Analysis of the Heat Balance of a Metal Hydride Separator Used for the Separation of Hydrogen from Syngas†
, Natália Jasminská, Jozef Živčák, Lukáš Tóth, Romana Dobáková, Filip Duda, Ľubomíra Kmeťová and Ľubomíra BednárováRound 1
Reviewer 1 Report
In this work the authors demonstrate the advantageous use of a metal hydride separator for the removal of hydrogen. It is very detailed and well written. I would view this as a well written technical report clearly demonstrating the advantage of using metal hydrides. I offer only a few very minor comments. Well done!
1) I believe there is a small typo in the title. "HM" should be "MH". However, I would recommend using the full name "metal hydride" as the MH abbreviation may not be known to all readers. I would further recommend appending to the title "for hydrogen separation" or something similar.
2) A small typo in the abstract, "metho16ds" should be "methods".
3) "Introduction". The section is very detailed and well written. It does an excellent job putting this work into context. My only comment would be that I wish there were a better way to present equation (1). However, I do not have any suggestions as how to do so.
4) Page 8, line 312. Change "chapter" to "section".
Author Response
Cover Letter
The changes carried out in the document, based on the comments provided by the reviewer, are listed below:
Reviewer 1:
1, “HM” in the title is a typo. In line with the reviewer's comment, “MH” has been removed and replaced with the term “metal hydride”.
Line: 2, 121, 123, 134, 162, 163, 172, 188, 204, 210, 225, 227, 251, 252, 258, 262, 293, 294, 311, 313, 322, 341, 357, 360, 361, 378, 381, 398, 401, 412, 419, 421, 425, 437, 442-446, 448, 450, 459, 512, 517, 519, 520, 522, 529
The title of the paper has been changed from “Analysis of the heat balance of a HM separator” to “Analysis of the heat balance of a metal hydride separator used for the separation of hydrogen from syngas”.
Line: 2-3
2, A small typo in the abstract, "metho16ds" has been corrected.
Line: 11
3, "Introduction". The section is very detailed and well written. It does an excellent job putting this work into context. My only comment would be that I wish there were a better way to present equation (1). However, I do not have any suggestions as how to do so.
The equation was deleted. The paper deals with the heat balance of a metal hydride separator and a possibility of streamlining the heat removal from the volume of the alloy during the hydrogen absorption process. For this reason, the description of the chemical decomposition of organic matter in a plasma reactor is useless information.
The equations have been renumbered.
Line:85, 86, 208, 217, 224, 231, 237, 245, 275, 282, 287, 416
4, Page 8, line 312. Change "chapter" to "section". The term “chapter” has been replaced with the term “section”.
Line: 193, 336
Other:
One literature was added. The references have been renumbered accordingly. English language and style were reviewed by native speaker.
Author Response File: 
Author Response.docx
Reviewer 2 Report
In this paper, the authors discuss the potential of a metal hydride device as separator of hydrogen from syngas obtained from the thermal recovery of wastes. A heat balance of the metal hydride is carried out by means of analytical calculations. The authors claim that the main problem of the metal hydride separator is the quite low thermal conductivity of the alloy powders that compose it. In order to overcome this drawback, the authors suggest implementing a heat transfer intensifier to improve the heat removal and the thermal homogeneity of the metal hydride separator. Finally, the authors compare three possible geometries to set the heat intensifier. From my sight, this work provides a good background with relevant references. Calculation methods and results are clearly presented. I recommend the publication of this paper after some minor changes. Comments can be found below:
- I recommend the authors avoiding the abbreviation (HM) in the title of the manuscript. When you read the title, you do not know what the work is about. Moreover, if HM refers to ‘Metal Hydride’ separator, it should be MH in any case.
- The authors refer to the ‘hydrogen absorbed’ or the ‘hydrogen absorption’ in the whole document. However, if the hydrogen is separated from the syngas by adhering to a solid, would not it be more accurate to say that this operation is an adsorption operation rather than an absorption?
