(312i) Plug Generation Mechanisms and Reaction Rates for Liquid-Liquid Flow Systems in Mesoscale

• Plugs are generated in liquid-liquid flow via squeezing, dripping or jetting.
• The corresponding generation mechanism affects the plug characteristics, which in turn affect the mass transfer and reaction rate.
• Understanding of plug flow is necessary for proper enhancement of reactions

Introduction

Mass transfer and chemical reactions during biphasic liquid-liquid co-flow is of interest as it is commonly encountered in a variety of processes ranging from synthetic organic chemistry to biomedical applications. Efforts have been made to enhance the rate of such reactions by a number of ways such as phase treatment, catalyst addition and change of state properties. Reaction enhancement by flow modification in reduced dimensions is one such method where the flow of biphasic mixtures are modified in such a way as to promote flow patterns that favors the reaction. The performance of a miniaturized biphasic plug flow reactor depends on the effective interfacial area­ and the local velocity profiles at the interface, both of which are decided by the plug characteristics. For vertical upward flow of two liquids in a milli-channel where phases are introduced via a T-junction, the plugs are generated either by squeezing, dripping or jetting mechanisms, each having their own plug characteristics. Squeezing is observed at low phase velocities where the dispersed phase fills majority of the conduit cross section, blocking way for the continuous phase which eventually squeezes the dispersed phase to form plugs. Dripping occurs when the high flowing continuous phase quickly shears the low flowing dispersed phase before it could grow in the conduit. Jetting is observed at increased toluene rates where the plugs get sheared off from the tip of the dispersed phase jet by the surrounding continuous flow.

Methodology

A mass transfer limited reaction between sodium hydroxide and iodine is tested in a 2.38mm diameter conduit of 1m length with distilled water and toluene as the respective solvents. The phases are introduced using high precision syringe pumps and the phase flow rates are varied from 1 mL/min to 80 mL/min. Iodine is transferred from organic phase to aqueous phase where it reacts instantaneously with sodium hydroxide. The extent of iodine depletion from toluene is determined by UV-vis spectrophotometric analysis. The plug generation mechanisms and the plug characteristics after attaining fully developed flow are recorded using two high- definition high- speed cameras placed at the inlet and at a point 40 cm away from the inlet respectively.

Selected Results

Long, capsule shaped identical plugs generated via squeezing exhibit better mass transfer characteristics compared to other mechanisms, due to short diffusion distances and countercurrent flow between the rising plugs and the falling annular film surrounding the plugs. Dripping produces short plugs the size of which are insensitive to varying flow rate ratio. Jetting generates monodispersed plugs at low to moderate organic rates and poly-dispersed plugs at high phase rates. Even though iodine depletion rates for jetting and dripping cases are lower than those for squeezing cases, they are still better than annular and inverted dispersed flows. The solute depletion rates obtained for the different flow regimes in moles of solute depleted per liter of organic phase per unit of residence time in seconds is presented in Figure 1.

Key Conclusions

Plug generation mechanisms decide the plug characteristics which in turn decide the mass transfer and reaction performance of the corresponding liquid-liquid flow. Knowing different ways by which plugs are generated within a flow domain, and understanding the performance of plugs in each of these mechanisms is necessary for proper enhancement of a biphasic reaction in reduced dimensions. The study sheds light on liquid-liquid flow behavior in reduced dimensions in the presence of an interfacial reaction and provides results that can be useful in continuous flow reactor design at industrial scale.