Agitation intensity

Inteifacial tension. A high interfacial tension promotes rapid coalescence and generally requires high mechanical agitation to produce small droplets. A low interracial tension allows drop breakup with low agitation intensity but also leads to slow coalescence rates. Interfacial tension usually decreases as solubility and solute concentration increase and falls to zero at the plait point (Fig. 15-10). 

The mass-transfer coefficients depend on complex functions of diffii-sivity, viscosity, density, interfacial tension, and turbulence. Similarly, the mass-transfer area of the droplets depends on complex functions of viscosity, interfacial tension, density difference, extractor geometry, agitation intensity, agitator design, flow rates, and interfacial rag deposits. Only limited success has been achieved in correlating extractor performance with these basic principles. The lumped parameter deals directly with the ultimate design criterion, which is the height of an extraction tower. 

Increase solids mixing. Improve powder flowahihty of feed. Increase agitation intensity (e.g., impeller speed, fluidization gas velocity, or rotation speed). 

FIG. 20-71 Classification of agglomeration processes by agitation intensity and compaction pressure. Relative density is with respect to primary particle density and equals (1 — e) where e is the solid volume fraction. Reprinted from Granulation and Coating Technologies for High-Value-Added Industties, Ennis and Litster (1996) with permission of E G Associates. All rights reserved. 

Decrease hinder viscosity Increase agitation intensity Increase particle density Increase particle size... 

Decrease impact velocity to reduce fragmentation Lower-formulation density. Decrease hed-agitation intensity (e.g., mixer impeller speed, fluid-hed excess gas velocity, drum rotation speed). Also strongly influenced hy distributor-plate design in fluid-heds, or impeller and chopper design in mixers. 

Pai+icle size enlargement equipment can be classified into several groups, with advantages, disadvantages, and applications summarized in Table 20-36. Comparisons of bed-agitation intensity, compaction pressures, and product bulk density for selected agglomeration processes are highlighted above in Fig. 20-71. 

Figure 7-15 shows plots of Pumping number Nq and Power number Np as functions of Reynolds number Np for a pitched-blade turbine and high-efficiency impeller. Hicks et al. [8] further introduced the scale of agitation, S, as a measure for determining agitation intensity in pitched-blade impellers. The scale of agitation is based on a characteristic velocity, v, defined by... 

Fig. 1. Relationships between agitation intensity and transfer rates at constant gas flow [after Gal-Or and Walatka (G9)]. At increased gas flow, the holdup fraction is increased mainly by an increase in the number of bubbles produced.

The main relationships between the agitation intensity of the dispersion and the total mass-transfer rate are summarized qualitatively for constant gas flow rate by Fig. 1 (G9) wherein interaction effects among the bubbles are indicated by dashed lines. Intermediate phenomena not shown, such as the direct and feedback effects between coalescence and mass transfer (G5, G9), should also be considered. 

In Section I, a qualitative schematic description of the main connection between increased agitation intensity and increased total mass-transfer rate was given. It can readily be seen from this description that further research in gas and liquid flow patterns and in the area of relative bubble velocities in dispersions will contribute to the basic knowledge necessary for understand ing the real mechanisms occurring in these systems. 

A change in the agitation intensity or gas flow rate will change 0. As a result, b, c0, and the total mass-transfer rate are affected. Thus, the effect of mixing in the vessel is considered by indirect mechanisms. 

The average absorption rate increases with the gas holdup in the vessel. This may be caused by a change in the gas flow rate or the agitation intensity. The effect is augmented as k increases. 

Jiisten P (1997) Dependence of PenicUlium Chrysogenum growth, morphology, vacuola-tion and productivity on impeller type and agitation intensity. PHD Thesis University of Birmingham... 

These mixing motions will tend to improve drug absorption for two reasons. Any factor that increases rate of dissolution will increase the rate (and possibly the extent) of absorption, especially for poorly water-soluble drugs (BCS Classes II and IV). Since rate of dissolution depends on agitation intensity, mixing movements will tend to increase dissolution rate and thereby influence absorption. As rate of absorption depends directly on membrane surface area, and since mixing increases the contact area between drug and... 

Ramtoola Z, Corrigan 01. Effect of agitation intensity on the dissolution rate of indomethacin and indomethacin-citric acid compressed discs. Drug Dev Ind Pharm 1988 14(15-17) 2241-2253. 

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