Droplet settling speed

The droplets settling speed for designing separators is calculated using the formulas ... 

The droplet settling speed quoted above is applicable to a continuous phase of water at 20 C. The speed is inversely proportional to the viscosity of this phase and there may be circumstances when it is better to carry out the separation at a higher than ambient temperature if the increased solvency of the solvent in water does not outweigh the advantage of faster settling. 

Diameter of droplets or particles of the discontinuous phase (this factor is very important because the settling speeds of the droplets or particles grow proportionately to the square of their diameters) ... 

Using Stokes law given by (9.42) we calculate v, = 120cms. Stokes law overestimates the settling speed of such a droplet by 60%. 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

The more finely the liquids are dispersed within one another, the more slowly will they settle, either in a separate decanter for a continuous operation or in the same vessel for a batch process. Most stable emulsions, those which settle and coalesce only very slowly if at all, are characterized by maximum particle diameters of the dispersed phase of the order of 1 to 1.5 microns. Presumably one could estimate through Eq. (6) what agitator speeds would produce such droplet sizes, but such calculations are not likely to yield completely useful results. For example, it has been observed on several occasions that the settling ability of some liquid dispersions passes through a minimum as agitator speed is increased. 

Herring and Marshall (3F) have sprayed the droplets into cells containing a fluid in which all droplets are allowed to settle to a common plane. Later investigations of drop distributions indicate that drop size varies with the 0.82-power of disk speed and the 0.85-power of disk diameter. 

Sample Solution Fill a 100-mL porcelain crucible halffull of ashless filter paper pulp. Place 2 g of the finished catalyst, in droplet or flake form and accurately weighed, on top of the paper pulp. Transfer the crucible to a muffle furnace set at room temperature, and slowly raise the temperature to 650° so that the stearine melts into the paper, and the organic mass bums and chars slowly. Continue heating at 650° for 2 h or until the carbon is burned off. Cool, add 20 mL of hydrochloric acid, quantitatively transfer the solution or suspension into a 400-mL beaker, and carefully evaporate to dryness on a steam bath. Cool, add 20 mL of hydrochloric acid, warm to aid dissolution (catalysts containing silica will not dissolve completely), transfer into a 500-mL volumetric flask, dilute to volume with water, and mix. Allow any solids to settle, pipet a clear, 50-mL aliquot into a 400-mL beaker, and dilute to 250 mL with water. (If there is suspended matter in the volumetric flask, filter a portion through a dry, medium-speed filter paper into a dry receiver, and pipet from the receiver.)... 

Settling and coalescence are common when the dispersed and continuous phases are of different density, and when agitation provides only minimal circulation throughout the vessel. It is, therefore, important to determine the minimum impeller speed, V in to completely incorporate the dispersed phase, as droplets, into the continuous phase (i.e., to remove the initial stratification of the immiscible liquids in a vessel). Most reported work is semiempirical and follows the approach of the just suspended state of solids in liquids described in an earlier entry of this book, as well as by Atiemo-Obeng, Penney, and Armenante. ... 

A major problem in settlers is emulsification, which occurs if the dispersed droplet size falls below 1 to 1.5 micro meters (/xm). When this happens coalescers, separator membranes, meshes, electrostatic forces, ultrasound, chemical treatment, or other ploys are required to speed the settling action. 

A consequence of the small size and large surface area in colloids is that quite stable dispersions of these species can be made. That is, suspended particles may not settle out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Charged species, when sedimenting, present a challenge to Stokes law because the smaller counterions sediment at a slower rate than the larger colloidal particles. This creates an electrical potential that tends to speed up the counterions and slow down the particles. At high enough electrolyte concentrations the electric potentials are quickly dissipated and this effect vanishes. 

Low shear process will produce larger liquid droplets (>30 microns), and heavy/light liquids can be separated using gravity settling method. Examples of low shear process are solvent extraction, mixer settlers, steam stripping, washing process, and counter-current tower. Coalescer pad can be used to speed up the liquid/liquid separation and reduce the separator size. 

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