Rising gas bubbles

A large deep bath contains molten steel, the surface of which is in contact with air. The oxygen concentration in the bulk of the molten steel is 0.03% by mass and the rate of transfer of oxygen from die ait is sufficiently high to maintain die surface layers saturated at a concentration of 0.16% by weight. The surface of die liquid is disrupted by gas bubbles rising to the surface at a frequency of 120 bubbles per in2 of surface per second, each bubble disrupts and mixes about 15 enr of the surface layer into the bulk. 

If pnliL is large, Q approaches unity. If ////// is small, Q approaches a value of 1.5. Thus the effect of circulation is small when a liquid drop falls in a gas although is large when a gas bubble rises in a liquid. If the fluid within the drop is very viscous, the amount of energy which has to be transferred in order to induce circulation is large and circulation effects are therefore small. 

The phenomenon of a gas bubble rising in a fluidized bed is similar to that of a gas bubble rising in a liquid bubbles rise through the bed at a constant velocity for a given size, this velocity being proportional to bubble diameter. The general relationship for a single bubble is of the... 

For a liquid drop falling in air or for a very viscous drop in a low viscosity field liquid, the correction term reduces to unity, and Eq. (23) becomes equivalent to Stokes law. For a gas bubble rising through a liquid or a low viscosity drop moving in a very viscous liquid field, the limiting correction factor of 1.5 may be realized for a fully circulating drop. These two limiting values have been confirmed by many experi-... 

In both the gassed (aerated) stirred tank and in the bubble column, the gas bubbles rise through a liquid, despite the mechanisms of bubble formation in the two types of apparatus being different. In this section, we shall consider some common aspects of the gas bubble - liquid systems in these two types of reactors. 

In the laboratory work of general chemistry, we have important applications of partial pressures. Gases (such as oxygen) that are not very soluble in water are collected in bottles by displacement of water. As the gas bubbles rise through the water, tney become saturated with vapor, and the collected gas is a mixture of water vapor and the original gas. When the bottle is filled, it is at atmospheric pressure, or... 

Actual values of SMD (or some other mean droplet size) for various types of fuel injectors are given for orifice injectors (23, 25, J+5, 47, 61, 95, 107, 113) swirl injectors (8, 18, 20, 26. 34, 46, 63, 76-78, 81, 87, 93, 99, 100, 104, 112, US, U8, 123) air-blast atomizers (5, 13, 33, 50, 64, 74, 94, 101) impinging-jet atomizers (30) rotating-element atomizers (7, 28, 53) and droplet formation caused by collapse of gas bubbles rising through a liquid surface (32, 91). Maximum values of x are reported for orifice injection (88) and for air-blast atomization (72). [The air-blast atomization mechanism may be controlling when the air blast is in reality only a high relative velocity between the air and the injected liquid (61).]... 

The bubble point test, while popular, has some deficiencies that must be realized. First, there is variation in the operator detection of the test end point that is, the first appearance of gas bubbles rising in the liquid. Some operators are able to see smaller bubbles than others. In a recent study, a panel of seven observers recorded the initial detection of a steady stream of air bubbles rising from a capillary held under water as the air pressure was gradually increased. The observers, who had received different degrees of training, identified the simulated bubble point as occurring at air flows of 5 to 50 mL/min corresponding to air pressures of 34 and 38 psi, respectively, for a 90-mm disc filter membrane [56]. 

Calc of Required Vessel length for gas bubble rise out of oi 1... 

Figure 4.8 Terminal velocity method for gas bubble rise in oil phase.

Figure 4.10 displays a typical vertical vessel program run printout. The program calculation output gives a minimum required diameter. This diameter is based on the terminal velocity of liquid drop fall velocity, gas bubble in oil rise velocity, or the water drop fall velocity. The smaller this terminal velocity, the greater the vessel cross-section area required and thus the greater the vessel diameter required. In this example, the oil-phase gas bubble rise terminal velocity is controlling. If you reduce the oil flow to, say, 10,000 lb/h, then the gas-phase liquid... 

Renewal of interfacial area when gas bubbles rise to the next stage, and also when they follow the secondary liquid motion in the same stage... 

The regime of gas bubble rise depends significantly on the hydrodynamic conditions, i.e. on Reynolds number Re... 

SEPRAN package (Cuvelierefci/., 1986). Recently Delnoij etal. (1997b) applied the VOF method to study the dynamics of single gas bubbles rising in a quiescent Newtonian liquid. They were able to demonstrate that the predicted bubble shape and the induced flow patterns in the liquid phase could be predicted very well as a function of the key physical properties of the liquid phase such as density, viscosity, and surface tension (see Section VI.D). The results reported by Tomiyama et al. are discussed in more detail below. 

Upflow reactors are sometimes used for small scale testing. The liquid phase is the continuous phase through which the gas bubbles rise and wetting problems are absent. Industrial reactors in which liquid and gas flow upward are rarely found, because it can be difficult to keep the catalyst bed immobile at large flow rates. 

The investigation of gas bubbles rising in vertical, cylindrical tubes filled with liquid is of technical interest, for example for the design of air-lift pumps and circulation evaporators among others. However the calculation of this process is difficult, even when it is highly idealised. 

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