Continuous Fixed Bed CO₂ Adsorption: Breakthrough, Column Efficiency, Mass Transfer Zone

Round 1

Reviewer 1 Report

The manuscript investigates the CO2 capture from a mixture of CO2/N2 using molecular sieve and silica gel type-III, highlighting the effect of temperature, superficial velocity and CO2 partial pressure on the breakthrough behaviour.

The topic is interesting and worthy of investigation. However, the paper cannot be accepted for publication in the present form. The Authors should address some issues, regarding the analysis/explanation of the obtained results, before publication on the Proceeses journal.

Specific comments

1. Introduction section: “There are three techniques to capture CO2 relying on the plant layout”. The three techniques should be specified.
2. The analysis of the breakthrough behavior, also in terms of the effect of the investigated operating variables, is quite shallow. The authors should provide a deeper explanation of the obtained results, clearly highlighting the link between the results and the fundamental thermodynamic/kinetic principle. To this aim, the Authors should consider the following works:
3. 1. Powder Technol 2020;373:446–58. https://doi.org/10.1016/j.powtec.2020.06.075
   2. Eng. J. 2019;372:526–535. https://doi.org/10.1016/j.cej.2019.04.165
   3. AIChE J 2013;59:923–35. https://doi.org/10.1002/aic.13870
   4. Chem Eng J 2013;223:795–805. https://doi.org/10.1016/j.cej.2013.02.041

Author Response

Please see the attachement as Reply to Reviewer 1

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript deals with a study on the CO2 capture from a mixture of CO2/N2 using molecular sieve and silica gel type-III. In particular, the breakthrough behaviour has been predicted as a function of temperature, superficial velocity and CO2 partial pressure.

Even though the topic is interesting, the paper is not acceptable for publication in the present form. Some issues should be addressed before publication on the Proceeses journal. In particular, the Authors should better motivate their work discussing about the novelty of their work.

Specific comments

1. The introduction section needs to be improved. Some important points should be discussed. Nothing is disclosed about the regeneration process. As a matter of fact, besides the adsorption step, the set-up of a reliable and economic sorbent regeneration strategy represents one of the crucial issues to be dealt with. The Authors should discuss about this, providing at least a brief description of the available technological alternatives (TSA, PSA, VSA…). The reading of the following works may be of use to the Authors:
2. 1. Ind Eng Chem Res 2020;59:3593–605. https://doi.org/10.1021/acs.iecr.9b04901
   2. Particuology 2015;23:8–15. https://doi.org/10.1016/j.partic.2015.02.001
   3. Chem Eng Sci 2004;59:3657–70. https://doi.org/10.1016/j.ces.2004.05.027
   4. Ind Eng Chem Res 2002;41:5802–11. https://doi.org/10.1021/ie011011j
   5. Int J Greenh Gas Control 2015;39:1–16. https://doi.org/10.1016/j.ijggc.2015.02.001
3. The Author should better motivate their work. There is plenty of works in the literature on the CO2 capture by adsorption, also focusing on the breakthrough behavior as affected by different operating variables (temperature, superficial velocity and CO2 partial pressure, etc.). Likewise, several works are available on the synthesis and development of novel cutting-edge materials with high-level adsorption/desorption performances. With the present work, the Authors investigated the breakthrough behavior, as affected by different operating variables, of commercially available and widely studied sorbents (molecular sieve and silica gel). Therefore, which is the novelty of the present work to make it worthy of being publishable?
4. Several grammatical errors/typos can be found throughout the manuscript. It should be carefully revised from this point of view.

