Where oil and water mix: Researchers explore use of near-critical water for replacing conventional solvents

August 25, 1999

Under normal conditions, oil and water don't mix. But "near-critical" water -- very hot but still liquid water at temperatures of 250 to 300 degrees C and pressures of 1,000 psi -- can be a good solvent for both salts and non-polar organic compounds, including oils. This makes ordinary water an ideal reaction solvent for certain chemical processes.

Researchers at the Georgia Institute of Technology are studying a wide range of chemical processes in search of applications where the special properties of this "near-critical water" might provide both economic and environmental advantages. Their work could lead to replacement of traditional organic solvents in certain specialty chemical processes.

"Our goal is to do the technical work to see where we can use this as a replacement process, and to couple that with an economic analysis to see where this can be used profitably," said Dr. Charles Eckert, director of Georgia Tech's Specialty Separations Center.

Certain types of chemical reactions operate well in near-critical water, and would be top candidates for the new process, he said. Use of water as a solvent could also be attractive for processes in which all traces of hazardous solvents must be removed -- such as pharmaceuticals.

Details of the work will be presented August 24 at the 218th American Chemical Society national meeting in New Orleans. In addition to Eckert, the research team includes Dr. Charles Liotta, Georgia Tech's Vice-Provost for Research and a professor of chemistry; Roger Glaser, a postdoctoral fellow, and two Ph.D. students: James Brown and Shane Nolen.

"Water is about as ideal a solvent as you could imagine," Eckert noted. "Not only is it benign, but the public perception is that it is benign."

Both the benign nature of water and its potential as a powerful solvent depend on its unique system of hydrogen bonding. As water is heated, its normally strong hydrogen bonds weaken, allowing dissociation that forms acidic hydronium ions and basic hydroxide ions. At the near-critical stage, the amount of dissociation is three times what it would be at normal temperatures and pressures.

"Simply from the dissociation of water into acidic and basic ions, a much larger amount of acid and base is present in near-critical water," Eckert explained. "We can use these to run acid-catalyzed and base-catalyzed reactions without the addition of mineral acid."

Near-critical water has properties similar to those of polar organic solvents like ethyl alcohol or acetone. Its dielectric constant drops from 80 to 20, and its density drops from one gram per cubic centimeter to 7/10 gram per cubic centimeter.

"What all this means is that molecules that would normally not be soluble in the same solvent become soluble together in near-critical water and can be processed together," he added. "Virtually all organics are soluble or completely miscible in water above about 250 degrees C."

Reactions performed using both oil and cold water simultaneously require rapid stirring or hazardous additives for good oil water contact. Dissolving organics in near-critical water allows some reactions now done in multiple phases to be completed in a single aqueous phase. This eliminates high cost and energy associated with stirring rapidly and separating unwanted additives from the final product .

Near-critical water also offers advantages in the most costly part of most chemical processes: separation of the chemical products. In near-critical processes, reaction products can be removed by cooling the water and reducing the pressure, dropping them out of solution where they can be decanted. Water-soluble catalysts remaining in the water can be re-used, Eckert said.

Use of near-critical water could also eliminate production of salts that result from the neutralization of acids used in conventional separations. Disposal of these salts can be a significant environmental issue, he added.

The Georgia Tech research team has learned to "tune" the reactions by varying the water temperature and pressure. That has allowed them to run a number of important chemical reactions in near-critical water, including condensations, ester-hydrolyses -- and Friedel-Crafts acylations. The latter were performed without added acids, neutralization steps, or production of salt byproducts.

Though they have found some reactions that work well in near-critical water, the researchers have also noted others that proceed too slowly for commercial use without addition of acids.

"We don't yet know which reactions will be worthwhile to do in near-critical water," Eckert noted. "They will not all work. The point of our research is to find out what we can do in this medium and sift through what is most worthwhile."

However, even processes with environmental or economic advantages in near-critical water face a critical obstacle: the large industrial investment in conventional processes. Companies cannot afford to abandon an existing manufacturing facility just because a more environmentally-benign process has been developed. For that reason, Eckert believes the near-critical processes will be used first in low-volume applications for non-commodity chemicals.

For more conventional processes, the higher cost of maintaining near-critical temperatures and pressures may be offset by the cost savings of eliminating hazardous organic solvents or salt byproducts that must removed from the product and disposed.

Use of near-critical water as a solvent is a relatively new science begun in the 1990s by researchers at several institutions. However, supercritical water -- produced at higher temperatures and pressures -- has been studied for nearly 25 years as a means of destroying hazardous materials.

Study of near-critical water for chemical processing is part of a larger Georgia Tech initiative aimed at applying sustainable technology to broad areas of manufacturing. Explained Eckert, "Sustainable development to us means producing something that has both economic and environmental advantage, then doing the technology transfer necessary to get someone to use it."

Work on near-critical water has been sponsored by the Georgia Research Alliance, the U.S. National Science Foundation, the U.S. Environmental Protection Agency, the Deutsche Forschungsgemeinschaft, Hoechst Celanese, and Georgia Tech's J. Erskine Love chair.

EDITOR'S NOTE: Following is an explanation of the term "near-critical water" and how it is differentiated from "supercritical water."

For every chemical substance, there exists a unique combination of temperature and pressure where the gas and liquid phases become identical. If a closed container half full of liquid water and half full of water vapor is heated, the liquid gradually becomes less dense and the vapor more dense as the temperature increases. At 374 degrees C and 220 bars of pressure, the two phases become identical.

Any point above that is called "supercritical." Significant amounts of work have been done on developing detoxification techniques using the special properties of this supercritical water, typically at 400-500 C and 200-500 bars.

Just below the critical point, water has different but still very useful properties. Georgia Tech researchers have studied "near critical" water at 250-300 C and pressures of 50-100 bars.
Technical Contact: Dr. Charles Eckert (404-894-7070);
E-mail: charles.eckert@che.gatech.edu.

Georgia Institute of Technology

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