Hydrogen extraction from Pb–17Li: Tests with a packed column

Fusion Engineering and Design 39 – 40 (1998) 787 – 792

Hydrogen extraction from Pb–17Li: Tests with a packed column N. Alpy, T. Dufrenoy, A. Terlain * CEA/CEREM/SCECF, Commissariat a` L’Energie Atomique, B.P. 6, 92265, Fontenay-aux-Roses, France

Abstract Tests of hydrogen extraction from the liquid metal Pb – 17Li have been performed in MELODIE loop at 673 and 713 K, using a gas liquid contactor. The tested extractor is a 60 cm high packed column equipped with three 54.7 mm diameter cylinders of Sulzer Mellapak 750Y structured packing. The investigated flow rates were 0.07 – 0.1 m3 h − 1 for Pb–17Li and 3–9 N l h − 1 for the extracting gas (argon). Most of the tests have been carried out with a hydrogen partial pressure of 1000 Pa in Pb–17Li at the extractor inlet. The results show that an extraction efficiency of about 25% can be achieved, in the present experimental conditions. However, it appears that the hydrogen mass balance is not correct when using our experimental data and the usual Sievert constant values. Several explanations have been discussed and calculations have been done assuming that the Ks values have to be corrected. © 1998 Published by Elsevier Science S.A. All rights reserved.

1. Introduction In the water-cooled Pb – 17Li (Pb with 17 at % Li) liquid blanket concept, the tritium generated from the interaction between neutrons and lithium atoms in Pb – 17Li, has to be removed from the liquid metal in a specific unit, outside the blanket, as described by Giancarli et al. [1]. A gas–liquid contactor is envisaged to achieve this operation. Different types of extractors have been considered by Pierini [2], Sze [3] or Malara [4] and some have been tested by Terlain et al. [5]. The low efficiencies obtained with a plate [5] or a bubble column by Sample et al. [6] seemed to

* Corresponding author. Fax.: + 33 1 42537231; e-mail: [email protected]

result from a narrow gas–liquid interfacial area. Therefore, in order to increase this surface, a packing has been installed in the extractor. The results obtained with this new contactor are presented and discussed here.

2. Experimental and results

2.1. Experimental de6ice The tests have been performed with a packed column installed on the MELODIE loop (Fig. 1), already described by Terlain et al. [5] and using hydrogen instead of tritium. This loop is composed of a Pb–17Li circuit and of a gas circuit. The Pb–17Li circuit consists of three interconnected vessels:

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1. A tank which acts as a buffer and is initially used to fill the loop with 450 kg of Pb –17Li, 2. A saturator, which is a bubble column allowing an hydrogen enrichment of Pb – 17Li before entering the extractor, 3. An extractor to transfer the dissolved hydrogen from Pb – 17Li to an argon stream. The gas comes into the bottom of this column by a perforated tube and flows against the Pb–17Li current. The extractor is equipped with three 54.7 mm diameter cylinders of AISI 410S stainless steel, Sulzer Mellapak 750Y, structured packing. Each cylinder is 20 cm high and is supported by a perforated plate. At the top of the packing, a liquid distributor with three slits was placed to favour the liquid distribution. Three electromagnetic pumps insure the Pb– 17Li circulation between each vessel. The liquid flow rates are measured with electromagnetic flowmeters. The whole Pb – 17Li circuit, except for the welds, pump and flowmeter ducts, is made of 316L stainless steel covered with an aluminide layer in order to decrease the hydrogen permeation. Iron membranes, immersed in Pb – 17Li and linked to pressure gauges, allow to measure the hydrogen pressure corresponding to the dissolved hydrogen concentration in Pb – 17Li. The gas circuit allows to

supply the vessels with gas. Before entering, the gas (Ar or Ar/H2 mixtures) goes through an H2O and O2 purifier. The hydrogen contents in the gas are determined by gas chromatography.

