Author/s: Samuel K. Moore
Fluoropolymers pilot plant at DuPont's Fayetteville, NC site, announced this spring, will constitute the largest application of supercritical carbon dioxide (scCO2) as a chemical reaction medium to date. Supercritical fluids are popular solvents for large-scale extraction processes, but even proponents acknowledge that they will never become the environmentally friendly wonder-solvents once promised.
"Overly optimistic forecasts and technical announcements have rendered potential users rather skeptical," says Michel Perrut, president of scCO2 process developer
Separex (Champigneulles, France). Chemists are churning out an increasing stream of reactions to perform in supercritical fluids, but few combine the efficiency, selectivity, and environmental or regulatory benefit needed to entice industry investment.
Industrial use of scCO2 and other supercritical fluids-including propane, butane, and water-has advanced farthest in separations such as extraction of natural ingredients and deasphaltization of petroleum. scCO2 has flourished in separations because of pressure to replace organic solvents. It has supplanted trichloro-ethylene in coffee decaffeination and is favored for extracting high-value fine chemicals such as pharmaceutical actives. scCO2 is challenging perchloroethylene in dry cleaning, helped by technology from Micell Technologies (Raleigh, NC), which was founded by University of North Carolina at Chapel Hill chemist Joseph DeSimone in 1997. DeSimone invented DuPont's fluoropolymers process (CW, Dec. 24, 1997, p. 30).
Compared to extraction, the adoption of scCO2 as a reaction medium has been "incredibly slow," says Joan Brennecke, professor of chemical engineering at Notre Dame University (Notre Dame, IN). Brennecke says that solvent replacement is not a sufficient driver for the chemical industry: scCO2 must also offer cost reductions or other benefits, such as improved selectivity.
Part of the delay stems from an incomplete understanding of the details of certain reactions, such as selectivity and reaction rates. "There are many things people don't fully understand," Brennecke says. "That makes them worry."
Efforts to apply the technology to production of high-volume chemicals failed, reorienting supercritical fluids researchers towards niche, high-value applications. "What people were trying to achieve a decade ago and what they are trying to achieve now are quite different," says Erdogan Kiran, a chemical engineering professor at the University of Maine (Orono, ME) and editor of The Journal of Supercritical Fluids.
The expense of reactors is also helping steer supercritical fluids toward high-value products. "Pressure costs money," says Brennecke. However, development of continuous processes should reduce costs compared to batch operations. Supercritical lab- and pilot-scale reactor makers, such as Applied Separations (Allentown, PA), are also lowering research costs. Lab-scale supercritical reactors have fallen from about $60,000 to $15,000 over the past decade, says Applied Separations president Rolf Schlake.
Specialty chemicals maker Thomas Swan (Consett, U.K.) is one of the few companies committed to scCO2 reactions, hoping to accelerate their expansion into pharmaceutical intermediates. The company teamed with Degussa-Huls and Nottingham University (Nottingham, U.K.) chemist Martyn Poliakoff to design continuous scCO2 hydrogenation and alkylation processes. Swedish process engineering firm Chematur is building a 500 m.t.-1,000 m.t./year pilot reactor for Swan to test the technology (CW, Dec. 2, 1998, p. 58).
Toll processing firm Phasex (Lawrence, MA) has used scCO2 for separations and extractions, but president Val Krukonis says the company is also using the fluids to replace conventional solvents in two reactions. One, the formation of a reactive multi-acrylate monomer, also offers improved yield.
DuPont's scCO2 process for fluoropolymers including polytetrafluoroethylene will replace hydrofluorocarbon solvents, which are potent greenhouse gases. (Previously, DuPont's process used ozone- depleting chlorofluorocarbon solvents.) "We also believe the operating costs will be lower," says Everett Baucom, technical manager/fluoropolymers. "How much lower is yet to be determined."
In the scCO2 process, fluoropolymers fall out of solution as they polymerize.
DeSimone says that the technology is attractive partly because fluoropolymer synthesis has unusual reactants. "The active species in the process is a radical so reactive that the polymerization can't be done in a solvent with any hydrogen present," he says. "There's a short list of those solvents."
But DeSimone maintains that scCO2 holds potential for other large-scale polymerizations that require an energy-intensive drying step, including polyvinyl chloride (PVC) and polyacrylic acid (PAA), because scCO2 can be driven out of the polymer far easier than water. He estimates that PVC makers could save 1 trillion btu/year using scCO2.
However, several polymer producers say efforts to apply the technology have fizzled. Geon says it considered PVC polymerization in scCO2 but decided not to pursue it because of an unattractive preliminary economic analysis.
Dow Chemical did some research on supercritical PAA processes but has since dropped the project. Ciba Specialty Chemicals explored supercritical fluids to extract residual monomers from PAA and other water-soluble polymers but discovered that the monomers were insufficiently soluble. "There was not much done on polymerizing directly in scCO2 mainly because of the monomer solubility problem," says Ian Johnson, Ciba's U.K. director/polymer technology.
Chemists have taken two paths to solve scCO2's solubility problem. DeSimone champions the use of surfactants to dissolve the reactants, while the other method uses a mixture of scCO2 and organic solvents such as toluene or ethanol. "At the end of the process you can drive off the unwanted solvent and unreacted monomer by decompressing the scCO2," says Kiran.
Another potential scCO2 extender may be ionic liquids, molten salts that have been hyped as alternative reaction media. Ionic liquids can dissolve many of the polar compounds scCO2 rejects, and Brennecke recently demonstrated that scCO2 can extend the use of ionic liquids to the synthesis of hydrophobic products (CW, May 12, p. 47).
Brennecke says companies including fluoropolymers giant Elf Atochem and UOP have shown interest in combined ionic liquid-scCO2 systems. However, commercial use of ionic liquids as reaction media is at least five to 10 years off, she says.
"It wasn't until 1992 that the first air- and water-stable liquids were developed," says Brennecke. "You can't even buy these things commercially yet."