Hydrothermal Treatment of Vegetable Oils and Fats Aiming at Yielding Hydrocarbons: A Review
Round 1
Reviewer 1 Report
The revised version of this manuscript does represent an improvement relative to the original submission. However, the manuscript still shows a number of issues.
The level of detail in which the literature is discussed shows considerable improvement with respect to the original submission. Nevertheless, results are still discussed in isolation, i.e., individual articles are summarized in a handful of lines but recurring trends that can be seen to emerge from several articles are not identified and discussed to a satisfactory extent. Indeed, the reviewer would be left with the overarching question of “what does it all mean? What are the underlying trends? How can the latter be leveraged?”, etc.
The context for the present review article relative to any previously published review literature on this or any closely related topics is still missing. The authors state “it must be borne in mind that although other reviews have been published regarding the aforementioned topic, they do not present the level of detail required to support the future research in the area”. However, these other reviews do not even seem to be cited and the reader is left wondering what are the other reviews available and how this one is different.
In Fig. 1, transesterification can also be considered a thermochemical conversion process. Moreover, some feedstocks can be hydrotreated directly.
In scheme 1 the caption may need revision since it displays the conversion of a monoglyceride.
The term “mechanism” is used too liberally in the manuscript. For instance, Scheme 4 does not depict a reaction mechanism, but a reaction sequence.
The manuscript has to be proof red thoroughly and carefully to rid it of many typos, just a handful of which are listed below for illustrative purposes.
Lines 104 and 105 are repeated.
“The” is unnecessarily capitalized in line 785, “Animal” is unnecessarily capitalized in line 934, “Acid” is unnecessarily capitalized in line 979.
There are superfluous periods in lines 833, 847, 861, 885.
There is a missing period in line 985.
The label (b) is missing in the caption of Figure 5. In this Figure, only integer numbers should be shown in the y axis.
Author Response
Point 1: The level of detail in which the literature is discussed shows considerable improvement with respect to the original submission. Nevertheless, results are still discussed in isolation, i.e., individual articles are summarized in a handful of lines but recurring trends that can be seen to emerge from several articles are not identified and discussed to a satisfactory extent. Indeed, the reviewer would be left with the overarching question of “what does it all mean? What are the underlying trends? How can the latter be leveraged?”, etc.
Response 1: We are grateful for this observation. We added to the revised manuscript the topic 6.2 “Trends for hydrothermal experiments using vegetable oils and fatty acids” (line 1092) to make the results and the discussions of this manuscript more relevant and valuable. In this topic, specifically, we discussed the temporal trend of published articles; the origin of the articles; the existence of partnerships; and the tendencies of hydrothermal studies of triglycerides and fatty acid in the short and long term. Also, parameters of temperature, residence time, biomass: water ratio, catalyst loading, feedstocks, and products were evaluated as well. Finally, we exposed the main challenges that could be further explored in hydrothermal articles and that could serve as potential scope for future articles.
Point 2: The context for the present review article relative to any previously published review literature on this or any closely related topics is still missing. The authors state “it must be borne in mind that although other reviews have been published regarding the aforementioned topic, they do not present the level of detail required to support the future research in the area”. However, these other reviews do not even seem to be cited and the reader is left wondering what are the other reviews available and how this one is different.
Response 2: Currently, no review focusing on hydrothermal studies for vegetable oils and fatty acids has been found. Concerning triglycerides and fatty acids, they briefly comment about the phase behavior and the solubility of them with water. Hence, all discussions about hydrothermal technology were interpreted taking into account previously published reviews of literature. To write the chapters 2, 3, 4, and 5, we used review articles as references to explain the hydrothermal technology, their regions in the subcritical and supercritical states of water and the reactions that occur under these states, the importance and characteristics of water under hydrothermal technology, important parameters in the hydrothermal process, and main feedstocks used in hydrothermal technology. We take this opportunity to explain this in the introduction of the revised manuscript (lines 93 and 94).
Furthermore, the lack of information regarding the parameters and results encouraged us to create this review. In fact, our review is, to the best of our knowledge, the first one to cover all aspects of hydrothermal treatment of vegetable oils and fats. We justified the lack of a previous review article in line 76.
Even so, we are thankful to the reviewer for the valuable comment. Besides, we carried out a bibliographic search in the database of Scopus and ScienceDirect, and no new articles or reviews were found.
Point 3: In Fig. 1, transesterification can also be considered a thermochemical conversion process. Moreover, some feedstocks can be hydrotreated directly.
