Extending Multilevel Statistical Entropy Analysis towards Plastic Recyclability Prediction

Round 1

Reviewer 1 Report

Conclusion section shall be limited to the scope of this work. In case authors prefer to discuss the potential future works, better do it at the discussions section (Lines 607 to 614) and mention it in short format at the conclusions. 

Author Response

Reviewer 1

Comments and Suggestions for Authors

Conclusion section shall be limited to the scope of this work. In case authors prefer to discuss the potential future works, better do it at the discussions section (Lines 607 to 614) and mention it in short format at the conclusions.

 

Response:

The authors thank the reviewer for the suggestion and have moved the discussion of the potential future works to the Discussions section (lines 602 to 605 ):

 

Other perspectives for future work are the refinement of the proposed recyclability metric and the inclusion of energy consumption aspects, the application of the proposed methodology to real-life plastic products and plastic waste streams and integration of different levels of entropy (e.g., at the molecular and geographical level)

 

And the authors have summarized in one sentence what was initially in the conclusions section with respect to future work (lines 616 to 617) :

Future work entails further refining using realistic case studies as well as complex processes as reactive extrusion.

 

 

Reviewer 2 Report

This paper presents the application of a multilevel SEA method to evaluate the recyclability of plastics. In a first stage the method is applied to a conceptual case study of a fictitious bag filled with plastics and then the method is extended with the computation of relative decomposition energies of components and products.

Although the extension of the proposed method allows estimating the recyclability of plastics, I would like to note the following comments to the Authors:

- The method used is only valid for plastic components whose constituents are insoluble and does not seem to take into account the chemical affinity of the mixed constituents, in which, for the separation process, different energy values per unit mass for the decomposition may have to be considered.

- For future works that are intended to be developed in the scope of this work, I suggest to evaluate the real usefulness of the application of this method to study the recyclability of real-life plastic products and plastic waste streams, namely in the works proposed in the conclusions of the paper.

Author Response

Comments and Suggestions for Authors

This paper presents the application of a multilevel SEA method to evaluate the recyclability of plastics. In a first stage the method is applied to a conceptual case study of a fictitious bag filled with plastics and then the method is extended with the computation of relative decomposition energies of components and products.

 

Although the extension of the proposed method allows estimating the recyclability of plastics, I would like to note the following comments to the Authors:

 

- The method used is only valid for plastic components whose constituents are insoluble and does not seem to take into account the chemical affinity of the mixed constituents, in which, for the separation process, different energy values per unit mass for the decomposition may have to be considered.

 

- For future works that are intended to be developed in the scope of this work, I suggest to evaluate the real usefulness of the application of this method to study the recyclability of real-life plastic products and plastic waste streams, namely in the works proposed in the conclusions of the paper.

 

Response:

The authors thank the reviewer for the suggestions and comments.

• A short comment has been added the following comment in the paper (lines 424 - 428):
  In addition, the case studies in this contribution are rather simple and do not account for the complexity of the constituents of the components and their mixtures. In practice, this can have implications for the effective decomposition energies.
• The authors agree and are studying the usefulness of the application of this method and the required complexity and concepts based on the real-life plastic products and plastic waste streams in future work.

Reviewer 3 Report

This article builds upon the existing method of multilevel statistical entropy analysis (SEA) with the purpose to assess the recyclability of plastics. The multilevel SEA was extended with energy consumption aspects (relative decomposition energy) to formulate recyclability metrics for components and products. The expanded methodology was used to assess the recyclability of plastics based on a conceptual example of a fictitious plastic bag.

The research presented in this article reflects an intermediate step towards developing applicable metrics for recyclability of plastics based on the multilevel SEA methodology. On the one hand, the article presents the theoretical extension of the multilevel SEA with energy consumption aspects, exemplified with a theoretical (conceptual) case of a fictitious plastic bag, but on the other hand it does not conclude to the level of a real and applicable case study of plastic recycling. Therefore, the contribution is valuable only for its theoretical premises and cannot be credited for any applicable results.

