Review Reports

Round 1

Reviewer 1 Report

The authors described the continuous biodiesel production from ultrasound-assisted line. The whole process and experiments are well described to give a clear and comprehensive paper. I recommend the manuscript for publication after minor corrections

• Line 48: correct typography "H2SO4"
• Line 159: The ultrasonic powers delivered by clamps and sonotrode must designated as output powers and not total powers.
• Adjust interlign from section 3.3

Author Response

Please see the attached documents.

Author Response File: Author Response.docx

Reviewer 2 Report

A paper entitled “Continous ultrasound-assisted esterification and transesterefication of palm fatty acid distillate for ethyl ester production” is submitted to processes for further reviewing and publication. This research work provided a a new method to ethyl esther production from PFAD with good yield and purity of the final product. The paper is well written and very interesting to readers. I recommended that this paper is acceptable for publication with its present form.

Minor issues:

In the introduction it would be interesting to have the worldwide production data for the by-product under study.

In section 2.3 it is necessary that the authors describe in a complete way the method of ethyl ester analysis by GC (equipment, column, Tª ramp, etc.) it is not enough with the norm that they indicate.

Author Response

Please see the attached documents.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper presents the results of investigations aimed to optimize the continuous ultrasound-assisted esterification and transesterification of palm fatty acid distillate with ethanol for biodiesel production. Three steps were applied for esterification and transesterification. All the steps were optimized by applying the response surface methodology and optimal conditions were selected for obtaining high ester yield. The performed research is interesting and gives new knowledge in the field of ultrasound application in biodiesel production aiming to increase the process effectiveness as well for selection of new possible raw materials including suitable fatty waste for biodiesel production.     

After evaluation of manuscript it could be stated that paper needs to be revised and substantially improved. The following comments and remarks are addressed to the authors:

• The equipment used for experiments is clearly described and demonstrated in the scheme, unfortunately the method of preparation of samples and determination of ester content is not clear. Authors say that after first and second stage they used thin layer chromatography for evaluation of ester content, in this case the equipment used in this analysis should be presented including the manufacturers and other details. More details about principle of TLC method and the accuracy should be included into the text. Did the paper, chromatography plates or columns were used for separation of components? The same is about gas chromatography method and equipment.
• Authors in the text use the term “ester purity”. This term should be explained or changed by other commonly used terms such us “ester content, ester yield or transesterification degree”.
• It is clear from the text that samples after esterification in the first and second stages were used for determination of ester purity without any cleaning with water for separation of glycerol, ethanol and catalyst residues. In this case the data obtained is incorrect and cannot be used for optimization of conditions.
• Some other statements in the text are also doubtful: In the page 2 lines 79-81 authors inform that during esterification glycerides are formed. It is not true; the ethyl esters are formed from free fatty acids and alcohol. In the page 3 lines 109-111 authors stated that ethanol is widely used for biodiesel production. It is also incorrect; the most part of biodiesel is produced by using methanol.
• Authors should check or explain the meaning presented in the 3 page 127 line of “PFADE aka biodiesel“.
• Taking into account that the esterification and transesterification processes are influenced by four independent variables, authors should explain why the process temperature was not included in optimization and why such temperature was selected for experiments.
• In the chapter 3.2 the responses surface plots are presented in the figures, but the data is not commented in the text. The interaction of independent variables is not discussed and reasons of such interaction are not analyzed. It seems that the length of US reactor has no influence on the ester purity in the first stage and has only small influence in the stage 2. These results show the equations EE1 and EE2 as well as contour plots. Authors should deeply analyze such findings and try to found and present the reasons.
• Authors report that during the first and second step of esterification the ester purity of 932 wt% was obtained. In this case less than 5 % of raw material (glycerides) had to be transesterified by using alkali catalyst. Authors should explain why such big amount of KOH was used as catalyst for third step of biodiesel production; also information about procedures used for removal of glycerol, residues of ethanol, soap, and water should be presented.
• In the conclusions section (line 527) authors stated that “The oil yields obtained under recommended conditions were 132.91 wt.% first-step esterified oil, 131.16 527 wt.% second-step esterified oil“. They should explain what it means such the percentage of esterified oil and how it was determined.
• Authors present the information considering the electricity consumption in continuous ultrasound-assisted esterification and transesterification of palm fatty acid distillate. The comparison of electricity consumption of investigated process with the process without application of ultrasound, batch processes of biodiesel production should be presented and analyzed for more clarity.

Author Response

Please see the attached documents.

Author Response File: Author Response.pdf

Reviewer 4 Report

Article quite interesting for Processes readers and, well structured, although needs some slightly improvements.

 

Nevertheless, the utilisation of homogeneous catalysts on the biodiesel production were overcome by heterogeneous ones several years ago. The article could be made from a different catalyst utilisation perspective.

 

Besides, it's not novelty that, when compared methanolysis with ethanolysis, at the same operating conditions, methanolysis shows always higher ester yields than ethanolysis process.

 

In this study, it lacks the experimental determination and correspondent results of acidity index, which, normally, is mandatory when it used raw materials, like these ones, with significant of free fatty acids.

 

It's missing also, reproductibility tests for the optimized operating conditions achieved.

Author Response

Please see the attached documents.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The submitted manuscript has been significantly improved.
From my point of view, it is suitable for publication.

Author Response

Thank you very much for your comments on our manuscript. 

Reviewer 3 Report

The authors have revised their manuscript properly.

 

The authors did not mark the changes made in the text by inserting them as required by the provisions of the journal (track changes or colored text), which makes it difficult to determine whether all suggestions and comments have been answered.

Author Response

Thank you very much for your comments on our manuscript. 

We have added the track changes and colored text in both of word and PDF in file's name "Revised processes-1102782 - with remark". 

Author Response File: Author Response.docx

Reviewer 4 Report

The main aspects mentioned before in previous revisions almost keep the same on this revised document. My recommendation is the same.

Author Response

Thank you very much for your comments on our manuscript. 

Author Response File: Author Response.pdf