Comparison of Two Different Designs of a Scraped Surface Crystallizer for Desalination Effect and Hydraulic and Thermodynamic Numbers

Round 1

Reviewer 1 Report

The manuscript entitled “Comparison of Two Different Designs of a Scraped Surface Crystallizer on Desalination Effect and Thermodynamic Numbers” from Erlbeck and coworkers explores the comparison between two different scraper designs of a scraped surface crystallizer. Supply of potable water is the most important need of humankind and desalination would ensure this supply in the future. However, the search for more economical and feasible processes is still in the development stages. Freeze desalination is one of the attractive technologies but a lot of research needs to perform towards its realization. Towards this aim, authors have compared a scraped surface crystallizer equipped with a screw conveyor and pressing cone. Some of these results were previously published but authors have never published the comparison of the different screws. Hence, the article lacks the novelty of the study but this is fine, as long as the criteria for comparison are presented rigorously, clearly, and consistently. The best section of the manuscript is the concluding paragraphs, which are insightful and informative. 

Based on this, I agree to accept this article for publication in the journal “Processes” in its present form.

Author Response

Dear reviewer.

Thank you for revising the manuscript. We hope the results and the discussion will contribute to the overall research of the community.

Reviewer 2 Report

The paper “Comparison of Two Different Designs of a Scraped Surface Crystallizer on Desalination Effect and hermodynamic Numbers”, submitted to Processes by L.

 Erlbeck  et al., addresses a very interesting topic, since desalination is a very important techonology for the supply of potable water to arid Mediterranean areas. 

The paper compares two different designs  for the proposed technology,  that is, scraped surface crystaller.

The two different designs are well described, as well as the overall test procedure.

The test solution was lab made; the most relevant parameters for assessing the effectiveness of this technology, such as removal efficiency, heat flow, heat transition coefficient and crystal area, were all determined. The dependence of these parameters on the operational parameters, such as temperature and rotational speed, is salso reported in the paper.

The explanation for the parameters variation along the different configuration has been proposed and is related to the Reynolds number (which represents the turbulence); the turbulence caused by the 2 different screws can be variable, largely affecting the efficieny of the system.

Furthermore, the authors proposed an improved configuration design for future applications and further research.

In short, the paper is worth publishing, although some minor revisions should be made as follows.

Please consider changing the word ‘thermodynamic’ with ‘hydraulic’ in the title, since Reynold number is more a hydraulic parameter (such as the Froude number Peclet, and so forth). Same observation for line 17 in the abstract.

In the Discussion subsection, the advanges and the drawbacks of the two different designs should be better addressed, and the addition of a Table illustrating both advantages and disadvantages, in my opinion, would be very helpful (though not compulsory, I leave this choise to the authors).

Figure I and Figure II could be somehow difficult to the reader. Please split both Figures in 2 parts, one for Screw pressed (Fig I a and Fig II a) and the other for Screw unpressed (Fig I b and Fig II b). Same advice applies to  Fig. 8

Please rephrase the following sentences (english is poor):

 L18

 In this research, a scraped screw crystallizer is presented  for which two different screw  designs were examined.

L 79 Desalination with a solution with low concentration was not feasible because of  the higher hardness of the ice and the insufficient power of the gear motor

L 143 This is due to the fact,  (delete comma)

Author Response

Thank you for revising the manuscript. The consideration of your comments led to an increase of quality of the manuscript. We hope the results and the discussion will contribute to the overall research of the community.

1) Please consider changing the word ‘thermodynamic’ with ‘hydraulic’ in the title, since Reynold number is more a hydraulic parameter (such as the Froude number Peclet, and so forth). Same observation for line 17 in the abstract.

In the title and the text line 17 the word “hydraulic” was added. The reason is that the thermodynamic numbers as the heat transition coefficient and the heat flow are as important as the Reynolds number and are also shown in the text.

2) In the Discussion subsection, the advanges and the drawbacks of the two different designs should be better addressed, and the addition of a Table illustrating both advantages and disadvantages, in my opinion, would be very helpful (though not compulsory, I leave this choise to the authors).

To increase the information for the reader one paragraph was added to the conclusion section naming the removal efficiencies which were achieved by the experiments (lines 258-264). Furthermore, another paragraph was added discussing the advantage of Screw II and the possibilities for a better desalination plant (lines 274-279).

3) Figure I and Figure II could be somehow difficult to the reader. Please split both Figures in 2 parts, one for Screw pressed (Fig I a and Fig II a) and the other for Screw unpressed (Fig I b and Fig II b). Same advice applies to 8

Figures 1 and 2 were split to Figure 1a, Figure 1b and so on. But the arrangement was kept the same because of the comparability. Furthermore, because of the different sizes the dimensions would not be readable if the screw and the plant were in the same arrangement. Figure 8 was also kept as it was because it allows a better comparability of the designs as well of the different concentrations. Furthermore, the results are also shown in Table 1 to have the numbers and another opportunity to compare them.

