Catalytic Microwave-Assisted Pyrolysis of the Main Residue of the Brewing Industry
Round 1
Reviewer 1 Report
This research paper describes the catalytic microwave-assisted pyrolysis of the brewer’s spent grains.
The paper is rejected for the following concerns.
1. The paper is not suitable for publication in catalysts. There are no details related to the catalyst.
2. Why the authors used CaO as the catalyst? Was it synthesized by the authors? No description is available in the text.
3. Why only XRD analysis was done? The authors should do a detailed characterization of fresh and used catalysts to analyze the difference in catalytic characteristics taking place after the reaction.
4. There is no mechanism of catalyst action. How does the catalyst contribute to enhancing the reaction? The authors should provide details. A complete mechanism of reaction should be provided.
5. The authors should provide a kinetic study for this reaction.
6. The authors should do stability tests to check the reusability and durability of the catalyst for the pyrolysis reaction.
Minor editing of English language required.
Author Response
We appreciate the time and effort that the reviewer have dedicated to providing valuable feedback on our manuscript. We are grateful to the reviewers for their insightful comments. We have been able to incorporate changes (highlighted in the revised manuscript) to reflect most of the suggestions provided by the reviewer and answered all comments.
Here is a point-by-point response to the reviewers’ comments and concerns. Our responses (R) are written in italics.
REVIEWER #1
01) The paper is not suitable for publication in catalysts. There are no details related to the catalyst.
R: New comments and data about the CaO used in this work were included in the revised manuscript (New pages 6, 12, 13, and 17 and New Fig.3).
02) Why the authors used CaO as the catalyst? Was it synthesized by the authors? No description is available in the text.
R: New comments about the choice of CaO as the catalyst were now added in the revised manuscript (New pages 2 and 3) and new information about it, were also inserted in the new text (New page 17).
03) Why only XRD analysis was done? The authors should do a detailed characterization of fresh and used catalysts to analyze the difference in catalytic characteristics taking place after the reaction.
R: In addition to XRD analysis, TG/DTG analyses of the catalyst were included in the revised version of the manuscript (New page 6 and New Fig.3). The comparison of the characterization of fresh and used catalysts is a very good suggestion that we will consider in future works. Thank you.
04) There is no mechanism of catalyst action. How does the catalyst contribute to enhancing the reaction? The authors should provide details. A complete mechanism of reaction should be provided.
R: A new section of catalyst action was included in the revised version of the manuscript (New Pages 12 and 13), in addition to the comments of the new Page 8.
05) The authors should provide a kinetic study for this reaction.
R: A kinetic study for the pyrolysis of BSG was carried out in a previous work performed by our research group. New comments were added to the revised manuscript (New Page 3).
06) The authors should do stability tests to check the reusability and durability of the catalyst for the pyrolysis reaction.
R: Thank you for your comment. This recommendation is valuable, and we will consider in future works. Thank you.
We would like to thank the Reviewer #1 for thoughtful comments. We tried to implement most of your suggestions in revised version of the manuscript
Reviewer 2 Report
In this study, the authors evaluate the effect of three essential experimental parameters of the microwave-assisted pyrolysis and catalytic pyrolysis process (temperature, feedstock moisture content, and catalyst loading), using brewer's spent grain as feedstock biomass.
In general terms, I consider that the study is very interesting, addressing several important aspects of the pyrolysis process, and analyzing the experimental results with statistical tools.
However, some issues should be improved before publication in "Catalyst", for example:
- Page 3, line 85 to 87: the authors claim that “As observed, high moisture contents of biomass, as is the case of run 8 (14%), favors the overall efficiency of microwave radiation energy absorption; however, the bio-oil yield is significantly affected by the aqueous fractions that dilute the bio-oil, reducing its quality [46].” However, it is not possible to corroborate that the quality of the bio-oil is reduced, since the bio-oils are not fully characterized; for example, physicochemical properties such as pH, calorific value, water content, density, among others, are not reported. The characterization that the authors carry out of the bio-oils only includes the GM/MS analysis, and not even the reported values ​​for the identified compounds are %wt, only %Area (relative composition). I believe that a more exhaustive characterization of the bio-oils obtained should be carried out, especially considering that "the quality of the liquid product" is the focus of this study, according to the objectives set out in the introduction.
- The authors only show the detailed composition of the liquid product obtained in the experiment carried out under the optimum conditions (a temperature of 570 °C and a catalyst percentage of 12.17%), where only 8 different oxygenated compounds appear: 5 phenols, 2 ketones and 1 aldehyde. Bio-oils are known to contain many different oxygenated compounds, including many acids. My question is: did all the bio-oils present only these oxygenated compounds in their composition?
- Authors should include the detailed composition obtained by GC/MS of the bio-oils as supplementary material.
- There are no characterizations of the solid products of the pyrolysis or of the gaseous products. Would it be possible to include some of these aspects to further improve the quality of the paper?
