(143d) Gas-Liquid Flow Characteristics in Microreactors

Motivation: Microreaction technologies are in the interest of industry and research in chemical engineering. The excellent heat-transfer properties and low volumes enable a safe operation of even highly exothermic and fast chemical reactions. Together with the high product quality and continuous operation mode, these reactors are advantageous for controlled mass transfer and safe operation. The hydrodynamics, as flow patterns, pressure drop and residence time distribution, are the key parameters for enhanced conversion rates: Optimized hydrodynamics result in good radial mixing and limited axial mixing. Especially for fast reactions, were the reaction kinetic is limited by the mixing, decreased conversion rates compared to conventional reactor design had been observed. Fundamental research on gas-liquid two-phase flow allows further improving in microprocesses.

Idea: As the resulting hydrodynamic characteristics of the flow depend strongly on the flowing media, the influence of fluid properties as density, viscosity and surface tension is as important as the influence of the flow rates and of the channel design. Therefore we investigated the hydrodynamics of gas-liquid flow in a rectangular microreactor with varying mixing zone for different fluids (gas: nitrogen, liquid: ethanol, water, glycerine) at different pressure values.

Experimental setup: Fabricating the microreactors via photolithographic processes of Silicone and Glass allows optical access to the flow to pressures up to 120 bar. These reactors have a rectangular cross section and the hydraulic diameters are ranging from 200 to 400 µm. Laser Induced Fluorescence (LIF) and µ-Particle Image Velocimetry (µ-PIV) are common methods to characterize two phase flow in microreactors. While LIF, where the colored liquid phase can be differed from the channel and the gaseous phase, contains information about flow pattern, holdup values, superficial velocities and bubble shapes, PIV is a method to characterize the relative flow inside the liquid phase.

Results: Using these methods, we investigated gas-liquid two-phase flow in channels with straight and meandering geometries. The flow pattern is defined by the static mixer and the flow rates; the channel geometry has no significant influence. In general, at a constant liquid flow rate, low gas flow rates result in bubble flow, characterized by small bubbles (diameter of spherical bubbles is smaller than the channel diameter). Increasing the gas flow rate results in segmented gas-liquid flow, where long gas bubbles with a diameter closed to the channel diameter alternate with liquid slugs. Further increasing of the gas flow rate results in annular flow, characterized by a continuous gas core in the center and surrounded by a liquid film. Flow pattern maps characterize the obtained flow pattern depending on the superficial velocities. Although this is a good possibility to predict the flow pattern, their use is limited to a special reactor design and to the fluids used in the experiment. Creating universal flow pattern maps with the Buckingham theorem, where the Re- and We-numbers allow the generalization of the fluid properties, expands the use of the flow pattern maps to varying fluids. We confirmed this theory for various fluids with different properties (water, ethanol, glycerine) and nitrogen for different pressure values. Beside the resulting flow pattern, fluid viscosity, density and surface tension have a strong impact on the resulting interfacial area. The slug and liquid length distribution and the curvature of the gas bubble depend at defined reactor geometry on these properties. Increasing the pressure shifts the transition lines from the bubbly flow to the segmented flow and from the segmented flow to the annular flow to lower liquid velocities at a constant gas velocity. In the segmented flow, high pressure values result in decreasing bubble radii; both bubble ends are more flattened at elevated pressure values. The liquid segment length distribution is broadened at elevated pressure values. In the segmented gas-liquid flow, obtained at intermediate gas and liquid flow rates, the liquid slug features a recirculation, symmetric to the channel geometry. This recirculation can be influenced by meandering channels. We observed a liquid segment elongation at the channel curvature, and, consequently, a decrease in the bulk velocity. The internal relative velocities dropped as well as the swirling motion. Another possibility of designed recirculation in the liquid segment is given by adapting the liquid properties. As the experiments with different liquids showed, lower surface tension increases the internal recirculation velocities and mixing quality. At constant flow rates, the highest relative velocities and swirling strength were measured for the system ethanol-nitrogen. [1]

Conclusion: We presented the influence of the flow rates, the liquid properties and the channel geometry on the flow regime and liquid velocity. As tailored hydrodynamics are the key function for advanced reaction engineering, this fundamental research offers the possibility for enhanced conversion rates.

References: [1] Wächli,S. and Rudolf von Rohr, Ph.: Two-Phase Flow Characteristics in Gas-Liquid Microreactors, International Journal for Multiphase Flow; accepted for publication (2006)