Author Response

Please see an attached file as Reply to Reviewer#2

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript presents study on using two different methods to capture carbon dioxide in order to decrease the level of greenhouse gases. Authors performed performed some very interesting experiments and they presented some thought on their results. However, I have some comments as below:

1. In Abstract, Line 14 and 15: Authors need to mention where the source of CO₂/N₂ and why they chose this mixture.
2. Abbreviation should not be used in Abstract unless they have been identified before. In Line 21, there is an abbreviation "MS" which was not mentioned before.
3. Line 23, using uncertainty words like "may be" does not sound scientific and appropriate for a research paper.
4. Lines 51-59 are basically reporting that there are some studies in this area. However, authors need to criticise these studies and show the research gap and the differences between literature and their current work.
5. Figure 1 needs to be captioned again to be more visible for readers. It also needs to add more details (i.e. the position of temperature sensors)
6. Line 102, what is IR? and what is the time interval in each experiment? These needs to be stated properly.
7. Lines 124-127 (.... the pore density .......) are repeating what authors mentioned in Lines 121-124. The paragraphs starts with mentioning that comparing Molecular sieve to Silica Gel, the porosity of MS is higher. Therefore, there is no need to repeat and say that the porosity of Silica is lower.
8. It is better to keep the time intervals and start/end time of the x-axis (time) in Figure 3(b) similar to Figure 3(a) in order to have better comparison.
9. Equations 1-4 can be moved to section 3.4 where authors are describing the mass transfer. Also, Equations 2 and 3 are same. Equ (3) is a generic formula for each time, and Equ (2) is showing specifically the breakthrough time. Suggest to remove Equ (2) and keep Equ (3)
10. Line 181, the temperature is supposed to be 50 degree not 40.
11. There is no information about the temperature profiles inside the column for Silica Gel, although this has been shown for MS in Figure 5. Suggest to add this for comparing the temperature profile by using two different methods.
12. Suggest to move errors and accuracy in a separate section.
13. English grammar and spelling needs to be reviewed and revised, for example:
    • Line 37, ".... mentioned THAT the CO₂ ...."
    • Line 37, ".... GHG produces (or is producing) as the ...."
    • Line 114, "..... surface characteristics OF ...", why OF is capital?
    • Line 141, ".... Prolonged break point (or breakthrough) ...."
    • Line 166, Cexit is column outlet and Cin is column inlet, but this is not reflecting to what you wrote. "..... are column inlet, outlet ...." should be changed to ".... are column outlet, inlet...."

Author Response

Please see an attached file-Reply to Reviewer#3

Author Response File: Author Response.docx

Reviewer 4 Report

Abstract

Page 1, line 17: Your data shows that breakpoint time reduced with increased superficial velocity.

Page 1, line 21: what is ‘MS’. Define acronyms on first use.

Page 1, line 23: last sentence is misleading: high adsorption performance in itself does not equal economical use.

*Emphasise what makes this work unique / important / new.

Introduction

Page 1, line 30: CO₂ is not most prevalent, it is the most abundant GHG

Page 1, line 32: change ‘a detriment threat to health’ to ‘detrimental to health’

Page 1, line 41: I think ‘relying’ should be ‘depending’

Page 2, line 42: adsorption not ‘physical or chemical absorption’

Page 2 , line 42: membranes employ absorption not adsorption

Page 2, line 43: References 10 and 11 should come before you mention cryogenic separation as these are only relevant to chemical absorption and membrane absorption.

*Make it clear that not all of these technologies are industrially used, yet. They are the leading proposed technologies being researched.

Page 2, line 44: this doesn’t make sense: ‘Practicable process for CO₂ captures for the power plant on a short time scale is post-combustion [13-16].’

*Introduction needs to be streamlined and re-ordered more logically.

Page 2, line 51-71: This is very weak for a literature review. It reads like a list and there is no clear link between examples or comparison between them. It has to be written like a story and the upshot and findings relevant to your own research have to be clear.

Page 2, line 73: You might want to introduce/define molecular sieves and silica gel type adsorbents. Why are these worth investigating?

Page 2, line 74: Rather than predict them, you calculate the bed capacity results do you not?

Materials and methods

Page 2, line 80: ‘Molecular sieve’ is very generic. Can you be more specific, so that someone may find it easily on Sigma Aldrich?

Page 3, line 82: explain why activation of the adsorbents is carried out.

Page 3, line 92: define ‘SS’ column.

Page 3, line 94: If this is a recirculator unit, it is not a ‘thermal’ jacket. You do not need to explain how the recirculator works.

