Scale-Up of the Semi-Continuous Chromatographic Process

Abstract

A review is given of the reported experimental and theoretical work which has led to a better understanding of the gas/- liquid chromatographic scale-up problem. The potential of several proposed process schemes for production-scale separations is discussed. Colum utilisation can be increased, with respect to the conventional batch process, by employing a counter-current movement between the soluteladen gas stream and the solvent-coated particulate solid to separate a continuous feed mixture into two fractions. A novel unit has been constructed in which this relative phase movement is simulated by sequencing the position of the input and output streams around a closed symmetrical system of twelve, fixed 7.6 om-diameter columns. Mechanical movement, save the automatically timed opening and closing of proven reliability solenoid valves, has thus been eliminated. The chemical system chosen for the study of the operating characteristics of the unit was an equivolume mixture of 1.1.2. trifluoro- 1.2.2, trichloro-ethane and 1.1.1. trichloro-ethane, The solvent phase was silicone oil DC 200/50 (25% wt./wt.) coated onto 500-355 m particles of Chromosorb P. In the separating mode, the effects of the solute mixture feedrate, gas rate and apparent liquid solvent rate were primarily investigated by determination of the on-colum concentration profiles. The operating limits for a successful separation were show, in keeping with theory, to be reduced by both a concentration dependent absorption isotherm and column pressure drop. Measured H-E.T.P, values lay in the range 0.7 to 2.0 cm, the average value increasing with both solute concentration and gas flowrate. Typically, purities in excess of 99.7% for both products have been achieved up to a feedrate of 0.7dm3 hr-1. A computer simulation of the process scheme has been written which incorporates the concentration and pressure effects. The predicted and experimental concentration profiles are compared.