Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present inven-tion relates to a ~w~-*i*-i~ps~wY~
process for the recombination of oxygen with heavy hydrogen
formed by the radiolysis of heavy water in, for example, a
heavy water-moderated ligh-t water-cooled reactor of pressure
tube type.
Heavy water D20 used as moderator reacts as Eollows
under an influence oE radiation:
2 ~2 2 .... (1)
, ~ ,
D20 ~ 2 2 2 2 ............................. (2)
' Heavy hydrogen D2 and oxygen 2 generated by the reac-tion
`; formula (1) are accumulated in a cover gas such as helium.
~ Heavy hydrogen content of the cover gas should be kept below
- 3 molar %, since if the heavy hydrogén content is increased,
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. there exists a fear of causing explosion. For this purpose,
there has been usually employed a process wherein heavy hydro-
;- gen and oxygen in -the cover gas are made to react by the
~ reverse reaction of the formula (1), i.e. by the following
I reaction (3j, in the presence of a catalyst comprising a noble
~,
~ metal oE Group VIII in the Periodic Table supported on a
~....................................... .
carrier of a large surface area such as alumina, to thereby
recombine heavy hydrogen with oxygen to form heavy water:
D2 + 2 2 ~ D20 ---- (3)
; ~ The reco~bination according to the reaction formula (3)
is carried out generally by passing the cover gas containing
heavy hydrogen and oxygen through a catalyst bed. However, a
part of oxygen formed by reaction (1) is consumed in the
production of heavy hydrogen peroxide according to the reaction
(2). Therefore, for converting all of heavy hydrogen contained
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in -the cover ~ras i.nto lleavy wa-ter, .it ls necessary to make
up the oxygen balance in the cover ~as prior to -the recombina--
~: tion according -to the reaction (3).
ur-ther, if heavy water vapor formed by the recombination
according to the reaction (3) is condensed on the surface of
the metal catalyst, the catalytic effec-t is deteriorated and,
. consequently, reaction velocity is reduced. Therefore, it is
required to heat the cover gas !to a temperature of abou-t 70-
150C so as to preven-t the condensa-tion of heavy wa-ter vapor
on the catalyst sur-Eace. ~ O
On the other hand, heavy hydrogen peroxide ~ formed
by the reaction (2) accumulates in the heavy water. Since
heavy hydrogen peroxide h.as a strong oxidizing power, i-t
oxidizes an ion exchange resin used for.purifying the heavy
.~. water to form H2SO~, H2CO3, H20, etc. As a result, the ele-
ctric conductivity of the heavy water is increased, the
. radiolysis of the heavy water is accelerated to reduce the
I ~ heavy water concentration and the heavy water circulation
system is corroded. Thus, heavy hydrogen peroxide in the
, heavy water must be decomposed con-tinuously according to the
followi.ng catalytic reaction (4):
D202 2 + 2 2 ----.(4)
:1 In coventional processes, the decomposition of heavy
hydrogen peroxide according to the reaction (4) is carried
out separately from the recombination of oxygen with heavy
hydrogen according to the reaction t3).
An object of the present invention is to provide a process
wherein th.e recombination of oxygen wlth heavy hydrogen and
; the decomposition of heavy hydrogen peroxide are effected
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simultaneous:Ly in a single step and wherein -the recombination
of oxygen with heavy hydrogen can be carried out efficiently.
According to -the present invention, the recombination
o-P oxygen with heavy hydrogen is carried out by allowing a
. flow of gas containing oxygen and heavy hydrogen to ascend in
` a reac-tion column charged with a noble me-tal catalyst and
allowing a Plow of water containing heavy hydrogen peroxide
.; to descend in said reaction column. Thus, both the recombina-
.' tion oP oxygen with heavy hydrogen and the decomposition o~;`!
heavy hydrogen peroxide are carried out simultaneously in one
:~ and the same reaction column.
; Therefore, in the present inven-tion, the oxygen .shortage
in the cover gas, which must be externally supplemented in the
conventional processes for the recombinatio`n of oxygen with
: heavy hydrogen in the cover gas, can be supplied continuously
by the decomposi-tion of heavy hydrogen peroxide. Thus, the
- recombination according to the reaction (3) can be carried
,
out efficiently and stoi.chiometrically without the external
; ~ supplementation of oxygen into the cover gas.