Nevertheless, this intermediate step in the methodological development is considered important for future studies with more applied content. For this reason, I recommend this article for publication. Some minor comments follow:

Comments:

1. Line 28, it is stated that “… plastic products are increasingly being designed for circularity.” Is there any reference that can support this statement? As far as the status quo is concerned, the vast majority of plastics produced today do not have any particular design improvements in terms of circularity.
2. Line 47, it is stated that “…linear economy alternative; producing virgin products with incineration at their end-of-life” Why is this mentioned as the only “linear economy alternative”? With the exception of NW Europe and Japan, the most common linear economy alternative in the world is landfilling (of land/water disposal). Please refine this statement, or delimit the scope of your article.
3. Lines 50 and 74, the term “transformative materials” is used. What is transformative materials? This term is not defined, thus does not mean anything for the reader. Please elaborate the meaning of this term.
4. Lines 82-83, the word experiment is repeated in the sentence. Please revise the sentence accordingly.
5. Line 121, please outline what P+MD is for readers not familiar with the collection system in Belgium.
6. Lines 225-226, the word “entity” is repeated to refer both to the component and the material. Could the authors devise a more precise language. For instance, one unit of component 1 and one entity of material e.g. 2kg. It is more appropriate to refer to the component as “unit” and to the material entity (which can consist of more units, e.g. 2kg). Please revise the sentence accordingly.
7. The example of the fictitious plastic bag is a very simple and presents an “ideal” situation of materials and components. This is never the case in a real life example of collected plastic waste (in a bag). It is understood that this example is used only to illustrate the elements of the expanded method and provide guidance for the calculations. The method could be applied in different product groups that would be more appropriate, e.g. WEEE (with varying components and materials). However, since the article is specifically focused on plastic recycling, then the specific example would suffice.

As mentioned already, the example is illustrating the method, however its applicability in real life is rather limited. This article holds significant scientific merit in terms of methodological development, however, it does not offer any applicable solutions in real life problems of recycling of plastics. It would be very interesting to see the follow-up research where the authors are planning to apply this method to the reactive extrusion technology for plastics.

 

Author Response

Response

The authors thank the reviewer for the suggestions and comments. The authors agree that the main contribution lies in the methodological and theoretical aspects that are presented and that real-life case studies are part of future work as indicated in our manuscript.

 

• Line 28, it is stated that “… plastic products are increasingly being designed for circularity.” Is there any reference that can support this statement? As far as the status quo is concerned, the vast majority of plastics produced today do not have any particular design improvements in terms of circularity.
  • The authors agree with this comment in the sense that it was too generically formulated. Therefore we have rephrased this statement and have added following references (lines 27 to 29):

At the front end of their lifecycle, some initiatives have been raised to design plastic products for circularity [13–15]. This can range from design-for-disassembly [16,17], design-for-recycling [18–20]

• Line 47, it is stated that “…linear economy alternative; producing virgin products with incineration at their end-of-life” Why is this mentioned as the only “linear economy alternative”? With the exception of NW Europe and Japan, the most common linear economy alternative in the world is landfilling (of land/water disposal). Please refine this statement, or delimit the scope of your article
  • The authors have added landfilling as an example of the linear economy alternative (lines 48 to 49):
    … linear economy alternative, e.g., producing virgin products with incineration at their end-of-life or landfilling (not taking resource scarcity or other environmental indicators into account).
• Lines 50 and 74, the term “transformative materials” is used. What is transformative materials? This term is not defined, thus does not mean anything for the reader. Please elaborate the meaning of this term.
  • The authors have clarified the term transformative materials (line 51)
    (based on novel disruptive technologies).
• Lines 82-83, the word experiment is repeated in the sentence. Please revise the sentence accordingly.
  • The authors have revised the sentence (line 84):
    linking SEA to exergy analysis applied to a crushed lithium-ion battery lab scale sieving experiment
• Line 121, please outline what P+MD is for readers not familiar with the collection system in Belgium.
  • The authors have added a brief clarification (lines 121-123)

such as the P+MD system, i.e., the waste collection bag for plastic packaging, metal packaging and drinking cartons in Belgium

• Lines 225-226, the word “entity” is repeated to refer both to the component and the material. Could the authors devise a more precise language. For instance, one unit of component 1 and one entity of material e.g. 2kg. It is more appropriate to refer to the component as “unit” and to the material entity (which can consist of more units, e.g. 2kg). Please revise the sentence accordingly.
  • The authors have revised the related sentences as follows, based on the suggestion of the reviewer (lines 225-227):

The multilevel SEA method is illustrated on the conceptual example of a fictitious plastic bag (product) which is filled with ten components (4 units of component 1 and 6 units of component 2). A unit of component 1 consists of 1 entity of red material (2g) and 1 entity of blue material (3g). A unit of component 2 consists of 1 entity of orange material (3g) and 1 entity of blue material (3g)