Simulation-Based Capacity Planning of a Biofuel Refinery

Round 1

Reviewer 1 Report

The paper is very technical oriented and focusing on particular case study. For broader audience I would suggest to focus on more general comments and applications.

Table 4 – could you please precise how you calculated daily production cost - it is not clear if it is OPEX or total cost it would be much more valuable to calculate NPV and create investment assessment of such application for decision making process. Unit production cost would be worth to add.

Could you provide any additional comment about feedstock transportation.

Application of different software can be interesting however in the text and also abstract could you provide any benefit and barriers of such application , particularly interested not only in technological optimisation i.e. modification capacity of a biofuel refinery for enhancement of the system efficiency and ethanol production but correlation or/and trade-off with environmental and economic impact.

Even it was underlined in the paper that using of use of various feedstock  for minimizes total ecological foot prints from production transportation and disposal are not included in the paper, however could you please add more general comments if the proposed solutions is applicable to corn or corn stover or more general to other feedstocks and what is the most important parameters which should be taken into account for modelling.

Author Response

Table 4 – could you please precise how you calculated daily production cost - it is not clear if it is OPEX or total cost it would be much more valuable to calculate NPV and create investment assessment of such application for decision making process. Unit production cost would be worth to add.

• The table has been updated according to reviewer’s comment.

Could you provide any additional comment about feedstock transportation.

• We add additional comment about feedstock transportation in Section 4.1.

“ ¹Unit price of ethanol is $1.25 per liter on Sep. 14, 2020 [32]; ²Production cost includes fixed cost and variable cost; ³Fixed cost includes maintenance cost (e.g., labor, equipment, and rent) and overhead cost; ⁴Variable cost includes any costs (e.g., associated with includes water, enzyme, and feedstock transportation and treatment) which can vary with the refinery throughput; ⁵Profit is computed by subtracting production cost from revenue.”

 We added a reference

Trading Economics. Ethanol 2005-2020 Data. Available online: https://tradingeconomics.com/commodity/ethanol (accessed on Sep. 14^th 2020).

Application of different software can be interesting however in the text and also abstract could you provide any benefit and barriers of such application , particularly interested not only in technological optimisation i.e. modification capacity of a biofuel refinery for enhancement of the system efficiency and ethanol production but correlation or/and trade-off with environmental and economic impact.

• The major challenge of the proposed approach is that it takes time to develop a CES model with understanding about the system. Even though there is a skilled simulation modeler, the engineer should understand detail operations and their interaction. Thus, the way to make supportive environment for the simulation modeling should be investigated. The relevant comment has been added to Conclusion.

Even it was underlined in the paper that using of use of various feedstock  for minimizes total ecological foot prints from production transportation and disposal are not included in the paper, however could you please add more general comments if the proposed solutions is applicable to corn or corn stover or more general to other feedstocks and what is the most important parameters which should be taken into account for modelling.

• This study has provided a general framework with the specific case (i.e., corn stover). Once detail operations or processes of a subject system is analyzed (see Section 2), the subject system can be modeled via a simulation tool called AnyLogic which is well-known simulation software used to model numerous production systems. Regarding that each refinery must have appropriate capacity for reduction of its operational cost, the proposed framework can be applied to a refinery with any types of feedstock. The relevant comment has been added to Conclusion.

Reviewer 2 Report

Here the suggestions to improve the work:

1. the abstract should be revised. It should be written as short introduction, aim and novelty of the work, used methodology, results, conclusions.
2. you should improve the literature analysis in the introduction, adding other works. You should add the gap in the literature and the novelty of your work.
3. you should define better material and method section and results
4. you should improve results section

Author Response

1. the abstract should be revised. It should be written as short introduction, aim and novelty of the work, used methodology, results, conclusions.

à we added sentences

For novelty, 

In line 13-14, Reduction of operational cost of a biofuel refinery is vitally important to make biofuel competitive with fossil fuels.

For methodology part,

Continuous-event simulation was utilized in this study to model complex operations of a refinery such as loading, unloading and treatment of feedstock over nine major phases (e.g., feedstock storage and handling, pretreatment and conditioning, fermentation and hydrolysis, and enzyme production) to produce ethanol. To improve the model predictions, reported data of corn yield produced in Tazewell County, Illinois, U.S. was used. The proposed simulation model is implemented in AnyLogic® simulation software, and the (near) optimal number of reactors for the hydrolysis and fermentation is found via optimization software known as OptQuest®

For result,

In line 24-25, Reduction of operational cost of a biofuel refinery has been is important to make biofuel competitive with fossil fuels.

2. you should improve the literature analysis in the introduction, adding other works. You should add the gap in the literature and the novelty of your work.

• In this manuscript, we have two introduction parts: section 1 and 2. And in section 2, we explained existing biorefinery and novelty.

In line 174-194, we have explained study novelty.

3. you should define better material and method section and results

à In line 299-304, Section 4.1 has been updated. Additional feedstock transportation information is provided. Table 4 is also updated.

“In the model, trucks deliver about 10 tons of feedstock per delivery to the refinery, the feedstock is received and immediately pretreated and conditioned for hydrolysis. Locations of feedstock farms are Minier, Delavan, Allentown, Morton, and Sewell lake in Tazewell County, Illinois, U.S. Since we assume that the subject facility is located in Schaeferville Tazewell County,  the annual transportation distance is 1,707,245 km with $1,929,186. Note that the unit transportation cost is $1.13 per km according to ATRI [28].”

And we added a reference.

ATRI, American Transportation Research Institute. An analysis of the operational costs of trucking: 2019 update. Available online: https://truckingresearch.org/wp-content/uploads/2019/11/ATRI-Operational-Costs-of-Trucking-2019-1.pdf (accessed on September 15^th 2020).

4. you should improve results section

• In result, we added a paragraph,

In line 372-377, The proposed model is generic so that it can be applied to a biofuel refinery with any type of feedstock. Once detail operations or processes of a subject system are analyzed (see Section 2), the subject system can be modeled via the proposed CES under AnyLogic®.  This is flexible simulation software used to model numerous production systems [33]. Regarding that each refinery must have appropriate capacity for reduction of its operational cost, it is necessary to use the proposed model for the facility design.

We also added a reference

AnyLogic, AnyLogic Simulation Software. Available online: https://www.anylogic.com/ (accessed on Oct. 28^th 2020).

In conclusion we added several sentences,

In line 395-400, In addition, barriers to utilize the proposed approach should be investigated and mitigated. The major challenge of the proposed approach is that it takes time to develop a CES model with understanding about the system. Even when there is a skilled simulation modeler, an engineer is needed to understand detail operations and their interaction. Thus, the way to make supportive environment for the simulation modeling should be investigated.

In line 402-403, Also, other applications (e.g., analysis of environmental and economic impact of the devised system) of the proposed approach should be investigated in future.

Round 2

Reviewer 1 Report

I accept as it is - thank you for proper changes

Reviewer 2 Report

the works was improved