J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Numerical Simulation of Detritiation System for NIFS with
Commercial Catalyst and Adsorbent
K. Munakataa,*, K. Haraa, T. Sugiyamab, K. Kotohc, M. Tanakad and T. Udad
*kenzo @gipc.akita-u.ac.jpaFaculty of Engineering and Resource Science, Akita University, Tegata-gakuen-machi 1-1, AKITA 010-8502 Japanb
Nagoya University, Furo-cho Chikusa-ku, Nagoya464-8603, Japanc
Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japand
National Institute for Fusion Science, Oroshi-cho 322-6, Toki, Gifu 509-5292 Japan(Received: 9 May 2012 / Accepted: 18 August 2012)

Establishment of tritium recovery system is necessary to prevent its leakage to the working area. The catalyticoxidation and adsorption is the most reliable method to recover tritium released into the working area of thosefacilities. Therefore, there is a necessity that the tritium recovery system with large-scale and higher integrity isdeveloped and constructed. For this purpose, it is required to obtain and accumulate updated database for chemicalengineering design based on most recently commercial and available catalysts and absorbents. In this work, we
selected a catalyst and an adsorbent, and examined their adsorption characteristics for water vapor and performanceof catalytic oxidation of hydrogen. Adsorption isotherms were studied with a volumetric gas adsorption instrument.
Various adsorption models were tested to correlate the experimental isotherms. Breakthrough experiments werealso performed to assess adsorption performances. Furthermore, numerical simulations were performed based on
the obtained experimental results.
Keywords: detritiation, catalyst, adsorbent, membrane dehumidifier, numerical simulation, mass balance
1. Introduction

The authors went through previous literature andinquired about catalysts or adsorbents used for
detritiation systems that are being used in several
research institutes. However, the result of such survey
reveals that the production or model number of thematerials such as catalyst and adsorbent is not clear, andthey are not considered to be available in the present
industrial market. Additionally, conduction of new
screening tests, in order to search feasible catalysts or
adsorbents, is time consuming. It should be also notedthat feasibility tests of catalysts and adsorbents are notlimited within catalytic activity or adsorption
performance; durability in long-time use, mechanicalstability, ease in their procurement with large quantities
are important points as well.

A large amount of tritium would be handled as fuelin nuclear fusion power plants. In these facilities, tritium
is confined in multiple confinement barriers. However, if
unexpected accidents take place, tritium leaks to theworking areas. Thus, establishment of tritium recovery
system is necessary to prevent its leakage to the working
area. The catalytic oxidation and adsorption is the mostreliable method to recover tritium released into theworking area of those facilities. In this method, tritium
gas is oxidized with catalysts, and then tritiated water
vapor is collected by adsorbents.
It is planned to conduct deuterium plasma
experiments in National Institute for Fusion Science(NIFS). In these experiments, D-D fusion reactions takeplace and tritium is produced, and thus the release oftritium to the environment has to be minimized.
Therefore, there is a necessity that the tritium recovery
system with large-scale and higher integrity is developed
and constructed. However the previous experimental andengineering database [1] obtained by several researchers
on absorbents cannot be applied to the design of updated
tritium recovery system since the absorbents used in theprevious studies are not been produced any longer in theindustry. Therefore, it is required to obtain andaccumulate updated database for chemical engineering
design by testing most recently commercial andavailable catalysts and absorbents in experiments. In thiswork, we selected an adsorbent and examined its
adsorption characteristics for water vapor.

For this reason, the authors selected a catalysts andan adsorbent, which are ones of the most widely andgenerally used in industries and are credited and reputedby engineers and designers as well.
In terms of catalyst, the authors selected a Pt/alumina
catalyst, DASH520, manufactured by N. E. Chemcat Co.,
which is widely used in industries for various purposes.
The catalyst was deposited with 4.1g/L of platinum. The
average diameter of the catalyst is 3.25 mm. the packing
density was 770 g/L.
With regard to the adsorbent, the author selected a
MS5A adsorbent that is produced by Union Showa Co.
in Japan (subsidiary company of Union Carbide Co.) andis widely used in industries. The production of zeolites inUnion Showa Co. is licensed by Union Carbide Co, andthe pellet-type MS5A adsorbent could be one of theworld-standard adsorbents.

