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The reaction between CO2 and liquid sodium
33 (1969)
JOURNALOFNUCLEARMATERIALS
THE
REACTION E. H. Reactor
328-332.
BETWEEN
and W.
Nederland,
Received
1.
COs AND
P. CORDFUNKE Centrum
Petten,
28 May
Introduction
Little information is available on the reaction of CO2 with liquid sodium. At red heat, sodium carbonate and carbon are formed, as stated by Gmelin 1). This has been confirmed by Gilbert s), who demonstrated the reaction by plunging a spoon containing burning sodium into COs, whereupon a cloud of finely divided carbon was observed. But although CO2 can evidently react with liquid sodium, quantitative information is lacking a). The present investigation was undertaken to obtain such information which, owing to the development of the sodiumcooled fast reactor, is now of considerable interest. 2.
0 NORTH-HOLLANDPUBLISHINGCO.,AMSTERDAM
Experimental
Purified sodium [ < 10 ppm 0] was used in all experiments. Samples were taken in a glass box that could be evacuated and filled with purified, dry argon. Reaction rates were measured in a stainless steel reaction vessel connected to a glass apparatus containing a mercury manometer. The stainless steel vessel was filled with sodium in the box, evacuated and then connected with the apparatus outside the box. The sample in the reaction vessel was brought to and held at the reaction temperature, using a molten salt bath. When the temperature had reached a constant value, purified CO2 was brought into the apparatus to an initial pressure of about one atmosphere. The pressures were then read on the manometer as a function of time. Leak tests showed that the final version of the stirrer worked quite satisfactorily.
LIQUID
SODIUM
OUWELTJES The
Netherlands
1969
It should be emphasized that it took special care to keep the sodium free of contamination, since, for instance, traces of water vapour absorbed on the inner surfaces of the reaction vessels rapidly formed surface layers on the sodium which might inhibit the reaction. After repeated evacuating of the box, which also contained P205, it proved possible to keep the contaminants at such a level that formation of surface layers on the sodium was hardly visible. The whole procedure lasted some days. 3. 3.1.
Results THE
REACTION
Preliminary experiments showed that when CO2 was passed over liquid sodium a rapid reaction occurred at about 450 “C ; the heat liberated by this reaction was so large 4) that
Fig.
1.
Experiment
in which CO2 has been passed
over liquid sodium. The reaction products are clearly visible in the pyrex
boat:
carbon in the form of a
black crust and NszC03 as a white sublimate walls of the boat.
on the
THE
REACTION
liquid sodium burned violently
BETWEES
in COs at this
temperature. The solid reaction product was examined by X-ray diffraction from which only the presence of
NaaCOa
carbon
could
be
deduced.
was in an amorphous
carbides were not formed.
Apparently
form,
whereas
During the reaction,
co2
AND
formed
LIQUID
329
SODIUM
during. a violent
temperatures
(350-400
place somewhat
reaction.
At
“C) the reaction
more slowly,
lower took
and an intimate
mixture of NaaCOa and carbon was formed. The product had a density of 2.4. It should be noted that the temperature which the reaction
started depended
gas samples were collected and examined by mass spectrometry; not even a trace of CO
on the purity
could be detected in the gas samples. Thus, the net reaction can be represented by:
passed over sodium at fixed temperatures, it was found that with pure sodium and purified CO2 a rapid reaction took place already at 425 “C, whereas with CO2 containing 40 ppm water vapour, a rapid reaction was not observed below 525 “C. The same observation was made
4 Na (liq.) + 3C0 2 +
2NaaCOs + C + 260 kcal.
Fig. 1 clearly shows the reaction products NasCOs (as a white sublimate) and carbon,
instance,
of both
in experiments
sodium
at
markedly
and COe. For
in which
CO2 was
800
100
150
200
250 time , minutes
Fig. 2.
Reaction rates (without stirring).
300
35-o
330
E. H. P. CORDFUNKE
with CO2 containing Apparently, the sodium 3.2.
W.
OUWELTJES
small amounts of oxygen,
an oxide layer is than formed on surface and inhibits the reaction,
REACTION
RATES
In order to collect mation
AND
more quantitative
on the sodium-CO2
reaction,
infor-
reaction
rates were measured. Two series of experiments were
done,
one
without
stirring
the
liquid
sodium, as described above; these experiments were carried out at temperatures between 419 and 453 “C. In a second series of experiments, the liquid sodium was stirred. 3.2.1. Reaction
rates without stirring
Reaction rates were measured at temperatures varying from 419 “C to 453 “C! (fig. 2); the initial rates were used to determine the order of the reaction. From the slope of the logarithm of these rates, plotted against the logarithm of the corresponding CO2 pressures, a value of 2 was found for the order of the reaction in COs. This implies that the reaction mechanism can be represented by two reactions, of which the first one is rate-determining: SKa+2COeA4NazO+2C, 4 NazO + 4 COS + 4 Nad203.
