Reports: UR951984-UR9: Fluid Flow and Solute Migration in Supercritical Fluid Chromatography

Donald P. Poe, PhD, University of Minnesota (Duluth)

A major objective of this research is to develop the theoretical framework for understanding how the hydrodynamics of supercritical fluids percolating through a packed column affect the chromatographically important properties related to retention and efficiency in supercritical fluid chromatography (SFC).  The primary focus during the first year has been on compressibility effects, modeling, and development of instrumentation:
  • Pressure, temperature and density drops in SFC
  • Effect of thermal conditions on retention and peak dispersion in SFC
  • Modeling separation processes in SFC
  • Construction of an experimental SFC system for fundamental studies

Pressure, temperature and density drops in SFC columns

            Experimental measurements and computer modeling of the pressure, temperature and density drops along analytical scale SFC columns packed with 3- and 5-micron diameter porous silica particles were conducted at outlet pressures from 80 to 300 bar and inlet temperatures from 293 to 373 K.  The columns were exposed to convective air or covered with thermal insulation.  The computed results agreed generally within 5% of the experimental values at outlet pressures above  120 bar (see paragraph on modeling).  Below 120 bar agreement was poor for mixtures of carbon dioxide and methanol due to the possible presence of two phases under these conditions.  We demonstrated that the temperature drop along thermally insulated (near-adiabatic) columns is due to isenthalpic expansion and can be readily calculated based on the pressure drop and the Joule-Thomson coefficient of the mobile phase.  Due to the variation in both velocity and density along the column, the pressure drop is best described by a modified form of Darcy’s law which is written in terms of the kinematic viscosity and the mass flow rate.  The experimental measurements were conducted by undergraduate students majoring in chemistry and chemical engineering, and who have gone on to professional school, or taken employment in the chemical engineering profession.

Effect of thermal conditions on retention and peak dispersion in SFC

             Isenthalpic cooling of the mobile phase along an SFC column results in the generation of axial and radial temperature gradients.  These temperature gradients become large when the outlet pressure of the column is near the critical conditions of the mobile phase, where the fluid is highly compressible.  We conducted experiments to demonstrate that excessive peak dispersion due to these radial gradients can be greatly reduced by operating the column in a near-adiabatic environment, by covering the column with a layer of thermal insulating foam or by suspending it in thermostatted still air.  The latter approach has so far been more successful.  This approach allows highly efficient SFC separations to be conducted at lower outlet pressures and higher temperatures than were previously thought possible.  The experimental studies were conducted by two undergraduate chemistry majors who have taken employment in the chemical profession, or entered graduate school in chemistry.

Modeling separation processes in SFC

            We have successfully modeled the distribution of pressure, temperature and density along SFC columns for carbon dioxide mobile phase with and without added methanol.  These distributions were computed numerically by solution of a heat-balance model, a mass-transfer model, and a velocity model.  Thermophysical properties were computed using REFPROP software (National Institute of Science and Technology).  We have also successfully modeled the retention and efficiency for n-alkane and alkylbenzene solutes.  These modeling efforts have provided new insights into the processes that lead to distorted peak shapes when operating under conditions where the mobile phase is highly compressible.  These modeling efforts have been completed in collaboration with Professor Krzysztof Kaczmarski, Rzeszow University of Technology, Poland.

Construction and characterization of an experimental SFC system for fundamental studies

            Commercial SFC instruments are designed to provide reliable separations over a range of operating conditions commonly used by SFC practitioners.  To meet market demands and serviceability, certain design features are typically included in such instruments that are not conducive to some types of fundamental investigations.  We have constructed an experimental system that is designed to provide such flexibility.  The use of commercially available high-pressure syringe pumps provide extremely precise flow rates and the capability of operating at pressures exceeding 500 bar.  While not convenient for routine separations, this will allow us to conduct studies near the zero-heat-balance condition.  At this temperature and pressure the Joule-Thomson coefficient is near zero, and no net heating or cooling of the mobile phase occurs.  Viscous heating limits the performance of ultra-high performance liquid chromatography, and isenthalpic cooling of supercritical fluids limits the operating range in SFC.  Another limiting feature of many commercial SFC systems is fixed-component configuration design which requires the use of excessive lengths of connecting tubing.  Our experimental system features an open design that will enable us to significantly reduce the overall length of the connecting tubing.  The resulting reduction in the extra-column band dispersion will provide for improved measurements of column dispersion properties.  The open format design also offers improved flexibility to optimize the thermal conditions.  This project has been conducted by a student in our M.S. program.