4) Please rephrase the following sentences (english is poor):
L18 “In this research, a scraped screw crystallizer is presented  for which two different screw  designs were examined.”
L 79 “Desalination with a solution with low concentration was not feasible because of  the higher hardness of the ice and the insufficient power of the gear motor”
L 143 This is due to the fact,  (delete comma)

All named sentences were checked and revised.

Reviewer 3 Report

Comments to the Author

This work suggests a comparison in screw designed freeze desalination configurations. Low pinch and high pinch configurations are investigated by changing operational variables like rotational speed and cooling temperature. Even though the authors tried to investigate the comparative study thoroughly, lack of experimental data and insufficient description made the manuscript weak for publication. I cannot recommend the manuscript for publication in the current form. Very significant revisions should be conducted before the publication of this paper. Please find the following concerns which should be properly addressed.

Major comments

1. The most significant problem in this paper is that the authors investigated two configurations by changing rotational speed and cooling temperature in most parts of the paper, but in the conclusions, they recommended a new configuration without the justification of experimental data. This conclusion not only weakens the value of the investigation but also doubt the validity of their work. I recommend the authors to conclude their work within their experimental data only.
2. In the Introduction, the authors did not describe why freeze desalination is a promising technology for the future. Please explain the advantages of the freeze desalination with relevant references in the Introduction.
3. In lines 114-115, the authors mentioned that ‘there is no difference visible if the initial concentrations were high or low’. However, the comparison was conducted only in the case of -9˚C. The reviewer cannot agree with the authors’ conclusion from only a single point. Also, in lines 124-125, the authors mentioned a conflict statement of ‘the removal efficiency was always lower compared to the same screw with high concentrations’. The statement should be arranged without conflict. Thus, more experimental data are required to support their conclusion. And, there is no clear reason why the concentration does not influence on the removal efficiency. Please explain the reason.
4. In Figure 4, there are no data of screw 1 at 54 min^-1 of rotational speed. Please include the data in Figure 4.
5. In lines 140-141, the authors mentioned that ‘The difference between low and high initial concentrations is mainly attributable to the different physical properties of each solution, especially to the melting point, which changes the logarithmic mean temperature difference and thus the heat flow.’ However, at the high concentration (4 wt%), the freezing point lowering is approximately -2˚C. At the low concentration, the extent of freezing point lowering is lower than -2˚C. The reviewer cannot easily understand why the small freezing point lowering attributes to the significant decrease of heat flow as shown in Fig. 5. Please explain more clearly how the freezing point lowering could cause a decrease in heat flow.

Minor comments

1. In lines 40-41, “Pressing thereby can be a potentially better way, but it is technologically harder to realize within a continuously operating plant.” There is no relevant reference.
2. In Section 2.1, it is unclear the exact value of low concentration. In the case of high concentration, it was described as 4 wt%. But, in the case of low concentration, it is not. Please explain the low concentration clearly, or it might be recommendable to include a table for compare the solution information.
3. In lines 55-56, “All samples were prepared, tested, and repeated as described in the previously published papers.” There is no relevant reference.
4. In lines 65-66, “All of this can also be looked up in the papers.”, All of these, not all of this.
5. In lines 108-110, ‘hard condition’ and ‘mild condition’ can be expected, but not clear. Please make these statements clearer. For example, ‘hard condition (from -12 to -10˚C)’.
6. What is the difference in ‘heat transfer coefficient’ and ‘heat transition coefficient’?? Please explain the difference in the main text.
7. In Section 3.3, what is the crystal area?? Is it the surface area of all precipitated crystals in the experimental apparatus?? If so, how can the authors measure the crystal area?? Please clarify the method.
8. In Section 3.3, the crystal area decreases as lowering the cooling temperature. The reviewer cannot understand why the crystal area is decreased as lowering the cooling temperature. When the cooling temperature is decreased, the amount of ice precipitated would be increased. Then, the crystal area of the ice precipitated would be larger. Please could you explain why the graph in Figure 8 showed a decreasing tendency depending on the cooling temperature??
9. In Table 1, the values have low quality for visibility, and the font size was not unified.

Author Response

Thank you for revising the manuscript. The consideration of your comments led to an increase of quality of the manuscript. We hope the results and the discussion will contribute to the overall research of the community.

Major comments

1) The most significant problem in this paper is that the authors investigated two configurations by changing rotational speed and cooling temperature in most parts of the paper, but in the conclusions, they recommended a new configuration without the justification of experimental data. This conclusion not only weakens the value of the investigation but also doubt the validity of their work. I recommend the authors to conclude their work within their experimental data only.

The recommendation is based on all results shown in this and in previously published papers. The design of the screw is similar to the one used for calculating the cone, which means it depends on the ice/water-ratio and thus the necessary tapering. In this case within the open space between two screw shafts instead of within the cone. The overall design is neither fixed nor a statement for a perfect design. The intention is to put a further possible design of an experimental test plant up to discussion for other researchers in this field. Nevertheless, an additional sentence was added to the text clarifying this (lined 290-293).

2) In the Introduction, the authors did not describe why freeze desalination is a promising technology for the future. Please explain the advantages of the freeze desalination with relevant references in the Introduction.

The introduction section was revised to improve the advantage of freeze desalination (lines 31-33).