TABLE
constants
1
for the second-order
reaction
I Temperature
(“C)
k x 106 (Torr -1. min -I)
*
Not
3.
419
429
440
453
2.15
5.4
8.82
*
measurable.
In the later stages of the reaction (in our experiment at pcoz < 600 Torr), the “order” of the reaction rapidly falls to a value of about 0.5 (fig. 3), because of the fact that the rate falls off more rapidly than would be expected on the basis of the “true” order. This means that reaction intermediates (Na&O& mixture) are formed, bringing about inhibition. This inhibition is also clearly demonstrated when COz
Determination
of the order
from
is contaminated
fig.
of the reactions
2.
with small amounts of oxidizing
gases, such as water vapour or oxygen. Experiments, using CO2 with 40 ppm Hz0 and 120 ppm H20, showed a pronounced decrease in the reaction rates with increasing concentration of water vapour. Obviously, the reaction rates depend on the purity of both sodium and CO2. 3.2.2.
Rate constants for the second-order reaction at different temperatures are collected in table 1.
Rate
Fig.
Reaction rates with stirring
Initial rate experiments in which the liquid sodium was stirred with a stainless steel stirrer, showed that reaction rates comparable with those without stirrer could be obtained at surprisingly lower temperatures ; in fact, at temperatures which are about 200 “C lower. This enabled us to construct the apparatus of Pyrex glass and to take a thermostated bath of silicone oil instead of a furnace. As a result, the whole reaction could be followed visually. Reaction rates were measured at temperatures varying from 177 “C to 260 “C. Fig. 4 shows the results obtained at 241 “C from which it can be seen clearly that the reaction rat’es depend markedly, not only on the temperature, but also on the effect of stirring. In fig. 4 three experiments have been collected, all at the same temperature, but differing in the conditions of stirring. The highest reaction rates are obtained
THE
REACTION
BETWEEN
cog
AND
331
SODITJM
20 “c
700
f
LXQUID
600
0
200
rfmin
*
350
rfmin
A
350
rfmin
L
t
I
500
0”” c, 0
400
0
300
200 A *
fO0 A
0
I
0
1
50
,
*
fO0
200
fS0
250
time , Fig.
4.
Reaction
the surface of liquid sodium is kept in motion by stirring through the surface. It should be noted that in all experiments the initial rates are of zero-order; this is in contrast to the experiments without stirrer. Obviously, the reaction mechanism is different in the experiments without stirrer and controlled by diffusion of CO2 through the surface layer. Figs. 5 and 6 show the reaction tube after the experiment, illustrat~i~lg the influence of the way of stirring on the course of the reaction. In fig. 5 (stirrer under the sodium surface) the formation of a layer of carbon on the sodium surface is clearly visible ; this layer inhibits the reaction markedly. The voluminous reaction when
rates
300
350
minutes
(with sCim3r).
products (Na&X& 1) finally creep up along the stirrer. In fig. 6 (stirrer ~~~o~g~ the sodium surface) reaction rates are much higher than in the experiment shown in fig. Ei.In this case rapid reaction was observed at temperatures as low as 177 “C.
4.
Con&&m
Liquid sodium reacts rapidly with COs at ~mperatures as low as 175 “C. The reaction rates are markedly dependent on the temperature and the contact surface. In static experiments, the sodium surface is rapidly covered with a layer of carbon, inhibiting
332
Fig.
E.
6.
Reaction
H.
I’.
CORDFUNKE
tube after the experiment
(stirrer
AND
W.
OUWELTJES
at 241 “C
under the surface).
the reaction strongly. But when the sodium surface is kept clean, for instance, by stirring, high reaction rates have been observed above 200 “C. At temperatures above 350 “C liquid sodium even burns in COZ.
Fig.
6.
Reaction
tube after the experiment
(stirrer
2)
H. N. Gilbert,
throuyh
at 241 “C
the surface).
Chem. Eng. News 26 (1948) p. 2604
and p. 2660 3)
References 1) Gmelin’s System
M. Sittig,
Sodium,
uses (Reinhold Handbuch nr. 21 (Berlin,
der
anorganischen
1928)
Chemie,
4)
C. E.
Wicks
Mines
(1963)
its manufacture,
Publ. and
F.
Corp., E.
New
Block,
properties York, Bull.
and
1956) 605. Bur.
