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The radiolysis of simple gas mixtures—I
Radiat. Phys. Chem. Vol. 20, No. 5--6, 315-321, 1982 Printed in Great Britain.
0146-57241821110315-07503.0010 Pergamon Press Ltd.
THE RADIOLYSIS OF SIMPLE GAS MIXTURES--I RATES OF PRODUCTION AND DESTRUCTION OF METHANE IN MIXTURES WITH CARBON DIOXIDE AS A MAJOR CONSTITUENT ALAN DYERt and GRAHAM E. MOORES~ Department of Chemistry and Applied Chemistry, The University, Salford, M5 4WT, England
(Received 9 July 1982) Abstract--Carbon dioxide based gas mixtures, similar to those used as coolants in the Advanced Gas-Cooled Nuclear Reactors have been radiolysed at the comparatively low dose rate of 3Gy s -~ using y-radiation from a 6°Co source. The variation in the methane concentration (initially in the range 60-760 volume parts per million) with dose, temperature, pressure and gas composition was determined. The gas mixtures were radiolysed in sealed stainless steel capsules and it was found that for a wide range of conditions a steady-state methane concentration was obtained irrespective of the initial methane content of the gas mixture, Packing the irradiation vessel with mild steel, stainless steel or graphite demonstrated that heterogeneous processes played a significant role in the reaction scheme. A mechanism involving the deposition of reactive carbon on surfaces is outlined.
INTRODUCTION THE COOLANT of the U.K. Advanced Gas-Cooled Reactors (AGR) contains methane as an additive designed to inhibit the radiation-induced reaction between the carbon dioxide coolant and the graphite moderator. (1-3) In the commercial versions of the A G R the methane concentration is maintained at around 200--400 vpm (volume parts per million) and the carbon monoxide concentration is allowed to rise to around 1 v/o (percent by v/o volume). The latter component is formed mainly from the oxidation of the methane which may be represented by the overall equation
(1)
CH4 + 3CO2
bria calculated from thermodynamic data. Fessler ~) showed that this is due to competition between radiolytic and thermal water gas shift reactions k
(2)
Hz+C02
~CO+H20 v
>4CO + 2H20.
The carbon monoxide also inhibits the reaction between carbon dioxide and graphite, by the interception of oxidising species, and so acts in competition with the methane. ~4) The four components carbon dioxide, carbon monoxide, hydrogen and water interact in a complicated reaction sequence and attain steady-state conditions which are significantly different from equilitAuthor to whom correspondence should be directed. *Present address: Department of Physical Chemistry, The University, Leeds, LS2 9JT, England.
with water production being favoured at high temperature and hydrogen production being favoured at high -/-radiation dose rates. Previous studies ~6"7~ of the production of methane in these gas mixtures showed that the steady-state concentration was dependent upon the hydrogeneous content of the mixture and upon the concentration of carbon monoxide. However, the effect of radiation on both carbon monoxide and carbon dioxide also has been shown (s'9) to produce carbon suboxide polymers, usually assigned the formula (CaO2)n. These polymers decompose progressively above 570 K; finally becoming pure carbon above 725 K, ~1°'1~) Hence, it remained unclear whether the radiolytic methane production mechanism involved wholly homogeneous reactions and previous studies ~12)have shown that iron can catalyse the methanation of carbon monoxide and carbon dioxide at 573-673 K. The objective of this study was therefore to establish the methane breakdown/production
315
316
A. DYER and G. E. MOORES
characteristics (G(_+CH4) values'D for carbon dioxide based gas mixtures in a low dose rate -/-irradiation facility and re-examine the role of heterogeneous reactions. EXPERIMENTAL The carbon dioxide based gas mixtures were prepared in steel cylinders by U.K.A.E.A., Springfields, and had compositions in the following ranges: carbon monoxide 0.25 v/o to 2.0 v/o; methane 60-760 vpm; hydrogen 3801400 vpm; water approximately 50 vpm. These mixtures were radiolysed in one of two stainless steel capsules of internal dimensions 16mmdia., 98ram length and 19 m m dia. 150 mm length. The y-radiation was from a nominal 300 TBq (10 kCi) ~Co source which gave a dose rate of approximately 3 G y s - t The dose rate was measured by Fricke dosimetry and corrected for carbon dioxide gas. The radiolyses were conducted at various temperatures in the range 300-750K and at various pressures in the range 0.5-5.5 MPa. The total dose absorbed by the sample was in the range 1 kGy-1 MGy. One of the stainless steel capsules was demountable and this facilitated the introduction of graphite, mild steel or stainless steel into the capsule. At the end of the radiolysis period the gas samples were allowed to flow into a helium carrier gas through a gas chromatograph containing a 5A molecular sieve column and then to a helium ionisation detector. The concentration of each component was determined, in triplicate, from a measurement of the peak area and a calibration curve derived from standard gas mixtures prepared by U.K.A.E.A., Springfields. The estimated error in the measurements (2a) was - 5%. In some experiments materials (See Table 1) were placed in the demountable capsule. The packed capsule was then degassed by heating at 750 K, in v a c u o , for 24 h prior to irradiation. RESULTS Typical variations in the methane concentration with dose are shown in Fig. 1. This figure shows the effect of three temperatures on two gas mixtures (see Table 2) each initially containing 2.0 v/o of carbon monoxide but gas A containing 60 vpm of methane and gas B containing 420vpm methane. The results showed that a steady-state methane concentration was established under all the experimental condition investigated except close to ambient temperature. The net rate of radiolytic methane destruction, expressed as G(-CH4),~, and defined by equation (3), was influenced by temperature and an Arrhenius plot for a typical gas mixture is shown in Fig. 2. The G(-CH4)~, values are derived from the slopes of [CH4] vs dose plots at zero dose. (3)
G(-CH4).,,= G(-CH4)
- G(
+
CH4).
i ' G value defined as the number of methane molecules produced or destroyed per 1.6× 10-17J of radiation absorbed by COs.
