Centrifugation BMS

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CENTRIFUGATION

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A settling process that is accelerated with a centrifugal field Used to separate particles as small as 0.1 µm in diameter Its effectiveness depends on particle size, density difference between the cells and the medium, and medium viscosity So used for separating solid particles from liquid phase Centrifuges can be compared using the relative centrifugal force (RCF) or g number

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Choice of centrifuge depends on particle size and density , and viscosity of the medium Separation of Smaller microorganisms (bacteria)- high speed centrifuge RCF 20,000 g may be required to recover bacterial cells, cell debris and protein precipitates from liquid media. For yeast cells – slow speed centrifuges are required Centrifuges can be divided in to small scale, pilot scale and industrial scale Laboratory batch centrifuges such as bench top, high speed and ultracentrifuges uses RCFs of 5000- 5,00,000g.

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For industrial purposes, semi-continuous and continuous centrifuges are required to process large volumes, but RCFs achieved are relatively low There will be Continuous feeding of slurry, and collection of clarified fluid and intermittent removal of deposited solids takes place; Four main types of industrial centrifuges are commonly used are Tubular centrifuge Multi-chamber bowl centrifuge Disc stack centrifuge Scroll decanter centrifuge

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1.Tubular Bowl Centrifuge known as super-centrifuge , develop a force about 13000 times the force of gravity . Consist of hollow tubular rotor bowl providing long flow path for suspension Suspension is usually fed through the bottom , and clarified liquid is removed from the top. Solid deposits on the bowl’s wall as a thick paste. When the particulate material accumulates on the inside of the bowl , the operating diameter becomes reduced Consequently there must be periodic removal of solids

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2. Multi-chamber centrifuge Modification of a tubular bowl type Capable of operating at 5000-10,000 g Consists of several chambers connected in such a way that the feed flows in a zigzag fashion Variation in the centrifugal force in different chambers Force is much higher in the outer chambers , this results in small particles settle down in the outermost chamber

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3. Disc stack Centrifuge Operate at 5000-13,000 g Centrifuge bowl contains a stack of conical discs whose close packing aids separation Feed is fed through the central tube and the particulate material is thrown outwards to the bowl wall, where they accumulate Periodic cleaning is required to remove solids deposited Disk bowl centrifuges are used in starch-gluten separation, concentration of rubber latex, and cream separation

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DISK STACK CENTRIFUGE

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4. Scroll decanter centrifuge Composed of a rotating horizontal bowl tapered at one end Operate continuously at 1500-5000 g Suitable for dewatering coarse solid materials at high solid concentrations Used in sewage systems for separation of sludge, harvesting yeast and fungal mycelium

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SCROLL TYPE OF DECANTING CENTRIFUGE Horizontal Type

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Advantages of Centrifugation Rapidly process large volumes in small centrifuges Centrifuges are steam sterilizable, allowing aseptic process No consumable cost for membranes, chemicals or filter aids Disadvantages of Centrifugation High initial capital costs ,Noise generated during operation and cost of electricity Physical rupture of cells may occur due to shear Bioaerosol generation also takes place

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