Centrifugation Lec 6 PDF

Centrifugation Course Title 3 4  Centrifugation  It is a process of separation of mixture of solid and liquid or two liquids by the action of centrifugal force. The centrifuge consists of a basket, in which a mixture of a solid and liquid, or mixture of two liquids, is rotated at a...

Centrifugation Course Title 3 4  Centrifugation  It is a process of separation of mixture of solid and liquid or two liquids by the action of centrifugal force. The centrifuge consists of a basket, in which a mixture of a solid and liquid, or mixture of two liquids, is rotated at a high speed  separation into its constituents by the action of centrifugal force. 5 In case of filtration: basket is perforated where liquid passes through perforation while solid is retained over the filter medium (paper). In case of decantation: liquids separated into layers where heavy liquid near the wall and light liquid near the center.  Significance of centrifugation: 1. As filtration process. 2. Separation of emulsion components. 3. Separation of light liquid from heavy. 4.In fractionation of biological products, blood and urine analysis. Applications of centrifugation: 1. Filtration 2. Separation 3. Decantation 4. Sedimentation. During centrifugation, the particles will be subjected to a simple gravity settling process vertically downwards and a centrifugal separation of solid particles from a liquid in a radial direction relative to the liquid. Suppose a particle of a mass m is subjected to a relative force at radius x with an angular velocity w this mass is subjected to a centrifugal force in a radial direction equal to mxw2. At the same time, it is subjected to a gravity force in vertical direction equal to mg, where g is the acceleration due to gravity.  The resultant force C, the net result of the two forces setting on the particle of the mass m is the ratio of the centrifugal force to the gravitational force, mxw2 / mg = xw2/ g x w2 /g is a measure of the separating power of the centrifuge and often has a value of several thousands.  Theory of centrifugation: If a particles with mass = m, rotated in an angular velocity =w, in an orbit with radius = x This particle will be affected by 2 forces: 1. a gravity force in vertical direction G = m.g (g = gravitational constant (9.807 m/s2) 2. Centrifugal force in a radial direction F = mxw2 The net force affecting on the particle = separation power of the centrifuge C C = F / G = mxw2 / m.g = x w2/ g  Centrifugal equipment I – Centrifugal filtration: 1-Batch Basket Centrifuge: Centrifugal dewatering filter 2- Continuous Pusher type centrifuge II – Centrifugal separation: Reverse-flow Cyclone Separator III- Centrifugal decantation Tubular Type Centrifuge IV- Centrifugal decantation with sedimentation The Disc-Bowl continuous Centrifuge  I – Centrifugal filtration: 1- Batch Basket Centrifuge: Centrifugal dewatering filter: Construction It has a perforated rotating basket, lined with a filter cloth, under driven by a directly electric motor.  An outer casing (shell) with an outlet for filtrate. The shell is closed by a cover which for safety is usually interlocked with the drive motor so that the centrifuge cannot be operated when the cover is raised.  Operation: 1. The basket is mounted on a vertical shaft by means of which it can be rotated at high speed. 2. The resultant centrifugal force throws the filtrate through the filter cloth which retains solid particles forming a cake. Centrifugal dewatering filter  Advantages: 1.The process is rapid and occupy little space. 2.Centrifugal force removes the liquid from the cake and produces almost a dry cake. 3.Used for highly concentrated slurries even to the extent of materials having a paste like consistency. Uses 1. Separation of precipitated protein from insulin. 2. Separation of crystals from its mother liquor. 2- Continuous Pusher type centrifuge Construction  Rotating basket with perforated wall covered with woven metal cloth, rotate at high speed.  The feed is introduced through a conical funnel and the cake deposited on the wall is pushed out by a piston. So it is a continuous. 16 Pusher type centrifuge Operation: The operation is completed through three stages. In the first stage: 1. The feed introduced through a centrally located conical funnel, the perforated basket rotates at high-speed producing a centrifugal force the filtrate passes through the filter cloth and cake retained. 2. The filtrate passes through the holes of the basket and is collected by a suitable piping.  In the second stage (cake washing): A washing is admitted by means of a suitable perforated head, and the wash liquid is collected through a separate piping. In the third stage (cake removal): by intermittent movement of pusher disc  cake out drying outside. Note:  A pusher disc intermittently moves the formed cake along the surface of the basket freeing the surface for further cake deposition.  The pusher makes one stroke forward and backward, after which it waits for a while until a further layer of solids is built up when a second stroke and so on.  Application: 1. This type used for separation of suspensions. 2. Centrifugal filtration of large amounts of slurry due to very large area. II – Centrifugal separation: Reverse-flow Cyclone Separator Mechanism: feeds introduced tangentially into the cylindrical upper section of the cyclone. Where the relatively high velocity produces a vortex which throws solid particles out on the wall of the cyclone (due to centrifugal force). Vortex: 1. Outer vortex: large particles near to walls then go down. 2. Inner vortex: fluid carrying the fine particles. Reverse-flow Cyclone Separator 22  Application: 1- Separation of heavy materials from dust. 2- Separation of solids from fluids. 3-Separation a powder into different particle size ranges by using series of cyclones having different flow rates or different dimensions. III- Centrifugal decantation decantation means (separation of 2 liquids) The heavy liquid is separated in the outer layer near the wall while light liquid forms the inner layer near the center. Tubular Type Centrifuge (sharples super centrifuge) Giant Centrifuge Principle: by  diameter and  rotation speed   centrifugal power for separation. “A high centrifugal effect could be obtained by using a centrifuge of small diameter rotate at high speed”.  The tubular centrifuge operates at 15.000 to 60.000 r.p.m thus giving accelerations up to 50.000 times the gravitational acceleration. Thus, the tubular centrifuge is an effective type even when the concentration of solids is very low. These are periodically discharged by scrapping the walls of the centrifuge-tube. Operation: 1. Formed of outer stationary vessel and inner rotating tube. 2. Feed enters from the bottom  by centrifugation liquid separated into its components where heavy phase near the wall and light phase near the center. 3. They are collected through 2 bent tubes. Tubular Type Centrifuge Uses: 1. Cleaning of fats and waxes, 2. Fractionation of the blood, 3. The recovery of viruses, 4. Separation of 2 immiscible liquids which differ in densities as oil and water, 5. Separation of solid with small particle size from liquids. 6. Separation of natural emulsions. IV- Centrifugal decantation with sedimentation: (separation of two liquids containing solids) The Disc-Bowl continuous Centrifuge Principle: 1. The centrifuge is a short basket with wide diameter  so,  efficiency of separation. 2. To  efficiency  use spinning discs to split the liquid into thin layers  easily separated. Operation: 1. Liquid enters through the distributor and is distributed between the discs. 2. The conical discs in the bowl enable the liquid stream to be split into a large number of very thin layers that can be easily separated into the heavier liquid (or solid particles) adjacent to the wall and light phase near the center. Disadvantage: The centrifuge is a short basket with wide diameter, so low degree of separation. Overcome: 1. A high degree of separation is obtained with a long basket of small diameter, but the speed required is then very high. 2. The introduction of conical discs. The Disc-Bowl continuous Centrifuge Uses: 1. separation of milk components. 2. Used for separation of 2 liquids containing solids (as with emulsifying agent in artificial emulsions).  Rate depends on and controlled by: 1.Feed rate and product removal. 2.The degree of separation required to be obtained. 3.Thickness of the liquid layer formed at the wall of the basket. 4.The total depth of the basket. 5.Number of discs. 6.Speed.