Vivek V. Buwa

Liquid-liquid disengagement in a continuous gravity settler

Several different equipment are used to disengage the liquid phases (immiscible liquids) following the solvent extraction processes. Centrifugal contactors and rotating disc contactors are preferred for phase separation when the process requires very short residence time. On the other hand, gravity settlers are used extensively for disengagement of liquid–liquid dispersions after solvent extraction process and are ideal for the processes that require longer residence time and most importantly when the dispersions are easily separable due to the difference in density. Their performance in terms of separation efficiency has a significant impact on the process economy of hydrometallurgical applications and of several other chemical processes (alkylation, sulphonation, crude desalting, etc). To improve the separation performance, the effects of settler operating parameters such as total flow rate, inlet drop size distribution, phase properties and dispersed phase volume fraction needs to be understand. It is also essential to understand the effects of design parameters such as settler size, the location of dispersion inlet/outlets, flow rates, physical properties of the fluid phases and settler internals (baffles, picket fences, end plate, etc.) on the separation performance. Therefore, our work is focused on investigations on the effects of the aforementioned parameters on the rate of phase separation in a laboratory–scale continuous gravity settler. In addition to the large–scale investigations, understanding of binary (drop˗drop) and interfacial (drop˗interface) coalescence, which is responsible for the phase separation process, is also important.

Apart from the experimental investigations, we have also performed computational fluid dynamics (CFD) simulations of liquid–liquid phase separation. Eulerian˗Eulerian two˗fluid simulations were performed to investigate the effects of total flow rate, inlet drop size distributions, physical properties of the liquids, the position of baffle opening/picket fence, length of the settler and position of continuous and disperse phase outlets for separation performance. The twoPhaseEulerFoam module of the open source code OpenFOAM was modified to simulate the flow of the immiscible phases (aqueous and organic phase) with user–defined drag correlations. A rigorous comparison with the measurements was made to improve the predictive capability and the two˗fluid module was further developed to incorporate multi˗fluid (multiPhaseEulerFoam) module. Further improvements in the solver was made by integrating the population balance module in the multi˗fluid CFD solver (multiPhaseCfdPbmFoam) to account the binary and interfacial coalescence in detail. The experimentally verified computational model used in this work will be useful in designing large–scale continuous gravity settlers and to achieve better separation efficiency.

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