Acetone–butanol–ethanol (ABE) fermentation, also known as theWeizmann process, is a process that uses bacterialfermentation to produceacetone,n-butanol, andethanol from carbohydrates such asstarch andglucose. It was developed bychemistChaim Weizmann and was the primary process used to produce acetone, which was needed to makecordite, a substance essential for the British war industry duringWorld War I.[1]
The process may be likened to howyeast ferments sugars to produce ethanol for wine, beer, or fuel, but the organisms that carry out the ABE fermentation are strictlyanaerobic (obligate anaerobes). The ABE fermentation produces solvents in a ratio of 3 parts acetone, 6 parts butanol to 1 part ethanol. It usually uses a strain of bacteria from classClostridia (familyClostridiaceae).Clostridium acetobutylicum is the most well-studied and widely used. Although less effective,Clostridium beijerinckii andClostridium saccharobutylicum bacterial strains have shown good results as well.[2][3]
The ABE fermentation pathway generally proceeds in two phases. In the initialacidogenesis phase, the cells grow exponentially and accumulateacetate andbutyrate. The low pH along with other factors then trigger a metabolic shift to thesolventogenesis phase, in which acetate and butyrate are used to produce the solvents.[4]
For gas stripping, the most common gases used are the off-gases from the fermentation itself, a mixture ofcarbon dioxide andhydrogen gas.[citation needed]
The production ofbutanol by biological means was first performed byLouis Pasteur in 1861.[5] In 1905, Austrian biochemist Franz Schardinger found that acetone could similarly be produced.[5] In 1910Auguste Fernbach (1860–1939) developed a bacterial fermentation process using potato starch as a feedstock in the production of butanol.[6]
Industrial exploitation of ABE fermentation started in 1916, during World War I, withChaim Weizmann's isolation of Clostridium acetobutylicum, as described in U.S. patent 1315585.[7]
The Weizmann process was operated byCommercial Solvents Corporation from about 1920 to 1964 with plants in the US (Terre Haute, IN, andPeoria, IL), andLiverpool, England. The Peoria plant was the largest of the three. It usedmolasses as feedstock and had 96 fermenters with a volume of 96,000 gallons each.[8]
AfterWorld War II, ABE fermentation became generally non-profitable, compared to the production of the same three solvents (acetone,butanol,ethanol) frompetroleum.[1] During the 1950s and 1960s, ABE fermentation was replaced by petroleum chemical plants. Due to different raw material costs, ABE fermentation was viable inSouth Africa until the early 1980s, with the last plant closing in 1983.[9] Green Biologics Ltd operated the last attempt to resurrect the process at scale but the plant closed in Minnesota in June 2019.
A new ABE biorefinery has been developed in Scotland by Celtic Renewables Ltd and will begin production in early 2022. The key difference in the process is the use of low value spent materials or residues from other processes removing the variable costs of raw feedstock crops and materials.[10]
The most critical aspect inbiomass fermentation processes is related to its productivity. The ABE fermentation viaClostridium beijerinckii orClostridium acetobutylicum for instance is characterized byproduct inhibition. This means that there is a product concentration threshold that cannot be overcome, resulting in a product stream highly diluted in water.[11]
For this reason, in order to have a comparable productivity and profitability with respect to thepetrochemical processes, cost and energy effective solutions for the product purification sections are required to provide a significant product recovery at the desired purity.The main solutions adopted during the last decades have been as follows:[citation needed]
In the second half of the 20th century, these technologies allowed an increase in the final product concentration in the broth from 15 to 30 g/L, an increase in the final productivity from 0.46 to 4.6 g/(L*h) and an increase in the yield from 15 to 42%.[3]
From a compound purification perspective, the main criticalities in the ABE/W product recovery are due to the water–alcohol mixture's non-ideal interactions leading to homogeneous and heterogeneousazeotropic species,[12] as shown by the ternary equilibrium diagram.This causes the separation by standarddistillation to be particularly impractical but, on the other hand, allows the exploitation of the liquid–liquid demixing region both for analogous[13] and alternative[citation needed]separation processes.
Therefore, in order to enhance the ABE fermentation yield, mainly in situ product recovery systems have been developed. These includegas stripping,[14][15]pervaporation,[16][17]liquid–liquid extraction,distillation via Dividing Wall Column,[18]membrane distillation,membrane separation,[19]adsorption, andreverse osmosis. Green Biologics Ltd. implemented many of these technologies at an industrial scale.
Moreover, differently fromcrude oil feedstocks,biomasses nature fluctuates over the year's seasons and according to the geographical location.[20][21] For this reasons,biorefinery operations need not only to be effective but also to be flexible and to be able to switch between two operating conditions rather quickly.[citation needed]
ABE fermentation is attracting renewed interest with a focus onbutanol as a renewable biofuel.[23]
Sustainability is by far the topic of major concern over the last years. Theenergy challenge is the key point of theenvironmental friendly policies adopted by all the most developed and industrialized countries worldwide. For this purpose Horizon 2020, the biggest EUResearch and Innovation programme, was funded by theEuropean Union over the 2014–2020 period.[24]
TheInternational Energy Agency definesrenewables as the centre of the transition to aless carbon-intensive and moresustainable energy system.Biofuels are believed to represent around 30% of energy consumption in transport by 2060. Their role is particularly important in sectors which are difficult to decarbonise, such asaviation,shipping and other long-haul transport. That is why severalbioprocesses have seen a renewed interest in recent years, both from a research and an industrial perspective.[25]
For this reason, the ABE fermentation process has been reconsidered from a different perspective. Although it was originally conceived to produceacetone, it is considered as a suitable production pathway forbiobutanol that has become the product of major interest.Biogenic butanol is a possible substitute of bioethanol or even better and it is already employed both asfuel additive and as pure fuel instead of standardgasoline because, differently fromethanol, it can be directly and efficiently used ingasoline engines. Moreover, it has the advantage that it can be shipped and distributed through existing pipelines andfilling stations.[26]
Finally biobutanol is widely used as a directsolvent forpaints,coatings,varnishes,resins,dyes,camphor,vegetable oils, fats,waxes,shellac, rubbers andalkaloids due to its higher energy density, lowervolatility, and lowerhygroscopicity.[citation needed] It can be produced from different kinds of cellulosic biomass and can be used for further processing of advancedbiofuels such as butyl levulinate as well.[27]
The application of n-butanol in the production ofbutyl acrylate has a wide scope for its expansion, which in turn would help in increasing the consumption of n-butanol globally.Butyl acrylate was the biggest n-butanol application in 2014 and is projected to be worth US$3.9 billion by 2020.[28]