Hydrodynamics And Liquid Mixing At Single Channel And Whole Bed Scales In Multiphase Monolithic Reactor

Multiphase monolithic reactors (MMRs) as novel reactors for process intensification offer superior advantages including low pressure drop, high geometric surface area, good mass transfer, and easy scale-up, compared with the traditional multiphase reactors such as trickle beds and slurry bubble columns. MMRs are promising catalyst packings suitable for catalytic gas-liquid-solid reactions encountered extensively in chemical, petrochemical, biochemical, and environmental processes. In such applications, understanding the gas-liquid hydrodynamics in the channels and liquid mixing in monolith bed is an important issue. The objective of present work is to study hydrodynamics and liquid mixing at a single channel scale and a monolith bed scale.Firstly, with a particular focus on the characterization of gas-liquid Taylor flow in individual channels of monolithic beds used for catalytic gas-liquid-solid reactions, a micro double-tip conductivity probe was developed for the measurements and characterization of bubble dynamics in capillary two-phase flows and then used to investigate the Taylor flow hydrodynamics in vertical capillaries with circular cross section of2.98mm in hydraulic diameter, including flow regime, bubble rise velocity, and liquid slug length in a wide range of gas and liquid superficial velocities. It is demonstrated that the micro double-tip conductivity probe method is implemental and accurate for identifying flow regimes in small-scale capillaries and measuring bubble dynamics parameters of the Taylor flow with the advantages of simple making, low cost, easy use and high accuracy. Furthermore, variation trends of bubble rise velocity and frequency, and liquid slug length with varying gas and liquid superficial velocities in the Taylor flow regime are demonstrated based on the data obtained, and correlations for prediction of the above parameters were obtained. CFD simulation is able to display Taylor flow in a capillary and the results matched well with experimental data.Secondly, two-phase flow and axial liquid dispersion characteristics in single capillary Taylor flow were experimentally investigated by CCD imaging method and conductivity probes using pulse tracer of KC1solution, respectively. Liquid mean residence time, axial dispersion coefficient and hydrodynamics parameters were measured with tap water as liquid phase and nitrogen as gas phase in a range of gas and liquid superficial velocities. It is demonstrated that gas holdup increase with the increase of superficial gas velocity and decrease with the increase of superficial liquid velocity; the mean residence time decreases and the axial diffusion increases with increasing either gas or liquid superficial velocity. A correlation of Peclet number was presented and well predicted the experimental data within a relative deviation of±20%.Then, experiments were carried to investigate the liquid flow distribution and hydrodynamic parameters including gas holdup, bubble frequency, bubble velocity and bubble length at high gas/liquid ratios in a cold model column packed with a0.048m diameter and400cpsi monolithic bed. Three types of distributor for the liquid distribution were adopted to evaluate their distribution performance. Local liquid saturation in individual channels was measured by16inserted single-point optical fiber probes within the bed. The results indicate that (1) The optical fiber probe method is efficient and convenient to measure phase distribution at channel or monolith scale, especially in opaque reactors like monolithic beds;(2) Within the range of high gas/liquid ratios under which experiments were conducted, two flow regimes occurred, i.e. Taylor and churn. The two phase flow structure remains unchanged along the axial direction of each channel, while liquid saturation distribution along the radial direction of monolith bed are non-uniform depending the distributor design and phase velocities; and (3) The tube array distributor provides superior liquid distribution performance over the showerhead and nozzle distributors;(4) Two correlations for predicting bubble velocity and bubble length applicable to not only single channel but also monolith bed were proposed and matched well with the experimental data. The scale-up effect is quantified within the range of-30%to30%.Finally,"tracer-response" experiments were conducted in three types of cell density (50cpsi,100cpsi,400cpsi) integral and segmented multiphase monolithic reactors separately, with tap water as liquid phase, air as gas phase, nozzle/glass beads static distributor as liquid distributor, operated at superficial liquid velocities of0.0283-0.0637m/s and superficial gas velocities of0.0490-0.2942m/s, in cocurrent downflow mode. The "tracer-response" concentration vs. time data were measured at different superficial gas/liquid velocities using conductivity probe method. Based on the experimental data, the influence of superficial gas/liquid velocities on the degree of axial dispersion was analyzed, the effect of superficial gas/liquid velocities and cell density on the average residence time, the bed pressure drop, the average liquid holdup and the average liquid slug length was studied, and the Peclet number correlation was obtained in nozzle liquid distributor integral multiphase monolithic reactors; the comparison of the influence of superficial gas/liquid velocities on the average residence time, the bed pressure drop, the average liquid holdup and the average liquid slug length, and of the degree of axial dispersion in the two liquid distributors integral and segmented multiphase monolithic reactors was done. The results show that:(1) The influence of superficial gas/liquid velocities on the average residence time, the bed pressure drop, the average liquid holdup and the average liquid slug length in segmented multiphase monolithic reactors is almost consistent with that in integral multiphase monolithic reactors;(2) The degree of axial dispersion is less when using nozzle as liquid distributor;(3) Setting redistribution segments could reduce liquid axial dispersion in multiphase monolithic reactors;(4) A Peclet number correlation is proposed and has an predictive accuracy within a relative deviation of±30%.