Response 3: We thank the reviewer for the comment. Therefore, as requested, we modified Figure 1. Transesterification was considered a thermochemical conversion process. We also included in Figure 1 the Catalytic hydroprocessing as a thermochemical conversion process too.
Point 4: In scheme 1 the caption may need revision since it displays the conversion of a monoglyceride.
Response 4: We thank the reviewer for the observation. We agree with the reviewer and we decide that the nomenclature used in scheme 1 will be better with the monoglyceride instead of the triglyceride. When we apply the hydrothermal process to glycerides, hydrolysis will be the first reaction that will occur and converts triglycerides, through diglyceride and monoglyceride intermediates, into fatty acids and one molecule of glycerol. The difference between triglyceride, diglyceride, and monoglyceride will be the number of fatty acid molecules that will be formed, but the reaction continues the same.
Point 5: The term “mechanism” is used too liberally in the manuscript. For instance, Scheme 4 does not depict a reaction mechanism, but a reaction sequence.
Response 5: We are especially grateful for this observation and suggestion. As suggested by the reviewer, we modified the title in Scheme 4 for the “Reaction sequence of the hydrolysis reaction from triglyceride.”
We took the opportunity to also modify the term “mechanism” in other paragraphs of the revised manuscript. The term “mechanism” was modified in lines 259, 262-263, 307, 1011-1012, 1029, 1152-1154.
Point 6: The manuscript has to be proof red thoroughly and carefully to rid it of many typos, just a handful of which are listed below for illustrative purposes.
- Lines 104 and 105 are repeated.
- “The” is unnecessarily capitalized in line 785, “Animal” is unnecessarily capitalized in line 934, “Acid” is unnecessarily capitalized in line 979.
- There are superfluous periods in lines 833, 847, 861, 885.
- There is a missing period in line 985.
- The label (b) is missing in the caption of Figure 5. In this Figure, only integer numbers should be shown in the y axis.
Response 6: We thank the reviewer for the observation. These were clear mistakes and we apologize for these errors.
- We removed line 107.
- “The”, “The”, “The”, “Animal”, “Acid”, were placed in lowercase in lines 742, 756, 845, 905, 949 respectively.
- We removed the superfluous period in line 510, 609, 766, 803, 820, 834, and 855
- We added label (a) “vegetable oils” and (b)”fatty acids” in the caption of Figure 5.
- We fixed line 1204, removing “;”
Reviewer 2 Report
The manuscript is complete and can be published in present form.Author Response
The authors are grateful for the opportunity to submit a revised draft of the manuscript entitled “Hydrothermal treatment of vegetable oils and fats aiming at yielding hydrocarbons: A review”. We truly believe that the suggestions made by the reviewers resulted in important and positive changes in the original manuscript.
Reviewer 3 Report
This version of the article is much improved. Some minor English editing might be required though.
Author Response
The authors are grateful for the opportunity to submit a revised draft of the manuscript entitled “Hydrothermal treatment of vegetable oils and fats aiming at yielding hydrocarbons: A review”. We truly believe that the suggestions made by the reviewers resulted in important and positive changes in the original manuscript. As requested, another careful English editing revision was made in the text.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The manuscript under consideration reviews hydrothermal technology for the conversion of biomass in general – and oleaginous biomass in particular – to fuel-like hydrocarbons. Albeit a review in this topic, which is attracting an increasing amount of attention, would be of interest to both academic and industrial researchers working in the field of biofuels, a number of major and minor issues must be addressed before this contribution can be deemed ready for publication.
The main problem with the manuscript in its present form is that the detail in which the literature is discussed in this review is insufficient. Using just the heterogeneous catalysis processes as an example, what are the metal loadings employed and the reactions conditions? To convert what kind of feedstock into what kind of products? With which conversion and selectivity? Line 461-462 offers another good example of this issue, i.e., what was the actual yield of H2 and fuel gas produced? Similarly, in lines 479-480: conversion to what, i.e., what were the products? One would assume fatty acids are the products in the entire paragraph, but this is never explicitly stated. Once again in line 578, in lines 586-590, in lines 592-598, and in lines 602-605, what are the products? Sadly, these are only a few examples, as the manuscript in its present form is riddled with instances in which additional and more specific details should have been given, perhaps in the form of tabular data that could replace some superfluous figures (see below). Lines 494-503 provide better example of a more useful amount of detail, which could be summarized in a table alongside the results of other reports as suggested previously.
Also missing is the context for the present review article relative to any previously published review literature on this or any closely related topics. Indeed, the reader is left wondering if there are other reviews available and if so, how is this one different?
