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Kinetic modeling of plasma methane conversion in a dielectric barrier discharge Manuscript id: FUPROC-D-07-00038 Author response of the 1st reviewer Question: In the Experimental section (page 5), the purity of only methane is listed and the source is not described. Please include the reactant gas purities and sources/manufacturers for all reactants. Answer: Thank you very much for the suggestion. In the revised manuscript, we added the information about the purity of the gases used in this experiment. Methane has purity of 99.97% while other gases have purity of 99.9%. All of those gases were provided by Kogas Co. (Korea Gas Co.) Question: In the Experimental section (p. 5), the flame ionization detector is abbreviated as "TCD", when it should be "FID". Answer: Thank you for the correction. Yes, it should be FID instead of TCD. We corrected it in the revised manuscript. Question: In the Experimental section (p. 6), the authors state that they assume the reactions to be first order, yet later in the paper, they discuss second order reactions to form the products (for example, reactions 12 and 13). What is the basis for this assumption? Please justify or support the assumption of first order kinetics and describe or estimate the error introduced by this assumption. Answer: Thank you very much for the question. In this case, I am afraid that the reviewer has a misinterpretation. In the kinetic calculation, we assumed that all of the reactions (both decomposition and formation reactions) follow the 1st order reaction. This assumption is clearly applied in the equation 3 and 4. There are some reasons why we proposed this idea: (1) Following the equation 3 and 4, giving a different order of the reaction to each species will not change the total reaction rate; it will only change the value of k. As the k consists both forward (kij) and backward (kji) reaction rate constant, this assumption is acceptable. (2) The reaction goes very fast, could be in millisecond. It is difficult (almost impossible) to conduct experiments to measure the variation of the reaction order for each reactant. (3) The plasma reaction is known as a very complex reaction. It is very difficult to distinguish for exact pathways that include the intermediates, such as: ion, charged particles, or other reactive species. The proposed way in our model is: we calculate the tendency of every single hydrocarbon (HC) molecule (1) converted to other HC molecules and (2) produced from other
HC molecules. Setting the order of reaction equals to one or it also means setting the order of reaction equals to one for each molecule will produce comparable value of k. By comparing the k value will enable us to draw a global major pathway of plasma methane conversion in a dielectric barrier discharge (DBD) as shown in Fig. 3. Regarding the correction of the k values, we added the s (standard deviation in percentage unit) in the revised manuscript. We did not put this value in the previous version because we think that it was not necessary. We thank reviewer for the suggestion. Question: Was temperature measured, either in the reactor or at the outlet? At 60 W of power input, I would expect some heating to occur. This should be reported in Section 2.1. On p. 4 in the Introduction, the model is described as being applicable to "ambient" temperature conditions. Is this 25°C? Please define this, especially as related to the question about the actual reactor temperature. On p. 7 in Results and Discussion, the reactor flow rate is listed as 30 ml/min. Is this at STP conditions or the undefined "ambient" conditions of temperature and atmospheric pressure? Please explicitly identify the temperature and pressure at which the volumetric flow was measured. Answer: Thank you very much. We did not measure the temperature inside the reactor. We just measured the inlet temperature of the reactant gas. In non-thermal plasma, the reaction goes under complicated temperature conditions. The bulk gas is in ambient condition, means at room temperature (~25oC) and 1atm of pressure, but the electron has much higher temperature. The existence of unbalanced charge inside the reactor (plasma zone) is the main reason of the plasma reactions which could be initiated by electron (known as high thermal species). In this model, the k values can be said to be independent from T (temperature). We did not measure and calculate the kinetic on the basis of temperature as it is almost impossible. It is wrong if we define the exact temperature, say 25oC or 100oC in the reactor, because of some reasons: (1) the temperature is not well distributed inside the reactor. (2) the reaction is mostly initiated by charged particle for example: excited molecule due to molecule charging. DBD is a type of non-thermodynamically equilibrium process as the distribution of charged species does not follow the statistical of Maxwell distribution. Regarding the suggestion to put clear information of the process, we thank for that. In the revised manuscript, we added the clear information of inlet gas temperature, inlet gas pressure, and volumetric flow. Question: In the Experimental section, please define the selectivity used for all products, as there are several definitions of selectivity in use. Also, please clarify whether conversion and selectivity are reported on a carbon basis. Please consider including another table (similar to
