Doctoral Dissertations

Date of Award

8-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

H. D. Cochran

Committee Members

Paul B. Bienkowski, Alexander Van Hook, Charles F. Moore

Abstract

A new method for dispersing aqueous liquid into supercritical carbon dioxide(SCO2) was demonstrated using pulsed high-voltage electric fields. System conditions were varied fi"om 35°C to 65°C at pressures from 10.34 MPato 17.24 MPa. Theapparatus was equipped with a laser light scattering system so that it was possible to determine the dependence of mean droplet size with various operating parameters such as field strength, pulse fi-equency, flow rate, temperature, and pressure. An extraction of ethanol from a 10 vol. % aqueous ethanol solution was performed, and compared with theoretical flash calculations using a ternary form of the Peng-Robinson equation of state.The extraction process was shown to occur with 100 % efficiency for a single theoretical stage.The effect of varying field strength, pulse frequency, and liquid flow rate was examined on p>, the number averaged droplet diameter, for dispersing pure water inSCO2 at 10.34 MPa and 40°C. This was the first study of electrospraying of a liquid into a dense, supercritical fluid. Increases in field strength caused p> to increase. This is opposite the effect normally observed when operating in the cone-jet mode of electrospray s. It is believed that the mode of electrospray in this research may extend past the cone-jet mode, because beyond the cone-jet mode increases in instability and polydispersity can increase p>. In all cases, whether increasing flow rate or field strength, lowering the pulse frequency appeared to give smaller p>. The effect of increasing p> with increasing flow rate demonstrated by other researchers was confirmed.A study varying temperature, pressure, and flow rate was completed using a liquid feed containing 10 vol. % aqueous ethanol (fermentation limit). These experiments examined the effect of flow rate (2-12 ml/min), pressure (10.34,13.79, 17.24 MPa), and temperature (35, 47, 65°C) on the mean droplet size of the dispersion while holding field strength and pulse frequency constant. For a pressure of 10.34 MPa, it was observed that increasing temperature increases p; however, at 17.24 MPa increasing temperature decreases p. This reversal was also seen when plotting the data for constant temperature. At 35°C, it was observed that increasing pressure increases p, againhowever, for 65°C increasing pressure decreased p. Although these observations initially appear puzzling, they were shown to correlate with surface tension, one of the several physical properties of the system that are changing with both temperature and pressure.Extraction of ethanol from a 10 vol. % aqueous ethanol solution using SCO2 inthe electrodispersion cell (EDC) appears to be possible. It occurs with essentially 100 %efficiency for a single theoretical stage. This was confirmed for four state conditions(10.34 and 17.24 MPa each at 35 and 65°C) and verified using a ternary Peng-Robinson Equation of state by examining the experimental and theoretical separation factors . Thisobservation was further supported by linear rates of extraction for both ethanol and water with molar flow rates of CO2. An important conclusion from the ethanol mass transfer experiments is that the micron-size ethanol-water dispersion was very efficiently coalesced by the wire-mesh electrode so that entrainment of liquid in the CO2 flowing from the cell was negligible. It was hoped that CO2 flow rates could be reached where the rate of extraction was much higher than that observed in the null experiment (no electric field) and where the ethanol extraction was mass transfer limited. Unfortunately,this was not achieved because of the limited capacity of the condensers.

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