Catalytic Hydrotreatment of Microalgae Biocrude from Continuous Hydrothermal Liquefaction: Heteroatom Removal and Their Distribution in Distillation Cuts

Round 1

Reviewer 1 Report

Dear authors,

This paper could be an interesting work about the catalytic hydrotreatment of the biocrude produced via hydrothermal liquefaction of spirulina.

Though it includes a dedicated analysis, some issues should be addressed:

1 - the aim of this work is "to obtain drop-in fuel properties from non-feed biomass...". However, Spirulina is a microalga well known by its nutritional potential and therefore it should not be considered “non-feed biomass”. Therefore, this sentence should be rephrased;

2 – The introduction section should be completed by including more updated (recent) references. Furthermore, some references cited in the text do not seem adequate for the purpose they have been used (ref. [2], [8], [19]). Thus, this section must be improved by better justifying the importance of previous and recent works towards the subject of this paper;

3 – The experimental section describes the use of factorial design of experiments including 8 experiments to understand the most influential process parameters during hydrotreating. Authors did not clarify if they have performed replicates of these experiments, and therefore if there is any statistical analysis of the results. There is no information about the reproducibility or SD of the results obtained. Further to this, a set of 3 new “optimized experiments” have been performed. However, it is not clear how this optimization was planned, as a result of /based on the previous set of experiments – it appears these experiments have been performed in conditions close to those found in the literature and not as a consequence of the modelization resulting from the previous set of 8 experiments;

4 – There is one reference missing in the list to ASTM D-7169 standard;

5 – Lines 173-177 are not formatted according to the requirements of this journal;

6 – There is at least one reference in the list that is not adequately formatted;

7 - a few typos along the text.

Please check details in the attached commented version of the manuscript.

Comments for author File: Comments.pdf

Author Response

Reviewer 1:

Comments to the Author:

This paper could be an interesting work about the catalytic hydrotreatment of the biocrude produced via hydrothermal liquefaction of spirulina. Though it includes a dedicated analysis, some issues should be addressed:

Please find our comments as following:

1. the aim of this work is "to obtain drop-in fuel properties from non-feed biomass...". However, Spirulina is a microalga well known by its nutritional potential and therefore it should not be considered “non-feed biomass”. Therefore, this sentence should be rephrased;

We are pleased to hear that the reviewer found this paper interesting. We used the phrase “non-feed biomass” for Spirulina as it has no direct competition with the food and humans do not directly consume it. However, we do agree with the reviewer that it has a potential in nutrition applications.  Therefore, we replaced it with 3rd generation biomass, which is the standard term used for aquatic biomass.

2. The introduction section should be completed by including more updated (recent) references. Furthermore, some references cited in the text do not seem adequate for the purpose they have been used (ref. [2], [8], [19]). Thus, this section must be improved by better justifying the importance of previous and recent works towards the subject of this paper;

We agree with the reviewer that some of the references used were quite old, such as reference [19]. We updated this with an adequate reference, which also provides the information about the hydroprocessing of microalgae. Furthermore, reference 1, 2 and 8 are also replaced as suggested by the reviewer. Additionally, we revised the introduction section in order to better highlight the position of our work in the context of literature.

3. The experimental section describes the use of factorial design of experiments including 8 experiments to understand the most influential process parameters during hydrotreating. Authors did not clarify if they have performed replicates of these experiments, and therefore if there is any statistical analysis of the results. There is no information about the reproducibility or SD of the results obtained. Further to this, a set of 3 new “optimized experiments” have been performed. However, it is not clear how this optimization was planned, as a result of /based on the previous set of experiments – it appears these experiments have been performed in conditions close to those found in the literature and not as a consequence of the modelization resulting from the previous set of 8 experiments;

Concerning reproducibility, the original manuscript at line 119 stated that: “in order to confirm reproducibility and comparability of results, experiments were measured in duplicates”. In the revised manuscript we have made it explicit that the measured results are the mean values and the reported errors are the standard deviations.

We agree with the reviewer that the term “optimized experiments” may be misleading. Actually, these 3 experiments are aimed at testing the outcomes from the factorial study, mainly the conclusion that temperature is the main factor affecting the result. In this sense, we conceived them as “optimal”, as they were expected to give the better results. Nevertheless, in order to avoid any misunderstanding, we renamed experiments 9-11 as “confirmatory experiments”.