Table 1) showing the selectivities for each experiment. Answer: In the revised manuscript we added the equation of selectivity (Eq. 2b). The calculation of selectivity is quite simple and can be simply derived from Table 1, following the equation of selectivity. The addition of selectivity table is not necessary and it does not have any meaning since the k values in Table 2 is the most important point of view. However, it is correct that the conversion and selectivity used in this experiment is in carbon (C) basis. We also put the carbon and hydrogen balance between input and output flow in Table 1 following reviewer suggestion as well as the conversion of reactants in the second column of table 2. Question: Table 1 is shown as the first item in the Results and Discussion section (p. 8), yet it is not discussed until the next paragraph. Consider relocating this table. The authors discuss the carbon balance in the text, but do not show this in Table 1. Please include a column for carbon balance in Table 1, showing the sum of the measured species, instead of requiring the reader to sum the columns to check how close the carbon balance is. Also, Table 1 is labeled "The reactant conversion . . .", yet conversion is not shown in the Table. Please rename the table to correctly identify the contents, or better yet, include a column showing the conversion of each reactant. Answer: Thank you very much for the suggestion. In the revised manuscript, we added the requested information about the species conversion, written in the 2 nd column Table 1, and the C and H balances between input and output stream. In the previous version, the conversion of species is written down as the concentration of reactant species. For example, in run-2, the conversion of C2H2 is the concentration of C2H2 itself as we injected pure C2H2. Anyway, in order to avoid the ambiguous, we put clearly the conversion of reactant in the 2nd column of Table 1. Again, thank you for the great suggestion. Question: Some discussion of accuracy, reproducibility, and experimental error is necessary to interpret the significance of the data in the tables. Table 1 reports data to four significant figures. Should the reader infer that the results are accurate and reproducible to the two decimal places shown? This information on experimental error and accuracy is required to properly analyze the results. Answer: The experiment was done three times for each data. The experiment flow is the 1 st and 2nd data done by continuously and the third one is conducted by random among all hydrocarbons (HCs) to ensure the reproducibility. All data were calculated and shown in average value. The reason why we showed 2 decimal digits of the data is simple. Showing only single digit could give a misinterpretation of products concentration, especially trace products. Simply, if the concentration is only 0.04 %, setting single digit of decimal will give zero value which is not
correct. As your GC is quite sensitive, capable to detect in hundreds ppm level, we are confidence enough to show up the data in two decimal digits. Question: On p. 9 in Results and Discussion, reactions 9 and 10 are not stoichiometrically balanced (for example, reaction 9 is shown as 4 C2H2 + H2 = i-C4H10, but should be 2 C2H2 + 3 H2 = i-C4H10). Answer: Thank you very much for the correction from the reviewer. We corrected the reaction 9 and 10 in the revised manuscript. Question: On p. 9 just before reactions 13 and 14, the authors state, "In the case of C 3H6 compounds, the reactions occurred by two main parallel ways." What evidence is there to support this statement? Many possible pathways exist among the radicals present in the reactor to form these products. How were these two reactions identified as the main pathways? Answer: Thank you for the question. The reason that becomes the basis of our statement is due to the result that we got. It is shown that the concentration of C 3H8 and C2H4 has increased significantly in almost same percentage value. The total of increment those two compounds are 56% from the conversion of C3H6. Although, we also believe that there are many other side-way reactions, this is clearly showing that those two products C3H8 and C2H4 are the main products of C3H6 decomposition. Question: In general, the reactions shown throughout the paper do not appear to be based on experimental evidence. They appear to be reasonable, but certainly not the only possible reactions. How were they selected? What basis was used to make the selections? (There are dozens of possible radical reactions. Some of these reactions are discussed and analyzed by Zhao et al. in the Chemical Engineering Journal, 125, 67-79, 2006.) Answer: Thank you very much. Yes it is correct that this paper, the kinetic study of methane plasma conversion did not conducted by direct measurement, for example by measuring the energetic intermediate species concentration. However, this calculation has some advantages rather than just showing the existence of those intermediate species. First, we built a global kinetic model which allows us to draw the tendency of each HC molecule evolutes or converts to other HC molecules. It will reduce the complexity of the plasma reaction and give a clear view on how molecular transformation occurs, for example, methane to C2, C3, or C4 or vice versa. Second, this reaction is not build on an assumed mechanism which could be wrong if the assumption is not correct. This calculation is calculating all of the possible way of HC transformation. The result of rate constant k in Table 2 will direct us to the conclusion of Figure 3.
