Bubble rise

Fig. 32). Using a fine pipette insert about i cm. length of the liquid into the bottom of the tube. Now place in the tube A a fine inverted melting-point tube B of about i mm. diameter, sealed at the upper end. Fasten the capillary tube to the ther- Fio. 32. mometer by means of a rubber band and place in a melting-point apparatus. Heat slowly until a stream of bubbles rises from the bottom... 

Bubbles rise through the bed in two different regimes. Slow bubbles rise at a gas velocity less than (equal to, and present an... 

A single bubble rises through a fluid bed at a velocity, proportional to the square root of its diameter, or more accurately, the diameter of a sphere of equivalent volume ... 

If the bed is slugging, bubble motion is retarded by the bed wall, and the bed or tube diameter, Z9, rather than the actual bubble diameter, determines the bubble rise velocity, ie... 

The velocity of a bubble ia a bubbling bed has been observed to be higher than equation 14 predicts, and it has been suggested that the actual bubble rise velocity in a bubbling bed (15) is... 

This empirical equation attempts to account for complex bubble coalescence, spHtting, irregular shapes, etc. Apparent bubble rise velocity in vigorously bubbling beds of Group A particles is lower than equation 16 predicts. 

As bubbles rise through the bed, they coalesce into larger bubbles. The actual bubble size at any height above the distributor, in the bed is a function of the initial bubble size as it emerges from the gas distributor and the gas flow rate (16) ... 

Fig. 6. Shape of drops and bubbles, (a) Bubble rising in sparged tower system air-water. Courtesy of Shell Development Co. (b) Bubble and droplet...

Studies of individual bubbles rising in a two-dimensional gas—Hquid—soHd reactor provide detailed representations of bubble-wake interactions and projections of their impact on performance (Fig. 9). The details of flow, in this case bubble shapes, associated wake stmctures, and resultant bubble rise velocities and wake dynamics are important in characteri2ing reactor performance (26). 

Viscosity can also be determined from the rising rate of an air bubble through a Hquid. This simple technique is widely used for routine viscosity measurements of Newtonian fluids. A bubble tube viscometer consists of a glass tube of a certain size to which Hquid is added until a small air space remains at the top. The tube is then capped. When it is inverted, the air bubble rises through the Hquid. The rise time in seconds may be taken as a measure of viscosity, or an approximate viscosity in mm /s may be calculated from it. In an older method that is commonly used, the rate of rise is matched to that of a member of a series of standards, eg, with that of the Gardner-Holdt bubble tubes. Unfortunately, this technique employs a nonlinear scale of letter designations and may be difficult to interpret. 

Foams Two excellent reviews (Shedlovsky, op. cit. Lemlich, op. cit.) covering the literature pertinent to foams have been published. A foam is formed when bubbles rise to the surface of a liquid and persist for a while without coalescence with one another or without rupture into the vapor space. The formation of foam, then, consists simply of the formation, rise, and aggregation of bubbles in a hquid in which foam can exist. The hfe of foams varies over many magnitudes—from seconds to years—but in general is finite. Maintenance of a foam, therefore, is a dynamic phenomenon. 

For a better understanding of the interactions between parameters, it is often helpful to calculate the effective bubble rise velocity from measurea valves of for example, the data of Mersmann (loc. cit.) indicated = 0.6 for = 0.05 iti/s, giving U, = 0.083 m/s, which agrees with the data reported in Fig. 14-43 for the rise velocity of bubble clouds. The rise velocity of single bubbles, for d - 2 mm, is about 0.3 m/s, for liquids with viscosities not too different from water. Using this value in Eq. (14-220) and comparing with Fig. 14-104, one finds that at low values of the rise velocity of the bubbles... 

GAS ABSORPTION AND GAS-LIQUID SYSTEM DESIGN TABLE 14-22 Terminal Velocity of Standard Air Bubbles Rising in Water at 20 C ... 

In design of separating chambers, static vessels or continuous-flow tanks may be used. Care must be taken to protect the flow from turbulence, which coiild cause back mixing of partially separated fluids or which could cany unseparated hquids rapidly to the separated-hquid outlet. Vertical baffles to protect rising biibbles from flow currents are sometimes employed. Unseparated fluids should be distributed to the separating region as uniformly and with as little velocity as possible. When the bubble rise velocity is quite low, shallow tanks or flow channels should be used to minimize the residence time required. 

Principle The adsorptive-bubble separation methods, or adsub-ble methods for short [Lemlich, Chem. Eng. 73(21), 7 (1966)], are based on the selective adsorption or attachment of material on the surfaces of gas bubbles passing through a solution or suspension. In most of the methods, the bubbles rise to form a foam or froth which carries the material off overhead. Thus the material (desirable or undesirable) is removed from the liquid, and not vice versa as in, say, filtration. Accordingly, the foaming methods appear to be particularly (although not exclusively) suited to the remov of small amounts of material from large volumes of hquid. 

Gas-liquid Released gas (air) bubbles rising in a large tank Mist formation from LPG evaporation Splash filling Cleaning with wet steam Mist formation from high pressure water jets... 

When the bubble diameter approaches the diameter of the containing vessel, slug flow is said to exist. In such cases, the bubble rise velocity is given by... 

Knowing the bubble rise velocity, the bed expansion can be predicted from a material balance on the bubble phase gas. Thus, total gas flow through the bubble phase equals absolute bubble velocity times the volume fraction E of bubbles in the bed. 

When a liquid is heated in an open container, bubbles form, usually at the bottom, where heat is applied. The first small bubbles are air, driven out of solution by the increase in temperature. Eventually, at a certain temperature, large vapor bubbles form throughout the liquid. These vapor bubbles rise to the surface, where they break. When this happens, the liquid is said to be boiling. For a pure liquid, the temperature remains constant throughout the boiling process. 

The temperature at which a liquid boils depends on the pressure above it. To understand why this is the case, consider Figure 9.3 (p. 231). This shows vapor bubbles rising in a boiling liquid. For a vapor bubble to form, the pressure within it, Plt must be at least equal to the pressure above it, P2. Because P1 is simply the vapor pressure of the liquid, it follows that a liquid boils at a temperature at which its vapor pressure is equal to the pressure above its surface. If this pressure is 1 atm (760 mm Hg), the temperature is referred to as the normal... 

In a gas and liquid system, when gas is introduced into a culture medium, bubbles are formed. The bubbles rise rapidly through the medium and dispersion of the bubbles occurs at surface, forming froth. The froth collapses by coalescence, but in most cases the fermentation broth is viscous so this coalescence may be reduced to form stable froth. Any compounds in the broth, such as proteins, that reduce the surface tension may influence foam formation. The stability of preventing bubbles coalescing depends on the film elasticity, which is increased by the presence of peptides, proteins and soaps. On the other hand, the presence of alcohols and fatty acids will make the foam unstable. 

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