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TRIEX facility: An experimental loop to test tritium extraction systems from lead lithium
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Fusion Engineering and Design 82 (2007) 2294–2302
TRIEX facility: An experimental loop to test tritium extraction systems from lead lithium A. Aiello a,∗ , A. Ciampichetti b , M. Utili a , G. Benamati a a
ENEA CR Brasimone, Fis Ing, 40032 Camugnano (BO), Italy b Politecnico di Torino, DENER, Torino, Italy Received 28 July 2006; accepted 18 July 2007 Available online 10 September 2007
Abstract The first and main step of the HCLL (helium cooled lithium lead) blanket fuel cycle consists of tritium extraction from the liquid breeder. In DEMO reactor a dedicated system, called TES (tritium extraction system), will be devoted to accomplish this task. Different technologies, belonging to the families of gas–liquid contactors, getters and tritium permeators have been studied in the last years, some of them mainly theoretically, others with much more experimental effort. Gas–liquid contactors, which have been more experimentally studied, showed contradictory results. Plate, spray and bubble columns resulted in a very low-extraction efficiency. On the contrary, packed columns gave better results, although some doubts arise about the accuracy of the experiments performed in the past. However, packed columns seem to be the most attractive technology and for this reason was decided for a further optimisation and testing of their performances. In order to accomplish this task, a dedicated facility called TRIEX (tritium extraction) was designed and built and is now available in ENEA-Brasimone. In TRIEX the extraction efficiency of different packed columns will be tested in a systematic way paying particular attention to the hydrogen monitoring system in gas and liquid metal phase. The design and the main instrumentation of TRIEX facility, as well as the operating modes and the planned experimental campaign are presented in this paper. © 2007 Published by Elsevier B.V. Keywords: Tritium; HCLL; Lead lithium
1. Introduction
∗ Corresponding author. Tel.: +39 0534 801380; fax: +39 0534 801225. E-mail address: [email protected] (A. Aiello).
0920-3796/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2007.07.037
Different systems for tritium extraction from liquid Pb–16Li (TES, tritium extraction system) have been studied and proposed during the past years, being this item of fundamental importance for the whole fuel cycle of a fusion reactor on DEMO scale.
A. Aiello et al. / Fusion Engineering and Design 82 (2007) 2294–2302
Different technologies, mainly belonging to the families of gas–liquid contactors, getters and tritium permeators have been evaluated, some of them mainly theoretically, others with much more experimental effort. Getters and tritium permeators have shown some aspects of potential interests, raising, however, significant questions about their feasibility for this type of application. Gas–liquid contactors, which have been more experimentally studied, have given contradictory results. Plate, spray and bubble columns exhibits a very lowextraction efficiency, although encouraging predictions are coming from different mathematical models. On the contrary, packed columns, tested in Melodie loop [1,2], have given better results, and so far seem to be the most attractive technology for further optimisation and testing, notwithstanding the doubts related to the accuracy in the experiments execution. Moving from this analysis a facility necessary to experimentally study at a significant scale the process of tritium recovery from Pb–16Li has been designed and constructed at ENEA-Brasimone research centre. The design and the main instrumentation of this facility, named TRIEX (tritium extraction), as well
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as the operating modes and the planned experimental campaign will be presented further on.
2. Design of TRIEX The TRIEX layout, as well as its original component, the extractor that adopts a segmented (modular) philosophy, with a filler consisting of waved steel sheets, can be seen in the synoptic P&I and 3D view shown in Figs. 1 and 2. Inside the TRIEX facility, a packed column of variable height will be tested, varying the flow rates both of gas and liquid metal as well as the hydrogen content and its partial pressure, in order to collect an adequate data set to be used for modelling. It would be also interesting to perform an experimental campaign using different numbers and geometries of the extractor modules, in order to demonstrate if they can be effectively employed in series and to verify if the extraction efficiency does not depend, as expected, on the hydrogen concentration at the extractor inlet. TRIEX facility is constituted by two main subsystems: the liquid metal loop and the gas circuit, as shown in Fig. 1. The main features of the two systems are listed below and subsequently described in detail.
Fig. 1. TRIEX synoptic P&I.
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(2) The saturator tank S2. (3) The extraction column S3. The main parts of the gas circuit are as follows: (a) Injector H1 in S2. (b) Hydrogen sensors in gas H2G. 2.1. Recirculation tank S1 The recirculation tank S1, acting also as draining tank, is filled with the alloy Pb–Li in eutectic composition and contains a centrifugal type recirculation pump P, with vertical axis and immersed impeller. The recirculation tank is placed, regarding to the other components of the system, at the lowest level to permit a complete gravity drainage of the liquid metal circuit. Fig. 2. Different views of TRIEX facility. S1: recirculation tank; S2: hydrogen saturator; S3: extractor column.
The liquid metal loop is composed of the following parts: (1) The recirculation tank S1, containing the mechanical pump P.
2.2. Hydrogen saturator S2 The hydrogen saturator is a tubular tank in which the hydrogen is injected, through the injector HI, from the bottom while the liquid metal flows from the top. The layout is shown in Fig. 3, while in Fig. 4 a detail of the injector is represented. The hydrogen concentration in the alloy at the exit of S2 is expected to be in
Fig. 3. TRIEX saturator view.
