(210f) Gas-Liquid and Liquid-Liquid Taylor Flow in Micro Channels: Hydrodynamics and Mass Transfer

Authors: 
Haase, S., Technische Universität Dresden
Bauer, T., Dresden University of Technology
Lange, R., Technische Universität Dresden
Mathematical model of gas-liquid and liquid-liquid Taylor flow was built on the base of continuity and Navier-Stokes equations for multiphase flows. The model allows predicting by known physicochemical properties of phases and the geometry of the microchannel following parameters of the flow: velocity of the bubbles/liquid plugs, film thickness, real gas hold-up, length of the bubbles and the slugs, circulation time for Taylor vortices and the inner geometry of these vortices: thickness of transit film, which is not involved in the Taylor circulation, the radius of two zones in the Taylor vortex: inner and outer layer moving correspondingly forwards and backwards.

Analysis on the basis of the mathematical model revealed that the circulation of liquid in the slugs take place, even if their length is 15 times smaller than the diameter of the capillary. CFD calculations was detected that although the circulation is not vanishing completely for short liquid slugs (approximately shorter than 0.7 of capillary diameter), its intensity (circulation flow rate) diminishes and dimensionless recirculation time is growing. Equation for radius of vortex center depending of slug length is proposed. It was revealed that for very short liquid slugs (shorter than 0.17 of capillary diameter) the inner circulation is less than the bypass flow rate. Effects of liquid slug length on dimensional and dimensionless recirculation times were defined.

 Hydrodynamics and mass transfer of slug flow in horizontal capillaries with different inner diameter (0.92 mm for hydrodynamics, 1.70 mm, 1.86 mm, 2.53 mm for mass transfer) in water-air and glycerol-air systems were studied. Experimental data of bubble velocity and pressure drop was compared with values calculated by mathematical model of slug flow in capillaries proposed earlier. A simple equation to calculate the gas bubble velocity was obtained. The intensification of mass transfer in slug flow regime at the expense of the Taylor circulation was demonstrated experimentally. Semi-empirical formula for Sherwood number in a wide range of capillary number was obtained.

A new method for the simultaneous measurement of hydrodynamics and reaction rates in a mini-channel with catalytically active walls at pressures up to 5 MPa is presented. A newly developed setup has led to an investigation of the influence of the operating conditions and of the gasâ??liquid feeding on the characteristics of Taylor flow and on chemical conversion. The detected parameters include gas bubble length, liquid slug length, bubble flattening in the cross-sectional area, change in the gas bubble velocity due to conversion and overall macrokinetics. The hydrogenation of alpha-methylstyrene was used as a test reaction system at a pressure of 1 MPa and a temperature of 343 K. The highest conversion was achieved at a two-phase velocity of 5 cm sâ??1 and a gas holdup of 0.75 and declined with decreasing gas holdup and increasing two-phase velocity. The presented method enables the study of reactor performance for a broad range of multiphase reactions and flow patterns with two immiscible fluids without any effects of maldistribution.

Three layer mathematical model was used to simulate mass transfer in microchannels for gas-liquid Taylor flow. The structure of slug (liquid between two adjacent bubbles) was stratified onto three layers: film at the wall with transit flow not involved in Taylor vortices, outer and the inner layers of Taylor vortex. Due to high Peclet number only convection was taken into account in both layers of the vortex, whereas pure diffusion was predominant in the transit film as well as in the film around bubble. Homemade program was implemented to solve the system of partial differential equations numerically. Patankarâ??s method was used for finite differences method implementation of convection and diffusion equation. Result of simulations revealed the existence of optimal velocity of two phase flow for fixed properties of phases and size of the micro channel.

Increase of viscosity of liquid and decrease of the surface tension results in fall of mass transfer coefficient. Both effects are coupled with the growth of film thickness with rising capillary number. The simulation has brought to light that velocity of two phase flow has ambivalent impact on the mass transfer. On the one hand, growing velocity of liquid slugs brings better mass exchange due to more frequent circulation by means of Taylor vortices. On the other hand, the higher the velocity of liquid slugs, the thicker is the film which limits overall mass transfer. In the aggregate these two phenomena result in an optimum of the dependence of mass transfer on two phase velocity.

Hydrodynamics and mass transfer in three-phase composite minichannel fixed-bed reactors was also recently studied. Industrial available and standardized catalyst particles or pellets may benefit from these microfluidic phenomena if they are packed into inert minichannels. Such packings form the key components of composite minichannel reactors. In order to evaluate this reactor concept, hydrodynamic phenomena, mass transfer, and pressure drop will be examined for a reactor consisting of a ceramic minichannel packing with a hydraulic diameter of 1.0 mm and dumped spherical catalyst particles of 0.8 mm in diameter. The experimental data, achieved in a setup combining hydrodynamic observation and chemical reaction, were used to derive universal applicable correlations to predict mass transfer coefficients and friction factors from Reynolds, Schmidt, and Sherwood numbers. This part of work concludes with an extensive comparison of composite minichannel reactors with conventional multiphase reactors and developing packed-bed reactors in terms of mass transfer capability, power consumption, and contacting efficiency.

At identical power consumption, the investigated composite minichannel reactor offered a remarkably higher overall mass transfer rate for the gaseous compound than conventional trickle-bed, slurry bubble column, or slurry stirred tank reactors. Similar rates or even higher rates were achieved in miniaturized packed-bed reactors with particles less 1.0 mm in diameter. Consequently, it is expected that structured and miniaturized packed-bed reactors are a promising concept to intensify multiphase reaction processes, e.g. by switching from batch to continuous processing.

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