There are four cleaning methods using carbon dioxide:
Macroscopic hard and dense dry ice pellets
Softer microscopic "snow" particles
Liquid CO2 Washing Systems
Supercritical fluid carbon dioxide (SFCO2).
Whichever process is used, cleaning depends on either the liquid carbon dioxide solvent properties, the energy and momentum transfer by the impacting solid phase, or a combination of solvent properties and momentum or energy transfer. Pellet systems rely upon the thermo-mechanical impact stresses related to the high impact velocity of macroscopic pellets for contamination removal - a momentum and energy transfer process. Snow sprays rely upon a combination of solvent action of liquid CO2 and the momentum transfer of high velocity microscopic snow particles. The liquid based CO2 washing systems rely upon the liquid phase solvent properties. Finally, the SFC systems rely exclusively upon carbon dioxide's unique supercritical fluid properties.
Applied Surface Technologies has recently wrote a short review paper for the 1999 McGraw-Hill Annual Review of Technology. Please contact us for a preprint.
Pellets systems accelerate macroscopic dry ice pieces for contamination removal. Cleaning is accomplished by thermo - mechanical impact shock. The key components and steps in a pellet system are first the formation of the pellets of desired size, maintaining the pellets at the proper conditions, pellet feed, acceleration by either mechanical methods or compressed gas and then the distribution of pellets on the item being cleaned. Overall, pellet systems are made for material removal and can only address cleaning situations when the object for cleaning can withstand the expected impact.
Snow Cleaning systems rely on the expansion of either gaseous or liquid carbon dioxide. The output stream is usually a high velocity solid and gas mix and focused at the surface for cleaning. Cleaning is accomplished by a combination of momentum transfer and solvent action of the dry ice with surface contamination. Cleaning mechanisms are discussed elsewhere on the site. CO2 Snow Cleaning can remove particles of all sizes, even submicron sized particles, and also hydrocarbon based deposits and films.
The most common commercial approach to the snow cleaning technology involves single expansion nozzles with high velocity outputs. The goal within the orifice and nozzle design is to have a constant enthalpy expansion and a high velocity stream. The asymmetric Venturi nozzles (supersonic nozzles) can yield these conditions. Other nozzle geometries give rise to high velocity snow streams but are less focused, may need nitrogen boosting, or can compromise organic removal abilities.
Carbon Dioxide Snow Cleaning units from Applied Surface Technologies use the asymmetric Venturi nozzle design and generate a high velocity snow stream. With this selection, the snow spray systems can remove both particulates and organic residues and can be formed with either a liquid or gas CO2 source.
SFCO2 systems rely upon the solvent properties of CO2 and other unique properties of a superfluid. This involves maintaining the pressure and temperature in the supercritical regime, above 31 C and 72.8 atm - see Phase Diagram. Generally, the SFCO2 units operate at much higher pressures and temperatures than the critical point. The superfluid has extremely low viscosity (low surface tension) and superior solvent properties than the liquid phase. In a SFCO2 system, the items for cleaning are sealed in a vessel, the vessel is filled, and the temperature and pressure are adjusted. This method is well suited for batch cleaning of small or delicate parts or complex assemblies where the major concern is organic contamination removal. The shortcomings of SFCO2 systems are the batch nature of the process and its poor particulate contamination. Recent improvements and equipment modifications have improved particle removal but it is still a secondary process.
Liquid CO2 Washing Systems
The liquid CO2 washing systems are simpler versions of the SFC process. By using lower pressures, i.e. cylinder pressure of 800 psi, equipment design is easier and large volumes can be accepted. Of course, by operating at lower pressures and temperatures than the SFC conditions, the liquid phase solubility of the CO2 are lower than the SFC phase and the unique penetrating power of the superfluid phase is gone. With the lower pressures and easier equipment design, agitation and spin cycles have been introduced and thus particle removal may occur in certain instances. Micell, of North Carolina, has added surfactants to the liquid CO2 - making the liquid CO2 washer systems just like a weekend laundry experience. Raytheon (find link) has also introduced new liquid CO2 washing technology.