Experimental Study on Distillation Column Parameters for Liquefaction Device of Low Concentration Coalbed Methane

Round 1

Reviewer 1 Report

Dear Authors

I revised the manuscript “ Experimental Study on Distillation Column Parameters for Liquefaction Device of Low Concentration Coalbed Methane” submitted to the “Processes” Journal. The paper is very interesting. However, I have some concerns, which need to be addressed.

Abstract (Line 7 – 22)

Line 17 - 18. "....However, in actual operation, the evaporation temperature at the bottom of the column should not be higher than -140℃.

The use of the wording "however" suggests that the author is concluding the results of the research from the temperature range given in the abstract. Please indicate another verbal term to link the content of the summary. The vocabulary used in the sentence does not logically or too briefly relate to the content of the results contained in the abstract (lines 13 - 17).

Because higher temperatures than -140 ^oC are the temperatures striving for 0 ⁰C so please make a less enigmatic sentence in this part of the statement. Please make a statement that builds a unified communication of the summary.

Line 18. Minor errors in writing units with numerical value. Please standardise the writing in the body of the article using text spaces.

Line 23 - 24. The key words are proposed correctly and relate to the methodological assumptions as well as to the described phenomena.

Introduction

Line 31 - 32. ".....about 240 trillion cubic metres....." Please use the notation of the power exponent in terms of describing unit values. Please use the SI system unit notations.

Line 45 ".....[12] and other methods [13]......". The presented level of generality of the statement in terms of method description is difficult to accept due to the possibility of describing common features or families of methods and their classification. Please increase the precision of the statement for the better understanding of the paper's content by the readers.

Line 47 Please correct the notation of the chemical formula of methane.

Line 51 - 53. The scope of the study is indicated in the text of the paper before the goal of the paper is indicated.

Please consider the order of the information in relation to the goal and scope of the work. This will improve readers' understanding of the article's content.

Line 53 - 69 The justification for undertaking the research topic is clearly stated.

It is worth to introduce verbal expressions to organize the content of the article. Such action improves readers' understanding of the article.

Line 69 - 73. The authors of the article have indicated the main goal of the work. The information is presented clearly and allows to read the content of the chapter in a comprehensive way.

Experimental Device

Line 81 - 82. Figure 1. The detailed description for illustration a) and illustration b) should be indicated in terms of the illustration name. This method of presentation is more conclusive for the reader.

Figure b) is unreadable. Please consider separating illustrations (a) and (b) to improve the scale of the figures and increase the readability of the descriptive information.

Line ? "....The purified oxygen-bearing coal-bed gas has been removed the CO2...." Please correct the notation of the chemical formulas in the text of the paper.

MDAE - please introduce a rule to expand abbreviated names. This improves readers' understanding of the content of the article.

The technological description at the end of the chapter is understandable. Despite the extensive content, it introduces the reader to the important substantive issues of the article.

Experimental methods

Line 89 - 90. Please correct the notation of measurement units and please correct the notation of the methane chemical formula. Please correct any punctuation errors in sentences in the paper's text.

Line 100. Please correct the notation of measurement units to the version with a power exponent. The used notation is closer to the popular science notification.

Line 117 – 118. Please correct the notation of measurement units to the version with a power exponent. The used notation is closer to the popular science notification.

The methodological assumptions in the chapter are extensively described and accompanied by limiting assumptions and disclaimers. I believe that the measurement methods used in the paper are correct. The notation of the used equations 1), 3), 4) has the signs of unprofessional notation. It should be considered to improve the notation of the equations to a mathematically correct one, without popular science symbols.

Analysis of experimental results

The abbreviated notation LNG, although well recognised, should be explained at least once in the text of the article. This improves the readers' understanding of the article's content.

Figure 2, Figure 3. Please remember that a trend line is not the same as developing an empirical model from discrete data or a statistical indication. Please indicate explicitly the function of the appearing trend line.  

Line 193 - 195. Please correct the formal notation of chemical compounds and units of temperature and pressure. The current form of notation is inappropriate and misleads the reader.

Technology optimization

Line 241.".....and the correlation is very strong...." Please indicate more precisely the range of work results relating to this statement.

