Cosolvent systems supercritical carbon dioxide

System Type. The three basic types of SCF cleaning systems are the direct system, in which supercritical carbon dioxide (SCCO2) is the only cleaning solvent, the single-cycle system, in which the SCCO2 is mixed with a cosolvent and the dual-cycle system in which the parts are exposed separately to cosolvent and supercritical carbon dioxide. 

Gii lu-Ustundag, O. and Temelli, F. 2005. Solubility behavior of ternary systems of hpids, cosolvents and supercritical carbon dioxide and processing aspects, J. Supercrit. Fluids 36 1-15. 

The reaction mixture contains a dilute solution of benzophenone solute, a few mole percent isopropanol and the remainder supercritical carbon dioxide solvent. This is analogous to a solute dissolved in a supercritical fluid/cosolvent mixture. These types of systems are important because in many applications researchers have found that the addition of a small amount of cosolvent (such as acetone or an alcohol) of volatility intermediate between that of the solute and the SCF can greatly enhance the solubility of the solute (Van Alsten, 1986). 

Enzymatic oxidation in a supercritical fluid medium has also been demonstrated for the formation of cholest-4-ene-3-one from cholesterol in supercritical carbon dioxide by Randolph et aL [32]. Many cholesterol oxidases showed catalytic activity. That isolated from Streptomyces spp. gradually degraded, but gave reaction rates comparable with those sustained in aqueous systems whilst it was yet catalytic. That from Gloecysticum chrysocrea is chemically stable at 35" C and 100 bar, and yielded reaction rates 75 times faster than in water (5 x 10 phosphate, pH 7, and 5% v/v propanol [34]) which were further increased by the addition of aggregating cosolvents. 

Supercritical fluids such as carbon dioxide can be used as solvents to extract organic compounds from aqueous solutions. In order to achieve recoveries of these products often in low concentration, cosolvents as methanol or other alcohols have been added to improve the solubility and the selectivity of the primary fluid. To optimize the extract recovery, the knowledge of phase equilibria of the ternary system carbon dioxide-methanol-water is required at different temperatures and pressures. 

Filters. Proper filtration is necessary to protect the equipment, the environment, and to prevent recontamination of the work. Good filtration adds to the initial cost, but saves maintenance cost as well as increasing overall system reliability. The filters must be compatible with the temperature and pressure of the system and installed with bypass circuits to facilitate periodic replacement. They must be fabricated from material compatible with the carbon dioxide in the supercritical, gaseous and liquid forms as well as any cosolvent and the contaminant to be removed. There are four areas where filtration is required for the safe and efficient operation of the cleaning system ... 

Phase Equilibria. From recent research (Schneider and Peters) it became apparent that in the near-critical region of certain ternary carbon dioxide mixtures, due to co-solvency effects of the two solutes relative to each other, the fluid multiphase behavior can be quite complex. Phenomena like immiscibility windows and holes are not unusual, which have their consequences for separations in near-critical processing. Peters stressed that for many applications in supercritical technology carbon dioxide is not an appropriate choice since for many solutes it is a poor solvent that would require the use of a cosolvents. If safety and environmental constraints permit, it is certainly worthwhile to consider alternatives for carbon dioxide. Gulari, Schneider and Peters emphasized the importance of studying representative model systems in order to obtain insight into the systematic variations of the complex phase behavior that may occur in near-critical multicomponent mixtures. Debenedetti stressed the importance of focusing on complex fluids like emulsions. 

Finally, it should be stated that ethyl lactate is a novel ecofriendly solvent for potential applications in supercritical fluid technology, as a cosolvent of carbon dioxide. Ethyl lactate can be readily dissolved in CO2 in the amounts usually employed for supercritical extraction processes (15-20 wt% or lower). Additionally, at the typical temperatures employed (35-70°C) the ethyl lactate -1- CO2 system presents homogeneous (single) phase at relative low pressures. 

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