Experimental and Simulation-Based Study of Acid Gas Removal in Packed Columns with Different Packing Materials

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Journal: Sustainability

Manuscript Number: sustainability-3930033

Tittle: Experimental and Simulation-Based Study of Acid Gas Removal in Packed Columns with Different Packing Materials

Authors: Ersin Üresin

General Comments:

The paper title Experimental and Simulation-Based Study of Acid Gas Removal in Packed Columns with Different Packing Materials a well-structured and comprehensive study combining both experimental and simulation approaches to investigate the removal efficiency of gaseous pollutants (H₂S and CO₂) using different packing materials. The experimental validation complemented by ASPEN Plus simulation adds strong credibility and practical relevance to the findings. The results are clearly presented, demonstrating meaningful trends regarding the effects of gas flow rate, column diameter, and packing type on removal performance. However, while the study provides valuable insights into process design and optimization, the discussion could improve, including the following comments:

1. Highlight the benefit from deeper interpretation of the underlying mass transfer mechanisms, especially the distinct behaviors of H₂S and CO₂ under similar operating conditions. Including a more detailed comparison with previous literature would also strengthen the novelty and contextual significance of the work.
2. A clearer explanation of the simulation assumptions, boundary conditions, and model validation metrics would enhance the reproducibility and scientific rigor (see methodology section).
3. Although the work claims novelty in combining simulations and experiments, recent studies have also explored hybrid approaches. A more critical literature positioning is needed to clearly delineate what aspect of integration or insight truly distinguishes this study. (See Page 3 Lines 125-137)
4. The manuscript should clarify how the operational parameters listed in Tables 1 and 2 were determined. Were these values obtained from experimental measurements, literature references, or simulation-based optimization? Providing this information is essential to ensure the reproducibility and validity of the study.
5. While the modeling and validation approach is clearly described, the section would benefit from a more rigorous justification of the scaling methodology and model assumptions used when extending simulations to larger column diameters. It remains unclear whether hydrodynamic and mass-transfer parameters were appropriately adjusted to reflect scale-dependent effects such as liquid maldistribution, pressure drop, or non-ideal flow patterns. Without explicit discussion or validation at larger scales, the reliability of the scale-up predictions and the resulting performance comparisons between packing types may be overstated or insufficiently supported. See Sub Section 2.2
6. Sub-section 3.1 remains largely descriptive and lacks sufficient quantitative analysis to substantiate the stated performance relationships. The discussion would benefit from including experimental or modeled data, such as measured mass transfer coefficients, pressure drops, or effective interfacial areas to validate the claims about flow maldistribution and hydraulic superiority. Additionally, the explanation assumes that the observed effects are solely due to surface area and void fraction, without considering other influential factors such as wettability, packing geometry, or liquid distribution quality. A more rigorous correlation or mechanistic linkage between structural parameters and the measured mass transfer performance would strengthen the scientific validity and general applicability of this section.
7. The caption in Fig. 2 states that experimental data are available only for the 10 cm diameter column. However, the plotted figure shows multiple experimental data markers (“×”) across all gas flow rates, which visually overlap with all simulated lines. This may incorrectly imply that experimental measurements exist for other diameters (20 cm and 40 cm). The legend and figure labeling must be revised to clearly distinguish which data points correspond to experimental versus simulated results. Additionally, the use of identical black “×” markers at all flow rates without differentiation causes confusion about which data points belong to the 10 cm experimental case. Distinguishing experimental data by color, shape, or by plotting them separately (e.g., as inset or secondary figure) would improve clarity. Please also check Figs. 4 and 6
8. The explanation of Fig. 9 remains largely qualitative and lacks quantitative validation. The authors should substantiate these claims by providing supporting correlations, dimensionless analysis, or experimental uncertainty ranges to confirm that the observed trends are not merely empirical but grounded in transport phenomena theory.
9. The discussion section would benefit from a more thorough comparison with previously published studies. At present, the absence of reference-based benchmarking makes it difficult to assess how the reported trends align with or diverge from established findings in the literature. Incorporating such comparative analysis would not only enhance the scientific rigor of the discussion but also help validate the reliability and broader relevance of the results within the existing body of knowledge.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments
1. Summary
I would like to sincerely thank you for the valuable and constructive comments provided. Your detailed feedback has significantly contributed to improving the clarity and overall quality of my manuscript entitled “Experimental and Simulation-Based Study of Acid Gas Removal in Packed Columns with Different Packing Materials”.   Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted files.   I am deeply grateful for your kind help and continuous support.   Best regards, Dr. Ersin ÜRESİN Vice President TÜBİTAK Marmara Research Center Material and Process Technologies Vice Presidency Barış Mah. Dr. Zeki Acar Cad. No:1 P.K. 21 41470 Gebze Kocaeli, TURKIYE T +90 262 677 2130  F +90 262 641 2309 GSM +90 542 763 60 73 www.mam.gov.tr ersin.uresin@tubitak.gov.tr
2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: Highlight the benefit from deeper interpretation of the underlying mass transfer mechanisms, especially the distinct behaviors of H₂S and CO₂ under similar operating conditions. Including a more detailed comparison with previous literature would also strengthen the novelty and contextual significance of the work.
Response 1: I sincerely thank the reviewer for this valuable and constructive suggestion, which has significantly improved the depth and clarity of my manuscript. To fully address the comment, i have introduced a new subsection 3.5 entitled "Deeper Interpretation of Mass Transfer Mechanisms: Distinct Behaviors of H₂S and CO₂ under Similar Operating Conditions" (Page 24, Lines 897–971. Additionally, to further strengthen the contextual significance and novelty new sentences have been added to the Abstract (Lines 22-25) and to the Introduction (Lines 132-145).
Comments 2: A clearer explanation of the simulation assumptions, boundary conditions, and model validation metrics would enhance the reproducibility and scientific rigor (see methodology section).
Response 2: I thank the reviewer for the valuable suggestion. To enhance reproducibility and scientific rigor, a new subsection (2.2.2) has been added (Page 8, Lines 319–376) to the Materials and Methods, explicitly detailing: simulation assumptions, inlet/outlet boundary conditions and quantitative model validation metrics.   Comments 3: Although the work claims novelty in combining simulations and experiments, recent studies have also explored hybrid approaches. A more critical literature positioning is needed to clearly delineate what aspect of integration or insight truly distinguishes this study. (See Page 3 Lines 125-137). Response 3: I thank the reviewer for this insightful comment. To address the reviewer’s concern about literature positioning, i have revised Page 3, Lines 125–137 (now Page 3, Lines 132–145 in the revised manuscript). While hybrid simulation-experimental studies exist, our work introduces a distinct integration by providing a validated, side-by-side comparison of random and structured packings across variable gas flow rates and column diameters using real syngas. Importantly, we identify a novel packing-dependent diameter scaling: increasing column diameter decreases H₂S removal for random packings but increases it (up to a limit) for structured packings, driven by differences in void fraction, interfacial area distribution, and radial flow maldistribution. Our Aspen Plus model closely matches experimental results (<5% deviation), establishing a robust framework for scale-up. This combination of systematic comparison and new scaling insight distinguishes our study from prior hybrid approaches. Additionally, i have updated the Abstract (Lines 25–29) to reflect this sharper novelty.   Comments 4: The manuscript should clarify how the operational parameters listed in Tables 1 and 2 were determined. Were these values obtained from experimental measurements, literature references, or simulation-based optimization? Providing this information is essential to ensure the reproducibility and validity of the study. Response 4: I thank the reviewer for this important comment. The manuscript has been revised to clarify the origin of the parameters in Tables 1 and 2 (Page 5, Lines 181-189 and Page 6, Lines 220-227). Operational parameters in Table 1 (gas and liquid flow rates, L/G ratio, inlet concentrations, column dimensions, and pressure) were directly obtained from laboratory-scale experimental measurements and fully controlled to ensure reproducibility. Packing properties in Table 2 (surface area, void fraction, packing factor) were sourced from manufacturer datasheets and validated literature, and directly applied in Aspen Plus simulations for 0.2 m and 0.4 m columns. No optimization was performed; values represent standard industrial characteristics to enable reliable scale-up comparisons.     Comments 5: While the modeling and validation approach is clearly described, the section would benefit from a more rigorous justification of the scaling methodology and model assumptions used when extending simulations to larger column diameters. It remains unclear whether hydrodynamic and mass-transfer parameters were appropriately adjusted to reflect scale-dependent effects such as liquid maldistribution, pressure drop, or non-ideal flow patterns. Without explicit discussion or validation at larger scales, the reliability of the scale-up predictions and the resulting performance comparisons between packing types may be overstated or insufficiently supported. See Sub Section 2.2.