The content of chapter 5 contains elements of discussion, however, in the scope of other chapters, which contain research results, the discussion of results has a modest representation of research results from other scientific centres. Please enhance the discussion of results with detailed literature references in key areas of the presented achievements. 

Line 257. Figure 4. The figure is unreadable in terms of the description of the components of the diagram. Please improve the readability of the diagram.

Conclusion

Conclusions have the character of technological guidance and respond widely to the issues addressed in the goal and scope of the work. You should avoid multiple reporting of research results, especially in a key chapter. Please reinforce in the chapter the information related to the realisation of the work' s goal.

Author Response

Dear Editor and Reviewers,

The authors would like to thank Editor and Reviewers for comments to improve the quality of the paper. Your comments will be of great help to our future thesis writing. We have studied comment carefully and have made correction. The revised content has been marked in the text. You may need to use the "show comments" function under "review" in Microsoft Word. Looking forward to your approval.  

Thank you.

Abstract (Line 7 – 22)

Line 17 - 18. "....However, in actual operation, the evaporation temperature at the bottom of the column should not be higher than -140℃.

The use of the wording "however" suggests that the author is concluding the results of the research from the temperature range given in the abstract. Please indicate another verbal term to link the content of the summary. The vocabulary used in the sentence does not logically or too briefly relate to the content of the results contained in the abstract (lines 13 - 17).

Because higher temperatures than -140 ^oC are the temperatures striving for 0 ⁰C so please make a less enigmatic sentence in this part of the statement. Please make a statement that builds a unified communication of the summary.

Reply：Theoretically, increasing the temperature at the bottom of the distillation column can enhance the evaporation rate of nitrogen and oxygen, which is conducive to improving the purity of LNG products. However, it is found that when the temperature is higher than - 140 ℃, the boiling at the bottom of the column is strong, the components at the bottom of the column often drift, and the purity is very difficult to control to the target value. Therefore, the author added that In actual operation, the evaporation temperature at the bottom of the column should not be higher than -140 ℃. The word "hover" is deleted here (see Q1 in the revised draft).

Line 18. Minor errors in writing units with numerical value. Please standardise the writing in the body of the article using text spaces.

Reply：Modified. (see Q41 in the revised version)

Line 23 - 24. The key words are proposed correctly and relate to the methodological assumptions as well as to the described phenomena.

Reply：Change "distillation column" to "distillation column operating temperature" and increase "leakage". (see Q3 in the revised version)

Introduction

Line 31 - 32. ".....about 240 trillion cubic metres....." Please use the notation of the power exponent in terms of describing unit values. Please use the SI system unit notations.

Reply：Modified. (see Q4 in the revised version)

Line 45 ".....[12] and other methods [13]......". The presented level of generality of the statement in terms of method description is difficult to accept due to the possibility of describing common features or families of methods and their classification. Please increase the precision of the statement for the better understanding of the paper's content by the readers.

Reply：Modified. (see Q5 in the revised version)

Line 47 Please correct the notation of the chemical formula of methane.

Reply：Modified. (see Q5 in the revised version)

Line 51 - 53. The scope of the study is indicated in the text of the paper before the goal of the paper is indicated.

Please consider the order of the information in relation to the goal and scope of the work. This will improve readers' understanding of the article's content.

Reply：The work mentioned in line 51 – 53 has been carried out before the research of this paper. The research content of this paper is at the end of this paragraph（In this paper, the effect of the operating temperature of the distillation column on the performance of the liquefaction cold box is investigated……）. The research work must be based on the work mentioned in line 51 – 53 (built-up device).

Line 53 - 69 The justification for undertaking the research topic is clearly stated.

It is worth to introduce verbal expressions to organize the content of the article. Such action improves readers' understanding of the article.

Line 69 - 73. The authors of the article have indicated the main goal of the work. The information is presented clearly and allows to read the content of the chapter in a comprehensive way.

Experimental Device

Line 81 - 82. Figure 1. The detailed description for illustration a) and illustration b) should be indicated in terms of the illustration name. This method of presentation is more conclusive for the reader.

