Bubble coalescence rate

The important variables that affect the bubble dynamics and flow regime in a bubble column are gas velocity, fluid properties (e.g. viscosity, surface tension etc.), nature of the gas distributor, and column diameter. Generally, at low superficial gas velocities (approximately less than 5 cm/sec) bubbles will be small and uniform though their nature will depend on the properties of the liquid. The size and uniformity of bubbles also depends on the nature of the gas distributor and the column diameter. Bubble coalescence rate along the column is small, so that if the gas is distributed uniformly at the column inlet, a homogeneous bubble column will be obtained. 

Models for the Binary Bubble Coalescence Rate Sink, fic di dj)... 

Since mechanical foam breaking with its high input E>ower requirements is expensive, anti foam agents are usually preferred in the fermentation industry. The presence of anti foam agents, however, deteriorates the efficiency of gas dispersion by increasing the bubble coalescence rate. 

For the bubble coalescence rate, the following function is used. 

This equation has been experimentally verified in liquids, and Figure 2 shows that it applies equally well for fluidized solids, provided that G is taken as the flow rate in excess of minimum fluidization requirements. In most practical fluidized beds, bubbles coalesce or break up after formation, but this equation nevertheless gives a useful starting point estimate of bubble size. 

The absorption rate increased with increasing nominal liquid velocity for all particle sizes and decreased with increasing particle size for all liquid velocities. The absorption rates were lower than those measured in an equivalent gas-liquid system with no solid particles present. The difference is explained as being due to a higher rate of bubble coalescence and, consequently, a lower gas-liquid interfacial area in the gas-liquid fluidized bed. 

The results of Massimilla et al., 0stergaard, and Adlington and Thompson are in substantial agreement on the fact that gas-liquid fluidized beds are characterized by higher rates of bubble coalescence and, as a consequence, lower gas-liquid interfacial areas than those observed in equivalent gas-liquid systems with no solid particles present. This supports the observations of gas absorption rate by Massimilla et al. It may be assumed that the absorption rate depends upon the interfacial area, the gas residence-time, and a mass-transfer coefficient. The last of these factors is probably higher in a gas-liquid fluidized bed because the bubble Reynolds number is higher, but the interfacial area is lower and the gas residence-time is also lower, as will be further discussed in Section V,E,3. 

The first flow pattern zone corresponds to the isolated bubble (IB) regime where the bubble generation rate is much higher than the coalescence rate. It includes both bubbly flow and/or slug flows and is present up to the onset of coalescence process domination. The second zone is the coalescing bubble (CB) regime, which is... 

Point C in Figure 15.14 is termed the critical heat flux or maximum boiling flux or peak boiling flux as bubbles coalesce on the surface creating a vapor blanket. Critical heat flux occurs because insufficient liquid is able to reach the heat transfer surface due to the rate at which vapor is leaving. Beyond Point D, the surface is dry and entirely blanketed by vapor and heat is transferred by conduction and radiation. 

Geldart (1970) showed a substantial distinction between bubble sizes in two dimensional and three dimensional beds. He used 128 pm river sand in a 30.8 cm round bed and a 68 1.27 cm rectangular cross section bed. The results, shown in Fig. 12, show that the bubbles in the three dimensional bed are larger. There were differences in the visible bubble flow rate at the same superficial velocity. Geldart ascribes the differences in bubble diameter to differences invisible bubble flow rate as well as to out-of-line coalescence in the three dimensional bed. 

Vaux (1978), Ulerich et al. (1980) and Vaux and Schruben (1983) proposed a mechanical model of bubble-induced attrition based on the kinetic energy of particles agitated by the bubble motion. Since the bubble velocity increases with bed height due to bubble coalescence, the collision force between particles increases with bed height as well. The authors conclude that the rate of bubble-induced attrition, Rbub, is then proportional to the product of excess gas velocity and bed mass or bed height, respectively,... 

The bubble column and spray tower depend on nozzles to disperse the drop or bubble phase and thus provide the high area and small particle size necessary for a high rate. Drop and bubble coalescence are therefore problems except in dilute systems because coalescence reduces the surface area. An option is to use an impeller, which continuously redisperses the drop or bubble phase. For gases this is called a sparger reactor, which might look as shown in Figure 12-16. 

It is a well-known fact that bubble sizes in aqueous electrolyte solutions are much smaller than in pure water with equal values of viscosity, surface tension, and so on. This can be explained by the electrostatic potential of the resultant ions at the liquid surface, which reduces the rate of bubble coalescence. This fact should be remembered when planning experiments on bubble sizes or interfacial... 

It is a well-known fact that bubbles produced by mechanical force in electrolyte solutions are much smaller than those in pure water. This can be explained by reduction of the rate of bubble coalescence due to an electrostatic potential at the surface of aqueous electrolyte solutions. Thus, k a values in aerated stirred tanks obtained by the sulfite oxidation method are larger than those obtained by physical absorption into pure water, in the same apparatus, and at the same gas rate and stirrer speed [3]. Quantitative relationships between k a values and the ionic strength are available [4]. Recently published data on were obtained mostly by physical absorption or desorption with pure water. 

Substituted phenols as well as phenol itself are typical constituents of (bio-)refractory waste waters and can increase a(0> 3 (Gurol and Nekouinaini, 1985). They studied the influence of these compounds in oxygen transfer measurements and attributed this effect to the hindrance of bubble coalescence in bubble swarms, which increases the interfacial area a. When evaluating the effect of these phenols on the ozone mass transfer rate, it is important to note that these substances react fast with ozone (direct reaction, kD= 1.3 103 L mol"1 s 1, pH = 6-8, T = 20 °C, Hoigne and Bader, 1983 b). 

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