~;'! In a preferred embodiment of the present invention, the
gas is introduced through the bottom of the reaction column'',
' ~ and discharged through the top thereof and the heavy water is
`~ allowed to flow downwardly from the top to the bottom, whereby
the ascending flow of the gas is countercurrently con-tacted
with the descending flow of the heavy water in the catalyst
bed. According to this countercurrent process, the recombina-
:, :
: tion of oxygen with heavy hydrogen can be accomplished effecti-
vely. The recombination by the reaction (3) is exothermic.
If, for example~ 1 vol. % heavy hydrogen in the cover gas is
combined with oxygen, temperature is elevated by about 70C.
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In SUC}l an exothermic reac-tion, it i5 desirable for maintain-
ing the ca-talyst l.ife to control -the -temperature elevation as
far as possible. Thus, by the countercurrent contact of -the
; heavy water with the gas, the temperature elevation can be
~ suppressed.
In a more preferred embodiment of the present i.nvention,
a hydrophobic noble meta]. catalyst is used as the noble metal
catalyst to be charged in the reaction column. The hydrophobic
, catalyst can be prepared by supporting a noble metal on the
., surface of a hydrophobic carrier such as, for example, poly-
tetrafluoroethylene or divinylbenzene styrene copolymer. As
the noble metals, there may be used the same metals of Group
. VIII in the Periodic Tab:Le as those used in the conventional
.~ processes such as platinum, palladium, rhodium and ruthenium.
When such a hydrophobic noble metal catalyst is used,
heavy water vapor formed by the recombination of oxygen and
~ .
:1: heavy hydrogen is not condensed on the catalyst surface and,
I therefore, the catalytic activity is not deteriorated.
~ Accordingly, heating of the catalyst bed necessitated for pre-
.
venting the condensation of heavy water vapor on the catalyst
~: surface in the conventional processes becomes unnecessary and
in addition, preheating of the gas to be in-troduced becomes
~!~ also unnecessary.
.. The present invention will be further illustrated by way
.~ of the accompanying drawings in which: .
Fig. 1 is a schematic diagram showing a reaction column
!
. used in the invention ,
Fig. 2 is a schematic diagram of a modified bottom portion
of Fig. 1 ;
Fig. 3 is a schematic diagram showing another reaction
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colwnn used in an embodiment of -the invention in wllich two
catalyst beds are separately provided in the column; and
Fig. 4 is a schema-tic diagram showing other reaction column
used in an embodiment of the invention in which mul-tiple ca-talyst
beds are provided :in the column.
Referring now to Fig. 1, there is shown an example of a
preferred reaction column employed Eor practising -the presen-t
invention. A gas containing oxygen and heavy hydrogen is in-
troduced into the column through a gas-supplying pipe 1. While
the gas ascends through a catalyst bed 2, oxygen and heavy
hydrogen are recombined together to form heavy-water according
to the catalytic effect. The gas is finally discharged from
the column through a gas-dischargin~ pipe 3. On the other hand,
heavy water containing heavy hydrogen peroxide is introduced
into the column through a heavy water-supplying pipe ~ at the
top of the column and contacted countercurrently with the
Z
ascending gas Elow while it descends in the catalyst bed 2.
During thi.s descending the flow of heavy water cools the heat
of reaction generated by the recombination of oxygen with heavy
hydrogen, and heavy hydrogen peroxide in the heavy water is -
decomposed to generate oxygen and finally discharged through a
heavy water-discharging pipe 5 disposed at the bottom of the
column. A given quantity of heavy water 6 is always stored at
the bottom of the column so that the gas supplied through the
gas-supplying pipe 1 would not be directly discharged through
the discharging pipe 5 together with the heavy water. In order
. ~ .
~; to store the given quantity of heavy water, heavy water supply
and heavy water discharge are controlled by valves 7 and 8 and
a controller 9. Another controlling device (not shown) may
also be providecl in order to control heavy water supply and
gas supply in connection with each o-ther so -that the decom-
position of heavy hydrogen peroxide and the recombination of
oxygen with heavy hydrogen are carried out properly.
