Accumulation and Emission of Water Vapor by Silica Gel Enriched with Carbon Nanotubes CNT-Potential Applications in Adsorption Cooling and Desalination Technology

Round 1

Reviewer 1 Report

The authors investigated the accumulation and emission of water vapor by silica gel enriched with carbon nanotubes with potential applications in adsorption cooling and desalination technology.

The authors aimed to investigate the effectiveness of a mixture of silica gels and CNTs for use in adsorption chillers.

This paper contributes to studying the effectiveness of a mixture of silica gels and CNTs in adsorption chillers. In my opinion, this work is worth publishing after some necessary changes.

In the section Discussion and Conclusion, the authors mention that "Three SG grain sizes were investigated: 100-160 μm, 200-250 μm, and 350-400 μm. Silica gels with granulation of 100-160 μm had the highest values of the sorption capacity in relation to nitrogen, specific surface area ???_L and total pore volume ?_t. These silica gels were selected for further study."

-Was not this an expected result? This granulation is the smallest in size, so it presents the most significant area for adsorption. The authors must discuss this point and perhaps eliminate this comment.

In the same section, the authors wrote: "The addition of CNTs can change the sorption capacity values of the bed. The addition of 10% wt. CNTs resulted in a decrease in the sorption capacity in mixtures."

Is this effect a result of hydrophobia of CNTs? The authors must discuss this point.

In the last paragraph of the same section, the authors wrote:" To summarize, besides adsorption properties, other properties should also be considered when discussing the performance of adsorption cooling and desalination systems."

The authors must be more specific on the "other properties" and make a short list to benefit the reader.

Finally, the authors must explain abbreviations before using them.

Author Response

Dear Reviewer,

 

I would like to express my gratitude for presenting my constructive suggestions and comments. I have studied all of them carefully and tried my best to revise the manuscript. I have made all the changes to the revised submission in a red color to make it easier to follow. Pointwise responses to the comments/suggestions raised by the reviewer are given.

The authors investigated the accumulation and emission of water vapor by silica gel enriched with carbon nanotubes with potential applications in adsorption cooling and desalination technology. The authors aimed to investigate the effectiveness of a mixture of silica gels and CNTs for use in adsorption chillers. This paper contributes to studying the effectiveness of a mixture of silica gels and CNTs in adsorption chillers. In my opinion, this work is worth publishing after some necessary changes.

1. In the section Discussion and Conclusion, the authors mention that "Three SG grain sizes were investigated: 100-160 μm, 200-250 μm, and 350-400 μ Silica gels with granulation of 100-160 μm had the highest values of the sorption capacity in relation to nitrogen, specific surface area ???_Land total pore volume ?_t. These silica gels were selected for further study."

-Was not this an expected result? This granulation is the smallest in size, so it presents the most significant area for adsorption. The authors must discuss this point and perhaps eliminate this comment.

As suggested by the Reviewer, the Discussion and Conclusion section was changed. The section describing the structural results of silica gels with three grain sizes was removed.

2. In the same section, the authors wrote: "The addition of CNTs can change the sorption capacity values of the bed. The addition of 10% wt. CNTs resulted in a decrease in the sorption capacity in mixtures." Is this effect a result of hydrophobia of CNTs? The authors must discuss this point.

In the last paragraph of the same section, the authors wrote: "To summarize, besides adsorption properties, other properties should also be considered when discussing the performance of adsorption cooling and desalination systems." The authors must be more specific on the "other properties" and make a short list to benefit the reader.

Finally, the authors must explain abbreviations before using them.

The Introduction section and the Discussion and conclusions section were further enhanced with a commentary on why sorption capacity changes in mixtures. In the Introduction section, text was added: „Since CNT also exhibit hydrophobic properties, this enables their use in a wide range of industries.”

and:

„The application of carbon nanotubes in adsorption is limited by their characteristics: hydrophobicity/hydrophilicity, presence of functional groups, size of specific surface area or surface charge. While the removal of contaminants from water by nanotubes is a successful method, the use of CNTs in the adsorption of inorganic compounds, including vapors and gases, may result in a reduction in sorption efficiency. This is due to the selectivity of CNTs and their unique properties.”

