Process Simulation of a Commercial Pervaporation Unit for Ethanol–Water Separation
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors- What are the advantages and limitations of the osmotic vaporization process mentioned in the text compared with the traditional distillation process? Especially when dealing with ethanol-water mixtures, how does osmotic vaporization overcome the azeotropic limitation of distillation?
- How is the UniSIM® model mentioned in the text constructed? Please describe the simulation process in detail. During the simulation process, what key parameters and assumptions were used to ensure the accuracy of the simulation results?
- How are the correction factors in the text obtained? How do correction factors affect the simulation results? Discuss the applicability of the correction factors in the text.
- Does this simulation take into account the energy consumption and economic issues of the osmotic vaporization process?
- The details of the thermodynamic model should be described in the text, and the calculation method of "non-ideal gas-liquid equilibrium" should be clarified.
- Is this model applicable to different operating conditions? It is suggested that different operating temperatures and pressures be changed to discuss the accuracy of the model.
- It is suggested to update the references.
Author Response
We are grateful to the reviewers for their valuable feedback. The comments were very important to improve the outcome of this manuscript. We tried to answer the comments to the best of our knowledge. Answers are given below.
Reviewer #2
- What are the advantages and limitations of the osmotic vaporization process mentioned in the text compared with the traditional distillation process? Especially when dealing with ethanol-water mixtures, how does osmotic vaporization overcome the azeotropic limitation of distillation?
Distillation is widely used in the industry because of its continuous process and high production rate. However, the distillation can produce up to 95 wt% ethanol stream. The distillation process is based on the difference in boiling points. The pervaporation membrane can produce ethanol with up to 99.9 wt% because the separation mechanism is based on a dense membrane that allows only water molecules to pass. The pressure difference across the membrane is the driving force. Despite the membrane producing a high-quality stream, the process suffers from a lower production rate. Therefore, in industry, the distillation is first used to produce 95 wt% ethanol, and after that, the membrane is used. The following was added to “Introduction”:
“A pervaporation membrane is used for further ethanol purification. Unlike distillation, where the separation is based on the difference in boiling points, the membrane uses a dense layer where only water passes due to the difference in solubility and diffusion between ethanol and water molecules [9]. In addition, the pressure difference across the membrane acts as the driving force for mass transport. However, the membrane has a lower production rate compared to that of distillation, and that is why it is usually integrated with distillation.”
- How is the UniSIM® model mentioned in the text constructed? Please describe the simulation process in detail. During the simulation process, what key parameters and assumptions were used to ensure the accuracy of the simulation results?
The used model is based on performing a material balance across the membrane. It is assumed that the stream has a plug-flow design with no back mixing. The UniSIM model was constructed using a “component splitter”, spreadsheet, and adjust functions. The component splitter is similar to a black box, where it only applies material balance. The component splitter needs permeate cuts to solve. The pervaporation equations were added to the spreadsheet, and the adjust functions were used to solve for permeate cuts and link them to the component splitter to solve for the pervaporation unit. The following text and figure were added to “Methodology”:
“In UniSIM®, the feed stream was first added using data of pressure, temperature, composition, and flowrate. After that, the above equations were entered manually using an integrated spreadsheet. A component splitter was selected as the pervaporation unit. The component splitter needs permeate cuts of ethanol and water to solve. To do so, adjust functions for ethanol and water were added to solve the material balance of Equation 1 in the spreadsheet. The adjust functions use numerical methods and therefore, the permeate cuts need to be guessed. After finding the solution, the permeate cuts are linked to the component splitter to determine permeate purity. Figure 2 shows the steps for constructing and solving the pervaporation unit in UniSIM®.”
- How are the correction factors in the text obtained? How do correction factors affect the simulation results? Discuss the applicability of the correction factors in the text.
The correction factor was obtained by multiplying the results by a number and comparing the simulation results with the experimental values. A trial-and-error procedure was used until the error is reduced to minimum.
The error in simulation results were linked to the linear fit of permeance data as the fitting accuracy was 95%. Furthermore, the use of the logarithmic-mean difference could also introduce error. It should be noted that the determined correction factor can only be used for ethanol feed range of 93 to 98 wt% at the current operating temperature and pressure. The following was added to “Results and Discussion”:
“Nevertheless, the developed correction factor can only be used for ethanol feed ranging from 93 to 98 wt% with the same operating temperature and pressure.”
- Does this simulation take into account the energy consumption and economic issues of the osmotic vaporization process?
No. The study focuses on the accuracy of simulation results in terms of product purity. The energy consumption and economic assessment will be considered in a future study.
- The details of the thermodynamic model should be described in the text, and the calculation method of "non-ideal gas-liquid equilibrium" should be clarified.
The thermodynamic package is based on the non-random two-liquid (NRTL). The model is widely used for non-ideal mixtures. For the “non-ideal gas-liquid equilibrium” step, it was found recently that if the step was ignored, a maximum error of 1% is expected. The following was added to “Methodology”:
- The selected thermodynamic package is the non-random two-liquid (NRTL) because it is widely used for non-ideal mixtures such as water and alcohols [11].