2. Catalyst and adsorbent_______________________________________________________________________________
Corresponding author: kenzo@gipc.akita-u.ac.jp

36©2013 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchK. Munakata et al., Numerical Simulation of Detritiation System for NIFS with Commercial Catalyst and Adsorbent

3. Experimental
3.1 Catalytic oxidation of hydrogenk r ,ox ,0a 1.16×105 exp 25400 /(RT )(4)

The experimental adsorption isotherms of water
vapor on the DASH520 catalyst and MS5A adsorbent
were found to be successfully correlated using a sort ofmulti-site Langmuir-Freundrich equation:nqad ,s ,mbm pv m

¦ 1  b pv

qad

m 1

m

(5)

m

bm b0,m exp(E m / RT )(6)

Parameters (optimized by non-linear least squares
analysis) for DASH520 catalyst (n=5) and MS5A
adsorbent (n = 4) are listed in Table 1.
Experimental results on the catalytic oxidation ofhydrogen in wet gases reveal that coexistence of water
vapor severely inhibits the catalytic oxidation ofhydrogen. It was found that the mass transfer coefficientfor catalytic oxidation in wet gases is expressed as afunction of the amount of water adsorbed on the catalyst
substrate:

The authors studied adsorption behavior of water
vapor on the DASH520 catalyst and MS5A adsorbent, aswell. The adsorption isotherms of water vapor on the
DASH520 catalyst and MS5A adsorbent were measured
using a volumetric gas adsorption instrument,
BELSORP-max, manufactured by BEL Japan Inc. The
instrument is designed for measurement of wide range ofadsorption isotherms on surface area and pore size
distribution analysis.

k r ,ox a

3.3 Summary of experimental results

k r ,ox ,0aConversion rate Rc and over all mass transfer
capacitance for oxidation (based on surface reaction) in
dry condition kr,ox,0a were computed to assess catalytic
performance. RC and KF,ox are defined as
SV Q /V(1)

100 u (C in C out )/C in(3)

The mass transfer capacitances for oxidation ofhydrogen (based on surface reaction) of DASH520 isexpressed as

The experiments were performed under the steadystate condition. The catalysts were packed in a reactor
made of quartz. The temperature of the reactor wascontrolled with the constant temperature bath. Argon gas
was used as a carrier gas. The argon gas containinghydrogen (about 300-800 ppm) and oxygen (20%) was
introduced to the reactor.
Experiments were also performed using wet processgases which contained water vapor (about 300-1000 Pa)
in order to study the influence of coexistent water vapor
on the catalytic activity for oxidation of hydrogen. The
concentrations of hydrogen at inlet and outlet stream of
the reactor were measured with a gas-chromatograph(GC-8A) manufactured by SHIMAZU Co.
3.2 Water adsorptionRCk r ,ox ,0a SV ln C in /C out

0.476exp ª  643qad
¬«

25.1

º
¼»

(7)

5.00
º
0.524exp ª  456qad¬«¼»

4. Simulation procedure and mass balance
The basic assumption on catalyst and adsorption bed
is one-dimensional dispersed plug flow in packed bed [2].
Chemical reactions such as catalytic oxidation ofhydrogen isotopes, adsorption of water vapor, isotope
exchange reaction were taken into consideration. The
mass balance equations in catalyst beds are expressed asfollows (for adsorption bed, catalytic reactions are notconsidered):(2)

Table 1 Parameters optimized for L-F equation.
m DASH520
MS5A
qs,m [mol/g]
1.52×10-4
19.8
3.86×10-13
2.02×10-15
b0,m [(mol/(gPa))1/ui]
1
39100
44700
Em [J/(molK)]vm [-]
3.16
0.908
qs,m [mol/g]
4.58
0.00703
1.49×10-22
8.90×10-20
b0,m [(mol/(gPa))1/ui]
2
75100
92000
Em [J/(molK)]vm [-]
1.76
2.67
qs,m [mol/g]
3.57×10-40.00191
1.43×10-7
1.42×10-9b0,m [(mol/(gPa))1/ui]
3
38800
36100
Em [J/(molK)]vm [-]
3.25
4.05
qs,m [mol/g]
1.70×10-50.000485
6.62×10-10
4.98×10-12b0,m [(mol/(gPa))1/ui]
4
34800
64400
Em [J/(molK)]vm [-]
1.93
6.33
qs,m [mol/g]
2.12×10-3-91/u
9.85×10b0,m [(mol/(gPa)) i]
532800
Em [J/(molK)]vm [-]
2.09-