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Fig. 4. Injector detail.
thermodynamic equilibrium with the hydrogen partial pressure in gas phase, according to Sievert’s law. S2 is configured as an absorption column in which the continuous phase is constituted by the liquid eutectic alloy Pb–16Li at a temperature in the range 620–770 K. The dispersed phase is constituted by a mixture of argon and hydrogen, with a design content of H lower than 5 vol.%. The gas mixture is injected in S2 through a PORAL filter with channels of diameter comprised between 1 and 10 m to ensure small gas bubbles. The PORAL was selected to:
dynamic parameters in the column and taking the equations from [3], to obtain:
• reduce as much as possible the bubble diameter; • ensure a homogeneous gas distribution in the lead lithium counter flow.
1.1 Evaluation of the gas bubble diameter free raising in the continuous phase. 1.2 Analysis of the gas rising speed; this analysis was performed assuming hypothetical column diameter, gas volumetric fraction value and lead lithium mass flow rate. 1.3 Calculation of mass and volumetric flow rates of the dispersed phase. 1.4 Check of the column diameter, dispersed phase fraction, gas and lead lithium flow rates.
In the upper dome the gas, passing through the lead lithium, realises an atmosphere at a certain pressure to ensure a stable equilibrium between the levels of the three tanks. A fluid-dynamic analysis was conducted in order to define the most suitable combination of heights and diameters of the column to optimise the saturation, possibly with a single pass in the column itself. An iterative procedure, which will be detailed later on, was used, starting from the definition of the fluid-
• The diameter of the column. • The dispersed (gas) and continuous (lead lithium) phase fractions. • The lead lithium mass flow rate. • The gas mixture (Ar + H2 ) mass flow rate. The abovementioned iterative procedure consists of the following steps:
Starting from point 1.2, the procedure shall be repeated up to the convergence of values. To determine the height of the column, a different procedure has been adopted, consisting in the following steps:
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2.1 An hypothetical value of the lead lithium level in the column (assumed as full height of the column) is fixed. 2.2 The inlet gas pressure is evaluated, as the pressure in the S2 dome plus the hydrostatic head of lead lithium. 2.3 The partial pressure of hydrogen in the gas phase is, therefore, determined, moving from the molar fraction of hydrogen in the inlet gas stream. 2.4 The gas and lead lithium inlet and outlet conditions are calculated as follows: • The H molar concentration in Pb–16Li at the column inlet is assumed to be zero. • The H2 molar concentration in the gas phase is assumed by design to be 5 vol.%, supposing to have the lead lithium hydrogen saturation in one pass. • The H molar concentration in the Pb–16Li at the column outlet is given by the Sievert’s equation, assuming to have liquid metal saturated in hydrogen. • The molar concentration of H2 in the gas phase at the column outlet is determined, as detailed in the paragraph, using the so-called working line. 2.5 The height of the column is, then, calculated. 2.6 The procedure shall be iteratively repeated up to the coincidence of the Pb–16Li starting and obtained hydrostatic head. 2.7 If the gas and lead lithium flow rates obtained are too high or low, the procedure is repeated starting from point 1.2.
• Gas superficial velocity inside the column: ug ∼ = 5.76 mm/s.
The column dimensioning was performed assuming an operative temperature of 723 K. The results of the procedure are reported further on:
where PMl is the molar weight of lead lithium, ρl its density and CH is the hydrogen molar concentration in lead lithium. The molar ratio between atomic hydrogen and lead lithium is x X= (4) 1−x
• • • • • • • • • •
Operative temperature of the column: T ∼ = 723 K. Internal diameter of the column: Db ∼ = 10.22 cm. Volume fraction of dispersed phase: α ∼ = 3.5%. Pb–16Li mass flow rate: Γ ∼ = 0.2 kg/s. Gas mass flow rate: Γ ∼ = 3.93E−02 kg/s. Pb–16Li volumetric flow rate: Q ∼ = 81.4 l/h. Gas volumetric flow rate: Q ∼ = 83.1 N l/h. Pb–16Li velocity in the column: vl ∼ = 2.83 mm/s. Gas velocity inside the column: vg ∼ = 160 mm/s. Pb–16Li superficial velocity inside the column: ul ∼ = 2.73 mm/s.
The Reynolds’ number obtained in correspondence to the reported parameters is Re ∼ = 120: a laminar flow regime is verified, and the assumption of single bubble regime is correct in determining Db . The H concentration in equilibrium conditions was evaluated using the Sievert’s equation, keeping in mind that hydrogen is in molecular form in gas phase and in atomic form in lead lithium: √ CH = KS pH2 (1) where CH is the hydrogen concentration at equilibrium in moles in Pb–16Li (mol/m3 ), pH2 the partial pressure of molecular gas at the column inlet and KS is the Sievert’s coefficient determined by Aiello et al. [4]: KS = 10.353 e−36447.74/RT
(2)
Using the data listed below: • T → operative temperature: 723 K; • R → 8.3144 J/mol K. From Eqs. (1) and (2) the following results have been obtained: • KS ∼ = 0.024 mol/(m3 Pa1/2 ); • CH ∼ = 1.95 molH /m3 . The molar fraction x of hydrogen in the continuous phase can be estimated with the formula: x=
CH PMl ρl
(3)
Using Eqs. (3) and (4) it is possible to determine x and X at the outlet of the column: • xout = 3.86E−05 molH /moltot ; • Xout = 3.86E−05 molH /molPb–16Li . The dimensioning of the column gave a minimum head of lead lithium respect to saturation conditions of about 5.6 cm.
