(235b) Liquid-Liquid Two-Phase Flow Patterns and Mass Transfer Characteristics in Rectangular Glass Microreactors

Intensification of chemical processes aiming at the effective use of raw materials and energy implies miniaturization of chemical reactors.

In order to define the reaction conditions and to design a microstructured reactor for a specific multiphase chemical transformation, detailed knowledge of the hydrodynamics in the microchannels is necessary. The common modes of interface in the case of liquid-liquid two-phase flow are ?slug flow? and ?parallel flow?. The main problem in the control of flow pattern is due to the flow sensibility to the experimental parameters like: linear velocity, ratio of the phase volume, fluid properties, channel geometries, and the construction material. Despite its importance, systematic experimental study of the effect of fluid properties on flow patterns has not been reported. The mass transfer performance is another key characteristic of microstructured devices applied for reacting systems. Only Burns and Ramshaw (2001) provided data on mass transfer performance for the slug flow regime during chemical reaction. The mass transfer characteristics for the parallel flow have not been reported so far.

In this context, the flow of two immiscible fluids was investigated in two rectangular glass microchannels with Y- and T- junctions and with equivalent diameters of 269 and 400 μm, respectively. The influence of the fluid properties on two-phase liquid-liquid flow patterns was studied using deionised water and toluene with volumetric flow rates between 1 - 6 ml/h and monitored by a photo-camera. Sodium hydroxide and/or trichloroacetic acid were added to both phases in order to change density, viscosity and interfacial tension of the fluid/fluid system. The mass transfer performance for the slug flow and parallel flow pattern was determined using the instantaneous neutralization reaction (controlled by mass transfer) between the trichloroacetic acid dissolved in the organic phase and sodium hydroxide present in the aqueous phase. The total volumetric flow rate was varied between 2 to 12 ml/h and the concentration of NaOH in the aqueous phase between 0.1- 0.3 M. The volume ratio of the organic/aqueous phases was kept constant 1:1.

The results showed that the shape of the interface between two immiscible liquids is controlled by a competition between the viscous forces and the interfacial tension. The viscous forces act to extend and drag the interface downstream (Squires and Quake, 2005) favouring the parallel flow. The increase of interfacial tension reduces the interfacial area resulting in a slug flow. The observed flow patterns were correlated with the mean Capillary and Reynolds numbers. The model allows prediction of the flow patterns depending on the values of the Reynolds and Capillary numbers for equal volumetric flow rates of the two phases.

For the conditions applied during the study, the slug flow and the parallel flow showed similar mass transfer performance with the global volumetric mass transfer coefficient (k_gl*a) in the range of 0.2 to 0.5 s^-1. The values of the mass transfer coefficients (k_gl) are strongly dependent on the way used to estimate the specific interfacial area. If the wall film of the continuous phase is taken into account in the case of slug flow, the global mass transfer coefficient was found to be in the range of 10^-5-10^-4 m/s for both flow patterns. For the parallel flow, the mass transfer coefficient was mainly affected by the base concentration in water. In the case of slug flow, no influence of the NaOH concentration on k_gl was observed. The k_gl values were influenced by the linear velocity only. The results can be rationalized using a film model, however the presence of secondary phenomena, like interfacial instabilities must also be taken into consideration.

References

Burns, J. R. ,Ramshaw, C., (2001). The intensification of rapid reactions in multiphase systems using slug flow in capillaries. Lab on a Chip, 1, 10-15.

Squires, T. M. ,Quake, S. R., (2005). Microfluidics: Fluid physics at the nanoliter scale. Reviews of modern physics, 77, 977-1026.