Physical and Chemical Phenomena during the Production of Hydrogen in the Microwave Discharge Generated in Liquid Hydrocarbons with the Barbotage of Various Gases

Round 1

Reviewer 1 Report

Lebedev et al. investigated the physical and chemical characteristics of process of hydrogen production in microwave 10 discharge in liquid hydrocarbon (petroleum solvent Nefras S2 80/120) with Ar, He, CO2 barbotage 11. The characterization results are robust and the writing is good. I thus recommend it to be accepted after the minor revisions.

1. The recent research progress should be given in the Introduction part to highlight the importance of the work.

2. More discussions should be made during the presentation of results, especially in the hydrogen yield part.

3. Did the authors consider the further work based on the findings? It is encouraged to make more perspectives for their future relared work. 

Author Response

Reply to Reviewer 1.

We are grateful to the Reviewer for interest to our paper and detail analysis of it. All comments are very useful for improvement of information presentation in the paper.

All Reviewer’s comments were carefully analyzed and taken into account in the revised version of the article.

Below the Reviewer’s comments are marked by red italic types and answers are given in black normal letters.

1. The recent research progress should be given in the Introduction part to highlight the importance of the work.

Thank you for the comment.

You are right, the introduction needs to be expanded. It is hardly necessary to prove and illustrate the possibilities of plasma chemistry. This is now common knowledge. It is expedient, apparently, to expand the description of the merits of discharge in liquids, since this is a rather new and little-studied type of discharge. It is they that stimulate interest in the study of discharge. Therefore, the text is added to the introduction:

"Since plasma is produced in a gas bubble inside the liquid and the bubble surface contacts the high-temperature zone, the inflow rate of molecules of liquid into such a minireactor (the bubble filled with plasma) is very high. Due to the intense influx of molecules from the liquid surface into the plasma, the densities of active particles (atoms, radicals, and charged particles) reach very high values. Therefore, the efficiency of physicochemical processes and, accordingly, the formation rates of the reaction products are also very high".

2. More discussions should be made during the presentation of results, especially in the hydrogen yield part.

Thank you for your question.

Discussing the mechanism of hydrogen production under experimental conditions is an interesting and important step. Unfortunately at the moment it can only be at the level of assumptions. An exhaustive answer can be given by modelling. We are currently solving this problem and would like to postpone detailed discussion to the future.

 Here we have obtained some experimental results that will be used in modelling.

We hope that this reasoning will be accepted by the honorable Reviewer in response to his recommendation.

3. Did the authors consider the further work based on the findings? It is encouraged to make more perspectives for their future relared work. 

Thank you for the comment.

We do plan to continue work on the subject of the article. The main activity will be related to process optimization, for which the information on process details obtained in this paper will be used. It should be noted that the discharge is very difficult both for diagnostics and modeling. Now there is a process of accumulation of knowledge about the relationship between physical and chemical phenomena in the discharge. One of the important areas of research is the influence of various additives on the characteristics of the hydrogen production process. This task is far from being completed. And the other, equally important task is to model the process. Only modelling together with experiments can give an answer about the mechanism of processes in plasma and determine the direction of their optimization.

We hope that our answers will satisfy the distinguished Reviewer.

Author Response File: Author Response.docx

Reviewer 2 Report

1. The authors used 1.5 paragraphs to introduce the importance of H2. More information should be given for the state-of-art for low-T plasma technique in H2 production.

2. It would be good if more detailed information could be provided regarding the Nefras, better with some table. Sometimes the commercial formula may change and it would be good to have some exact information.

3, Please keep the formal, especially for the dot in the number.

4. If a low amount Ar or other added gas can have a better H2 flow rate, what is the disadvantage for this system to have a lower Ar rate? 

5. Is there any other hydrocarbon generated during the reaction? The author may need to take a look of the liquid component change.

6, What is meaning of changing the kinetics of plasma processing when adding CO2?

7. The scale should be added to the photos.

Author Response

Reply to Reviewer 2

We are grateful to the Reviewer for interest to our paper and detail analysis of it. All comments are very useful for improvement of information presentation in the paper.

All Reviewer’s comments were carefully analyzed and taken into account in the revised version of the article.

Below the Reviewer’s comments are marked by red italic types and answers are given in black normal letters.

1. The authors used 1.5 paragraphs to introduce the importance of H2. More information should be given for the state-of-art for low-T plasma technique in H2 production.

Thank you for comment.

You are absolutely right and this kind of information is very important.

It is contained in Section 3.4, where a table summarizing the characteristics of hydrogen production is given for different conditions of low-temperature plasma.  We considered that such information is more appropriate at the end of the paper because it allows us to compare the known results with the data of this paper.

2. It would be good if more detailed information could be provided regarding the Nefras, better with some table. Sometimes the commercial formula may change and it would be good to have some exact information.

Thank you for comment.

That's a really important question. We have answered it by investigating the composition of gas products in microwave discharge in a wide range of different hydrocarbons. It turned out that the composition of the main products weakly depends on the hydrocarbon (Lebedev Yu.A., Averin K.A., Tatarinov A.V. Main Gaseous Products of Microwave Discharge in Various Liquid Hydrocarbons // High Energy Chem. 2019, V.54, No 4, 331-335. DOI: 10.1134/S0018143919030032). Therefore, knowledge of the detailed composition of nephras, in our opinion, will not provide additional information. Nevertheless, following the recommendation of the honourable Reviewer, we give the composition of Nefras in the article for reference.

Table 1. Composition of Nefras С2 80/120

3, Please keep the formal, especially for the dot in the number.

Thank you. The text was corrected.

4. If a low amount Ar or other added gas can have a better H2 flow rate, what is the disadvantage for this system to have a lower Ar rate? 

Thank you for the question.