Table 1. Packing materials Area
Type
Volume
10 3 m m 2 M i l d Steel Stainless
10 3 m m 2
120
3
50
12
Steel
Graphite*
* Anglo-Great Lakes
4
Ltd. (Oilsonite) Moulded Production.
Table 2. Gas mixtures used [ca3]
[~21
[H2ol
[COl
vpm
vpm
vpm
v/o
Gas A
60
600
40
2
Gas B
420
640
50
2
Gas C
530
380
50
0.25
400
300
[CH < , ~ ] 200 vpm I
J00
.
,~
2~
3~
A~
4~o 5& 6& 7&
IL.
8;0
,ooo
Oos_.~e kGy
FiG. 1. Typical variations in the methane concentration with dose. II Gas A, 525 K; I Gas B, 625 K; IV Gas A, 68K; III Gas B, 68K; VI Gas A, 750K; V Gas B, 750 K. Two regions were found to exist, the first (below approximately 680 K) gave an apparent activation energy of less than 4 k J m o l -j and the second (above approximately 700 K) an apparent activation energy of 81 LI m o l - t The results on the effect of mild steel, stainless steel and graphite over the temperature range 300715 K are summarised in Table 3. The presence of mild steel in the capsule resulted in large changes in hydrogen and carbon monoxide concentrations. The resulting G ( CH4).e, values were either very small or negative, i.e. there was net production of methane, even in gas mixtures containing over 400 vpm initially. An
The radiolysis of simple gas mixtures--I
317
Table 3. The effects of various materials on G(--CH4),¢t T/K
300
500
Gas A, empty capsule Gas A, stainless steel added
-
680
715
0.53
0.57
0.86
1.1
0.49
0.86
1.9
1.3
1.0
0.48
0.31
Gas A, graphite added
I
625
Gas B, e~pty capsule
0.41
0.31
0.39
Gas B, mild steel added
0.14
-0.08
-0.09
those close to ambient. The variation in the steady-state concentration with temperature for a typical gas mixture is shown in Fig. 3. The peak steady-state concentration occurred in the range 500-550 K. The variation in the steady-state concentration with pressure is shown in Fig. 4, where the pecked line represents a curve of the form
t
(4) [CH4]p = [CH4]st,ady-stat,{1- exp - k(P - 0.6)}
x
i
J 2
i
i 3
i
103K .-?-
FIG.
2. Arrhenius plot of methane destruction rates (net) for gas mixture C.
increased area of stainless steel increased G ( CH4) at temperatures above 500 K. The presence of graphite appeared to enhance the net destruction rate of methane at the lower end of the temperature range but to reduce it at the higher end. The effect of carbon monoxide was marked: at all temperatures and pressures an increase in the carbon monoxide concentration from 0.25 rio to 2.0 vlo reduced G(-CH4)net by a factor of approximately 1.5-2.0 except at 500K where the reduction was much greater, see Table 4. The hydrogen concentration appeared to have little effect on G(-CH4),ot except at 750K when a reduction in G(-CH4)n,t was noted. A steady-state methane concentration was observed at high doses under all conditions except
and P is the pressure in MPa. The effect of an increased area of stainless steel on the steady-state methane concentration varied, depending upon the gas composition and experimental parameters. The effect of mild steel, however, was to markedly increase the steadystate concentration and even to result in a standing methane concentration at 300 K, see Table 5.
A [CH4] vpm
;
2001
// j.,cf
3o0
x
o/
~'x
'~
J
\
4oo
~
RPC Vol. 20, No. 5 - 6 - - B
7& "
FIO. 3. Variation in the steady-state methane concentration with temperature at a pressure of 4.1 MPa.
Table 4. The effect on G(--CH4)netof carbon monoxide concentration T/K
640 T
300
500
625
680
750
Gas A, 0.25 V/o CO
0.39
0.53
0.57
0.86
2.9
Gas B, 2 . 0 V/o CO
0.32
41x10- 3
0.32
0.61
1.8
A. DYER and G. E. MOORES
318
The effect of an increased carbon monoxide concentration was similar at 500 K and 680 K although less marked at the latter temperature.
4OO
300
DISCUSSION In all experiments (except those at 300K) methane was always destroyed and produced, i.e. the observed G(-CH4) contained a contribution from a G ( + C H 4 ) term and hence all measured changes in the methane concentration were net Changes, The observed G(-CH4),et values increased with increasing temperature in nearly all the empty capsule experiments. The exceptions to this trend were two gas mixtures containing a high carbon monoxide concentration. These mixtures gave a low or negative G(-CH4)not value at 500 and 625 K. It was possible that G(-CH4) was low under these conditions but more probable that G ( + CH4) was comparatively large. This suggested that carbon monoxide played a role in methane production, at least in this temperature range. The values of G(-CH4),et obtained from the initial set of experiments were considerably larger than the later values and it seemed probable that the s t e e l was acting catalytically. The Arrhenius plots showed the existence of two temperature regimes. The lower temperature regime, up to approximately 680 K, yielded an apparent energy of activation of less than 4 k J m o l - ' , which was a lower value than had been observed previously. (~3't4) The higher temperature regime, above approximately 700K, yielded a value of 81 kJ mol -~ which agreed well with these workers. The results on the effect of pressure on G ( CH4)net were inconclusive. The value was increased at 300 and 400 K but decreased at the
.u.-
Ioo
6 P MPo
FIG. 4. Variation in the steady-state methane concentration with pressure at a temperature of 5110K. Table 5. The effect of stainless steel and mild steel on the steady-state methane concentration (vpm) T/K Empty capsule Mild steel
300
500
625
680
2
180
II0
75
90
335
520
-
160
190
60
Stainless steel
The effect of gas composition on the steadystate methane concentration is shown in Fig. 5 for gases radiolysed at 625 K. The abscissa of Fig. 5 is in units of total hydrogenous concentrations (THC) as defined by equation (5)
(5)
THC = 2[CH4] + [H20] + [H2]. 400
0 /s /
/
i / 300
1I /I r j
[CH,] v!am
//J 200
~,t
•
~'~"
/
s aa
I
500
IOOO
1500
2000
2500
TH_~C vpm
FIG. 5: Variation in the methane concentration with THC at 625 K. O, 2% CO; O, 1% CO; ©,
0.25 vie CO.
