(174bs) A Novel Hollow Fiber Membrane Embedded Co-Axial Microdevice for Simultaneous Extraction and Stripping

Abstract

The micro-chemical systems developed in
the 1990s opened a new vision for chemical industry. The studies of micromixers,
micro heat exchangers, micro reactors and micro dispersers have been comprehensively
carried out. As for liquid-liquid extraction process,
mass transfer distance and size of droplet are in the dimension of 10~1000 ¦Ìm
in microdevices. In this characteristic dimension, the specific surface area
can be more than 1000 times larger than the traditional extraction devices. Because
of these advantages, microdisperion extraction has showed a brilliant
application prospect in the purification of wet phosphoric acid, the separation
of metal ionsand organics, and the clean-up of alkaloids.

Nevertheless, microdisperion extraction
has its own limitation. In general, multistage process is necessary to realize
higher recover ratio due to thermodynamic and phase ratio controls, and in microdevices
this control can be more prominent. Therefore, how to achieve multistage
process has become a frontier in microscale separation systems. In microchannels,
the viscous force and interfacial tension replace the gravity and inertial
forces as the dominant factors, which determines that the dispersion and flow
regimes in microchannels are totally different from the traditional equipment. For
example, the multistage countercurrent operation mode in extraction columns,
which is driven by density difference between the two phases, cannot be
performed in microchannels.

In the 1960s, Liproposed the
concept of liquid membrane permeation (LMP) and applied it to separation
processes. According to the different configurations and modes of operation,
LMP can be divided into two types, emulsion membrane (liquid surfactant
membrane) and supported liquid membrane. The emulsion membrane involves three
liquid phases, which can be respectively called external phase, internal phase
and membrane phase. External phase and internal phase are separated by membrane
phase by forming stable W/O/W or O/W/O double-emulsion system. Meanwhile, the
membrane phase is also a bridge for transmission of solute from external phase
to internal phase. In this way,
the simultaneous extraction and stripping process is realized by transmission
across the liquid membrane. Based on this concept, the membrane phase can be
further absorbed in the micro pores of a porous support, which is called
supported liquid membrane permeation. The liquid membrane formed on a support
like hollow fiber membrane can be more stable than the emulsion membrane, and
the specific surface area for mass transfer can also be increased.

Based on the concept of LMP, realizing the
dispersion and mass transfer process of liquid-liquid-liquid three phases in
the same microdevice and coupling extraction and stripping to carry out multistage
process are core contents of our study. We embedded a hollow fiber membrane in
a microchannel to form the supported liquid membrane. Meanwhile, in order to
solve the problem of high mass transfer resistance caused by the thickness of
the membrane, we introduced a pressure control equipment to regulate the tube
and shell pressures precisely and simultaneously, and in this way, the
extractant could flow in the membrane pores oscillatorily to change the mass
transfer process from original static state into convective flow state (see Fig.
1).

As for the specific content of our study,
we used the extraction of phenol as a model system, and
realized the steady liquid-liquid-liquid three-phase flow of the feed,
extractant, and back-extractant in the designed microdevice. By precisely
controlling the pressure of the tube side, the extractant was separated from
the feed in situ and passed into the shell side to contact with the
back-extractant directly (see Fig. 2). In this way, both of the
extraction yield and stripping yield reached 95%. And by introducing a
transmembrane pressure with periodic oscillation, 5 theoretical stages were
successfully achieved within 3 min residence time in the single microdevice.

 Hopefully, the method proposed in
this study might bring a new idea for multistage microscale extraction
processes.

Fig.
1. Simultaneous extraction and stripping process
for separation of phenol in the microdevice

1 - pre-dispersion channel, 2 - hollow fiber membrane embedded
channel, 3&4 - UV
spectrophotometer  5&6 - buffer tank, 7 - pressure-control equipment,
8 - filter, 9 - valve, 10 - air tank

Fig. 2. Schematic diagram of
experimental set-up