Page 3, line 101: again, if you have already made it clear that the water recirculating unit controls the temperature, you do not need to say ‘The desired column temperature is controlled by the water circulator fitted with unit.’

Results and discussion

Page 4, line 114: undo caps ‘OF’

Page 4, line 115: how do these values for surface area match to the values supplied by the supplier of the materials - do Sigma provide this information?

Page 4, line 116: define BJH

Page 4, line 118: Is this average pore size?

Page 4, Table 1: To report pore diameter (or width) is more conventional

Page 4, line 123: Intuitively, this seems to contradict what the surface area and pore volume and pore size data says. Can you explain this? Do you have pore size distribution data? This will help interpret the SEM images.

Page 4, line 124: Remove ‘The pore density is not very high in case of silica gel compared to molecular sieve.’ It is repetition.

Page 4, Figure 2: Can you get images where the scale bar is clearer as it is hard to read.

Page 5, lines 133-143: Make this paragraph about the breakthrough results more succinct. Deal first with Figure 3(a) to describe how breakthrough time changes with temperature. Do not repeat sentences.

*Why did you choose 40 °C for the adsorption response at different superficial velocities if the longest breakthrough time was at 30 °C?

*What definition do you set for ‘breakthrough’? E.g. What % of C_in is detected in C_exit when you record the breakthrough times?

Page 5, line 145: What is meant by ‘stoichiometric wave front’? Mass-transfer zone?

Page 5, lines 147-149: These sentences need to be reworded.

Page 6, line 154: Re-order sentence: lower superficial velocity leads to increased breakpoint time.

Page 6, Eq. 1: This is an equation for stoichiometric time, not total column capacity.

Page 6, Eq. 2 and 3: define t_ca

Page 7, line 180-181: Check ‘The adsorption capacity of 0.488 mmol/g adsorbent was obtained at a column temperature of 40 °C.’ You might mean 50 °C.

Page 7, line 183: What do you mean by ‘selective’ here?

Page 8, line 207: why use N/m² when you use bar in the Figure?

Page 9, Table 2: This would be useful in the introduction/literature review.

Page 10, line 235: Are these breakthrough curves steeper? It is difficult for the reader to appreciate without overlaying them.

Page 10, line 246: ‘and is more pronounced at lower superficial velocity’. It is not clear what this means.

Page 12, line 277: Correct ‘Isothers’

Page 12, line 283: where is the data for the ‘repeatability measurements’?

Page 12, line 304: (LMTZ) is length of zone of mass-transfer.

Page 12, line 305: ‘A lean MTZ means the proficient exploitation 305 of the adsorbent leading to minimize the energy regeneration cost.’ Reconsider this and the relationship with regeneration.

Page 13, Eq. 6: define L_br

Page 13, line 316: You mean Eq. (7).

Page 13, Eq. 7: define L_ub

Page 13, line 329: What is 5 lpm? Why not use the same units as before (m/s)?

Conclusions

Page 14, line 348: Typo - superficial velocity was not 0.062 m/s.

Page 14, line 348: This is incorrect, ‘It was suggested that breakpoint time enhanced with increased feed superficial velocity employing both the adsorbents’. Breakpoint time decreased with increasing superficial velocity as per Fig 3(b) and Fig 6(b).

Page 14, line 349: Which adsorbent is this referring to? This needs to be stated: ‘The prolonged break-point of 1515 s was noticed at a superficial feed velocity of 0.032 m/sand reduced to 1185 s on raising the velocity to 0.042 m/s at a temperature of 40 °C.’

*The conclusions need to emphasise the significance of this work, point to further work that may arise on completion of this study.

Page 14, line 360: On what basis can it be suggested that the Molecular Sieve adsorbent may be ‘economically viable’? The adsorbent’s CO₂ capacity is not particularly high. It has not been extensively tested. What adsorbents does this material compare favourably against in the literature?