Response 5: I thank the reviewer for the constructive suggestion. A new paragraph entitled “Scale-up Methodology and Model Assumptions” has been added to Subsection 2.2 (Page 8, Lines 295-317). It explicitly justifies the scaling methodology, details the adjustment of hydrodynamic (maldistribution, pressure drop, axial dispersion) and mass-transfer parameters using established correlations, and cites pilot/industrial-scale validation studies to support the reliability of scale-up predictions. These additions address the concerns regarding scale-dependent effects and strengthen the credibility of performance comparisons between packing types.

 

Comments 6: Sub-section 3.1 remains largely descriptive and lacks sufficient quantitative analysis to substantiate the stated performance relationships. The discussion would benefit from including experimental or modeled data, such as measured mass transfer coefficients, pressure drops, or effective interfacial areas to validate the claims about flow maldistribution and hydraulic superiority. Additionally, the explanation assumes that the observed effects are solely due to surface area and void fraction, without considering other influential factors such as wettability, packing geometry, or liquid distribution quality. A more rigorous correlation or mechanistic linkage between structural parameters and the measured mass transfer performance would strengthen the scientific validity and general applicability of this section.

Response 6: I sincerely thank the reviewer for the constructive feedback and helpful remarks. Sub-section 3.1 has been revised (Page 11, Lines 439-508) to include quantitative analysis of gas-phase mass transfer coefficients, incorporating measured pressure drops and gas velocities to support the observed performance differences between structured and random packings. Mechanistic correlations have been added to link packing geometry, flow regime, and mass transfer performance. Additional factors such as wettability and liquid distribution quality are now considered, with effective interfacial area calculations explaining differences in hydraulic behavior. These revisions strengthen the correlation between structural parameters and measured performance, enhancing both the scientific rigor and general applicability of the section.

 

Comments 7: The caption in Fig. 2 states that experimental data are available only for the 10 cm diameter column. However, the plotted figure shows multiple experimental data markers (“×”) across all gas flow rates, which visually overlap with all simulated lines. This may incorrectly imply that experimental measurements exist for other diameters (20 cm and 40 cm). The legend and figure labeling must be revised to clearly distinguish which data points correspond to experimental versus simulated results. Additionally, the use of identical black “×” markers at all flow rates without differentiation causes confusion about which data points belong to the 10 cm experimental case. Distinguishing experimental data by color, shape, or by plotting them separately (e.g., as inset or secondary figure) would improve clarity. Please also check Figs. 4 and 6.

Response 7: I sincerely thank the reviewer for pointing out this inconsistency and for the constructive comment. Figures 2, 4, and 6 have been revised. Experimental and simulation data are now clearly distinguished using different colors and symbols. A note has been added to clarify that experimental data are only available for the 10 cm diameter column. The figures have been made more readable, and separate panels have been prepared to avoid confusion.

 

Comments 8: The explanation of Fig. 9 remains largely qualitative and lacks quantitative validation. The authors should substantiate these claims by providing supporting correlations, dimensionless analysis, or experimental uncertainty ranges to confirm that the observed trends are not merely empirical but grounded in transport phenomena theory.

