(401c) Liquid-Liquid Extraction in Intensified Small Scale Units

Liquid-Liquid
Extraction in Intensified Small Scale Units

Garciadiego Ortega Eduardo, Tsaoulidis
Dimitrios, Angeli Panagiota*

Department of Chemical Engineering, University College London, Torrington
Place, WC1E 7JE, London, UK. * Corresponding author; e-mail: p.angeli@ucl.ac.uk

ABSTRACT

Intensification
of solvent extraction processes leads to reduction of costs and solvent
inventories, and improves the safety and sustainability of the operations in a
wide range of industries. The use of small channels as contactors improves
mixing and increases the surface area available for mass transfer as compared
to conventional contactors. Extractions performed in millimetric channels operating
in slug flow regime, show high volumetric mass transfer coefficients (k_La)
and extraction efficiencies (%E) [1,2]. These studies have shown the links
between hydrodynamic characteristics like plug lengths (and thus interfacial
area) or internal circulation patterns in each phase with the mass transfer
performance of the contactors.

The
throughput achieved in one channel, however, is small compared to industrial
rates. To increase throughput in small scale devices, scale out (instead of
scale up) is used; this can however, require a prohibitively large number of
small channels. To overcome this problem we followed two strategies. In the
first strategy, the size of the channels is increased enough to allow large
throughputs while still preserving the benefits of small scale operation, such
as laminar flows and well defined, regular patterns. Scale out to desired
throughputs would then involve a smaller number of channels. In the second one,
the velocities of the two liquid streams are increased when they are brought
into contact (impinging-jets mixer). It has been found that when jets of the
same phase collide very high energy dissipation rates are achieved, which can
be almost two orders of magnitude higher than in conventional contactors [3]. Impinging-jet
mixers have not been studied for jets of immiscible liquids, but the high
energy dissipation rates are expected to produce very small drops and increased
interfacial areas.

For
the scale-out strategy, the main challenge is to develop manifolds which
produce uniform flow distribution in the many parallel channels. The two single
phases can be distributed in separate manifolds and fed into the contacting
channels (split-combine), or alternatively, the two phases can be first
mixed before the two-phase mixture is distributed into several channels (combine-split).
Both these configurations have been studied for gas-liquid flows operating in
Taylor flow regime [4, 5]. Al-Rawashdeh et al. (2012) found for the
split-combine approach that the manifold was successful for only one flow rate
condition [4]. For the combine-split approach good flow distribution was again
achieved only at limited conditions [5]. Neither of these approaches has been
tested for liquid-liquid flows.

The
scale up of liquid-liquid flows in small channels was investigated in this work
following both strategies. Water and kerosene were used as the immiscible
liquids. Flow patterns and drop sizes were studied with high speed imaging. For
the manifold studies both the split-combine and combine-split concepts were
used to construct set-ups with 2 and 4 outlet channels with 1 and 2 mm internal
diameter. For the design of the impinging-jet mixer (Figure 1) two different
nozzles have been used (0.25 and 0.5 mm internal diameter) and two different
mixing chambers (2 and 3 mm). Drop size measurements showed that interfacial
area in the case of the impinging-jet mixer is increased up to 8 times compared
to T-junction capillaries for the same operating conditions. In addition, it
was found that smaller droplets formed with increasing energy dissipations. The
goal of the current work is to identify the different flow configurations that
produce small drops with narrow size distribution.

Figure
1.
Schematic and photograph of the experimental setup of the confined
impinging-jet mixer.

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