STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2001
COMPARISON BETWEEN WATER DISTILLATION PROCESS AND
HYDROGEN ISOTOPE EXCHANGE PROCESS FOR DEPLETION
AND ENRICHMENT OF TRITIUM IN LIGHT WATER
MASAMI SHIMIZU* and KENJI TAKESHITA**
* Isotope Science Laboratory 1198 Issiki, Hayama, Kanagawa-ken 240-0111, Japan
** Dept. of Environmental Chemistry and Engineering, Tokyo Institute of Technology,
4259 Nagatsuda, Midori-ku, Yokohama 226-8502, Japan
ABSTRACT. The dimensions of typical tritium separation processes, water distillation
process and the hydrogen-isotope exchange process with hydrophobic Pt-catalyst, were
evaluated numerically for the recovery of tritium from wastewater generated by the
decommissioning of Magnox reactor. Both the column diameter and the column height
required for the hydrogen-isotope exchange process were much smaller than these for the
water distillation process. The application of hydrogen-isotope exchange process to the
treatment of the tritiated wastewater is effective for not only the reduction of process
dimensions but also the depression of energy consumption.
1. Introduction
When a 165-MWe Magnox reactor is decommissioned, a large amount of tritiated
wastewater is generated by cutting the concrete and the steel structure of reactor (1). Then,
the quantity of tritiated wastewater and the content of tritium in the wastewater are given
as 7600 m3 and 2.2×1011 Bq, respectively (1). If tritium is recovered by operating an
appropriate separation process for 5 yrs and then the operating time of separation process
per year is given as 8000 h/y, the waste water flow rate, F, and the tritium concentration
of the waste water, xF , are evaluated as
F
xF 
7600  10 3 kg
1

 10.56kmol / h
5y  8000h / y 18kg / kmol
(2.2 10 Bq)(3.12 10
11
10
3
m / Ci)
(7600 m )(3.7 10 Bq / Ci)
3
10
(1-1)
 2.44 10 13 T  atom fraction
(1-2)
respectively. In this study, typical separation processes, the water distillation process and
the hydrogen-isotope exchange process using Pt-catalyst, were applied for the recovery
of tritium in the wastewater. The dimensions of these processes were evaluated
numerically under the following same initial conditions,
COMPARISON BETWEEN WATER DISTILLATION PROCESS AND HYDROGEN ISOTOPE …
- tritium concentration of T-enriched waste water : xp = 104xF = 2.44×10-9 T-atom fraction
- tritium concentration of T-depleted waste water : xD ≤ 1×10-14 T-atom fraction.
Namely, the ratio of (xP / xF) = 104 and that of (xD / xF) ≤ ( 10-14 ) / ( 2.44×
-13
10 ) = 0.041. When tritium concentration of the waste water xD is less than 10-14
T-atom fraction, the wastewater is dischargeable directly to the atmosphere. When either
the water distillation process or the hydrogen isotope exchange process is applied for the
recovery of tritium from the wastewater, it is necessary to remove the impurities in the
wastewater as a pretreatment of the wastewater. In this study, the process dimensions
will be evaluated for the wastewater without impurities.
2. Isotope Separation factors
Two isotope exchange reactions between hydrogen and tritium,
H 2 O(g)  HT(g)  HTO(g)  H 2 (g)
(2-1)
(2-2)
H2O(l)  HTO(g)  HTO(l)  H2O(g)
have to be considered for the numerical evaluation of proposed processes. In the
previous papers (2),(3) , the isotope separation factor for Reaction (2-1),  gH / T , is given as
gH / T 
[HTO(g)][H 2 (g)]
336.5
 exp{0.292  log( t  273) 
 1.055}
[H2O(g)][HT(g)]
t  273
(2-3)
and that for Reaction (2-2),  lH / T , as
lH / T 
[HTO(l)] [H2O(g)] PH 2O
47.98
23122

 exp{0.00791 

}
[H2O(l)] [HTO(g)] PHTO
t  273 (t  273)2
(2-4)
where t denotes Celsius temperature. From Eqs.(2-2) and (2-4), the separation factors of
 gH / T and  lH / T can be calculated as the function of temperature.
3. Water distillation process
Fig.3-1 shows the schematic description of water distillation process. Tritium is
enriched at the bottom of column. The separation performance of tritium in water
distillation column was described mathematically in the previous paper(4). The ratios of
tritium fraction between bottom and feed, (xP/xF) and that between top and feed, (xD/xF),
are given as
 P 
( P / V )  (1  1 )

