Compaction, Flow and Dispersion in a Chromatographic Column

Sammanfattning: An experimental and theoretical study of compaction, flow and hydrodynamic dispersion in a chromatographic column containing non-rigid particles is presented. Experiments were performed using chromatographic columns packed with either a dextran-based chromatographic gel or glass beads. It was found that both the velocity variations and the dispersion irregularities in the column containing the compressed gel was smaller in the wall region than in the column packed with glass beads. This indicated that the compression made the packing more uniform. The hydrodynamic dispersion coeffcients measured at high Peclet numbers were found to be approximately twice that expected for a rigid packing, which was interpreted as an increase in the correlation (dispersion) lenght. A cubic plane strain tester was used to determine Young's modulus and the Poisson ratio of the compressible, water-saturated gel at stress levels < 7kPa. The wall friction coeffcient was determined using a modified version of a Jenike shear cell. A model for flow and compaction based upon Darcy's law and elastic analysis was presented, which incorporated a constant Poisson ratio and a void fraction dependent Young's modulus. The packed bed was assumed to be locally isotropic and the permeability was calculated using the Kozeny-Carman equation. A control-volume method was used to solve the set of coupled equations obtained. Simulations were performed without any parameter fitting. Experimental velocity and pressure profiles, reflecting the structural changes along the column, were predicted well. The predicted velocity profiles across the column agreed qualitatively with experimental measurements and the flow rate could be predicted within 30%. To investigate the influence of compression on the overall hydrodynamic dispersion a second control-volume based program was developed. A moving-point technique was successfully applied to eliminate numerical dispersion.