Bubble size control

Bubble size control is achieved by controlling particle size distribution or by increasing gas velocity. The data as to whether internal baffles also lower bubble size are contradictory. (Internals are commonly used in fluidized beds for heat exchange, control of soflds hackmixing, and other purposes.)... 

Bubbles, in fluidized beds, 11 805-806 Bubble size control, 11 805 in fluidized beds, 11 819, 821 Bubble size distribution, 12 14 in foams, 12 11 Bubble tear-offs, 20 229 Bubble tray absorbers, 1 27, 29 design, 1 83-86 Bubble-tube reactor, 25 194 Bubble tube viscometer, 21 739 Bubble two-phase theory of fluidization, 11 805-806... 

Bubble-size control was also critical. The intensity of scattering by nonresonant gas bubbles is proportional to the sixth power of the radius of the bubble. Hence, the larger the bubble, the better the scattering intensity. However, the acceptable upper size limit for in vivo administration is determined by the need for bubbles to cross capillary beds. Bubbles larger than 6-8 pm should be avoided as they are trapped in the lung capillaries. The current accepted sizes are in 1-7 pm, preferably around 3 pm, with as narrow a size distribution as possible. Bubble shell material needs to be biodegradable. Soft shells are generally preferable, as they minimally impede US backscatter. 

Nemec L (1980) The behavior of bubbles in glass melts Part 1. Bubble size controlled by diffusion. Glass Technology 21 134-138... 

Part A gives general guidelines for the design of large commercial fluidized bed reactors with respect to the following aspects (1) solids properties and their effect on the quality of fluidization (2) bubble size control through small solid particle size or baffles (3) particle recovery by means of cyclones (4) heat transfer tubes (5) solids circulation systems (6) instrumentation, corrosion and erosion, mathematical models, pilot plants and scale-up techniques. 

Kukizaki M, Nakashima T, Song J, Kohama Y (2004) Monodispersed nano-bubbles generated from porous glass membrane and bubble size control, Kagaku Kogaku Ronbunshu 30-5 654-660... 

Uses Mineral processing surfactant frothing agent maintaining effective bubble size control provides superior str. and stability necessary for effective removal of min. particles Properties M.w. 200 Use Level 5-50g/fon Unifioth 250 [Huntsman]... 

The maximum bubble size for Group A powders is of great significance for design. The single most important parameter controlling bubble size is... 

Steam-liquid flow. Two-phase flow maps and heat transfer prediction methods which exist for vaporization in macro-channels and are inapplicable in micro-channels. Due to the predominance of surface tension over the gravity forces, the orientation of micro-channel has a negligible influence on the flow pattern. The models of convection boiling should correlate the frequencies, length and velocities of the bubbles and the coalescence processes, which control the flow pattern transitions, with the heat flux and the mass flux. The vapor bubble size distribution must be taken into account. 

In bubbling, the control of the bubble diameter is a little easier. In these methods bubbles are made at an orifice or a multitude of orifices. If there is only one orifice, of radius r, and if bubble formation is slow and undisturbed, the greatest possible bubble volume is 27rry/gp] y is the surface tension of the liquid, p the difference between the densities of liquid and gas (practically equal to the density of the liquid), and g is acceleration due to gravity. Every type of agitation lowers the real bubble size. On the other hand, if there are many orifices near enough to each other, the actual bubble may be much larger than predicted by the above expression. 

The primary factor controlling how much gas is in the form of discontinuous bubbles is the lamellae stability. As lamellae rupture, the bubble size or texture increases. Indeed, if bubble coalescence is very rapid, then most all of the gas phase will be continuous and the effectiveness of foam as a mobility-control fluid will be lost. This paper addresses the fundamental mechanisms underlying foam stability in oil-free porous media. 

The acoustic bubble size, determined through a pulsed MBSL method developed by Lee at al. [30], was also found to obey a similar dependence on gas concentration as did the coalescence in the same electrolyte solutions [41], as can be seen in Fig. 14.8. It can be inferred from these results that gas concentration controls the extent of coalescence, which itself is the main determinant of the bubble size in an acoustic field. 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

In some multiphase reactors, stirring with an impeller or the flow pattern caused by gravity will control the interfacial area. By suitably designing and positioning propellers and reactant injection orifices or by using static mixers, it is possible to provide very efficient breakup of hquids into drops and bubbles. A factor of two decrease in drop or bubble size means a factor of four increase in interfacial area. 

Surface tension can be very important in deternhning drop and bubble sizes and shapes. This ultimately controls the size of drops and the breakup of films and drops. The presence of surface active agents that alter the interfacial tension between phases can have enormous influences in multiphase reactors, as does the surface tension of sohds and the wetting between solids and liquids. 

Bubble columns rely on nozzles, mixing plates, and impellers within the reactor to control the bubble size, which determines the interfacial area between gas and liquid phases. Clearly, the interfacial area can be varied over a wide range by suitable design of the mixer and flow pattern. 

Beer and champagne bubbles are usually smaller than 1 mm in radius and are hence spherical. Because a beer bubble grows as it rises, the growth is controlled by convective CO2 transport into the bubble. With theories developed in Section 4.2.5.3, both the ascent velocity U and growth rate u of a single bubble in an infinite reservoir of beer or champagne can be calculated. The two must be calculated together because bubble size affects the ascent velocity, which in turn... 

Dickinson, E. (1994). Emulsions and droplet size control. In Wedlock, D.J. (Ed.). Controlled Particle, Droplet and Bubble Formation, Oxford Butterworth, pp. 191-216. 

In addition to the usual reactor design parameters, height of the fluidized bed is controlled by the gas contact time, solids retention time, bubble size, particle size, and bubble velocity. 

Several disadvantages are associated with the fluidized bed. The equipment tends to be large, gas velocities must be controlled to reduce particle blowout, deterioration of the equipment by abrasion occurs, and improper bed operation with large bubble sizes can drastically reduce conversion. 

---