Research on the Chlorine Removal and Upgrading of Waste Plastic Pyrolysis Oil Using Iron-Based Adsorbents

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript explores the mixing of red mud with a series of adsorbents for Cl removal and pyrolysis oil upgrading. The chemical recycling of waste plastics has become a growing global concern, and utilizing existing refining processes to recycle waste plastics represents a promising approach. It is recommended that the authors address the following issues before recommending publication in Energies.

 

1. The experimental results (e.g., oil yield, Cl removal rate, etc.) lack error bars or standard deviations, and there is insufficient reproducibility data from repeated experiments. It is recommended to provide averages with error ranges (or standard deviations) based on at least three independent experimental trials.

 

2. Although characterization techniques such as XRD, BET, and NH3-TPD were provided, the comparison of adsorption performance and structural stability among RM1–RM3 remains somewhat subjective. It is recommended to supplement thermogravimetric analysis (TGA) of the adsorbents before/after use or Cl element distribution analysis to provide a more scientific explanation for the dechlorination mechanism.

 

3. All experiments were conducted at fixed temperatures (lower section at 420°C and upper section at 350°C) without exploring the influence of temperature on adsorbent performance. It is recommended to supplement the study with temperature gradient experiments or provide a theoretical justification for the rationale behind the current experimental conditions.

 

4. The mixed adsorbent contains multiple components (red mud, activated carbon, kaolin, methyl cellulose), but control groups such as "activated carbon only" or "kaolin only" were not included. It is recommended to supplement these control experiments to clarify the roles of individual components in dechlorination and pyrolysis.

 

5. The labeling of units is unclear (e.g., the "%" in Figure 3 is not specified as weight percent (wt%) or volume percent).

 

6.Figure 3, is there a direct linear relationship between the oil yield and each RM adsorbent mixture obtained through pyrolysis of waste vinyl fragments?

7.The conclusion section excessively reiterates content from the main text and lacks a succinct synthesis of the innovative aspects,practical implications, and future application prospects of the work. It is recommended to rewrite the conclusion to emphasize the key contributions of this study while also addressing current limitations, thereby enhancing its scientific rigor and guiding value for subsequent research.

Author Response

Comments 1: The experimental results (e.g., oil yield, Cl removal rate, etc.) lack error bars or standard deviations, and there is insufficient reproducibility data from repeated experiments. It is recommended to provide averages with error ranges (or standard deviations) based on at least three independent experimental trials.

Response 1: The results presented are based on three repeated experiments, and the values shown represent their averages. The error margin for each experiment was within 3%.

Comments 2: Although characterization techniques such as XRD, BET, and NH3-TPD were provided, the comparison of adsorption performance and structural stability among RM1–RM3 remains somewhat subjective. It is recommended to supplement thermogravimetric analysis (TGA) of the adsorbents before/after use or Cl element distribution analysis to provide a more scientific explanation for the dechlorination mechanism.

Response 2: After the reaction, the redmud contains chemically adsorbed chlorine, which cannot be removed physically under a nitrogen atmosphere. The table below shows the ion chromatogram analysis of the adsorbent before and after treatment at 350 °C in a nitrogen atmosphere, confirming that the chlorine content remained unchanged.

In the TGA analysis (temperature range: 30~600 °C, heating rate: 10 °C/min, flow rate: 10 ml/min N₂), a slight weight loss was observed in the 250~500 °C range. However, this can be attributed to the residual oil remaining on the adsorbent rather than desorption of chlorine.

	Before regeneration	After regeneration
Cl concentration of redmud	982 ppm	960 ppm
Redmud weight	3.97g	3.94g

Comments 3: All experiments were conducted at fixed temperatures (lower section at 420°C and upper section at 350°C) without exploring the influence of temperature on adsorbent performance. It is recommended to supplement the study with temperature gradient experiments or provide a theoretical justification for the rationale behind the current experimental conditions.

Response 3: Experiments conducted using the pyrolysis reactor shown in Fig. 1 confirmed that wax components with boiling points of 450℃ or higher were removed in the pyrolysis reactor. Based on the results, subsequent experiments were conducted with the reaction temperature fixed at 420℃. (line 93-96)

Comments 4: The mixed adsorbent contains multiple components (redmud, activated carbon, kaolin, methyl cellulose), but control groups such as "activated carbon only" or "kaolin only" were not included. It is recommended to supplement these control experiments to clarify the roles of individual components in dechlorination and pyrolysis.