The radiolysis of simple gas mixtures--I higher temperatures. At 625K, a reduction in pressure from 3.6 to 1.6MPa reduced the G(CH4)n~t value to approximately 0,7 of its value. Previous work on the effect of pressure "3) showed that a 625 K reduction in pressure from 3.7 to 2.1 MPa reduced the value of G(-CH4),~ to approximately 0.9 of its value: The effect of an increased carbon monoxide concentration was to reduce the value of G(-CH4) at all temperatures. This was in line with the well established theory of gas phase competition for the oxidising species proposed by Dominey. ~5 Increasing the carbon monoxide concentration from 0.25 to 2.0% halved G(-CH4) .... This was in good agreement with recent work by Norwood° ° who showed that the concentration of active species in radiolysed carbon dioxide (determined by the rate of arrival at a carbon surface) was inversely proportional to [CO]°'~ as predicted by classical apparent mean free path theory. The effect of carbon monoxide was much more marked at 500 K and this was probably due to the sum of the two processes given above, viz. gas phase competition and a reduction in G(-CH4)~, at 500K in high carbon monoxide gases due to the increase in ~( + CH,). The presence of mild steel in the capsule gave rise to large changes in gas composition. At 500 K, the hydrogen concentration in the gas was very high--typically 1500--2000vpm (640 vpm initially in the gas). at 625 K the carbon monoxide concentration was approximately doubled to 1.9 v/o at high dose. A possible explanation for these phenomena was that hydrogen was evolved from the steel during the experiments. This remained as hydrogen at 500K; the water gas shift reaction proceeding at a very slow rate at this temperature. At 625 K, the hydrogen converted carbon dioxide to carbon monoxide via the shift reaction. To produce the observed quantity of carbon monoxide would have required some 10,000vpm of hydrogen. An alternative explanation was that water was desorbed from the steel during the experiments and at 500 K the shift reaction moved in favour of hydrogen. However, this would have resulted in the oxidation of carbon monoxide and a change in its concentration. This was not observed. at 625 K, carbon monoxide could have been produced from carbon dioxide via reaction (6). (6)
4CO2 + 3Fe
~Fe304 + 4CO.
The presence of mild steel gave rise to a net production of methane at 500 and 625 K, The effect was more marked at 625 K though less
319
marked in later experiments under similar conditions. In one experiment an additional 1100 vpm of methane was produced. Dwyer and Somorjaicm showed that iron foils catalysed the hydrogenation of carbon monoxide and carbon dioxide at 573 K and proposed reaction (7) for the production of methane. (7)
Fe
3H: + CO
~ CH4 + H20.
They found that the iron was rapidly poisoned by carbonaceous deposition and continued to produce methane only at a very low rate. An alternative route for methanisation could be the disproportionation of carbon monoxide, already present and produced via reaction (6), at the iron surface. Brown et al. "7) proposed reaction (8) to represent the disproportionation and the carbon formed could have been methanated at the hydrogen-rich surface via a reaction of the type given in equation (9). (8) (9)
2CO + Fe C + 2H2
~(FeC) ~(CH4)
~,Fe + C + CO2. ' CH4.
surface
Again the active sites were probably poisoned by the carbon and this resulted in a lower rate of methane production. When the mild steel was removed from the capsule it was found to be covered with two types of deposit: a black, amorphous deposit and, where this covered more sparsely, a brownish film. The former was probably carbonaceous and the latter an iron oxide. The possibility of iron carbonyl formation arose with the mild steel present. At the temperatures employed in the experiments any carbonyls formed would have decomposed into iron and carbon monoxide and their influence on the reactions was thought to be negligible. The effect of an increased area of stainless steel was to reduce G(-CH4)n~t for a gas mixture containing 1 v/o carbon monoxide at 500, 625 and 680 K, but to reduce it only at 500K for a gas mixture containing 0.25 v/o carbon monoxide. In the latter c a s e G(--CH4)net was increased at 625, 680 and 715K. Clearly heterogeneous reactions were taking place but if the steel affected both G(-CH4) and G(+ CH4) the effect on G(-CH4)not might have appeared complex. A possible mechanism to explain the observations was as follows. The stainless steel affected both G(-CH4) and G(+ CH4). The former was in-
320
A. DYERand G. E. MOORES
creased by the catalytic action of the steel, particularly at the higher temperatures. The production of methane was a heterogenous reaction, at least in part, and occurred via a reaction between deposited carbon and adsorbed hydrogen. The carbon was formed from the carbon suboxide polymer on the steel surfaces. The polymer decomposed progressively as the temperature increased. Thus the amount of available carbon increased with the surface area available for deposition, the carbon monoxide concentration of the gas and increasing temperature. However, above 500-600K the hydrogen concentration was progressively reduced by increasing temperature, i.e. by the shift reaction. Now the production rate of methane, G(+ CH4), depended upon the availability of both carbon and hydrogen and thus tended to peak around 500-600 K. For a gas mixture containing 0.25 v/o carbon monoxide and 380vpm hydrogen, the formation of carbon via the suboxide was limited. The effect of stainless steel on the G(-CH4) value outweighed the effect on G(+ CH4) except near the most favourable temperature for methane production, viz. K. For a gas mixture containing 1 v/o carbon monoxide and 640vpm hydrogen, the carbon formation was greater and g(-CH4),,t was reduced at 500, 625 and 680 K. This scheme was summarised by equations (10)(12). (lO)
(11) (12)
CO
i. . . . . . ing) C --~ C 0 2 3, ) C 3 0 2 .__)(C302)n temp. . . . . . .