JOURNALOFNUCLEARMATERIALS
THE
REACTION E. H. Reactor
328-332.
BETWEEN
and W.
Nederland,
Received
1.
COs AND
P. CORDFUNKE Centrum
Petten,
28 May
Introduction
Little information is available on the reaction of CO2 with liquid sodium. At red heat, sodium carbonate and carbon are formed, as stated by Gmelin 1). This has been confirmed by Gilbert s), who demonstrated the reaction by plunging a spoon containing burning sodium into COs, whereupon a cloud of finely divided carbon was observed. But although CO2 can evidently react with liquid sodium, quantitative information is lacking a). The present investigation was undertaken to obtain such information which, owing to the development of the sodiumcooled fast reactor, is now of considerable interest. 2.
0 NORTH-HOLLANDPUBLISHINGCO.,AMSTERDAM
Experimental
Purified sodium [ < 10 ppm 0] was used in all experiments. Samples were taken in a glass box that could be evacuated and filled with purified, dry argon. Reaction rates were measured in a stainless steel reaction vessel connected to a glass apparatus containing a mercury manometer. The stainless steel vessel was filled with sodium in the box, evacuated and then connected with the apparatus outside the box. The sample in the reaction vessel was brought to and held at the reaction temperature, using a molten salt bath. When the temperature had reached a constant value, purified CO2 was brought into the apparatus to an initial pressure of about one atmosphere. The pressures were then read on the manometer as a function of time. Leak tests showed that the final version of the stirrer worked quite satisfactorily.
LIQUID
SODIUM
OUWELTJES The
Netherlands
1969
It should be emphasized that it took special care to keep the sodium free of contamination, since, for instance, traces of water vapour absorbed on the inner surfaces of the reaction vessels rapidly formed surface layers on the sodium which might inhibit the reaction. After repeated evacuating of the box, which also contained P205, it proved possible to keep the contaminants at such a level that formation of surface layers on the sodium was hardly visible. The whole procedure lasted some days. 3. 3.1.
Results THE
REACTION
Preliminary experiments showed that when CO2 was passed over liquid sodium a rapid reaction occurred at about 450 “C ; the heat liberated by this reaction was so large 4) that
Fig.
1.
Experiment
in which CO2 has been passed
over liquid sodium. The reaction products are clearly visible in the pyrex
boat:
carbon in the form of a
black crust and NszC03 as a white sublimate walls of the boat.
on the
THE
REACTION
liquid sodium burned violently
BETWEES
in COs at this
temperature. The solid reaction product was examined by X-ray diffraction from which only the presence of
NaaCOa
carbon
could
be
deduced.
was in an amorphous
carbides were not formed.
Apparently
form,
whereas
During the reaction,
co2
AND
formed
LIQUID
329
SODIUM
during. a violent
temperatures
(350-400
place somewhat
reaction.
At
“C) the reaction
more slowly,
lower took
and an intimate
mixture of NaaCOa and carbon was formed. The product had a density of 2.4. It should be noted that the temperature which the reaction
started depended
gas samples were collected and examined by mass spectrometry; not even a trace of CO
on the purity
could be detected in the gas samples. Thus, the net reaction can be represented by:
passed over sodium at fixed temperatures, it was found that with pure sodium and purified CO2 a rapid reaction took place already at 425 “C, whereas with CO2 containing 40 ppm water vapour, a rapid reaction was not observed below 525 “C. The same observation was made
4 Na (liq.) + 3C0 2 +
2NaaCOs + C + 260 kcal.
Fig. 1 clearly shows the reaction products NasCOs (as a white sublimate) and carbon,
instance,
of both
in experiments
sodium
at
markedly
and COe. For
in which
CO2 was
800
100
150
200
250 time , minutes
Fig. 2.
Reaction rates (without stirring).
300
35-o
330
E. H. P. CORDFUNKE
with CO2 containing Apparently, the sodium 3.2.
W.
OUWELTJES
small amounts of oxygen,
an oxide layer is than formed on surface and inhibits the reaction,
REACTION
RATES
In order to collect mation
AND
more quantitative
on the sodium-CO2
reaction,
infor-
reaction
rates were measured. Two series of experiments were
done,
one
without
stirring
the
liquid
sodium, as described above; these experiments were carried out at temperatures between 419 and 453 “C. In a second series of experiments, the liquid sodium was stirred. 3.2.1. Reaction
rates without stirring
Reaction rates were measured at temperatures varying from 419 “C to 453 “C! (fig. 2); the initial rates were used to determine the order of the reaction. From the slope of the logarithm of these rates, plotted against the logarithm of the corresponding CO2 pressures, a value of 2 was found for the order of the reaction in COs. This implies that the reaction mechanism can be represented by two reactions, of which the first one is rate-determining: SKa+2COeA4NazO+2C, 4 NazO + 4 COS + 4 Nad203.
