Fusion Engineering and Design 88 (2013) 2413– 2416
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Fusion Engineering and Design
journa l h om epa ge: www.elsev ier .com/ locat e/ fusengdes
omputation and comparison of Pd-based membrane reactorerformances for water gas shift reaction and isotope swamping iniew of highly tritiated water decontamination
lessia Santuccia,∗, Claudio Rizzellob, Silvano Tosti a
Associazione ENEA-Euratom sulla Fusione, C.R. ENEA Frascati, Via E. Fermi 45, 00044 Frascati, RM, ItalyTesi Sas, Via Bolzano 28, Roma, Italy
i g h l i g h t s
A dedicated detritiation process for highly tritiated water (HTW) has to be identified.Water gas shift and isotopic swamping via Pd–Ag membrane reactor are possible processes.A parametric analysis through two simulation codes is performed.A comparison in terms of the decontamination factor is provided.
r t i c l e i n f o
rticle history:eceived 14 September 2012eceived in revised form 9 May 2013ccepted 21 May 2013vailable online 2 July 2013
eywords:ighly tritiated waterd-based membrane reactorater gas shift
sotopic swamping
a b s t r a c t
In a D–T fusion machine, due to the possible reaction between tritium and oxygen, some potential sourcesof highly tritiated water (HTW) can be identified. Therefore, a dedicated detritiation process has to beassessed either for economic and safety reasons. In this view, the use of a Pd-based membrane reactorperforming isotopic exchange reactions can be considered since hydrogen isotopes exclusively permeatethe Pd–Ag membrane and their exchange over the catalyst realizes the water detritiation.
In this activity, the treatment of highly tritiated water, generated by an ITER-like machine (i.e. 2 kg ofstoichiometric HTO containing up to 300 g of tritium), via a Pd-membrane reactor is studied in terms ofdecontamination capability. Especially, a parametric analysis of two processes (water gas shift and iso-topic swamping) performed in a Pd-based membrane reactor is carried out by using two mathematicalmodels previously developed and experimentally verified. Particularly, the effect of the reactor temper-
ature, the membrane thickness, the reaction pressure and the protium sweep flow-rate is investigated.Moreover, a comparison in terms of the decontamination factor and the number of reactors necessaryto detritiate the HTW are provided. Generally, the results reveal a higher decontamination capability ofthe WGS reaction respect with the IS (maximum DF values of about 120 and 1.6 in the case of WGS andIS, respectively). However some drawbacks, mainly related with the formation of tritiated species, can
During the operation of a D–T fusion machine (like ITER andEMO) several tritiated products will be generated. Particularly,
he interaction between tritium and oxygen will inevitably pro-uce some quantities of highly tritiated water (HTW) especiallyuring vacuum vessel venting, cryopumps regeneration and in case
f accidental release of tritium in a glove box or in a room [1–3].lthough the amount of the produced HTW will be small, it can con-
ain important quantities of tritium (during ITER operation about
A Association sulla Fusione. Published by Elsevier B.V. All rights reserved.
2 kg of HTW having up to 300 g of tritium are expected). The mainissues of this HTW are related with its high radiotoxicity, radio-chemical decomposition (self-radiolysis) and corrosivity [4]. Forthese reasons a strategy to handle and treat HTW has to be clari-fied, by considering that dilution cannot be taken as an option sinceit will produce an unacceptable large amount of tritiated water aswell as the long term storage should be avoided.
Conventionally, recovery of tritium from water can be accom-plished by using hot metal beds, high temperature electrolysis,water gas shift (shift catalyst/permeator combination) or isotopicswamping in a palladium membrane reactor [5]. By using hot metal
bed, tritium is removed from water according to the followingreaction:
here x is the number of molecules of water reacting stoichiomet-ically with the metal. Several metals have the ability to reducehe water, but iron, magnesium and uranium are the most used forTO processing [6]. However in such metals the formation of a sta-le oxide layer prohibits the occurring of reaction (1). Even if suchroblem can be overcome by heating the bed, alternative metalsre under investigation. The group of Luo et al. has tested a Zr–Nilloy metal bed to reduce the HTO arising in the breeder materialo HT. Their results show that at 400 ◦C, the Zr–Ni alloy is able tory the gas flow to less than 1 ppm of H2O [7].
High temperature electrolysis is the most straightforwardpproach currently available to produce hydrogen from water. Theroup of Konishi et al. has proposed a solid oxide electrolysis cellor the decomposition of tritiated water. They have experimentallyerified the feasibility of this method in the temperature rangeetween 500–950 ◦C. High conversion ratios from both light andeavy water up to 99.9% were obtained. From these results, similaronversion from tritiated water is expected [8]. However, in pres-nce of tritium, the high operative temperature could represent aroblem for safety and design issues. Another strategy to removeritium from water is represented by the water gas shift reactionWGS).