Question: On p. 10 and in Table 2, what are the units of the rate constants? Since they are all first order, they must have units of inverse time, but is it per s, per minute, or what unit? Please show in both the text and the table. Answer: Thank you for the correction. And it is correct that all k values are in reverse time unit (per m means per minute). Question: On p. 11, the authors state, "Low value of k13 (CH4 C2H4) and k12 (CH4 C2H2) shows that acetylene and etylen was not coming from methane conversion." k13 and k12 are not zero (unless the reported results are zero within the accuracy of the fitting method), so this statement is not accurate. It should be softened to say that acetylene and ethylene are not predominantly formed directly from methane. ("Ethylene" is misspelled in the text, as shown above.) Answer: As it is a problem of linguistic, we thank reviewer for the correction (also thank to reviewer 2 for making the correction). We tried to clean up and put more discussion on our revised manuscript. We hope that the revision version will satisfy the requirement to be a scientific manuscript. Question: Were mixtures of the reactant gases ever tested in the reactor? For example, if an equimolar mixture of all 8 reactants were feed simultaneously, the model should be able to predict the product composition. If this were verified experimentally, it would strongly validate the model. Even results for binary mixtures of reactants could help prove the model accuracy. Answer: Yes, we conducted a binary mixture experiment and we put those result on the revised manuscript. Those are written at the end of the result and discussion section. The result shows the close point between experimental and model calculation. Unfortunately, we did not conduct more than binary mixture experiment as we have limited number of MFC (mass flow controller) to control the flow of input gases. Question: Is Figure 2 meant to show only the major reaction pathways? This should be clarified in the caption because there are no zero values in Table 2 (other than the diagonal self-reaction rate constants), which indicates that any product can be formed from any reactant, at least to some degree. Again, a discussion of the accuracy or confidence intervals of the data would help to establish the significance of the data. Answer: Thank you very much. In the Figure 2, we just put the major reaction pathways as it is the main objective of this experiment besides the kinetic model itself. We did not put the sidepathway reactions in the diagram because some reasons:
(1) As we mention that this model shows the tendency of the majority conversion of certain HC species to others HCs. (2) Sideways reaction which the value is relatively small can be interferer with error value of the data. The standard deviation of the calculation error was put on Table 2 (revised manuscript). By knowing the standard deviation (s) lower that 30% (in most cases), we confidence enough that the k is acceptable. Question: In the Conclusions on p. 12, the authors state that "C2H6 has an important role in the global mechanism." However, on p. 11 and in Figure 2, C2H4 is identified as the important intermediate. Is this a typographical error in the Conclusions and should they state that "C2H4 has an important role in the global mechanism"? Answer: Thank you very much for the correction. We really appreciate with the deep review of the reviewer and we are sorry very much as we could not find this error when we submitted the manuscript. Yes, following the Figure 2, it should be C2H4 that has an important role on the branching of the reactions.