- It is also important to know the composition of the biomass raw material used in the pyrolysis process. In the information provided by the authors regarding the brewer's spent grain used in the experiments, its lignocellulosic composition and elemental composition (CHONS) are not shown; I considered that this information is relevant and should be included, since from it, it is possible to establish relationships with the composition of the bio-oil, especially with the oxygenated and nitrogenous compounds.
- In the methodology, the authors must mention which software tool they used to carry out the statistical calculations.
Then some minor formatting issues should be fixed as well:
- Page 2, line 69: the expression “experimental design (CCD)” appears, and it is not understood what4 the acronym CCD means.
Author Response
We are grateful to the reviewers for their insightful comments. Here is a point-by-point response to the reviewers’ comments and concerns. Our responses (R) are written in italics.
REVIEWER #2
In this study, the authors evaluate the effect of three essential experimental parameters of the microwave-assisted pyrolysis and catalytic pyrolysis process (temperature, feedstock moisture content, and catalyst loading), using brewer's spent grain as feedstock biomass. In general terms, I consider that the study is very interesting, addressing several important aspects of the pyrolysis process, and analyzing the experimental results with statistical tools. However, some issues should be improved before publication in "Catalyst", for example:
R: We thank the reviewer for these positive comments
01) Page 3, line 85 to 87: the authors claim that “As observed, high moisture contents of biomass, as is the case of run 8 (14%), favors the overall efficiency of microwave radiation energy absorption; however, the bio-oil yield is significantly affected by the aqueous fractions that dilute the bio-oil, reducing its quality [46].” However, it is not possible to corroborate that the quality of the bio-oil is reduced, since the bio-oils are not fully characterized; for example, physicochemical properties such as pH, calorific value, water content, density, among others, are not reported. The characterization that the authors carry out of the bio-oils only includes the GM/MS analysis, and not even the reported values for the identified compounds are %wt, only %Area (relative composition). I believe that a more exhaustive characterization of the bio-oils obtained should be carried out, especially considering that "the quality of the liquid product" is the focus of this study, according to the objectives set out in the introduction.
R: Thank you for your comment. The authors included some additional data related to the analysis of the liquid product from catalytic MAP, as well as new comments and references (New pages 4 and 12).
02) The authors only show the detailed composition of the liquid product obtained in the experiment carried out under the optimum conditions (a temperature of 570 °C and a catalyst percentage of 12.17%), where only 8 different oxygenated compounds appear: 5 phenols, 2 ketones and 1 aldehyde. Bio-oils are known to contain many different oxygenated compounds, including many acids. My question is: did all the bio-oils present only these oxygenated compounds in their composition?
R: New comments and explanations were inserted on the manuscript (New page 12), and a Supplementary Material file was also added. Under the optimum condition, only these compounds were found. This behavior is also reported in other publications that mention that the CaO, as a basic metal oxide catalyst, is effective in removing acids with high oxygen content from the bio-oil from pyrolysis.
03) Authors should include the detailed composition obtained by GC/MS of the bio-oils as supplementary material.
R: As recommended by the Reviewer, a detailed composition obtained by GC/MS of the bio-oils from the catalytic MAP was added as Supplementary Material.
04) There are no characterizations of the solid products of the pyrolysis or of the gaseous products. Would it be possible to include some of these aspects to further improve the quality of the paper?
R: A new section about the solid product characterization was included in the revised manuscript (New page 12, and new Figure 6).
05) It is also important to know the composition of the biomass raw material used in the pyrolysis process. In the information provided by the authors regarding the brewer's spent grain used in the experiments, its lignocellulosic composition and elemental composition (CHONS) are not shown; I considered that this information is relevant and should be included, since from it, it is possible to establish relationships with the composition of the bio-oil, especially with the oxygenated and nitrogenous compounds.
R: As recommended by the Reviewer, the ultimate analysis and the chemical composition of the BSG were added in the revised manuscript in the “Materials and Methods” section (New page 14).
06) In the methodology, the authors must mention which software tool they used to carry out the statistical calculations.
R: The details about the software tool were added in the manuscript in the “Materials and Methods” section (New page 16).
07) Then some minor formatting issues should be fixed as well: Page 2, line 69: the expression “experimental design (CCD)” appears, and it is not understood what4 the acronym CCD means.
R: Thank you for your comment. The expression CCD means central composite design and the term “experimental design” was replaced by “central composite design”. New comments and explanations about the configuration of the central composite design (CCD) were included in the revised manuscript (New page 16).
The authors would like to thank you very much for your comments, which increased the paper quality. We have revised our paper accordingly your comments and suggestions.
Reviewer 3 Report
The microwave-assisted pyrolysis of brewer’s spent grain was investigated in this research. The article achieves its stated goal, however, the authors need to correct some of the following points to make the article more complete before it is published.
