Operation of a Pilot-Scale CO₂ Capture Process with a New Energy-Efficient Polyamine Solvent

Round 1

Reviewer 1 Report

The present study used a pilot-scale carbon capture process (PCCP) to evaluate the carbon-capturing capability of a new solvent blend formulation containing polyamine and other auxiliary amines. Analysis of the physical properties of several candidate solvent blend formulae were first performed and the blend of 3,3′-iminobis (N, N-dimethylpropylamine), 2-amino-2-methyl-1- propanol and Piperazine (referred as IAP solvent therein) was chosen as the solvent for tests on its carbon capture capacity (working capacity) due to reasonable viscosity and excellent ability to dissolve carbon dioxide gas. A 30 mass% monoethanolamine (MEA) was used as a baseline solvent. It appears that, according to this study, at 1.7 solvent to flue gas ratio, IAP was more than 2.5 times more effective in absorbing carbon dioxide than MEA solvent. Also, IAP absorbs carbon dioxide at a faster rate with a more stable operation and moderately less energy consumption. The manuscript offers a better engineering solution to carbon dioxide capture and storage (CCS) technology, which addresses the environmental and climate issue of raising carbon dioxide in the atmosphere due to the fossil fuel combustion. As a result, this manuscript appears to be suitable for the scope of the Applied Sciences.

 

The research group in this manuscript demonstrated strong track records of conducting flue gas carbon dioxide measurements, which effectively convinced me the validity of their measurement results. However, I have a couple of major concerns regarding the rationale and design of the pilot experiments in this study: (1) As the authors mentioned in the Introduction, a common amine-based CCS solvent is aMDEA. Why not using aMDEA as the baseline solvent in this study? If shown to be better than aMDEA, the new IAP solvent blend will have a better potential to be used in industrial-scale applications. (2) The solvent blend tested using the PCCP facility was selected based on three physical properties of several candidate blends, which was valid based on various engineering and application considerations. However, within the viscosity limit, why not test several different IAP solvents with various amine mass ratios to achieve the best working capacity? In other words, how can the authors prove that the presented 30:6:6 IAP blend was the best solvent formulation?

 

Minor concerns: In general, the manuscript omitted some details that can be confusing to some readers. For example, I assume the solvent formulae discussed in this manuscript were made to 100 mass% by adding water. Yet this is not mentioned briefly as a general comment at the beginning of the manuscript.

A list of smaller editorial comments is as following and a thorough proofread of the manuscript is warranted:

Line 20: AMP not defined when it first appears in the manuscript.

Line 40: “aMDEA” is registered brand name

Line 47-48: meaning of the sentence?

Line 78: space after the first period mark

Line 109/116: degree symbol. Similar problem in the figure captions.

Line 117: operate with IAP

Line 133: Stainless steel (SUS316)

Line 235: extra “increasing”

Author Response

At the outset, we wish to thank the distinguished reviewers for going through our manuscript and giving us useful comments.

 

We acknowledge that the solvents in this study do not perform well as innovative solvents in the field of CO₂ capture research. However, the IAP in this study is unique as a polyamine-based solvent. In fact, research based on polyamines is not active, and no pilot demonstration has been promoted. In this sense, this study is unique and interesting. We look forward to providing good information to interested authors on polyamines in this field.

We hope for a positive response from renowned reviewers.

 

The revised manuscript and “Response to Reviewer Comments(Line 332)” are attached as a single word file.

The remarks of the reviewers are retyped in normal font and our responses and the actions that we took to address the comments are in red color. And manuscript additions and corrections are marked in blue and highlights.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript ‘operation of a pilot-scale CO₂ capture process with a new energy-efficient polyamine solvent’ is well-written and well-organized. The manuscript looked at three different mixtures of amines for CO₂ sorption from a flue gas, and compared the performance of one of them with aqueous MEA in the pilot plant. The manuscript is not groundbreaking but does offer some interesting experimental results. It may be published but I do have suggestions to improve the manuscript.

