SEPARATIONS BASED ON THE MOTION OF PARTICLES THROUGH FLUIDS

SEPARATIONS BASED ON THE MOTION OF PARTICLES THROUGH FLUIDSGRAVITY SETTLING PROCESSESCLARIFIER A settler that removes virtually all the particles from a liquid.

CLASSIFIER A device that separates the solids into two fractions.Gravity classifiersMost classifiers in a chemical processing separate particles on the basis of size, in situations in which the density of the fine particles is the same as the larger ones.Mechanical classifiers are used specially in metallurgical operations. The relatively coarse particles are called sands and the slurry of fine particles are called slimes.

Gravity classifiersSorting Classifiers. Devices that separate particles of differing densities.Sink-and-float methods it uses a liquid sorting medium, the density of which is intermediate between that of the light material and that of the heavy.

Differential settling methods it utilizes the difference in terminal velocities that can exist between substances of different density.

CLARIFIERS AND THICKENERSGravity separation under hindered settling conditions is often used to convert a dilute slurry of fine particles into a clarified liquid and a concentrated suspension. Flocculation In many fine suspensions, the particles form agglomerates or cluster of particles that settles at reasonable rate. Batch sedimentation there are several stages in the settling of a flocculated suspension, and different zones are formed as sedimentation proceeds.

EQUIPMENT FOR SEDIMENTATIONThickeners are specially valuable when large volumes of dilute slurry must be thickened. Used extensively in sewage treatment and water purification.

PRINCIPLES OF CENTRIFUGAL SEDIMENTATIONParticle trajectoryIf rB < r2, the particle leaves the bowl with the liquidIf rB = r2, it is deposited on the bowl wall and removed from the liquid

Stokes law rangeVolumetric flow rateThickness of liquid layerIf the thickness of the liquid layer is small compared to the radius of the bowl, r1 r2. Volumetric flow rate corresponding to the cut diameter can be solved using this equation

Sigma value; scale upFor application to industrial centrifuges equation 10 is modified as follows. Radius r2 and thickness s are replaced by re and se, respectively, which are appropriate average values of r and s for the type of centrifuge under consideration.

= 2 ut

Characteristics of sedimenting centrifugesTypeBowlDiameter, in.Speedr/min value,ft2 x 10-4Tubular4.12515,0002.7Disk9.56,50021.513.74,65039.319.54,240105Helical Conveyor144,0001.34253,0006.1Axial-flow Conveyor No vanes292,6004.05 96 vanes292,60012.7PROBLEMSTwo centrifuges rotate at the same peripheral velocity of 53.34 m/s. The first bowl has a radius of r1=76.2 mm and the second r2=305 mm. Calculate the rev/min and the centrifugal forces developed in each bowl

A centrifuge bowl is spinning at a constant 2000 rev/min. What radius bowl is needed for the following?a) A force of 455 gsb) A force four times that in part a.What is the capacity in cubic meters per hour of a clarifying centrifuge operating under the following conditions?

Diameter of bowl, 600 mms.g. of liquid 1.2Thickness of liquid layer, 75mms.g. of solid, 1.6Depth of bowl, 400mmViscosity of liquid 2 cPSpeed 1200 r/minCut size of particle, 30micrometer