Mild Hydrothermal Pretreatment of Microalgae for the Production of Biocrude with a Low N and O Content
, Rosalía Rodríguez 2, Victoria Morales 1, Luis Fernando Bautista 1 and Gemma Vicente 2,*Round 1
Reviewer 1 Report
In the present study, the authors used a hydrothermal liquefaction (HTL) process to produce a biocrude from the microalga Nannochloropsis gaditana with reduced N and O contents. The authors focused on the evaluation of temperature, reactor loading and time (factors) to maximize the yield of the pretreated biomass and the heteroatom contents transferred from the microalga biomass to the aqueous phase (responses). They found that temperature and time are critical factors with a negative effect on the pretreated biomass yield, but a positive one on the N and O recovery in the aqueous phase. In addition, they discovered that the slurry concentration has to be low to increase heteroatom recovery and high to maximize the pretreated microalga yields.
Basically, I consider that this manuscript did provide certain critical results which benefit the industries. However, there are some concerns needed to be addressed before publication.
Concerns:
1.I recommend the conclusion of this study should be incorporated into abstract.
2.For RSM, the results of R square, R square adjust, lack of fit, and model should be presented.
3.In lines 132 and 293, why the NRAP is different.
4.Why there is no “discussion”?
5.In conclusion, the authors should point out their findings which contribute to the industries.
Author Response
REVIEWER #1
General Comment. In the present study, the authors used a hydrothermal liquefaction (HTL) process to produce a biocrude from the microalga Nannochloropsis gaditana with reduced N and O contents. The authors focused on the evaluation of temperature, reactor loading and time (factors) to maximize the yield of the pretreated biomass and the heteroatom contents transferred from the microalga biomass to the aqueous phase (responses). They found that temperature and time are critical factors with a negative effect on the pretreated biomass yield, but a positive one on the N and O recovery in the aqueous phase. In addition, they discovered that the slurry concentration has to be low to increase heteroatom recovery and high to maximize the pretreated microalga yields.
Basically, I consider that this manuscript did provide certain critical results which benefit the industries. However, there are some concerns needed to be addressed before publication.
Answer: We agree with this reviewer´s general comment. In this sense, we have taken into account all the reviewer’s suggestion in the revised version of the manuscript.
Comment 1. I recommend the conclusion of this study should be incorporated into abstract.
Answer: In accordance with this comment, we have included the principal conclusion of this study in the abstract:
Response equations were obtained for the analyzed responses, which facilitates the accurate prediction of the operating conditions required to obtain a given value of these responses.
Comment 2. For RSM, the results of R square, R square adjust, lack of fit, and model should be presented.
Answer: We had already included the models and the R2 in the original manuscript. Acting on the reviewer´s comment we have included also the R2 adjust and the lack-of-fit.
Comment 3. In lines 132 and 293, why the NRAP is different.
Answer: The reviewer is right. There is an erratum in line 293. In this sense, we have changed NRAP in line 293 (295 in the revised version) for the appropriate term (NRAP) to refer to the N recovery in the aqueous phase in relation to the N content in the microalga dry matter
Comment 4. Why there is no “discussion”?
Answer: The discussion had been included in the original manuscript together with the results. Particularly, the subsections 3.2, 3.3. and 3.4 comprised the discussion of the results obtained for the yield of the solid phase (YSP), the N recovery in the aqueous phase in relation to the N content in the microalga dry matter (NRAP) and the O recovery in the aqueous phase referred to the O content in the dry microalga (ORAP), respectively. We have change the title of the section 3 and subsection 3.1 to clarify where the discussion is located in the revised version of the manuscript.
Comment 5. In conclusion, the authors should point out their findings which contribute to the industries.
Answer: Acting on this comment, we have pointed out how our results could contribute to the industry in the conclusion section:
The use of these models is essential to carry out a mild hydrothermal pretreatment process of microalgae on an industrial scale to obtain appropriate yield of pretreated biomass with a relatively low N and O contents
Reviewer 2 Report
The manuscript by Vicente and co-workers deals with a mild hydrothermal pretreatment of microalgae for the production of biocrude.
The research topic is interesting and essential, the manuscript is well-written and demanding, however the reviewer has a few observations:
Figure 1: The time scale of Fig. 1a is inversely compared to the others.
In the text, there is often no space between the value and the unit.
The reviewer didn’t get answer from the manuscript about the following questions:
Were the experimental results repeated?
What is the reason of the worse measured-calculated fit of the O content?
Was the predicted optimum for each target value verified with the experimental results?
I believe that this work might be of interest to the readers of Processes, and recommend for publication after the minor revisions noted above were properly taken into account.
Author Response
REVIEWER #2
General Comment. The manuscript by Vicente and co-workers deals with a mild hydrothermal pretreatment of microalgae for the production of biocrude. The research topic is interesting and essential, the manuscript is well-written and demanding, however the reviewer has a few observations.
Answer: The authors thank the reviewer for the positive comments regarding the manuscript.
Comment 1. Figure 1: The time scale of Fig. 1a is inversely compared to the others.
Answer: In accordance with this comment, we have changed the time scale on Fig.1a in the revised version.
Comment 2. In the text, there is often no space between the value and the unit.
Answer: Acting on the reviewer´s comment, we have revised carefully the manuscript and we have included a space between the value and the corresponding unit.
Comment 3. Were the experimental results repeated?
Answer: We have used the factorial design and response surface methodology, which includes the repetition of one experiment to assess the experimental error. In this study, we have carry out four times the central point experiment (see experiments 9 to 12 in Table 1), which we had already indicated in the original manuscript:
The central point experiment was carried out four times in order to determine the variability of the results and to evaluate the experimental error.
Comment 4. What is the reason of the worse measured-calculated fit of the O content?
Answer: The elemental analyses were carried out using an analyser equipped with a thermal conductivity detector (TCD). The contents of C, N, S, and H were determined by an oxidation/reduction reactor kept at a temperature of 900 °C. However, the O content was determined through an Oxygen-specific pyrolysis reactor heated at 1060 °C. Although triplicate analyses were conducted for each sample and the average values were taken, the quantification of the O content probably has slightly high error than the determination of the N content. On the other hand, the analysis of the pretreated biomass yield was determined gravimetrically, the experimental error being very low.
Comment 5. Was the predicted optimum for each target value verified with the experimental results?
Answer: Yes, but we had not included these experimental results in the original manuscript. Accordingly, we have incorporated these results in the revised version:
At these operating conditions, the yield of solids predicted by the non-linear models (Eqs. (1) or (4)) was 63.5 %. This predicted value was confirmed with the experimental result at the same operating conditions (63.24 %).
At these operating conditions, the NRAP predicted by the non-linear models (Eqs. (2) or (5)) and the corresponding experimental value were 71.1 % and 73.0%, respectively.
At these operating conditions, the O recovery in the aqueous phase predicted by the non-linear models (Eqs. (3) or (6)) was 74.1 %. The experimental result of O recovery at the optimal operating conditions was slightly higher (76.3 %).
Comment 6. I believe that this work might be of interest to the readers of Processes, and recommend for publication after the minor revisions noted above were properly taken into account.
Answer: The authors thank again the reviewer for the recommendation for publication after a minor revision of the manuscript. We have taking into account all the reviewer´s suggestions.