(1) The novelty of the work must be clearly addressed and discussed, compare your research with existing research findings and highlight novelty.
(2) Introduction section must be written on more quality way. Research gap should be delivered on more clear way with directed necessity for the conducted research work.
(3) Authors should refer to some of the latest research in the field, thereby clarifying the scientific significance of the research. Do not use this form of citation for references, like “[29-32],[37-42] “ and so on.
(4) Please give us more information about the catalysts, such as the source, the microstructure, what’s the meaning of the catalyst concentration ?
(5) Some figures are of low resolution, please modify them.
(6) “In this work, a multi-response optimization was performed using the desirability function [54]”, please provide more details.
English language should be carefully checked and carefully check paper for language typos.
Author Response
We are grateful to the reviewer for their insightful comments. Here is a point-by-point response to the reviewer’ comments and concerns. Our responses (R) are written in italics.
REVIEWER #3
The microwave-assisted pyrolysis of brewer’s spent grain was investigated in this research. The article achieves its stated goal, however, the authors need to correct some of the following points to make the article more complete before it is published.
01) The novelty of the work must be clearly addressed and discussed, compare your research with existing research findings and highlight novelty.
R: The novelty of the work and a comparison of this research with existing research findings were highlighted and the most recent and relevant references that have been published in highly ranked journals have been added in the revised version of the manuscript (New pages: 1-3)
02) Introduction section must be written on more quality way. Research gap should be delivered on more clear way with directed necessity for the conducted research work.
R: The gaps that exist in the literature that this work aimed to investigate and therefore the innovativeness of the catalytic MAP of BSG were presented in new comments inserted in the new introduction section (New pages 1-3).
03)Authors should refer to some of the latest research in the field, thereby clarifying the scientific significance of the research. Do not use this form of citation for references, like “[29-32],[37-42] “ and so on.
R: Some of the latest research in the field was addressed in the new introduction section (New page 2). The citation format was adjusted as recommended.
04) Please give us more information about the catalysts, such as the source, the microstructure, what’s the meaning of the catalyst concentration?
R: New comments about the CaO used in this work were inserted (New pages: 6, 13 and 17). The term “catalyst concentration” was updated to “catalyst ratio” (catalyst/biomass ratio).
05) Some figures are of low resolution, please modify them.
R: The figures were updated to a version with a better resolution.
06) “In this work, a multi-response optimization was performed using the desirability function [54]”, please provide more details.
R: Regarding the desirability function, its aimed to find a balance between the high content of hydrocarbons and the generation of a substantial amount of liquid product. The conditions that lead to the maximum bio-oil yield are not the same conditions that lead to maximum hydrocarbon content in bio-oil. Therefore, the multi-objective optimal point does not correspond to the largest amount of bio-oil that can be achieved, nor does it correspond to the largest amount of hydrocarbons, but rather a balance between these two responses. This type of optimization strategy is widely used in the RSM framework and consists of converting multiple responses into a single one by combining the individual responses in a composite function named Global Desirability to achieve the best joint responses. The desirability values remain between 0 and 1, where 0 corresponds to an undesirable response, and 1 represents a completely desirable value. New comments about the desirability function were included in the revised manuscript (New Page 9).
We are very grateful to the reviewer for the careful and meticulous reading of this manuscript. Your comments and suggestions were valuable and very helpful for revising and improving our manuscript. We tried to implement most of your suggestions in revised version of the manuscript.
Round 2
Reviewer 1 Report
The authors did not provide satisfactory revision on catalytic part. For most of the important comments, they responded they will include in the next paper. For paper to be published in Catalysts Journal, the catalytic details must be provided.
Author Response
REVIEWER #1
01) The authors did not provide satisfactory revision on catalytic part. For most of the important comments, they responded they will include in the next paper. For the paper to be published in Catalysts Journal, the catalytic details must be provided.
R: New comments and references about the reusability of the spent CaO catalyst were included in the revised manuscript, including the difficulty of accomplishing this in the situ configuration (New page 3). In addition TGA data of spent catalyst were also included in the results section (new pages 6 and 7 and new Figure 3).
Changes in the text from the second revision have been highlighted in pink color to easily identify the changes, while the previous revision were yellow highlights.
We would like to thank the Reviewer #1 for the time spent and for the important suggestions and contributions to our manuscript. We have made an effort to perform most of the corrections requested by all reviewers.
Reviewer 2 Report
The authors have made all suggested changes. They have also incorporated new experimental results and discussions regarding the liquid and solid products of pyrolysis.
I consider that the manuscript is of sufficient quality to be published in the journal Catalysts
Author Response
REVIEWER #2
The authors have made all suggested changes. They have also incorporated new experimental results and discussions regarding the liquid and solid products of pyrolysis.
I consider that the manuscript is of sufficient quality to be published in the journal Catalysts
R: We thank the reviewer#2 for this positive feedback to our revised text and for all the excellent suggestions and contributions given in the previous step