3) In lines 114-115, the authors mentioned that ‘there is no difference visible if the initial concentrations were high or low’. However, the comparison was conducted only in the case of -9˚C. The reviewer cannot agree with the authors’ conclusion from only a single point. Also, in lines 124-125, the authors mentioned a conflict statement of ‘the removal efficiency was always lower compared to the same screw with high concentrations’. The statement should be arranged without conflict. Thus, more experimental data are required to support their conclusion. And, there is no clear reason why the concentration does not influence on the removal efficiency. Please explain the reason.

The comment is correct, therefore the sentence in lines 114-115 was deleted because of lacking data points. Thus the statement in lines 124-125 (now lines 135-136) is correct and now not in conflict with the previous statement anymore.

4) In Figure 4, there are no data of screw 1 at 54 min-1 of rotational speed. Please include the data in Figure 4.

The data point of Screw II was possible because the plant was optimized during the change of the design of the plant and thus another point was selectable. This statement was added in the text in lines 141-142.

5) In lines 140-141, the authors mentioned that ‘The difference between low and high initial concentrations is mainly attributable to the different physical properties of each solution, especially to the melting point, which changes the logarithmic mean temperature difference and thus the heat flow.’ However, at the high concentration (4 wt%), the freezing point lowering is approximately -2˚C. At the low concentration, the extent of freezing point lowering is lower than -2˚C. The reviewer cannot easily understand why the small freezing point lowering attributes to the significant decrease of heat flow as shown in Fig. 5. Please explain more clearly how the freezing point lowering could cause a decrease in heat flow.

The difference of 2 K is based on different melting temperatures but results in a logarithmic mean temperature difference of 4.4 K when calculated with the coolant. Furthermore, this is a linear factor to the heat flow, which means the influence is not negligible even if it would have been 2 K. The fact of 4.4 K is added to the text to clarify this (lines 154-156).

Minor comments

1) In lines 40-41, “Pressing thereby can be a potentially better way, but it is technologically harder to realize within a continuously operating plant.” There is no relevant reference.

The word in the sentence was changed to “hard” instead of “harder” changing the overall statement (lines 42-44).

2) In Section 2.1, it is unclear the exact value of low concentration. In the case of high concentration, it was described as 4 wt%. But, in the case of low concentration, it is not. Please explain the low concentration clearly, or it might be recommendable to include a table for compare the solution information.

It could be looked up in the named papers but was added to the text as a table (line 64-65). A reference to the table was also added to the text (line 58).

3) In lines 55-56, “All samples were prepared, tested, and repeated as described in the previously published papers.” There is no relevant reference.

The source is given at the end of the paragraph, which means it counts for all information’s within this paragraph. Nevertheless, the sources were also added to the end of the sentence (line 59).

4) In lines 65-66, “All of this can also be looked up in the papers.”, All of these, not all of this.

The sentence was revised as suggested (line 73).

5) In lines 108-110, ‘hard condition’ and ‘mild condition’ can be expected, but not clear. Please make these statements clearer. For example, ‘hard condition (from -12 to -10˚C)’.

A sentence clarifying “hard” conditions was added in lines 118-119. Furthermore, another sentence was added to clarify the word “mild” (lines 120-121).

6) What is the difference in ‘heat transfer coefficient’ and ‘heat transition coefficient’?? Please explain the difference in the main text.

The difference between heat transfer coefficient and heat transition coefficient should be clear to the reader because these are common numbers in the thermodynamic field.

Nevertheless several sentences were added to the text to explain what is meant (lines 74-79).

7) In Section 3.3, what is the crystal area?? Is it the surface area of all precipitated crystals in the experimental apparatus?? If so, how can the authors measure the crystal area?? Please clarify the method.

Each crystal of a sample was measured and the mean value was used. It is described in the previous papers. However, a sentence clarifying this was added to the text (lines 61-63).

8) In Section 3.3, the crystal area decreases as lowering the cooling temperature. The reviewer cannot understand why the crystal area is decreased as lowering the cooling temperature. When the cooling temperature is decreased, the amount of ice precipitated would be increased. Then, the crystal area of the ice precipitated would be larger. Please could you explain why the graph in Figure 8 showed a decreasing tendency depending on the cooling temperature??

At crystallization processes this usually happens because of the higher supersaturation and the corresponding fast crystallization of lots of small crystals. An additional sentence was added to the text (lines 214-215).

9) In Table 1, the values have low quality for visibility, and the font size was not unified.

The whole table was revised and the numbers are now text and not produced with the formula editor (lines 238-239).

Round 2

Reviewer 3 Report

This manuscript is now improved and much of my concerns are well addressed.

I have only one recommendation. In Conclusions, it is too lengthy. This is because there are summary of this study and explanation of the authors' recommendation at the same time. I suggest the authors to make an additional section before Conclusion for introducing the author's recommendation (design). Then, much parts of Conclusion (including Fig. 11) will be moved to the new section. This might be better to deliver the authors' message effectively.

Author Response

Dear reviewer.

Thanks for the suggestion. The part dealing with the recommendation is now shifted and diverted from the discussion section.