The Performance of Low-Pressure Seawater as a CO2 Solvent in Underwater Air-Independent Propulsion Systems
Round 1
Reviewer 1 Report
This work seems very interesting. I think that an extensively improved version should be published and encourage the authors to do it. However, as it is this work cannot be published. I wonder whether the flaws leading me to this temporary rejection are only due to the authors not mastering the English language and having used the wrong words in critical issues. Therefore, I am not certain about my evaluation in the manuscript’s check list. Overall, the poor English and poor elaboration of some figures and tables make the presentation hard to follow. I got lost with what the authors are trying to prove/show. Extensive English editing is required. Better structuring of the Methods and Results is possible. But the fundamental flaws in this work are the authors mistakes about solubility: solubility is not matter (and thus not in units of grams), it does not change with time, it is not a rate, and it increases with salinity. The authors probably mean something else than “solubility”. However, as it is, it is wrong and also because of it I do not understand the Results and Discussion. The text does not help clarifying. I also suggest comparison with the solubility models by Weiss (1974) and Johnson (2010), both available in the FuGas (Vieira 2016,2018).
Specific comments:
Introduction:
An explanation of the reforming process is required. Its vague explanation in the abstract and introduction, together with the available on the net, was confusing. Is it Steam Reforming, Autothermal Reforming or Internal Combustion Reforming? What are the objective propulsion systems? To use conventional diesel engines, and “recycle” the exhaust producing CH4, thus improving the efficiency? Is the CO2 from the diesel combustion used for the Reforming system? The CO2 solubility in seawater is of interest for the Reforming system input? Or just for the propulsion system final exhaust to the water column? When/where is CO2 being input or output? What are the reactants, temperatures and pressures commonly used in operating conditions? Maybe this is all clear for the experts on this field, but surely not for anyone else. the manuscript would benefice significantly from clarifying these questions in a dedicated first paragraph of the introduction.
Section 2:
Experimental setup (2.1) is vague and poorly written. Line 115 is redundant. Delete and insert “(Fig.1)” in the next sentence. This next sentence is too long and dispersed. Temperature and pressure measurements should be cut from this sentence and inserted later, together with the explanation of the Thermocouples and pressure gauges. Figure 1 legend with just 2 words?
Experimental procedure (2.2) is vague and poorly written. Why was the operating pressure set to 2, 3, and 5 bar. Is this experimental design or subsidiary consequence of the reactor equilibrium pressure? Lines 129-131 are confusing. As it is written it suggests that this could be random. It seems to me that “was maintained” could (should) be replaced by “stabilized”. This way the text starts making sense. However, I am not sure this is what the authors intend to say. Line 130 “under the operating conditions” is unnecessary. Table 1 does not clarify anything.
Line 136: 6000 mL are 6L.
Table 2: These are not really concentrations but proportions. In atmospheric and oceanic sciences, proportions are usually given in ppt, which can be in g/Kg or mmol/mol. What is it, in this case?
Line 141 and 142: sentence poorly written.
Line 146: “in the experiments” is unnecessary. The sentence is poorly written. Maybe better as “the CO2 pressure was estimated from the pressure change of the system”.
Line 147: meaning the Ideal Gas Law?
Equation 1 should come immediately after line 149.
Define Vactual and Videal and how where they estimated.
Define the units of the variables and R used in equation 1.
Line 161 is unnecessary.
Section (3.1): why is this section called “numerical modelling”?
Line 174-175 “flow” is repeated.
In equation 3, u=0 and dC/dt reduces to DdeltaC, right? Then, (1) this must be explicit and (2) this presentation could be simplified.
For equation (4) to be correct, the Kg (line 183) must be (mol fuel gas)∙cm-2∙s-1∙atm-1. I highlight that it is mol of Fuel Gas!
Lines 182 to 189: units are mis-written. As an example, line 183 should be either
mol∙cm-2∙s-1∙atm-1
or
mol/(cm2∙s∙atm).
Use journal standard.
Equation (5): Define H units. This is hardly the best way to define Henry’s constant. Henry’s Law and constant do not require referencing. But if you insist, then, reference 28 is not suited. I suggest the original work by Henry (1803) or Sander (1999,2015) or Vieira (2016,2018).
Henry, W. (1803). "Experiments on the quantity of gases absorbed by water, at different temperatures, and under different pressures". Phil. Trans. R. Soc. Lond. 93: 29–274. doi:10.1098/rstl.1803.0004
Sander, R. Compilation of Henry’s law constants (Version 4.0) for water as solvent. Atmos. Chem. Phys. 2015, 15, 4399–4981.
Vieira, V.; Sahl ée, E.; Jurus, P.; Clementi, E.; Pettersson, H.; Mateus, M. Improving estimates of atmosphere-ocean greenhouse gas fluxes for Earth-System modelling. In ESA SP, Proceedings of the Living Planet Symposium, Prague, Czeck Republic, 9–13 May 2016; European Space Agency: Paris, France, 2016;
Volume 740.
Vieira, V., Jurus, P., Clementi, E., Mateus, M. The FuGas 2.3 Framework for Atmosphere–Ocean Coupling: Comparing Algorithms for the Estimation of Solubilities and Gas Fluxes. Atmosphere 2018, 9, 310; doi:10.3390/atmos9080310
How was Henry’s constant estimated for given pressure, temperature and salinity?
Line 230: Poor English. “… main governing factors that affect” is grammatically incorrect and redundant. By definition, “governing factors” obviously “affect”.
Line 230-233: this explanation is unnecessary.