A. Aiello et al. / Fusion Engineering and Design 82 (2007) 2294–2302
For the lead alloy head, instead of the abovementioned obtained value a different dimension has been adopted, to have a more flexible management of the operative parameters, i.e. gas and lead lithium mass flow rates, temperature or hydrogen concentration. In more detail, being the 5% of hydrogen concentration in argon extremely high, it has been chosen to adopt a head about six times higher than the calculated value to obtain a one pass saturation with less than 1% of hydrogen content in Ar, a positive aspect, this one, also for safety reasons. The following real geometry, therefore, has been adopted: • internal diameter: 10.2 cm; • lead alloy head: 35 cm. Having a head of 35 cm and a hydrogen concentration of 5% it is also possible to adopt the following reported parameters: • • • • • •
ptot = pdome + ρPb–16Li gh ∼ = 1.30E+05 Pa. pAr = xptot ∼ = 1.24E+05 Pa. pH = (1 − x)ptot ∼ = 6.50E+03 Pa. CH ∼ = 1.95 mol/m3 . Xout ∼ = 3.86E−05 molH /molPb–16Li . Yout ∼ = 6.54E−03 molH2 /molAr .
where x is the molar fraction of argon. The characteristic fluid-dynamic parameters are as follows:
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• Operative temperature: 720 K. • Volume fraction occupied by the dispersed phase: 3.5%. • Argon–hydrogen flow rate (gas injector): 10–100 N l/h. • Lead lithium mass flow rate: 0.2 kg/s. • Lead lithium volumetric flow rate: 81.4 l/h. • Lead lithium velocity inside the column: 2.83 mm/s. • Gas velocity inside the column: 160 mm/s. 2.3. Extractor column S3 The extractor column, used for the stripping of the hydrogen contained in the eutectic alloy Pb–16Li, is of the filled type, in counter flow. The liquid phase, represented by the Pb–16Li alloy, enters in the column from the top, passing through the filler, in hydrogensaturated conditions. The real hydrogen content is read by a hydrogen sensor in liquid metal. The gaseous phase, represented by pure argon, is injected in the column from the bottom through an appropriate system of distribution that has the function to uniform and fragment the gas bubbles. Such system is constituted by a PORAL sintered steel disc. The lower part, with the gas injector and a filler module, is shown in Fig. 5. The disc is characterized by porosity with diameter between 1 and 10 m. The top part of the stripping column is a dome, devoted to
Fig. 5. Detail of filler module and gas injector.
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collect the gas after its passage through the liquid phase.
Concentrating on the diameter of the column, it can be stated that its value is of basic importance because it has:
2.3.1. Design criteria A detailed fluid-dynamic analysis of the gas/liquid system in the column has been carried out to design the column and evaluate the expected extraction efficiency corresponding to a certain height of the column itself. The extraction efficiency has been defined as the ratio between the hydrogen content variation passing through the column and the inlet hydrogen concentration in lead lithium. Analysing the column fluid dynamic [3] it has been possible to determine:
1. to ensure a continuous and uniform circulation of gas and liquid metal; 2. to be large enough to reduce gas pressure drops.
• • • •
the type of filler; the hydraulic section of the column; the optimum Ar mass flow rate; the hydrogen extraction efficiency. The following procedure has been adopted:
1.1 Assumption of the characteristic diameter of the filler. 1.2 Calculation of the diameter of the column under the hypothesis to avoid any flooding effect (this calculation has been carried out iteratively optimising both the fluid dynamic and the column extraction efficiency). 1.3 Evaluation of the minimum lead lithium flow rate necessary to wet the selected type of filler. 1.4 Evaluation of the maximum liquid mass flow rate to produce the flooding effect. 1.5 Evaluation, for the given filler, of the gas pressure drop in the column. The efficiency of the column has been, then, evaluated as follows: 2.1 The height of the filler has been fixed. 2.2 The inlet gas pressure has been determined, being approximately equal to the pressure in the S3 dome plus the lead lithium head. The pressure in S3 dome is almost the same as in S2 bottom. 2.3 The hydrogen concentration at S3 inlet has been fixed, being the same one calculated at S2 outlet. 2.4 The hydrogen content in lead lithium at S3 outlet has been evaluated. The operative temperature was fixed at 723 K.
A careful and detailed fluid-dynamic analysis, therefore, has been carried out to calculate the column diameter. Starting form the hypothesis of a liquid metal flow rate high enough to wet the filler and adopting reduced gas mass flow rates the flow regime in the column will be turbulent and the gas pressure drops will be proportional, roughly, to G1.8 , were G is the gas mass flow rate. Increasing the gas flow rate this condition will be respected up to the loading point after which the pressure drops will increase rapidly. The exponent of G will be higher than 1.8. The fluid-dynamic behaviour will, then, remain the same up to the flooding point, at which the gas flow rate will be so high to obstruct the liquid metal flow, and a mixture of gas and liquid metal will exit from the gas line on the top of the column. The working point of the column is close to the loading point, and the diameter of the column can be evaluated from the flow rate Gf at which the flooding condition starts. Once the flooding flow rate Gf is known, the operative flow rate of the column G can be determined imposing a gas flow rate lower than the flooding flow rate, for example: G = 60%Gf Considering the reference filler and assuming a pure argon gas flow rate of about 5.00E−05 kg/s, corresponding to 100 N l/h, the result is that: • The flooding gas flux is about 5.50E−02 kg/m2 s (1.11E+05 N l/m2 h). • The specific gas flux, assumed lower of about 60% of the flooding flow rate, is equal to 6.64E+04 N l/m2 h. • The column diameter shall be larger than 5 cm. To guarantee a stable operation of the column, considering the errors introduced in calculations, and the fact that the abovementioned relationships are mainly used with light liquids (water, oils) a value of 12.8 cm,
A. Aiello et al. / Fusion Engineering and Design 82 (2007) 2294–2302
two times higher than the one determined, has been adopted. Focusing on the liquid mass flow rate in the extraction column, it can be stated that it strongly affects the contact surface involved in the mass transfer process. If the liquid flow rate is too low, the column will became unstable, if it is too high the loading effect will appear. The liquid flow rate, therefore, shall be varied between those limits to have a stable operation of the column. The minimum and maximum flow rates of lead lithium have been fixed according to the indications given by the filler manufacturer. More in detail, assuming to use a structured filler Baretti B1-350 type, the range of operative flow rates is between 0.2 and 250 m3 /m2 h. Considering the column diameter previously defined the mass flow rate of lead lithium is • minimum: 5.60E−03 kg/s; • maximum: 4.20E+00 kg/s. The results obtained during the design of the column are summarised in the following list: • • • • • • • •
Internal diameter of the column: D ∼ = 12.8 cm. Void factor of the filler: 95%. Specific surface of the filler: 350 m2 /m3 . Mass flow rate of Pb–16Li: Γ ∼ = 0.2 kg/s. Mass flow rate of argon: Γ ∼ 5.00E−02 kg/s. = Volumetric flow rate of Pb–16Li: Q ∼ 80.8 l/h. = Volumetric flow rate of gas: Q ∼ 100 N l/h. = Gas pressure drops: p ∼ = 1 Pa.
