Gas bubble formation

As already indicated, when a liquid is the fluidising agent, substantially uniform conditions pervade in the bed, although with a gas, bubble formation tends to occur except at very low fluidising velocities. In an attempt to improve the reproducibility of conditions within a bed, much of the earlier research work with gas fluidised systems was carried out at gas velocities sufficiently low for bubble formation to be absent. In recent years, however, it has been recognised that bubbles normally tend to form in such systems, that they exert an important influence on the flow pattern of both gas and solids, and that the behaviour of individual bubbles can often be predicted with reasonable accuracy. 

In addition, the hydrodynamics were monitored. The main features were gas bubble formation (HC1) and particulate formation (Et3NHCl) and agglomeration, both due to the reaction. The flow in the glass tube seemed to be rather undisturbed [53] only from time to time, bubble formation due to HC1 gas evolution and passing of EtjNHCl lumps were observed. These bubbles and lumps moved with the liquid mixture and were rinsed out of the tube and hence did not behave as obstacles which could cause a breakdown of the flow. 

A four-electrode capacitively coupled (contactless) detector has been integrated on a Pyrex glass chip for detection of peptides (1 mM) and cations (5 mM K+, Na+, Li+). The A1 electrode (500 nm Al/100 nm Ti) was deposited in a 600-nm-deep trench and was covered with a thin dielectric layer (30-nm SiC). The other parts of the channel were covered and insulated with Si3 N4 (160 nm). To avoid gas bubble formation after dielectric breakdown, the electric field for separation was limited to 50 V/cm [145]. This four-electrode configuration allows for sensitive detection at different background conductivities without the need of adjusting the measurement frequency [328]. 

Next we come to phase transitions. Chapter 14 mentions the various phase transitions that may occur, such as crystallization, gas bubble formation, or separation of a polymer solution in two layers it then treats the nucleation phenomena that often initiate phase transitions. Chapter 15 discusses crystallization, a complicated phase transition of great importance in foods. It includes sections on crystallization of water, sugars, and triacylglycerols. Chapter 16 introduces glass transitions and the various changes that can occur upon freezing of aqueous systems. 

Supersaturation. This is not applied in food emulsion making but is fairly common in foams. A gas can be dissolved in a liquid under pressure, and then the pressure is released, so that gas bubbles are formed. The gas should be well soluble in water to obtain a substantial volume of bubbles, and carbon dioxide is quite suitable. It is applied in most fizzy beverages. C02 can also be formed in situ by fermentation, as in beer and in a yeast dough. Initiation of gas bubble formation is discussed in Section 14.4. 

Gas bubble formation and blistering effects have been widely observed in high-dose implantations of inert-gas ions. Backscattering measurements of depth distributions often show very low concentrations of implanted species in the nearsurface region. This indicates that the inert-gas atoms can escape from the material even without sputtering. In these cases, the simple model described in the previous sections does not apply. 

Interactions between cells and the mechanical forces associated with gas bubble formation at the sparger and with bubble rise phenomena... 

Figure 4.6 Filter efifect phenomenon Gas bubble formation [Nemeth, 1979]...

Foam A sohd or liquid matrix containing macropores or gas bubbles Formate species Methanoate species or ion CHOO ... 

Nucleation The initial assembly of molecules or atoms that forms a semistable structure or aggregate. Often the formation of this aggregate comes with a large entropic price (as in crystallization or gas bubble formation). 

La Nauze RD, Harris IJ. Gas bubble formation at elevated system pressures. Trans Instn Chem Engrs 52 337 348, 1974. 

The good agreement of theory and experiment in the case of boihng, as seen by inspection of Tables lA and IB, contrasts with the somewhat worse agreement for cavitation at room temperature (Table 2) and the generally very poor agreement for gas bubble formation in liquids, the data for which are given in Tables 6-8, and are discussed below. This may be a reflection of the... 

It is good to realize that the problem domains as defined above are generally characterized by a great complexity with regard to the composition of the samples. Moreover, very often the samples will contain not only dissolved material but also solid particles may be present with inherent risks of clogging of the analyzing equipment, etc. Dissolved gas may also cause problems, especially when the sample is heated and gas-bubble formation may occur. These problems may require special precautions to be taken in combination with pTAS such as filtration and degassing of the solutions. However, there are many more aspects to be considered which can be better discussed for the various domains separately. 

The high viscosity of heavy oils has been ascribed to high concentrations of asphaltenes [42, 43]. That high concentrations of asphaltenes and resins can lead to high crude oil viscosities is exemplified by the data presented in Figure 10.4 for a series of degassed oils at ambient temperature. It has also been argued that asphaltenes represent nucleation sites for gas bubble formation [39] and can adsorb... 

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