Lines 239-242: several flaws. (1) poorly written. Very unclear. (2) Was it adding and subtracting 50% to the reference value? Explain better! It should be clearer defining the multiplicative factor for the sensitivity. I guess that in this case was x1.5 and x0.5. These seem too wide changes, particularly if non-linear effects may occur. Lines 240-242 are Results, not Methods.
Lines 244-247: Four lines for a single sentence without any punctuation?!
Section (4.1) and (4.2): Solubility is not in units of grams and does not change with time. It is not a rate. The authors probably mean something else than “solubility”. As it is, I do not understand these sections. The text does not help clarifying.
Line 314-315: This is false. Solubility is directly proportional to salinity. Solubility increases with increasing salinity as the saults break the hydrogen bounds between water molecules, decreasing the cohesion of their 3D structure.
I got lost in the results and discussion, since the authors base it in something that it is not true: (1) that solubility decreases with increasing salinity, (2) that solubility changes with time, and (3) that solubility corresponds to mass (solubility in grams?).
I suggest comparison with the solubility models by Weiss (1974) and Johnson (2010), both available in the FuGas (Vieira 2016,2018).
Author Response
Dear Reviewer,
Sincerest thanks for reviewers’ comments on our manuscript. The authors have revised the manuscript considering all your comments carefully and have revised the manuscript accordingly.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript examines the solubility of CO2 in seawater. It is interesting and worth publishing but the following comments must be addressed:
The current title is confusing. The guess the authors mean to say that they estimated the CO2 solubility in seawater. Therefore the title should be: "Estimation of low-pressure CO2 solubility in Seawater" The language of the manuscript needs to be improved. The authors should have compared their predictions with similar predictions in the literature. Many papers have addressed CO2 physical solubility in water and amines, e.g. Journal of Natural Gas Science and Engineering 34 (2016) 841-848. Could they use their method to predict the solubility in this study? The authors use the term "artificial water", which is not proper. They should say "synthetic seawater". Caption for Table 2 should be changed to "Chemical composition of synthetic seawater used in this study". They should refer to their work in the figures as "This worK" not "Matlab".
Author Response
Dear Reviewer
Sincerest thanks for reviewers’ comments on our manuscript. The authors have revised the manuscript considering all your comments carefully and have revised the manuscript accordingly.
Best Regards,
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Most of the changes were not done, with the authors untruthfully replying about having done them, when they have not! Falsely replying to reviewers is not acceptable. Adding to this problem, several changes were wrongly implemented, including mistaking absolute pressure for temperature (line 360). Examples are shown below, in the “Specific Comments”. Such mistakes suggest a lack of care unsuited for submissions to scientific publications. It is time for the authors to pause and re-consider their actions. I highlight that it is Christmas eve and I am uselessly spending my time reviewing a manuscript falsely alleged to have been changed, because the editorial office asked me to provide the review in 3 days.
Reading the new manuscript was difficult with all the tracked changes. The standard protocol is to provide a clean version and a version with tracked changes. Apart this problem, the new manuscript reads much better than its former, representing a significant improvement. The Introduction presents the topic in a clear fashion, and the same for the presentation of the Methods.
The English is still very poor and needs extensive editing. Besides grammatically incorrect sentences, the authors sometimes have difficulty in putting their ideas into paper. The authors should ask a native English speaker to help with the text.
Solubility increases with salinity. There is no way around it. Saying otherwise is scientifically incorrect. No article can be published saying otherwise!
The authors must be aware that the definition of solubility is the amount of gas dissolved in water per partial pressure of that same gas in the overlying air. Therefore, original formula and units are mol/(L*atm). Meanwhile, alternative formulations and units have been developed for practical proposes, as its scalar form (Cair/Cwater). Nevertheless, solubility, in its correct scientific definition, increases with salinity. The text must clearly state this.
Specific comments:
The tittle is very confusing. Simpler tittles are possible and should be preferred. As an example, the option below elucidates much better than the current tittle what the article is about:
“Performance of low-pressure sea-water as CO2 solvent in underwater air-independent propulsion systems”.
Line 56: This sentence is too long, redundant and confusing. Replace either by:
“…by underwater weapon systems to design and optimize the CO2 treatment process”,
or by:
“…by underwater weapon systems to design and optimize the CO2 dissolution in seawater.”
Line 70-73: Four lines with one single sentence without any punctuation makes a terrible reading.
Line 254: 9000mL = 9L. The authors untruthfully replied that this had been changed.
Line 293: 6000mL = 6L. The authors untruthfully replied that this had been changed.
Table 2: Concentration still comes as %. The authors untruthfully replied that this had been changed to g/L.
Line 360: T is NOT absolute pressure.
Line 375, Equation 3: The authors untruthfully replied that this had been changed.
Line 376-377: “… u [m/s] is the flow velocity flow field of the fluid”. This is very confusing! The authors untruthfully replied that this had been changed.
Lines 362, 367 and others: “Modeling” is American spelling. “Modelling” is British spelling.
Lines 384-391: The units are still incorrect. They have not been changed as the authors said they would.
Many more examples could be shown here. However:
(i) I will not waste more of my time reviewing a manuscript where most of the alleged changes were in fact not implemented, with the response being untrue.
(ii) given the very narrow time-frame requested for the review, I will not go thoroughly over them. The bottom line is that the text needs extensive language editing.
Author Response
Many thank you for your review until Christmas Eve. There seems to be some confusion, but the revised manuscript faithfully was modified following the reviewer's opinion. Please see the responses from the reviewer's comment in attachment.
Author Response File: 
Author Response.docx
