Mechanisms of liquid extraction in a pilot plant sieve plate column

Abstract

The literature pertinent to droplet hydrodynamics and mass transfer in perforated (sieve)plate liquid-liquid extractor columns has been reviewed. An experimental investigation was conducted in a 450 mm diameter perforated plate extractor with four plates using four different designs with holes of 1.588 mm, 3.175 mm, 4.763 mm and 6.350 mm, each with adjustable downcomers. Hydrodynamics were studied with the system Clairsol 350 (dispersed) and water.The parameters investigated included jet length, dispersed phase hold-up, coalesced dispersed phase and flocculation zone height beneath the plates, mean drop size and drop size distribution as a function of phase velocities. The results have been compared with the published correlations based on data from laboratory scale perforated plate columns or single ground glass nozzles; a wide divergence was found. The experimental data was therefore correlated independently to predict the jet length, dispersed phase hold-up, flocculation zone height and mean drop size. The correlation for mean drop size was modified to include terms involving column dimensions. The mean drop size was correlated within ±10%. Drop size distributions were obtained by analysis of photographic prints. Mugele-Evans upper-limit function gave a better fit compared to the log-nonnal. Mass transfer operation was with the system Clairsol (dispersed)­ acetone-water.Both directions of transfer were investigated In the calculation of the overall experimental mass transfer coefficient CKaD) expt the mean driving force was determined from the concentration profile along the column using Simpson's Rule. A novel method was developed to calculate the theoretical overall mass transfer coefficient using the drop size distribution diagram to determine the volume percentage of stagnant, circulating and oscillating drops in the sample population. Individual mass transfer coefficients were estimated for the corresponding droplet states using published single nozzle drop mass transfer models. A fairly wide divergence was found between the experimental and theoretical overall mass transfer coefficients based on the other workers' models. Recommendations for industrial column design include: (i) using drilled and punched plate to obtain effective use of all orifices and (ii) reduction in plate spacing to reduce back rnixing.