(153f) Modelling of the Multiphase Hydrodynamics in Capillary Micro-Separators

Abstract for the AIChE 2015 Annual Conference

Modelling of the
Multiphase Hydrodynamics in Capillary Micro-separators

Lu Yang, Nopphon Weeranoppanant, and Klavs F. Jensen

Department of Chemical Engineering, MIT, 77 Massachusetts
Avenue, Cambridge MA 02139

With benefits including
enhanced heat/mass transfer and improved safety profile, microscale
flow chemistry systems have attracted considerable attention in the past
decade. To streamline the production process, microscale
separation has achieved significant development in tandem with upstream
chemical synthesis. A widely used configuration for liquid-liquid extraction and
gas-liquid separation is the capillary micro-separators. Unlike traditional
lab-scale extraction equipment, which depends on the gravitational force to
drive separation, the capillary micro-separator separates alternating segments
of immiscible liquids based on interfacial tension. The wetting phase is
preferentially drawn into the micron-scale capillaries with the help of an
externally-imposed pressure gradient, which results in the complete separation
of the wetting and non-wetting phases.

The capillary micro-separator
has proven to be an effective tool in separating multiphase flows in a wide
range of microscale systems. However, in order to
predict the operating range and to optimize separator design, it is necessary
to obtain deeper insights into the hydrodynamics behavior of the multiphase
flows. Here, we present a CFD-based simulation strategy that reveals the physical
details of the multiphase separation processes in the capillary micro-separator.
The simulations were performed using OpenFOAM, an
open-source C++ package designed for computational fluid dynamics computations.
We ran the simulations in parallel on a 128-core high-performance computing
cluster to support high-resolution meshing while maintaining computation speed.
The simulation results were in good agreement with experimental observations,
and the simulation provided high spatial and temporal resolutions that were
difficult to achieve experimentally. With the numerical model, we examined how
flow velocity, interfacial tension, liquid properties and capillary size
distribution affect the operating range and separation efficiency of the
device. Moreover, based on the understanding of the working mechanism of the
micro-separators, we established an analytic model that extrapolated such knowledge
to membrane-based separators, which contain micron-scale pores that serve as
capillaries for separation. Together, the numerical and analytic models
provided a comprehensive framework to examine and understand the underlying
multiphase flow behavior dominated by interfacial tension force. Such knowledge
will enable prediction of device performance and optimization of separator
design.

Figure
1 Computational fluid dynamic (CFD) simulation of multiphase flow in the capillary
micro-separator (red: wetting phase; blue: non-wetting phase)