INTRODUCTION

Heterogeneous catalytic reaction is one of the most important reactions in the industrial production of fine chemicals.1-3 The multiphase mass transfer efficiency can be intensified by stirring, reducing catalyst particle size, increasing reaction pressure and gas-liquid ratio.4-6 In a fixed bed reactor, a continuous flow equipment, traditional gas-liquid distributors have many shortcomings, such as poor gas-liquid dispersion and unclear mixing, resulting in a low concentration of gas-phase reactants in the liquid phase, which seriously affects the performance of the multiphase catalytic reaction. In order to overcome this limitation, many techniques have been taken. Pickering emulsions,7 microfluidic devices,8 higee technology,9,10ultrasound method,11 microwave method12 and tube-in-tube technique13 have been developed to boost interfacial area and gas adsorption. However, these methods require additional additives or complex equipment. It is crucial to develop a low-energy, continuous-feed gas distributor that strengthens gas-liquid mixing.
There are three approaches to enhance gas-liquid mass transfer: (a) increasing the liquid mass transfer coefficient; (b) increasing the saturated concentration of gas phase in liquid phase; (c) increasing the gas-liquid interfacial area.14 The enhancement of mass transfer coefficient and saturated concentration requires high energy consumption, and the relatively easy method of increasing the gas-liquid interfacial area makes it the best way of gas-liquid mass transfer. The gas-liquid interfacial area can be increased by reducing the diameter of the bubbles in the gas-liquid system. Bubbles of millimeters and microns in size are capable of continuously produced by the micro-nano porous structure of ceramic membranes.15 The gas-liquid two-phase interfacial area is elevated by increasing the gas holdup and reducing the size of the bubbles. Small size bubbles have a large specific surface area, which is beneficial to heighten the gas-liquid mass transfer rate. The microchannels can effectively reduce bubble size and have the advantages of uniform mixing, fast mixing rate and good stability, while the fluid cleanliness requirements are high and the pressure drop of the channel is relatively large due to the small mixing area.16-18 Another method is membrane dispersion technology, which uses porous membrane materials as dispersion media to achieve micro-scale mixing. Compared to T- and Y-mixers, the porous structure of the ceramic membrane acts as a gas distributor, generating microbubbles in the liquid phase. Bubbles prepared through the nano-micro pores of ceramic membranes are typically several hundred micrometers in size. Micrometer-sized bubbles coalesce to form millimeter-sized bubbles in the rising process, which undoubtedly reduces the phase interfacial area with the liquid phase. The stability of bubble rising process deserves to be noted and the solutions should be presented.
In recent years, ceramic membranes have attracted more and more attention due to their excellent gas-liquid dispersed property. Khirani et al.16 investigated the effects of different combinations of dispersed and continuous phases on microbubble generation, microbubble size proposing to the physicochemical properties of the two phases and the properties of the membrane surface. Chen et al.19-23 evaluated the macroscopic dispersion of bubbles enhanced by membrane dispersion. In addition, they coupled the numerical simulation method of Navier-Stokes equation and Darcy equation to predict the gas permeation process in porous ceramic membranes. Zhang et al.24-26 compared the effects of hydrophilic ceramic membranes and hydrophobic PTFE membranes, solvent type, and internals on bubble size. However, most works have focused on the structural parameters of ceramic membranes and the preparation of microbubbles. Very few reported shape and size changes of bubbles and the effects of trajectory on bubble stability during rising process.
In this work, a single-channel ceramic membrane was applied as a gas distributor, and high-speed photograph technology was used to investigate the effect of operating conditions on the gas-liquid two-phase flow pattern and bubble distribution, aiming to controllably prepare bubbles of corresponding sizes. Firstly, a CMGD was built to prepare micrometer-sized bubbles, and a high-speed photograph setup was constructed above the CMGD. Secondly, the effects of ceramic membrane length and gas pressure on bubble size were investigated. Subsequently, the bubble flow patterns were classified and the flow pattern transition line was obtained. Finally, the effects of gas flow rate, liquid flow rate and rising distance on bubble size were discussed, and the modeling of bubble size was proposed and verified. In addition, visualization experiment also analyzed the coalescence process and motion trajectory of the bubbles, and installation of baffle-type internals influence the trajectory of bubbles, leading to inhibiting bubble coalescence and making the bubbles rise stably.