- The scheme of a hydrogen separator in Figure 1 is not as clear as desirable. The syngas inlet seems not to achieve the metal hydride separator. The line entering and leaving the metal hydride separator seems to be the same, leading to confusion. I recommend the authors reorganizing the scheme to make it more intuitive if possible.
- When authors refer to secondary syngas components, what are the authors referring to? What is the composition of the syngas in this work? Are not these secondary components ‘absorbed’ in any way in the metal hydride? Does this mean that the selectivity of the alloy powder to the hydrogen is 100 %? Some discussion about this is needed.
- Equation 4 is not introduced in the text of the manuscript. It should be introduced before appearing in the manuscript to avoid confusion.
- The symbol ‘τ’ in equation 7 is not defined in the text. Does it refer to time?
- What is the difference between Figure 13 and 14? Although the authors claim that Figure 14 shows the heat flux as a function of the separator length, it seems to be the same. I think this must be an error. It must be remedied.
- If the mass of the MH alloy is reduced by the implementation of the intensifier, will this decrease have a negative impact on the capacity of hydrogen ‘absorption’? Will this negative impact be overcome by the positive effect of the increase in the heat flux? In brief, have these intensifier implementations net benefit in hydrogen obtention? I recommend some discussion about which geometry is the best among the studied in terms of heat flux increase, but also in terms of hydrogen separation capacity if possible.
- Several sections of the manuscript are numbered with the number 3. This problem should be solved to avoid confusion.
Comments for author File: Comments.docx
Author Response
The changes carried out in the document, based on the comments provided by the reviewer, are listed below:
1, I recommend the authors avoiding the abbreviation (HM) in the title of the manuscript. When you read the title, you do not know what the work is about. Moreover, if HM refers to ‘Metal Hydride’ separator, it should be MH in any case
The “MH” abbreviation has been removed and replaced with the term “metal hydride”.
Line: 2, 121, 123, 134, 162, 163, 172, 188, 204, 210, 225, 227, 251, 252, 258, 262, 293, 294, 311, 313, 322, 341, 357, 360, 361, 378, 381, 398, 401, 412, 419, 421, 425, 437, 442-446, 448, 450, 459, 512, 517, 519, 520, 522, 529
2, The authors refer to the ‘hydrogen absorbed’ or the ‘hydrogen absorption’ in the whole document. However, if the hydrogen is separated from the syngas by adhering to a solid, would not it be more accurate to say that this operation is an adsorption operation rather than an absorption?
In this case, we are talking about the absorption of hydrogen into the structure of the alloy. Hydrogen is catalytically cleaved and stored in the intermetallic space of the alloy by forming a metal hydride with the alloy.
Explanation added in the text: Hydrogen gas molecules stick to the metal surface and break down into hydrogen atoms (H). The hydrogen atoms then penetrate into the interior of the metal crystal to form a new solid substance called a metal hydride.
Line: 99 – 101
3, The scheme of a hydrogen separator in Figure 1 is not as clear as desirable. The syngas inlet seems not to achieve the metal hydride separator. The line entering and leaving the metal hydride separator seems to be the same, leading to confusion. I recommend the authors reorganizing the scheme to make it more intuitive if possible.
The scheme was corrected.
Line: 170
4, When authors refer to secondary syngas components, what are the authors referring to? What is the composition of the syngas in this work? Are not these secondary components ‘absorbed’ in any way in the metal hydride? Does this mean that the selectivity of the alloy powder to the hydrogen is 100%? Some discussion about this is needed.