TABLE
constants
1
for the second-order
reaction
I Temperature
(“C)
k x 106 (Torr -1. min -I)
*
Not
3.
419
429
440
453
2.15
5.4
8.82
*
measurable.
In the later stages of the reaction (in our experiment at pcoz < 600 Torr), the “order” of the reaction rapidly falls to a value of about 0.5 (fig. 3), because of the fact that the rate falls off more rapidly than would be expected on the basis of the “true” order. This means that reaction intermediates (Na&O& mixture) are formed, bringing about inhibition. This inhibition is also clearly demonstrated when COz
Determination
of the order
from
is contaminated
fig.
of the reactions
2.
with small amounts of oxidizing
gases, such as water vapour or oxygen. Experiments, using CO2 with 40 ppm Hz0 and 120 ppm H20, showed a pronounced decrease in the reaction rates with increasing concentration of water vapour. Obviously, the reaction rates depend on the purity of both sodium and CO2. 3.2.2.
Rate constants for the second-order reaction at different temperatures are collected in table 1.
Rate
Fig.
Reaction rates with stirring
Initial rate experiments in which the liquid sodium was stirred with a stainless steel stirrer, showed that reaction rates comparable with those without stirrer could be obtained at surprisingly lower temperatures ; in fact, at temperatures which are about 200 “C lower. This enabled us to construct the apparatus of Pyrex glass and to take a thermostated bath of silicone oil instead of a furnace. As a result, the whole reaction could be followed visually. Reaction rates were measured at temperatures varying from 177 “C to 260 “C. Fig. 4 shows the results obtained at 241 “C from which it can be seen clearly that the reaction rat’es depend markedly, not only on the temperature, but also on the effect of stirring. In fig. 4 three experiments have been collected, all at the same temperature, but differing in the conditions of stirring. The highest reaction rates are obtained
THE
REACTION
BETWEEN
cog
AND
331
SODITJM
20 “c
700
f
LXQUID
600
0
200
rfmin
*
350
rfmin
A
350
rfmin
L
t
I
500
0”” c, 0
400
0
300
200 A *
fO0 A
0
I
0
1
50
,
*
fO0
200
fS0
250
time , Fig.
4.
Reaction
the surface of liquid sodium is kept in motion by stirring through the surface. It should be noted that in all experiments the initial rates are of zero-order; this is in contrast to the experiments without stirrer. Obviously, the reaction mechanism is different in the experiments without stirrer and controlled by diffusion of CO2 through the surface layer. Figs. 5 and 6 show the reaction tube after the experiment, illustrat~i~lg the influence of the way of stirring on the course of the reaction. In fig. 5 (stirrer under the sodium surface) the formation of a layer of carbon on the sodium surface is clearly visible ; this layer inhibits the reaction markedly. The voluminous reaction when
rates
300
350
minutes
(with sCim3r).
products (Na&X& 1) finally creep up along the stirrer. In fig. 6 (stirrer ~~~o~g~ the sodium surface) reaction rates are much higher than in the experiment shown in fig. Ei.In this case rapid reaction was observed at temperatures as low as 177 “C.
4.
Con&&m
Liquid sodium reacts rapidly with COs at ~mperatures as low as 175 “C. The reaction rates are markedly dependent on the temperature and the contact surface. In static experiments, the sodium surface is rapidly covered with a layer of carbon, inhibiting
332
Fig.
E.
6.
Reaction
H.
I’.
CORDFUNKE
tube after the experiment
(stirrer
AND
W.
OUWELTJES
at 241 “C
under the surface).
the reaction strongly. But when the sodium surface is kept clean, for instance, by stirring, high reaction rates have been observed above 200 “C. At temperatures above 350 “C liquid sodium even burns in COZ.
Fig.
6.
Reaction
tube after the experiment
(stirrer
2)
H. N. Gilbert,
throuyh
at 241 “C
the surface).
Chem. Eng. News 26 (1948) p. 2604
and p. 2660 3)
References 1) Gmelin’s System
M. Sittig,
Sodium,
uses (Reinhold Handbuch nr. 21 (Berlin,
der
anorganischen
1928)
Chemie,
4)
C. E.
Wicks
Mines
(1963)
its manufacture,
Publ. and
F.
Corp., E.
New
Block,
properties York, Bull.
and
1956) 605. Bur.