In the Discussion and conclusions section, text was added:

„These results indicate the unfavorable applications of CNTs in the context of sorption. „This is due to the hydrophobic properties of the tested CNTs. This is confirmed by SEM-EDS analysis, proving the purity of the nanotubes used. The presence of functional groups, both carboxyl (-COOH) and hydroxyl (-OH), is undesirable for industrial use. These functional groups block the pores of the nanotube surface and can adsorb water due to their more hydrophilic nature. The specific characteristics of CNTs related to electrical conductivity and mechanical properties, make this material nevertheless interesting for applications related to sorption as a function of temperature and pressure.”

and:

To summarize, besides adsorption properties, other properties should also be considered when discussing the performance of adsorption cooling and desalination systems. „The efficiency of devices using physical adsorption depends primarily on the heat and mass transport parameters of the adsorbent bed.”

 

Thank you also for giving me the opportunity to resubmit the revised manuscript and I hope it will now be accepted for publication.

I look forward to your feedback.  Thank you!

 

Yours sincerely,

Anna Pajdak

Reviewer 2 Report

The manuscript entitled "Accumulation and emission of water vapor by silica gel eriched with carbon nanotubes CNT - potential applications in adsorption cooling and desalination technology" reports a study on silica gel samples enriched with carbon nanotubes for their use in the design of adsorption beds in adsorption cooling.

Although the topic may be interesting, I found the work a bit confusing. It is not clear the importance of the results achieved also because the authors conclude by saying that carbon nanotubes are unfavorable in the context of absorption. Furthermore, there is confusion about the figures referred to in the text. I therefore believe that the manuscript must undergo a thorough revision before being considered for its publication. Here are some tips for authors:

 

*) Table 1 is not very clear. Perhaps it could be eliminated by writing the parameters in the text. Or using a single line by entering the different temperature values.

*) Is it necessary to clarify what substantial difference is there between SG1 and SG2 materials? Why use two apparently different SGs only for the manufacturer?

*) Thecharacterization of the SGs. To compare them, it is necessary to know what the measurement error is? How many times have the tests been repeated ?.

*) Have the nanotubes used been purified?

*) Figure 9.c. what are the parameters on the axes? How are the presence of Mg, Al and Mn justified in the EDS analysis of nanotubes?

*) The thermo-gravimetric analyzes (shown in figures 4 and 5) refer to SG1 and 2 with which granulometry?

*) Table 2 is unclear. Why indicate with the same symbols samples of different granulometry? For better clarity I believe it is best to distinguish the ssamples  throughout the text.

*) In the text there is a lot of confusion with the number of figures:

Line 246: Figures 2a; 2b ??

line 253 figure 2c?

line 272: Figure 3a?

Line 273/277: Figure 3b?

Line 283/284: figure 4a, 4b, 4c, 4d ??

etc .. In the text the number of the figures does not coincide .. Check all the figures.

*) The data obtained for the different systems containing SG1 / CN and SG2 / CN do not seem very discordant between them. Therefore, before drawing conclusions, it is necessary to know what the error is and how many times the analyzes have been repeated.

*) The conclusions should more clearly motivate the value of the results obtained.

Author Response

Dear Reviewer,

 

I would like to express my gratitude for presenting my constructive suggestions and comments. I have studied all of them carefully and tried my best to revise the manuscript. I have made all the changes to the revised submission in a red color to make it easier to follow. Pointwise responses to the comments/suggestions raised by the reviewer are given.

The manuscript entitled "Accumulation and emission of water vapor by silica gel eriched with carbon nanotubes CNT - potential applications in adsorption cooling and desalination technology" reports a study on silica gel samples enriched with carbon nanotubes for their use in the design of adsorption beds in adsorption cooling.

1. Although the topic may be interesting, I found the work a bit confusing. It is not clear the importance of the results achieved also because the authors conclude by saying that carbon nanotubes are unfavorable in the context of absorption. Furthermore, there is confusion about the figures referred to in the text. I therefore believe that the manuscript must undergo a thorough revision before being considered for its publication. Here are some tips for authors:

Thank you for your suggestion, the article has been rewritten and the most significant aspects of the paper have been highlighted in red.