Response 4: The objective of this study is to remove sulfur and chlorine compounds contained in pyrolysis oil by utilizing redmud, a waste product from the aluminum industry. To achieve this objective, the performance of adsorbents was evaluated by varying the types and contents of additives based on redmud. The chlorine removal performance of each adsorbent was evaluated, and as shown in Figure 7, it was confirmed that the chlorine removal performance varied depending on the types and contents of additives contained in the adsorbents. Based on your suggestion, experiments will be conducted to investigate the effects of individual additives in future studies. (line 47-53)

 

Comments 5: The labeling of units is unclear (e.g., the "%" in Figure 3 is not specified as weight percent (wt%) or volume percent).

Response 5: Following your comments, the units in Figure 3 have been corrected to wt.%. (line 193)

 

Comments 6: Figure 3, is there a direct linear relationship between the oil yield and each RM adsorbent mixture obtained through pyrolysis of waste vinyl fragments?

Response 6: As shown in Figure 3, the effect of kaolin as an additive can be confirmed through the results of RM1 and RM2. However, it was not determined in this study which component between activated carbon and methyl cellulose has a negative impact on chlorine removal. To address this issue, in accordance with your previous suggestion, individual experiments for each additive will be conducted in future studies to investigate how each additive affects chlorine removal. (line 192)

 

Comments 7: The conclusion section excessively reiterates content from the main text and lacks a succinct synthesis of the innovative aspects, practical implications, and future application prospects of the work. It is recommended to rewrite the conclusion to emphasize the key contributions of this study while also addressing current limitations, thereby enhancing its scientific rigor and guiding value for subsequent research.

Response 7: This research compares the properties of the pyrolysis oil of composite waste vinyl chips according to the type of adsorbent mixed with redmud. The cracking behavior was significantly influenced by the presence or absence of an adsorbent. The pyrolysis oil obtained from the pyrolysis reaction conducted without an adsorbent contained approximately 58% hydrocarbons with a carbon number of C13 or higher, mostly in the form of paraffin. In contrast, when pyrolysis oil was collected after passing through the adsorbent layer, the amount of light hydrocarbons discharged as gas increased, and the composition of the pyrolysis oil showed a high contents of hydrocarbons in the C5–C12 range, exceeding 70%. As the oil vapor generated from the thermal decomposition of the composite waste vinyl chips passed through the adsorbent layer, cracking occurred due to the pore characteristics and acid sites of the adsorbent. The strength of the acid sites of the adsorbent varies depending on the kaolin content, and the influence of weak acid sites was significant under the thermal decomposition and Cl adsorption conditions below 500°C. The observed trend showed that more weak acid sites produced lighter hydrocarbons. Moreover, the amount of oil obtained was small compared to the weight of the raw materials supplied, indicating that a large amount of hydrocarbons (C5–C12) were present in the obtained oil. Although cracking occurred, cyclization also occurred due to the influence of the adsorbent.

Regarding the Cl removal performance of the RM adsorbent, the Cl content of the pyrolysis oil obtained without the adsorbent was 115 ppm, while the adsorbent demonstrated a Cl removal efficiency of more than 45%. Significant differences were observed in the Cl removal performance of each adsorbent based on the degree of cracking. When the pyrolysis oil composition consists of lighter hydrocarbons, the adsorbent experiences a decrease in surface area for Cl adsorption due to the reduction of internal pores, resulting in increased surface dependence. Consequently, it was found that the structural stability of the adsorbent is important to achieve both upgrading and Cl removal effects to enable waste plastics to be utilized as alternative resources.

Future studies will investigate the individual characteristics of each additive to better understand their specific effects on the pyrolysis process and chlorine removal performance. (line 286-315)

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Line 145 It is necessary to give the accuracy of the analysis ± value. Which method was used to calculate concentrations (internal/external standard, etc.)

Line 163 and 227. The pore size is located at the junction of micro- and mesopores. It is necessary to calculate the volume and size using the BJH and t-Plot methods.

Line 163. It is necessary to explain how weak and strong centers were calculated. Since the determination is at different temperatures, is the value additive and what is the measurement error?

Line 234 There is no description of the RFI. It is necessary to clearly establish the composition and number of phases and compare them with known data. There is no discussion of the results.

The article is presented in the form of facts

The methodology should specify the basic formulas

Author Response

Comments 1: Line 145 It is necessary to give the accuracy of the analysis ± value. Which method was used to calculate concentrations (internal/external standard, etc.)

Response 1: Adsorbents were prepared by physical mixing with varying ratios of redmud and additives. The content of components in each adsorbent was quantified using XRF, NH3-TPD, and XRD analytical methods to determine the amount of additives contained in the adsorbents, as shown in Figure 2. (line 100-105, 106, and 114-121)

Comments 2: Line 163 and 227. The pore size is located at the junction of micro- and mesopores. It is necessary to calculate the volume and size using the BJH and t-Plot methods.