Response 8: I thank the reviewer for highlighting this important point. A new subsection (3.3.1) and Table 7 were added (Page 22, Lines 835-866) to include a quantitative validation through dimensionless analysis (Re, Sc, Sh). The results show <2% deviation between experimental and theoretical correlations, confirming that the trends in Figure 9 (now Figure 12 in the revised manuscript) are physically consistent and not purely empirical.

 

Comments 9: The discussion section would benefit from a more thorough comparison with previously published studies. At present, the absence of reference-based benchmarking makes it difficult to assess how the reported trends align with or diverge from established findings in the literature. Incorporating such comparative analysis would not only enhance the scientific rigor of the discussion but also help validate the reliability and broader relevance of the results within the existing body of knowledge.

Response 9: I sincerely thank the reviewer for this insightful and constructive comment. Sections 3.1–3.3 have been revised (Section 3.1. (Page 11, Lines 467-473), Section 3.2. (Page 17, Lines 661-667), Section 3.3. (Page 22, Lines 827-833)) to provide a more thorough discussion situating our findings within the broader context of gas absorption studies. The effects of packing type on mass transfer efficiency and flow distribution are now mechanistically linked to observed efficiency trends. The decrease in removal efficiency with increasing column diameter and gas flow rate is discussed in terms of reduced superficial velocity and radial maldistribution. Finally, the improved predictive performance of the BRF-93 model compared to BRF-85 is highlighted to validate my approach for both structured and random packings. These revisions strengthen the discussion, clarify the mechanistic understanding, and demonstrate the broader relevance and reliability of my results.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. The novelty of comparing structured and random packings is clearly stated, but the study would benefit from a deeper mechanistic discussion linking void fraction, flow regime, and gas–liquid interfacial area to the observed efficiency trends.
2. The paper mentions simultaneous CO₂ and H₂S absorption but does not quantify the competition kinetics or selectivity; adding a kinetic model or selectivity ratio would strengthen the conclusions.
3. The discussion on column diameter effects could be enhanced by introducing dimensionless correlations (e.g., Reynolds or Sherwood numbers) to generalize the findings beyond the tested scale.
4. While Aspen Plus was used effectively, the paper lacks sensitivity analysis for key parameters such as temperature or NaOH concentration, which could provide design-relevant insights.
5. The validation results are promising, yet statistical metrics (e.g., R², RMSE) for simulation–experiment agreement are not presented and should be included for quantitative rigor.
6. The CO₂ removal section would benefit from more emphasis on absorption chemistry and mass transfer limitations to distinguish chemical versus physical contributions.
7. The study briefly cites BRF-85 and BRF-93 models but does not explain their limitations or suitability for non-ideal flow conditions in larger columns.
8. Conclusions could be expanded to highlight potential implications for industrial design or environmental performance (e.g., energy consumption, solvent regeneration, or scalability).
   
   Technical comments:
   
   
   
9. Figure captions should be more descriptive—especially Figures 2–9—to ensure standalone understanding without referring to the main text.
10. Experimental uncertainties, measurement errors, or repeatability data are missing; their inclusion would improve reproducibility.
11. Tables 1–5 contain useful data, but the formatting (units alignment and spacing) should be standardized for clarity.
12. The Aspen Plus simulation section should specify which thermodynamic property method (e.g., ELECNRTL, NRTL, etc.) was applied.
13. Some typographical inconsistencies (e.g., “m3/h” vs. “m³/h”) and subscript formatting (H₂S, CO₂) should be corrected throughout.
14. Figures could benefit from higher resolution and consistent axis labeling; several plots lack clear legends or font uniformity.

