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On the oxidation behaviour of C-C composites
Carbon Vol. 19.No.5, p,401, 1981 Printed inGreal Britain.
03@-6223/81/0544014)1502.00/0 Pergamcn Press Ltd.
LETTERS TO THE EDITOR
On the oxidation behaviour of C-C composites (Receioed 8 September 1980) The authors do not quote the flow rate values used in all their experiments. They only give figures for those runs in which flow rate was intentionally varied. These experiments were intended to confirm, in their understanding, the presence of external diffusion conditions [2]. However, the reaction rate changes found in the referred experiments (Table 4 of their paper), can be explained on a daerent basis. At the same time, the data quoted allow us to propose another explanation to the low activation energy value reported. From the data presented by Chang and Rusnak (Table 4 of Ref. [1]) we have calculated values shown in Table 1. A mass of 7.7g, corresponding to an apparent density of 1.5Ogjcc, was assumed for the samples. Then, O2 consumption rates have been calculated assuming that either CO1 or CO are exclusively formed. Real O2 consumption rates must be between these two extreme values. When these values are compared with O2flows it turns out that, in most cases, either the reaction is limited by the O2 input or a considerable depletion of O2 in the gas stream exists. The observed dependence of reaction rates on air flow is obviously explained by this fact. In the absence of flow data for the other experiments, the low activation energy obtained for sample A, is suspected to be caused by a similar artifact. It must be emphasized that, if meaningful results are to be expected, oxidant gas consumption must be very small so that the concentration can be considered constant [4]. If this is not realized, the actual concentration must be known and properly accounted for in the calculations.
Chang and Rusnak [1] have recently reported a study on the air oxidation behaviour of carbon-carbon composites. Reaction rates were thermogravimetrically measured on 5 mg samples of several materials powdered to sizes below 5 pm, at temperatures between 580 and 700°C. Activation energies ranging between 35 and 44 kcal/mol were obtained, which supports zone I reaction conditions [2]. By changing sample weight, the authors showed that the reaction is, at 65O”C,free of interparticle diffusion limitations. Oxidation rates were also measured on bulk samples with dimension 3.18x 2.54x 0.64cm. In this case, the experiments were performed in a quartz tube furnace and the samples were periodically removed in order to measure the weight loss. Although good Arrhenius plots are shown in the paper, the apparent activation energies determined from them, range between 10.5 and 29.5kcal/mol. A value of 10.5kcal/mol was obtained between 480 and 600°Cfor sample A, which is the most reactive one. The authors claim that this low activation energy value is due to stagnant film diffusion limitations on the reaction. It is quite unusual to find such a low value at the temperatures used by the authors. Moreover, it is hard to understand how external diffusion conditions could exist for a bulk sample, when the powdered sample was shown to react in zone 1 conditions and free of interparticle diffusion limitations at a higher temperature. This note aims to point out that the experimental approach used is not rigorous enough and ignores two serious considerations which may invalidate completely the conclusions reached. We have studied the oxidation of bulk carbon specimens with air and found [3] that, even for reaction rates lower than those used by Chang and Rusnak, special caution must be taken to keep constant the temperature of the sample. This is not surprising in view of the strongly exothermic nature of the C-O2 reaction. In consequence, the procedure of weighing before and after the oxidation in a tube furnace, does not seem to be adequate to determine kinetic parameters of the reaction. However, as heat evolution increases with the reaction rate, heating of the sample would tend to increase the measured value of the activation energy. Therefore, despite the objections about their procedure on general grounds, the anomalous low value reported should be explained in terms of some other artifact.
ALUAR AluminioArgentino SAIC Carbon Department Research and Development 9120 Puerto Ma&y-cc. 52 Chubut, Argentina
JORGE
F.
REY B~ERO
REFERENCES
1. H. W. Chang. R. M. Rusnak, Carbon 17,407 (1979). 2. P, L. Walker, F. Rusinko, L. G. Austin, Ado. Catal. 11. 133 (1959). 3. J. F. Rey Boero, to be submitted to Carbon. 4. H. Guerin, In Grouse Francaise d’Etude Des Carbones: Les carbones, Vol. II, hiason et tie. Paris (1965).
100
20
0.11
0.0085
16
8
0,29
0,022
41
500
100
OS13
0,0101
19
9,5
0,71
0,055
102
51
1000
200
0.15
0,0115
21
10,5
0,97
0,075
140
70
1500
300
0,16
0,0123
23
11.5
I,03
0,079
147
73.5
*Taken from Ref. [I], Table IV. 401
20.5
03@-6223/81/0544014)1502.00/0 Pergamcn Press Ltd.