Figure b) is unreadable. Please consider separating illustrations (a) and (b) to improve the scale of the figures and increase the readability of the descriptive information.

Line ? "....The purified oxygen-bearing coal-bed gas has been removed the CO2...." Please correct the notation of the chemical formulas in the text of the paper.

MDAE - please introduce a rule to expand abbreviated names. This improves readers' understanding of the content of the article.

The technological description at the end of the chapter is understandable. Despite the extensive content, it introduces the reader to the important substantive issues of the article.

Reply：Modified. (see Q6, Q7 in the revised version)

Experimental methods

Line 89 - 90. Please correct the notation of measurement units and please correct the notation of the methane chemical formula. Please correct any punctuation errors in sentences in the paper's text.

Reply：Modified. (see Q8 in the revised version)

Line 100. Please correct the notation of measurement units to the version with a power exponent. The used notation is closer to the popular science notification.

Reply：Modified. (see Q9 in the revised version)

Line 117 – 118. Please correct the notation of measurement units to the version with a power exponent. The used notation is closer to the popular science notification.

Reply：Modified. (see Q10 in the revised version)

The methodological assumptions in the chapter are extensively described and accompanied by limiting assumptions and disclaimers. I believe that the measurement methods used in the paper are correct. The notation of the used equations 1), 3), 4) has the signs of unprofessional notation. It should be considered to improve the notation of the equations to a mathematically correct one, without popular science symbols.

Reply：Modified. (see Q42, Q43, Q44 in the revised version)

Analysis of experimental results

The abbreviated notation LNG, although well recognised, should be explained at least once in the text of the article. This improves the readers' understanding of the article's content.

Reply：Modified. (see Q11 in the revised version)

Figure 2, Figure 3. Please remember that a trend line is not the same as developing an empirical model from discrete data or a statistical indication. Please indicate explicitly the function of the appearing trend line.  

Reply：Modified. (see Q12, Q13 in the revised version)

Line 193 - 195. Please correct the formal notation of chemical compounds and units of temperature and pressure. The current form of notation is inappropriate and misleads the reader.

Reply：Modified. (see Q14 in the revised version)

Technology optimization

Line 241.".....and the correlation is very strong...." Please indicate more precisely the range of work results relating to this statement.

Reply：Modified. (see Q45 in the revised version)

The content of chapter 5 contains elements of discussion, however, in the scope of other chapters, which contain research results, the discussion of results has a modest representation of research results from other scientific centres. Please enhance the discussion of results with detailed literature references in key areas of the presented achievements. 

Reply：Modified. (see Q46 and Q47 in the revised version)

Line 257. Figure 4. The figure is unreadable in terms of the description of the components of the diagram. Please improve the readability of the diagram.

Reply：The original picture is not clearly visible, now enlarged.

Conclusion

Conclusions have the character of technological guidance and respond widely to the issues addressed in the goal and scope of the work. You should avoid multiple reporting of research results, especially in a key chapter. Please reinforce in the chapter the information related to the realisation of the work' s goal.

Reply：Modified. (see Q48 in the revised version)

Author Response File: Author Response.docx

Reviewer 2 Report

The paper studies the production of LNG from coalbed methane by low-temperature distillation with experiments. The effect of the condenser and reboiler temperature on the methane recovery rate and the LNG purity (methane concentration), respectively, is studied. The temperature set points providing acceptable recovery and purity are determined, and some modifications of the process are suggested.

The paper fits the scope of the journal, and the objective of the paper is clearly defined and accomplished. However, the details of the article are sometimes insufficient. The paper contains few grammatical errors, but the phrasing of the sentences and the choice of words must be improved to make the paper easier to understand. The phrasing of the text is often imprecise or vague (e.g. “temperature and component migration” instead of heat and mass transfer, “volatilizes”, “the liquid phase drops downward”, “upward-evaporating”, “dewdrop”).

I propose the paper to be accepted with the following corrections.

Questions and major comments:

• What is the refrigerant used?
• How is it possible that the refrigerant first enters the reboiler and only after this, the condenser?
• Are the concentrations mole fractions?
• Did you measure the oxygen content of the LNG?
• It is not at all clear how exactly the reboiler and condenser temperatures were controlled? Did you use automatic control? Even if not, what was the procedure? You have two controlled variables and three manipulated ones.
• What is the tray type used? What is the diameter and height of the column?
• What happens to the LNG produced if its oxygen content is too high?
• What was the equation of state used?

