Fishing for butanol
In too high concentrations, biobutanol is toxic for our clostridial bacteria. At the end of the process, you have a mixture of bacteria, waste, and butanol. But how do you recover this butanol? To do this, you can boil the mixture very hard, which requires a lot of energy. Developing alternatives is part of my research.
Such a fermentation mixture is a like a thick soup. Which method you use, in a first step, the microbes and all other waste are removed. You can do this by using a filter or centrifuge. In a centrifuge, the mixture spins around at extreme velocities, precipitating all solids.
The produced butanol is now diluted in clear and clean water. The next step, however, is the most difficult. The dilute butanol mixture is boiled at high temperature. In this way, butanol evaporates from the liquid mixture. The vapors containing butanol are subsequently cooled down. The vapor condenses and you obtain pure butanol. With a technical term, we call this distillation.
The full purification is slightly more complicated. Acetone and ethanol are two side products who are also present in the mixture. Acetone is an important solvent in chemical industry. Ethanol is known better as the alcohol in our beer. Those two products are also interesting to recover.
That is why we boil our mixture in different steps. First, steam is injected in a big vessel containing our clarified soup. The mixture starts boiling very hard, creating a vapor contain butanol. Next, the vapors are cooked in different large vessels. Acetone, which has the lowest boiling point is purified first. Ethanol, which has a slightly larger boiling point is purified secondly. And in the final vessel, we obtain fully pure butanol.
At home, we used to cook with an electric cooking plate. This took a huge amount of time. It took a long time for water, to cook pasta, became hot. One of the main reasons is that heating water requires a huge amount of energy. This makes biobutanol recovery also extremely energy intensive. Per kilogram of butanol, we need about 20 kWh of heat. For bio-ethanol recovery, we only need 2.7 kWh of energy, or 10 times less!
A big challenge. But alternatives exist which use less energy. During my PhD thesis, I investigated the use of materials which behave like a sponge. They can selectively remove butanol, directly from the liquid mixture. You can find a detailed explanation here.