Supercritical fluid parts cleaning

This chapter discusses general parameters, processes, and procedures for scaling up supercritical fluid parts cleaning (SFPC) vessels at the mechanistic level. A section is also provided which includes work at Pacific Northwest Laboratory and... 

Figure 1. Supercritical fluids parts cleaning test stand shown with recycle.

For supercritical fluid parts cleaning, the geometry will be more complex than the case of a flat plate. The cleaning system will likely contain a variety of objects with complex shapes. A simplification of this case would be a system packed with uniformly shaped objects. For this simplification, an empirical expression similar to that used for packed beds may be developed ... 

The following discussion introduces the considerations applicable to initial design and subsequent scaleup of any supercritical or near-critical fluids parts cleaning vessel, then centers on dimensionless variables and principles for applying these variables to actual scaleup of a parts cleaning vessel. The last part of the discussion focuses on the application of these concepts to an agitated, supercritical carbon dioxide, parts cleaning vessel. 

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 CO2 make it an excellent choice for cleaning applications. 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

Green chemistry can be applied to devices and processes other than automobiles to reduce smogforming emissions. This is especially true in the area of organic solvents used for parts cleaning and other industrial operations, vapors of which are often released into the atmosphere. The substitution of water with proper additives or even the use of supercritical carbon dioxide fluid can eliminate such emissions. 

Supercritical fluid carbon dioxide is an excellent solvent for organic solutes. This has led to its uses in place of organochlorine solvents for cleaning metal parts and in dry cleaning. A major advantage of supercritical fluid carbon dioxide in some... 

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