O + HTO ↔ CO2 + HT (2)
The performance of this reaction can be largely improved byemoving the HT from the product site. Such HT removal is usuallyccomplished by applying a Pd-based membrane reactor that offerslso the possibility to combine the chemical reaction and the hydro-en recovery in a single unit. Few groups have investigated thisrocess and at the Savannah River Site (SRS) a demonstration sys-em was successfully used to recover tritium from tritiated water9]. A typical drawback of this process is linked with the CO con-umption and with the formation of a waste gas stream containingritiated CO2. In this view, the group of Tosti et al. has proposed alosed loop in which the produced CO2 can be converted into CO10].
Also the HTW detritiation via isotopic swamping can be accom-lished by using a membrane reactor (i.e. the PERMCAT concepteveloped at the Tritium laboratory of Karlsrhue [3]). Such reactorombines a Pd–Ag membrane exclusively permeable to hydrogensotopes and a catalytic bed that promotes the isotopic swampingIS) described in the following reaction:
2 + HTO ↔ H2O + HT (3)
As reported in literature, with a proper design of the reactorodule almost all the tritium can be recovered in a pure molecular
orm within the hydrogen purge gas [11,12].
. HTW purification through Pd-based membrane reactor
During the operation of a fusion machine (i.e. ITER) low amountsf water with high activity (up to about 1.5 × 106 Ci/kg, correspond-ng to stoichiometric DTO) will be produced. This water shall beecontaminated down to an activity value feasible for the Wateretritiation System, WDS, (which is <300 Ci/kg) [3].
.1. Code for the water gas shift reaction
The code postulates a Langmuir–Hinshelwood kinetic model:he kinetic constants adopted are the ones evaluated by Podolskind Kim [13] at 400 ◦C for a catalyst made of 93 wt.% iron and 7 wt.%
hromium. Table 1 reports the main kinetic parameters.
A complete description of the finite elements code modeling theGS reaction is reported in a previous work [14]. Fig. 1 presents
he scheme of the simulated membrane reactor.
Fig. 1. Scheme of the membrane reactor used for the simulation of the WGS process.
The reactants (CO and H2O) are fed inside the tubular membranereactor filled with the catalyst bed, where reaction (2) occurs. Theproduced hydrogen permeates from the inner (lumen side) to theouter (shell side) of the Pd–Ag membrane. At the shell side, thehydrogen partial pressure is kept low by vacuum pumping. Themodel takes into account the kinetics of the water gas shift reac-tion and the permeation through the Pd–Ag membrane. The modelis based on the following main assumptions: steady state condi-tions, constant temperature, negligible pressure losses, perfect gasbehavior, negligible isotopic effect (both for the reaction and forthe permeation steps). Moreover, the velocity and the gas concen-trations have been considered uniform through every cross sectionof the catalyst bed (i.e. one dimensional model).
Also, the WGS code does not take into account side reactions likemethanation of CO and CO2 that produce tritiated methane and theformation of coke. From the thermodynamic point of view such anhypothesis is very optimistic because under equilibrium conditionsthe CH4 selectivity is about 20% at 400 ◦C. However, this detrimentalaspect could be avoided by using new catalysts that have exhibitedvery low methanation selectivity and negligible formation of coke[15].
2.2. Code for the isotopic swamping
The main assumptions postulated in the IS code are: plug flowfluid dynamic regime, perfect gas behavior, isothermal conditions,negligible pressure losses, tritium in elemental form always in equi-librium with tritium in tritiated water.
A detailed description of the simulation code used for the iso-topic swamping process is given in a previous work [16]. Fig. 2reports the scheme of the membrane reactor simulated by the code.In practice, a flux of HTO is fed into the lumen of the reactor, whilein the shell side a H2 flux is counter-currently sent.
The Pd–Ag membrane allows the permeation of H2 from theshell to the lumen side. The catalyst inside the membrane promotes
isotopic swamping, so that the permeated hydrogen can replace thetritium in the water. The released tritium permeates to the shellside of the reactor and can be collected downstream of the process.On the contrary from to the case of the WGS reaction, in the IS due
A. Santucci et al. / Fusion Engineering and Design 88 (2013) 2413– 2416 2415
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100 and 150 N cm3 min−1 have been considered corresponding toa swamping gas ratio of 1.6, 3.2 and 4.8, respectively.
From the two codes, it is possible to evaluate the total cascadenumber (n) necessary to detritiate all the water in case of WGS or IS.
ig. 2. Scheme of the membrane reactor used for the simulation of the isotopicwamping process.
o the process reactants (only HT and HTO) no side reactions areossible.