 

(
P
/
V
)  (1  1 )(1 /  )M 1
 F
  D  (1 /  )1 (1  1/  )  (1 /  ) N 1 (1  1 )

 
N
  F  (1 /  ) 1 (1  1/  )  (1 /  )(1  1 )
(3-1)
(3-2)
respectively.
71
MASAMI SHIMIZU and KENJI TAKESHITA
In the conditions that
the process is operated at
70°C, the separation factors
are calculated as
 lH / T  1.062
and
Con d en s er
D(1 )
x
'
N 1
(R ’+1 )
(  x D ) (Ls / D=R’)
y N 1
x 'N1
N
Ls
y 1
HP
1
s tr ip p in g
s ection
x1'
F
xF
x M +1
yM
xM
M
Le
V
x2
E n r ich in g
s ection
y1
x1
1
(R+1 )
(R=V / P)
Still
x0
P ?l ?
HB
x 0 (= x P )
Fig. 3 -1 Sch em a tic d es cr ip tion of wa ter d is tilla tion colu m n
1  1 /  lH / T  1 / 1.062  0.942
As shown in Eq.(1-2), the
atom fraction of tritium in
the feed solution is estimated
as xF = 2.44x10-13. When
the overall separation factor
for the enrichment of tritium
is given as104, the atom
fraction of tritium in the
production and the fraction
ratio of (xD /xF) are given as
xP = 2.44×10-9 and 0.041,
respectively, from the mass
balance equations of tritium,
FxF=PxP+DxD and F=P+D.
Therefore, the molar flow
rates of product water and
stripped water are calculated
as
PF
(x F  x D )
 10.56 x(9.59 x105 )  1.013x103
(x P  x D )
kmol / h
D  F  P  10.56  1.013x10 3  10.559 kmol / h
The molar ratio of the production flow to the vapor flow, (P/V), is given as
(P / V ) 
P
D(1  R)
where R’ denotes the reflux ratio at the top of distillation column. The molar ratio of the
production flow to the depletion flow is described as
(P / D) 
1  (x D / x F )
(x P / x F )  1
from the overall mass balance equation of tritium. From (xD /xF)=0.041 and (xP/xF)=104,
(P / V) 
72
1  0.041 1
1
 (9.591  10  5 )
(10 4  1) (1  R )
(1  R )
COMPARISON BETWEEN WATER DISTILLATION PROCESS AND HYDROGEN ISOTOPE …
As shown in Appendix (A), the reflux ratio at the top of the distillation column,
R’, has to be given as more than 23.2. Table 3-1 shows the relations among the reflux
ratio (R’), the number of theoretical plates of the enriching section (M), the vapor flow
rate (V) and the measure of process dimension (M·V), which were calculated from
Eq.(3-1). The optimum reflux ratio, R’, and the optimum number of theoretical plates of
the enriching section, M, are evaluated as 24 and 170, respectively. Then, the number of
theoretical plates of the stripping section, N, is given as 111 from Eq.(3-2) substituting ’,
which was calculated by the relation of ’=Ls/V = (D·R’) / [D(R’+1)].
When a commercial
packing, Sulzer Pack- Table3-1 The relations between the reflux ratio (R’), the number
ing, was used in the of theoretical plates of the enriching section (M), the vapor flow
distillation column, the
rate (V) and the measure of process dimension (M·V)
HETP of distilltion
column was reported to
R’[-]
24
25
26
be 0.1m at the operatM[-]
170
169
168
ing conditions that the
V[kmol/h]
264
275
285
column temperature is
4
4
4
M·V[kmol/h
4.49x10
4.65x10
4.79x10
70°C and the linear
]
velocity of vapor is 1.0
m/s (op.c) (5). ThereTable 3-2 Summary of water distillation process
fore, the heights of the
enriching section and
Operating
70
Remarks
the stripping section are
temperature
evaluated as 17m and
Vapor pressure
233.7 mm Hg
11.1m, respectively. As
1.062