4. There is one reference missing in the list to ASTM D-7169 standard;

We fully agree with the reviewer that the reference for ASTM D-7169 standard is missing. We updated the reference list, which would suffice the requirement.

5. Lines 173-177 are not formatted according to the requirements of this journal;

We again agree with the reviewer, that the lines 173-177 are not formatted according to the requirements of this journal. Thus, they have been updated accordingly.

6. There is at least one reference in the list that is not adequately formatted;

The above-mentioned reference by “Doornbosch R., Steenblik R.”, is highlighted and formated in proper way.

7. A few typos along the text.

We appreciate the reviewer for highlighting the typographical mistakes along the text. All the typographic errors are addressed accordingly.

Reviewer 2 Report

The paper addresses the catalytic hydrotreatment of microalgae biocrude produced from hydrothermal liquefaction of Spirulina. Experimental investigations have been performed to assess the effect of temperature, initial H2 pressure, and residence time on the removal of heteroatoms.

In order to find the most influencing parameters during hydrotreating. The work distinguishes through the setup of a factorial-based experiment – an approach that allows to identify the most influencing parameters from the above mentioned, during hydrotreating.

The paper is well written and easy to follow. The existing literature is used wherever required. Both the theoretical and experimental setups, as well as the characterization and analytical techniques, are appropriate, explained in detail, allowing the readers to have a deep view and understanding on the results, and to ensure (if necessary) the reproducibility of the experiments. Results appear to be valid. The most important and interesting conclusion that was drawn based on the obtained results and on the processing of experimental data show that the operating temperature is a key parameter that influences the degree of deoxygenation, while the operating pressure and the temperature-pressure interaction have a high influence on the degree of denitrogenation.

I found only a few minor concerns regarding the use of English language (please see the comments and suggestions that I made on the following rows from the attached file: 140-141, 190, 203, 207, 359).

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

Comments to the Author:

“The paper addresses the catalytic hydrotreatment of microalgae biocrude produced from hydrothermal liquefaction of Spirulina. Experimental investigations have been performed to assess the effect of temperature, initial H₂ pressure, and residence time on the removal of heteroatoms. In order to find the most influencing parameters during hydrotreating. The work distinguishes through the setup of a factorial-based experiment – an approach that allows to identify the most influencing parameters from the above mentioned, during hydrotreating. The paper is well written and easy to follow. The existing literature is used wherever required.”

1. I found only a few minor concerns regarding the use of English language (please see the comments and suggestions that I made on the following rows from the attached file: 140-141, 190, 203, 207, 359).

We are pleased to hear that the reviewer finds the paper interesting, appropriate, well explained and easy to follow. We appreciate the reviewer for pointing out minor concerns about the use of definite article in English language. All the linguistic errors addressed in row 140-141, 190, 203, 207 and 359, are corrected accordingly.

Reviewer 3 Report

Please find my comments in the attached file

Author Response

Reviewer 3:

Comments to the Author:

“The current research article carry out the catalytic hydrotreatment of microalgae biocrude and Heteroatom removal and their distribution in distillation cuts. As authors, discussed in introduction section that in microalgae can accumulate relatively large amount of lipids, therefore it could viable option for production of biofuels. Also, the experimental conditions of this research paper well organized and discussed very well. This MS should be accepted with major revision. There are several comments, which should be addressed before publication in Energies.”

We hope that the reviewer will find our response satisfactory.

1. Abstract is not clear and should be redrafted. Abstract should be given a pertinent overview of the study. Authors should highlight the purpose of the study, describe briefly the main methods and finally summarize the article's main findings.

We do appreciate the time and the efforts the reviewer has spent on this research work. We are pleased to hear that the reviewer has found this research sound, well discussed and well organized. First of all the typographical error, which is mentioned in comment 3, is taken into account. We do not fully agree with the reviewer comment that the “Abstract is not clear and should be redrafted”, but we have gone through the abstract to make sure that it is clear and follows the structure as stated by the reviewer. We have reworded the abstract slightly to make sure that it has a proper flow, and follows the same pattern as mentioned by the reviewer. We would appreciate more specific suggestions for improvements, if this revision is not sufficient. However, w[THP1] e believe that all relevant information is included in the revised version, including main methods and most important analytical instruments such as GC-MS, Sim-Dis and fractional distillation unit (ASTM D 1160).

