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)