Biologically produced alcohols, most commonly ethanol, and less commonly propanol and butanol, are produced either by the action of microorganisms and enzymes through the fermentation of sugar beets, corn, grass, leaves, agricultural waste, or cellulose. The last named is a more difficult process. It can be produced from biomass (“biobutanol”) as well as from fossil fuels (“petrobutanol”). Production of industrial butanol and acetone via fermentation started in 1916, during World War I. A student of Louis Pasture (Chime Wizemann) isolated the microbe that made acetone. England approached the young microbiologist and asked for the rights to make acetone for cordite. Up until the 1920s acetone was the product sought, but for every pound of acetone fermented, two pounds of butanol were formed. A growing automotive paint industry turned the market around, and by 1927 butanol was primary and acetone became the byproduct.
The production of butanol by fermentation declined from the 1940s through the 1950s, mainly because the price of petrochemicals dropped below that of starch and sugar substrates such as corn and molasses. The labor intensive batch fermentation system’s overhead combined with the low yields contributed to the situation. Fermentation-derived acetone and butanol production ceased in the late 1950s. From the 1970s on the primary focus for alternative fuels was on ethanol.
But bio-butanol does has superior properties to bio-ethanol when used as a biofuel. Biobutanol (also called biogasoline) is often claimed to provide a direct replacement for gasoline without modification to the engine or the car. It will produce more energy a better and is less corrosive and less water soluble than ethanol. One promising development came from Tulane University, and announced in the late summer of 2011 – the university’s alternative fuel research scientists discovered a Clostridium-genus bacteria, which they codenamed “TU-103”, that can convert nearly any form of cellulose into butanol, and is the only yet-discovered strain of Clostridium-genus bacteria that can do so in the presence of oxygen.
When making biobutanol through fermitation, the sugars are broken down into various alcohols, which include ethanol and butanol. Unfortunately, a rise in alcohol concentration causes the butanol to be toxic to the microorganisms, killing them off after a period of time. That toxicity limits the amount of fuel that can be made in one batch. This made the fermentation process expensive. The next challenge lies with the separation costs of butanol from the fermentation broth at the high concentrations used by the industry. Hao Feng says both problems have now been solved.
In the study, funded by the Energy Biosciences Institute, Feng’s team successfully tested the use of a non-ionic surfactant, or co-polymer, to create small structures that capture and hold the butanol molecules. “This keeps the amount of butanol in the fermentation broth low so it doesn’t kill the organism and we can continue to produce it,” he said. This process, called extractive fermentation, increases the amount of butanol produced during fermentation by 100 percent or more.
But that’s only the beginning. Feng’s group then makes use of one of the polymer’s properties — its sensitivity to temperature. When the fermentation process is finished, the scientists heat the solution until a cloud appears and two layers form. “We use a process called cloud point separation,” he said. “Two phases form, with the second facing the polymer-rich phase. When we remove the second phase, we can recover the butanol, achieving a three- to fourfold reduction in energy use there because we don’t have to remove as much water as in traditional fermentation.” A bonus is that the co-polymers can be recycled and can be reused at least three times after butanol is extracted with little effect on phase separation behavior and butanol enrichment ability. After the first recovery, the volume of butanol recovered is slightly lower but is still at a high concentration, he said.
According to Feng, alternative fuel manufacturers may want to take another look at butanol because it has a number of attractive qualities.