HYDRODYNAMICS AND LIQUID-SOLID MASS-TRANSFER MECHANISMS IN PACKED-BEDS OPERATING IN COCURRENT GAS-LIQUID DOWNFLOW - AN EXPERIMENTAL-STUDY USING ELECTROCHEMICAL SHEAR RATE SENSORS

Overall and local hydrodynamics and liquid-solid mass transfer mechani sms were investigated in a laboratory scale packed bed reactor operati ng in gas-liquid downflow. The mean liquid saturation and the liquid-s olid mass transfer rate were determined using classical electrochemica l techniques and the local instantaneous hydrodynamics were analyzed u sing electrochemical shear rate sensors. The experimental results as w ell as theoretical considerations enabled us to contribute to the eluc idation of the gas-liquid flow mechanisms, especially in high-interact ion regimes. In pulse flow at low liquid flow rates (L < 10 kg m-2 s-1 ) the wall is, on average, not entirely wetted, this might explain hot spot occurence in industrial fixed-bed reactors. In dispersed bubble flow and in the liquid rich slugs of pulse flow, the local instantaneo us liquid-wall shear rate is characterized by high-amplitude-high-freq uency fluctuations. The comparison of the space averaged shear rate me asurements with the overall mass transfer rate indicates that the liqu id-solid mass transfer mechanism is laminar in nature and may be model ized by a succession of developing laminar boundary layers. An overall mechanical force balance on the liquid shows that the average drag of the liquid by the gas is very small, compared to the total energy dis sipated by the gas in the reactor. All the experimental results obtain ed in this work as well as several literature data can be explained by a flow mechanism in dispersed bubble flow, where the liquid flow is d ominated by viscous forces whereas the gas bubbles pass through the pa cked bed by pressure pulses.