- The dispersed phase should be the one that has the higher volumetric flowrate, except in equipment subject to backmixing, where it should be the phase with the smaller rate. It should be the phase that wets the material of construction less well. Since the holdup of the continuous phase usually is greater, that phase should be the less expensive or less hazardous material.
- Mixer settlers are limited to five stages. Mixing is done with rotating impellers or circulating pumps. Settlers are designed assuming that droplet sizes are about 150 Ám diameter. In open vessels, residence times of 30-60 min, or superficial velocities of 0.5-1.5 ft/min are provided in settlers. Extraction stage efficiencies commonly are taken as 80%.
- Spray towers even as high as 20-40 ft cannot be depended on to function as more than a single stage.
- Packed towers are employed when 5-10 stages will suffice. Pall rings of 1-1.5 in. are best. Dispersed phase loadings should not exceed 25 gal/(min)(ft2). An HETP of 5-10 ft may be realisable. The dispersed phase must be redistributed every 5-7 ft. Packed towers are not satisfactory when surface tension is more than 10 dynes/cm.
- Sieve tray towers have holes of 3-8 mm diameter. Hole velocities are kept below 0.8 ft/s to avoid formation of small drops. Re-dispersion of either phase at each tray can be designed for. Tray spacings are 6-24 in.; efficiencies are 20-30%.