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.