H2+CO2 C + 2H2
3".i...... i.~ C O + H 2 0 temperature
~ (CH4) surface
~. CH4
The effect of graphite was to reduce G(--CH4)net in all experiments except those at 500 K, where it was increased. The data on the affect of graphite were limited but they tended to support the mechanics outlined above. Experiments with a gas mixture containing a high hydrogen concentration and a mixture containing a high carbon monoxide concentration produced large reductions in G ( CH4)net. Not only did the graphite provide a large increase in surface area but also an additional source of carbon atoms. The first experiment to be performed with the graphite present produced an additional l l 5 v p m of methane and a possible explanation was that carbon atoms at active sites on the graphite were hydrogenated to methane. Campioncm observed that G ( - C H 4 ) was reduced in graphite pores and this phenomena could have contributed to the overall reduction.
At 680 K the high hydrogen concentration in one of the gas mixtures appeared to have little effect on G ( - C H 4 ) .... At 750K the hydrogen was rapidly converted to water via the shift reaction and G(-CH4)net was approximately halved. This reduction might have been bought about by gas phase competition, for oxidising species, between methane and water. A contribution might also have arisen from the blocking of catalytic sites on the steel by adsorbed water. The reduction in G(-CH4)net compared well with the results of Campion. "8) However, the latter study did not differentiate between hydrogen and water in their effect on G(-CH4)n~, and Pritchard et al. ~19~ expected gas phase competition between methane and water to be significant only in the graphite pores. At high doses a steady-state methane concentration was observed in all the experiments except those at 300 K. The highest steady-state concentration was usually at 500 K, although occasionally it was at 625 K. In all experiments where pressure was a variable it increased the steady-state coecentration. The composition of the gas mixture had a substantial effect on the steady-state concentration, increasing either the carbon monoxide or the "total hydrogenous concentration" increased the methane concentration. Previous studies <2°)have shown that the steadystate methane concentration is increased by increasing the pressure. The present results gave a good fit to the exponential relationship given in equation (4). The steady-state methane concentration, for a given gas mixture and pressure increased with increasing temperature up to a maximum value around 500-625 K. The maximum for a 1 v/o carbon monoxide gas mixture which was investigated more closely in this respect, was around 550 K. The steady state concentration then decreased with further increase in temperature. The mechanism already proposed, in which suboxide polymer was carbonised and hydrogen was converted to water could be invoked to explain this variation with temperature. Another mechanism was that any prodtiction process was temperature dependent and that G(+ CH4) increased with increasing temperature in the region where G ( CH4) was essentially constant. At higher temperatures G ( - C H 4 ) entered a region of high activation energy and consequently the steadystate concentration fell. However, the G(-CH4) term did not enter the high activation energy region until approximately 580 K and the steadystate concentration might have been expected to
The radiolysis of simple gas mixtures--I increase until this temperature. This would have been more pronounced if both hydrogen and water were involved in the production mechanism since the water gas shift reaction could not then be invoked to reduce the hydrogen concentration at temperatures below 680 K. However, any mechanism involving hydrogen alone could not be ruled out; for example a purely gas phase reaction between carbon monoxide and hydrogen having a non-zero activation energy. The presence of graphite in the capsule tended to increase the steady-state concentration but it was not clear whether or not this was due to a reduction in G ( - CH4) or an increase in G ( + CH4) or both. As stated previously, graphite provided both a large surface area and a source of carbon atoms and might have been expected to increase G( + CH,). The steady-state concentrations in the presence of mild steel were much higher than the corresponding empty capsule results. This was attributed to catalytic action resulting in larger G ( + CH4) values similar to those observed at low doses.
Acknowledgements--The authors are grateful to Dr. P. Campion, Mr A. Blanchard, Dr. P. A. V. Johnson and their colleagues at the Springtlelds Nuclear Power Developments Laboratories for many helpful discussions and to the U.K. Atomic Energy Authority Northern Division for financial support during the tenure of this work.
321
REFERENCES 1. M. F. SHEPPARD,H. C. COWENand H. L. ROBINSON, U.K.A.E.A. Report TRG-R767(c), 1964. 2. J. WRIGHTet al., 4th Int. Conf. Peaceful Uses Atomic Energy, Geneva, 1971, 10, 415. 3. D.A. DOMINEY,H. MORLEYand R. J. WAITE,A.E.R.E. Report R4987, 1967. 4. A. J. WICKHAMet al., Radiat. Phys. Chem. 1977, 10, 107. 5. J. -C. FESSLER, Ph.D. Thesis, University of Grenoble, 1971. 6. W. R. GODFREYand P. PHENNAH,J. B N E S 1968, 7, 224. 7. P. CAMPIONand A. BLANCHARD,Unpublished work. 8. P. HARTECKand S. DONDES,J. Chem. Phys. 1955, 23, 902. 9. P. HARTECKand S. DONDES,J. Chem. Phys. 1957, 26, 1727. 10. A. R. BLAKE,W. T. EELES and P. P. JENNINGS,Trans. Far. Soc. 1964, 60, 691. 11. A.R. BLAKEand A. F. HYDE, Trans. Far. Soc. 1964,60, 1775. 12. D. J. DWYERand G. A. SOMORJAI,J. Catalysis 1978, 52, 291. 13. P. CAMPION, M.Sc. Thesis, University of Salford, 1972. 14. D. J. NORFOLK et al., Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), p. 67. Heyden, London, 1980. 15. D. A. DGMINEY,A.E.R.E. Report R3481, 1961. 16. K. S. NORWOOD,private communication. 17. A. B. BROWNet al., Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), p. 26. Heyden, London, 1980. 18. P. CAMPION, Ph.D. Thesis, University of Salford, 1977. 19. J. A. PRrrCHARD,Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), p. 239. Heyden, London, 1980. 20. P. CAMPION,Private communication.
