(36e) Gas/Liquid Mass Transfer and Hydrodynamics in Microreactor: Effect of Geometry and Liquid Properties

Manufacture of chemicals often involves gas-liquid reactions and recently has been tested in microreactors aiming at the effective use of raw materials and energy [1]. Microreactors have an equivalent hydraulic diameter up to few hundreds of microns and therefore, provide high mass- and heat transfer increasing drastically performance as compared to the conventional reactors [2].

In order to design a microreactor to carry out a gas-liquid reaction, the knowledge of hydrodynamics and mass transfer performance for various capillary geometries is needed. There are a great number of studies suggesting correlations for flow characteristics in pipes with hydraulic diameter above 1mm. However, these correlations cannot be applied for micro-devices.

The complex interaction between interfacial, viscous and inertia forces is responsible for a variety of flow patterns with regularly shaped interface [3]. In the case of gas-liquid flow in micro-capillaries, bubbly, slug, churn, slug-annular and annular flow can be observed [4]. The main problem for controlling flow pattern is due to the flow dependence on many experimental parameters like linear velocity, the ratio of phases, fluid properties, channel geometry (dimensions and cross-sectional shape) and the micro-reactor material.

In this work, the flow of CO₂ - deionised water system was investigated by a high speed camera using three different glass microreactors: 1) T-shaped with rectangular cross-section and hydraulic diameter of 400 µm; 2) T-shaped with trapezoidal cross-section and hydraulic diameter of 400 µm; 3) Y-shaped chicane mixer with a diameter of 1 mm. Moreover, the influence of liquid properties on flow pattern formed was studied by adding ethylene glycol, acetone or glycerol to water changing viscosity and surface tension. The values of the liquid side volumetric mass transfer coefficients were determined for physical absorption of CO₂ in water. Based on the design of experiment (DOE) approach, correlations of the mass transfer coefficients as a function of the gas and liquid linear velocities for the slug flow and annular flow regimes were developed.

During the presentation, the influence of liquid phase properties and of the micro-capillary geometry on flow pattern, flow pattern transitions and the flow stability domains will be discussed. Dimensionless number (Reynolds, Capillary & Weber) will be used to develop a universal tool for the prediction of hydrodynamics of different fluids in micro-capillaries. The correlations describing the mass transfer performance of the slug and annular flow regimes in different microreactors will be suggested.

References

[1] Stankiewicz et al. Chemical Engineering Progress 96, 22-34 (2000).

[2] Kiwi-Minsker et al. Catalysis Today, 110, 2-14 (2005).

[3] Gunther et al. Lab on a Chip 6, 1487-1503 (2006).

[4] Yue et al. Chemical Engineering Science 62, 2096 ? 2108 (2007).