Biorefineries built on the biochemical conversion platform can take advantage of their fermentative capacity to produce various organic acids, which can then be reacted with ethanol to make a number of different higher-valued ester compounds.
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.
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.
"Reactive distillation has been around for a long time-it's not new," Glass tells Biomass Magazine. "But what is new is our application. Generally the problems with reactions is that to separate them you have to distill them, and when you distill them you have boiling points that are very close together, very hard to separate. But reactive distillation allows us to produce compounds called hemiacetals, which are stable in the system and have boiling points completely different than what they might be for the original chemical." Thus, once distillation is complete the hemiacetal can be broken and the pure compound recovered with a high percent of purity.
Glass calls the chemistry involved in reactive distillation "elegant" because it is unusually effective and simple. Miller says from equipment and energy standpoints, reactive distillation is an efficient way to carry out a number of chemical reactions. "If you look at conventional processing, the reaction goes part way then stops, part way and stops, and the idea with reactive distillation is you keep pushing the reaction all the way until it's complete," Miller says.
At least two feeds are used in the reactive distillation column, with the least volatile reactant, the acid, entering the top and the more volatile ethanol entering the bottom. The goal is to provide a reactive zone where the ester and byproduct water move in opposite directions in the column. This process requires two columns because in solution lactic acid forms oligomers, and the researchers note that accurate modeling of oligomer behavior and mixture phase equilibria are integral aspects of this particular project's design. NCGA currently seeks companies that might be interested in buying a license for this ethyl lactate production process, which can be retrofitted into a dry-grind ethanol plant.
Markets and Product Diversity
"If all you produce from a biorefinery is ethanol, that is fine for a nascent industry but, in essence, all you have is a one-trick pony," Glass says. "My dream is the integrated biorefinery where the only limits are your imagination and ability to make the system." Lira says ethanol refineries are one-dimensional. "In a biorefinery you really want that diversity," he says. "But now much of the effort is to make a single organism-to make a single product-because that simplifies separation downstream, and if you can make only ethanol then you can convert the design into a turnkey system." The word "turnkey" hasn't been associated with the biomass ethanol refineries as it has in the starch-based ethanol industry.
A Classic Example: Methyl Acetate Versus Ethyl Lactate
This diagram compares the major differences of process flow and design used in petrochemical manufacturing of ethyl lactate, left, versus the much simpler reactive distillation process, right.
source: MICHIGAN STATE UNIVERSITY
Glass says from a 25 MMgy ethanol plant, an ethanol side stream of 1 MMgy diverted to make chemicals like ethyl lactate could bring in the same amount of revenue as the remaining 24 MMgy of ethanol sold as fuel. One might wonder why not divert more ethanol and produce even more value-added chemicals instead of producing the lower-valued ethanol. The problem with this is that the markets are fragile and what some might consider small increases in production could drive prices way down. Ethyl lactate use in the United States, for instance, is between 10 million and 20 million pounds a year, and sells for about $1.50 a pound according to MSU. "Keep in mind there are some big players out there and if you come in and try to take their market away from them, what are they going to try to do?" Glass poses. "They're not going to be happy." Natureworks LLC operates the world's largest polylactic acid plant in Blair, Neb., which produces 300 million pounds per year.
According to a report titled, "Succinic Acid Production and Market in China," China produces about a quarter to a third of the world's succinic acid. The report says major production methods in play in today's China are electrochemical reduction and hydrogenation. Globally most succinic acid is produced via fermentation, "which can significantly reduce the manufacturing cost," the report states. Lira says much of the acetic acid produced today is made from methanol feedstock-methane to methanol to acetic acid. "Acetic acid can be made by fermentation too, but I'm not sure how cost competitive it is with the petroleum process," Lira says. "We're working now on succinic acids but I can't share details on that yet because work is ongoing." The work entails esterifying succinic acid with ethanol to make diethyl succinate.
Given that many of these acids are produced through methods other than fermentation, this will provide a great opportunity for biorefineries built on the biochemical platform to diversify their product streams. But developing new markets for organic acids and ester compounds would be a critical component of this approach. "The big lesson we're learning is defining the markets," Lira says. "That is going to be the big challenge-breaking into the markets, finding companies to be the first one to take advantage of these opportunities."
Ron Kotrba is a Biomass Magazine senior writer. Reach him at firstname.lastname@example.org or (701) 738-4942.