0146-57241821110315-07503.0010 Pergamon Press Ltd.
THE RADIOLYSIS OF SIMPLE GAS MIXTURES--I RATES OF PRODUCTION AND DESTRUCTION OF METHANE IN MIXTURES WITH CARBON DIOXIDE AS A MAJOR CONSTITUENT ALAN DYERt and GRAHAM E. MOORES~ Department of Chemistry and Applied Chemistry, The University, Salford, M5 4WT, England
(Received 9 July 1982) Abstract--Carbon dioxide based gas mixtures, similar to those used as coolants in the Advanced Gas-Cooled Nuclear Reactors have been radiolysed at the comparatively low dose rate of 3Gy s -~ using y-radiation from a 6°Co source. The variation in the methane concentration (initially in the range 60-760 volume parts per million) with dose, temperature, pressure and gas composition was determined. The gas mixtures were radiolysed in sealed stainless steel capsules and it was found that for a wide range of conditions a steady-state methane concentration was obtained irrespective of the initial methane content of the gas mixture, Packing the irradiation vessel with mild steel, stainless steel or graphite demonstrated that heterogeneous processes played a significant role in the reaction scheme. A mechanism involving the deposition of reactive carbon on surfaces is outlined.
INTRODUCTION THE COOLANT of the U.K. Advanced Gas-Cooled Reactors (AGR) contains methane as an additive designed to inhibit the radiation-induced reaction between the carbon dioxide coolant and the graphite moderator. (1-3) In the commercial versions of the A G R the methane concentration is maintained at around 200--400 vpm (volume parts per million) and the carbon monoxide concentration is allowed to rise to around 1 v/o (percent by v/o volume). The latter component is formed mainly from the oxidation of the methane which may be represented by the overall equation
(1)
CH4 + 3CO2
bria calculated from thermodynamic data. Fessler ~) showed that this is due to competition between radiolytic and thermal water gas shift reactions k
(2)
Hz+C02
~CO+H20 v
>4CO + 2H20.
The carbon monoxide also inhibits the reaction between carbon dioxide and graphite, by the interception of oxidising species, and so acts in competition with the methane. ~4) The four components carbon dioxide, carbon monoxide, hydrogen and water interact in a complicated reaction sequence and attain steady-state conditions which are significantly different from equilitAuthor to whom correspondence should be directed. *Present address: Department of Physical Chemistry, The University, Leeds, LS2 9JT, England.
with water production being favoured at high temperature and hydrogen production being favoured at high -/-radiation dose rates. Previous studies ~6"7~ of the production of methane in these gas mixtures showed that the steady-state concentration was dependent upon the hydrogeneous content of the mixture and upon the concentration of carbon monoxide. However, the effect of radiation on both carbon monoxide and carbon dioxide also has been shown (s'9) to produce carbon suboxide polymers, usually assigned the formula (CaO2)n. These polymers decompose progressively above 570 K; finally becoming pure carbon above 725 K, ~1°'1~) Hence, it remained unclear whether the radiolytic methane production mechanism involved wholly homogeneous reactions and previous studies ~12)have shown that iron can catalyse the methanation of carbon monoxide and carbon dioxide at 573-673 K. The objective of this study was therefore to establish the methane breakdown/production
315
316
A. DYER and G. E. MOORES
characteristics (G(_+CH4) values'D for carbon dioxide based gas mixtures in a low dose rate -/-irradiation facility and re-examine the role of heterogeneous reactions. EXPERIMENTAL The carbon dioxide based gas mixtures were prepared in steel cylinders by U.K.A.E.A., Springfields, and had compositions in the following ranges: carbon monoxide 0.25 v/o to 2.0 v/o; methane 60-760 vpm; hydrogen 3801400 vpm; water approximately 50 vpm. These mixtures were radiolysed in one of two stainless steel capsules of internal dimensions 16mmdia., 98ram length and 19 m m dia. 150 mm length. The y-radiation was from a nominal 300 TBq (10 kCi) ~Co source which gave a dose rate of approximately 3 G y s - t The dose rate was measured by Fricke dosimetry and corrected for carbon dioxide gas. The radiolyses were conducted at various temperatures in the range 300-750K and at various pressures in the range 0.5-5.5 MPa. The total dose absorbed by the sample was in the range 1 kGy-1 MGy. One of the stainless steel capsules was demountable and this facilitated the introduction of graphite, mild steel or stainless steel into the capsule. At the end of the radiolysis period the gas samples were allowed to flow into a helium carrier gas through a gas chromatograph containing a 5A molecular sieve column and then to a helium ionisation detector. The concentration of each component was determined, in triplicate, from a measurement of the peak area and a calibration curve derived from standard gas mixtures prepared by U.K.A.E.A., Springfields. The estimated error in the measurements (2a) was - 5%. In some experiments materials (See Table 1) were placed in the demountable capsule. The packed capsule was then degassed by heating at 750 K, in v a c u o , for 24 h prior to irradiation. RESULTS Typical variations in the methane concentration with dose are shown in Fig. 1. This figure shows the effect of three temperatures on two gas mixtures (see Table 2) each initially containing 2.0 v/o of carbon monoxide but gas A containing 60 vpm of methane and gas B containing 420vpm methane. The results showed that a steady-state methane concentration was established under all the experimental condition investigated except close to ambient temperature. The net rate of radiolytic methane destruction, expressed as G(-CH4),~, and defined by equation (3), was influenced by temperature and an Arrhenius plot for a typical gas mixture is shown in Fig. 2. The G(-CH4)~, values are derived from the slopes of [CH4] vs dose plots at zero dose. (3)
G(-CH4).,,= G(-CH4)
- G(
+
CH4).
i ' G value defined as the number of methane molecules produced or destroyed per 1.6× 10-17J of radiation absorbed by COs.