H/T
shown in Appendix (B), No. of theor. Plates
M
170
the cross-sectional area
N
111
of the distillaion column,
(M+N)
281
A, is calculated as 2.06
H=(0.1)(M+N) [m]
28.1
HETP=0.1 m(4)
m2 in the conditions of
u [m/s (op.c)]
1.0
=233.7 mmHg and
F [kmol/h]
10.56
V=264 kmol/h, as
V/F
25.0
shown in Ref. (5). Thus,
V [kmol/h]
264
the column diameter, Di, A [m2]
2.06
Appendix(B)
is given as 1.62 m.. As
Di [m]
1.62
Appendix(B)
the latent heat of
CEV[kw]
3062
Appendix(B)
evaporation, L, is given
as 1.0x104 kcal/kmol at 70°C, the energy consumption for the phase conversion between
liquid and vapor, CEV, is calculated as 3062 kW from the relation of CEV= VL/860
(1kw=860 kcal/h). The operating conditions and the calculation results were
summarized in Table3-2.
73
MASAMI SHIMIZU and KENJI TAKESHITA
4. Hydrogen isotope exchange process
4-1 Separation system
zA
NA
zW
N1
xR
NR
z nW 2
yW
N
y nW1
x Hn 1
scrubbing bed
NW
L
N GW
NH
G
catalyst bed
Scrubbing bed
(n+1)th
unit
x nW1
z1W
zF N
F
z nW1
yW
0
W
H
W
z N1
Catalyst bed
x0
yM
y nW
H
x Hn
xM
Scrubbing bed
W
NL
W
z1
W
NG
W
y0
H
NG
nth
unit
x nW
Catalyst bed
H
z nW
x0
W
zB
zp N
P
electrolyser
σw
Fig.4-1 Schematic description of hydrogenisotope exchange process
y nW1
x Hn 1
σV
σH
Fig.4-2 Schematic description of
exchange column
Figures 4-1 and 4-2 show the hydrogen-isotope exchange process using Pt-catalyst,
which consists of an exchange column with Pt-catalyst and an electrolyser. It should be
noted that the process does not have a phase converter, H2/O2-recombiner at the top of
column. Natural water is fed to the top of the exchange column and the tritium-depleted
hydrogen gas is discharged to the atmosphere. The electrolyser plays a role as a phase
converter, in which H2O is decomposed electrochemically to H2 and O2. The hydrogen
gas obtained is transferred to the bottom of exchange column. The exchange column has
many units, which consists of a packed bed with a hydrophobic Pt catalyst and a
scrubbing bed with a commercial packing, Dixon packing. The hydrophobic Pt-catalyst
is a porous stylene-divinylbenzene copolymer impregnated with 0.5 wt% Pt (0.5wt%
Pt/SDBC) and is 4 mm in diameter. Tritium is transferred from H2(g) to H2O(l) in the
exchange column. The overall exchange reaction of hydrogen isotope can be described as
HT (g)  H 2 O(l)  H 2 (g)  HTO (l)
74
(4-1)
COMPARISON BETWEEN WATER DISTILLATION PROCESS AND HYDROGEN ISOTOPE …
The hydrogen-isotope exchange reaction between H2(g) and H2O (v),
HT (g)  H 2 O( v)  H 2 (g)  HTO ( v)
(4-2)
takes place in the Pt-catalyst bed and that between H2O (l) and H2O (v),
HTO ( v)  H 2 O(l)  H 2 O( v)  HTO (l)
(4-3)
in the scrubbing bed. Tritium is enriched at the bottom of exchange column.
4-2 Numerical calculation of hydrogen-isotope exchange process
In the n-th unit, the exchange efficiency c of Pt-catalyst bed and the scrubbing
efficiency b of scrubbing bed are defined as
b 
 nW  ynW
 nW  ( ynW )e
C 
 nH1   nH
 nH1  (  nH )e
and
(4-4)
respectively, where the suffix, e, denotes equilibrium. As seen in Figs.4-1 and 4-2, the
relationships among xH, yW and zW are obtained as follows,
x Hn  Ax Hn 1  BynW1
x nW  Cx Hn 1  DynW1
y nW  Ez nW  Fx nW
z nW1

