Synthesis database: Ethanol synthesis

Ethanol fermentation is a biological process in which sugars are converted to ethanol and carbon dioxide. This is accomplished with the help of yeasts, which convert sugars to ethanol in the absence of oxygen. Because of that, the fermentation of ethanol is an anaerobic process. Ethanol fermentation has been used for quite some time, mainly for the production of alcoholic beverages. Today this process is also important in the production of various types of fuel which can serve, to a greater or lesser degree, as substitutes for common fuels which are derived from oil. The following chemical equations describe the process:

C₁₂H₂₂O₁₁ + H₂O -> 2C₆H₁₂O₆

and

2C₆H₁₂O₆ -> 4C₂H₅OH + 4CO₂

The first one shows the hydrolysis of sucrose, which gives double the amount of glucose. The formed glucose then further gives double the amount of ethanol.

The equipment and compounds needed for the fermentation are very common. Water, sucrose, a larger container in which the fermentation will be done and yeasts are the main needed components. In this case, 5 kilograms of sucrose was added to 20 liters of water. Instead of commercial yeast, a bit more than a liter of fermenting cherry juice, which already contained the yeasts needed for the fermentation, was added. By using the mentioned values, the mass fraction of sucrose in the solution was around 20%. If most of the sucrose gets used up, and if most of the produced carbon dioxide leaves the solution, the final mass fraction of ethanol in the solution should be around 12%, which is close to the value at which yeasts stop producing ethanol.

After the solution has been prepared, the container has been closed and left alone for about a month. During that period, the temperature was mostly between 20 °C and 25 °C. Every now and then, the lid was slightly lifted in order to inspect the progress of the fermentation. Up to the last few days, the production of carbon dioxide could be heard and seen in form of emerging bubbles. The fermentation was considered to be over when the formation of carbon dioxide stopped.

When the fermentation was finished, the solution was added to a copper cauldron which is ordinarily used for the distillation of various alcoholic beverages. Then the cauldron was connected to the condenser, and the heating was turned on. After about an hour, the distillate started coming over.

The volume fraction of the alcohol in the first 700 mL of the distillate was a bit lower than 70%. During the distillation, additional measurements were made. The measured values are shown in the table and graph below:

The distillate was collected until the volume fraction of ethanol dropped below 15%. Approximately 7.1 L of ethanol solution was obtained, with a volume fraction of 44% (matches the mass fraction of approximately 38%). A part of the fraction which came over at the end, was not added to the rest, in order to avoid further reduction of the ethanol content. The result of the first distillation can be seen on the picture below.

After that, about 700 mL of the first distillate was distilled once again by using a laboratory distillation apparatus (picture below). This gave approximately 270 mL of ethanol solution, most of which came over at a temperature around 80 °C.

The density of the obtained solution was equal to 0.84 g/mL which, together with the boiling point, shows that the mass fraction of ethanol in the solution was approximately 82%.

The result of the second distillation can be seen above. Such ethanol can be further purified, i.e. concentrated by implementing other methods, such as removal of water from the solution with the help of calcium oxide. The mentioned method is described here.

When the amount of ethanol present in the already fermented cherry juice is deducted, the yield of the fermentation is approximately 94%.