Solvent engineering of compressed and supercritical fluid solvents for bioprocessing applications

Supercritical fluids (SCFs) make good bioprocessing solvents because of their excellent mass transfer characteristics, 'tunable' solvent power, near-ambient processing temperatures, and easy and complete removal from the final product. The use of near and supercritical fluid technology for the processing of model proteins, pharmaceuticals, and biodegradable polymers is investigated to develop novel products and improve fundamental understanding of existing processes.; The enzymatic catalysis of a model reaction, the esterification of cholesterol using vinyl acetate (VA), in pressurized hexane and SCF ethane was studied. The initial reaction rate in pressurized hexane varied linearly with the cholesterol concentration. Thus, the increase in solubility of cholesterol in SCF ethane because of the cosolvent effect of VA also increased the rate in ethane. The reaction rate was also a function of pressure---the effect being significantly more pronounced in ethane than in hexane. The reactions were carried out at constant water activity, making this the first valid comparison of enzymatic catalysis in pressurized organic and supercritical media. The results suggested that the reaction is faster in SCF ethane than in hexane.; Precipitation using a compressed antisolvent (PCA) is a demonstrated technology for microparticle formation. The extension of PCA to the recovery of dry protein powders from aqueous-organic solutions was investigated. Lysozyme, chymotrypsin, and trypsin were precipitated from DMSO solutions containing up to 10 vol% water. The protein morphology was a function of water content. The precipitation pressure increased with water content and varied with protein type, suggesting the use of PCA as a separation technique.; Microparticle precipitation using CO2-philic antisolvents was studied to improve fundamental understanding of PCA. The ability of fluorinated CO2-philic liquid antisolvents to micronize small solutes (griseofulvin) and polymeric systems (poly(lactic acid), PLA) was compared to precipitation with CO2 and traditional antisolvents. Analogies are made between PCA and CO2-philic antisolvent precipitation based on thermodynamic driving forces and the dynamics of the spray process.; A mathematical model was developed to describe the dissolution of a solvent droplet suspended in a miscible antisolvent continuum. The governing differential equations were based on two-way mass transfer and unsteady state mass balances. The model correctly predicts that saturation and, hence, precipitation occurs earlier in liquid CO2 compared to other liquid antisolvents.