㺃Mass balance of Q2 (i = H, D or T) in gas phasew(uC Q 2,i )wz

HwC Q 2,iwt

k F ,ox aC Q 2,i1

k F ,ox a1k r ,ox aHw § wC Q 2,i ·¨D¸wz ¨© L wz ¸¹

1

(8)

(9)

kga

㺃Mass balance of Q2O (i = H, D or T) in gas phasew(uC Q 2O ,i )
wz

37

H

wC Q 2O ,iwt

Hw § wC Q 2O ,i ·¨D¸wz ¨© L wz ¸¹(10)

K. Munakata et al., Numerical Simulation of Detritiation System for NIFS with Commercial Catalyst and Adsorbent

vapor in equilibrium increases almost linear with theactivity of water vapor [5]. In the present study, water
vapor content in the matrix of support layer in
equilibrium is expressed by the following equation.

k F ,ox aC Q 2,i k F ,ad ,i a cQ 2O ,H  cQ 2O ,D  cQ 2O ,T T1
k F ,ex aT2

㺃Mass balance of Q2O (i = H, D or T) in solid phaseJ

wqQ 2O ,iwz

qQ 2O ,ik F ,ad ,i a cQ 2O ,H  cQ 2O ,D  cQ 2O ,T T1

k F ,ex aT2

qad ,Q 2O ,i  qst ,Q 2O ,i

T1 cQ 2O ,i 

qQ 2O ,H  qQ 2O ,D  qQ 2O ,T

when cQ 2O ,H  cQ 2O ,D  cQ 2O ,T t 0

T2 cQ 2O ,i / cQ 2O ,H  cQ 2O ,D  cQ 2O ,T

(11)

(13)



wz

H

w §
wT ·
¨ Oeff
¸
wz ©
wz ¹

wt

J

(20)

���
= 0 = ��,� � ��,��� � ���,� �� ,
��
���,�
���,�
= ���,� �� � ���,� �� � ����

w c p ,sT

��,�

(15)

���,�
= ��,� ��,� � ��,��� ��,��� � ���,�,� �� ,�����,�,�
���,�
= ���,�,� �� � ���,�,� �� � �� ,�����,�,�
���,�
= ���,�,� �� � ���,�,� �� ,��4hw (T Tair )/ Db

The assumptions used in the modeling of membrane
dehumidifier are: (i) the feed gas is a binary mixture ofwater vapor and air, (ii) the pressure drops and diffusive
mass transfer in the feed side and permeation side are
negligible, (iii) laminar films on the surface of themembrane are negligible and (iv) the system is adiabaticand the temperature of the gas mixture is constant [3].
The molar fluxes of penetrants through the dense andporous layers can be expressed as,��,� = ��,�

���,�
.
��

���
= 0 = ��,��� � ��,� � ���,� �� .
����,� ��

k F ,ad ,T a(C Q 2O ,H  CQ 2O ,D  CQ 2O ,T C )'H ad���,� ��,� ��=���,� � ��,� �
��
��

(22)

(23)

The mass balances for each component are, then,*��,� = ��,�

(21)

and

wt

(k F ,ox ,H C Q 2,H  k F ,ox ,DC Q 2,D  k F ,ox ,T C Q 2,T )a 'H ox= ��,� ���,� � ��,� �,��,� = ��,�,� � �� ���,� /�� ���,�The heat balance equation is given as

w Ug c p ,gT(19)

The membrane is divided into small sections along
the axis, and a kind of plate model was applied. The total
mass balances are expressed as

T2 qQ 2O ,i / qQ 2O ,H  qQ 2O ,D  qQ 2O ,T
w u U g c p , gT�̅ � � ��,�
� = ���� ����� ���,��.

It is known that a very low partial pressure of acondensable species, such as water vapor in the feedstream, can significantly reduce the permeability of theother constituent relative to its permeability as a purecomponent. In the present study, the permeance of air isexpressed by the following equation at relatively low
water vapor activity.

