Some call it biology’s intelligent defense mechanism against microbial infiltrators and decomposers. “It is nature’s structural material, and it’s put together very securely,” says Dennis Miller, a professor of chemical engineering at Michigan State University. Deconstructing plant material, separating the lignin from cellulose and hemicellulose in order to utilize five- and six-carbon sugars, is much trickier than other forms of biomass utilization, like chipping wood and combusting it in a solid-fuel boiler. Dartmouth College professor Lee Lynd says the “recalcitrance of cellulosic biomass” is the biggest obstacle to cost-effective biorefining. “If this is solved, conversion of sugars to ethanol and recovery of ethanol is well established,” he says. “For organic acids, there are more challenges including fermentation titer and product recovery.”
Some experts consider the class of compounds known as organic acids to be one of the most promising groups of products to arise from the fermentation of biomass. A National Renewable Energy Laboratory study conducted a few years ago identified eight of the top 12 value-added chemicals from sugars as being carboxylic acids. Acetic acid is an example of a carboxylic acid. When alcohol is reacted with an acid an ester is made. One common ester in today’s renewable fuels world is biodiesel—methanol reacted with fatty acids to make methyl esters.
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Corn dry-grind ethanol producers are all too familiar with lactic and acetic acid bacteria, which stealthily infiltrate the ethanol production process and ferment sugars into acids instead of alcohol, robbing saccharomyces cerevisiae of vital nutrients and minerals, therefore reducing yield and grinding production to a halt until the contamination is under control. In a corn ethanol plant, only a couple of huge fermentors are used at giant refineries, but a lignocellulosic biochemical refinery would likely have many more fermentors, according to a subcontractors report conducted for NREL by Lynd et al, titled “Strategic Biorefinery Analysis: Analysis of Biorefineries.” It reads, “The number of fermentors in even a moderately sized biorefinery is so large—greater than 25 for many designs—that the cost of fermentation capacity does not depend on whether this capacity is devoted to one product or to several products.” Given this, a biorefinery could easily dedicate a fermentor to biochemical production of lactic, acetic or succinic acid, which can be sold on the open market as such, or reacted with a slip stream of the biorefinery’s primary product, ethanol, to make a variety of useful esters.
Ethyl Lactate Via Reactive Distillation
For companies developing ethanologens to ferment both five- and six-carbon sugars, what must be dealt with is the natural tendency for these beasts to want to produce acids. A company called TMO Renewables Ltd. developed an organism with an appetite for five- and six-carbon sugars, and “turned off” the genes in the organism that produce lactic and acetic acids.
“As you look at some of the organisms out there, some of the common products you can get that nature has already designed are fermentations to acids,” says MSU professor of chemical engineering and thermodynamics, Carl Lira. “Instead of trying to get the organism to make another product, let it make the organic acid it wants to make and then we can figure out how to convert that into other intermediates.” And that is exactly what Lira, Miller and other MSU professors, along with Richard Glass, vice president of research and development with the National Corn Growers Association, have done.
“Wouldn’t it be wonderful if we could take a product that’s currently produced by the petrochemicals industry and find a competitive peer for it—one that’s renewable, competitive and green,” Glass says. “That was our mission. We got the model system out there because ethyl lactate is commercially produced today from petrochemicals, and we know exactly what it costs because the model exists.” The MSU-NCGA project began downstream of fermentation and involved reacting separate streams of lactic acid and ethanol. Ultimately the researchers intended to license the retrofitting of existing dry-grind corn ethanol plants to diversify their narrow line of products, but it is also entirely applicable to the lignocellulosic biorefinery concept. “Our process is independent of feedstock,” Miller says. “It doesn’t matter if we use glucose from corn grain to make the lactic acid or if we use sugars from corn stover or woody biomass. The sugar stream used to make ethanol is the same sugar stream we’d use to make lactic acids.”
Ethyl lactate is an ester compound derived from reacting ethanol with lactic acid. According to Lira, ethyl lactate is not widely used today because of its high cost, but has applications in the electronics industry for micro-circuit fabrication, mainly because it’s a clean solvent. Lira says during the time he and his colleagues were working on this project, the results of which were published in 2007, the cost of producing ethyl lactate was between $1.30 and $1.60 a pound. MSU and NCGA researchers were able to cut that cost by half using a process called reactive distillation.
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