Author Response

Response to Reviewer 2 Comments
1. Summary
I am deeply grateful to you for the insightful remarks and thoughtful suggestions for my manuscript entitled “Experimental and Simulation-Based Study of Acid Gas Removal in Packed Columns with Different Packing Materials”. Your comments have helped me to refine our analyses and strengthen the discussion of my findings.   Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted files.   I am deeply grateful for your kind help and continuous support.   Best regards, Dr. Ersin ÜRESİN Vice President TÜBİTAK Marmara Research Center Material and Process Technologies Vice Presidency Barış Mah. Dr. Zeki Acar Cad. No:1 P.K. 21 41470 Gebze Kocaeli, TURKIYE T +90 262 677 2130  F +90 262 641 2309 GSM +90 542 763 60 73 www.mam.gov.tr ersin.uresin@tubitak.gov.tr
2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: The novelty of comparing structured and random packings is clearly stated, but the study would benefit from a deeper mechanistic discussion linking void fraction, flow regime, and gas–liquid interfacial area to the observed efficiency trends.
Response 1: I thank the reviewer for this insightful suggestion. Section 3.1.1 (Page 11, Lines 475-508) has been added to mechanistically link packing characteristics to efficiency trends. Specifically, high void fraction in structured packings promotes uniform flow and sustained interfacial area, while low void fraction in random packings leads to maldistribution and early loading. Flow regime transitions and gas velocity effects further explain the observed differences. These interpretations are fully consistent with our experimental, Aspen Plus, and BRF-93 results.
Comments 2: The paper mentions simultaneous CO₂ and H₂S absorption but does not quantify the competition kinetics or selectivity; adding a kinetic model or selectivity ratio would strengthen the conclusions.
Response 2: I thank the reviewer for this valuable suggestion. To quantify the competition and selectivity between CO₂ and H₂S, i added subsection 2.3 (Page 9, Lines 378-396) with a kinetic model and 3.4 (Page 23, Lines 868-895) with results, reporting a selectivity ratio of 51.3. This explains the high H₂S removal (79–98%) despite excess CO₂, strengthening the conclusions.   Comments 3: The discussion on column diameter effects could be enhanced by introducing dimensionless correlations (e.g., Reynolds or Sherwood numbers) to generalize the findings beyond the tested scale. Response 3: I am very grateful to the reviewer for the thoughtful suggestions. A new subsection has been added to Section 3.2 immediately following the explanation of superficial velocity reduction (Page 14, Lines 556-587). It introduces dimensionless analysis using Reg, Shg, and Scg, supported by established correlations. The analysis quantifies the ~16 fold Reg reduction across diameters, predicts ~45% Shg drop, and directly links it to observed efficiency trends (99% to 92%). This generalizes the findings to any scale and packing geometry, addressing scale-up predictability.   Comments 4: While Aspen Plus was used effectively, the paper lacks sensitivity analysis for key parameters such as temperature or NaOH concentration, which could provide design-relevant insights. Response 4: I sincerely appreciate the reviewer’s insightful comments. Accordingly, a new subsection titled “3.6. Sensitivity Analysis of Key Operating Parameters” (Page 25, Lines 973-1009) has been added. This section presents a sensitivity analysis of liquid inlet temperature (288–318 K) and NaOH concentration (0.1–2.0 wt%) using the validated Aspen Plus model. The results (Figures 10–11) show that increasing temperature slightly reduces H₂S removal due to the exothermic nature of the reaction, while NaOH concentrations above 1.0 wt% offer minimal improvement. These findings provide useful guidance for optimizing industrial gas treatment systems.   Comments 5: The validation results are promising, yet statistical metrics (e.g., R², RMSE) for simulation–experiment agreement are not presented and should be included for quantitative rigor. Response 5: Thank you very much for your insightful comment. A new subsection “3.2.1. Quantitative Validation of Simulation–Experiment Agreement” has been added (Page 18, Lines 675-695). It reports R² = 0.974 and RMSE = 0.54% based on the average of four experimental replicates compared to simulation results for the 10 cm column. These values demonstrate excellent model accuracy and satisfy established validation standards in the field.   Comments 6: The CO₂ removal section would benefit from more emphasis on absorption chemistry and mass transfer limitations to distinguish chemical versus physical contributions. Response 6: I thank the reviewer for this valuable suggestion. To better emphasize the absorption chemistry and mass transfer limitations, a new subsection titled “3.2.2. Absorption Chemistry and Mass Transfer of CO₂” (Page 18, Lines 697-718.) has been added. This section clarifies the two step CO₂ absorption mechanism physical dissolution followed by chemical reaction with OH⁻ and discusses the transition from mass transfer to kinetic diffusion controlled regimes at lower NaOH concentrations. The contrast with H₂S absorption and relevant literature references have also been included to highlight the chemical and physical contributions to CO₂ removal.   Comments 7: The study briefly cites BRF-85 and BRF-93 models but does not explain their limitations or suitability for non-ideal flow conditions in larger columns Response 7: I thank the reviewer for the insightful and valuable comment. I fully agree that a more detailed discussion of the BRF-85 and BRF-93 models’ applicability and limitations particularly under non-ideal flow conditions in larger diameter columns is essential for a comprehensive evaluation of the simulation results. To address this concern, i have revised Section 3.3 (Impact on Mass Transfer Coefficients) by inserting a new, detailed paragraph on Page 20, Lines 747-764.   Comments 8: Conclusions could be expanded to highlight potential implications for industrial design or environmental performance (e.g., energy consumption, solvent regeneration, or scalability). Response 8: I sincerely thank the reviewer for the constructive feedback and helpful remarks. The Conclusions section has been significantly expanded (Page 27, Lines 1010–1063) to explicitly address industrial design implications, including scalability challenges in large-diameter columns, energy savings through reduced pressure drop with structured packing (up to 50%, equivalent to 50–100 MWh/year in a 500 m³/h biogas plant), solvent regeneration costs due to CO₂ co-absorption (60–70% NaOH consumption in high-CO₂ feeds), and environmental performance (waste stream management and integration with sulfur recovery processes).   Comments 9: Figure captions should be more descriptive—especially Figures 2–9—to ensure standalone understanding without referring to the main text. Response 9: I am very grateful to the reviewer for the constructive feedback and thoughtful suggestions. All figure captions, particularly for Figures 2–9, have been extensively revised to ensure complete standalone interpretability without reference to the main text. Each caption now includes; clear description of the packing type, column diameter, and gas flow rate range, distinction between experimental and simulation data, summary of major trends and quantitative outcomes, units and axis information.These revisions eliminate ambiguity and enhance the accessibility of the figures for readers. The updated captions are provided in the revised manuscript.   Comments 10: Experimental uncertainties, measurement errors, or repeatability data are missing; their inclusion would improve reproducibility.

