Performance of a natural convection circulation system for supercritical fluids

A closed-loop circulation system for supercritical fluids that operates on the principle of density differences induced by a heating and a cooling source has been developed. Performance of the system was determined by measuring average flow velocities for CO₂ over a range of conditions from 7.8 to 15 MPa and from 15 to 55 °C for the given initial loading densities, ρ_ini, of 550-800 kg/m³ and density differences, Δρ_eff, between heating and cooling sources of the loop of 62-121 kg/m³. One-dimensional finite-difference simulation could predict the velocities at most conditions to within 35%. The flow rates achieved in the system could be correlated in terms of Grashof and Prandtl numbers and a dimensionless effective density difference between heating and cooling sources to within 25% and by an empirical equation in terms of the system pressure, loaded density and heating and cooling source average density difference to within 10%. Average flow velocities as high as 4 m/min could be obtained with heating and cooling source (wall) temperature differences of 3-8 °C. The system should find use in applications such as extended-time extractions, sample preparation and enrichment for analytical applications, catalytic reactions and for transporting slurry suspensions and solutions.