1037 Korean J. Chem. Eng., 32(6), 1037-1045 (2015) DOI: 10.1007/s11814-014-0283-0 INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: bzelic@fkit.hr Copyright by The Korean Institute of Chemical Engineers. Mass transfer coefficient of slug flow for organic solvent-aqueous system in a microreactor Ana Jurinjak Tušek * , Iva Anić * , Želimir Kurtanjek * , and Bruno Zelić ** ,† *Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia **Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, HR-10000 Zagreb, Croatia (Received 16 April 2013 • accepted 23 September 2014) Abstract-Application of microreactor systems could be the next break-through in the intensification of chemical and biochemical processes. The common flow regime for organic solvent-aqueous phase two-phase systems is a segmented flow. Internal circulations in segments cause high mass transfer and conversion. We analyzed slug flow in seven sys- tems of organic solvents and aqueous phase. To analyze how slug lengths in tested systems depend on linear velocity and physical and chemical properties of used organic solvents, regression models were proposed. It was shown that models based on linearization of approximation by potentials give low correlation for slug length prediction; however, application of an essential nonlinear model of multiple layer perceptron (MLP) neural network gives high correlation with R 2 =0.9. General sensitivity analysis was applied for the MLP neural network model, which showed that 80% of variance in slug length for the both phases is accounted for the viscosity and density of the organic phases; 10% is accounted by surface tension of the organic phase, while molecular masses and flow rates each account for 5%. For defined geometry of microreactor, mass transfer has been determined by carrying out the neutralization experiment with NaOH where acetic acid diffuses from organic phase (hexane) into aqueous phase. Estimated mass transfer coeffi- cients were in the range k L a=4,652-1,9807 h -1 . Keywords: Liquid-liquid Slug Flow, Regression Model, Mass Transfer Coefficient, Microreactors INTRODUCTION Reducing the reactor scale from “macro” to “micro” opens many new experimental possibilities. This approach in process intensifi- cation is a new concept in chemical and biochemical engineering that aims to reduce capital and energy costs [1]. Due to the small dimensions of their microchannels (diameter in range from 10 μm to 500 μm), microreactors have greater surface-to-volume ratio, ensure effective heat and mass transfer, low amounts of chemical are needed and reaction times are very short. According to Mills et al. [2], multiphase reactions that are catalyzed by homogeneous catalysts, heterogeneous catalysts or biocatalysts provide the basis for most commercial-scale processes for production of diverse prod- ucts, ranging from petroleum-derived product to fine chemical and pharmaceuticals. Multiphase reactions are still more difficult to conduct than homogeneous reactions, because the efficiency of interactions and mass transfer between phases must be taken into account [3]. New reaction systems being widely used for multi- phase (gas-liquid, liquid-liquid, gas-liquid-liquid, etc.…) reactions are microreactors [4-8]. To define the optimal process conditions for reaction taking place in a multiphase microreactor, it is neces- sary to analyze the hydrodynamics in microchannels [9,10]. Sta- ble flow patterns with uniform interfacial areas allow precise mass transfer tuning [11]. Introducing two immiscible liquids in the microreactor, the most usual two flow patterns that can occur are parallel flow or slug (seg- mented) flow [12]. Flow pattern formation depends on linear veloc- ity [13], ratios of the phases, fluid properties, the channel geometry [11] and the microreactor construction material; all these parame- ters have to be considered when controlling the flow pattern. Liq- uid-liquid slug flow is characterized by a series of slugs of one phase separated by slugs of other phase. Each slug served as an individual processing sub volume [14]. Slug flow development and its char- acteristics depend on two individual transport mechanisms, con- vection and diffusion, and the physical force, which is the result of interfacial surface tension. Convection takes place in each slug due to internal circulations, while diffusion takes place between two slugs due to concentration gradients [14]. Convection depends on physical properties of fluids, slug geometry and flow velocities, while diffusions depend upon interfacial area available for mass transfer. The internal circulations within both slugs caused by the shear be- tween the wall surface and slug renew the interface enhance the diffusive penetration and increase the reaction rates [15]. This fluid motion enhances mixing within the fluid segments and also im- proves the rate of diffusion of solute across the liquid/liquid inter- face by the shearing disruption of concentration gradients within the segments. In slug flow, mass transfer and chemical reaction take place at the interfacial area. According to Ghaini et al. [16], knowledge of interfacial area between two liquid phases is essen- tial in understanding and modelling the flow and transport pro- cesses in a given liquid–liquid contactor. Dimensionless analysis is usually used to understand the driving forces of two-phase flows.