(14)

when cQ 2O ,H  cQ 2O ,D  cQ 2O ,T  0(18)

Sorption rate of water vapor is, then, expressed as(12)

cQ 2O ,H  cQ 2O ,D  cQ 2O ,T qQ 2O ,i̅ � = ����� ���,� /�� ���,�

(1 � �)��,�
and��,� ��

����,�,�
= ��
��

���,�
= ��,��� ��,��� � ��,� ��,� � ���,�,� �� .
��

(24)
(25)
(26)
(27)

(28)

Equations (25) and (26) represent the mass transfers in
the pore of the support layer. On the other hand, Eq. (27)represents the concentration change of water vapor in thematrix of the support layer. Equations (29)-(31) and therelationships, ���,�,� = ��,� = ��,� and ���,�,� = ��,�give Eqs. (32) and (34) as fluxes in which the massconservations across the membrane is taken into account.
In the same way, Eqs. (33) and (35) are obtained for the
air.

(16)

(17)

In Eq. (16), a constant pressure gradient is assumed anda permeance is applied because the thickness of thedense layer is very thin and the value is not disclosed by
the vendor. The diffusion coefficient ��,� in Eq. (17) isestimated as Knudsen’s diffusion coefficient.

��,� =��,������� �� ��,� � �� ��,�,�
���
��

���
���
= ��,� ��� ��,� � ��,�,�

Polyimide membranes absorb moisture easily at highpressures of water vapor. The sorption capacity of water

38

(29)

K. Munakata et al., Numerical Simulation of Detritiation System for NIFS with Commercial Catalyst and Adsorbent���� =

����� ���������� − ������ ����� ����
������������ −������������ =����� ������ � − ������ ������� �� �� � ����� � ���������� =����� �������� �� �� � �����

������� =��������� − �� ���� ��� �� �����

������� =� ��� �� − ���� � − ������ ���������� �� − �� �� − ���� ���� �� �����

����� = ������� � �������

����� = ������� � �������

Table 2 Operational condition of detritiation system.

(30)

Detritiation of vacuum
vessel purge gas
CQ2,H: 0.5 ppm
CQ2,D: 90.0 ppm
CQ2,T: 1×10-8 ppm
CQ2O,H: 1500.0 ppm
CQ2O,D: 90.0 ppm
CQ2O,T: 1×10-8 ppm
Amount of catalyst:
100 kgTemp. of catalyst䠖30Υ,
100Υ, 200Υ
Heating䠖㻌 heatexchanger, adiabatic
Recovery䠖 Polymer
membrane-type
dehumidifier UM-XC5
䠄Φ90 mm×710 mm䠅, 60
sets䠈high pressure 0.6
MPa䠈low presasure0.1
MPa, 30 Υ䠈100 L/min
Dehumidifier cooling䠖㻌
heat exchanger, adiabatic

(31)
(32)

(33)

(34)
(35)
(36)
(37)

The mass balance equations discredited and thenmatrices generated were solved by direct methods such
as LU decomposition method. The generalized CrankNicholson scheme was applied for time-marchingprocedure.

Detritiation of plasma
exhaustCQ2,H: 130.0ppm
CQ2,D: 2983.3 ppm
CQ2,T: 9.25 ×10-6 ppm
CQ2O,H: 3.6667 ppm
CQ2O,D: 28.667 ppm
CQ2O,T: 6×10-8 ppm
Amount of catalyst:
100 kgTemp. of catalyst䠖30Υ,
100Υ, 200Υ, 400Υ
Heating䠖㻌 heatexchanger, adiabatic
Recovery䠖㻌 MS5A
adsorbent,100 kg

Adsorbent cooling䠖㻌
heat exchanger,
adiabatic

plasma exhaust gases are treated using catalyst andadsorption beds. One of the major differences isthroughput of the gases processed. The concentrations oftritium and deuterium are considerably higher in theplasma exhaust gas. Numerical simulations were
performed for both cases described above.