Author Response

Please see an attached file-Reply to Reviewer #4

Author Response File: Author Response.docx

Reviewer 5 Report

1. Any of abbrevitions such as MS, MOF, BET, OF, BJH, MTZ etc. should be defined in the first use. 
2. No duplicated results were shown to let the reader see the data were qualified.
3. L121-L27 : there should be results related to Figure 2 instead of Figure 1. Also, the authors should keep the consistence of object name such as MS in Figure 2(a)(b) and molecular sieve in the text.
4. Table 2 is a good template for comparsion between the data in this study and the others. The authors should make more effort on the discussion to relveal the significance of this study.
5. The description in L336 "This makes ...." is not clear enough to address the scopes of efficiency and cost.

Author Response

Please see an attached file-Reply to Reviewer #5

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript can be accepted for publication.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

All the Reviewer's comments have been addressed and the manuscript has been revised accordingly. Therefore, it can be accepted for publication.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Thanks for replying to Round-1 comments. Overall the paper looks OK now with all changes. However, there is still couple of comments which have not yet been explained properly:

• First, how authors justify that N2 is a major constituent of greenhouse gas whereas the air itself contains 79% of Nitrogen.

"The CO2 capture from a mixture of CO2/N2 has been studied using molecular sieve (MS) and silica gel type-III (SG)."

• The source of CO2 and N2 is still not clear. We understand that authors used gas bottles in their experiments as mentioned in Section 2.1, however in reality where can we find this mixture and where the source is? and why this mixture has been chosen?

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 4 Report

Reviewer’s Comments:

Section: “Abstract”

Comment 3: Page 1, line 23: last sentence is misleading: high adsorption performance in itself does not equal economical use.

Response: We thank the reviewer for valuable comment. The last sentence has been rephrased as “The findings suggest that molecular sieve is suitable for CO₂ capture due to high adsorption performance owing to outstanding adsorption characteristic parameters (page-1 line-23-25).

There is still no basis to ascribe MS as having ‘outstanding adsorption characteristic parameters’ with an uptake of 0.665 mmol/g, when compared to materials from the literature which you report have much higher CO₂ capacities. You have demonstrated however, that MS is better than SG.

Section: “Introduction”

Comment 8: Page 2, line 42: adsorption not ‘physical or chemical absorption’

Response: We thank the reviewer for valuable comment. This is the other absorption based technologies to separate CO₂ from flue gases and highlighted in the references [10, 11].

Leading technologies also include physical and chemical adsorption. You can say ‘sorption’ to refer to both adsorption and absorption.

Comment 10: Page 2, line 43: References 10 and 11 should come before you mention cryogenic separation as these are only relevant to chemical absorption and membrane absorption.

Response: We thank the reviewer for the comment. I think, the references 10 and 11 are rightly placed as in the preceding section the general aspects of environmental issue has been covered.

You should find a specific reference for cryogenic separation. References 10 and 11 are not about cryogenic separation.

Comment 14: Page 2, line 51-71: This is very weak for a literature review. It reads like a list and there is no clear link between examples or comparison between them. It has to be written like a story and the upshot and findings relevant to your own research have to be clear.

Response:  We thank the reviewer for the comment. The literature has been rephrased. Various researchers studied the CO₂ capture from different viewpoints. It is not always possible to find numerous literature having the same conditions, same parameter estimation. The updated and relevant literatures have been added to the revised manuscript under the “Introduction” section.

The literature review still needs attention. Don’t only mention what was studied – explain what was learned from the study. What are the main scientific findings you have observed in the literature and what are the general trends that you have found? A lot of the detail you report from the papers could be gained from the title of the paper alone – go deeper. What work has previously been carried out on silica gel type adsorbents?

Comment 15: Page 2, line 73: You might want to introduce/define molecular sieves and silica gel type adsorbents. Why are these worth investigating?

Response:  We thank the reviewer the valuable comment. Numerous studies devoted on the use of commercial adsorbent for CO₂ adsorption study. Molecular sieves and silica gel are well known adsorbent used for adsorption purposes. These adsorbents have been used to carry out in depth investigation for CO₂ capture using adsorption. Under current investigation, bed characteristics has been analyzed in terms of breakthrough, adsorption capacity, column efficiency, usable bed height, width of mass transfer zone and temperature profiles. The novelty of the current work has been provided at the end of section “Introduction”(page 3, line 103-109).

