Temperature Dependence of Density and Viscosity of Biobutanol-Gasoline Blends

Round 1

Reviewer 1 Report

An international team of authors is what I evaluate positively.

The authors used 4 models to describe the dependences of density and kinematic viscosity on temperature for different gasoline-biobutanol blends.

They compared the models and also their coefficients of determination, which is important. On three-dimensional graphs, the effect of temperature and percentage of biobutanol on density and kinematic viscosity can be seen. In this case, multivariate polynomial model was used.

Why did the authors not use the percentage of biobutanol in blends in the range from 10% to 85%?

 

The manuscript contains some formal errors highlighted in the text.

Line 16 – “(RON 95 and RON 98)” – the abbreviations have to be explained at its first appearance in the text. Also line 29 – “B5 and B10” are explained later, in Table 3 “SSE” - the explanation missing.

In Table 2, instead of weight, right will mass. Is volumetric mass density the same quantity as density? Between number and physical unit has to be a free space “15°C”.

Line 97 – instead of “2 minutes”, right will be “2 min”.

Line 102, 220, 221, 222, … – Symbols of physical quantities have to be in italics – ν , ρ, R, E_a, …

Lines 152, 180 – There should be spaces around the <, = characters.

Line 155 – Instead of “five times”, right will be “5 times”.

In Tables 5 and 9 – the correct unit for E_a is J · mol^-1.

In Table 12 – in unit of kinematic viscosity, number 2 there should be an upper exponent.

In Tables A5 and A6 – the correct unit for kinematic viscosity is mm² × s^-1.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thank you very much for the very useful comments and helpful remarks and we apologize for the formal errors. Please, see attached document with detailed response for your comments.

We firmly believe that we have complied with all your comments.

 

Author Response File: Author Response.docx

Reviewer 2 Report

The current work deals with specific physical properties examination of biofuel-fuel blends and the use of them as ultimate fuels. It is a well presenting paper, with a well-established research methodology and a comprehensive experimental section. The paper deserves publication after the following minor revisions:

1. Introduction, lines: 38-39: Taking into consideration the contemporary green energy transmission, this statement regarding fossil fuels is too risky. It would be a good idea to revise this statement appropriately.
2. Introduction, lines: 42-43: It would be essential to mention that another successful approach is the in situ gasoline upgrade into strong anti-knocking alcohol mixtures (https://doi.org/10.1016/j.cattod.2014.07.058 and 10.2118/177687-ms).
3. Introduction, lines: 53-62: Authors ought to comment as well that on the other hand microalgae strains also gaining attention as an advanced energy feedstock for biofuel production (e.g. 10.4155/bfs.09.10 and 10.25103/jestr.084.01)
4. Section 2.5: How did authors statistically examine normality and homogeneity of data?
5. Line 192: Use section instead of chapter.
6. Explain in your main text what does SSE stand for?

Author Response

Dear reviewer,

Thank you very much for the very useful comments and helpful remarks and we apologize for the formal errors. Please, see attached document with detailed response for your comments.

We firmly believe that we have complied with all your comments.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper focuses on the evaluation of fluid properties and density of butanol-gasoline blends as a response to temperature. For this purpose, the authors measured the density and viscosity of a selection of butanol-gasoline blends, ranging from 0 to 100% for temperatures between -10 °C and 40 °C. The main findings of this study are that blends up to 10% butanol in gasoline show fairly similar properties as gasoline alone, implying that it would be feasible to use such blends in existing vehicles throughout the evaluated temperature ranges.

Overall, the article is well written, and the topic appears to be relevant and important for the increased integration of renewables into existing fuel mixtures.

The introduction presents the background well and provides a good basis for the reader to understand the rest of the text.

The methods describe all the work that was performed, but there are some unclarities that prevent the work from being completely replicable (see below for more details). It is of high importance that this information is provided!

The results section systematically covers the gathered data, as well provides an overview of the different mathematical models used to describe the relationships between butanol concentration, temperature, viscosity, and density. This section needs to be strengthened mainly by providing more information on how the appropriateness of each model was evaluated. The authors are advised to use more than just R² to evaluate the goodness of fit for the models (more information is again available below) and to reevaluate whether or not the chosen model was truly the most appropriate.

