CO2 Processing Technology Overview
CO2 as a Solvent, Coolant, and Lubricant
It has been known for more than 80 years that CO2 behaves in a manner similar to an organic “solvent” when compressed to liquid-like densities and as an organic “solute” when compressed into and mixed with another organic solid or liquid. Examples include CO2 plasticization of polymers such as low density polyethylene (LDPE) and solid CO2 mixed into and complexed with cold hydrocarbon solvents. CO2 modifies the physical and chemical properties of solids and liquids having similar cohesion energies, for example expanding organic solvents to reduce viscosity and increasing permeability of gases into polymers.CO2 is also useful as a one-carbon (C1) feedstock for forming useful organic compounds in industry. From a physicochemical perspective, CO2 behaves much like a low molecular weight version of a fluorocarbon with a Hildebrand solubility parameter ranging from 14 MPa1/2 to 22 MPa1/2, which is dependent upon its phase, pressure, and temperature. CO2 can be compressed to a range of liquid-like densities, yet it will retain the diffusivity of a gas with extremely low viscosity. CO2 cleaning agent densities may be adjusted from 0.5 g/cm3 to 0.9 g/cm3 with surface tensions of 0 dynes/cm (supercritical state) and 5 dynes/cm (liquid phase). Moreover, CO2 can be cooled (boiled) to form solid crystals or particles having a density of 1.6 g/cm3, similar to many fluorocarbon solvents [2-2 – 2-3].
Practical processing benefits of CO2 solvent properties is rapid (or even spontaneous) penetration into, wetting and expansion of a surface contamination such as an oil film, or an organic pre-wash solvent, and access to microscopic pores, crevices and gaps present on or within complex substrates. However, a limitation of dense phase CO2 (liquid and supercritical fluid) is weak solvency (or high selectivity) with regards to complex contamination such as water-soluble and RMA flux residues, waxes, heavy greases, and inorganic contamination such as particles and ionic residues. This is related in part to its extremely low Kauri-Butanol (KB) value (solvent power), estimated to be between 20 and 30; also similar to many fluorocarbon solvents. Still moreover, CO2 gas can be partially ionized using a strong electromagnetic field to form low-pressure or atmospheric plasma. CO2 plasma, considered the 4th state of matter, provides thermal energy, ultraviolet radiation, electrons, and oxygenated ions that promote surface cleaning and functionalization. As such, a veritable tool box of surface processing capabilities are provided by solid, liquid, plasma and supercritical-state CO2, and combinations of same.
CO2 is also an excellent refrigeration agent, termed R744. It is used in both closed-cycle heat pumps and open-cycle refrigeration systems such as Joule Thomson (JT) expansion coolers and gas-solid cooling sprays. Close-cycle R774 refrigeration systems - using mixed fluid (ammonia-CO2) and transcritical (liquid-supercritical) fluids - have been employed in the early part of the last century but fell out of favor due to the development of lower-pressure and (at the time) safer CFC-based refrigerants. However, due to more recent ecological concerns regarding CFCs and the advancement of high pressure compressor technology, R744 refrigeration technology is gaining traction worldwide. Discussed in this blog and eBook, CO2 technology is used as an open-cycle spray cooling process to manage thermal contamination in hard machining and thermal spray operations. Although open-cycle R744 refrigeration is an exclusive topic, current research and development work in adjunct technology such as CO2 purification, gas blending, and charging systems for closed-cycle R744 systems will be included as a subtopic in a future edition.
References:
2-2. Bos A. et al. “CO2-induced Plasticization Phenomena in Glassy Polymers”, Journal of Membrane Science, 1999, Vol. 155, pp. 67-78
2-3. Myers, A.L. and Prausnitz, J. M., Thermodynamics of solid carbon dioxide solubility in liquid solvents at low temperatures, Ind. Eng. Chem. Fund., 4, 209, 1965
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