Focusing the attention, finally, on the column efficiency, the design procedure can be summarised as follows: 1. The column efficiency is defined as the ratio between the difference in hydrogen concentration in lead lithium from inlet to outlet and the inlet concentration. 2. From the assumed efficiency it is possible to evaluate the outlet hydrogen concentration in lead lithium. 3. Knowing the difference in hydrogen concentration in lead lithium from the inlet to the outlet of the stripping column and the gas and lead lithium flow rates it is possible to determine the hydrogen con-
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centration in gas phase coming out from S3. 4. Using the mass balance equations at interfaces and the Sievert’s equation, hydrogen concentrations at interfaces in liquid and gas phase are calculated. 5. The hydrogen concentration in both phases at S3 outlet can be calculated by means of concentration differential equations. 6. The column efficiency can be finally determined and used as starting value for an iterative calculation from point 2 up to the convergence of values. Using this procedure, the column efficiency has been estimated to be 25%. Being the value determined above coherent with the design assumptions, the geometry of the component can be fixed: • column diameter: 12.8 cm; • height of filler: 80 cm. The design and operating conditions of the stripping column are hereafter summarised: • Efficiency of about 25%.1 • Maximum operative pressure of 2.21 bar in the upper dome and 3.50 bar in the bottom of the column. • Design pressure of 7 bar. • Operative temperature between 620 and 770 K. • Design temperature of 800 K. The extraction column is constituted by standard removable modules, to vary the column height. In such a way it will be possible to experimentally evaluate the column efficiency varying the exchange surface between phases. With one module the height of the column will be 20 cm while installing all the four modules the total height will be the design one of 80 cm.
3. Experimental activities to be carried out The main objective to be reached in TRIEX facility is to verify the efficiency of the hydrogen extraction system in operating conditions relevant for the European Breeding Blanket Test Facility (EBBTF). 1 It is useful to point out that this value was obtained with a draft estimation, as the adopted filler modules were never operated in lead lithium and empirical parameters used in the design are merely hypothetical.
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The foreseen operating conditions are as follows: • • • •
Temperature range of lead lithium: 620–770 K. Lead lithium flow rate: maximum 0.2 kg/s. Gas stripping flow rate range (S3): 5–120 N l/h. Argon–hydrogen flow rate (gas injector): 10–100 N l/h. • Maximum hydrogen concentration in the Ar–H flow rate (gas injector): 5 vol.%. The loop will be operated in isothermal mode, at a maximum temperature of 770 K. Saturator includes a PORAL diffuser to ensure small and dispersed argon and hydrogen bubbles. It is assumed that saturation should be achieved in one pass. The hydrogen concentration at the gas outlet of the extractor will be measured by thermal conductivity sensors. Three permeation hydrogen sensors are foreseen to measure hydrogen content in lead lithium at the extractor inlet, outlet and at the saturator outlet.
S1, S2 and S3, and a common discharge pressure, the atmospheric one. A continuous operating mode needs a rigorous remote control of the pressure in the tanks dome, managing the inlet and outlet gas flows. The experimental campaign will start in September 2006 after the completion of the qualification phase.
Acknowledgements This work, supported by the European Communities under the contract of Association between EURATOM/ENEA, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
References 4. Conclusion The design was based on the diffusivity data of Reiter [5] confirmed by the LEDI/SOLE experiment and on solubility data coming form SOLE experiment conducted in ENEA [4]. The design was optimised to be compatible, in the future, with an installation in the EBBTF (European Breeding Blanket Test Facility) that will be operative in ENEA Brasimone at the end of 2007. It has to be pointed out that the loop is operated having three different internal pressures, in
[1] N. Alpy, T. Dufrenoy, A. Terlain, Hydrogen extraction from Pb–17Li: tests with a packed column, Fusion Eng. Des. 39–40 (1998) 787–792. [2] N. Alpy, A. Terlain, V. Lorentz, Hydrogen extraction from Pb–17Li: results with 800 mm high packed column, Fusion Eng. Des. 49–50 (2000) 775–780. [3] R.B. Perry, C.H. Chilton, Chemical Engineers’ Handbook, McGraw-Hill, NY, 1973. [4] A. Aiello, A. Ciampichetti, G. Benamati, Determination of hydrogen solubility in lead lithium using Sole device, Fusion Eng. Des. 81 (2006) 639–644. [5] F. Reiter, Solubility and diffusivity of hydrogen isotopes in liquid Pb–17Li, Fusion Eng. Des. 14 (1991) 207–211.