2.2. Experimental conditions of the tests The tests have been performed at two temperatures: 673 and 713 K. The saturator was fed with a Ar +2 vol.% H2 or Ar + 5 vol.% H2 gas at a 3.6 NTP l h − 1 flow rate. Most of the experiments have been performed with a 1000 Pa hydrogen partial pressure in Pb–17Li at the extractor inlet, which corresponds to that expected in the European water-cooled blanket as described by Giancarli et al. [1]. The pure argon flow rate in the extractor was varied between 3 and 9 NTP l h − 1. The range of Pb–17Li flow rate, 0.07–0.1 m3 h − 1, was imposed by the pump capabilities.

2.3. Results The results are summarised in Table 1. The data given in this table are averages obtained from values registered for several days (about 5 days) at steady state.

Fig. 1. MELODIE loop.

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Table 1 Results of the tests No.

TK

L m3 h−1

G Nm3 h−1

PIE Pa

POE Pa

YH2 vppm

1 2 3 4 5 6 7 8 9

673 673 673 673 713 713 713 713 713

0.071 0.066 0.069 0.100 0.061 0.068 0.068 0.100 0.070

3×10−3 6×10−3 9×10−3 6×10−3 3×10−3 6×10−3 9×10−3 6×10−3 6×10−3

1085 1003 927 1188 1258 1152 1158 1228 2490

653 613 557 858 682 670 612 712 1215

477 259 190 234 825 575 375 515 775

h= FGH/Fmax

3. Discussion

and therefore

3.1. Hydrogen flux and extraction efficiency

h= 2GYH210 − 6  (0.0224LKs PIE) − 1 The hydrogen exchanged between the liquid and the gas can be calculated in two ways. First it can be considered as the hydrogen flux extracted from Pb–17Li. In that case, using the Sievert’s law to calculate the hydrogen concentration in Pb–17Li, the exchanged atomic hydrogen flux is given by: FLH =LDCH =LKs( PIE − POE)

(1)

Secondly, it can also be considered as the atomic hydrogen flux in the gas at the extractor outlet and is given by: FGH = 2G(YH210 − 6)/0.0224

(2)

The extraction efficiency is defined as the ratio of the hydrogen removed from Pb – 17Li in the extractor to the maximum removed hydrogen flux, thermodynamically possible. As the argon entering the extractor does not contain hydrogen, this maximum hydrogen flux corresponds to the dissolved hydrogen flux in Pb – 17Li entering the extractor. Then, the extractor efficiency can be calculated in two different ways: first, h =FLH/Fmax,

with

Fmax =LKs × PIE

which leads to h =( PIE − POE)/ PIE secondly,

(3)

(4)

The values of FLH have been calculated using the Ks values given by Reiter et al. [7]. As we can see from Table 2, the FLH/FGH ratios for the different experiments, are different from (1) which is the value expected to satisfy the hydrogen mass balance. Pressures in the membranes, hydrogen content in the gas and gas flow rates have been checked and found reliable with an uncertainty lower than 5%. The uncertainty on the liquid alloy flow rate is about 10%. Therefore, the uncertainties on these measurements cannot explain alone the difference between the two hydrogen flux, FLH and FGH.

Table 2 FLH/FGH and h values for the different tests No.

L m3 h−1

FGH 10−4 mol h−2

FLH/FGH

h%

1 2 3 4 5 6 7 8 9

0.071 0.066 0.069 0.100 0.061 0.068 0.068 0.100 0.070

1.28 1.39 1.52 1.26 2.21 3.08 3.01 2.76 4.15

4.2 3.5 3.2 4.2 2.7 1.9 2.2 3.1 2.6

22.5 22 22.5 15 26 24 27.5 24 30

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Hydrogen losses through the structures are not excluded but they cannot explain alone our mass balance incoherence. Indeed, in that case, the difference between FGH and FLH should be more significant at 713 K than at 673 K whereas the opposite is observed during our tests (see Table 2). Assuming that no hydride-like compounds are formed, another possibility could be a wrong estimation of the Ks values. To satisfy the hydrogen mass balance in our experiments, the Ks value should be about 2.5 times less at 713 K and 3.7 times less at 673 K than the lowest ones given in the literature (Reiter’s values [7]). A wrong value of Ks cannot be discarded because the Ks data in the literature are spread in a large range of values (a few orders of magnitude) and seem to depend on the determination method as mentioned by Caorlin et al. [8]. However, if it was the only reason of our mass balance incoherence, it should produce the same difference for all the tests performed at a same temperature whereas the results in Table 2 shows fluctuations which are just larger than experimental measurement uncertainties. Therefore, in the absence of a clear explanation of the mass balance discrepancy, it seems that the most reliable data are those independent of Ks and L. Consequently, the efficiencies of the extractor have been calculated using Eq. (3) (see Table 2).