Minor corrections:

• You can just use “tail gas” instead of “nitrogen and oxygen tail gas”.
• 1, line 8: Change “decided” to “determined”.
• Do not capitalize common nouns in the text (e.g. “Measures”, “Theory”, “Engineering Thermodynamics”, “Experiment”).
• 1, line 19: Control loops have interaction, not correlation.
• 2, lines 53-56: What is the relation between the parts of this phrase?
• 2, line 63: Methane is a single component.
• 2, lines 65-66: Temperature control is not a value, so it cannot be too low.
• Figure 1: The caption is only valid for Figure 1a.
• 3, first two lines: Review this phrase; it is not grammatical. The subject should not be the gas (but the CO2 and the acidic gas). Change “at front” to “before”.
• 3, second paragraph, second line: Substances are either methane or not, not “mainly”.
• 3, first two paragraphs: Is the feed cooled to -172 or -162 °C. Both values are mentioned in the text.
• 3, second paragraph: Change “lower saturated vapor content” to “less volatile component”.
• 2, line 105: What do you mean by “mixing refrigerant ratio”?
• 4, lines 137-138: What do you mean by “generally”? What did you actually use?
• 5, line 162 and elsewhere: Use “outlier” instead of “bad points”.
• 5, line 169: What do you mean by the term “the components […] drift”?
• 6, line 185: Something is either sufficient or not.
• 6, line 191: Which dew point do you refer to here?
• 6, line 198: Add “it is” before “carried away”.
• 6, line 203: This phrase is incomplete.
• 6, line 208: Nitrogen and oxygen are individual components; they cannot be “more”.
• 6, line 211: Which component?
• 6, line 216: Change “dew” to “condensate”.
• 7, line 225: We select the compressor, not the refrigeration cycle.
• 7, lines 231-232: I do not understand this phrase.
• 7, lines 243-244: Where is this bypass valve?
• 8, lines 259-267: I do not understand this part. Will the purity of the product be acceptable?

Author Response

Dear Editor and Reviewers,

The authors would like to thank Editor and Reviewers for comments to improve the quality of the paper. Your comments will be of great help to our future thesis writing. We have studied comment carefully and have made correction. The revised content has been marked in the text. You may need to use the "show comments" function under "review" in Microsoft Word. Looking forward to your approval.  

Thank you.

• What is the refrigerant used?

Reply：Modified. (See revised Q16, chapter "Experimental Methods")

• How is it possible that the refrigerant first enters the reboiler and only after this, the condenser?

Reply：After the gas phase in the mixed refrigerant is cooled to about-120 ℃ by plate fin heat exchanger, it enters reboiler, and heats the liquid at the bottom of distillation column to evaporate the trace oxygen and nitrogen. After the refrigerant comes out of reboiler, the temperature is about-141 ℃, and then it returns to plate fin heat exchanger (III) for further cooling. After the temperature is reduced to about-182 ℃ through throttle valve (III), it is fed into the condenser to provide the cooling capacity for the distillation column.

• Are the concentrations mole fractions?

Reply：Yes，they are. (See revised Q17)

• Did you measure the oxygen content of the LNG?

Reply：Compared with oxygen and methane, the saturated vapor pressure of nitrogen is the highest, so the impurities in the bottom product of distillation column are basically only oxygen and no nitrogen. The mole fraction of nitrogen, oxygen and methane at the bottom of distillation column is measured by On-line Chromatograph. (See revised Q18)

• It is not at all clear how exactly the reboiler and condenser temperatures were controlled? Did you use automatic control? Even if not, what was the procedure? You have two controlled variables and three manipulated ones.

Reply：The reboiler and condenser temperatures are controlled by Throttle Valve (I), Throttle Valve (I), and Throttle Valve (I). The normal operation of the device should be automatically controlled, and the control variables are mainly methane content at the top and oxygen content at the bottom of the column. In order to change the temperature during the experiment, manual control was used to change the opening of each throttle valve to record the top temperature, flow rate and molar fraction of each component, as well as the bottom temperature, flow rate and molar fraction of each component.