 

Minor comments:

1. Introduction:
   • Line 38-39: The term “will become the most valuable energy source” can infer a very different meaning from the one the authors likely intended. Considering the context, I suggest it is changed to “the most expensive energy source”. Furthermore, the article by Festel (2008) does not at any point state this, so using this reference here is inappropriate and needs to be removed!
   • Line 56: It would be good to spell-out what FAME stands for the first time it is used.
   • Line 60: Please follow the taxonomic nomenclature rules when writing species names, i.e. the genus needs to start with a capitalized letter, and the whole name should be in italics.
2. Materials and Methods:
   • Table 2: What is the purpose of this table? Firstly, it is never referred to in the text, and secondly, it shows information about diesel and bioethanol, neither of which is the focus of this manuscript, making the table seem quite out of place. The text under Table 1 states that table 2 will also contain information on biobutanol, but it does not. Please remove this table or incorporate it better.
   • Line 93: The authors state which viscometer was used, but not which spindle or which measurement speed. Since this affects the accuracy of the measurement, such information is critical to the reader! This is primarily for replication purposes, but also to be able to estimate the reliability of the collected data, since and incorrect choice may widen the uncertainty of the measurement. Please provide this information!
   • Section 2.3: Considering that each sample was placed into the viscometer, cooled to -10 °C, and gradually heated by 5°C, while performing 20 measurements at each temperature, how did the authors ensure that no significant sample evaporation occurred? This is particularly important for measurements at higher temperatures, where evaporation is faster, and the samples will have been exposed to the atmosphere for longer. Providing this information in the methods section will help strengthen the results and conclusions, by showing how data accuracy was ensured.
   • Line 95: In which way was the used procedure equal to that of citation nr. 21 (Alptekin & Canakci, 2008)? Those authors used a capillary viscometer for their measurements, so there does not appear to be much apparent similarity. Similarly, while citation nr. 22 refers to previous work by the authors using the same instrument, it is written in Czech. Consequently, most international readers will not be able to reproduce this work without some more basic information in this manuscript, such as the spindle used, as described above, and the rotational speeds. This kind of information will also help the authors’ argumentation, since it will reflect the quality of the laboratory work performed (i.e. measurements performed within the measuring range of the instrument).  
3. Results and Discussion:
   • Lines 155-157: The information on the number of replicate measurements would fit better in the methods section.
   • Why is kinematic viscosity listed with a unit of mm/s? Is it meant to be m^²/s?
   • Sections 3.4.1, 3.4.2, and 3.4.3: Based on the manuscript it appears the authors evaluated how well the different models represent the analytical data by only comparing the R² values, which may be problematic. The authors should at least examine the distribution of residuals to make sure they have a normal distribution around 0 and no discernable patterns when plotted along the x-axis (temperature). On a related note, it is very much expected that the Vogel model would have the highest R² values, since it has the highest number of parameters (unless the R2 value in the text represents the adjusted R², in which case that should be made clearer in the methods section).
   • Tables 7 and 11: When explaining under the table that “*” stands for cases where the coefficient is not statistically significant, please add information on what p-value was used as a cutoff value for statistical significance. This information will enable the tables to serve as standalone units of information.
   • Figures 3-7: Since the viscosity was measured at each temperature in 20 replicates, does the individual dot at each temperature represent the mean of those measurements? This needs to be clarified in the text describing the figures. It would be better if the plots visualized the distribution of the measuring points by e.g. standard deviations. This would aid the reader in understanding how well each of the models fits the data, since different points may have more variance linked to them.
   • Figure 5: It Is quite clear that the Vogel model is not appropriate for describing the data for B100, since it systematically undershoots the data until around 285 K, after which it systematically overshoots it. This is also reflected in the R² It seems the exponential model was much more appropriate in this case, so why are all butanol-gasoline mixtures on figures 6 and 7 represented with the Vogel model? This is a misleading presentation of the data, even if the general conclusions are not really affected by it.
4. Conclusions:
   • Lines 305-306 and 309-310: Is the conclusion that densities and viscosities of the butanol-gasoline blends decrease with increasing temperature novel in any way? Was this not be entirely expected?

Author Response

Dear reviewer,

Thank you very much for the very useful comments and helpful remarks and we apologize for the formal errors. Please, see attached document with detailed response for your comments.

We firmly believe that we have complied with all your comments.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Dear authors,

thank you for the clarifications and updates to the text. I have no further questions and comments.