Design constraints for gas-liquid catalytic microreactors

Üner, Necip Berker
Microreactors are a promising class of chemical reactors, which can provide high conversion, selectivity, heat-mass transfer rates and safety in production. They can accommodate gas-liquid or gas-liquid-solid reactions very well. The aim of this study is to present some new mass transfer and kinetics oriented physical phenomena and operation strategies that can emerge in multiphase microreactors, which are usually overlooked in macroscale reactors. The first example is demonstrated experimentally by absorbing NO into ferrous sulfate solutions with help of a novel contactor. It is observed that when the liquid layer is almost stagnant with respect to the gas flow, Marangoni convection currents occur upon gas absorption. These are dimmed by adding chemical reactants to the liquid, but then a surface poisoning effect may occur, which decreases uptake rates significantly. In addition to this homogeneous reaction-diffusion example, the applicability of classical mass transfer theories to finite films is questioned and quantitative limitations are given for the use of penetration theory for gas-liquid mass transfer in thin films. The saturation-depletion limits for contact times are also provided. A general solution for diffusion into a flowing liquid film with nth order reaction is presented, in order to demonstrate the effects of velocity field on mass transfer rates. As a gas-liquid-solid example, low-temperature Fischer-Tropsch synthesis is investigated. Effectiveness factors for diffusion with negative order reaction are presented. Conceptual periodic operation of Fischer-Tropsch synthesis is discussed and a model for Taylor flow is generated. Analogous to the homogeneous reaction part, a general solution of mass transfer to a flowing liquid film with surface reaction is presented, whereby the reaction initiation times are deduced for any flow field in the film.