Kinetic Study on CO-Selective Methanation over Nickel-Based Catalysts for Deep Removal of CO from Hydrogen-Rich Reformate

Round 1

Reviewer 1 Report

 

 

Comments to the Authors

This is a fascinating work, which is of pronounced importance for understanding the kinetic study on CO selective methanation over nickel-based catalysts for deep removal of CO from hydrogen-rich reformate, which is endowed for superb hydrogen production.

The authors have elaborated on the issue of CO removal and its conversion to CH₄. Two efficient nickel-based catalysts showing high activity and selectivity to CO methanation were considered and their kinetic models were investigated adopting the mechanistic kinetic models based on the Langmuir chemisorption theory. The kinetic parameter study was carried out by global optimization handling all the rate equations for the plausible reactions at once.

 Although the present article can be accepted for publication due to relevance and undoubted advantages of the work, in the “Catalysts’ However, there are some necessary comments to be considered before final acceptance.

 

1-      The English language should be thoroughly revised; long statements should be split for easy understanding (lines30-35)

2-       In equation (12), the value of n should be stated.

3-      Correction: in line 172, the experimental description is given in section 3, not chapter 4. Use (sections instead of chapters)

4-      Correction: in line 184, review the CO value, is it 0.4 or 0.04?

5-      Large numbers of references are massed. It recommended using only a few specific citations. See lines (51,56 and 105)

6-      It is found inconsistency in the determination of balance equations of the species. particularly equation (6) for the hydrogen in line131.

7-      The authors should explain broadly and justify the setting values of the variables:  Crossover fraction = 0.8; Elite count= 4 and the number of populations =200

8-      The authors should state clearly the constraints of the present objective Function?

9-      The presentation quality of figures is not clear and need to be revised

10-  In the parity plot for the Ir/Ni-MgO/g-alumina catalyst figure S1(a), no comparison is given for the intermediate concentrations, authors are urged to comment on the reason for these aspects.

11-  Compare the obtained results with the results obtainable from using the Particle Swarm Optimization (PSO).

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for a critical evaluation of the manuscript and the constructive comments and suggestions. The detailed explanations for the reviewer's comments are summarized in the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors,

The paper is interesting but it requires some improvement in order to be considered for publication. Please find the comments in no specific order.

 

1. Authors cited lists of 3 or more references without proper justification of the relevance of each one. Authors must rearrange text and cite each reference placing it closer to the place where it is needed and/or add at least a few words/a single sentence to justify why each reference is relevant and must be cited.
2. As in the development of a kinetic model, the crucial part is the experimental work, my suggestion is that the Results and discussion paragraph to be rearranged, in order to begin with the presentation of experimental kinetic study and then with the theoretical work (development of the kinetic model, estimation of parameters, model adequacy testing, etc.)
3. Page 4, Lines 164-165, it is stated that : “For the kinetic experiments, a fixed-bed quartz reactor was used and the pressure 164 was maintained at atmospheric level.” Could the authors explain how the pressure was kept in the quartz reactor at atmospheric value? Have they used a backpressure regulator, or is just an estimation based on pressure reading devices?
4. The usual practice in chemical engineering is to compare the prediction of a model (as a theoretical tool) with experimental determination. The authors are mentioning the opposite (page 4, lines 165-167). Please recheck. Also, given the data in Table 2, it does not seem that the gaseous compositions have been varied in a wide range, as the authors are stating in the same paragraph.
5. Page 9, lines 317 – 318: In the phrase “In this work, the two aforementioned outstanding catalysts, Ir/Ni-MgO/γ-alumina 317 and Ni0.5Zr0.5O2 catalysts prepared according to the published works [5,21]” seems that something is missing. Please rephrase.
6. One of the aspects very important in the development of a kinetic model is the evaluation of the physical steps (internal and external diffusion). Have the authors performed any experimental/theoretical evaluations of these influences. Please mention it in the manuscript.
7. Given that the catalysts used in this kind of processes are subjected to deactivation, information about the reproducibility of the experiments and/or catalysts activity over-time have to be presented.
8. Page 3: The meaning of the variables appearing in the rate expressions (1) – (3) as well as the ones in relations at line 126 should be mentioned.
9. Page 3, lines 105-107: The authors are mentioning that “Using the kinetic models and stoichiometry, the generation or consumption rate of each species can be expressed as the ordinary differential forms (equations (4) - (10)).”. The kinetic models are practically rate expressions, that refers or to a specific chemical reaction, or is expressed as formation/consumption reaction rate for a specific chemical compound. The equations (4) – (10) represents the mathematical model of the chemical reactor. In order to be more consistent, the hypotheses used to develop this mathematical model should be provided. Also is not very clear what w means in these mass balance equations. Please present this paragraph in a more clear way.
10. It is not very clear how the composition is calculated using relation (11). Also, the variables denoted with F, are molar or mass flowrates at the reactor outlet? If so, how were these flowrates evaluated/measured?
11. Page 4, lines 151-153: Mentioning the type of computer processor used for parameter estimation is not relevant if no comparison regarding the run time of the estimation procedure is presented. Also, the authors have to better explain the meaning of the variables appearing in the objective function and its mathematical form that is not an usual one.
12. Page 5, lines 176-178: The authors are stating that “The experimental data and parameter estimation results for Ir/Ni-MgO/γ-alumina 176 are illustrated in Figure 2 for the comparison between the predicted and the experimen-177 tally measured composition of the carbonaceous species.”. In the catalyst science, as carbonaceous species are usually referred the heavy chemical compounds or the coke leading to catalyst deactivation. In the same statement, the parameter estimation results is presented in Table 3, not in Figure 2. Probably Figure 2 presents a comparison of the model’s predictions in terms of chemical compositions at the reactor outlet and the experimental results. Please reformulate.
13. The paragraph 2.2 (Parameter estimation results) should start with the numerical values resulting from the estimation procedure. Also, details regarding the adequacy of the estimated parameters should be provided (correlation coefficient, parity diagrams or other statistical/thermodynamic model validation procedures). The value of the objective function for the optimal values of the kinetic parameters should be provided.
14. A comparison of the model’s predictions for other operating conditions than the ones used in the estimation should be provided.
15. Figure 4 seems to be irrelevant for the present study as one can draw the diagrams if the parameters in Table 3 are provided.
16. There is an inconsistency in Table 4. The DH_H2 is negative for the current study and positive for the value taken from the work of Xu and Froment. Same issue with the value of DH_H2O. Please comment.
17. My suggestion is to include Figure S1 in the main manuscript. Also, details for the two regions in Figure S1 (a) should be presented in order to be easier to compare the predicted with the experimental data.

 

Author Response

We thank the reviewers for a critical evaluation of the manuscript and the constructive comments and suggestions. The detailed explanations for reviewer's comments are summarized in the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Dear authors,

The manuscript must follow the usual structure of a paper dealing with  the development of a kinetic model for a synthesized catalyst. Consequently, the quality of the paper must be improved.