Gz nW

(4-5)
Hx nW
where the coefficients of A to H are defined as follows,
 
 g c
c
b
 g  g c
B
A  1 g c ,
, C ,
, D  1
, E
,
g   g
g   g
g   g
1  1b
g  g
F  (1  b ) 
12b
1
, G
1  1b
1  1b
and H 
 11b
1  1b
Eqs.(4-5) are transformed to the following finite difference equation concerning xH.
x Hn  3  px Hn  2  qx Hn 1  r x Hn  0
(4-6)
where, the coeficients of p, q and r deonte as p = – ( A + DF + G ), q = (ADF – BDF +
AG + DFG + DEH ) and r = (BC – AD )(FG – EH ). The solution of Eq.(4-6) is given as
follows,
(4-7)
x Hn  K1g1n  K 2g n2  K3g3n (n=0)
where, g1, g2 and g3 denote the real roots of the following characteristic equation,
g3+pg2+qg+r=0
(4-8)
K1, K2 and K3 mean constants to determine the boundary conditions shown in the
Section 4-3. By Combining Eqs.(4-5) with Eq.(4-7), the atom fractions of tritium in
75
MASAMI SHIMIZU and KENJI TAKESHITA
hydrogen gas, water vapor and water are given as
x nW 
3
 Ki[C  B (g
D
 A)]g in 1
i
i 1
(n≥1)
(4-9)
3
 Ki(
y nW 
i 1
gi  A n
)g i
B
(n≥0)
(4-10)
z nW 
3
Ki
 BE {(g
 A)g i  F[BC  D(g i  A)]}g ni 1
i
(n1)
i 1
(4-11)
The concentration profiles of tritium in hydrogen gas, water vapor and water of the
stripping section of the multiunit hydrogen isotope exchange column are calculated by
Eqs.(4-7) to (4-11). Similarly, these of the enriching section are calculated by the
following equations,
H
xn 
3
K g
i
n
i
(n≥0)
i 1
W
xn 
3
 K [C  B (g
n 1
D
i
i
 A)]g i
(n≥1)
(4-13)
(n≥0)
(4-14)
i 1
3
W
yn 
K (
i
gi  A
B
i 1
W
3
Ki
i 1
BE
zn  
(4-12)
n
)g i
n 1
{(g i  A )g i  F[ BC  D(g i  A )]}g i
(4-15)
where the bars on these symbols denote the enriching section.
4-3. Boundary conditions for calculation
The boundary conditions for the exchange column of Fig.4-1 were given as
W
H
H
(1) z B ( N A  N F )  z P N P  x 0 N G
(2)
z lW
(3)

