Two Phase Flow And Mass Transfer And Conversion Characteristics In Microchannel With Catalytically Reactive Boundaries

As an emerging development direction, the micro-chemical technology is becoming an increasingly hot spot in the field of chemical engineering. It is important to study the flow, mass and heat transfer characteristic in gas-liquid-solid micro-reactor, but until now, few researchers have deeply looked into the mechanism of three phase micro-reactor, most researchers have focused on mass transfer and flow characteristic in gas-liquid, gas-solid, liquid-liquid micro-reactor without reaction or with homogeneous reaction.In this paper, we use numerical simulation method to obtain an insight into two phase flow, mass transfer and heterogeneous catalytic reaction in microchannel with catalytically reactive boundaries. And the hydrodynamics and mass transfer and conversion in a planar catalytic membrane micro-reactors is also studied. The main studies and conclusions are as follows.① Mass transfer and conversion characteristic during annular flow in circular microchannel is studied. The reaction rate increases with inlet nitrobenzene concentration, reaction temperature and gas pressure increasing but it decreases rapidly in downstream of reactor. The concentration of hydrogen mainly depends on gas pressure. Meanwhile the nitrobenzene conversion increases with reaction temperature, gas pressure increasing and inlet nitrobenzene concentration decreasing. For this second order reaction, gas pressure and reaction temperature have a greater effect on nitrobenzene conversion than inlet nitrobenzene concentration.② Mass transfer and conversion characteristic during Taylor flow in circular microchannel is studied. The volumetric mass transfer coefficient increases with bubble velocity and bubble length but is independent of surface reaction. For the first order reaction, the volumetric reaction rate of catalytic layer increases with reaction rate constant, catalytic layer thickness and porosity increasing, while the overall volumetric reaction rate coefficient depends on volumetric mass transfer coefficient and volumetric reaction rate of catalytic layer. When the reaction kinetics constant is less than 6.5s-1, or the catalytic layer thickness is less than 5.7 mm, or the catalytic layer porosity is greater than 0.3 in this basic case, the catalyst effectiveness will be bigger than 0.95, the internal mass transfer can be negligible. If the catalytic layer thickness continues decreasing, when catalytic layer thickness is less than 0.752 mm the volumetric mass transfer coefficient will be much larger than catalytic layer volumetric reaction rate coefficient, which means that the external mass can also be negligible and the reaction is controlled by intrinsic kinetics. If the catalyst activity is high enough, when the reaction kinetic constant is bigger than 153.4s-1, the volumetric mass transfer coefficient will be much less than catalytic layer volumetric reaction rate coefficient, which means reaction rate will be controlled by mass transfer. At the same time, α-methyl styrene conversion(XAMS) increases with reaction kinetics constant, catalytic layer thickness, effective diffusion constant, bubble velocity and bubble length increasing. While the bubble velocity markedly impacts the residence time of the reactants which is also very important to the reactants conversion③ Mass transfer and conversion characteristic during annular flow in rectangle microchannel is studied. The liquid mainly flow in corner of rectangle microchannel and the liquid velocity is very small in the thin film region. The reaction rate is obviously faster in the thin film region. Meanwhile the concentration distribution of nitrobenzene is uniform except the direction of motion in liquid side and catalytic layer, which means that internal and external mass transfer can be neglected and the reaction is controlled by intrinsic kinetics in this case. While the internal mass transfer become dominated with the reaction kinetics constant, gas pressure and inlet concentration of nitrobenzene increasing and catalytic layer porosity decreasing.④ Mass transfer and conversion characteristic during annular flow in rectangle microchannel is studied. The hydrogen concentration increases more quickly and the nitrobenzene concentration deceases faster near thin film region because of shorter diffusion distance. The catalyst effectiveness decreases with catalyst activity and gas pressure. There is distinct difference between second order reaction and first order reaction to calculate the catalytic layer reaction rate coefficient. For the second order reaction, the catalytic layer reaction rate coefficient is proportional to the nitrobenzene concentration at the liquid-solid interface. So the catalytic reaction rate coefficient will decrease more quickly with increasing catalyst activity, gas pressure and bubble velocity. The volumetric mass transfer coefficient increases with bubble velocity, and overall volumetric reaction rate coefficient depend on both volumetric mass transfer and catalytic layer volumetric reaction rate coefficient.⑤ Mass transfer and heterogeneous catalytic reaction and conversion characteristic in a planar catalytic membrane micro-reactor is studied The nitrobenzene conversion decreases with liquid velocity increasing and inlet concentration increasing, because the increasing of liquid velocity will reduce the residence time and the increasing of inlet concentration will lower the catalyst activity. However, the mass transfer capacity in liquid can improve with increasing liquid velocity. At the beginning, the performance of micro-reactor is enhanced with catalytic thickness, catalyst activity, and gas pressure increasing, and its performance tends to be constant afterwards. However the catalyst effectiveness decreases with reactants concentration, catalytic layer thickness and catalyst activity increasing.