Agitated vessels dispersion

This correlation is valid when turbulent conditions exist in an agitated vessel, drop diameter is significantly bigger than the Kohnogoroff eddy length, and at low dispersed phase holdup. The most commonly reported correlation is based on the Weber number ... 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

Liquid and insoluble Dispersion Agitated vessel, planetary mixer,... 

Combine (1) through (4), with vigorous agitation, continuing until well dispersed. In another vessel, disperse (5), (6),... 

The physical technique just described directly measures the local surface area. The determination of the overall interfacial area in a gas-liquid or a liquid-liquid mechanically agitated vessel requires the application of this technique at various positions in the vessel because of variations in the local gas (or the dispersed-phase) holdup and/or the local Sauter mean diameter of bubbles or the dispersed phase. The accuracy of the average interfacial area for the entire volume of the vessel thus depends upon the homogeneity of the dispersion and the number of carefully chosen measurement locations within the vessel. 

Treybal (T5) has reported that the volume-fraction of dispersed phase actually present in an agitated vessel may be considerably less than the volume-fraction in the feed, especially in an upflow system when the dispersed phase is the lighter one. For baffled vessels and flat-blade turbines, with cocurrent upward flow, the volume-fraction of dispersed phase in the vessel is about 20% of the volume-fraction in the feed at energy inputs less than 100 ft.-lb./ft. of feed. This ratio rises rapidly with increased power above this level and is between 80% and 100% at energy inputs above 400 ft.-lb./ft. of feed. This effect is still incompletely understood. 

Figure 10.21. Chart for determining which phase is most likely to disperse for liquid-liquid dispersions in agitated vessels.

Example 10.16 calculate Njs, which phase is dispersed and the equilibrium drop size for a liquid-liquid dispersion in an agitated vessel. ... 

G.B. Tatterson, Fluid Mixing and Gas Dispersion in Agitated Vessels, McGraw-Hill, New York, 1991. 

A.H.P. Skelland and R. Seksaria, Minimum impeller speeds for hquid-liquid dispersions in agitated vessels, I ECProcess Des. Dev., 17(1) 56-61 (1978). 

W.R. Penney and H.X. Vo, Scale-up of liquid-hquid dispersions in agitated vessels to duplicate (1) time scales and (droplet size distribution), Paper 150a, AIChE Annual Meeting, Los Angeles, 1997. 

In a turbulent flow, the entire spectrum of the continuous phase eddies is imparted to the droplets or particles of the dispersed phase present. Theory and experiments (K15, K16, L8, S14) indicate that small drops or particles follow the behavior of the fluid eddies very closely. Drops or particles larger than the integral scale of turbulence follow the mean fluid flow. Experiments (K16) with solid particles (0.013-0.20 cm in diameter) in an agitated vessel show that the particle velocity fluctuations are given by the Maxwell distribution. In addition, the micromotion of the particles was determined mainly by eddies of size comparable to particle diameters. 

Coalescence frequencies can have a pronounced effect on the rate of mass transfer or chemical reaction in a liquid-liquid dispersion. Various investigators have attempted to model and measure coalescence frequencies in agitated vessels. A review of the experimental techniques is given by Rietema (R12) and Shah et al. (S16). 

There is little work on direct determination of coalescence frequencies in a turbulent dispersion. Kuboi et al. (KI7) and Park and Blair (P4) have used high-speed cinephotography to directly observe and measure the drop coalescence frequencies in pipe flow and in an agitated vessel, respectively. The difficulty with this method is that only a few coalescences are observed at great expense of cine film after tedious/examination. Thus there is doubt regarding the statistical representation of coalescence in the dispersion from a small sample. 

A variety of models are employed to predict extent of reaction and selectivity for complex reactions which occur in liquid-liquid dispersions in agitated vessels. The nature of the models depends on which phases the reaction occurs in, the relative magnitude of the time scales of the mass transfer and reaction processes compared to the mixing processes, com-... 

Zeitlin and Tavlarides (Z2-5) developed a simulation model which attempts to account for the macroflow patterns of the dispersed phase in a turbine-agitated vessel, the droplet mixing via breakage or coalescence, and nonuniform drop size on mass transfer or reaction in dispersions. 

Skelland and Ramsay [Ind. Eng. Chem. Res., 26(1), pp. 77-81 (1987)] correlated the minimum impeller speed needed to completely disperse one liquid in another in an agitated vessel with standard baffles as follows ... 

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