Another major issue is that results are discussed in isolation, i.e., individual articles are summarized in a handful of lines – which often lack detail (see above) – but recurring trends that can be seen to emerge from several articles are not identified and discussed. If, as is suggested, these results were to be presented in tabular form, the reviewer would be left with the overarching questions of “what does it all mean? What are the underlying trends? How can the latter be leveraged? What are the gaps in the literature?”, etc.
Also of note is the fact that there are several areas of the literature that have high relevance to the reviewed field but are not included in the manuscript. Just to name a few examples, the manuscript ignores several articles [Fuel 166 (2016) 302; ACS Sus Chem Eng 6 (2018) 4521, Energy Tech 6 (2018) 1832] related to the hydrothermal conversion of lipids to hydrocarbons on ZnO2-supported metal catalysts, a significant current direction in hydrothermal deoxygenation research. Additionally, a recent and relevant example of a continuous process was also omitted [ACS Omega 6 (2018) 7046]. Also, in spite of the significant amount of attention given to carbon-supported catalysts as well as the use of many types of feedstocks, an article reporting in great detail on the use of Pt-Re/C catalysts and a collection of different feedstocks is also missing [ACS Sus Chem Eng 4 (2016) 1775].
In lines 35-36, the authors state that CO2 is a co-product of the steam reforming or naphta or natural gas, but the latter process produces syngas, i.e., a mixture of CO and H2. The authors may be referring to the production of additional hydrogen via the steam reforming of the resulting CO, which is technically the water gas shift reaction.
The organization of Fig. 1 is rather odd since transesterification can also be considered a thermochemical conversion process, most – if not all – other processes besides transesterification also rely on chemical reactions, liquefaction can be considered a hydrothermal process (HTL), thermocatalytic processes are not considered separately, etc. The authors may want to consider if there are better ways to present the information in this figure.
In line 117 the authors suggest that biodiesel can be produced hydrothermally. Biodiesel conventionally comprises fatty acid methyl esters. Perhaps the authors mean green or renewable diesel (i.e., diesel-range biomass-derived hydrocarbons)?
The 3 arrows on the top right corner of Scheme 1 need to be corrected. In addition, Scheme 1 rightly differentiates hydrodeoxygenation, decarbonylation and decarboxylation reactions (all of which are deoxygenation reactions) but other passages in the manuscript either conflate or do not differentiate these reactions clearly enough.
It is unclear how in Scheme 2 the decarboxylation of the fatty acid can afford an aldehyde, particularly if decarboxylation is defined as the removal of oxygen as CO2. Also, the labels for intra- and inter-cyclization seem to be wrongly exchanged.
Figures 4 and 5 seems rather superfluous. In addition, why are glucose and fructose listed separately from carbohydrates?
In line 526, how can alkanes have any oxygen content?
In line 647, the authors mention the regeneration of metal catalysts but the first catalyst discussed is activated carbon. Immediately thereafter (lines 651-652), the catalyst regenerated is not even mentioned; and immediately thereafter (lines 653-655) the products are not identified.
Lines 658-660 once again illustrate a frustrating lack of detail since neither the catalyst(s), nor the reaction conditions, nor conversion, nor selectivity are identified.
Most of section 6.3 doesn’t really discuss reaction mechanisms, rather it discusses side reactions (e.g., isomerization, hydrogenation, etc.) in what at beast could be described as reaction schemes. In short, the caption for Scheme 3 is more apposite than the title for section 6.3.
Reviewer 2 Report
The review describes the hydrocarbons production by hydrothermal treatment of vegetable fats and oils.
It is well structured and divided into seven sections.
in all the paper there are some typos and errors that need to be corrected (such as in row 284 where CO2 needs to be corrected with CO2).
Please improve the resolution of scheme 4 that results hard to read.
Reviewer 3 Report
This review article presents recent progress on hydrothermal treatments on lipids and fats to liquid fuels. The idea of this review article is novel. However, the authors need to narrow down hydrothermal treatments specifically on lipids and fats. In this current manuscript, the authors have presented irrelevant sections describing hydrothermal treatments of lignocellulosic biomass, cellulose, glucose. The authors have also presented various treatment processes other than hydrothermal treatment, which are irrelevant for this manuscript. On the other side, the authors have little or no information on how various fats and lipids reacts during hydrothermal treatments, there is no information on reaction kinetics, not adequate information on byproducts and undesired products. The application of catalysts on hydrothermal treatments need to discussed in detailed including reaction kinetics, selectivity, yields differing from various catalysts. Catalyst stability, regeneration, and poisoning due to hydrothermal treatment of these oils and fat are highly important for this review.
