Drainage rate

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

Tallmadge and Gutfinger Jnd. Eng. Chem., 59[11], 19-34 [1967]) discuss prediction of drainage rates from liquid films on flat and cylindrical surfaces. 

Most drum fUters operate at a rotation speed in the range of 0.1 to 10 r/min. Variable-speed drives are usuahy provided to ahow adjustment for changing cake-formation and drainage rates. 

FIGURE 9.1 Typical drag-out drainage rates. (Adapted from U.S. EPA, Meeting Hazardous Waste Requirements for Metal Finishers, Report EPA/625/4-87/018, U.S. Environmental Protection Agency, Cincinnati, OH, 1987.)... 

Any additives that can act to reduce the viscosity of foam films, and thereby increase the liquid drainage rate, will tend to reduce foam stability as a result. This includes any chemicals that can reduce surface viscosity and/or surface elasticity. Some alcohols can be use to produce these effects. 

A relative drainage rate test in which a foam is formed in a vessel and thereafter the remaining foam volume is determined as a function of time. The foam number is the volume of bulk liquid that has separated after a specified interval, expressed as a percentage of the original volume of liquid foamed. 

Singer, P. C. and W. Stumm. 1970. Acid mine drainage Rate-determining step. Science. 167 1121-1123. 

The cul-de-sac normally holds 7-9 pL of tear fluid, with the normal tear flow rate being 1.2-1.5pL/min [4], The loss from the precorneal area by drainage, tear fluid turnover, and noncorneal absorption plays an important role in determining the ocular bioavailability of a drug. As the drainage rate is much faster than the ocular absorption rate, most of the topically applied drug is eliminated from the precorneal area within the first minute [4],... 

Some of these problems have been encountered by addition of viscosityenhancing agents such as cellulose derivates, which are believed to increase the viscosity of the preparation and consequently reduce the drainage rate. The use of viscosity enhancers will be discussed later in this section. 

Recently Joye et at. [74] have reported a numerical simulation of instability causing asymmetric drainage in foam films. The results obtained confirmed the rapid increase in drainage rate. 

Eqs. (5.34-5.36), (5.38), (5.42), (5.44) and (5.46) describe the process of foam drainage at complete immobility of border surfaces. Calculation of drainage rate with the correction p being a function of surface viscosity is possible only by numerical methods. 

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