Liquid dispersion mechanism

Chang TPK, Tatterson GB Dickey DS (1981) Liquid dispersion mechanisms in agitated tanks Part II. Straight blade and disc style turbines. Chemical Engineering Communications 10 215-222. 

In this chapter we shall refer mainly to mechanically agitated gas-liquid dispersions. However, most of the theoretical and experimental conclusions also apply to any type of gas-liquid dispersion. 

No exact theoretical analysis has as yet been possible because of the large number of variables involved and the complex mechanisms governing the transfer mechanism in a gas-liquid dispersion. The following section analyzes in a qualitative manner some of the effects produced by the mixing impeller in the disperser. It will serve to show some of the interrelationships involved as well as to illustrate the difficulties in the path of arriving at an exact mechanism. 

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

Certain hydrodynamical problems, as well as mass-transfer problems in the presence of surface-active agents, have been investigated theoretically under steady-state conditions (L3, L4, L10, R9). However, if we take into account the fact that in gas-liquid dispersions, the nonstationary term must appear in the equation of mass- or heat-transfer, it becomes apparent that an exact analysis is possible if a mixing-contacting mechanism is adopted instead of a theoretical streamline flow around a single bubble sphere. 

Gas or air may be dispersed into a liquid by mechanical means (beaters) or by introducing bubbles of desired size directly by disintegrating massive streams or bubbles of gas. Bubbles are introduced either by sprayers (simple bubbling) or by porous dispensers (diffusers). 

Gas Sparging with Mechanical Agitation Calderbank (1958) correlated gas hold-up for the gas-liquid dispersion agitated by a flat-blade disk turbine impeller as... 

The general usage of the terms lyophilic and lyophobic in describing colloidal systems is somewhat illogical. Lyophobic traditionally describes liquid dispersions of solid or liquid particles produced by mechanical or chemical action however, in these so-called lyophobic sols (e.g. dispersions of powdered alumina or silica in water) there is often a high affinity between the particles and the dispersion medium - i.e. the particles are really lyophilic. Indeed, if the term lyophobic is taken to imply no affinity between particles and dispersion medium (an unreal situation), then the particles would not be wetted and no dispersion could, in fact, be formed. Lyophilic ... 

In a gas-continuous impinging stream device with liquid as the dispersed phase, the liquid is usually atomized into fine droplets with nozzles of an appropriate type, and ejected into gas flows to form droplets-in-gas suspensions before impingement. This can be called the Primary Atomization, and it defines the primary dispersity of liquids. The mechanism of primary atomization and the methods for predicting size distribution (SD) and mean diameter (MD) of the sprayed droplets have been widely reported and some sources of references may be found, e.g., in Ref. [69]. 

While a number of good correlations for the gas holdup in mechanically agitated reactors are available (Joshi et ai, 1982), the best correlations are those by Hughmark (1980) and Sridhar and Potter (1980), and they are recommended. The critical impeller speed, N0, required for effective gas-liquid dispersion, and the impeller speed at which gas above liquid is first entrapped, Nc, can be reasonably well calculated using Eqs. (2.4) and (2.5), respectively. 

For absorption in a mechanically agitated gas-liquid dispersion in the absence of surface foam, they proposed... 

Dispersions of gas in solids are also called foams but the foam cells (bubbles) formed are isolated from one another. An example of such foams are the natural porous materials, cellular concrete, cellular glass and polymer foams. However, if in such disperse systems both phases are continuous (such as in many foamed polymers), they are called sponges. Many porous materials are partially sponge and partially solid foam. The properties of solid foams differ drastically from those of foams with liquid dispersion medium. At the same time the strength and other physical and mechanical characteristics of solid foams depend significantly... 

Woodburn55 showed thai, for Re], 650, the correlations proposed by DeMaria and White, J Sater and Levenspiel,43 and Dunn et al.16 could correlate his data. However, for 650 < ReL < 1,500, the axial dispersion in the gas phase was independent of the liquid rate. Under these liquid flow conditions, the reverse gas flow induced by the counterflowing liquid was measured. Thus, he concluded that an additional dispersive mechanism associated with reverse gas flow becomes operative at ReL 650. 

Physical measurements can be made of gas holdup a, bubble size, and specific surface area a in gas-liquid dispersions, as usually encountered in bubble columns, plate columns, mechanically agitated tanks, and spray towers. Any two of these interfacial parameters are sufiicient to define all three, since they are interrelated ... 

The second collision mechanism comes about only if there is a significant difference between the densities of the fluid and the drops or particles. Because of this significant difference, the drops or particles are not completely entrained by turbulent eddies. Drops or particles with different diameters move with different velocities, which results in collisions between them. Researchers (El, L7, P3, S3) have accounted for this acceleration" collision mechanism in their derivation of collision expressions for drops in air. It should be noted that for liquid-liquid dispersions (small density differences) this acceleration mechanism is insignificant. 

The latter mechanism assumed is the well-known Taylor dispersion (T9, TIO, Til), which has been studied extensively (All, G6, L9, T14, S2). High-speed motion pictures taken by Towell et al. (T23) in a 40-cm bubble column (R3) have shown the presence of turbulent eddies, on a scale roughly equal to the column diameter, with systematic large-scale circulation patterns superimposed. Their pictures showed that liquid near the wall flowed downward, while liquid near the center of the column flowed upward, consistent with the flow theory developed earlier and with the Taylor dispersion mechanism. 

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