Table 1. Packing materials Area
Type
Volume
10 3 m m 2 M i l d Steel Stainless
10 3 m m 2
120
3
50
12
Steel
Graphite*
* Anglo-Great Lakes
4
Ltd. (Oilsonite) Moulded Production.
Table 2. Gas mixtures used [ca3]
[~21
[H2ol
[COl
vpm
vpm
vpm
v/o
Gas A
60
600
40
2
Gas B
420
640
50
2
Gas C
530
380
50
0.25
400
300
[CH < , ~ ] 200 vpm I
J00
.
,~
2~
3~
A~
4~o 5& 6& 7&
IL.
8;0
,ooo
Oos_.~e kGy
FiG. 1. Typical variations in the methane concentration with dose. II Gas A, 525 K; I Gas B, 625 K; IV Gas A, 68K; III Gas B, 68K; VI Gas A, 750K; V Gas B, 750 K. Two regions were found to exist, the first (below approximately 680 K) gave an apparent activation energy of less than 4 k J m o l -j and the second (above approximately 700 K) an apparent activation energy of 81 LI m o l - t The results on the effect of mild steel, stainless steel and graphite over the temperature range 300715 K are summarised in Table 3. The presence of mild steel in the capsule resulted in large changes in hydrogen and carbon monoxide concentrations. The resulting G ( CH4).e, values were either very small or negative, i.e. there was net production of methane, even in gas mixtures containing over 400 vpm initially. An
The radiolysis of simple gas mixtures--I
317
Table 3. The effects of various materials on G(--CH4),¢t T/K
300
500
Gas A, empty capsule Gas A, stainless steel added
-
680
715
0.53
0.57
0.86
1.1
0.49
0.86
1.9
1.3
1.0
0.48
0.31
Gas A, graphite added
I
625
Gas B, e~pty capsule
0.41
0.31
0.39
Gas B, mild steel added
0.14
-0.08
-0.09
those close to ambient. The variation in the steady-state concentration with temperature for a typical gas mixture is shown in Fig. 3. The peak steady-state concentration occurred in the range 500-550 K. The variation in the steady-state concentration with pressure is shown in Fig. 4, where the pecked line represents a curve of the form
t
(4) [CH4]p = [CH4]st,ady-stat,{1- exp - k(P - 0.6)}
x
i
J 2
i
i 3
i
103K .-?-
FIG.
2. Arrhenius plot of methane destruction rates (net) for gas mixture C.
increased area of stainless steel increased G ( CH4) at temperatures above 500 K. The presence of graphite appeared to enhance the net destruction rate of methane at the lower end of the temperature range but to reduce it at the higher end. The effect of carbon monoxide was marked: at all temperatures and pressures an increase in the carbon monoxide concentration from 0.25 rio to 2.0 vlo reduced G(-CH4)net by a factor of approximately 1.5-2.0 except at 500K where the reduction was much greater, see Table 4. The hydrogen concentration appeared to have little effect on G(-CH4),ot except at 750K when a reduction in G(-CH4)n,t was noted. A steady-state methane concentration was observed at high doses under all conditions except
and P is the pressure in MPa. The effect of an increased area of stainless steel on the steady-state methane concentration varied, depending upon the gas composition and experimental parameters. The effect of mild steel, however, was to markedly increase the steadystate concentration and even to result in a standing methane concentration at 300 K, see Table 5.
A [CH4] vpm
;
2001
// j.,cf
3o0
x
o/
~'x
'~
J
\
4oo
~
RPC Vol. 20, No. 5 - 6 - - B
7& "
FIO. 3. Variation in the steady-state methane concentration with temperature at a pressure of 4.1 MPa.
Table 4. The effect on G(--CH4)netof carbon monoxide concentration T/K
640 T
300
500
625
680
750
Gas A, 0.25 V/o CO
0.39
0.53
0.57
0.86
2.9
Gas B, 2 . 0 V/o CO
0.32
41x10- 3
0.32
0.61
1.8
A. DYER and G. E. MOORES
318
The effect of an increased carbon monoxide concentration was similar at 500 K and 680 K although less marked at the latter temperature.
4OO
300
DISCUSSION In all experiments (except those at 300K) methane was always destroyed and produced, i.e. the observed G(-CH4) contained a contribution from a G ( + C H 4 ) term and hence all measured changes in the methane concentration were net Changes, The observed G(-CH4),et values increased with increasing temperature in nearly all the empty capsule experiments. The exceptions to this trend were two gas mixtures containing a high carbon monoxide concentration. These mixtures gave a low or negative G(-CH4)not value at 500 and 625 K. It was possible that G(-CH4) was low under these conditions but more probable that G ( + CH4) was comparatively large. This suggested that carbon monoxide played a role in methane production, at least in this temperature range. The values of G(-CH4),et obtained from the initial set of experiments were considerably larger than the later values and it seemed probable that the s t e e l was acting catalytically. The Arrhenius plots showed the existence of two temperature regimes. The lower temperature regime, up to approximately 680 K, yielded an apparent energy of activation of less than 4 k J m o l - ' , which was a lower value than had been observed previously. (~3't4) The higher temperature regime, above approximately 700K, yielded a value of 81 kJ mol -~ which agreed well with these workers. The results on the effect of pressure on G ( CH4)net were inconclusive. The value was increased at 300 and 400 K but decreased at the
.u.-
Ioo
6 P MPo
FIG. 4. Variation in the steady-state methane concentration with pressure at a temperature of 5110K. Table 5. The effect of stainless steel and mild steel on the steady-state methane concentration (vpm) T/K Empty capsule Mild steel
300
500
625
680
2
180
II0
75
90
335
520
-
160
190
60
Stainless steel
The effect of gas composition on the steadystate methane concentration is shown in Fig. 5 for gases radiolysed at 625 K. The abscissa of Fig. 5 is in units of total hydrogenous concentrations (THC) as defined by equation (5)
(5)
THC = 2[CH4] + [H20] + [H2]. 400
0 /s /
/
i / 300
1I /I r j
[CH,] v!am
//J 200
~,t
•
~'~"
/
s aa
I
500
IOOO
1500
2000
2500
TH_~C vpm
FIG. 5: Variation in the methane concentration with THC at 625 K. O, 2% CO; O, 1% CO; ©,
0.25 vie CO.
