Unit OperatiOn MECHANICAL SEPARATIONS 3.1 Mechanical .

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Unit OperatiOnMECHANICAL SEPARATIONS3.1Mechanical separation of particles from a fluid uses forcesacting on these particles. So the separating action depends onthe character of the particle being separated and the forces onthe particle which cause the separation. Mechanical separationscan be divided into four d-gassolid -gassolid -gasGravity ForceCentrifugal ForceLecturer . Shymaa Ali HameedPressure ForceGravity Force2013-2014

Unit OperatiOnSedimentationIntroductionIn setting and sedimentation the particle are separated from thefluid by gravitational forces acting on the particles .Free setting : when a particles is at a sufficient distance from thewalls of the container and from other particles so that its fall isnot affected by then .( The ratio of the particle dia. to thecontainer dia. is less than 1/200 or if the particle cen. is less than0.2vol % in the solid ) .Hindered setting : when the particles are crowded , the settlealower rate . The separation of a dilute slurry or suspension bygravity setting in a clear fluid and a slurry of higher solidscontent is called sedimentation .Gravitational sedimentationReminal falling ( setting ) velocity of a particleForces Balance :FbFdFggravitation Force Bouyancy force Dray forceFg FbLecturer . Shymaa Ali Hameed Fd2013-2014

Unit OperatiOn ( . ) .For spherical particle (6 4 ) (4.)2terminal falling ( setting )velocity of spherical particlewhere :Ut : terminal falling velocity ( m/s)Ds : dia. of solid particleA proj : projected area of solid particle (m2): density of solid ( kg / m3 ): density of fluid ( kg / m3 )CD : dreg coeff.g : gravitational acc. ( m/ s2)Lecturer . Shymaa Ali Hameed2013-2014

Unit OperatiOnIn the deriving the terminal falling velocity eq. it has been ass.a) That the setting is not affected by the presence of otherparticles in the fluid ( i.e. free setting ) .b) The walls of the container a enatexert an appreciableretarding effect .c) The fluid can be considered as a continuous modia .CD is a function of Reynolds num. of particle (Re)p where :(Re)p ( Ut ds / µ )For a flow of (Re)p 0.2 around a spherical particle stokesobtained the following formula from theoretical consideration 3stokes lawWhere :. 3. 3 (for (Re)p 0.2)Sub. Interminal velocity eq. gives ()( stokes eq. for steady streamline flowpast a sphere (Re)p 0.2 )Lecturer . Shymaa Ali Hameed2013-2014

Unit OperatiOnFor non – spherical particle , the terminal falling velocity eq.can be written in the modified formas ()( for non-spherical particle ): sphericityLecturer . Shymaa Ali Hameed2013-2014

Mechanical separation of particles from a fluid uses forces acting on these particles. So the separating action depends on . liquid-gas solid -gas Centrifugal Solid-Liquid liquid-liquid liquid-gas solid -gas Filtration Solid-Liquid Sieving Solid-Solid. Unit OperatiOn