Fusion Engineering and Design 82 (2007) 2294–2302
TRIEX facility: An experimental loop to test tritium extraction systems from lead lithium A. Aiello a,∗ , A. Ciampichetti b , M. Utili a , G. Benamati a a
ENEA CR Brasimone, Fis Ing, 40032 Camugnano (BO), Italy b Politecnico di Torino, DENER, Torino, Italy Received 28 July 2006; accepted 18 July 2007 Available online 10 September 2007
Abstract The first and main step of the HCLL (helium cooled lithium lead) blanket fuel cycle consists of tritium extraction from the liquid breeder. In DEMO reactor a dedicated system, called TES (tritium extraction system), will be devoted to accomplish this task. Different technologies, belonging to the families of gas–liquid contactors, getters and tritium permeators have been studied in the last years, some of them mainly theoretically, others with much more experimental effort. Gas–liquid contactors, which have been more experimentally studied, showed contradictory results. Plate, spray and bubble columns resulted in a very low-extraction efficiency. On the contrary, packed columns gave better results, although some doubts arise about the accuracy of the experiments performed in the past. However, packed columns seem to be the most attractive technology and for this reason was decided for a further optimisation and testing of their performances. In order to accomplish this task, a dedicated facility called TRIEX (tritium extraction) was designed and built and is now available in ENEA-Brasimone. In TRIEX the extraction efficiency of different packed columns will be tested in a systematic way paying particular attention to the hydrogen monitoring system in gas and liquid metal phase. The design and the main instrumentation of TRIEX facility, as well as the operating modes and the planned experimental campaign are presented in this paper. © 2007 Published by Elsevier B.V. Keywords: Tritium; HCLL; Lead lithium
1. Introduction
∗ Corresponding author. Tel.: +39 0534 801380; fax: +39 0534 801225. E-mail address: [email protected] (A. Aiello).
0920-3796/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2007.07.037
Different systems for tritium extraction from liquid Pb–16Li (TES, tritium extraction system) have been studied and proposed during the past years, being this item of fundamental importance for the whole fuel cycle of a fusion reactor on DEMO scale.
A. Aiello et al. / Fusion Engineering and Design 82 (2007) 2294–2302
Different technologies, mainly belonging to the families of gas–liquid contactors, getters and tritium permeators have been evaluated, some of them mainly theoretically, others with much more experimental effort. Getters and tritium permeators have shown some aspects of potential interests, raising, however, significant questions about their feasibility for this type of application. Gas–liquid contactors, which have been more experimentally studied, have given contradictory results. Plate, spray and bubble columns exhibits a very lowextraction efficiency, although encouraging predictions are coming from different mathematical models. On the contrary, packed columns, tested in Melodie loop [1,2], have given better results, and so far seem to be the most attractive technology for further optimisation and testing, notwithstanding the doubts related to the accuracy in the experiments execution. Moving from this analysis a facility necessary to experimentally study at a significant scale the process of tritium recovery from Pb–16Li has been designed and constructed at ENEA-Brasimone research centre. The design and the main instrumentation of this facility, named TRIEX (tritium extraction), as well
2295
as the operating modes and the planned experimental campaign will be presented further on.
2. Design of TRIEX The TRIEX layout, as well as its original component, the extractor that adopts a segmented (modular) philosophy, with a filler consisting of waved steel sheets, can be seen in the synoptic P&I and 3D view shown in Figs. 1 and 2. Inside the TRIEX facility, a packed column of variable height will be tested, varying the flow rates both of gas and liquid metal as well as the hydrogen content and its partial pressure, in order to collect an adequate data set to be used for modelling. It would be also interesting to perform an experimental campaign using different numbers and geometries of the extractor modules, in order to demonstrate if they can be effectively employed in series and to verify if the extraction efficiency does not depend, as expected, on the hydrogen concentration at the extractor inlet. TRIEX facility is constituted by two main subsystems: the liquid metal loop and the gas circuit, as shown in Fig. 1. The main features of the two systems are listed below and subsequently described in detail.
Fig. 1. TRIEX synoptic P&I.
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(2) The saturator tank S2. (3) The extraction column S3. The main parts of the gas circuit are as follows: (a) Injector H1 in S2. (b) Hydrogen sensors in gas H2G. 2.1. Recirculation tank S1 The recirculation tank S1, acting also as draining tank, is filled with the alloy Pb–Li in eutectic composition and contains a centrifugal type recirculation pump P, with vertical axis and immersed impeller. The recirculation tank is placed, regarding to the other components of the system, at the lowest level to permit a complete gravity drainage of the liquid metal circuit. Fig. 2. Different views of TRIEX facility. S1: recirculation tank; S2: hydrogen saturator; S3: extractor column.
The liquid metal loop is composed of the following parts: (1) The recirculation tank S1, containing the mechanical pump P.
2.2. Hydrogen saturator S2 The hydrogen saturator is a tubular tank in which the hydrogen is injected, through the injector HI, from the bottom while the liquid metal flows from the top. The layout is shown in Fig. 3, while in Fig. 4 a detail of the injector is represented. The hydrogen concentration in the alloy at the exit of S2 is expected to be in
Fig. 3. TRIEX saturator view.