Response 10: I appreciate the reviewer’s valuable suggestion. In the revised manuscript, a new subsection titled “2.4. Experimental Uncertainty and Repeatability” has been added to the Materials and Methods section (Page 10, Lines 398-410). This subsection now describes the accuracy of the measurement instruments, the number of replicate experiments, and the procedure used to estimate the combined uncertainties. The overall uncertainty of the removal efficiency was determined through the propagation of measurement errors and deviations among replicates. Corresponding error bars have also been added to the relevant figures.

 

Comments 11: Tables 1–5 contain useful data, but the formatting (units alignment and spacing) should be standardized for clarity.

Response 11: I thank the reviewer very much for the helpful comment. All tables have been thoroughly reviewed, reformatted, and standardized in terms of unit presentation. The internal layout and borders of the tables have been adjusted in accordance with the journal’s formatting guidelines. For data-rich (multi-column) tables, the extended table format recommended by the journal has been adopted.

 

Comments 12: The Aspen Plus simulation section should specify which thermodynamic property method (e.g., ELECNRTL, NRTL, etc.) was applied.

Response 12: I sincerely appreciate the reviewer’s insightful comments and valuable suggestions. The thermodynamic property method has now been explicitly specified in Section 2.2 (Simulation Approach and Model Validation) on Page 5, Lines 198-210. The ELECNRTL property method was used to model the electrolyte system, as it is the most appropriate for aqueous solutions containing NaOH, CO₂, and H₂S with ionic speciation. A detailed justification has been added following the introduction of the Aspen Plus model.

