When sucrose is fermented to produce ethanol why is it not possible to obtain a final solution containing more than 10-15% ethanol?
Does this have to do with azeotropes? However, I thought this was possible? I am confused, please help.
The limit of obtaining a final solution containing more than 10-15% ethanol when fermenting sucrose is indeed related to azeotropes. Azeotropes are mixtures of two or more substances that have a constant boiling point and composition. In the case of ethanol-water mixtures, there are two azeotropes to consider: the minimum boiling azeotrope and the maximum boiling azeotrope.
The minimum boiling azeotrope, also known as the positive azeotrope, occurs at 95.6% ethanol and 4.4% water by weight. This means that no matter how much heat you apply or how long you distill the mixture, you cannot separate ethanol from water with a concentration greater than 95.6% ethanol. Therefore, it is not possible to obtain a solution with a higher ethanol concentration through simple distillation.
The maximum boiling azeotrope, also known as the negative azeotrope, occurs at 89.4% ethanol and 10.6% water by weight. This azeotrope has a higher boiling point than pure ethanol, which means that if you distill a mixture of ethanol and water with a concentration lower than the negative azeotrope, you will not be able to remove all the water even at the boiling point of ethanol. This limits the maximum ethanol concentration obtainable through distillation.
In the case of fermenting sucrose to produce ethanol, the fermentation process yields a mixture of ethanol, water, and other by-products. Even when distilling this mixture, it is impossible to achieve a solution containing more than 10-15% ethanol due to the azeotropic limitations mentioned earlier.
To obtain higher ethanol concentrations, additional processes such as molecular sieve adsorption or fractional distillation with added entrainers can be employed. These methods can break azeotropes and achieve solutions with ethanol concentrations beyond the azeotropic limits.