5. Results and discussion
5.1 Detritiation system design of NIFSIn D-D experiments in NIFS, (a) detritiation ofdownstream of outgas generated in cleaning of plasma
container (vacuum vessel purge gas) and (b) plasma
exhaust gases is required. Thus, it is necessary todevelop a detritiation system to recover lowconcentration tritium generated in LHD (Large Helical
Device). The schematics of the detritiation systemdesigned at present are shown in Fig. 1. Representative
operational conditions are summarized in Table 2. With
regard to the vacuum vessel purge gas, the process gas isfirst introduced to 100 kg of catalyst bed and thentritiated water vapor is collected by condenser andpolymer membrane-type dehumidifier. In contrast, the

5.2 Results of numerical simulation for detritiation of
vacuum vessel purge gas

CQ2,T,out [ppm]Figure 2 shows changes in the concentration ofmolecular form of tritium in the outlet stream of thecatalyst bed. With regard to the temperature of inlet gas,
three cases (30 qC, 100 qC and 200 qC) were tested. It
can be seen in the figure that catalytic activity is
considerably decreased in the cases where the inlet gas
temperatures are 30 qC and 100 qC. This is because thatwater vapor contained in the process gas inhibits
10

-8

10

-9

10

-10

10

-11

10

-12

10

-13

10

-14

㻌

30 qC

100 qC㻌���� =

200 qC

0

5

10

15

20

Fig. 1 Detritiation systems designed in NIFS

Time [h](a) outgas generated in cleaning of plasma container(b) downstream of plasma exhaust gases.

Fig. 2 Change in concentration T2 in outlet stream of
Pt/alumina catalyst beds.

39

K. Munakata et al., Numerical Simulation of Detritiation System for NIFS with Commercial Catalyst and Adsorbent

catalytic activity with increase in the amount of water
adsorbed on the catalyst substrate. However, this effectcould be considerably avoided if the inlet gas
temperature is raised to 200 qC as shown in the figure.
Thus, preheating of the process gas with heat exchangers
is necessary for the treatment of vacuum vessel purgegas.

increases in the same trend. Figure 4 shows changes inconcentrations of molecular form of tritium and tritiatedwater vapor at the outlet of the duct. The concentrationof tritiated water vapor increases almost linearly withlapse time. The molecular form of tritium sharply
increases 20 h after the commencement of operation of
the detritiation system, since the inhibition of catalytic
activity by water vapor becomes a dominant factor.
However, the result of this simulation indicates that
regulatory level of tritium concentration at the outlet ofthe duct is fulfilled.

Subsequent numerical simulation for condenser andpolymer membrane-type dehumidifier was performed in
the case where the inlet gas temperature is raised to 200
qC. The transient variations of chemical components
such as hydrogen isotopes and isotopes of water vapor in
the outlet stream of the catalyst bed were used as theinlet condition of the polymer membrane-type
dehumidifier. Figure 3 shows changes in amounts ofbulk water and concentration of tritiated water in draintank. The amount of bulk water increases almost linearlywith lapse time. The concentration of tritiated water

5.3 Results of numerical simulation for detritiation ofplasma exhaust gas
Figure 5 (a) shows changes in T2 concentration inoutlet stream of the catalysts bed. In this case, preheatingof the process gas before the catalyst bed is unnecessary
because of the help of higher concentration of D2; its
reaction can generate heat and raise the reactor

(a)

(b)

Fig. 3 Change in (a) amount of bulk water and (b) concentration of tritium in drain tank.

(a)

(b)

Fig. 4 Change in (a) concentration of molecular form of tritiumand (b) concentration of tiritiated water vapor on outlet stream of duct.
-5

10

(a)

-10

10

-15

10

CQ2O,T,out [ppm]

CQ2,T,out [ppm]

-14

10

-15

10

-16

10

-17

30 qC

-20

10

-25

10

-30

10

-35

10

-40

10

-45

10

-50

10

-55

(b)
200 qC

100 qC

400 qC

30 qC

100 qC

10

-18

-60

10

10

0

5

10

15

10

20

Time [h]

0

5

10

15

20

Time [h]Fig. 5 Change in (a) concentration of molecular form of tritium in outlet gas of catalysts bed
and (b) concentration of tiritiated water vapor on outlet stream of adsorption bed.
(Temperatures shown in the figures represent the temperature of the process as at inlet of the catalyst bed.)40

K. Munakata et al., Numerical Simulation of Detritiation System for NIFS with Commercial Catalyst and Adsorbent

temperature. Figure 5 (b) shows breakthrough curve forT2O vapor, which indicates that breakthrough starts at 10
h after the start of operation of 100 kg of MS5A
adsorbent regardless of variation in temperature ofprocess gas at the inlet of the catalyst bed.

t
T
u
V
V
X
Z

6. Conclusion
Catalytic oxidation rate of hydrogen and water
adsorption characteristics are investigated with regards
to commercial catalyst and adsorbent. Based on theexperimental results, simulation models wereconstructed for catalyst bed, adsorption bed andpolymer-type membrane dehumidifier. Test processsimulations were carried out on the basis of the
detritiation systems designed in NIFS.