 l y 0W
 (1 
 l ) l y 0W
x  (1 /  El )z P
H
0
(4-16)
(4-17)
(4-18)
for the enriching section and
(4)
(5)
76
x 0H  x HM
y
W
0
y
W
M
(4-19)
(4-20)
COMPARISON BETWEEN WATER DISTILLATION PROCESS AND HYDROGEN ISOTOPE …
(6)
z lW N LW  z F N F  z MW1 ( N LW  N F )
xF  z
(7)
(8)
(4-21)
(4-22)
W
M 1
W
W
W
zW
N 1  N L  z A  N A  y N  N G
(4-23)
for the stripping section. Then, the separation factor of tritium in the water electrolysis,
 El , is 5.6 at 30°C (7). The values of K1 , K 2 , K 3 , K 1 , K 2 and K 3 are calculated by
way of the above boundary conditions and Eqs.(4-4) to (4-15).
4-4 Summery of calculation conditions and results
The calculation conditions and results were summarized in Table 4-1. The flow
rate of natural water from the top of column, NA, was assumed as a half of feed rate.
Namely, the molar flow ratio, (NA/NF), was given as 0.5 instead of the minimum flow
ratio, 0.183, which is required to
Table 4-1 Summery of calculation results
Item
Calculation
conditions&results
NF[kmol/h]
ZF[T atom-fraction]
NA[kmol/h]
ZA[T atom-fraction]
NR[kmol/h]
XR[T atom-fraction]
Np[kmol/h]
Zp/ZF
P[mmHg]
tC
Ug(H+V)[Nm/s]
c[-]
b[-]
Hg[m]
Hl[m]
nE
nS
10.56
2.4410-13
5.26
210-17
15.84
110-14
1.0310-3
1104
760
70
0.6
0.9
0.9
0.1
0.15
13.8
7.7
HE[m]
4.14
Hs[m]
2.31
Dex[m]
Vcat[m3]
CEL[kw]
0.55
0.57
1774
Remarks
2.895107 Bq/m3 water
Assumed as half of feed rate
Concentration of tritium in natural water
Dischargeable directly to the atmosphere
Enrichment factor of tritium
Operating preasure
Operating temperature
(H2+vap.) gas velocity
Ref.(8)
Ref.(8)
Height of catalyst bed
Height of scrubbing bed
No. of unit in enriching section
No. of unit in stripping section
Height of enriching section. Height of unit is
assumed as 0.3 m/unit.
Height of stripping section. Height of unit is
assumed as 0.3 m/unit
Eq.(4-24)
Eq.(4-25)
Eq.(4-26)
obtain the enrichment factor of 104. The column diameter (DEX), the total volume of
Pt-catalyst (Vcat) and the energy consumption of electrolyser (CEL) were evaluated as
77
MASAMI SHIMIZU and KENJI TAKESHITA
D EX



 ( N F  N A  N P )( 22.4)(   p ) 
H 2O


3.14


3600  u g ( H  V ) (
)
4


Vcat  H g (n E  n S )(D 2EX )(3.14 / 4)
[m]
(4-24)
[m3]
(4-25)
C EL  N R  22.4   (  5kwh / Nm  H 2 ) [kW]
3
(4-26)
5. Conclusion
Table 5-1 shows the comparison of calculation results between the hydrogen-isotope
exchange process and the water distillation process. It is found that the diameter and
height of hydrogen-isotope exchange process are much smaller than those of water
distillation process and the energy consumption of elecrolyser is reduced to about 55%
of that of still of water distillation process. It is concluded that the hydrogen-isotope
exchange process is suitable for the treatment of tritiated wastewater generated by the
decommissioning of Magnox reactor.
Table 5-1. Comparison between hydrogen-isotope exchange process and water
distillation process
F or NF [kmol/h]
XF or ZF [T atom fraction]
R’ or A[-]
D or NR [kmol/h]
xD or xR [T-atom fraction]
NA [kmol/h]
zA [T-atom fraction]
10.56
2.4410-13
R’=LS/D=24
A =NA/NF=0.5
D=10.559
NR=15.839
xD 10-14
xR 10-14
5.28
2 10-17
P or NP [kmol/h]
xp/xF or zP/zF
(Operating conditions)
Pressure [mmHg]
Temperature (C)
uV or ug(H+V)
(Operating constants)
HETP [m]
c [-]
b [-]
Hg [m]
Hl [m]
(Results of calculation)
No of theor. plates
No of units
Height of Colum [m]
Diameter of Column [m]
Vcat [m3]
CEV or CEL [kw]
P=1.01 10-3
78
NP=1.03 10-3
1104
233.7
70
uV=1.0 m/s (op.c)
Feed flow
Tritium in feed
Reflux ratio
Depleted flow
Release to atmosphere
Tritium in Natural
water
Production
Enrichment factor
760
70
ug(H+V)=0.6 Nm/s
0.1
0.9
0.9
0.1
0.15
Sulzer packing (Ref.5)
Ref.8
Ref.8
Ref.8
22
6.6
0.55
0.57
1774
M=170 N=111
ne=14 ns=8 units
Height of unit=0.3 m
App.(B), Eq(4-24)
Eq(4-25)
App(B), Eq(4-26)
281
28.1
1.62
3062
COMPARISON BETWEEN WATER DISTILLATION PROCESS AND HYDROGEN ISOTOPE …
Appendix ( A ) Reflux ratio, R’, at the top of the water distillation column
From Eq.(3-1), the following equation
 1 
 