The radiolysis of simple gas mixtures--I higher temperatures. At 625K, a reduction in pressure from 3.6 to 1.6MPa reduced the G(CH4)n~t value to approximately 0,7 of its value. Previous work on the effect of pressure "3) showed that a 625 K reduction in pressure from 3.7 to 2.1 MPa reduced the value of G(-CH4),~ to approximately 0.9 of its value: The effect of an increased carbon monoxide concentration was to reduce the value of G(-CH4) at all temperatures. This was in line with the well established theory of gas phase competition for the oxidising species proposed by Dominey. ~5 Increasing the carbon monoxide concentration from 0.25 to 2.0% halved G(-CH4) .... This was in good agreement with recent work by Norwood° ° who showed that the concentration of active species in radiolysed carbon dioxide (determined by the rate of arrival at a carbon surface) was inversely proportional to [CO]°'~ as predicted by classical apparent mean free path theory. The effect of carbon monoxide was much more marked at 500 K and this was probably due to the sum of the two processes given above, viz. gas phase competition and a reduction in G(-CH4)~, at 500K in high carbon monoxide gases due to the increase in ~( + CH,). The presence of mild steel in the capsule gave rise to large changes in gas composition. At 500 K, the hydrogen concentration in the gas was very high--typically 1500--2000vpm (640 vpm initially in the gas). at 625 K the carbon monoxide concentration was approximately doubled to 1.9 v/o at high dose. A possible explanation for these phenomena was that hydrogen was evolved from the steel during the experiments. This remained as hydrogen at 500K; the water gas shift reaction proceeding at a very slow rate at this temperature. At 625 K, the hydrogen converted carbon dioxide to carbon monoxide via the shift reaction. To produce the observed quantity of carbon monoxide would have required some 10,000vpm of hydrogen. An alternative explanation was that water was desorbed from the steel during the experiments and at 500 K the shift reaction moved in favour of hydrogen. However, this would have resulted in the oxidation of carbon monoxide and a change in its concentration. This was not observed. at 625 K, carbon monoxide could have been produced from carbon dioxide via reaction (6). (6)
4CO2 + 3Fe
~Fe304 + 4CO.
The presence of mild steel gave rise to a net production of methane at 500 and 625 K, The effect was more marked at 625 K though less
319
marked in later experiments under similar conditions. In one experiment an additional 1100 vpm of methane was produced. Dwyer and Somorjaicm showed that iron foils catalysed the hydrogenation of carbon monoxide and carbon dioxide at 573 K and proposed reaction (7) for the production of methane. (7)
Fe
3H: + CO
~ CH4 + H20.
They found that the iron was rapidly poisoned by carbonaceous deposition and continued to produce methane only at a very low rate. An alternative route for methanisation could be the disproportionation of carbon monoxide, already present and produced via reaction (6), at the iron surface. Brown et al. "7) proposed reaction (8) to represent the disproportionation and the carbon formed could have been methanated at the hydrogen-rich surface via a reaction of the type given in equation (9). (8) (9)
2CO + Fe C + 2H2
~(FeC) ~(CH4)
~,Fe + C + CO2. ' CH4.
surface
Again the active sites were probably poisoned by the carbon and this resulted in a lower rate of methane production. When the mild steel was removed from the capsule it was found to be covered with two types of deposit: a black, amorphous deposit and, where this covered more sparsely, a brownish film. The former was probably carbonaceous and the latter an iron oxide. The possibility of iron carbonyl formation arose with the mild steel present. At the temperatures employed in the experiments any carbonyls formed would have decomposed into iron and carbon monoxide and their influence on the reactions was thought to be negligible. The effect of an increased area of stainless steel was to reduce G(-CH4)n~t for a gas mixture containing 1 v/o carbon monoxide at 500, 625 and 680 K, but to reduce it only at 500K for a gas mixture containing 0.25 v/o carbon monoxide. In the latter c a s e G(--CH4)net was increased at 625, 680 and 715K. Clearly heterogeneous reactions were taking place but if the steel affected both G(-CH4) and G(+ CH4) the effect on G(-CH4)not might have appeared complex. A possible mechanism to explain the observations was as follows. The stainless steel affected both G(-CH4) and G(+ CH4). The former was in-
320
A. DYERand G. E. MOORES
creased by the catalytic action of the steel, particularly at the higher temperatures. The production of methane was a heterogenous reaction, at least in part, and occurred via a reaction between deposited carbon and adsorbed hydrogen. The carbon was formed from the carbon suboxide polymer on the steel surfaces. The polymer decomposed progressively as the temperature increased. Thus the amount of available carbon increased with the surface area available for deposition, the carbon monoxide concentration of the gas and increasing temperature. However, above 500-600K the hydrogen concentration was progressively reduced by increasing temperature, i.e. by the shift reaction. Now the production rate of methane, G(+ CH4), depended upon the availability of both carbon and hydrogen and thus tended to peak around 500-600 K. For a gas mixture containing 0.25 v/o carbon monoxide and 380vpm hydrogen, the formation of carbon via the suboxide was limited. The effect of stainless steel on the G(-CH4) value outweighed the effect on G(+ CH4) except near the most favourable temperature for methane production, viz. K. For a gas mixture containing 1 v/o carbon monoxide and 640vpm hydrogen, the carbon formation was greater and g(-CH4),,t was reduced at 500, 625 and 680 K. This scheme was summarised by equations (10)(12). (lO)
(11) (12)
CO
i. . . . . . ing) C --~ C 0 2 3, ) C 3 0 2 .__)(C302)n temp. . . . . . .