Author response of the 2nd reviewer Title: Only need to provide one version (the complete and descriptive one). Throughout the manuscript there are awkward and confusing statements. To assist the authors with revision of the manuscript, a marked copy of the manuscript is attached to this review with specific suggestions and identification of awkward wording. The problem of scholarship is that the authors do little to discuss their results in the context of the literature, whether from work on plasma conversion of methane or other methods for methane conversion. It would be very worthwhile for the authors to compare and contrast their analyses of reaction kinetics and reaction pathways with other techniques for methane conversion, to highlight how the plasma influences the process. But, in the Results and Discussion section, only one reference is cited by the authors that was not written by H.K. Song and co-workers. This must be addressed before the present manuscript will be suitable for publication. Answer: Thank you very much for the suggestion addressed to our manuscript. We added a lot of discussions on the result and discussion section mostly in the part of kinetic model. The comparison with similar system [Thanyachotpaiboon et al.] was also added in the revised manuscript. As plasma system has many types including thermal and non-thermal plasmas which produce different product distribution and phenomena, we could not easily make a comparison. Most of the published papers in this area only put suggested mechanism without calculating the quantitative number of the probability or the reaction rate occurred in the real process. However, as we were thinking those sources are useful to support our data, we put some of those results and compared with ours in the revised manuscript. We thank reviewer for helping us to check the manuscript especially for the grammatical problem. All sentences that probably make an ambiguous meaning have been changed to make it clearly understable for the reader.
HC molecules. Setting the order of reaction equals to one or it also means setting the order of reaction equals to one for each molecule will produce comparable value of k. By comparing the k value will enable us to draw a global major pathway of plasma methane conversion in a dielectric barrier discharge (DBD) as shown in Fig. 3. Regarding the correction of the k values, we added the s (standard deviation in percentage unit) in the revised manuscript. We did not put this value in the previous version because we think that it was not necessary. We thank reviewer for the suggestion. Question: Was temperature measured, either in the reactor or at the outlet? At 60 W of power input, I would expect some heating to occur. This should be reported in Section 2.1. On p. 4 in the Introduction, the model is described as being applicable to "ambient" temperature conditions. Is this 25°C? Please define this, especially as related to the question about the actual reactor temperature. On p. 7 in Results and Discussion, the reactor flow rate is listed as 30 ml/min. Is this at STP conditions or the undefined "ambient" conditions of temperature and atmospheric pressure? Please explicitly identify the temperature and pressure at which the volumetric flow was measured. Answer: Thank you very much. We did not measure the temperature inside the reactor. We just measured the inlet temperature of the reactant gas. In non-thermal plasma, the reaction goes under complicated temperature conditions. The bulk gas is in ambient condition, means at room temperature (~25oC) and 1atm of pressure, but the electron has much higher temperature. The existence of unbalanced charge inside the reactor (plasma zone) is the main reason of the plasma reactions which could be initiated by electron (known as high thermal species). In this model, the k values can be said to be independent from T (temperature). We did not measure and calculate the kinetic on the basis of temperature as it is almost impossible. It is wrong if we define the exact temperature, say 25oC or 100oC in the reactor, because of some reasons: (1) the temperature is not well distributed inside the reactor. (2) the reaction is mostly initiated by charged particle for example: excited molecule due to molecule charging. DBD is a type of non-thermodynamically equilibrium process as the distribution of charged species does not follow the statistical of Maxwell distribution. Regarding the suggestion to put clear information of the process, we thank for that. In the revised manuscript, we added the clear information of inlet gas temperature, inlet gas pressure, and volumetric flow. Question: In the Experimental section, please define the selectivity used for all products, as there are several definitions of selectivity in use. Also, please clarify whether conversion and selectivity are reported on a carbon basis. Please consider including another table (similar to