3.2. Influence of the gas and liquid flow rates on efficiency Fig. 2 represents the influence of the gas flow rate on the extraction efficiency at 673 and 713 K. It shows that an increase of Ar flow rate in the extractor does not significantly improve the extractor efficiency. It could result either from a bypass of the flow or from a too minor gas flow rate range of investigation since we have worked far from the hydrodynamic conditions corresponding to the flooding. The insignificant variation of the efficiency with the liquid flow rate observed in our experiments (see Table 2), could indicate that the involved superficial velocities (ratio between the liquid flow

Fig. 2. Efficiency of the extractor as a function of the gas flow rate, for a liquid flow rate of about 0.07 m3 h − 1.

rate and the column section) are sufficient to ensure the formation of a uniform liquid film on the packing. This would imply that the superficial velocities required to use correctly the packing with Pb–17Li should be about five times higher than the recommended one by the packing manufacturer (F. Pichon, Sulzer, personal communication), for their reference liquids (0.01 m s − 1 against 3× 10 − 4 –4× 10 − 3 m s − 1). These larger values of the superficial velocity could result from the high superficial tension of our liquid alloy in comparison with those of Sulzer reference liquid (petrochemicals) (about a factor 6). It must be confirmed by working with higher and lower superficial velocities (corresponding respectively to the progressive column flooding and to a liquid film non uniformly distributed on the packing) for which the efficiencies, with the same gas flow rates, should be lower.

3.3. Comparison between a bubble and this packed column The results of similar extraction tests performed in the MELODIE loop with a bubble column were reported by Sample et al. [6]. The bubble column had dimensions similar to those of the packed column (68 cm high and a 13 cm diameter). To compare the present packed column with the bubble column, the height of transfer units is

N. Alpy et al. / Fusion Engineering and Design 39–40 (1998) 787–792

convenient since it expresses the capacity of the extractor to perform the mass transfer: HTU=

V1 a  K1

HTU is reached by calculating the number of transfer units as reported elsewhere [10]. According to the results achieved with the bubble column, at 723 K and with a 0.095 m3 h − 1 Pb–17Li liquid flow rate and a 5.4 × 10 − 3 N m3 h − 1 argon flow rate, HTU was about 7 m. In similar hydrodynamic conditions with the packed column, HTU is close to 2 m while operating at 713 K. Consequently, this packed column would be at least two times smaller than our previous bubble column to reach the same hydrogen extraction performances.

3.4. Tentati6e of extrapolation to the water cooled Pb– 17Li ITER test module In the water cooled Pb – 17Li ITER test module, the nominal flow rate of Pb – 17Li should be 1.43 kg s − 1 (which is only about five times larger than our studied maximum Pb – 17Li flow rate) and the tritium partial pressure in Pb – 17Li extractor inlet, 1000 Pa, as described by Fu¨tterer et al. [11]. A tentative of extrapolation of our results to this test module was done, assuming: 1. A mean corrective factor of 2.8 for Ks and 2. A constant HTU value equal to that deduced from our tests at 713 K (1.7 m). With these hypotheses, the height of the column can be calculated as a function of the extraction efficiency and of the liquid to gas flow rates ratio, using the superficial velocity V1 involved in our contactor (0.01 m s − 1). These calculations indicate that an efficiency of 60% could be achieved by means of a 2 m high packed column of 14 cm diameter and of an extracting gas flow rate of 9 mol h − 1. If we take into account the size of our experimental device (60 cm high and 5.5 cm diameter) and the reached efficiencies (25%), an extrapolation beyond a 60% efficiency is not relevant since it would be too far from our experimental conditions and performances.