2. Table 1 is not very clear. Perhaps it could be eliminated by writing the parameters in the text. Or using a single line by entering the different temperature values.

Table 1 has been removed as suggested. The information in Table 1 has been included in the text.

3. Is it necessary to clarify what substantial difference is there between SG1 and SG2 materials? Why use two apparently different SGs only for the manufacturer?

In this study, two types of silica gels were used intentionally. These materials differed in particle shape and structural parameters, which can significantly affect the sorption process. The use of two types of silica gels gives the possibility of a broader comparative aspect and possible correlation of sorption parameters with structural ones. Besides, the reader is able to compare the experimental results for two similar materials and the paper is thus more interesting.

4. The characterization of the SGs. To compare them, it is necessary to know what the measurement error is? How many times have the tests been repeated ?.

All measurements were performed on standardized apparatus. Measurement uncertainties of selected structural parameters on material structure determined by low pressure N₂ gas adsorption method are added to Table 3. Uncertainties on water vapor sorption measurements are added to Table 4. Low pressure N₂ adsorption tests were performed once, water vapor sorption tests were performed twice for each sample.

5. Have the nanotubes used been purified?

According to the manufacturer, the nanotubes are purified and are a high purity product.

6. Figure 9c. what are the parameters on the axes? How are the presence of Mg, Al and Mn justified in the EDS analysis of nanotubes?

In Figure 9c, the x-axis parameter is the energy (keV), the y-axis parameter is the number of counts of processed X-ray signal per second (cps). The descriptions are supplemented in Figure 9c in the manuscript. The presence of trace elements may be due to surface contamination. The presence of aluminium and magnesium is very probably due to the fact that the holder to which the samples are adhered is made of Al alloy and contains a few percent Mg in its composition. Nanotubes are not very compact materials. Electrons hitting the sample, also penetrate deep into the sample and can collect information also to the substrate - holder. Surface impurities can also be oxidized.

7. The thermo-gravimetric analyzes (shown in figures 4 and 5) refer to SG1 and 2 with which granulometry?

Thermogravimetric tests were performed on the finest grain size of silicagels, i.e. 100-160 mm.

8. Table 2 is unclear. Why indicate with the same symbols samples of different granulometry? For better clarity I believe it is best to distinguish the samples  throughout the text.

Table 2 and Figure 6 were corrected for better communication and clarity of the article. Names for silica gels with different grain sizes were introduced so that the reader can distinguish between them Further tables and sample descriptions were also improved as suggested by the reviewer.

9. In the text there is a lot of confusion with the number of figures:

Line 246: Figures 2a; 2b ?? line 253 figure 2c? line 272: Figure 3a? Line 273/277: Figure 3b? Line 283/284: figure 4a, 4b, 4c, 4d ?? etc .. In the text the number of the figures does not coincide .. Check all the figures.

Thank you for comment, the numbering of Figures has been checked and corrected.

10. The data obtained for the different systems containing SG1 / CN and SG2 / CN do not seem very discordant between them. Therefore, before drawing conclusions, it is necessary to know what the error is and how many times the analyzes have been repeated.

The DVS VACUUM Dynamic Dual Vapor/Gas Gravimetric Sorption Analyzer used for water vapor sorption testing has guidelines from the manufacturer. According to these guidelines, the measurement error is 0.1 mg. The sorption measurements were performed twice for each sample. Each time the measurement results coincided or the differences were negligible.

11. The conclusions should more clearly motivate the value of the results obtained.

Discussion and conclusion section, has been revised. All changes are highlighted in red.

 

Thank you also for giving me the opportunity to resubmit the revised manuscript and I hope it will now be accepted for publication.

I look forward to your feedback.  Thank you!

 

Yours sincerely,

Anna Pajdak

Round 2

Reviewer 1 Report

The authors made the required changes to their manuscript. I recommend publication.

Reviewer 2 Report

The authors revised the manuscript and made many improvements.

I believe that in the current version it can be considered for its publication.