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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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