 

Comments 13: Some typographical inconsistencies (e.g., “m3/h” vs. “m³/h”) and subscript formatting (H₂S, CO₂) should be corrected throughout.

Response 13: I am deeply thankful to the reviewer for the careful evaluation and valuable insights. All typographical inconsistencies have been corrected, and chemical formulas and units are now formatted consistently throughout the manuscript.

 

Comments 14: Figures could benefit from higher resolution and consistent axis labeling; several plots lack clear legends or font uniformity.

Response 14: I sincerely thank the reviewer for the helpful comment regarding the figures. All figures have been carefully revised, and their resolution has been increased (4638×2572 pixels) to enhance clarity. The figures have been made more readable, and separate panels have been prepared to avoid confusion.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

1. The manuscript contains a critical inconsistency regarding the column diameters used. Table 1 lists "Column diameter (m)" with values of "10-40". The text and figures (e.g., Figure 2, 3) consistently refer to columns with diameters of 10 cm, 20 cm, and 40 cm. It is physically implausible that the experimental setup had a column diameter of 10 meters. This appears to be a unit error (cm vs. m). The authors must clarify and correct this fundamental unit error throughout the manuscript. Furthermore, the scale-up logic from a 10 cm lab column to a 40 cm column via simulation alone needs justification. What specific models or correlations within Aspen Plus were used to account for the significant change in scale, particularly regarding flow distribution and wall effects, which are known to differ greatly between lab-scale and pilot-scale columns?
2. The methodology is ambiguous regarding what data is experimental and what is purely from simulation. The abstract and results suggest experiments were conducted with structured packing, while all data for random packing and for larger diameters (20 cm, 40 cm) are from simulations. Please explicitly state in the "Materials and Methods" section which scenarios were tested experimentally and which were solely simulation-based. For the simulated cases, particularly the random packing, how was the model validated for a packing type that was not experimentally tested? Providing a comparative table of key simulated outputs (e.g., pressure drop) against literature data for random packing would strengthen the validity of these extensive simulation-based conclusions.
3. There is a direct contradiction in the description of the packing materials' surface areas. In Section 3.1, it is stated: "the random packing provided approximately 1.5 times higher surface area than the structured packing." However, the data in Table 2 shows the random packing surface area is 363 m²/m³ and the structured is 245 m²/m³, meaning the random packing has a surface area 1.48 times higher, confirming the statement. Yet, later in Section 3.3, the authors state, "the greater void fraction in random packing... may reduce the available surface area per unit volume."  This latter statement is incorrect and contradicts the provided data. Please correct this and ensure all discussions about the physical properties of the packings are consistent with the data in Table 2. The discussion should focus on why random packing, despite its higher surface area, often underperforms compared to structured packing, likely due to flow maldistribution and inefficient use of that surface area.

