Microbubbles

In exceptional cases, the formation of seeds can be prevented by thin layer melting. In this case one layer of the batch is melted after the other, which however results in rather small throughput rates. A typical example that utilises this principle is the chemical vapour deposition (CVD) of pure or doped Si02 for light-guide fibre preforms [167]. 

If the bubbles do not rise fast enough through the melt, it can be supported additionally by melt flow in the melting vessel, which will help to move the bubbles to the surface. This process is described in the following section. 

In most cases a combination of all the methods described above allows for the effective elimination of seeds. If it is not possible to avoid the formation of seeds or remove them satisfactorily, it is still possible to increase the fining temperature, which will result in a reduced melt viscosity. However, the temperature cannot be increased without limits as the melt will start to evaporate. In particular, the light oxides, such as boron oxide, tend to evaporate. 

If the gas bubbles reach the surface of the melt bath, then they must be able to push through the melt/air interface, which requires additional energy. 

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Microbond NP Microbond NP2 Microbubble flotation Microbubbles Microcarriers Microcline [12251-43-3] Micrococcin P... 

The actual flotation phenomenon occurs in flotation cells usually arranged in batteries (12) and in industrial plants and individual cells can be any size from a few to 30 m in volume. Column cells have become popular, particularly in the separation of very fine particles in the minerals industry and coUoidal precipitates in environmental appHcations. Such cells can vary from 3 to 9 m in height and have circular or rectangular cross sections of 0.3 to 1.5 m wide. They essentially simulate a number of conventional cells stacked up on top of one another (Fig. 3). Microbubble flotation is a variant of column flotation, where gas bubbles are consistently in the range of 10—50 p.m. 

Another modification is the use of microbubble column flotation (13). In this process, smaller bubbles are generated to enhance the recovery of micrometer-sized particles. A countercurrent flow of feed slurry is also used to further enhance the bubble—particle attachment. The process is capable of produciug ultraclean coals containing less than 0.8% ash. 

The most common application of carbon adsorption in municipal water treatment is in the removal of taste and odor compounds. Figure 12 provides an example of a process flow diagram for a municipal water treatment plant. In this example water is pumped from the river into a flotation unit, which is used for the removal of suspended solids such as algae and particulate matter. Dissolved air is the injected under pressure into the basin. This action creates microbubbles which become attached to the suspended solids, causing them to float. This results in a layer of suspended solids on the surface of the water, which is removed using a mechanical skimming technique. Go back to Chapter 8 if you need to refresh your memory on air flotation systems. 

The use of ultrasonic (US) radiation (typical range 20 to 850 kHz) to accelerate Diels-Alder reactions is undergoing continuous expansion. There is a parallelism between the ultrasonic and high pressure-assisted reactions. Ultrasonic radiations induce cavitation, that is, the formation and the collapse of microbubbles inside the liquid phase which is accompanied by the local generation of high temperature and high pressure [29]. Snyder and coworkers [30] published the first ultrasound-assisted Diels-Alder reactions that involved the cycloadditions of o-quinone 37 with appropriate dienes 38 to synthesize abietanoid diterpenes A-C (Scheme 4.7) isolated from the traditional Chinese medicine, Dan Shen, prepared from the roots of Salvia miltiorrhiza Bunge. 

An interesting phenomenon that some gas microbubbles emerged in the thin liquid film in a nanogap under an EEF was observed by Luo s group [78-80]. They have investigated the influence factors on and the mechanism of the emergence of these micro-bubbles. 

The overheating effect is believed to play a dominant role in explaining the physical mechanism of the microbubble formation, and three main reasons could be underpinned First, it was observed that a small amount of the glycerin injected into the gap would eventually disappear after a few hours at the positive EEF intensity of 1 MV/cm. Second, no remarkable difference in the chemical composition between the glycerin before and after the EEF was applied could be found in the experiment, which indicated physical effects might predominate. Besides, a rough estimation of the temperature rise in the contact region due to the electro-thermal effect will be conducted as follows. 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

Perfectly monodisperse microbubbling by capillary flow focusing Phys. Rev. Lett. 87,... 

The retention time in the flotation chamber is usually about 3 to 5 min, depending on the characteristics of the process water and the performance of the flotation unit. The process effectiveness depends upon the attachment of air bubbles to the particles to be removed from the process water.57 The attraction between the air bubbles and particles is primarily a result of the particle surface charges and bubble size distribution. The more uniform the distribution of water and microbubbles, the shallower the flotation unit can be. 

Calvisi ML, Lindau O, Blake JR, Szeri AJ (2007) Shape stability and violent collapse of microbubbles in acoustic traveling waves. Phys Fluids 19 047101 (15 pages)... 

Yasui K, Lee J, Tuziuti T, Towata A, Kozuka T, Iida Y (2009) Influence of the bubble-bubble interaction on destruction of encapsulated microbubbles under ultrasound. J Acoust Soc Am 126 973-982... 

Li et al. [76] confirmed that efficacy of phenol degradation depends on microbubble formation. In their experiments, they observed no change in phenol concentration if micro-bubble formation was stopped. The phenol decomposition rate was found maximum in the case when O2 was passed in the solution due to highest micro-bubble formation followed by air and N2 respectively. 

Hanajiri K, Maruyama T, Kaneko Y, Mitsui H, Watanabe S, Sata M, Nagai R, Kashima T, Shibahara J, Omata M, Matsumoto Y (2006) Microbubble-induced increase in ablation of liver tumors by high-intensity focused ultrasound. Hepatology Research 36 308-314. 

It is advantageous to generate bubbles of micron-size when the particles to be floated are very small. The generation of such bubbles is almost impossible in conventional equipment which relies on mechanical means of breaking down the gas. If air, or another gas, is dissolved under pressure in the suspension before it is introduced into the cell, numerous microbubbles are formed when the pressure is reduced and these then attach themselves to the hydrophobic particles. Similar effects can be obtained by operating the cells under vacuum, or producing gas bubbles electrolytically. Dissolved and electro filtration are discussed later. 

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