| Hollow fiber membrane contactors offer a number of advantages over dispersed phase contactors for extraction of aqueous feeds. In addition, dense gases provide benefits that traditional extraction solvents do not. A mathematical model of a membrane contactor was developed that predicts the steady state fluid velocities and solute concentrations by solving the applicable conservation equations, with the gravitational force term included. Model predictions were compared to experimental data obtained in our laboratory for the extraction of isopropanol or acetone into dense CO2, and to data reported by others for extraction of various solutes into dense CO2 or propane.; Generally, predicted mass transfer coefficients and yields were in reasonable agreement with experimental values, except for data obtained using a module that was particularly susceptible to flow maldistribution and the resulting loss of efficiency. The model predicted that the portion of the mass transfer resistance attributable to the aqueous phase decreased with decreasing solute partition coefficient as expected. Mass transfer coefficients and yields were higher for solutes with higher partition coefficients.; A theoretical study of acetone extraction from aqueous solution into supercritical CO2 was performed, with tube side CO2 flow with or against gravity. Buoyancy-induced flow was important for large (1.8 mm inside diameter) but not small (0.6 mm ID) fibers, consistent with our expectation that such flow is more difficult to achieve when the characteristic length is small. The importance of buoyancy-induced flow decreased with increasing imposed fluid velocity, as forced convection masked the effects of free convection. For the range of conditions studied, the mass transfer coefficient obtained with flow in the direction of gravity was as much as 33% higher than for flow opposing gravity.; Ethanol and isopropanol extractions performed by others using spray, sieve tray or packed columns were run on a simulated membrane contactor, and the resulting values for the height of an equivalent theoretical stage (HETS) were compared to the reported values for the conventional contactors. In most instances the simulated membrane contactor offered a significantly (in some cases, substantially) lower HETS than the corresponding traditional column, indicating that the membrane contactor was more efficient. |