.3. Parametric analysis
The described codes have been used to perform a comparisonf the WGS and IS processes through a parametric analysis. In suchnalysis the geometry of the Pd–Ag membrane was fixed: lengthf 0.3 m and a diameter of 0.01 m. The tritiated water feed flow-ate was 31 N cm3 min−1. This feed flow-rate has been calculatedy assuming that the 2 kg of water has to be treated in 10 days andhe work will be performed by a number “n” of cascades with 5
embrane reactors in parallel for each cascade. For case of WGSeaction a CO:H2O feed molar ratio of 1:1 has been always consid-red. The performance of the membrane reactor has been assessedn terms of decontamination factor (DF):
F = HTOin
HTOout(4)
here HTOin and HTOout are the moles of tritiated water enter-ng and leaving the membrane lumen, respectively. The DF couldlso take into account the moles of HT leaving the lumen sidef the membrane reactor, but for all the cases of our calculationuch amount was very low and has been neglected. In order toeduce the activity of the HTW from about 1.5 × 106 Ci/kg down to
× 102 Ci/kg, a total DF of 5000 is required. Accordingly, the n num-er of cascade (each cascade composed of 5 membrane reactors inarallel) results from the following equation:
= logDF 5000 (5)
The parameters investigated by the two simulation tools haveeen: reactor temperature, wall thickness of the Pd–Ag membraneube, lumen pressure (only for the WGS), protium flow-rate in thehell side (only for the IS).
. Results
Fig. 3 reports the DF values obtained by the model code forhe WGS process in the temperature range of 573–673 K for threed–Ag membrane thicknesses. From the graph it is evident thathe temperature has a large influence on the performance of the
embrane reactor: at 573 K the DF is about 3.6 while at 673 K itsalues is 123.5 for the all thicknesses. Since the conversion of theGS reaction is promoted by low temperature, such a behavior
s demonstrating that the effect of the temperature on the kinet-cs overcome the thermodynamic one. Conversely, the membrane
hickness does not significantly affect the DF value, in fact at eachemperature, the DF values corresponding to the three thicknessesre overlapped. This behavior indicates that the permeation is notffecting significantly the WGS under the tested conditions.
Fig. 3. DF values for the WGS process for three different membrane thicknesses(lumen pressure of 105 Pa, reactor length of 0.3 m).
A similar analysis has been performed in the IS case and theresults are illustrated in Fig. 4. In this case both the reactor temper-ature and the membrane thickness have a large impact on the DFof the process.
In the case of the WGS reaction the influence of the lumen pres-sure has been investigated in the range of 105–4 × 105 Pa. As shownin Fig. 5, by increasing the lumen pressure, the DF of the membranereactor increases from a value of 60 to about 105.
The beneficial effect of the pressure is justified by the fact thatone of the products of reaction (2), the HT, permeates through themembrane thus promoting the reaction conversion (shift effect ofthe membrane). Conversely, in the IS case the reactor perform-ances are not strongly affected by the lumen pressure because ofthe mutual permeation of the hydrogen isotopes from lumen toshell, and vice versa. In this case a significant parameter for inves-tigating the reactor behavior is the protium flow-rate in the shellside. Therefore, as reported in Fig. 6, protium shell flow-rates of 50,
Fig. 4. DF values for the IS process for three different membrane thicknesses (lumenpressure of 9 × 104 Pa, reactor length of 0.3 m, swamping ratio of 1.6).
2416 A. Santucci et al. / Fusion Engineering a
Fig. 5. DF values for the WGS process at different lumen pressures (membranethickness of 100 �m and reactor temperature of 623 K).
Fig. 6. DF values for the IS process at the different protium flow-rates in the shellside (membrane thickness of 100 �m and reactor temperature of 623 K).
Table 2Number of cascades and total permeating area necessary to reach a DF of 5000. Thevalues refer to the case of 100 �m membrane thickness, lumen pressure of about105 Pa and HTO feed flow of 31 N cm3 min−1 for each tube. The H2O:CO molar ratiofor the case of the WGS is equal to 1, while the sweep ratio for the IS process is 1.6.
Moreover, taking into account that each cascade has 5 Pd–Ag tubesof length 0.3 m and diameter 0.01 m, it is possible to establish therequired permeating area for the entire HTW treatment process.Accordingly, Table 1 reports the total number of the Pd–Ag tubes(5 × n) and the total permeating area (Area) for the two processes atthe three different temperatures (see cases described in Figs. 3 and 4and Table 2).
4. Conclusion
Two different processes have been simulated for the detritia-tion of highly tritiated water through Pd-membrane reactors: thewater gas shift and the isotopic swamping. The simulation of theseprocesses has been carried out by computer codes previously devel-oped and experimentally verified. The performances in terms ofdecontamination factor have been evaluated at different operatingconditions. As a general conclusion, the water gas shift demon-strates a higher decontamination performance with fixed reactorgeometry compared to the isotopic swamping. However a weakpoint of the WGS is represented by the presence of side reac-tions that produce tritiated species, particularly methane. The WGScode does not take into account such side reactions: this is a veryoptimistic hypothesis and should be validated by new studies ofselective catalysts. Then, in a final comparison between the pro-cesses WGS and IS, it has been considered that the latter results ofmore practical application. Moreover, better performances of theIS process can be achieved by a proper design of the membranereactor.
The results and considerations reported in the paper encouragefuture investigation of new catalysts for the WGS. Moreover, sincethe WGS consumes large quantities of CO and produces CO2, exper-iments with a closed loop able to re-convert the CO2 into CO willbe performed.
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