H2+CO2 C + 2H2
3".i...... i.~ C O + H 2 0 temperature
~ (CH4) surface
~. CH4
The effect of graphite was to reduce G(--CH4)net in all experiments except those at 500 K, where it was increased. The data on the affect of graphite were limited but they tended to support the mechanics outlined above. Experiments with a gas mixture containing a high hydrogen concentration and a mixture containing a high carbon monoxide concentration produced large reductions in G ( CH4)net. Not only did the graphite provide a large increase in surface area but also an additional source of carbon atoms. The first experiment to be performed with the graphite present produced an additional l l 5 v p m of methane and a possible explanation was that carbon atoms at active sites on the graphite were hydrogenated to methane. Campioncm observed that G ( - C H 4 ) was reduced in graphite pores and this phenomena could have contributed to the overall reduction.
At 680 K the high hydrogen concentration in one of the gas mixtures appeared to have little effect on G ( - C H 4 ) .... At 750K the hydrogen was rapidly converted to water via the shift reaction and G(-CH4)net was approximately halved. This reduction might have been bought about by gas phase competition, for oxidising species, between methane and water. A contribution might also have arisen from the blocking of catalytic sites on the steel by adsorbed water. The reduction in G(-CH4)net compared well with the results of Campion. "8) However, the latter study did not differentiate between hydrogen and water in their effect on G(-CH4)n~, and Pritchard et al. ~19~ expected gas phase competition between methane and water to be significant only in the graphite pores. At high doses a steady-state methane concentration was observed in all the experiments except those at 300 K. The highest steady-state concentration was usually at 500 K, although occasionally it was at 625 K. In all experiments where pressure was a variable it increased the steady-state coecentration. The composition of the gas mixture had a substantial effect on the steady-state concentration, increasing either the carbon monoxide or the "total hydrogenous concentration" increased the methane concentration. Previous studies <2°)have shown that the steadystate methane concentration is increased by increasing the pressure. The present results gave a good fit to the exponential relationship given in equation (4). The steady-state methane concentration, for a given gas mixture and pressure increased with increasing temperature up to a maximum value around 500-625 K. The maximum for a 1 v/o carbon monoxide gas mixture which was investigated more closely in this respect, was around 550 K. The steady state concentration then decreased with further increase in temperature. The mechanism already proposed, in which suboxide polymer was carbonised and hydrogen was converted to water could be invoked to explain this variation with temperature. Another mechanism was that any prodtiction process was temperature dependent and that G(+ CH4) increased with increasing temperature in the region where G ( CH4) was essentially constant. At higher temperatures G ( - C H 4 ) entered a region of high activation energy and consequently the steadystate concentration fell. However, the G(-CH4) term did not enter the high activation energy region until approximately 580 K and the steadystate concentration might have been expected to
The radiolysis of simple gas mixtures--I increase until this temperature. This would have been more pronounced if both hydrogen and water were involved in the production mechanism since the water gas shift reaction could not then be invoked to reduce the hydrogen concentration at temperatures below 680 K. However, any mechanism involving hydrogen alone could not be ruled out; for example a purely gas phase reaction between carbon monoxide and hydrogen having a non-zero activation energy. The presence of graphite in the capsule tended to increase the steady-state concentration but it was not clear whether or not this was due to a reduction in G ( - CH4) or an increase in G ( + CH4) or both. As stated previously, graphite provided both a large surface area and a source of carbon atoms and might have been expected to increase G( + CH,). The steady-state concentrations in the presence of mild steel were much higher than the corresponding empty capsule results. This was attributed to catalytic action resulting in larger G ( + CH4) values similar to those observed at low doses.
Acknowledgements--The authors are grateful to Dr. P. Campion, Mr A. Blanchard, Dr. P. A. V. Johnson and their colleagues at the Springtlelds Nuclear Power Developments Laboratories for many helpful discussions and to the U.K. Atomic Energy Authority Northern Division for financial support during the tenure of this work.
321
REFERENCES 1. M. F. SHEPPARD,H. C. COWENand H. L. ROBINSON, U.K.A.E.A. Report TRG-R767(c), 1964. 2. J. WRIGHTet al., 4th Int. Conf. Peaceful Uses Atomic Energy, Geneva, 1971, 10, 415. 3. D.A. DOMINEY,H. MORLEYand R. J. WAITE,A.E.R.E. Report R4987, 1967. 4. A. J. WICKHAMet al., Radiat. Phys. Chem. 1977, 10, 107. 5. J. -C. FESSLER, Ph.D. Thesis, University of Grenoble, 1971. 6. W. R. GODFREYand P. PHENNAH,J. B N E S 1968, 7, 224. 7. P. CAMPIONand A. BLANCHARD,Unpublished work. 8. P. HARTECKand S. DONDES,J. Chem. Phys. 1955, 23, 902. 9. P. HARTECKand S. DONDES,J. Chem. Phys. 1957, 26, 1727. 10. A. R. BLAKE,W. T. EELES and P. P. JENNINGS,Trans. Far. Soc. 1964, 60, 691. 11. A.R. BLAKEand A. F. HYDE, Trans. Far. Soc. 1964,60, 1775. 12. D. J. DWYERand G. A. SOMORJAI,J. Catalysis 1978, 52, 291. 13. P. CAMPION, M.Sc. Thesis, University of Salford, 1972. 14. D. J. NORFOLK et al., Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), p. 67. Heyden, London, 1980. 15. D. A. DGMINEY,A.E.R.E. Report R3481, 1961. 16. K. S. NORWOOD,private communication. 17. A. B. BROWNet al., Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), p. 26. Heyden, London, 1980. 18. P. CAMPION, Ph.D. Thesis, University of Salford, 1977. 19. J. A. PRrrCHARD,Gas Chemistry in Nuclear Reactors and Large Industrial Plant (Edited by A. Dyer), p. 239. Heyden, London, 1980. 20. P. CAMPION,Private communication.