Formation of drops at narrow slots in a centrifugal liquid–liquid contactor

Abstract

iquid-liquid contacting in a centrifugal field has important advantages that arise from the effect of large radial forces both on drop L formation and on drop transport through the continuous phase.Among many designs of rotary extractor now available (Hafez, 1983), the Hi-Gee packed rotary contactor reported by Balasundaram e t al. (1990) initially for use in gas-liquid contacting is noteworthy for its simplicity and effectiveness. In subsequent adaptations of the Hi-Gee design for liquid-liquid contacting discussed by Howarth et al. (1 993), Lee (1993) and Lee et al. (1995), the dispersed phase is injected through small bore nozzles or holes directly into a high voidage packing immersed in the continuous phase. The packing helps to maintain a small drop size, provide a large area for countercurrent flow and ensure that the phases flow in a radial direction.The small bore injection nozzles needed to generate suitably fine dispersions in centrifugal contactors are difficult and expensive to manufacture. Moreover, their fixed position and separation distance help to create contrasting regions of high and low volume fraction of the dispersed phase within the packing, thereby reducing the effectiveness of contacting. This paper examines the use of narrow slot-nozzles as an alternative method of dispersing an injected liquid phase which may help to avoid some of these shortcomings. Southern and Wraith (1 990), Southern (1991) and Li et al. (1994) have shown that gas injection into a liquid through narrow slots produces a prolific linear array of small bubbles whose sizes relate to the slot opening width, the superficial injection velocity and the physical properties of the fluids. A key feature is the spontaneous appearance of uniformly spaced bubble formation sites along the length of the slot. These dispersion sources occur a t nodes attributed to the Rayleigh-Taylor instability on the interface between the injected and the continuous phase at the slot opening.When an immiscible liquid is injected, we demonstrate that drops form likewise as a result of the instability and that slot-nozzles exhibit a spontaneous and continuous distribution of drop sources along the slot length. The inherent simplicity of such nozzles suggests that they may be cheap and simple to manufacture and, furthermore, that they could be stacked in layers to multiply the number of drop sources thereby easily increasing the axial thickness of a dispersion. The principle of the slotnozzle assembly and its adaptation to the centrifugal contactor are shown schematically in Figure 1. In experiments on the dispersion of organic phases into aqueous phases, drops were generated continuously at narrow slots of width 12 pm 5 w I 125 pm mounted peripherally and normal to the radius in a 'Hi-Gee' centrifugal contactor. The drops formed at an array of sources attributed to the Rayleigh-Taylor instability distributed an average distance h = 29/= apart where u is the interfacial tension of the phases, Ap