Question

You hear about an opportunity for a process engineer with a company that is starting a new commercial plant to produce ethanol from hybrid poplar trees. You apply and soon get called for an interview!

As a good candidate for the position, you do your own search about the company before visiting it, and find out that their current process uses direct fermentation of sugars to obtain ethanol. They are heavily considering an alternative option, though, which employs fermentation of sugars to acetic acid, followed by hydrogenation of the acetic acid to produce ethanol. The article you read about this only mentions that this alternative “reduces carbon emissions”.

As it turns out, during the interview you are asked to provide a detailed, technical opinion about each one of these two routes. Specifically, you are asked about the challenges involved in separation: in one case, you need to separate ethanol from water after fermentation, in the other case you need to separate acetic acid from water after fermentation. You remember the concepts you learned in class about multiphase, multicomponent systems and attempts to answer the following questions: How would you design a system for separation in each case? What type of separation? What would be the challenges involved?

In addition, your response could include other aspects such as the stoichiometry of each reaction, and potential energy requirements. Be as quantitative as possible.

Answer 1

a) the separation in case of ethanol-water will involve fractional distillation.

We can also use the technique of additon of sulphur to the mixture of ethanol and water. Sulphur is insoluble in water while one form of sulphur is soluble in ethanol. so on addition of sulphur to the mixture the ethanol with sulphur will settle down. The sulphur can be bled off easily.

A significant amount of energy is saved in this process as compared to fractional or conventional distillation.

The separation of acetic acid from water is sa little difficult process as compared to ethanol-water mixture.

Acetic acid forms zeotropic mixture with water. The conventional method of distiilation is less effective method as with progressive distillations the solution will become less in amount with water and the process of separation becomes more difficult with the decreas. Economicaly it is not a practical advise to distill off the solution.