Flow patterns and their influence on trickle-bed reactors: Experiments and theory.

摘要

Gas-liquid downflow packed-bed reactor, also termed trickle-bed reactor, is one of the most commonly used and studied multiphase reactor configurations. However, it is also one of the least understood, as inadequate quantitative knowledge of the extremely complex gas-liquid flow conditions; such as the effects of phase distributions and dynamics on transport coefficients thus overall reaction outcome, prevent a priori design of such reactors.; This dissertation is directed towards understanding the influence of transient flow environments, especially pulsing flow, on reactor performance. This is accomplished by first characterizing the various gas-liquid flow regimes. A novel high-speed heat transfer measuring technique was introduced to detect the flow fluctuations, which performs better than traditional pressure transducer or conductance probe methods. Based on the measurements, a four-parameter heat transfer model involving heat transfer in liquid-rich pulsing and gas-continuous bases, pulsing frequency and pulse fraction was developed to unify the various flow regimes. Such square-wave model can be extended to other fluctuating transport quantities as well, and be used in any trickle-bed reactor simulations that account for the dynamic interactions of catalytic reactions and fluid dynamics.; It was found that fluid-solid heat transfer coefficients in pulses and bases corresponded to liquid-solid and gas-solid heat transfer respectively, and their values in pulses were roughly 3 to 4 times higher than those in bases for air and water system. Pulsing frequency, which can significantly impact reaction, may be tuned by selecting appropriate packing size, since smaller sizes generate higher frequency pulses (frequency doubled in 6mm packing as compared to that with 8mm packing). Careful examination indicated that there existed an intermediate liquid flow rate where the heat removal efficiency, which accounted for pressure drop penalty, was optimal.; The effects of flow regimes on trickle-bed reactor performance were investigated in reactor simulations. Isothermal model results were found to be in good agreement with experimental data obtained from a laboratory-scale reactor. Results from various qualitative, quantitative and nonisothermal models all showed significant reactor performance enhancement (up to 10% conversion and 40% selectivity) under pulsing flow versus steady trickling flow environment, and the highest selectivity percent improvement happened under slightly liquid-limiting conditions.; The results of this work provide a general understanding that can be applied to other multiphase reaction systems.