The research on a possibility to separate hydrogen from a mixture of various gaseous components includes the examination of the effects of the basic gas components (CO2, CO, CH4, N2, and H2) on the metal hydride properties. Due to the high percentage contents of carbon oxides in the syngas, which may amount to more than 50% of the total volume of the syngas produced during the waste gasification, it is important to investigate their effects on the degradation of the metal hydride. The degradation will depend not only on the metal hydride composition, but also on a maximum temperature of the alloy generated during the absorption process. The elimination of temperature differences in the metal hydride volume, as well as the improved homogeneity of the metal hydride thermal field, reduces the risk of catalytic reactions between the individual gas mixture components and the metal hydride components. This goal may be achieved by installing the intensifiers of the heat transfer from the metal hydride volume.
The ongoing research on the efficient removal of the generated heat from the metal hydride volume is carried out simultaneously with the investigation, production and testing of metal hydrides which exhibit a higher resistance to the degradation caused by the presence of carbon oxides in the mixture of gases. Such metal hydrides will be presented to the public over the next few years. Furthermore, the accompanying benefit consists in the designs of novel systems facilitating a rapid reduction of the contents of carbon oxides in the mixture of gases which enters the process of hydrogen separation carried out with the use of metal hydrides. Such systems may be based on the chemical or gravitational principles.
Line: 478 - 498
5, Equation 4 is not introduced in the text of the manuscript. It should be introduced before appearing in the manuscript to avoid confusion.
Explanation added in the text: When these metal powders absorb hydrogen to form hydrides, the heat is released. The generated heat output in a metal hydride alloy during the hydrogen absorption can be defined as follows:
Line:221 - 223
6, The symbol ‘τ’ in equation 7 is not defined in the text. Does it refer to time?
Explanation added in the text: τ is the time (s).
Line:247
7, What is the difference between Figure 13 and 14? Although the authors claim that Figure 14 shows the heat flux as a function of the separator length, it seems to be the same. I think this must be an error. It must be remedied.
It was an error, an inappropriate image was inserted into the article. The images uploaded to the MDPI system were correct.
Line: 431
8, If the mass of the MH alloy is reduced by the implementation of the intensifier, will this decrease have a negative impact on the capacity of hydrogen ‘absorption’? Will this negative impact be overcome by the positive effect of the increase in the heat flux? In brief, have these intensifier implementations net benefit in hydrogen obtention? I recommend some discussion about which geometry is the best among the studied in terms of heat flux increase, but also in terms of hydrogen separation capacity if possible.
Explanation added in the text: The increase in the linear heat flux was affected by the primary and secondary fins. The application of the cross-flow fins did not increase the removal of heat through the jacket; nevertheless, it positively affected the homogeneity of the metal hydride thermal field.
An analysis of the temperature gradient Δt between the maximum and minimum temperatures of the metal hydride alloy revealed that Geometry 2 exhibited a 64% decrease, when compared to the Δt in the alloy with Geometry 1. With Geometry 3, the decrease in the temperature gradient Δt amounted to 71% and with Geometry 4 as much as 77%. With Geometry 2, the amount of the stored hydrogen was 9% smaller than the amount stored with Geometry 1. With Geometry 3, the decrease amounted to 10.9%; and with Geometry 4, the amount of the stored hydrogen was 13.7% smaller. However, a positive effect of the increase in a linear heat flux and of a better temperature distribution within the thermal field significantly exceeds the decrease in the capacity of the stored hydrogen, which was caused by the application of the intensifier. Apparently, the most optimal shape of the intensifier for maximising the linear heat flux was Geometry 3. Where the required outcome is to achieve a minimum temperature difference, the optimal alternative is Geometry 4, as it facilitated the reduction of the total generated heat in the metal hydride volume; as a result, the internal heat source was 3.1% smaller than that in Geometry 3.
Line: 449, 454 – 457, 462 – 472
9, Several sections of the manuscript are numbered with the number 3. This problem should be solved to avoid confusion.
It was a typo, the sections was renumbered.
Line: 260, 337, 436, 501
Other:
One literature was added. The references have been renumbered accordingly. English language and style were reviewed by native speaker.
Author Response File: Author Response.docx
