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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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Cost Comparison
Figure 6. Normalized annual costs for a 5-year accounting life of the equipment. Comparison between an aqueous cleaning process and a supercritical C02 cleaning process.
Cost Comparison
Figure 7. Normalized annual costs for a 5-year accounting life of the equipment. Comparison between a xylene solvent cleaning process and a two-step supercritical fluid cleaning process.
The initial equipment costs and operating costs for the more conventional processes are higher than for supercritical carbon dioxide. The higher equipment costs are due to the additional parts needed for environmental treatments (i.e., scrubbers, vapor incinerators, etc.) The consumables costs are lower for the supercritical fluid processes due to the closed-loop recycle design of the supercritical fluid system, elimination of water for rinsing and the reduced electricity costs associated with not having to dry the parts.
For the two cases we studied, supercritical fluid cleaning processes resulted in a lower cost of ownership. This cannot be generalized to all processes, but it does indicate that the overall operational costs of supercritical fluid processes can be competitive with other cleaning processes.
Process and equipment developments continue to make supercritical carbon dioxide cleaning a more competitive option and to expand the applications for this process. These developments are primarily aimed at reducing the requirements for continuous carbon dioxide flow, producing effective cleaning at lower temperatures and pressures, and the construction of equipment with less expensive materials. These objectives are being reached primarily because of improved understanding of the effectiveness of carbon dioxide as a solvent and the required performance of the equipment. It appears that the dual-cycle (two step) process will continue to receive primary attention as the most effective supercritical carbon dioxide alternative to conventional cleaning methods.
The authors would like to thank Roger Hoffman from Vision Inc., Duluth, GA for his assistance and helpful comments and Laura Rothman and Mike H. S. Tseng of IBM for their contribution to the SEMATECH cost of ownership model.
1. Phelps, M. R. Hogan, M. O. and Silva, L. J. PNL-SA-24365, Pacific Northwest Laboratory, Richland, WA (1994).
2. Barton, J., LA-12786, Los Alamos National Laboratory, Los Alamos, NM (1994)
3. TWO COOL by Wright, Williams and Kelly, 11875 Dublin Boulevard, Suite D262, Dublin, CA 94568
4. Phelps, M. R., Hogan, . O., and Snowden-Swan, L. J. (Pacific Northwest Laboratory), Barton, J. C., Spall, W. D. and Williams, S. B., (Los Alamos National Laboratory), Waste Reduction Using Carbon Dioxide: A Solvent Substitute For Precision Cleaning Applications, Precision Cleaning 95 Proceedings (1995)
A Practical Guide to Supercritical Fluid Cleaning
Robert J. Purtell
Why would you want to do supercritical fluid cleaning? In fact, what is a supercritical fluid (SCF)? Is it a supercold, mysterious fluid that physicists study? What is it good for? What can you dissolve in it: organics, salts, particles, resins? Why not just use water and add a little detergent? These are some of the questions which have been discussed by authors in previous chapters and are reviewed in this chapter.
First of all, the term supercritical fluid does not refer to superfluid helium, a state of matter for 3He and 4He near absolute zero.!1! As shown in the phase diagram in Ch. 1, a supercritical fluid refers to the state of matter above its critical point. The critical point is a temperature and pressure, Tc and Pc, at which two phases of a substance in equilibrium with each other become identical, forming one phase. Above the critical temperature, Tc, a substance can not
exist in the liquid phase no matter how much pressure is applied. Likewise, the critical pressure, Pc, is the pressure above which a fluid can not be liquefied by increasing the pressure for temperatures greater than Tc.
Typical materials that might be used for supercritical fluid cleaning or extraction are given in Ch. 1 by Manivannan and Sawan. The only fluid that will be discussed in this chapter, however, is carbon dioxide. The combination of low critical temperature and pressure, 31.7C and 72 atm, nontoxicity, and low cost make it an ideal candidate for cleaning applications.
Why use a supercritical fluid for cleaning, especially since elevated pressures are required? One reason is that supercritical fluids offer a combination of liquid-like solubilities and gas-like transport properties which are superior to many liquids. The economic viability of a cleaning process often depends on how much solvent is needed to remove a given amount of material and how much energy must be added to the system to remove the dirt from the part. The low cost and ease of recyclability of SCF C02 make it an excellent choice for cleaning applications.
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