LETTERS TO THE EDITOR
On the oxidation behaviour of C-C composites (Receioed 8 September 1980) The authors do not quote the flow rate values used in all their experiments. They only give figures for those runs in which flow rate was intentionally varied. These experiments were intended to confirm, in their understanding, the presence of external diffusion conditions [2]. However, the reaction rate changes found in the referred experiments (Table 4 of their paper), can be explained on a daerent basis. At the same time, the data quoted allow us to propose another explanation to the low activation energy value reported. From the data presented by Chang and Rusnak (Table 4 of Ref. [1]) we have calculated values shown in Table 1. A mass of 7.7g, corresponding to an apparent density of 1.5Ogjcc, was assumed for the samples. Then, O2 consumption rates have been calculated assuming that either CO1 or CO are exclusively formed. Real O2 consumption rates must be between these two extreme values. When these values are compared with O2flows it turns out that, in most cases, either the reaction is limited by the O2 input or a considerable depletion of O2 in the gas stream exists. The observed dependence of reaction rates on air flow is obviously explained by this fact. In the absence of flow data for the other experiments, the low activation energy obtained for sample A, is suspected to be caused by a similar artifact. It must be emphasized that, if meaningful results are to be expected, oxidant gas consumption must be very small so that the concentration can be considered constant [4]. If this is not realized, the actual concentration must be known and properly accounted for in the calculations.
Chang and Rusnak [1] have recently reported a study on the air oxidation behaviour of carbon-carbon composites. Reaction rates were thermogravimetrically measured on 5 mg samples of several materials powdered to sizes below 5 pm, at temperatures between 580 and 700°C. Activation energies ranging between 35 and 44 kcal/mol were obtained, which supports zone I reaction conditions [2]. By changing sample weight, the authors showed that the reaction is, at 65O”C,free of interparticle diffusion limitations. Oxidation rates were also measured on bulk samples with dimension 3.18x 2.54x 0.64cm. In this case, the experiments were performed in a quartz tube furnace and the samples were periodically removed in order to measure the weight loss. Although good Arrhenius plots are shown in the paper, the apparent activation energies determined from them, range between 10.5 and 29.5kcal/mol. A value of 10.5kcal/mol was obtained between 480 and 600°Cfor sample A, which is the most reactive one. The authors claim that this low activation energy value is due to stagnant film diffusion limitations on the reaction. It is quite unusual to find such a low value at the temperatures used by the authors. Moreover, it is hard to understand how external diffusion conditions could exist for a bulk sample, when the powdered sample was shown to react in zone 1 conditions and free of interparticle diffusion limitations at a higher temperature. This note aims to point out that the experimental approach used is not rigorous enough and ignores two serious considerations which may invalidate completely the conclusions reached. We have studied the oxidation of bulk carbon specimens with air and found [3] that, even for reaction rates lower than those used by Chang and Rusnak, special caution must be taken to keep constant the temperature of the sample. This is not surprising in view of the strongly exothermic nature of the C-O2 reaction. In consequence, the procedure of weighing before and after the oxidation in a tube furnace, does not seem to be adequate to determine kinetic parameters of the reaction. However, as heat evolution increases with the reaction rate, heating of the sample would tend to increase the measured value of the activation energy. Therefore, despite the objections about their procedure on general grounds, the anomalous low value reported should be explained in terms of some other artifact.
ALUAR AluminioArgentino SAIC Carbon Department Research and Development 9120 Puerto Ma&y-cc. 52 Chubut, Argentina
JORGE
F.
REY B~ERO
REFERENCES
1. H. W. Chang. R. M. Rusnak, Carbon 17,407 (1979). 2. P, L. Walker, F. Rusinko, L. G. Austin, Ado. Catal. 11. 133 (1959). 3. J. F. Rey Boero, to be submitted to Carbon. 4. H. Guerin, In Grouse Francaise d’Etude Des Carbones: Les carbones, Vol. II, hiason et tie. Paris (1965).
100
20
0.11
0.0085
16
8
0,29
0,022
41
500
100
OS13
0,0101
19
9,5
0,71
0,055
102
51
1000
200
0.15
0,0115
21
10,5
0,97
0,075
140
70
1500
300
0,16
0,0123
23
11.5
I,03
0,079
147
73.5
*Taken from Ref. [I], Table IV. 401
20.5