Bubbles breakup

Based on the same assumption, the results by Viswanathan et al. on gas holdup in beds of larger particles are in agreement with the results of Lee (L3) on bubble breakup in beds of larger particles (05). No work on the residencetime distribution of the gas phase in gas-liquid fluidized beds has come to the author s attention. 

The most efficient turbulent eddies for bubble breakup are eddies of the same size as the bubbles. Large eddies will merely move the bubbles and smaller eddies do not have sufficient energy to break up the bubbles. Assuming that the most efficient eddies to break up fluid particles are eddies of the same size as the bubble, that is, A. 2d, gives the required turbulent energy dissipation... 

Breakup will occur due to high turbulence and high shear rate. This will occur in the impeller region and close to the walls. In a stirred tank, almost all breakup of the bubbles occurs in the impeller region. According to Eq. (15.3), the energy required to break up a 5-mm bubble is on the order of 1W m 3, while 35 W m-3 is required to break up a 1-mm air bubble in water. A high rate of bubble breakup... 

Small bubbles and flow uniformity are important for gas-liquid and gas-liquid-solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m. 

Keywords Airlift reactor Internal Radial profile Gas holdup Bubble size Bubble breakup... 

When a gas stream is introduced into a turbulent liquid flow in a motionless mixer, the gas is broken up into bubbles. The breakup is due mainly due to the turbulent shear force of the liquid but, for motionless mixers, also partly to the collision between the gas and the leading edge of an element. Gas dispersion is a physical process and involves bubble breakup and coalescence, which can both take place in the same mixer/reactor. 

Bubble breakup and coalescence are both complex processes. In a turbulent-flow held, bubbles are broken up mainly due to the turbulent shear force, and the eventual bubble size is a balance between this force and the surface tension force. For a given gas-liquid system and how held, a maximum bubble size exists. Any bubbles larger than this size will be broken up. According to theory (14), this maximum bubble size relates to gas-liquid physical properties and flow characteristics ... 

Figure 9.13. Bubble coalescence and breakup processes (a) Bubble coalescence (b) Bubble breakup.

Favelukis et al. (37,38) dealt with the problem of droplet deformation in exten-sional flow with both Newtonian and non-Newtonian Power Law model fluids, as wellas bubble breakup. For the Newtonian case, they find that as an inviscid droplet (or bubble) deforms, the dimensionless surface area is proportional to the capillary number... 

The breakup or bursting of liquid droplets suspended in liquids undergoing shear flow has been studied and observed by many researchers beginning with the classic work of G. I. Taylor in the 1930s. For low viscosity drops, two mechanisms of breakup were identified at critical capillary number values. In the first one, the pointed droplet ends release a stream of smaller droplets termed tip streaming whereas, in the second mechanism the drop breaks into two main fragments and one or more satellite droplets. Strictly inviscid droplets such as gas bubbles were found to be stable at all conditions. It must be recalled, however, that gas bubbles are compressible and soluble, and this may play a role in the relief of hydrodynamic instabilities. The relative stability of gas bubbles in shear flow was confirmed experimentally by Canedo et al. (36). They could stretch a bubble all around the cylinder in a Couette flow apparatus without any signs of breakup. Of course, in a real devolatilizer, the flow is not a steady simple shear flow and bubble breakup is more likely to take place. 

The gas axial mixing is due to the bubble size distribution resulting in a distribution of bubble rise velocities, which varies along the column due to bubble breakup and coalescence. There are a variety of correlations in the literature, with varying results and reliability, for instance, the correlation of Mangartz and Pilhofer [Verfahrenstechn., 14 40 (1980)]. 

For a gas bubble in liquid, Henriksen and Ostergard (H7) have observed that the bubble is broken up by disturbances created by the downward jet of liquid. According to their observation, a finger of liquid projects down from the roof and eventually divides the bubble in two. The reason that bubble breakup in methanol is easier than in water is due to lower surface tension. Based on these observations, they support the hypothesis of Clift and Grace that the bubble is broken up as a result of the Taylor instability. 

Rigby, G.D., Evans, G.M. and Jameson, G.J. (1997), Bubble breakup from ventilated cavities in multiphase reactor, Chem. Eng. Sci., 52, 3677-3684. 

Luo H, Svendsen HF (1993) Theoretical Model for Drop and Bubble Breakup in Turbulent Dispersions. AIChE J 42(5) 1225-1233. 

Lee C-K, Erickson LE, Glasgow LA (1987) Bubble Breakup and Coalescence in Turbulent Gas-Liquid Dispersions. Chem Eng Comm 59(l-6) 65-84... 

Luo H (1993) Coalescence, break-up and liquid circulation in bubble column reactors. Dr ing Thesis, the Norwegian Institute of Technology, Trondheim Luo H, Svendsen HF (1996) Theoretical Model for Drop and Bubble Breakup in Turbulent Dispersions. AIChE J 42(5) 1225-1233... 

---