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4. Conclusion Extraction tests have been performed with a packed column installed on the MELODIE loop. The tested packing, supplied by Sulzer Company, has allowed to reach efficiencies up to about 25% at 713 K. These performances have been obtained without any preliminary optimisation since this packing is generally used for petrochemical applications. Our physical system is offside of the Sulzer reference data, and in particular the density and the superficial tension of Pb–17Li are unusually high. The results of these tests have shown that the hydrogen mass balance was not correct even if several explanations have been discussed. The main objective of our next work will be to clarify this point. Calculations and extrapolation to ITER device have been performed assuming that the Ks values have to be corrected. An optimisation of the using conditions of the packing will be performed by investigating a larger range of gas and liquid flow rates. Pumps with higher performance to move Pb–17Li will be required and a device to trap Pb-17Li vapours and very fine particles has to be defined to work with higher gas flow rates.

5. Nomenclature L G POS PIE POE CH YH2 Ks FLH FGH

Pb–17Li flow rate (m3 h−1) Extractor Ar flow rate (N m3 h−1) H2 pressure in the membrane at the saturator outlet (Pa) H2 pressure in the membrane at the extractor inlet (Pa) H2 pressure in the membrane at the extractor outlet (Pa) Atomic hydrogen concentration in Pb– 17Li (mol m−3) H2 content in Ar at the extractor outlet (vppm) Sievert’s constant (mol m−3 Pa−1/2) Atomic hydrogen flux in Pb–17Li leaving the extractor (mol h−1) Atomic hydrogen flux in Ar leaving the extractor (mol h−1)

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h HTU V1 a K1

Extractor efficiency Height of transfer unit (m) Superficial velocity (m3 s−1 m−2) Specific gas – liquid contact surface (m2 m−3) mass transfer coefficient (m s−1)

References [1] L. Giancarli, Status and further development of the European liquid metal breeder blanket, presented at Annual Meeting on Nuclear Technology, Mannheim, Germany, May 21 – 23, 1996. [2] G. Pierini, R. Baratti, A.M. Polcaro, P.F. Ricchi, A. Viola, Mass transfer operational units applied to the liquid breeding material of fusion reactors for tritium recovery, Fusion Technol. 8 (1985) 2121–2126. [3] D.K. Sze, Counter current extraction system for tritium recovery from 17Li–83Pb, Fusion Technol. 8 (1985) 887 – 890. [4] C. Malara, Tritium extraction from Pb–17Li by bubble columns, Fusion Technol. 28 (1995) 693–699.

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[5] A. Terlain, T. Sample, M. Futterer, Hydrogen extraction tests from Pb – 17Li using gas – liquid contactors, in: K. Herschbach, W. Maurer, J.E. Vetter (Eds.), Proceedings of the 18th Symposium On Fusion Technology, Karlsruhe 1994, Elsevier, Amsterdam, 1995, pp. 1265 – 1268. [6] T. Sample, A. Terlain, T. Dufrenoy, M. Perrot, Results of the bubble column extractor in MELODIE, CEA Report, RT-SCECF 367, December, 1994. [7] F. Reiter, J. Camposilvan, G. Gervasini, R. Rota, Interaction of hydrogen isotopes with the liquid eutectic alloy 17Li – 83Pb, in: Proceedings of the 14th Symposium on Fusion Technology, Avignon, vol. 2, Pergamon, Oxford, 1986, pp. 1185 – 1190. [8] M. Caorlin, G. Gervasini, F. Reiter, The impact of tritium solubility and diffusivity on inventory and permeation in liquid breeder blanket, Fusion Technol. 14 (1988) 663 – 674. [10] P.C. Wankat, Separations in Chemical Engineering, Equilibrium staged separations, Elsevier, Amsterdam, 1988. [11] M. Futterer, B. Bielak, J.P. Deffain, L. Giancarli, N. Morley, J.F. Salvadory, J. Szczepanski, Design description document (DDD) for the European water-cooled Pb – 17Li test blanket, Chapter 2, CEA Report, Report DMT 96/349, SERMA/LCA/1911, December, 1996.