Response 2: The BJH desorption cumulative pore volume decreased from approximately 0.031–0.039 cm³/g before the reaction to 0.005–0.01 cm³/g after the reaction, indicating the disappearance of micropores as observed in the t-plot analysis. The detailed pore distribution is shown in the figure below.

After the adsorption reaction, hydrocarbon-based chlorines are adsorbed and block the pores, resulting in a significant loss of porosity. In the case of RM3, the material shows a more developed pore structure compared to other adsorbents, and some mesopores remain on the surface even after the reaction.

Comments 3: Line 163. It is necessary to explain how weak and strong centers were calculated. Since the determination is at different temperatures, is the value additive and what is the measurement error?

Response 3: Adsorbents were prepared by physical mixing with varying ratios of redmud and additives. The content of components in each adsorbent was quantified using XRF, NH3-TPD, and XRD analytical methods to determine the amount of additives contained in the adsorbents, as shown in Figure 2. As presented in Chapter 2.2, the acid sites of the adsorbents were characterized using NH3-TPD, and it was confirmed that both weak and strong acidity increased when the kaolin content increased [22, 23]. (line 115-121, 168-171)

Comments 4: Line 234 There is no description of the RFI. It is necessary to clearly establish the composition and number of phases and compare them with known data. There is no discussion of the results. The article is presented in the form of facts. The methodology should specify the basic formulas

Response 4: According to your comments, additional references have been included. [28-32]

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presented is based on a promising idea of converting two very hazardous and long-lived wastes (red mud and PVC) to useful products: efficient catalyst-absorbent and synthetic oil, respectively. The authors demonstrated that specially prepared compositions of red mud, kaolin, activated carbon with a number of additives can be successfully applied as (1) the catalysts of hydrocarbons cracking and (2) HCl trapping absorbents. They also constructed the labor setup to show the feasibility of PVC recycling through pyrolysis accompanied by hydrocarbon cracking and HCl capture. Unfortunately, the authors didn’t say a word about how long the system can operate without replacing red mud catalyst-absorber. Also, they gave no chemical reactions that could occur when HCl interacts with the catalysts components. These reactions are probably more important for the catalyst deactivation then the cracking process.

Here are several comments:

1. The manuscript Title. In fact, the authors write about the removal of HCl, not about the removal of Cl₂, as they write throughout the manuscript! It seems to very important to give a scheme for PVC decomposition in the beginning of Experimental part.
2. Line 20. limited.
3. Lines 43-44. “If the Cl…” (1). What means Cl?; (2) HCl is the product of PVC decomposition.
4. Lines 53-54. What means “activation energy of plastic”? This sentence has to be revised.
5. Line 69. “in” -?
6. Table 2. What was the measurement accuracy? Two decimal places are enough.
7. Lines 165-168. To revise the beginning of this sentence.
8. Line 173. Interval in front of (Figure 3).
9. Line 174. “and” instead of “to”.
10. For Table 3. It seems better to present the pore and specific surface area parameters before and after reaction in one Table. Additionally: is the measurement accuracy that high?
11. Line 229. “kaolin”
12. Line 250-270. Once again about scheme of PVC decomposition (pyrolysis).                  This part is too complex to understand the author’s idea. It is necessary to simplify and systematize the discussion of such a large amount of data.      

Author Response

We sincerely appreciate the reviewer’s thoughtful and constructive comments. Your feedback has been invaluable in improving the quality and clarity of our manuscript. We have carefully addressed all the points raised and revised the paper accordingly. Thank you again for your time and effort in reviewing our work.

 

Comments 1: The manuscript Title. In fact, the authors write about the removal of HCl, not about the removal of Cl2, as they write throughout the manuscript! It seems to very important to give a scheme for PVC decomposition in the beginning of Experimental part.

Response 1: In this study, pyrolysis oil was produced using waste vinyl. The collected waste vinyl was crushed to a uniform size and fed into the pyrolysis reactor, and pyrolysis was conducted at 420°C under oxygen free conditions[17]. During this process, when polymer chains are broken by thermal energy, they decompose into low molecular weight substances such as gas, solids (residue, wax), and liquid (pyrolysis oil) [18-20]. The pyrolysis gas is converted to pyrolysis oil through a condensation process. (line 83-88)

Comments 2: Line 20. limited.

Response 2: According to your comments, we changed limited to limited. (line 20)

Comments 3: Lines 43-44. “If the Cl…” (1). What means Cl?; (2) HCl is the product of PVC decomposition.

Response 3: In this study, waste vinyl was used, which may contain chlorine-containing substances in the raw material structure and may exhibit high chlorine content due to impurities not being completely removed during the pretreatment process. (line 88-91)

Comments 4: Lines 53-54. What means “activation energy of plastic”? This sentence has to be revised.

