INTRODUCTION MANY METHODS OF MECHANICAL SEPARATION ARE BASED ON THE MOVEMENT OF THE SOLID PARTICLES OR LIQUID DROPS THROUGH A FLUID. IN THIS TOPIC WE ARE FOCUSING ON SOME SITUATIONS OF THE PARTICLES DELIBERATELY SUSPENDED FROM A LIQUID STREAM IN ORDER TO OBTAIN SEPARATIONS OF THE PARTICLES INTO FRACTIONS DIFFERING IN SIZE OR DENSITY. THE FLUID IS THEN RECOVERED, SOMETIMES FOR REUSE, FROM THE FRACTIONATED PARTICLES. SOME SEPARATION METHODS, SUCH AS JIGGING AND TABLING, DEPENDING ON DIFFERENCES IN PARTICLE BEHAVIOR DURING THE ACCELERATION PERIOD. MOST COMMON METHODS, HOWEVER, INCLUDING ALL THOSE DESCRIBED HERE, MAKE USE OF THE TERMINAL-VELOCITY PERIOD ONLY.
THEORY: SETTLING PROCESSES ALSO REFERRED AS SEDIMENTATION TANKS. SETTLING - PROCESS BY WHICH PARTICULATES SETTLE TO THE BOTTOM OF A LIQUID AND FORM A SEDIMENT. PARTICLES EXPERIENCE A FORCE, EITHER DUE TO GRAVITY OR DUE TO CENTRIFUGAL MOTION; TEND TO MOVE IN A UNIFORM MANNER IN THE DIRECTION EXERTED BY THAT FORCE. GRAVITY SETTLING - THE PARTICLES WILL TEND TO FALL TO THE BOTTOM OF THE VESSEL, FORMING A SLURRY AT THE VESSEL BASE. FOR DILUTE PARTICLE SOLUTIONS, TWO MAIN FORCES ENACTING UPON PARTICLE. PRIMARY FORCE IS AN APPLIED FORCE, SUCH AS GRAVITY, AND A DRAG FORCE THAT IS DUE TO THE MOTION OF THE PARTICLE THROUGH THE FLUID. THE APPLIED FORCE IS NOT AFFECTED BY THE PARTICLE'S VELOCITY; THE DRAG FORCE IS A FUNCTION OF THE PARTICLE VELOCITY.
SETTLING OR SEDIMENTATION SETTLING - A UNIT OPERATION IN WHICH SOLIDS ARE DRAWN TOWARD A SOURCE OF ATTRACTION. THE PARTICULAR TYPE OF SETTLING THAT WILL BE DISCUSSED IN THIS SECTION IS GRAVITATIONAL SETTLING. IT SHOULD BE NOTED THAT SETTLING IS DIFFERENT FROM SEDIMENTATION. SEDIMENTATION - THE CONDITION WHEREBY THE SOLIDS ARE ALREADY AT THE BOTTOM AND IN THE PROCESS OF SEDIMENT. SETTLING IS NOT YET SEDIMENT, BUT THE PARTICLES ARE FALLING DOWN THE WATER COLUMN IN RESPONSE TO GRAVITY. OF COURSE, AS SOON AS THE SOLIDS REACH THE BOTTOM, THEY BEGIN SEDIMENT. IN THE PHYSICAL TREATMENT OF WATER AND WASTEWATER, SETTLING IS NORMALLY CARRIED OUT IN SETTLING OR SEDIMENTATION BASINS.