As for contro]ling means for s-toring a given quantity of
heavy wa-ter at the bottom of the column, there may be employed
a method wherein heavy water level is detec-ted by photoelectric
cells 10 and 11 and opening degree of the heavy water-discharg-
ing valve 8 is controlled by a controller 12 as shown in Fig.
2.
It is desirable to select variety of the noble metal
catalysts and modes of the supply of the gas and water depending
on the heavy hydrogen content of tlle gas and the heavy hydrogen
"
peroxide content of the heavy water to be introduced into the
reaction column.
, In the case where the heavy hydrogen content of the gas
is relatively high and the heavy hydrogen peroxide content of
the heavy water is relatively low, it is preferred to charge
~i a hydrophobic noble metal catalyst in the react:ion column as
shown in Fig. 1. In this case, the hydrophobic catalyst is
used so as to prevent the condensation of the resulting heavy
water vapor on the catalyst surface in order to effect the
recombination of oxygen with heavy hydrogen particularly
efficiently. The heat of reaction generated by the recombi-
`` nation of oxygen and heavy hydrogen can be cooled effectively
~ by the heavy water descending countercurrently to the ascending ~
.~ , -
gas fl~ow. Though the decomposition of heavy hydrogen peroxide
is accelerated more effectively when a hydrophilic noble metal
.:
catalyst is used than a case wherein a hydrophobic noble metal
catalyst is used, said decomposition can also be carried out in
the presence of a hydrophobic catalyst without being influenced
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i. by the recombination wl~en -the heavy hydrogen peroxide con-
ten-t of the heavy water is rela-tively low.
In the case wherein both the heavy hydrogen content of
the gas and the heavy hydrogen peroxide content of the heavy
water are relatively high, it is necessary to carry out both
i recombination of oxygen with heavy hydrogen and the decomposi-
tion of heavy hydrogen peroxide efficiently. For this purpose,
it is preferred to charge a mixture of the hydrophobic noble
metal catalys-t and the hydrophilic noble metal caralyst in the
~ reaction column shown in Fig. 1. By using the mixed catalys-t
;` bed, the oxygen/heavy hydrogen recombining reaction and the
heavy hydrogen peroxide-decomposing reaction ~can be promoted
effectively by the hydrophobic catalyst and the hydrophilic
catalyst, respectively.
Also when the heavy hydrogen content of the gas is
. ~ .
~, relatively low and the heavy hydrogen peroxide content of the
;, heavy water is relatively high, the effective treatement is
made possiblé if the mixed catalyst bed comprising hydrophobic
catalyst and hydrophilic catalyst is used. The hydrophilic
catalysts have been used heretofore, and can be prepared by
supporting noble metals on carriers of a large surface area such
. ~,
as alumina and diatomaceous earth. ;
In the case where both the heavy hydrogen content of the
' ~ gas and the heavy hydrogen peroxide content of the heavy water
l~ are relatively low, a modified method as shown in Fig. 3 can
be employed favorably. In an embodiment shown in Fig. 3, a
; hydrophob1c noble metal catalyst bed 21 and a hydr~philic noble
metal catalyst bed 22 are placed separately in an upper part
and a lower part of a reaction column, respectively, with
, leaving a space between the catalyst beds. The gas is introduced
:~ - 8 -
~L~3!5~
into a center por-tion of the column -through a gas-supplying
pipe 23 disposed in a side wall of the column. While the
gas ascneds through the hydrophobic catalys-t bed 21, the
oxygen/heavy hydrogen recombining reaction is carried ou-t
efficiently. The gas is finally discharged -from the column
through a gas-discharging pipe 2~ provided at the top of the
column. On the other hand, -the heavy water is introduced into
the center portion of the column through a heavy water-supply-
ing pipe 25 disposed in a side wall of the column. While the
heavy water descends through the hydrophilic catalyst bed 22,
the heavy hydrogen peroxide-decomposing reaction is carried
out efficiently. The heavy water is finally discharged from
the column through a heavy water-discharging pipe 26 provided
at the bottom of the column. In the embodiment shown in Fig.
3, the countercurrent contact of the descerding flow of heavy
water with the ascending flow of gas is not always required,
since the heat ~uantity generated by the recombination of
heavy hydrogen with oxygen is relatively small because of the
relatively low heavy hydrogen content of the gas. However,
. : :
like the embodiment shown in Fig. 1, oxygen formed by the
decomposition effectively makes up the oxygen deficiency in the
~ recombination, since the oxygen/heavy hydrogen recombining reac-
,~ tion and the heavy hydrogen peroxide-decomposing reaction pro-
ceed simultaneously in one and the same reaction column.
