(123a) Measurement of Critical Locus for Carbon Dioxide + Acetonitrile System with Variable-Volume View Cell

Phase equilibrium data are most important information for the process development with supercritical fluids. In recent years, a mixture of supercritical carbon dioxide (scCO2) and organic solvent, expanded liquid and co-solvent, are paid much attention to expand the capability and the applicability of scCO2 as a solvent.

The critical locus data determine the boundary conditions, temperature and pressure, for the phase separation of the scCO2 and organic solvent system. Based on the critical locus data of the scCO2 and organic solvent mixture, the phase of the system, homogeneous or phase separation, can be controlled to adjust the temperature and pressure of the system. However, the critical locus data for the scCO2 and organic solvent systems are very scarce and difficult to measure. A rapid and accurate measurement of the critical locus data is desired for an effective design of a solvent with scCO2 and organic solvent mixture.

From this viewpoint, we have developed a variable-volume view cell with data acquisition system for the critical locus data measurements. The cell consists of 70 mm in outer diameter and 15 mm in inner diameter. The maximum temperature and pressure of the cell are 473 K and 70 MPa, respectively. The piston is moved with geared motor and the piston speed is variable in the range of 0.001 to 3.5 mm/minutes. The volume of the cell can be changed from 3.87 to 19.95 mL. The piston position is detected continuously with linear scale and inputted with the inside pressure data of the cell in the control system. The piston movement is strictly and automatically controlled by the sequence using both data for ensuring precise measurements. Data acquisition system enables to record the temperature and pressure inside the view cell continuously. Critical temperatures and critical pressures for the CO2 + acetonitrile system were measured at different compositions with this system. Rapid and accurate critical locus measurement was ensured with this variable-volume view cell system.