Biorefineries developed to produce ethanol from cellulose sources such as trees and fast-growing plants could get a significant economic boost from the sale of high-value chemicals—such as vanillin flavoring—that could be generated from the same feedstock.
To that end, researchers from the Georgia Institute of Technology are exploring the use of three environmentally-friendly tunable solvent and separation systems—gas-expanded liquids, supercritical fluids and near-critical water—to produce specialty chemicals, pharmaceutical precursors and flavorings, from a small portion of the ethanol feedstock. The green processes could produce chemicals worth up to $25 per pound.
It seems unlikely that fuel from a biorefinery—at least in the beginning—is going to be as cost-effective as fuel from traditional fossil sources. To make the biorefinery sustainable, we must therefore do everything we can to help the economics. If we can take a chemical stream worth only cents per pound and turn it into chemicals worth many dollars per pound, this could help make the biorefinery cost effective.—Charles Eckert, Georgia Tech
Using near-critical water and gas-expanded liquids, Eckert and his colleagues have already demonstrated the production of vanillin, syringol and syringaldehyde from a paper mill black liquor side stream. They have also proposed a process that would generate levulinic acid, glucaric acid and other chemicals from the pre-pulping of wood chips. That process would use an alcohol-carbon dioxide mixture, followed by depolymerization and dehydration in near-critical water.
Research aimed at producing high-value products from cellulose feedstocks is being done through the AtlantIC Alliance for BioPower, BioFuels and Biomaterials, a coalition of three research institutions in the United States and the United Kingdom.
The alliance, which includes Oak Ridge National Laboratory, Imperial College and Georgia Tech, seeks to solve the complex issues involved in economically producing ethanol fuel from cellulose materials such as wood chips, sawgrass, corn stovers and municipal waste. (Earlier post.)
The Alliance is taking a comprehensive approach to the biorefinery, conducting studies of how to maximize plant growth through genetic engineering, developing new microbial techniques for digesting cellulose, and applying environmentally-friendly chemical processes for reactions and separations. The organizers decided to pursue only non-food sources as their feedstock.
Using tunable solvent systems in the biorefinery would avoid the generation of wastes associated with processes that depend on strong acids—which must be neutralized at the end of the reaction.
For instance, near-critical water—familiar H2O but at 250° to 300° C under pressure—separates into acid and base components that can be used to dissolve both organic and inorganic chemicals. When the pressure is removed, the water returns to its normal properties.
Gas-expanded liquids, such as carbon dioxide in methanol, provide a flexible solvent whose properties can be adjusted by changing the pressure. When the reaction is over, the pressure is released, allowing the carbon dioxide to separate from the methanol.
Supercritical fluids, such as carbon dioxide under high pressure, simplify separation processes. Separation of the carbon dioxide from chemicals dissolved in it requires only that the pressure be reduced, allowing the CO2 to return to its gaseous state.
Eckert described the green processes today at the 232nd national meeting of the American Chemical Society in San Francisco.
In 2004, Eckert and collaborator Charles Liotta received a Presidential Green Chemistry Challenge Award for their development and promotion of benign tunable solvents that couple reaction and separation processes.