  
M 1

P V1  x
P
  1   
1
x F 
(A-1 )
 x P 1   
 x 
1
F

is obtained. As (1/)M+1 is positive, the numerator of Eq.(A-2 )
 P  x P 
 1    1  1   0
 V  x F 
(A-2)
has to be positive. From the mass balance equation, (P/V) is expressed by
1  1   1 
and (1-l) is

1 
  0.058
1.062 
1   x D 
P
 xF   1
 
x
V

   P   1 1  R '
 xF 
From these results, the limitation of reflux ratio, R’, is given as
(A-3)
R' 23.17
Appendix ( B )
Cross-sectional area of distillation column
The cross-sectional area of distillation column can be calculated by
V 22.4 PH 2O  273  t 
A
[m2]
   273 
3600  u v
(B-1)
When uV = 1.0 m / s ( op.c )(5) for Sulzer Packing ( op.c ) ,
 V  PH O 
A    2 273  t  2.27920 103
 u   


[m2]
(B-2)
The column diameter calculated from Eq.(B-1) was shown in Table B-1.
Table.B-1 Evaluation of column diameter, Di, by Eq.(B-1)
233.7
Remarks
Operating pressure  [mmHg]
Operating Temperature t [°C]
70
Vapor flow rate V[kmol/h]
264
Vapor superficial velocity
1.0
Reference (4)
uV[m/s(op.c)]
Sectional area A[m2]
2.06
Eq (B-1)
Diameter of column
1.62
79
MASAMI SHIMIZU and KENJI TAKESHITA
SYMBOLS
P : T-enriched water flow rate [ kmol / h ]
V : Vapor flow rate [ kmol / h ]
: flow ratio = Le / V
’:flow ratio = Ls / V
Le, Ls : Flow rates of water in the enriching section and stripping one [kmol /h]
M, N : Numbers of theoretical plate in enriching section and stripping one [-]
x Hn , x nW , y Hn and z nW : T-atom fractions of hydrogen and water vapor leaving the nth
catalytic bed and those of water vapor and liquid leaving the nth scrubbing bed
respectively [-]
 g  H / V  (  PH 2O ) / PH 2O  l   V /  W
H/T
g  gH / T  l   l
H, V and W : Hydrogen, water vapor and water liquid flow rate [kmol / m2h]
REFERENCES
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
80
Proposal of Fuji Electric Co.Ltd (1997)
J.F.Black : J.Chem.Phys , 11 , 395 ( 1943 )
M.M.Popov : Atom Energ. SSSR , 8 , 420 ( 1960 )
M.Shimizu and S.Hibino : Some Analyses on the Operation of Water Distillation Column ,
Nippon Genshiryoku Gakkaishi ( Japanese ) 4 , 313 ( 1962 )
M.Asahara : Hydrogen Isotope Separation by Water Distillation , “Separation of Deuterium
and Tritium “ ed.by Nakane , Isomura and Shimizu ( in Japanese ) , Gakujyutsu – Shuppan
Centre ( Japan ) , 1982.
M.Shimizu , T.Doi , A.Kitamoto , and Y. Takashima : Numerical Analysis on Heavy Water
Separation Characteristics for a pair of Dual Temperature Multistage-Type
H2/H2O-Exchange Columns , J. of Nucl. Sci. Technol. 17 , 448 ( 1980 )
K.Takeshita, Y.Wei, M.Shimizu, M.Kumagai and Y.Takashima: Application of
H2/HTO-Isotopic Exchange Method to Recovery of Tritium from Waste Water generated in
Spent Nuclear Fuel Reprocessing Plant , Fusion Technol. 28 , 1572 ( 1995 )
Private communications on the design data and the operating performance of the IPCR-PNC
type tritiated D2O upgrader in the Fugen site.