Flow regime microreactors

The microreactor contained a single channel with two feeds for gas and liquid [309,273]. The gas flows were set so high that an annular-flow regime was reached with a central gas core surrounded by a liquid film wetting the channel. This flow pattern has a very high interface and low liquid-side resistance due to the thin film. Formic acid was used as a solvent. 

The standard convection-diffusion equations rely on the assumption of local thermal equilibrium. For gas flow, this means that a Maxwell-Boltzmann distribution is assumed for the velocity of the particles in the frame-of-reference comoving with the fluid. When gas flow in microreactors at high temperature or low pressure is considered, this assumption may break down. The principle quantity determining the flow regime of gases and deviations from the standard continuum description is the Knudsen number, defined as... 

In the first two cases the Navier-Stokes equation can be applied, in the second case with modified boundary conditions. The computationally most difficult case is the transition flow regime, which, however, might be encountered in microreactor systems. Clearly, the defined ranges of Knudsen numbers are not rigid they rather vary from case to case. However, the numbers given above are guidelines applicable to many situations encountered in practice. 

Unfortunately, the extreme numbers are somewhat misleading. In order to properly understand their context for biological application, an understanding of microbubble column flow regimes is necessary. Bubble velocity in microreactors is defined by a gas space velocity. This velocity is similar to the superficial gas velocity, but it tends to be slower due to very significant wall effects. So, the gas space velocity is approximated using a sample bubble velocity. 

Kashid, M. N., Agar, D. W. (2007). Hydrodynamics of liquid-liquid slug flow capillary microreactor flow regimes, slug size and pressure drop. Chemical Engineering Journal, 131, 1-13. 

Interdigital mixers and micromixer with the split-recombine technique Single microchannel operating in annular flow regime Microstructured falling film reactor Mesh microreactor... 

Weber and coworkers reported catalyst screening of the Stille coupling reaction using a capillary microreactor (75 pm i.d., 6.7 m length) [17]. Optimum palladium catalysts and ligands were screened effectively by the flow regime, where the reaction products were analyzed by on-line gas chromatography. 

S. McGovern, G. Harish, C. S. Pai, W. Mansfield, J. A. Taylor, S. Pau, R. S. Bessei Multiphase flow regimes for hydrogenation in a catalyst-trap microreactor, Chem. Eng. J. 2008, 135, S229-S236. 

Figure 0.4 Observed flow regimes in the capillary microreactor (Y-junction ID = 1 mm, capillary ID = 1 mm), (a) Slug flow, (b) drop flow, and (c) deformed interface flow. (Adapted from Kashid, M.N. and Agar, D.W., Chem. Eng. J. 131, 1, 2007.)...

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