A. Aiello et al. / Fusion Engineering and Design 82 (2007) 2294–2302
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Fig. 4. Injector detail.
thermodynamic equilibrium with the hydrogen partial pressure in gas phase, according to Sievert’s law. S2 is configured as an absorption column in which the continuous phase is constituted by the liquid eutectic alloy Pb–16Li at a temperature in the range 620–770 K. The dispersed phase is constituted by a mixture of argon and hydrogen, with a design content of H lower than 5 vol.%. The gas mixture is injected in S2 through a PORAL filter with channels of diameter comprised between 1 and 10 m to ensure small gas bubbles. The PORAL was selected to:
dynamic parameters in the column and taking the equations from [3], to obtain:
• reduce as much as possible the bubble diameter; • ensure a homogeneous gas distribution in the lead lithium counter flow.
1.1 Evaluation of the gas bubble diameter free raising in the continuous phase. 1.2 Analysis of the gas rising speed; this analysis was performed assuming hypothetical column diameter, gas volumetric fraction value and lead lithium mass flow rate. 1.3 Calculation of mass and volumetric flow rates of the dispersed phase. 1.4 Check of the column diameter, dispersed phase fraction, gas and lead lithium flow rates.
In the upper dome the gas, passing through the lead lithium, realises an atmosphere at a certain pressure to ensure a stable equilibrium between the levels of the three tanks. A fluid-dynamic analysis was conducted in order to define the most suitable combination of heights and diameters of the column to optimise the saturation, possibly with a single pass in the column itself. An iterative procedure, which will be detailed later on, was used, starting from the definition of the fluid-
• The diameter of the column. • The dispersed (gas) and continuous (lead lithium) phase fractions. • The lead lithium mass flow rate. • The gas mixture (Ar + H2 ) mass flow rate. The abovementioned iterative procedure consists of the following steps:
Starting from point 1.2, the procedure shall be repeated up to the convergence of values. To determine the height of the column, a different procedure has been adopted, consisting in the following steps:
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2.1 An hypothetical value of the lead lithium level in the column (assumed as full height of the column) is fixed. 2.2 The inlet gas pressure is evaluated, as the pressure in the S2 dome plus the hydrostatic head of lead lithium. 2.3 The partial pressure of hydrogen in the gas phase is, therefore, determined, moving from the molar fraction of hydrogen in the inlet gas stream. 2.4 The gas and lead lithium inlet and outlet conditions are calculated as follows: • The H molar concentration in Pb–16Li at the column inlet is assumed to be zero. • The H2 molar concentration in the gas phase is assumed by design to be 5 vol.%, supposing to have the lead lithium hydrogen saturation in one pass. • The H molar concentration in the Pb–16Li at the column outlet is given by the Sievert’s equation, assuming to have liquid metal saturated in hydrogen. • The molar concentration of H2 in the gas phase at the column outlet is determined, as detailed in the paragraph, using the so-called working line. 2.5 The height of the column is, then, calculated. 2.6 The procedure shall be iteratively repeated up to the coincidence of the Pb–16Li starting and obtained hydrostatic head. 2.7 If the gas and lead lithium flow rates obtained are too high or low, the procedure is repeated starting from point 1.2.
• Gas superficial velocity inside the column: ug ∼ = 5.76 mm/s.
The column dimensioning was performed assuming an operative temperature of 723 K. The results of the procedure are reported further on:
where PMl is the molar weight of lead lithium, ρl its density and CH is the hydrogen molar concentration in lead lithium. The molar ratio between atomic hydrogen and lead lithium is x X= (4) 1−x
• • • • • • • • • •
Operative temperature of the column: T ∼ = 723 K. Internal diameter of the column: Db ∼ = 10.22 cm. Volume fraction of dispersed phase: α ∼ = 3.5%. Pb–16Li mass flow rate: Γ ∼ = 0.2 kg/s. Gas mass flow rate: Γ ∼ = 3.93E−02 kg/s. Pb–16Li volumetric flow rate: Q ∼ = 81.4 l/h. Gas volumetric flow rate: Q ∼ = 83.1 N l/h. Pb–16Li velocity in the column: vl ∼ = 2.83 mm/s. Gas velocity inside the column: vg ∼ = 160 mm/s. Pb–16Li superficial velocity inside the column: ul ∼ = 2.73 mm/s.
The Reynolds’ number obtained in correspondence to the reported parameters is Re ∼ = 120: a laminar flow regime is verified, and the assumption of single bubble regime is correct in determining Db . The H concentration in equilibrium conditions was evaluated using the Sievert’s equation, keeping in mind that hydrogen is in molecular form in gas phase and in atomic form in lead lithium: √ CH = KS pH2 (1) where CH is the hydrogen concentration at equilibrium in moles in Pb–16Li (mol/m3 ), pH2 the partial pressure of molecular gas at the column inlet and KS is the Sievert’s coefficient determined by Aiello et al. [4]: KS = 10.353 e−36447.74/RT
(2)
Using the data listed below: • T → operative temperature: 723 K; • R → 8.3144 J/mol K. From Eqs. (1) and (2) the following results have been obtained: • KS ∼ = 0.024 mol/(m3 Pa1/2 ); • CH ∼ = 1.95 molH /m3 . The molar fraction x of hydrogen in the continuous phase can be estimated with the formula: x=
CH PMl ρl
(3)
Using Eqs. (3) and (4) it is possible to determine x and X at the outlet of the column: • xout = 3.86E−05 molH /moltot ; • Xout = 3.86E−05 molH /molPb–16Li . The dimensioning of the column gave a minimum head of lead lithium respect to saturation conditions of about 5.6 cm.