Greek lettersΔ
Thickness of the layer [m]ΕVoid ratio in the porous layer
Oeff
Effective heat conductivity in axial direction [J
m-1 s-1]
Ug
Density of gas [g m-3]
Overscript
Int
Interface between dense and porous layer¯
Equilibrium state'
Matrix of the porous layer

Nomenclature

a
asorp
ap
b0

cp,g
cp,s
C
D
Db
'Had
'Hox
kF,ad
kF,ox
Kg

kr,ox

kr,ox,0
ksorp
l
N
p
P
q
qad
qst
q

Q
r
R
Rc
S
SV
Sw

Time [s]Temperature [K]Superficial gas velocity [m s-1]
Volume [m3]Constant related to Langmuir-Freundrichequation [-]Mole fraction of water vapor [-]Length in axial direction [m]Specific surface area [m2 m-3]
Slope of the adsorption isotherm [mol m−3]
Coefficient in Eq. (20) [mol m−2 s−1 Pa−1]
Constant related to Langmuir-Freundrichequation [arbitrary]Specific heat of gas [J g-1 K-1]
Specific heat of solid phase [J kg-1 K-1]
Molar concentration [mol m−3]
Diffusion coefficient [m2 s−1]
Bed diameter [m]Adsorption heat [J mol-1]
Reaction heat for catalytic oxidation [J mol-1]
Overall mass transfer coefficient for adsorption[m/s]Overall mass transfer coefficient for catalyticoxidation [m/s]Mass transfer coefficient for gas film layer[m/s]
Mass transfer coefficient for catalytic oxidation
based on surface reaction [m/s]Mass transfer coefficient for catalytic oxidation
based on surface reaction in dry condition[m/s]Rate constant of the adsorption [s−1]Position in the permeate direction [m]Permeation flux [mol m−2 s−1]
Pressure or partial pressure [Pa]Permeance [mol m−2 s−1 Pa−1]
Flow rate [mol s−1]
Concentration of water adsorbed [mol g-1]
Concentration of water strongly bound tosurface (structural water) [mol g-1]
Net Concentration of water species on thesurface [mol g-1]
Volumetric gas velocity [m3 s-1]
Adsorption rate of water vapor [mol s−1]
Gas constant [J K−1 mol−1]
Conversion rate [-]Outside wall area of a calculation cell [m2]Space velocity [s-1]
Solubility coefficient of water [mol m−3 Pa−1]

Subscripts
0Initial value
1
Feed side
12
From feed side to the porous layer
2
Porous layer
23
From porous layer to the permeation side
3
Permeation sideA
AirA
Indicator in Fig. 2
B
Indicator in Fig. 2
C
Indicator in Fig. 2
D
Dense layer
HHigh pressure sideI
Component (H, D or T) or (w or a)
In
inlet
J
Number of the calculation cell along the axis
L
Low pressure sideM
Adsorption site number
N
Maximum adsorption site numberOutoutletQ
H, D, or T (1H, 2H or 3H)
S
Saturated state
W
Water vapor

References
[1]

[2]

[3]

41

K. Kotoh and K. Kudo, “Applicability of Commercial
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K. Munakata, M. Nishikawa, T. Takeishi, N. Mitsuishi
and M. Enoeda, “Recovery of Tritium in Room Air by
Precious Metal Catalysts with Hydrophilic Substrate”,
Journal of Nuclear Science and Technology. 25 , 383(1988)
T. Sugiyama, N. Miyahara, M. Tanaka, K. Munakata andI. Yamamoto, “A Simulation Model for Transient
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Fiber Membrane”, Fusion Engineering and Design, 86,2743(2011).