An Analysis of Fixed-Bed Catalytic Reactors Performances for One-Stage Butadiene Synthesis from Ethanol

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper deals with a comparison of 3 different reactor technologies for the conversion of ethanol to butadiene through the Lebedev process. This technology has already been implemented industrially log time ago, and has renewed interest in view of the demand for more sustainable products. 

I would recommend to the authors to have a look at the publications from Pr Cavani (Bologna Universtity) who recently revised the reaction mechanisms for that reaction. It would be useful for the reader to have a figure with the reactions involved such as the figure 7.6 in Tommaso Tabanelli et al. (doi.org/10.1515/9783110646856-007).  It would then appear that many of the reactions which are described in the usual ethanol to butadiene process are not described in list of reactions R1 to R7. For example, the route from 2 acetaldehyde to crotonaldehyde goes through the intermediate formation of 3-hydroxybutanal, and similarly the route from ethylene to butadiene is also likely to occur via ethylene reacting with acetaldehyde rather than 2 ethylene reacting together, simply because the partial pressures of intermediates are more likely to favor adsorption of acetaldehyde. 

I can understand that the authors favor a reaction mechanism versus another one, but that should be more justified and simplifications should be explained in more details. Pr Cavani has many publications on this topic, so I recommend to the author to select the most appropriate ones.

Concerning the reaction itself, the authors explain that it is limited by diffusion. Since they operate at a low conversion per path, I assume that it would not change between the entrance and the exit of the reactor. Nevertheless, the experts on catalysis would then recommend several things to reduce the diffusion limitation inside the catalyst particle: 1) change the particle size: a smaller particle has less chance to have intraparticle diffusion limitation; if that's not possible because the pressure drop on the reactor would increase, then change the shape of the particle. Instead of the sphere it could be a cylinder, hollow cylinder, a trilobe or quadrilobe... or a coated particles. There are multiple strategies to reduce the mass transfer limitations in a catalyst particle, which have not bee considered here. Also, and especially in the case where we have multiple beds of catalyst like in the present case, an appropriate strategy is to have different catalysts for the different layers. That's particularly relevant if there is a strong mass and heat transfer limitation at the entrance of the bed, and much less at the exit of the bed.

Here the reactor is designed around a catalyst particle which is poorly efficient, and not with the perspective that the catalyst will improve. New catalyst will be developed in the furture and will have solved this mass transfer limitations. Since the reaction is endothermic, the first consequence as the catalyst will become more productive is that the temperature drop on an adiabatic bed will be larger (until the reaction is limited by the heat transfer). So to anticipate all the catalysts improvements, the reactor technology that would best avoid the heta transfer limitation should be preferred. 

There is however a case where it should not be the case: if the selectivity is improved with another reactor design. The reactions with the lowest activation energies (R3 and R6) should be compared with the reactions having the highest activation energies (R1 mostly, R2 and R7) in their model. What it means is as the temperature decreases along the bed, R3 and R6 are favored (ethylene and butadiene production, versus acetaldehyde production).. But I see nothing in the paper that would justify to have a temperature gradient on the bed.

Other comments: Figure 1: why the heat carrier in the multitubular reactor is cocurrent and not counter current as usual. In fact the fluid is most often flowing perpendicalar to the tube for a better efficiency and has baffles to redirect it to the edge and the center.

Heat carrier: why did you choose an oil, which is limited in temperature? When one need to operate above 300 °C, and for sure above 350 °C, the usual choice is to select molten salts. It would then have been possible to consider to operate even at 450 °C and not be limited by temperature. In some cases, some reactors are operated with boiling water under pressure, but above 370 °C that would not be a good choice, and the reactor cost would increase significantly unless a thermoplate reactor is selected. 

Why the recycling of acetaldehyde and other coproducts is not considered since the reactor is operating at low conversion. That would significantly increase the yield.

I had difficulties to understand why the reactor temperature is limited to 400 °C. Is it because the kinetic data have been taken at low conversion and so only low temperature, or is it because of the choice of the heat carrier (oil). In either case, the choice made is not appropriate. 

The development/selection of the reactor technology shold not be done independantly of the catalyst shape, porosity, mass and heat transfer limitations and possible improvements. Here, selecting an adiabatic reactor would strongly limit the possibility to implement a better catalyst in the future. So this choice should be further justified. 

 

Comments on the Quality of English Language

There are several "words" or "fragment of sentence" that would need to be corrected. I recommend to have the english checked, for example-

Decennia line 38, should it be decades instead

Favorized ==> favored

line 88: a "bench of parallel reactors" ==> do you mean a "Bank" or better a "Bundle"

Superunitary ==> does not seem to be in my dictionnary..

Author Response

Our reply point by point is in the attached word file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper deals with the simulation and optimization of the fixed bed resorts MBAR and MTR for one-stage butadiene synthesis from ethanol. The influence of internal and external diffusion was studied and the empirical expressions were used to calculate the internal effectiveness factors. The work is interesting and can be published in Catalysts with a minor revise.

The author is requested to consider the following issues:

(1)   Please revise the English of the full text carefully to improve the readability of the paper;

(2)   In each expression, the meanings of notations should be explained directly.

(3)   The author uses equation (17) to fit the internal efficiency factor. What is the fitting accuracy? What is the source of the expression of Thiele modulus φ?

Author Response

Our reply point-by-point is presented in the attached word file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am satisfied with the improvement made by the authors. 

However, I should say that if I had to implement such a process, most probably I would select a molten salt heated multitubular reactor, and not a Heating oil at the limit of its operating temperature. An adiabatic reactor is probbaly not the best choice, because currently the conversion per pass is low and the reaction is endothermic. If a better catalys can be developed (more active), more heat would have to be provided, and that would not be easy with the adiabatic reactor.

What is also important is the selectivity and the recycling of side products. Recycling can affect the selectivity, but can improve the yield significantly. So I recommend to investigate this point also.

Comments on the Quality of English Language

corrections made.