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For the lead alloy head, instead of the abovementioned obtained value a different dimension has been adopted, to have a more flexible management of the operative parameters, i.e. gas and lead lithium mass flow rates, temperature or hydrogen concentration. In more detail, being the 5% of hydrogen concentration in argon extremely high, it has been chosen to adopt a head about six times higher than the calculated value to obtain a one pass saturation with less than 1% of hydrogen content in Ar, a positive aspect, this one, also for safety reasons. The following real geometry, therefore, has been adopted: • internal diameter: 10.2 cm; • lead alloy head: 35 cm. Having a head of 35 cm and a hydrogen concentration of 5% it is also possible to adopt the following reported parameters: • • • • • •
ptot = pdome + ρPb–16Li gh ∼ = 1.30E+05 Pa. pAr = xptot ∼ = 1.24E+05 Pa. pH = (1 − x)ptot ∼ = 6.50E+03 Pa. CH ∼ = 1.95 mol/m3 . Xout ∼ = 3.86E−05 molH /molPb–16Li . Yout ∼ = 6.54E−03 molH2 /molAr .
where x is the molar fraction of argon. The characteristic fluid-dynamic parameters are as follows:
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• Operative temperature: 720 K. • Volume fraction occupied by the dispersed phase: 3.5%. • Argon–hydrogen flow rate (gas injector): 10–100 N l/h. • Lead lithium mass flow rate: 0.2 kg/s. • Lead lithium volumetric flow rate: 81.4 l/h. • Lead lithium velocity inside the column: 2.83 mm/s. • Gas velocity inside the column: 160 mm/s. 2.3. Extractor column S3 The extractor column, used for the stripping of the hydrogen contained in the eutectic alloy Pb–16Li, is of the filled type, in counter flow. The liquid phase, represented by the Pb–16Li alloy, enters in the column from the top, passing through the filler, in hydrogensaturated conditions. The real hydrogen content is read by a hydrogen sensor in liquid metal. The gaseous phase, represented by pure argon, is injected in the column from the bottom through an appropriate system of distribution that has the function to uniform and fragment the gas bubbles. Such system is constituted by a PORAL sintered steel disc. The lower part, with the gas injector and a filler module, is shown in Fig. 5. The disc is characterized by porosity with diameter between 1 and 10 m. The top part of the stripping column is a dome, devoted to
Fig. 5. Detail of filler module and gas injector.
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collect the gas after its passage through the liquid phase.
Concentrating on the diameter of the column, it can be stated that its value is of basic importance because it has:
2.3.1. Design criteria A detailed fluid-dynamic analysis of the gas/liquid system in the column has been carried out to design the column and evaluate the expected extraction efficiency corresponding to a certain height of the column itself. The extraction efficiency has been defined as the ratio between the hydrogen content variation passing through the column and the inlet hydrogen concentration in lead lithium. Analysing the column fluid dynamic [3] it has been possible to determine:
1. to ensure a continuous and uniform circulation of gas and liquid metal; 2. to be large enough to reduce gas pressure drops.
• • • •
the type of filler; the hydraulic section of the column; the optimum Ar mass flow rate; the hydrogen extraction efficiency. The following procedure has been adopted:
1.1 Assumption of the characteristic diameter of the filler. 1.2 Calculation of the diameter of the column under the hypothesis to avoid any flooding effect (this calculation has been carried out iteratively optimising both the fluid dynamic and the column extraction efficiency). 1.3 Evaluation of the minimum lead lithium flow rate necessary to wet the selected type of filler. 1.4 Evaluation of the maximum liquid mass flow rate to produce the flooding effect. 1.5 Evaluation, for the given filler, of the gas pressure drop in the column. The efficiency of the column has been, then, evaluated as follows: 2.1 The height of the filler has been fixed. 2.2 The inlet gas pressure has been determined, being approximately equal to the pressure in the S3 dome plus the lead lithium head. The pressure in S3 dome is almost the same as in S2 bottom. 2.3 The hydrogen concentration at S3 inlet has been fixed, being the same one calculated at S2 outlet. 2.4 The hydrogen content in lead lithium at S3 outlet has been evaluated. The operative temperature was fixed at 723 K.
A careful and detailed fluid-dynamic analysis, therefore, has been carried out to calculate the column diameter. Starting form the hypothesis of a liquid metal flow rate high enough to wet the filler and adopting reduced gas mass flow rates the flow regime in the column will be turbulent and the gas pressure drops will be proportional, roughly, to G1.8 , were G is the gas mass flow rate. Increasing the gas flow rate this condition will be respected up to the loading point after which the pressure drops will increase rapidly. The exponent of G will be higher than 1.8. The fluid-dynamic behaviour will, then, remain the same up to the flooding point, at which the gas flow rate will be so high to obstruct the liquid metal flow, and a mixture of gas and liquid metal will exit from the gas line on the top of the column. The working point of the column is close to the loading point, and the diameter of the column can be evaluated from the flow rate Gf at which the flooding condition starts. Once the flooding flow rate Gf is known, the operative flow rate of the column G can be determined imposing a gas flow rate lower than the flooding flow rate, for example: G = 60%Gf Considering the reference filler and assuming a pure argon gas flow rate of about 5.00E−05 kg/s, corresponding to 100 N l/h, the result is that: • The flooding gas flux is about 5.50E−02 kg/m2 s (1.11E+05 N l/m2 h). • The specific gas flux, assumed lower of about 60% of the flooding flow rate, is equal to 6.64E+04 N l/m2 h. • The column diameter shall be larger than 5 cm. To guarantee a stable operation of the column, considering the errors introduced in calculations, and the fact that the abovementioned relationships are mainly used with light liquids (water, oils) a value of 12.8 cm,
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two times higher than the one determined, has been adopted. Focusing on the liquid mass flow rate in the extraction column, it can be stated that it strongly affects the contact surface involved in the mass transfer process. If the liquid flow rate is too low, the column will became unstable, if it is too high the loading effect will appear. The liquid flow rate, therefore, shall be varied between those limits to have a stable operation of the column. The minimum and maximum flow rates of lead lithium have been fixed according to the indications given by the filler manufacturer. More in detail, assuming to use a structured filler Baretti B1-350 type, the range of operative flow rates is between 0.2 and 250 m3 /m2 h. Considering the column diameter previously defined the mass flow rate of lead lithium is • minimum: 5.60E−03 kg/s; • maximum: 4.20E+00 kg/s. The results obtained during the design of the column are summarised in the following list: • • • • • • • •
Internal diameter of the column: D ∼ = 12.8 cm. Void factor of the filler: 95%. Specific surface of the filler: 350 m2 /m3 . Mass flow rate of Pb–16Li: Γ ∼ = 0.2 kg/s. Mass flow rate of argon: Γ ∼ 5.00E−02 kg/s. = Volumetric flow rate of Pb–16Li: Q ∼ 80.8 l/h. = Volumetric flow rate of gas: Q ∼ 100 N l/h. = Gas pressure drops: p ∼ = 1 Pa.