2. Conclusion is very general, it should be conclude about the best results of this study. Conclusion should be revised. For example authors stated that “aim of this study was to investigate”, please remove this phrase.

We do agree with the reviewer about the use of phrase “aim of this study was to investigate”. We rephrased the sentence to the best of our understanding of the reviewers comment.

3. Line 18: Please insert the space after . and beginning of new sentence. “interaction.Based” to interaction. Based.

We thank the reviewer for mentioning this typographical mistake. This comment is addressed in updated version.

4. Line 13, 25, 86, 97….. and through the MS name of microalgae (Spirulina) should be written in Italic.

We fully agree with the reviewer in this regard, and modifications have been done according to the reviewer’s observation.

5. Line 37: As authors, discussed in introduction section that in microalgae can accumulate relatively large amount of lipids. Please could you provide the actual number and also authors should discuss the chemical composition of Spirulina such as proteins, carbohydras and proteins.

“Authors can find these information in following article and should cite the same article. Molino, A.; Iovine, A.; Casella, P.; Mehariya, S.; Chianese, S.; Cerbone, A.; Rimauro, J.; Musmarra, D. Microalgae Characterization for Consolidated and New Application in Human Food, Animal Feed and Nutraceuticals. Int. J. Environ. Res. Public Health 2018, 15, 2436”

The purpose of Line 37 is only to identify that microalgae is a potentially attractive energy source due to the presence of carbohydrates, proteins along with lipids; in HTL, all three biomass components are valorized. As we do not work with the Spirulina algae itself, but only the derived HTL biocrude, we do not think it is appropriate to go into a more detailed description of the original feedstock, but rather refer to the original HTL work (reference [17]), as we have done in the manuscript although a bit later in the text, line 99-103. The original composition of the algae has an impact of the composition of the biocrude, and therefore the discussion of the HTL part naturally includes this. However, we agree that it might be helpful to readers to be able to find detailed information about the composition of microalgae, and therefore we have included the reference as suggested by the reviewer. The reference [5] also shows it’s potential in biodiesel, which in our opinion is more related to our work and support the fact for its further upgrading.

6. Line 96, 97: why the biocrude was produced at sub-critical condition at 220 bar and 350 °C. Please explain it.

As the HTL experiments were reported in reference [17] and carried out in a different lab as part of a collaborative multi-partner research project, we can only refer to this reference for background on the choice of operating conditions for the HTL experiments. Our purpose here has only been to report activities on the HTL biocrude as received from these experiments.

7. Line 98: Please remove the double space after “used to produce biocrude” and before “with”.

We again thank the reviewer for bringing into notice the above typographical error. This comment is corrected and addressed. 

Round 2

Reviewer 1 Report

Dear authors, 

Thank you for having taken into consideration the reviewer's comments.

The quality of the paper has improved.

Still:

- Table 3 is split en two pages;

- Table 4 should be inserted in an horizontal page, so that each value ± SD fits in only one line;

- Ref. [15] is incomplete;

- line 63 "Chlorella" should be written in italic.

Comments for author File: Comments.pdf

Author Response

Reviewer 1:

Comments to the Author:

Thank you for having taken into consideration the reviewer's comments. The quality of the paper has improved. Please find our comments as following:

1. Table 3 is split in two pages;

We are grateful to hear that reviewer finds all the updates satisfactory; and found that quality of the article has improved after first revision. We have taken into account the comment from the reviewer and fixed it accordingly.

2. Table 4 should be inserted in an horizontal page, so that each value ± SD fits in only one line;

We do understand the reviewers point, and Table 4 is edited by following the reviewer’s suggestion. The editorial experts in the Energies; will hopefully carry out the final editing and the table alignments.

3. Ref. [15] is incomplete;

We appreciate the reviewer for pointing out, that the ref. 15 is incomplete, as some of the information was missing. We updated this reference, which would suffice the requirement.

4. line 63 "Chlorella" should be written in italic.

We fully agree with the reviewer in this regard, and modifications have been done according to the reviewer’s observation.

Reviewer 3 Report

well done. good luck

Author Response

Comments to the Author:

1. “well done. good luck.”

We are pleased to hear that the reviewer finds the paper interesting and appropriate; and thanks him for his nice and motivating words “Well done. Good luck”, regarding our efforts.