Table 1) showing the selectivities for each experiment. Answer: In the revised manuscript we added the equation of selectivity (Eq. 2b). The calculation of selectivity is quite simple and can be simply derived from Table 1, following the equation of selectivity. The addition of selectivity table is not necessary and it does not have any meaning since the k values in Table 2 is the most important point of view. However, it is correct that the conversion and selectivity used in this experiment is in carbon (C) basis. We also put the carbon and hydrogen balance between input and output flow in Table 1 following reviewer suggestion as well as the conversion of reactants in the second column of table 2. Question: Table 1 is shown as the first item in the Results and Discussion section (p. 8), yet it is not discussed until the next paragraph. Consider relocating this table. The authors discuss the carbon balance in the text, but do not show this in Table 1. Please include a column for carbon balance in Table 1, showing the sum of the measured species, instead of requiring the reader to sum the columns to check how close the carbon balance is. Also, Table 1 is labeled "The reactant conversion . . .", yet conversion is not shown in the Table. Please rename the table to correctly identify the contents, or better yet, include a column showing the conversion of each reactant. Answer: Thank you very much for the suggestion. In the revised manuscript, we added the requested information about the species conversion, written in the 2 nd column Table 1, and the C and H balances between input and output stream. In the previous version, the conversion of species is written down as the concentration of reactant species. For example, in run-2, the conversion of C2H2 is the concentration of C2H2 itself as we injected pure C2H2. Anyway, in order to avoid the ambiguous, we put clearly the conversion of reactant in the 2nd column of Table 1. Again, thank you for the great suggestion. Question: Some discussion of accuracy, reproducibility, and experimental error is necessary to interpret the significance of the data in the tables. Table 1 reports data to four significant figures. Should the reader infer that the results are accurate and reproducible to the two decimal places shown? This information on experimental error and accuracy is required to properly analyze the results. Answer: The experiment was done three times for each data. The experiment flow is the 1 st and 2nd data done by continuously and the third one is conducted by random among all hydrocarbons (HCs) to ensure the reproducibility. All data were calculated and shown in average value. The reason why we showed 2 decimal digits of the data is simple. Showing only single digit could give a misinterpretation of products concentration, especially trace products. Simply, if the concentration is only 0.04 %, setting single digit of decimal will give zero value which is not
correct. As your GC is quite sensitive, capable to detect in hundreds ppm level, we are confidence enough to show up the data in two decimal digits. Question: On p. 9 in Results and Discussion, reactions 9 and 10 are not stoichiometrically balanced (for example, reaction 9 is shown as 4 C2H2 + H2 = i-C4H10, but should be 2 C2H2 + 3 H2 = i-C4H10). Answer: Thank you very much for the correction from the reviewer. We corrected the reaction 9 and 10 in the revised manuscript. Question: On p. 9 just before reactions 13 and 14, the authors state, "In the case of C 3H6 compounds, the reactions occurred by two main parallel ways." What evidence is there to support this statement? Many possible pathways exist among the radicals present in the reactor to form these products. How were these two reactions identified as the main pathways? Answer: Thank you for the question. The reason that becomes the basis of our statement is due to the result that we got. It is shown that the concentration of C 3H8 and C2H4 has increased significantly in almost same percentage value. The total of increment those two compounds are 56% from the conversion of C3H6. Although, we also believe that there are many other side-way reactions, this is clearly showing that those two products C3H8 and C2H4 are the main products of C3H6 decomposition. Question: In general, the reactions shown throughout the paper do not appear to be based on experimental evidence. They appear to be reasonable, but certainly not the only possible reactions. How were they selected? What basis was used to make the selections? (There are dozens of possible radical reactions. Some of these reactions are discussed and analyzed by Zhao et al. in the Chemical Engineering Journal, 125, 67-79, 2006.) Answer: Thank you very much. Yes it is correct that this paper, the kinetic study of methane plasma conversion did not conducted by direct measurement, for example by measuring the energetic intermediate species concentration. However, this calculation has some advantages rather than just showing the existence of those intermediate species. First, we built a global kinetic model which allows us to draw the tendency of each HC molecule evolutes or converts to other HC molecules. It will reduce the complexity of the plasma reaction and give a clear view on how molecular transformation occurs, for example, methane to C2, C3, or C4 or vice versa. Second, this reaction is not build on an assumed mechanism which could be wrong if the assumption is not correct. This calculation is calculating all of the possible way of HC transformation. The result of rate constant k in Table 2 will direct us to the conclusion of Figure 3.
