The formation of gas bubbles and their subsequent rise due to buoyancy are very important fundamental phenomena that contribute significantly to the hydrodynamics in gas-liquid reactors. The rise of a bubble in dispersion can be associated with possible coalescence and dispersion followed by its disengagement from the system. The phenomenon of bubble formation decides the primitive bubble size in the system (which latter attains an equilibrium size), whereas the rise velocity decides the characteristic contact time between the phases which governs the interfacial transport phenomena as well as mixing. In view of their importance, we herein present a comprehensive review of bubble formation and bubble rise velocity in gas-liquid systems. The emphasis of this review is to illustrate the present status of the subjects under consideration and to highlight the possible future directions for further understanding of the subject. The bubble formation at a single submerged orifice and on multipoint sieve trays in Newtonian as well as non-Newtonian stagnant and flowing liquids is discussed in detail, which includes its mechanism as well as the effect of several system and operating parameters on the bubble size. The comparison of results has shown that the formulation of Gaddis and Vogelpohl 22 is the most suitable for the estimation of bubble size in stagnant liquids. The special cases, such as bubble formation in reduced gravity conditions and weeping and in flowing liquids, are discussed in detail. The section on the rise of a gas bubble in liquid covers the various parameters governing bubble rise and their effect on the rise velocity. A comprehensive comparison of the various formulations is made by validating the predictions with experimental data for Newtonian as well as non-Newtonian liquids, published over last several decades. The results highlight that for the estimation of rise velocity in (i) pure Newtonian liquids, (ii) contaminated Newtonian liquids, and (iii) non-Newtonian liquids, the formulation based on the wave theory by Mendelson,190 Nguyen's formulation, 155 and the formulation by Rodrigues, 153 (last two, based on the dimensional analysis), respectively are the most suitable. The motion of bubbles in non-Newtonian liquids and the reason behind the discontinuity in the velocity are also discussed in detail. The bubble rise is also analyzed in terms of the drag coefficient for different system parameters and bubble sizes.

Metal oxide nanoparticles are an important class of nanomaterials that have found several applications in science and technology.

Segmented flow is often used in the synthesis of nanomaterials to achieve narrow particle size distribution. The narrowness of the distribution is commonly attributed to the reduced dispersion associated with segmented flows. On the basis of the analysis of flow fields and the resulting particle size distribution, we demonstrate that it is the slip velocity between the two fluids and internal mixing in the continuous-phase slugs that govern the nature of the particle size distribution. The reduction in the axial dispersion has less impact on particle growth and hence on the particle size distribution. Synthesis of gold nanoparticles from HAuCl(4) with rapid reduction by NaBH(4) serves as a model system. Rapid reduction yields gold nuclei, which grow by agglomeration, and it is controlled by the interaction of the nuclei with local flow. Thus, the difference in the physical properties of the two phases and the inlet flow rates ultimately control the particle growth. Hence, a careful choice of continuous and dispersed phases is necessary to control the nanoparticle size and size distribution.

Hydrodynamics and mass transfer of gas−liquid flow are explored under ambient conditions in an Advanced-Flow Reactor (AFR), an emerging commercial system designed for continuous manufacture. Carbon dioxide/water is the model system used in this study for a range of flow rates for gas and liquid of 5.6−103 mL/min and 10−80 mL/min, respectively. Bubble size distribution, gas holdup, specific interfacial area, pressure drop, and mass transfer coefficients are determined from flow visualization experiments and compared with conventional gas−liquid contactors. These variables are mainly influenced by the inlet flow rates and inlet composition. Average bubble sizes (d B ) of 0.9−3.8 mm, gas holdup (ε G ) of 0.04−0.68, specific interfacial areas (a) of 160−1300 m 2 /m 3 , and overall mass transfer coefficients (k L a) of 0.2−3 s −1 were obtained for the vertical orientation of the AFR. Although effect of gravity is present for this system, no significant effect on the hydrodynamic properties was observed. The measured pressure drop for vertical orientation (3.6−53.4 kPa) was used to estimate power consumption, which is used as a metric to compare mass transfer efficiency among different gas−liquid contactors. A power law relationship was obtained for the overall mass transfer coefficients in terms of power input and gas holdup, given by k L a = 0.101P w 0.443 ε G 0.459 . The design of the AFR with a series of heart-shaped confined sections with obstacles enhances continuous breakup and coalescence of bubbles providing interfacial areas and mass transfer coefficients 1 order of magnitude larger than other gas−liquid contactors, such as bubble columns (50−600 m 2 /m 3 ; 0.005−0.24 s −1 ) and spray columns (75−170 m 2 /m 3 ; 0.015−0.022 s −1 ), and 1 order of magnitude smaller than gas−liquid microchannels (3400−9000 m 2 /m 3 ; 0.3−21 s −1 ) or falling film reactors (20,000 m 2 /m 3 ).