• What is the tray type used? What is the diameter and height of the column?

Reply：The number of trays in the design of distillation column is 32 (including the condenser and reboiler), and the feeding position is the 10th tray from top to bottom. In actual processing, regular packing was used instead of trays, with inner diameter of 253mm and packing layer height of 7.2m. (See revised Q19)

• What happens to the LNG produced if its oxygen content is too high?

Reply：见Q13. High oxygen content of the product will affect the safety of the LNG storage tank. The valve for LNG to enter the storage tank is linked to the oxygen content at the bottom of the distillation column through an automatic control system. Once the molar fraction of oxygen exceeds 1%, the valve is closed and the unqualified LNG product is vented through the bypass valve. (See revised Q13)

• What was the equation of state used?

Reply：P-R equation is adopted, which is widely used in the calculation of physical properties in natural gas industry.

Minor corrections:

• You can just use “tail gas” instead of “nitrogen and oxygen tail gas”.

Reply：Modified.

• 1, line 8: Change “decided” to “determined”.

Reply：Modified. (See revised Q20)

• Do not capitalize common nouns in the text (e.g. “Measures”, “Theory”, “Engineering Thermodynamics”, “Experiment”).

Reply：Modified. (See revised Q49, Q50, Q51, Q52)

• 1, line 19: Control loops have interaction, not correlation.

Reply：Modified. (See revised Q21)

• 2, lines 53-56: What is the relation between the parts of this phrase?

Reply：The distillation column is inside the cold box. (See revised Q22)

• 2, line 63: Methane is a single component.

Reply：Modified. (See revised Q23)

• 2, lines 65-66: Temperature control is not a value, so it cannot be too low.

Reply：Modified. (See revised Q24)

• Figure 1: The caption is only valid for Figure 1a.

Reply：Modified. (See revised Q6)

3, first two lines: Review this phrase; it is not grammatical. The subject should not be the gas (but the CO2 and the acidic gas). Change “at front” to “before”.

Reply：Modified. (See revised Q7)

• 3, second paragraph, second line: Substances are either methane or not, not “mainly”.

Reply：The principle of low concentration coalbed methane separation in distillation column is as follows: the substances at the bottom of the column are mainly methane, and accompanied by trace amounts of oxygen, nitrogen and other components. In the process of heating, the oxygen, nitrogen and other components with high saturated vapor pressure are evaporated into gas and escape upward.  Modified. (See revised Q25)

• 3, first two paragraphs: Is the feed cooled to -172 or -162 °C. Both values are mentioned in the text.

Reply：Here is a clerical error, it should be - 172 ° C. (See revised Q26)

3, second paragraph: Change “lower saturated vapor content” to “less volatile component”.

Reply：Modified. (See revised Q27)

• 2, line 105: What do you mean by “mixing refrigerant ratio”?

Reply：Here, it should be mole fraction of each component in mixed refrigerant. (See revised Q28)

4, lines 137-138: What do you mean by “generally”? What did you actually use?

Reply：The word "generally" has been deleted here. (See revised Q29)

I'll use Newton's iteration. The calculation here can also use commercial software, but that involves copyright issues.

• 5, line 162 and elsewhere: Use “outlier” instead of “bad points”.

Reply：Modified. (See revised Q30, Q31, Q32)

• 5, line 169: What do you mean by the term “the components […] drift”?

Reply：Under normal conditions, as long as the column bottom temperature is lower than - 143 ° C, the mole fraction of each component measured by on-line chromatographic analysis is very stable. When the column bottom temperature is higher than - 140 ° C, the mole fraction of each component fluctuates irregularly, which is very frequent. Only when the temperature is restored to - 143 ° C can the purity of the product return to normal.

• 6, line 185: Something is either sufficient or not.

Reply：Modified. (See revised Q33)

• 6, line 191: Which dew point do you refer to here?

Reply：dew point temperature. Modified. (See revised Q34)

• 6, line 198: Add “it is” before “carried away”.