You could still introduce/define molecular sieves and silica gel type adsorbents in the literature review – SG adsorbents are not mentioned here.  

Section: “Results and discussion”

Comment 22: Page 4, line 115: how do these values for surface area match to the values supplied by the supplier of the materials - do Sigma provide this information?

Response: We thank the reviewer for valuable comment. The surface characteristics of both the adsorbents obtained using BET analyzer as presented in now Table 2 (Table 1 of original manuscript) of revised manuscript (page 6).

As these are commercially available materials, do their surface area and porosity characteristics match any information given by suppliers?

Comment 36: Page 6, Eq. 2 and 3: define t_ca

Response: We thank the reviewer for the valuable comment. As the Eqn. 2 from the original manuscript has been deleted to avoid any confusion and now the Eqn. (1) is for stoichiometric time and Eqn. (2) is generic one for any time. The Eqn. (3) is used to determine the adsorption capacity any time of adsorption.

It is still unclear what the definition of t_ca is.

Comment 38: Page 7, line 183: What do you mean by ‘selective’ here?

Response: We thank the reviewer for the valuable comment. Here selective means adsorption of CO₂ only considering that N₂ is not adsorbed. It means that adsorption of CO₂ is selective in comparison with the adsorption of N₂ which is a carrier gas.

‘Selective CO₂ capacity’ should just be called ‘CO₂ capacity’.

Do you have any qualitative data to demonstrate selectivity of CO₂ over N₂?

Comment 40: Page 9, Table 2: This would be useful in the introduction/literature review.

Response: We thank the reviewer for the valuable comment. The table 2 (now table 1) and corresponding description has been moved to introduction “section”(page 4) according to the suggestion.

Table 1 is under ‘2. Materials and methods’, not the Introduction. Is there particular reason for this?

Comment 46: Page 12, line 305: ‘A lean MTZ means the proficient exploitation of the adsorbent leading to minimize the energy regeneration cost.’ Reconsider this and the relationship with regeneration.

Response: We thank the reviewer for the valuable comment. Smaller width of mass transfer zone signifies the better use of adsorbent capacity at breakpoint. It makes efficient use of adsorbent for CO₂ capture and decreases the cost of adsorbent regeneration (page 14, line 366). The above mention concept is very well supported by literature [1. C.J. Geankoplis, Transport Processes and Unit Operations, 3^rd Edition, Prentice Hall, 2.  W.L.McCabe, J.C.Smith, P.Harriott, Unit Operation of Chemical Engineering, 4^th Edition, McGraw-Hill Company] 

Reference this literature                                               

Section: “Conclusions”

Comment 55: Page 14, line 360: On what basis can it be suggested that the Molecular Sieve adsorbent may be ‘economically viable’? The adsorbent’s CO₂ capacity is not particularly high. It has not been extensively tested. What adsorbents does this material compare favourably against in the literature?

Response: We thank the reviewer for the valuable comment. The adsorption capacity determined for molecular sieve is considerably high compared to that of silica gel. Also the determined bed characteristic parameters i.e. column efficiency, utilized bed height and width of mass transfer zone are superior for MS. Based on these aspects, it can be clearly stated that molecular sieve is suitable for CO₂ capture from CO₂/N₂ mixture.

Only in comparison to SG is MS a better adsorbent, and therefore might be more ‘economical’ for CO₂ separation. In comparison to adsorbents discussed in the literature, MS has a lower capacity and therefore may be less economical. It is completely misleading to suggest that MS is an ‘economical’ CO₂ adsorbent. The findings in this paper do not suggest that - they only show that MS is better than SG.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 5 Report

The authors have to define "MS" in the first use in the "Introduction" section since the "Abstract" part can be islolated from your article. In addition, the authors should indicate how many data you use to caculate the uncertainty on each of your controlled parameters (ex. n=20) to enhance your data quality since your repeatability is only supported by the consistence between batch1 and batch2 in section 3.4. 

Author Response

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Round 3

Reviewer 3 Report

The paper looks OK now.