, ~ If the recombination and the~decomposition are not com-
: .
pleted by passing the flows of gas and water through only one
hydrophobic catalYst bed and one hydrophilic catalyst bed as
,~, .
shown in Fig. 3, the reactions can be carried out more com-
pletely by employing a column of multiple catalyst beds as
shown in Fig. 4. In this embodiment, hydrophobic noble metal
, ~ .
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catalys-t beds 31a and 31b and hydrophilic nob:Le me-tal catalyst
beds 32a and 32b are placed alterna-tely and separately Prom
one another in the column. A fl,ow of heavy water contaning
heavy hydrogen peroxide is introduced through a wa-ter-supply-
ing pipe 33 and allowed to descend -through the hydrophilic
catalys-t bed 32a. The descending heavy wa-ter is once guided
outside the column -through a wa-ter-guiding pipe 34 by means o~
an inclined par-ti-tion 35 and then introduced again in-to a par-t
above -the second hydrophilic catalys-t bed 32b and allowed to
descend through the catalys-t bed 32b. On the other haDd, the
heavy hydrogen-containing gas is introduc~d through a gas-
supplying pipe 36 and allowed to ascend through the lower
hydrophobic catalyst bed 31a, once guided outside the column
through a gas-guiding pipe 37 by the partition 35, then intro-
duced again into a part below the second hydrophobic catalyst
bed 3lb and allowed to ascend through the catalyst bed 3lb.
In order to prevent the gas introduced into the column through
the gas-supplying pipe 36 and the gas-guiding pipe 37 from
being directly discharged through the water-guiding pipe 34 and
the water-supplying pipe 33, respectively, downstream end por-
tions of the water-supplying pipe 33 and -the water-guiding pipe
34 are provided with water seal means (not shown). Though -two
hydrophobic catalyst beds and two hydrophilic catalyst beds are
provided in the embodiment of Fig. 4, more catalyst beds may be
used in one and the same reaction column as a matter of course.
Thus, by providing multiple hydrophobic catalyst beds and hydro-
philic catalyst beds alternately in one and the same react1on
column, -the recombination and decomposition can be carried out
completely in a compact device.
.
, ~ - 1 0 _
, ~ . . .
The foregoing descrip-tion has been made on the recombina-
tion of oxygen with heavy hydrogen and the decomposition of
heavy hydrogen peroxide contained in heavy water. It will be
easily understood, however, that the present invention is also
applicable to the recombination of oxygen with hydrogen and
the decomposition o~ hydrogen peroxide con-tained in wa-ter.
Example
A granular hydrophobic platinum carried on porous poly-
tetrafluoroethylene was charged in a co:Lumn of a diameter of
Scm to obtain a catalys-t bed of a length of 30 cm. A nonpre-
; heated cover gas having the hydrogen content o-f 3 vol. % was
introduced through the bottom of the catalyst bed at a flow
rate of 1 Nm3/hr. and discharged through -the top of the column.
On the other hand, water containing 10 ppm of hydrogen peroxide
was introduced through the top of the catalyst bed at a rate
of 2 Kg/hr. and discharged through the bottom. Thus, the ascen-
~ ing gas flow was contacted countercurrently with the descending
` water flow. The cover gas discharged through the top had the
hydrogen content of less than O;l vol. % and water discharged
through the bottom had the hydrogen peroxide content of less
than 1 ppm. Temperature elevation in the catalyst bed was less
than 50C.
~ It can be seen from the above description of the present
i~ invention that the oxygen deficiency in the recombination of
"~ oxygen with hydrogen can be made up continuously by oxygen
formed by the decomposition of hydrogen peroxide, since the
recombination and the decomposition are carried out simulta-
neously in one and the same reaction column. Further, the
activity of the catalyst is not deteriorated, since the
-- 1 1 --
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condensation of wa-ter vapor Eormed by -the recombina-tion on the
catalyst surface can be prevented by using the hydroph~bic noble
:~ metal catalyst and, therefore, a step of prehea-ting of the gas
for the prevention of condensation is not required.
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