Research on chemical exchange process of boron isotope separation

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Procedia Engineering

ProcediaProcedia Engineering 00 (2011) 000–000 Engineering 18 (2011) 151 – 156 www.elsevier.com/locate/procedia

The Second SREE Conference on Chemical Engineering

Research on chemical exchange process of boron isotope separation Yiping Huang*, Shuang Cheng, Jiao Xu, Weijiang Zhang School of Chemical Engineering and Technology, Tianjin University, Tianjin300072, China

Abstract The chemical exchange process of boron isotope separation was studied in order to enrich rare stable isotope 10B. Specially, the relations among the pyrolysis temperature, the number of theoretical plates, the exchange reaction time, the enrichment factor and the abundance of 10B were investigated. The conclusion was obtained by experiment that the abundance of 10B could reach to 36.3% with the pyrolysis temperature of 160℃, and the exchange reaction time to 192 hours. It will lay better foundation for the industrial scale production in the next step.

© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering Key words: Chemical exchange; Boron isotope separation; pyrolysis temperature; 10B

1. Introduction Naturally occurring boron consists of 19.8% 10B and 80.2% 11B [1]. 10B isotope is a good absorbent of thermal neutrons with 3837 barns of neutron absorption cross sections, that is more effective about five times than natural boron, and 7.7×104 times than 11B isotope [2]. The excellent features of 10B isotope make it possible to be widely used in most of industries. For examples as follow: 1) Nuclear Industry, 10B isotope was mainly applied in the neutron shielding materials, or with lithium, chromium and other elements made into rods used for the reactor emergency protection [3]. 2) Military Industry. 10B isotope can be made into light weight alloy to ensure the nuclear military equipment running safely under some terrible environment after smelted with some metal materials [4]. 3) Modern Industry. 10B isotope could shield gamma radiations efficiently caused by high-cycle elements. In addition, it was also applied widely in nuclear physics devices based on the following reaction: 10

B  n [11 B] 7 Li  4 He

* Corresponding author. Tel.:+86-022-27402028. E-mail address: [email protected].

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.024

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So, 10B isotope can be made into the neutron counters by capture of a neutron through ion reaction. 4) Medical Medicine Field, 10B isotope compounds can be made into targeted medicine to cure cancer by Boron Neutron Capture Therapy (BNCT) [5]. This method can protect normal cell away from hurting by X ray or other radiations in actinotheraphy. A Japanese journal of “Nikkan Kogyo Shimbun” reported that 10 B isotope as the main component was applied successfully in curing skin cancer by medical practice. In a word, 10B isotope is attracting more and more attention in the world. Back in the Second World War, the research of boron isotope separation begun. Many kinds of process of 10B isotope enrichment was developed such as low temperature rectification of BF3, chemical exchange rectification of BF3, ion exchange of boric acid, and counter-current circulation of BF3 in the series film and so on [6-10]. So far, it has reported in some developed countries that the industrial production of 10B isotope enrichment was realized by virtue of chemical exchange rectification. Nowadays only Sichuan 812 plants and Tianjin University in China 10B isotope enrichment research was studied in the pilot stage, and the abundance of the experimental samples did not meet the product requirement yet [11]. The chemical exchange rectification of boron isotope separation passed through three periods: the first one was the chemical exchange rectification of (C2H5)2O·BF3 complex in UK, and Switzerland prior to 1961. Later, it was replaced by (CH3)2O · BF3 complex because of the decomposition rate of the complex decreasing about 20 times, and the unit production capacity increasing by 100 times, that is the second period. At last, they were all replaced by the chemical exchange rectification of C6H5OCH3 ·BF3 complex as a result of superior operational conditions with normal temperature and pressure [12]. In 1997, Professor Cartagena Cove, worked in the Moscow Mendeleev Institute of Chemical Technology, gave some related lectures to clarify the chemical exchange rectification of boron trifluoride-anisole complex and he pointed out that it was the most practical method of 10B isotope enrichment industrially. In a word, the separation progress of boron isotopes was briefly introduced in this paper, and the better operational conditions were described by virtue of the investigation among the pyrolysis temperature, the number of theoretical plates, the exchange reaction time, the enrichment factor and the abundance of 10B in the progress of chemical exchange rectification. 2. Process overview of boron isotopes separation A schematic diagram of the chemical exchange rectification of boron trifluoride-anisole is presented in Fig.1. It contained four parts: recombiner, isotope chemical exchange column, decomposer and rectification column, which were consisted of different diameter packed column severally, connected by advanced diaphragm metering pump. As follows:

Vent port Sampling port

1.Boron trifluoride cylinder 2.Abosorber 3.Isotope exchang column 4.Decomposer 5.Anisole tanks 6.Rectification column

Fig.1 Process overview of boron isotopes separation

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The recombiner performs the formation of the liquid complex about anisole and BF3 from its corresponding to specialized storage cylinders. The process was followed by the energy release of 51.53KJ/mol, which should be removed to avoid anisole decomposition. Therefore, the recombiner comprised the counter-current column filled with packing supplied with a cooling jacket. Anisole from the upper end of the column took counter-current reaction to BF3 from the bottom in Equ.2. And then, the liquid complex was transported by the diaphragm metering pump into the chemical exchange unit, the key part of the enrichment process, that the 10BF3 pyrolysised from the decomposer took reaction to the anisole· 11BF3 from the upper of the exchange column as follows Equ.3. After repeated exchange process, the one side 10B gradually enriched in the liquid phase, while 11B gradually enriched in the gas phase. Over the above process, pyrolysised anisole should be purified, dehumidified and recycled back into the system. The basic one was the anisole dehumidification up to the water concentration of 0.0030.005%. (2) C6 H 5 OCH 3 (l )  BF3 ( g )  C 6 H 5OCH 3  BF3 (l ) 10

BF3 ( g )  C6 H5OCH3  11 BF3 (l )  11 BF3 ( g )  C6 H5OCH3  10 BF3 (l )

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3. Analysis and discussion 3.1. The effect of the pyrolysis temperature Based on that having more number of theoretical plates, 2*2 θ packing was applied in the experimental device. The operational temperature and pressure was corresponding to 298.15K, 0.02MPa(T = 298.15K; P = 0.02Mpa), and the total rate of cyclic flow was about 1.2L / h. Meantime, the different pyrolysis temperature was set severally at 80,100,120,140,160, and 165℃ in the research, and the relation between the pyrolysis temperature and the abundance of enriched 10B was shown in Fig.2: 27

80℃ 100℃ 120℃ 140℃ 160℃ 165℃

26

B10 abundance/%

25 24 23 22 21 20 19

0

24

48

72

96

120

Sampling time/h

Fig.2 the relation between the pyrolysis temperature and the abundance of enriched 10B

As shown in Fig.2, the pyrolysis temperature was an important operational parameter that has a impact on the abundance of 10B obviously in the chemical exchange reaction. The abundance of enriched 10 B increased gradually with the rising of the pyrolysis temperature, and it obtained better result when the temperature reached to 160℃. Under this conditions, it not only got higher pyrolysis levels of the CH3OC6H5·10BF3 Complexes, but also provided enough 10BF3 gases exchanging with 11B liquid complex into the exchange column. Furthermore, the increase of enriched 10B was not obvious when the pyrolysis temperature got to 165℃. The by-products such as phenol, cresol and other impurities could easily come

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out, which would cause a series of side effects under the higher pyrolysis temperature. So, it hadn’t better choose higher pyrolysis temperature in the process. On the other side, the abundance of 10B was close to the theoretical limit of enrichment with the growth of the exchange time. That was because the driving force of mass transfer decreased gradually, and the separation was more difficult with the growth of exchanging equilibrium time. Therefore, there would be the leveling off of 10B abundance in the relationship curve. 3.2. The relation between the number of theoretical plates and 10B abundance The relation between the number of theoretical plates and the abundance of enriched 10B was treated with the conditions of the pyrolysis temperature 160℃, and the operational pressure 0.02MPa. The analysis between the different height of the exchange column and the abundance of 10B at the same conditions was shown in Fig.3: 38