Reply：Modified. (See revised Q35)

• 6, line 203: This phrase is incomplete.

Reply：Modified. (See revised Q36)

6, line 208: Nitrogen and oxygen are individual components; they cannot be “more”.

Reply：Modified. (See revised Q37)

• 6, line 211: Which component?

Reply：LNG products at the bottom of the distillation column.

• 6, line 216: Change “dew” to “condensate”.

Reply：Modified. (See revised Q38)

• 7, line 225: We select the compressor, not the refrigeration cycle.

Reply：Modified. (See revised Q39)

7, lines 231-232: I do not understand this phrase.

Reply：Modified. (See revised Q40)

7, lines 243-244: Where is this bypass valve?

Reply：The bypass valve connects the inlet and outlet of the hot fluid of the reboiler. It has been added in Figure 5.

• 8, lines 259-267: I do not understand this part. Will the purity of the product be acceptable?

Reply：By increasing the bottom heat of distillation column, the oxygen and nitrogen in the liquid at the bottom of the column can be fully evaporated. Only when the purity of the product meets the user's requirements, the liquid at the bottom of the column can be introduced into the LNG tank as an acceptable product.

Author Response File: Author Response.docx

Reviewer 3 Report

• Graphs are blurred and lack description (What are the points means? The curve are the polynomial? Which type and why?)
• If the points in the graphs are the samples. in the text are missing tables with the samples (measured samples)
• How does the progress of the curves look like as a function of time?
• How large was thethe leakage rate of refrigerant
• Is it possible to prove this experiment  or its parts with a model in some suitable software (f.e. CFD )?
• The figures 1 and 4 are to small and there are missing the highlighted differences.
• The descriptions are in too long text. A breakdown in bullets would help.

Author Response

Dear Editor and Reviewers,

The authors would like to thank Editor and Reviewers for comments to improve the quality of the paper. Your comments will be of great help to our future thesis writing. We have studied comment carefully and have made correction. The revised content has been marked in the text. You may need to use the "show comments" function under "review" in Microsoft Word. Looking forward to your approval.

Thank you.

• Graphs are blurred and lack description (What are the points means? The curve are the polynomial? Which type and why?)

Reply: The original picture is fuzzy and has been modified. The points in the coordinate diagram describe the experimental data, and the curve represents the change trend. See Q12 and Q13 in the revised draft for their functions. If the points in the graphs are the samples. in the text are missing tables with the samples (measured samples)

Reply: In this paper, the points in the coordinate diagram describe the experimental data, and the corresponding ordinate and abscissa are the specific values of the experiment, which can be supplemented if necessary, but it will be repeated.

• How does the progress of the curves look like as a function of time?

Reply: The abscissa of the coordinate is temperature, not time.

• How large was the leakage rate of refrigerant？

Reply: The amount of refrigerant leakage is not measured for the time being. The device is a small-scale test device, with relatively large leakage, and needs to be supplemented immediately to maintain the operating pressure of the refrigeration cycle.

• Is it possible to prove this experiment or its parts with a model in some suitable software (f.e. CFD )?

Reply: The process flow design needs to be calculated by software. According to the calculation results of equipment design and selection, engineering construction. The calculation results need to be tested experimentally after the installation is completed. Software or model calculation and experiment are different means of technical research. The results obtained in this experiment should also be verified by appropriate software or models. The figures 1 and 4 are to small and there are missing the highlighted differences.

Reply: Has been modified. The two drawings have been enlarged and the difference is obvious after amplification. The thick black solid line on the left side of Fig. 5 (original figure 4) is the increased process pipeline.

• The descriptions are in too long text. A breakdown in bullets would help.

Reply: OK, thank you. I will pay attention to it when writing the article.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

• The graphs are still blurred. Do you now MS Excel???
• Do you think enlarging the image will solve my comment? Do you know another color than black?

Author Response

The graphs are still blurred. Do you now MS Excel??? Do you think enlarging the image will solve my comment? Do you know another color than black?

Reply: I have used different colored lines to represent different fluids in the revised draft. I apologize that I thought the content in the article could only be printed in black. See the revised version for details.

Author Response File: Author Response.docx