Heat Capacities and Enthalpies of Normal Alkanes in an Ideal Gas State
Round 1
Reviewer 1 Report
Well done.
Author Response
Thank you very much for evaluating our research!
Reviewer 2 Report
The article proposes an approach for alkanes thermodynamic properties prediction. Although the abstract is promising, the article does not provide the necessary input to verify its claims. And in current form cannot be accepted.
Below are my main comments:
The article is rather brief and lacks a clear presentation of the novelty compared with the literature. The newest cited reference is almost two decades old. The data are compared with literature data from 1975 but it lacks the comparison with largely available and used reference databases from NIST Refprop or opensource alternative Coolprop. Which is built on newer literature such as:
"Reference Equations of State for the Thermodynamic Properties of Fluid
Phase n-Butane and Isobutane," J. Phys. Chem. Ref. Data, 35(2):929-1019, 2006.
E. K. Michailidou, M. J. Assael, M. L. Huber, and R. A. Perkins. Reference Correlation of the Viscosity of n-Hexane from the Triple Point to 600 K and up to 100 MPa. J. Phys. Chem. Ref. Data, 42(3):033104:1–12, 2013. doi:10.1063/1.4818980.
etc.
Data comparison is performed for 4 and 10 carbon atoms hydrocarbons and approximated for 20 atoms but is not compared with available data on other particles properties. The data representation in the article needs to be improved.
In the conclusions authors write :
"Knowledge of this dependences allows accurate estimates of the heat capacities and enthalpies of vapors of n-alkanes with scarce
or lacking data, thus providing the accurate interpolation/extrapolation of available experimental data without applying various semi-empirical methods. Also, they can be readily used in CFD applications instead of high-order polynomials." - but the authors correlation is in form of a complex fourth-order polynomial and is a semi-empirical equation as well.
Author Response
Thank you very much for evaluating our research! We made our best to follow all the comments in the manuscript. All changes are marked in blue.
Comment #1
The article is rather brief and lacks a clear presentation of the novelty compared with the literature. The newest cited reference is almost two decades old. The data are compared with literature data from 1975 but it lacks the comparison with largely available and used reference databases from NIST Refprop or opensource alternative Coolprop. Which is built on newer literature such as:
"Reference Equations of State for the Thermodynamic Properties of Fluid
Phase n-Butane and Isobutane," J. Phys. Chem. Ref. Data, 35(2):929-1019, 2006.
- K. Michailidou, M. J. Assael, M. L. Huber, and R. A. Perkins. Reference Correlation of the Viscosity of n-Hexane from the Triple Point to 600 K and up to 100 MPa. J. Phys. Chem. Ref. Data, 42(3):033104:1–12, 2013. doi:10.1063/1.4818980.
etc.
Reply to Comment #1
As a matter of fact, we have used NIST Refprop (our Ref. 2, retrieved in 2021) to check the validity of our approach (see Appendix A). Also, following this comment we have checked our approach against recent theoretical calculations of n-alkane heat capacities using MP6 and G4 methods (see the new Ref. 12 and new Appendix B). In view of it, we have added 5 more references to the literature review. It appeared that our Eq. (5) has excellent accuracy (better than 0.1%), if one consistently applies each of the methods: determines function f(T) using Eq.(4) and calculates C_p using Eq. (5) within a chosen method (MP6 or G4).
As for the references suggested by the reviewer, they deal with n-alkane properties other than ideal gas heat capacities and enthalpies of n-alkanes and are beyond the scope of our study.
Comment #2
Data comparison is performed for 4 and 10 carbon atoms hydrocarbons and approximated for 20 atoms but is not compared with available data on other particles properties. The data representation in the article needs to be improved.
Reply to Comment #2
We did not include tables with other n-alkanes just to avoid unnecessary increase in the volume of the manuscript. Actually, for all n-alkanes, the average accuracy of Eq. (5) appeared to be on the level of 0.1% and less. It is very easy to check with a pocket calculator, using Eqs. (4) and (5). Alternatively, we used 3 sets of different reference data: from Ref. 1, Ref. 2, and Ref. 12 and included tables for n=3, 4, 5, 6, 8, 10 and 20 indicating high accuracy in all cases. Moreover, based on the discovered generality, we believe that a discrepancy from Eq. (5) can be treated as the indication of error in a set of reference data used for analysis.
Comment #3
In the conclusions authors write:
"Knowledge of this dependences allows accurate estimates of the heat capacities and enthalpies of vapors of n-alkanes with scarce
or lacking data, thus providing the accurate interpolation/extrapolation of available experimental data without applying various semi-empirical methods. Also, they can be readily used in CFD applications instead of high-order polynomials." - but the authors correlation is in form of a complex fourth-order polynomial and is a semi-empirical equation as well.
Reply to Comment #3
Our dependences for heat capacities and enthalpies include a single set of coefficients for all normal alkanes, whereas the high-order polynomials available in the literature have different sets of coefficients for each compound. We have included this note to the Conclusions.
Reviewer 3 Report
Based on the analysis of hydrocarcon data, the simple unified dependence of heat capacity and entalpy on the number of carbon atom in a molecule were derived. The error of estimations is about 0.1% or less. Obtained dependence can be potentially used in computational fluid dynamics.
In my opinion, the paper is suitable for publication.
Some minor editorial corrections are possible:
on page 2
for "computational fluid dynamics" the abreviation CFD will be needed, because the notion "CFD" is used on page 6.
for "high-order NASA polinomials" maybe "high-order polinomials" will be corrected (or explanation what difference is for "NASA polinomials").
Author Response
We have introtuced the abbreviation "CFD."
We have used "high-order polynomials" as you suggested.
Thank you very much for evaluating our research!
Round 2
Reviewer 2 Report
The authors made substantial changes in the article according to the previous comments.
