The effects of a surfactant on the mass transfer in spray-tower extraction column

Abstract

For an extraction process, the overall efficiency of mass transfer is determined by the mass transfer coefficient and the interfacial area. A small amount of surfactant has the effect of decreasing the mass transfer coefficient while increasing the interfacial area. The extraction efficiency thus varies in a complex manner with the concentration of surfactant due to the two competing factors. To obtain a better understanding of these effects, the two factors were studied individually. A single-drop extraction apparatus was used to study the effects of a surfactant on the mass transfer coefficient in a previous work. In this work, the effects of a surfactant on the overall efficiency and mass transfer area were investigated in a spray-tower extraction column. The results show that sodium lauryl sulphate (SLS) has a negative effect on the overall efficiency. The value of K R a decreases rapidly with increasing SLS concentration, approaches a minimum at about 10 ppm, and then increases monotonically with further addition of SLS. With increasing surfactant concentration, the interfacial area increases more rapidly at a low SLS concentration, which is mainly due to the inhibition of drop coalescence by the dynamic surface effect. However, the slower increasing rate of mass transfer area at higher SLS concentrations results from a decrease in drop size with decreasing interfacial tension.