Column Height of 5m Column Height of 8m Column Height of 11m Column Height of 14m Column Height of 20m

36

B10 abundance/%

34 32 30 28 26 24 22 20 18

0

24

48

72

96

120

144

168

192

Sampling time/h

Fig.3 The relation between number of theoretical plates and 10B abundance According to the usual experience, the number of theoretical plates was reflected in the height of the packing column, and the more number of theoretical plates increased, the easier boron isotopes separated clearly. In Fig.3, the abundance of enriched 10B was greatly enhanced with the increasing of the packing height, and it reached to 36.3% when the packing height was 20 meters. As the balance time reached to a certain level, the abundance of 10B enriched more difficultly due to dramatic increase of mass transfer resistance in the enrichment process. So a higher targeted abundance of 10B became less likely to get owing to this. From another viewpoint, the separation coefficient was so little that the process required considerable the number of theoretical plates in order to achieve the targeted abundance. 3.3. The relation between the exchange time and 10B abundance According to the analysis of above relation curves, it could be concluded that 10B abundance enrichment degree increased with the growth of chemical exchange time. There was a rapid growth trend at the first 96hours, after a certain time, the follow-up 10B enriched abundance became more stable, and the smooth growth trend was coming when the exchange time reached to 192hours. In other words, 10B abundance has got to the limit of theoretical abundance. The reason was that at the beginning of the enrichment, 10B abundance needed to enrich was less and the mass transfer driving force was easily conductive to the positive direction of the exchange reaction. With the growth of 10B abundance, the change of 10B abundance became fewer and fewer both in the bottom and the top of exchange column from Eq.3, and it was close to a limited value, namely the system was in the equilibrium state.

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3.4. The influence of the enrichment factor q Supposed to the concentration of boron isotopes was x0 at the cascade (or column) feed, and increased to xp after the circle of cascade, the enrichment factor was defined as follows: q

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x p /(1  x p ) x0 /(1  x0 )

It showed clearly about a good separation in enrichment, but also led an important role in the isotopes separation research. The analysis of the relation between the enrichment factor and the exchange time was studied with the pyrolysis temperature of 160℃, the positive pressure, and the packing height of 20meters. As shown in Fig.4. 5

Enrichment factor q

4

3

2

1

0

48

96

144

192

240

Sampling time/h

Fig.4 The influence of the enrichment factor q

In Fig.4, the enrichment factor q increased with the growth of the exchange reaction time, and it got to a flat trend when the exchange time was to 192 hours. The conclusion was that the relation was consistent with the trend of 10B enrichment abundance, which met to simulation of least squares methods. So, the enrichment degree of 10B abundance increased obviously both in the bottom and the top of the exchange column at the beginning, so the enrichment factor q rises rapidly. As the system was to an equilibrium state, the change trend of the exchange column reduced to a certain value, and the enrichment factor q was also to be stable. It was just consistent with the changing relation between the exchange time and 10B abundance. 4. Conclusion Based on the experiment and analysis, the pyrolysis temperature was better at 160℃, and in this case higher pyrolysis degree of complex and lower decomposition rate of anisole were all achieved easily. Besides, it indicated that isotopes separation with a less separation factor should require enough number of theoretical plates in order to achieve better separation. Furthermore, the analysis report of impact relations between enrichment factor and 10B enriched abundance was consistent with the impact of exchange time, further guide the follow-up research. References [1] A.A.PALKO, J.S.DRURY. The chemical fractionation of boron isotopes[J]. Advances in Chemistry, American Chemical Society, 1969, 40-41.

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