Focusing the attention, finally, on the column efficiency, the design procedure can be summarised as follows: 1. The column efficiency is defined as the ratio between the difference in hydrogen concentration in lead lithium from inlet to outlet and the inlet concentration. 2. From the assumed efficiency it is possible to evaluate the outlet hydrogen concentration in lead lithium. 3. Knowing the difference in hydrogen concentration in lead lithium from the inlet to the outlet of the stripping column and the gas and lead lithium flow rates it is possible to determine the hydrogen con-
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centration in gas phase coming out from S3. 4. Using the mass balance equations at interfaces and the Sievert’s equation, hydrogen concentrations at interfaces in liquid and gas phase are calculated. 5. The hydrogen concentration in both phases at S3 outlet can be calculated by means of concentration differential equations. 6. The column efficiency can be finally determined and used as starting value for an iterative calculation from point 2 up to the convergence of values. Using this procedure, the column efficiency has been estimated to be 25%. Being the value determined above coherent with the design assumptions, the geometry of the component can be fixed: • column diameter: 12.8 cm; • height of filler: 80 cm. The design and operating conditions of the stripping column are hereafter summarised: • Efficiency of about 25%.1 • Maximum operative pressure of 2.21 bar in the upper dome and 3.50 bar in the bottom of the column. • Design pressure of 7 bar. • Operative temperature between 620 and 770 K. • Design temperature of 800 K. The extraction column is constituted by standard removable modules, to vary the column height. In such a way it will be possible to experimentally evaluate the column efficiency varying the exchange surface between phases. With one module the height of the column will be 20 cm while installing all the four modules the total height will be the design one of 80 cm.
3. Experimental activities to be carried out The main objective to be reached in TRIEX facility is to verify the efficiency of the hydrogen extraction system in operating conditions relevant for the European Breeding Blanket Test Facility (EBBTF). 1 It is useful to point out that this value was obtained with a draft estimation, as the adopted filler modules were never operated in lead lithium and empirical parameters used in the design are merely hypothetical.
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The foreseen operating conditions are as follows: • • • •
Temperature range of lead lithium: 620–770 K. Lead lithium flow rate: maximum 0.2 kg/s. Gas stripping flow rate range (S3): 5–120 N l/h. Argon–hydrogen flow rate (gas injector): 10–100 N l/h. • Maximum hydrogen concentration in the Ar–H flow rate (gas injector): 5 vol.%. The loop will be operated in isothermal mode, at a maximum temperature of 770 K. Saturator includes a PORAL diffuser to ensure small and dispersed argon and hydrogen bubbles. It is assumed that saturation should be achieved in one pass. The hydrogen concentration at the gas outlet of the extractor will be measured by thermal conductivity sensors. Three permeation hydrogen sensors are foreseen to measure hydrogen content in lead lithium at the extractor inlet, outlet and at the saturator outlet.
S1, S2 and S3, and a common discharge pressure, the atmospheric one. A continuous operating mode needs a rigorous remote control of the pressure in the tanks dome, managing the inlet and outlet gas flows. The experimental campaign will start in September 2006 after the completion of the qualification phase.
Acknowledgements This work, supported by the European Communities under the contract of Association between EURATOM/ENEA, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
References 4. Conclusion The design was based on the diffusivity data of Reiter [5] confirmed by the LEDI/SOLE experiment and on solubility data coming form SOLE experiment conducted in ENEA [4]. The design was optimised to be compatible, in the future, with an installation in the EBBTF (European Breeding Blanket Test Facility) that will be operative in ENEA Brasimone at the end of 2007. It has to be pointed out that the loop is operated having three different internal pressures, in
[1] N. Alpy, T. Dufrenoy, A. Terlain, Hydrogen extraction from Pb–17Li: tests with a packed column, Fusion Eng. Des. 39–40 (1998) 787–792. [2] N. Alpy, A. Terlain, V. Lorentz, Hydrogen extraction from Pb–17Li: results with 800 mm high packed column, Fusion Eng. Des. 49–50 (2000) 775–780. [3] R.B. Perry, C.H. Chilton, Chemical Engineers’ Handbook, McGraw-Hill, NY, 1973. [4] A. Aiello, A. Ciampichetti, G. Benamati, Determination of hydrogen solubility in lead lithium using Sole device, Fusion Eng. Des. 81 (2006) 639–644. [5] F. Reiter, Solubility and diffusivity of hydrogen isotopes in liquid Pb–17Li, Fusion Eng. Des. 14 (1991) 207–211.