curves are normalized withsaturatedpressures of 300Paand 460 Parespectively forH 2 and D2.Thebuild-up curvesdemonstratethe behaviorof H 2 and D2 leaving

from sieving connement within crystalline pores, released by thesieves increasingtheir meshsizewithelevatingtemperature.Thesesequential experiments were begun from thestep of loading gasonadsorbent at a pressure of 8.5 103 Pa. The change of curves indi-cates the connement release starting around 120K for D 2 or 130Kfor H2 and then establishing at about 180 K for D 2 or about 190 Kfor H2. In this transient region, the curves exhibit a temperaturedifference around several degrees, D 2 indicating the temperaturelower than that of H 2. Since the experiments are in temperature-transitional process, the sieving release occurs sequentially in the

Fig. 4. Schematic illustration of molecular-sieving release in temperature transient process.


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Fig. 5. Schematic process of adsorption of hydrogen isotopes isolated/separated bysieving dependent on temperature.

packed-bed as illustrated in Fig. 4. The range of T W from CT to T Ccorresponds to the changing range shown by the curves in Fig. 4.The temperature ranges of sieving for H 2 and D2 agree with thosefrom pseudo-isobars in the previous work [16].

Referring to the result from preliminary examinations, threeseries of experiment with a H 2HDD2 sample were performed at130K, 140K and 160K of T W selected as the returning point fromsieving adsorption to connement, where we assumed the adsorp-tion/connement of hydrogen isotopes separated by the sieving ina manner explained in Fig. 5. In the desorption process after isola-tion of conned sample, the heating-up operation was regardlessof moderation in a rate of increase in temperature,anticipating thatthe release of D2 and H2 would begin respectively at net temper-atures around 115K and 125K. The released sample-gas build-uppressures were saturated at 20 Pa, 41Pa and 140 Pa in the experi-ments under the conditions of 130 K, 140 K and 160 K, respectively.If the same pseudo-thermostatic operation as in the preliminaryexperiments were carried out, each normalized build-up pressurecurve would be observed behaving between D 2 and H2 curvesshown in Fig. 3.

When the sieving critical temperature CT for every hydro-gen isotope is within the temperature distribution formed in thepacked-bed, conned at T W the crystalline gas is D2-enrichedbecause D2 diametrically smaller than H 2 is isolated locally in thepacked-bed. Problem is the behavior of HD. As shown in Fig. 5, if the critical temperature for HD is between those for H 2 and D2, thehybrid HD would be also enriched in conned samples because of formationoftheintermediateregioncontainingisolatedHDandD 2,butreduced relatively toD 2, of course. This behaviorof HD matcheswith the model of sieving for two-atomic molecules assumed asCase (b) in Fig. 1. On the other hand, if according to the modelassumed as Case (a) of dependence on the molecular larger atomicdiameter, the behavior would be same as that of H 2 because of thesame sieving critical temperature.

Table 1Experimental separation factors of HD/H 2 and D2/H2 in molecular-sieving adsorp-tion of SZ-3A.

T W HD/ H2 D2 / H2

130 K 3.19 3.71 (4.33)a140 K 2.60 3.14 (3.04)a160 K 2.20 2.24 (2.32)a

a Obtained in the previous study [16].

Experimental results are listed in Table 1, where the separationfactor HD/ H2 or D2/ H2 is dened as

i/j =( yi/y j)conned( xi/x j)loaded

(1)

Here, x and y are the mole fractions of a component in the loadedgas-phase and the conned crystalline-adsorbed phase, respec-tively. The subscript i denotesasubjectiveHDorD 2,and j representsH2 as reference. The experimental HD/ H2 values 3.71, 3.14 and2.24 respectively at 130K, 140K and 160K are consistent with onespreviously obtained from similar experiments using H 2D2. How-ever, these values from sieving experiments are abnormally largecompared with those predictable in physical adsorption charac-teristics of hydrogen isotopes at these temperatures, referring tosuch as a study of multi-component behavior of hydrogen isotopesonto SZ-5A or -13X at 77.4 K [12]. In consideration on a variationof the experimental values, a rising trend in D2 / H2 with decreas-ing temperature can be interpreted with the sieving model, due toreduction of the region of H 2D2 or H2HDD2 with reducing thereturning-point temperature.

The values of HD/ H2 clearly larger than unity suggest that themolecular sieving occurs in the mechanism considered in Case (b).Andalso, thevalues reduced relatively to D2 / H2 reect the diamet-ric order of hydrogen isotopes in the sieving model of Case (b). Theenrichment of HD in the SZ-3A packed-bed results from the isola-tion of HD-D2 in the intermediate region. Accordingly, the trend of an increase in HD/ H2 withdecreasingtemperature meetswith thatin D2/ H2 , due to reduction of the H 2HDD2 region.

The sieving behavior of SZ-3A adsorbing the hybrid-atomicmolecules hence suggests the possibility notonly of a new methodfor production of deuterium from natural hydrogen, but alsoof a convenient but remarkably effective technique for isolatingremoval or recovery of HT diluted in hydrogen. In diametric com-parisonamonghydrogen isotopes,however,the differencebetweenD2 and HT is sensitive. When the effective diameter of a hydrogen-isotope molecule passing through sieves is assumed corresponding

to its rotational diameter, hybrid-atomic molecules exhibit largeeffectivediametersrelatively to twinones.Reason is that thehybridmolecules are rotating with wobbling motion because the centerof mass differs from that of the quantum-mechanical wave func-tion [12,19]. Thus, the sieving takes place in the order of H2, HD,HT, D2, DT and T2. Therefore, we would be able to develop variousprocesses of isolating hydrogen-isotope separation, if temperaturecan be controlledpreciously between the molecular-sieving criticaltemperatures.

5. Conclusions

The molecular sieving ofSZ-3A for HDcontained in a H2HDD2mixture was examined by the sequential experiments of sieving

adsorption, connement and recovery. The separation factor of HD/H2 obtained from the sieving operation was larger than unitybut reduced relatively to that of D 2/H2. This result endorses themechanism of sieving for hydrogen isotopes in which the dynamicdiameter of a molecule in rotation is determinant. The sieving ispossible to separate between H 2, HD, HT, D2, DT and T2. Moreover,drastic separation is expectable, since the sieving is of utilizing thegeometric difference. The molecular sieving of SZ-3A would makeit possible to develop a convenient butremarkably effective systemfor tritium separation, removal or recovery.

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Fusion Engineering and Design Molecular-sieving Effect of Zeolite