Microfluidics
Technology based on microfluidic devices can be used for the process intensification in chemical and bio-chemical process industries. We explore the fundamental aspects of micro-reactor technology, categorized into (i) hydrodynamics i.e. fluid flow characteristics (ii) mass transfer and (iii) reactions (Fig. 1). This involves both theoretical and experimental work for single phase and two-phase systems. Theoretical work is based on development of mathematical models using a first principle approach. The experimental study involves the analysis of fluid flow characteristics, mass transfer performance in systems such as liquid-liquid extraction and two-phase reactions. Our focus is on understanding the relation between hydrodynamics and masstransfer/reactions, and thereby develop strategies for enhancing the performance of micro-reactors/separators.
Fig. 1: Three-pronged approach to investigating the fundamental principles underlying micro-reator technology.
Single phase flow: Due to the small length scales of microchannels, fluid flow in these devices is laminar and mass transfer is controlled predominantly by diffusion. Mass transfer can be enhanced by inducing secondary vortices, which improve the mixing. This can be obtained via passive and active methods. One of the methods to induce passive mixing in microfluidics is curving the channel. We have tried to understand the systems such as curved catalytic reactors which incorporate coupled mass, momentum and reaction phenomena.We have extended this idea to further enhance mixing by inducing periodic slip on the curved microchannel walls [1]. Another method to enhance mixing is through active methods which use external energy. We have proposed chaotic mixer using periodic applied electric field which induces the secondary motion in electrolytic solutions via electro-osmosis. In recent we have proposed a new idea for recycle flows in microreactors using electro-osmosis [2] (Fig 2).
Fig. 2: Schematic of electroosmotic flow for inducing recycle in a microchannel E is the electric field applied across the side branch, in the opposite direction to the pressure gradient (Pin-Pex).[2]
Two-phase flow: Two-phase flows in microchannels can occur in a variety of flow regimes, depending on the realtive magnitude of capillary, inertial and viscous forces. Images of typical liquid-liquid flow patterns captured in experiments are shown in Fig 3.. These flow regimes are categorized as stratified, core annular, slug and droplet flows. The nature of the flow pattern can play a major role in the performance of micro-reactors/separators, by modifying the interfacial area and the rate of inter-phase mass transfer. Understanding this coupling between momentum and mass transport can help in designing better microsystems for carrying out processes such as liquid-liquid extraction and phase transfer catalytic reactions.
Fig 3: Experimental images of typical liquid-liquid flow patterns in microchannels.
We have been studying these systems in two stages. First the hydrodynamics is analyzed in detail, with emphasis both on fundamental aspects (e.g. the role of curved geometries in generating vortex flows [3,4], cf. Fig 4), as well as quantities of practical interest (e.g. phase holdup vs flow-rate fraction relationship [5]). This knowldge is then applied to mass transfer systems, such as liquid-liquid extraction, in various configurations – stratified and core-annular [6,7], cocurrent and counter-current [8].
Fig 4: Schematic and one possible circulation pattern for (a) Laterally stratified two-phase flow and (b) vertically stratified two-phase flow, in a curved channel. The deformation of the interface, caused by the centrifugally driven circulations, is also shown [3,4].
Apart from layered flows, we have also been studying the slug flow regime. Recently we experimentally compared micro-extraction in slug flow with stratified flow, and demonstrated its superiority over conventional macro batch systems [9]. We are currently developing a mathematical model for extraction in slug flow, taking care to account for the presence of strong internal circulations. This is motivated by our recent experimental results which demonstrate the strong influence of these circulations on the conversion in phase transfer catalytic reactions (where mass transfer may be the rate limiting step) [10]. The circulations are found to underlie a rather paradoxial observation, in which conversion decreases with increasing residence time!
References:
[1] P. Garg, J.R. Picardo, S. Pushpavanam, Chaotic mixing in a planar, curved channel using periodic slip, Phys. Fluids. 27 (2015). doi:10.1063/1.4915902.
[2] T. Krishnaveni, T. Renganathan, S. Pushpavanam, Recycle flows in lab on chip applications using electroosmotic effects, Industrial & Engineering Chemistry Research (2017)
[3] J.R. Picardo, S. Pushpavanam, Laterally stratified flow in a curved microchannel, Int. J. Multiph. Flow. 75 (2015) 39–53. doi:10.1016/j.ijmultiphaseflow.2015.04.017.
[4] P. Garg, J.R. Picardo, S. Pushpavanam, Vertically stratified two-phase flow in a curved channel: Insights from a domain perturbation analysis, Phys. Fluids. 26 (2014). doi:10.1063/1.4889738.
[5] A.B. Vir, S.R. Kulkarni, J.R. Picardo, A. Sahu, S. Pushpavanam, Holdup characteristics of two-phase parallel microflows, Microfluid Nanofluid. 16 (2014) 1057–1067. doi:10.1007/s10404-013-1269-7.
[6] A.B. Vir, A.S. Fabiyan, J.R. Picardo, S. Pushpavanam, Performance Comparison of Liquid − Liquid Extraction in Parallel Micro fl ows, Ind. Eng. Chem. Res. 53 (2014) 8171–8181. doi:dx.doi.org/10.1021/ie4041803.
[7] J.R. Picardo, T.G. Radhakrishna, A.B. Vir, S. Ramji, S. Pushpavanam, Modelling Extraction in Microchannels with Stratified Flow: Channel Geometry, Flow Configuration and Marangoni Stresses, Indian Chem. Eng. 4506 (2015) 1–37. doi:10.1080/00194506.2015.1044027.
[8] J.R. Picardo, S. Pushpavanam, On the conditional superiority of counter-current over co-current extraction in microchannels, Microfluid. Nanofluidics. 15 (2013) 701–713. doi:10.1007/s10404-013-1173-1.
[9] A. Sahu, A.B. Vir, L.N.S. Molleti, S. Ramji, S. Pushpavanam, Comparison of liquid-liquid extraction in batch systems and micro-channels, Chem. Eng. Process. Process Intensif. 104 (2016) 190–200. doi:10.1016/j.cep.2016.03.010.
[10] A.B. Vir, S. Pushpavanam, Phase transfer catalysis in a microchannel: Paradoxical effect of transition from kinetic control to mass transfer control, Chem. Eng. J. 317 (2017) 1047–1058. doi:10.1016/j.cej.2017.02.131.