Author Response

Response to Reviewer 3 Comments
1. Summary
I would like to express my sincere appreciation to you for the careful review and helpful recommendations for my manuscript entitled “Experimental and Simulation-Based Study of Acid Gas Removal in Packed Columns with Different Packing Materials”. Your observations were instrumental in enhancing both the methodological and conceptual aspects of my work.   Please find the detailed responses below and the corresponding revisions/corrections highlighted in the re-submitted files.   I am deeply grateful for your kind help and continuous support.   Best regards, Dr. Ersin ÜRESİN Vice President TÜBİTAK Marmara Research Center Material and Process Technologies Vice Presidency Barış Mah. Dr. Zeki Acar Cad. No:1 P.K. 21 41470 Gebze Kocaeli, TURKIYE T +90 262 677 2130  F +90 262 641 2309 GSM +90 542 763 60 73 www.mam.gov.tr ersin.uresin@tubitak.gov.tr
2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: The manuscript contains a critical inconsistency regarding the column diameters used. Table 1 lists "Column diameter (m)" with values of "10-40". The text and figures (e.g., Figure 2, 3) consistently refer to columns with diameters of 10 cm, 20 cm, and 40 cm. It is physically implausible that the experimental setup had a column diameter of 10 meters. This appears to be a unit error (cm vs. m). The authors must clarify and correct this fundamental unit error throughout the manuscript. Furthermore, the scale-up logic from a 10 cm lab column to a 40 cm column via simulation alone needs justification. What specific models or correlations within Aspen Plus were used to account for the significant change in scale, particularly regarding flow distribution and wall effects, which are known to differ greatly between lab-scale and pilot-scale columns?
Response 1: I sincerely thank the reviewer for this astute observation. The column diameter in Table 1 was incorrectly listed as "10–40 m" due to a typographical error and has been corrected to "10–40 cm". All experimental and simulation results consistently use 10 cm, 20 cm, and 40 cm, and this correction resolves the unit inconsistency.To justify the scale-up from a 10 cm laboratory column to a 40 cm pilot-scale column using Aspen Plus simulations, we employed the RadFrac absorber model with rate-based (non-equilibrium) approach,These improvements have been incorporated into Section 2.2 (Simulation Approach) to clarify the scale-up methodology, under a new subheading 2.2.1. Scale-up Methodology and Model Assumptions (Page 8, Lines 295–317), and into Section 3.2 (Effect of Column Diameter) (Page 14, Lines 540–555).
Comments 2: The methodology is ambiguous regarding what data is experimental and what is purely from simulation. The abstract and results suggest experiments were conducted with structured packing, while all data for random packing and for larger diameters (20 cm, 40 cm) are from simulations. Please explicitly state in the "Materials and Methods" section which scenarios were tested experimentally and which were solely simulation-based. For the simulated cases, particularly the random packing, how was the model validated for a packing type that was not experimentally tested? Providing a comparative table of key simulated outputs (e.g., pressure drop) against literature data for random packing would strengthen the validity of these extensive simulation-based conclusions.
Response 2: I sincerely appreciate the reviewer’s insightful comments. I have clarified in Section 2.2 that experimental data were collected only for the 0.1 m column with structured packing, while random packing and larger diameters (0.2 m, 0.4 m) were evaluated solely via simulation (Page 7, Lines 275-289). To validate the random packing model, Table 6 has been added (Page 8, Lines 292-295), comparing Aspen Plus simulated pressure drops (using Billet-Schultes correlation) with literature values. All deviations are <5%, confirming the model’s accuracy for untested configurations. This strengthens the reliability of simulation-based results.   Comments 3: There is a direct contradiction in the description of the packing materials' surface areas. In Section 3.1, it is stated: "the random packing provided approximately 1.5 times higher surface area than the structured packing." However, the data in Table 2 shows the random packing surface area is 363 m²/m³ and the structured is 245 m²/m³, meaning the random packing has a surface area 1.48 times higher, confirming the statement. Yet, later in Section 3.3, the authors state, "the greater void fraction in random packing... may reduce the available surface area per unit volume."  This latter statement is incorrect and contradicts the provided data. Please correct this and ensure all discussions about the physical properties of the packings are consistent with the data in Table 2. The discussion should focus on why random packing, despite its higher surface area, often underperforms compared to structured packing, likely due to flow maldistribution and inefficient use of that surface area. Response 3: We sincerely thank the reviewer for pointing out this inconsistency and for the constructive comment. The descriptions of the physical properties of the packing materials have been carefully revised for consistency with Table 2. The statement regarding the “greater void fraction in random packing” has been corrected to reflect that structured packing possesses a higher void fraction (0.95) compared to random packing (0.67). Additionally, the discussion in Section 3.3 has been expanded to clarify that random packing, despite its higher geometric surface area, often exhibits lower mass-transfer performance due to flow maldistribution and inefficient utilization of the available surface area. The revised text now appears in Section 3.3 (Page 20, Lines 771–783, Page 22, Lines 806–809).

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have considered all comments and suggestions raised in the previous review round. The revisions have improved the overall presentation of the manuscript. The responses provided are satisfactory, and the revised version reflects substantial effort to refine both the content and structure of the paper.