1. The featured blend should have been compared to other state-of-the-art solvents in addition to aqueous MEA. This will help assess whether the new blend offer advantages over other alternatives to MEA. Merely pointing out the blend has lower reboiler heat duty requirement than MEA is not impressive without comparing it to more state-of-the-art solvents such as other piperazine blends. Cf. 10.1039/c3ee42350f. I concede that running MEA in the pilot plant does offer valuable benchmark, however, in calibrating the expected vs actual heat duty.

2. Quality of the graphs can be improved. Figure 1 should be redrawn with x-axis on the bottom and with larger symbols denoting the datapoints. Figure 2a was supposed to show the CO₂ absorption rate in terms of the overall mass transfer coefficient; instead, it was the absorption rate vs. CO₂ loading, while 2b was also supposed to be viscosity vs. absorption rate. Please check the figure caption and the paragraph starting at line 108.

3. IBP was supposed to show the slowest CO₂ absorption among the three candidate solvents – isn’t the CO₂ absorption rate supposed to reflect the reaction kinetics? Figure 2a showed that the three solvent blends were similar in absorption characteristics.

4. Is there a reason why 30 cP is selected as the cutoff viscosity for this study? All three blends exhibit viscosity significantly higher than 30 wt% MEA, which can be a major cost driver in this process. 

5. The authors chose to use a simulated flue gas in lieu of an actual flue gas to control the CO₂ concentration. Did the authors introduce any SO₂ into the test to the expected level (2 ppm)? Is the O₂ concentration of 16.5 vol% is correct, or should it be 6.5% instead? How long were these tests performed? To assess the claim that the operation was highly stable, it would be convincing to provide operation data over the evaluation time instead of just providing steady-state data, especially if SO₂ is being introduced.

6. Please pay attention to the consistency on the use of subscripts and degree sign (°C) throughout the manuscript.

Author Response

At the outset, we wish to thank the distinguished reviewers for going through our manuscript and giving us useful comments.

 

We acknowledge that the solvents in this study do not perform well as innovative solvents in the field of CO₂ capture research. However, the IAP in this study is unique as a polyamine-based solvent. In fact, research-based on polyamines is not active, and no pilot demonstration has been promoted. In this sense, this study is unique and interesting. We look forward to providing good information to interested authors on polyamines in this field.

We hope for a positive response from renowned reviewers.

 

The revised manuscript and “Response to Reviewer Comments(Line 332)” are attached as a single word file.

The remarks of the reviewers are retyped below in normal font and our responses and the actions that we took to address the comments are in red color. And manuscript additions and corrections are marked in blue and highlights.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I appreciate the authors' thoughtful responses to my comments. I have further follow-up minor comments, but these are completely optional to address - the paper is fine as submitted. I am numbering them according to my original comment numbers (i.e. comment number 5 refers to my original point 5).

1. The authors pointed out that there are few operational results of polyamines in a pilot process, and few new recipes were published. I think this should be pointed out in the introduction, as it strengthens the authors' case for publishing this paper.

4. Thank you for addressing the viscosity concern - adding that reference helps. I was more thinking of the viscosity issue in light of technoeconomic analysis, which shows the solvent viscosity really making a difference in operational cost. But since the authors are not focused on technoeconomic analysis, I think adding that reference is enough.

5. The pilot data is a very helpful addition to the paper. I would still suggest the authors clarify in the operational section that despite the ability to add SO2 into the flue stream, the authors chose not to do it. This is because the authors described some unit ops (such as pre-scrubber) that is less relevant for this study. A simple row in Table 6 (SO2 in the flue gas = 0 ppm) will be enough.  For the O2, in my experience, the O2 content corresponding to 14 vol% CO2 flue is much lower than 16.5%, as 16.5% O2 implies a huge excess of air (and thus expected the CO2 level to be much lower than 14%). But again, I will not ask the authors to redo the experiment over this.