REMOVAL MECHANISMS GRAVITY SEPARATION SETTLING TANKS, TUBE SETTLERS AND HYDRO CYCLONES FILTRATION SCREEN, GRANULAR MEDIA, OR POROUS MEDIA FILTER FLOTATION FOAM FRACTIONATION
SETTLING BASINS ADVANTAGES Simplest technologies Little energy input Relatively inexpensive to install and operate No specialized operational skills DISADVANTAGES Low hydraulic loading rates Poor removal of small suspended solids Large floor space requirements Re-suspension of solids and leeching Easily incorporated into new or existing facilities
SOLIDS PHYSICAL CHARACTERISTICS PARTICLE SPECIFIC GRAVITY PARTICLE SIZE DISTRIBUTION
CENTRIFUGE SETTLING SEDIMENTATION WHEN A SUSPENSION IS ALLOWED TO STAND, THE DENSER SOLIDS SLOWLY SETTLE UNDER THE INFLUENCE OF GRAVITY. CENTRIFUGATION A SETTLING PROCESS THAT IS ACCELERATED WITH A CENTRIFUGAL FIELD.
COMPARISON BETWEEN FILTRATION AND CENTRIFUGATION: Feature Filtration Centrifugation Separation principal Employment Particle size Removal of insoluble which are dilute, large and rigid Density Used when filtration is ineffective Product obtained Expense of equipment Dry cake Less A paste or a more concentrated suspension More
COMPARISON BETWEEN FILTRATION AND CENTRIFUGATION: Feature Filtration Centrifugation Separation principal Particle size Density Employment Removal of insoluble which are dilute, large and rigid Used when filtration is ineffective Product obtained Dry cake A paste or a more concentrated suspension Expense of equipment Less More
SEPARATION COST FOR RECOVERING WHOLE CELLS OR CELL DEBRIS: Ultrafiltration more economical Centrifugation more economical Ultrafiltration Centrifugation
CARE OF CENTRIFUGES (1) AVOID IMBALANCE IN THE ROTOR, WHICH MAY BE CAUSED BY: A. TUBE CRACKING DURING THE RUN * CONVENTIONAL GLASS (PYREX) CENTRIFUGE TUBES WITHSTAND ONLY 3-4000 G. USE CENTRIFUGE TUBES MADE FROM POLYPROPYLENE OR POLYCARBONATE. B. MISBALANCE OF THE TUBES IN THE FIRST PLACE SMALL TUBES BALANCED BY VOLUME BY EYE; LARGE TUBES (> 200 ML) SHOULD BE WEIGHED. (2) ANY SPILLAGE SHOULD BE IMMEDIATELY RINSED AWAY. AVOID CORROSION OF CENTRIFUGE ROTORS. (3) DO NOT USE THE MACHINE AT TOP SPEED CONSTANTLY.
TYPE OF CENTRIFUGE TUBULAR BOWL CENTRIFUGE DISK CENTRIFUGE
TUBULAR BOWL CENTRIFUGE 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. THE SUSPENSION CAN BE FED UNTIL SOLID LOSS IN THE EFFLUENT BECOMES PROHIBITIVE. AN INTERMITTENT OPERATION.
DISK CENTRIFUGE
DISK CENTRIFUGE A SHORT, WIDE BOWL 8 TO 20 IN. IN DIAMETER TURNS ON A VERTICAL AXIS. INSIDE THE BOWL AND ROTATING WITH IT ARE CLOSELY SPACED DISKS, WHICH ARE ACTUALLY CONES OF SHEET METAL SET ONE ABOVE THE OTHER. IN OPERATION, FEED LIQUID ENTERS THE BOWL AT THE BOTTOM, FLOWS INTO THE CHANNELS, AND UPWARD PAST THE DISKS.
DISK CENTRIFUGE
A PROPERLY OPERATED DISC CENTRIFUGE SHOULD SEPARATE 99% OF THE SOLIDS FROM THE LIQUID STREAM AND PRODUCE AN 80-90% WET SOLIDS CONCENTRATE. THE SMALLER THE PARTICLE DIAMETER, THE LOWER THE FLOW RATE, AND THE LONGER THE INTERVAL BETWEEN DISCHARGES. FLOW RATE IS PROPORTIONAL TO THE SQUARE OF THE DIAMETER OF THE PARTICLE. Q v ; v 2 d ( s - g 18 g g ) CELL DEBRIS (PARTICLE SIZE 0.5 MM) CAN BE SEPARATED WITH FLOW RATES OF 300-500 L/H.
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