- “The above step was suggested by Davis [10] and it was investigated in this study by using a membrane module without the adiabatic flash separator. It was found that the maximum calculated error was only 1%, but it is still suggested to use that step for the best accuracy.”
- Is this model applicable to different operating conditions? It is suggested that different operating temperatures and pressures be changed to discuss the accuracy of the model.
We expect the accuracy of the model to change if the temperature and pressure are altered. The following was added to “Results and Discussion”:
“Nevertheless, the developed correction factor can only be used for ethanol feed ranging from 93 to 98 wt% with the same operating temperature and pressure.”
- It is suggested to update the references.
The following references were added:
- Schwarz, C. Effect of variation between different experimental VLE data sets on thermodynamic model and separation predictions: NRTL correlation of the ethanol + water system. Ind. Eng. Chem. Res. 2024, 63, 10721-10734.
- Zheng, L.; Wu, H.; Zhou, Z.; Fan, X. Distillation-pervaporation hybrid process for ethanol dehydration: process optimization and economic evaluation. Chemie Ingenieur Technik 2025, 97, 311-323.
- Cardinali, V.; de Souza, T.; da Costa, R.; Pinto, G.; Roque, L.; Vidigal, L.; Frez, G.; Pérez-Rangel, N.; Coronado, C. Exploring possible pathways for green hydrogen-based transportation in Brazil: Fuel cells, hydrogen engines and dual-fuel combustion. Int. J. Hydrogen Energy 2025, 120, 238-253,
- Zentou, H.; Abidin, Z.; Yunus, R.; Awang Biak, D.; Korelskiy, D. Overview of alternative ethanol removal techniques for enhancing bioethanol recovery from fermentation broth. Processes 2019, 7, 458
Thank you
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsNice Communication that will be interesting for the researchers working in this and related fields, contains new scientific information and well written. Nevertheless, reviewer recommends some minor revision of this manuscript before publishing for the following reasons:
1) I think that Conclusion section should be increased by more detailed indicating of research novelty.
2) The list of references is too short. I suggest to add more literature data in Introduction section.
3) All abbreviations on Figure 1 should be explained and deciphered for clarity.
4) It will be interesting for more visibility to add in Introduction Figure with schematic structure of discussed commercial membrane.
5) In my opinion, some formulas used in current methodology should be provided with references from the literature.
Author Response
We are grateful to the reviewers for their valuable feedback. The comments were very important to improve the outcome of this manuscript. We tried to answer the comments to the best of our knowledge. Answers are given below.
Reviewer #3
- I think that Conclusion section should be increased by more detailed indicating of research novelty?
The following was added to “Conclusion”:
“The model was not readily available in UniSIM®, but it was manually constructed using a component splitter, a spreadsheet, and adjustment functions. The model was solved numerically, and the results show that the simulation calculations differ by an acceptable error of 4.6% from pilot-plant data. The error was further reduced to 0.7% by using a correction factor. This study confirmed that the current simulation model is reliable for estimating the performance of the pervaporation membrane for ethanol purification.”
- The list of references is too short. I suggest to add more literature data in Introduction section.
The following references were added:
- Schwarz, C. Effect of variation between different experimental VLE data sets on thermodynamic model and separation predictions: NRTL correlation of the ethanol + water system. Ind. Eng. Chem. Res. 2024, 63, 10721-10734.
- Zheng, L.; Wu, H.; Zhou, Z.; Fan, X. Distillation-pervaporation hybrid process for ethanol dehydration: process optimization and economic evaluation. Chemie Ingenieur Technik 2025, 97, 311-323.
- Cardinali, V.; de Souza, T.; da Costa, R.; Pinto, G.; Roque, L.; Vidigal, L.; Frez, G.; Pérez-Rangel, N.; Coronado, C. Exploring possible pathways for green hydrogen-based transportation in Brazil: Fuel cells, hydrogen engines and dual-fuel combustion. Int. J. Hydrogen Energy 2025, 120, 238-253,
- Zentou, H.; Abidin, Z.; Yunus, R.; Awang Biak, D.; Korelskiy, D. Overview of alternative ethanol removal techniques for enhancing bioethanol recovery from fermentation broth. Processes 2019, 7, 458
- All abbreviations on Figure 1 should be explained and deciphered for clarity:
The figure was updated.
- It will be interesting for more visibility to add in Introduction Figure with schematic structure of discussed commercial membrane
The following figure was added.
- In my opinion, some formulas used in current methodology should be provided with references from the literature
All the equations were taken from the following cited source:
- Davis, R. Simple gas permeation and pervaporation membrane unit operation models for process simulators. Chem. Eng. Technol. 2002, 25, 717-722
Thank you.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe author has made revisions in accordance with the review comments, so I agree to accept this manuscript.