You are right, at lower argon flow rates the efficiency of the hydrogen production process increases. But at the same time the soot yield increases, which is a disadvantage. Therefore, it is necessary to find a compromise between hydrogen yield and soot yield when creating the process. The following has been added to the text of the article:

"Note that when the argon content decreases, the hydrogen yield increases, but the soot yield also increases, which is a disadvantage. Therefore, it is necessary to find a compromise between these processes when developing the process".

5. Is there any other hydrocarbon generated during the reaction? The author may need to take a look of the liquid component change.

Thank you for the question.

The answer is based on our previous studies in which we worked with liquid n-heptane and specifically investigated the composition of the liquid before and after the experiment. These results are described in the article: Lebedev Yu A, Averin K A, Borisov R S, Garifullin A R, Bobkova E.S, and Kurkin T S Microwave Discharge in Liquid Hydrocarbons: Study of a Liquid Hydrocarbon after Exciting the Discharge. //High Energy Chemistry, 2018, Vol. 52, No. 4, pp. 324-329.  DOI: 10.1134/S0018143918040100.

It shows that only trace amounts of new compounds appear in the liquid after treatment in the discharge. That is, the effect of discharge on the liquid phase is negligible.

6, What is meaning of changing the kinetics of plasma processing when adding CO2?

Thank you for the question.

Unfortunately, there is no comprehensive answer to it. As a possible mechanism of CO₂ influence on hydrogen yield we can cite the known reaction CO₂+CH4 ->2CO+H₂. Methane is present in the discharge products in any liquid hydrocarbons. But this problem requires further research.

In the present work, based on the analysis of experimental data, we came to the conclusion that the main factor is the increase in the absorbed power.

7. The scale should be added to the photos.

Thank you for comment.

As the article says, the size of the glowing area is difficult to relate to the size of the bubble. Therefore, it makes no sense to introduce a scale there and the photographs only show that the intensity of the discharge radiation changes.  The scale is important for shadow photos, which show the real size of the bubble. The shadow photos have an image that shows the scale - it is the dark image of the antenna at the bottom of each photo. Its diameter is 2 mm. That's why the caption under the shadow photos adds:

" A cylindrical antenna with a diameter of 2 mm can be seen in the lower part of the pictures."

We hope that our answers will satisfy the distinguished reviewer

Author Response File: Author Response.docx

Reviewer 3 Report

1.      The proposed solution, as Authors mentioned, is relatively new. I think that in the introduction section examples from literature should be provided regarding particular application, description of the mechanism representing the idea of the process. The current description is not enough, especially for beginners in the field,

2.      The novelty of the research should be clearly underlined by comparison to other recent reports,

3.      Is it possible to estimate the bubble size and their distribution in the reactor? Is there any impact of bubble size on the composition of outlet stream/efficiency of hydrogen generation?

1.      The proposed solution, as Authors mentioned, is relatively new. I think that in the introduction section examples from literature should be provided regarding particular application, description of the mechanism representing the idea of the process. The current description is not enough, especially for beginners in the field,

2.      The novelty of the research should be clearly underlined by comparison to other recent reports,

3.      Is it possible to estimate the bubble size and their distribution in the reactor? Is there any impact of bubble size on the composition of outlet stream/efficiency of hydrogen generation?

Author Response

Reply to Reviewer 3

We are grateful to the Reviewer for interest to our paper and detail analysis of it. All comments are very useful for improvement of information presentation in the paper.

All Reviewer’s comments were carefully analyzed and taken into account in the revised version of the article.

Below the Reviewer’s comments are marked by red italic types and answers are given in black normal letters.

1. The proposed solution, as Authors mentioned, is relatively new. I think that in the introduction section examples from literature should be provided regarding particular application, description of the mechanism representing the idea of the process. The current description is not enough, especially for beginners in the field,

Thanks for the comment.

In the introduction, text has been added that describes the difference between the described method of plasma production and traditional gas discharges.

"Since plasma is produced in a gas bubble inside the liquid and the bubble surface contacts the high-temperature zone, the inflow rate of molecules of liquid into such a minireactor (the bubble filled with plasma) is very high. Due to the intense influx of molecules from the liquid surface into the plasma, the densities of active particles (atoms, radicals, and charged particles) reach very high values. Therefore, the efficiency of physicochemical processes and, accordingly, the formation rates of the reaction products are also very high".

2. The novelty of the research should be clearly underlined by comparison to other recent reports,

 Thank you for your question.

There are few publications on microwave discharge in liquids, and only a few on the problem of hydrogen production.  This is the novelty of the study. The obtained results are compared with the data for other discharges in Section 3.4. It is clear that these results are still far from the technology, but this is the first step on the way to it, since process optimization is required.

3. Is it possible to estimate the bubble size and their distribution in the reactor? Is there any impact of bubble size on the composition of outlet stream/efficiency of hydrogen generation?

Thank you for your question.

The answer has to do with the peculiarity of microwave discharge in liquids. Namely, the size of the bubble is determined by the condition of zero forces of gravity, Archimedean and surface tension then the bubble detaches from the antenna and the plasma disappears in it. as soon as the bubble detaches from the antenna, the processes in it stop and the plasma disintegrates. The size of the gas bubble is determined by these forces and it can only be controlled by changing the pressure above the liquid surface.  After detachment the process is repeated.

Interestingly, the size does not depend on the power absorbed in the discharge and it determines only the time to reach a given size. in our conditions, the diameter of the bubble is about 1 cm. The frequency of bubble appearance and, accordingly, the yield of target products depend on the power, and in each plasma bubble the processes are practically the same and the composition of products is also the same.

We hope that our answers will satisfy the distinguished Reviewer.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

This is an interesting work and has been improved after modification. It is good for publication.