Response 4: According to your comments, we revised the sentence. The catalytic properties possessed by the metal oxides contained in redmud lowered the activation energy of pyrolysis reactions, thereby accelerating the reaction rate. (line 53-55)

Comments 5: Line 69. “in” -?

Response 5: According to your comments, we revised the sentence. Pyrolysis, chlorine removal, and upgrading of waste plastic were conducted in a reactor with a diameter of 3 inches and a height of 1.5 m. (line 69-70)

Comments 6: Table 2. What was the measurement accuracy? Two decimal places are enough.

Response 6: The values were rounded to the second decimal place from the third decimal place. (Table. 2) The accuracy is ≥95%.

Comments 7: Lines 165-168. To revise the beginning of this sentence.

Response 7: According to your comments, we revised the sentence. The thermal decomposition results obtained from 650 g of composite waste vinyl chips showed no wax products, and since the pyrolysis reaction of raw materials occurs under the adsorbent layer, no difference was observed in the amount of char recovered from the lower part of the reactor regardless of the type of adsorbent used. (line 178-181)

Comments 8: Line 173. Interval in front of (Figure 3).

Response 8: According to your comments, we interval was in front of (Figure 3).

Comments 9: Line 174. “and” instead of “to”.

Response 9: According to your comments, we changed to > and. (line 188)

Comments 10: For Table 3. It seems better to present the pore and specific surface area parameters before and after reaction in one Table. Additionally: is the measurement accuracy that high?

Response 10: Another reviewer requested additional information regarding the table, which led us to include new content. Unfortunately, due to formatting constraints during the revision process, it was difficult to combine the two tables. We apologize for this inconvenience.

Comments 11: Line 229. “kaolin”

Response 11: According to your comments, we changed Kaolin to kaolin. (line 242)

Comments 12: Line 250-270. Once again about scheme of PVC decomposition (pyrolysis). This part is too complex to understand the author’s idea. It is necessary to simplify and systematize the discussion of such a large amount of data.

Response 12: According to your comments, we revised the sentence. During the pyrolysis of waste vinyl-based chips, chlorine present in the raw material is released as hydrogen chloride (HCl) gas, which is subsequently captured by various metal oxides contained in the redmud (RM) adsorbent, including Fe₂O₃, CaO, and MgO. The adsorption process involves the conversion of these oxides into chloride compounds, while oxygen atoms are simultaneously reduced to water (H₂O) [24, 25]. SEM-EDX analysis conducted to examine the elemental distribution on RM adsorbent surfaces before and after the pyrolysis reaction confirmed that oxygen content decreased while chlorine content increased on all adsorbent surfaces; however, significant variations were observed in the rates of surface oxygen reduction and chlorine accumulation among different adsorbents (Figure 8). RM3, which achieved the highest chlorine removal rate, exhibited distinct characteristics where oxygen remained present on the surface after the reaction and the rate of chlorine accumulation was the lowest among all tested adsorbents. In contrast, RM2 demonstrated the poorest chlorine removal performance, with no oxygen detected on the surface after the reaction and a high rate of chlorine accumulation observed. Post-reaction pore analysis (Table 3) revealed that RM2 exhibited the greatest decrease in pore characteristics, indicating substantial structural degradation, while RM3 maintained relatively high structural stability with minimal changes in pore properties. During the thermal decomposition process, oil vapor passes through the internal pores following initial surface reactions, facilitating chlorine adsorption onto metal oxides and promoting catalytic cracking reactions. RM2 exhibited a high cracking occurrence rate; however, due to its small internal pores, surface reactions were predominantly enhanced, resulting in rapid reduction of surface oxides and oxygen, increased chlorine adsorption concentrated on the surface, and accelerated surface deactivation. Conversely, RM3 demonstrated superior performance due to its structural characteristics, where minimal pore structure changes maintained reaction accessibility, chlorine adsorption occurred throughout the internal structure rather than merely on the surface, and the highest chlorine removal rate was achieved through distributed reaction sites. Despite high chlorine removal efficiency, oxygen remained on the RM3 surface with reduced chlorine accumulation, indicating sustained catalytic activity. The superior performance of RM3 can be attributed to the beneficial effects of activated carbon, which provides enhanced surface area for increased availability of reaction sites, improved structural stability to prevent pore collapse and maintain internal reaction pathways, and reduced surface deactivation to preserve catalytic activity throughout the reaction process. This study demonstrates that effective chlorine removal in waste vinyl pyrolysis is critically dependent on the adsorbent's pore structure and stability, where RM3's combination of adequate pore accessibility and structural integrity enables distributed chlorine adsorption throughout the material, resulting in superior performance compared to RM2's surface-limited reaction mechanism. (line 263-299)

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

 Accept in present form