"Precision Cleaning - The Magazine of Critical Cleaning Technology"
Parts Cleaning

Surfactant Solutions Advance Liquid CO2 Cleaning Potentials

by: Charles H. Darvin R. Bradley Lienhart
Pages: 25 - 27; February, 1998

Following the phase-out of many cleaning chemicals, industry has been struggling to find suitable and efficient alternatives. Most are addressing the challenge of surface cleaning by depending on their suppliers for an answer, utilizing "no-clean" options where possible, or, to a large extent, by attempting to find workable aqueous cleaning processes. While significant improvements have been made in aqueous technology, it is clear that a number of users are still looking for solvent alternatives that perform chemically and mechanically like more familiar solvents.

The potential of carbon dioxide (CO2) as an extractive solvent has been known and exploited in the food processing and pharmaceutical industry for a number of years. One common process using supercritical CO2 extraction is the decaffeination of coffee and tea.1

Since the early 1980s, research has been underway to investigate the use of supercritical CO2 as a solvent substitute in the polymerization of hydrocarbon monomers.2 A significant outcome of that research was that CO2 might also be applicable as a surface-cleaning agent. However, CO2 alone is a poor solvent and will not dissolve a broad range of typical contaminants. Thus, subsequent research has concentrated on the development of additives to enhance the solvent capability of CO2. This has resulted in the development of a class of CO2-compatible surfactants that may make the technology a viable and nonpolluting surface-cleaning option.

 

Not So Aggressive

CO2 is a common, non-ozone-depleting compound that is nontoxic, nonflammable, and recyclable. It can be acquired as a by-product gas from various production processes; thus, it does not add to or encourage greenhouse gas. In either the supercritical or liquid phase it has the capability to solubilize certain classes of compounds.

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Unfortunately, CO2 in either the supercritical or liquid state is a relatively weak solvent for most contaminants. Larger molecular-weight molecules such as heavy oils, greases, waxes, and polymers are generally not soluble in CO2.3 Thus, it alone is incapable of removing many contaminants encountered in typical manufacturing operations. Even when compressed to extremely high pressures to form the supercritical state, CO2 has weaker solvency power than those of typical hydrocarbon solvents.4

 

Surpassing the Supercritical

Most past research and development activities by various agencies have focused primarily on the use of supercritical CO2 as the cleaning medium. The physical properties of supercritical CO2 are similar to those of conventional liquid solvent systems. In the supercritical state, however, its viscosity, surface tension, and diffusion properties are more characteristic of a gas. Thus, unlike a liquid, it will not readily transfer mechanical energy within the system to enhance its cleaning efficiency.5 This generally rules out the use of mechanical enhancement devices in a supercritical CO2 system to improve its cleaning capability.

Liquid CO2 (LCO2), however, may provide greater advantages for surface cleaning. This is due to the functional benefits that a liquid system can provide in cleaning and in cleaning equipment specifications. Although supercritical CO2 has a marginally greater solvent capability than LCO2, neither medium using virgin carbon dioxide has been shown to have broad potential for widespread surface-cleaning applications.

In a LCO2 cleaning system, however, efficiency can be improved by incorporating mechanical - or sonic - agitation or centrifugal forces to the liquid. In addition, the liquid system can penetrate small and complex spaces and openings similar to conventional solvent systems. The liquid system has a lower energy consumption and smaller equipment footprint due to the lower pressures compared to the supercritical system.

Liquid systems will operate at ambient temperatures and at pressures of only 800 to 1200 psi (5.5 to 7.6 MPa), which most components can withstand without damage. Supercritical systems operate at 1200 psi (7.6 MPa) and higher, resulting in significantly greater equipment weights and footprints.

 

Production Partner

The key to the eventual use of CO2 for general cleaning is the expansion of its capability to address the broad spectrum of contaminants typically found in a manufacturing environment. In chlorinated- and aqueous-based systems, cleaning efficiency is enhanced by the addition of surfactants, detergents, or other chemical stabilizers.

The newly developed class of CO2-compatible surfactants serves the same purpose that detergents provide to conventional cleaning systems. However, the CO2 medium must be capable of removing all potential contaminants in one system - which these surfactants make possible.

The LCO2 cleaning process is performed as a batch operation. Contaminated items are placed in a chamber, sealed, cleaned, and removed. Within the sealed system the contaminants removed from the surface are collected and recycled by decreasing the pressure to convert the LCO2 to its natural gas state and allowing the contaminants to precipitate and fall to a collection reservoir. The recovered CO2 gas is then repressurized to the liquid state and recycled to the system for further use. Figure 1 presents the process steps in an LCO2 cleaning system.

 

Surfactant Solutions

In 1990, working under the leadership of Dr. Joseph DeSimone of the Chemistry Department at the University of North Carolina at Chapel Hill, research colleagues at several other universities made a major breakthrough in the development of CO2-surfactant chemistry. Their research to polymerize hydrocarbon monomers in supercritical CO2 led to the development and synthesis of a class of unique CO2-compatible, or

-philic, surfactants. This was significant since conventional surfactants are not compatible with CO2.

Further research also led to the development and synthesis of a class of compatible carbon dioxide-phobic surfactants. The synthesis of these hybrid surfactant molecules has opened the door to the potential use of CO2, in the liquid or supercritical state, as a viable solvent substitute. It is also apparent that the solution to surface cleaning using carbon dioxide, as with dispersion polymerization using CO2, is a surfactant system which enhances the efficiency of the solvent to emulsify the target molecule.

Laboratory testing at MiCell Technologies (Raleigh, NC) and North Carolina State University's Chemical Engineering Department has demonstrated significant cleaning performance improvement using LCO2 and candidate CO2-compatible surfactants on various contaminants and material surfaces.

Surfactant packages are being developed, tailored, and tested for various cleaning requirements and specifications. To date, the synthesis and production of these surfactants is considered too costly for economical application. However, continued research should develop both the surfactant manufacturing process and commercial markets

for LCO2 technology.

 

References

1. Krukanis, V. "Processing of Polymers With Supercritical Fluids," Polymer News, Vol. 11, 1985.

2. DeSimone, J.M., Maury, E.E., Menceloglu, Y.Z., McClain, J.B., Romack, T.J., and Aombes, J.R. "Dispersion Polymerizations in Supercritical Carbon Dioxide," Science, Vol. 265, July 15, 1994.

3. Beckman, E.J. "Carbon Dioxide Extraction of Biomolecules," Science, Vol. 271, February 2, 1996.

4. Rotman, D. "CO2 Challenges Organic Solvents," Chemical Week, January 1-8, 1996.

5. Cline, C.M., "Emerging Technology; Emerging Markets," Precision Cleaning, Vol. IV, No. 10, October 1996, pp. 11-19.

 

About the Authors

R. Bradley Lienhart, president and CEO of MiCell Technologies (Raleigh, NC), started his 28-year career with the Dow Chemical Company and founded their Advanced Cleaning Systems Company in 1991. Upon retirement, he became president and COO of Corpex Technologies before joining MiCell in 1996. Lienhart also served as the first managing director of the Chlorine Chemistry Council, a proactive arm of the Chemical Manufacturers Association.

Charles H. Darvin has been a research engineer for the U.S. Environmental Protection Agency in Research Triangle Park, NC, since 1976, specializing in industrial process development for surface cleaning and coating. He holds a bachelor's degree in mechanical engineering from the University of Evansville, IN. Darvin has been awarded a patent in the area of surface coating and one silver and two bronze medals from the EPA for his research programs. He has written numerous applied research papers.


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