Question: On p. 10 and in Table 2, what are the units of the rate constants? Since they are all first order, they must have units of inverse time, but is it per s, per minute, or what unit? Please show in both the text and the table. Answer: Thank you for the correction. And it is correct that all k values are in reverse time unit (per m means per minute). Question: On p. 11, the authors state, "Low value of k13 (CH4 C2H4) and k12 (CH4 C2H2) shows that acetylene and etylen was not coming from methane conversion." k13 and k12 are not zero (unless the reported results are zero within the accuracy of the fitting method), so this statement is not accurate. It should be softened to say that acetylene and ethylene are not predominantly formed directly from methane. ("Ethylene" is misspelled in the text, as shown above.) Answer: As it is a problem of linguistic, we thank reviewer for the correction (also thank to reviewer 2 for making the correction). We tried to clean up and put more discussion on our revised manuscript. We hope that the revision version will satisfy the requirement to be a scientific manuscript. Question: Were mixtures of the reactant gases ever tested in the reactor? For example, if an equimolar mixture of all 8 reactants were feed simultaneously, the model should be able to predict the product composition. If this were verified experimentally, it would strongly validate the model. Even results for binary mixtures of reactants could help prove the model accuracy. Answer: Yes, we conducted a binary mixture experiment and we put those result on the revised manuscript. Those are written at the end of the result and discussion section. The result shows the close point between experimental and model calculation. Unfortunately, we did not conduct more than binary mixture experiment as we have limited number of MFC (mass flow controller) to control the flow of input gases. Question: Is Figure 2 meant to show only the major reaction pathways? This should be clarified in the caption because there are no zero values in Table 2 (other than the diagonal self-reaction rate constants), which indicates that any product can be formed from any reactant, at least to some degree. Again, a discussion of the accuracy or confidence intervals of the data would help to establish the significance of the data. Answer: Thank you very much. In the Figure 2, we just put the major reaction pathways as it is the main objective of this experiment besides the kinetic model itself. We did not put the sidepathway reactions in the diagram because some reasons:
(1) As we mention that this model shows the tendency of the majority conversion of certain HC species to others HCs. (2) Sideways reaction which the value is relatively small can be interferer with error value of the data. The standard deviation of the calculation error was put on Table 2 (revised manuscript). By knowing the standard deviation (s) lower that 30% (in most cases), we confidence enough that the k is acceptable. Question: In the Conclusions on p. 12, the authors state that "C2H6 has an important role in the global mechanism." However, on p. 11 and in Figure 2, C2H4 is identified as the important intermediate. Is this a typographical error in the Conclusions and should they state that "C2H4 has an important role in the global mechanism"? Answer: Thank you very much for the correction. We really appreciate with the deep review of the reviewer and we are sorry very much as we could not find this error when we submitted the manuscript. Yes, following the Figure 2, it should be C2H4 that has an important role on the branching of the reactions.
Author response of the 2nd reviewer Title: Only need to provide one version (the complete and descriptive one). Throughout the manuscript there are awkward and confusing statements. To assist the authors with revision of the manuscript, a marked copy of the manuscript is attached to this review with specific suggestions and identification of awkward wording. The problem of scholarship is that the authors do little to discuss their results in the context of the literature, whether from work on plasma conversion of methane or other methods for methane conversion. It would be very worthwhile for the authors to compare and contrast their analyses of reaction kinetics and reaction pathways with other techniques for methane conversion, to highlight how the plasma influences the process. But, in the Results and Discussion section, only one reference is cited by the authors that was not written by H.K. Song and co-workers. This must be addressed before the present manuscript will be suitable for publication. Answer: Thank you very much for the suggestion addressed to our manuscript. We added a lot of discussions on the result and discussion section mostly in the part of kinetic model. The comparison with similar system [Thanyachotpaiboon et al.] was also added in the revised manuscript. As plasma system has many types including thermal and non-thermal plasmas which produce different product distribution and phenomena, we could not easily make a comparison. Most of the published papers in this area only put suggested mechanism without calculating the quantitative number of the probability or the reaction rate occurred in the real process. However, as we were thinking those sources are useful to support our data, we put some of those results and compared with ours in the revised manuscript. We thank reviewer for helping us to check the manuscript especially for the grammatical problem. All sentences that probably make an ambiguous meaning have been changed to make it clearly understable for the reader.
