HANDOUT 7.1 Table 7-1. Four Stages of Solid Liquid Separations

Transcription

1 HANDOUT 7.1 Table 7-1. Four Stages of Solid Liquid Separations PRETREATMENT To increase particle size, reduce viscosity. SOLIDS CONCENTRATION SOLIDS SEPARATION POST TREATMENT CHEMICAL PHYSICAL coagulation crystallization freezing/heating flocculation aging filteraid admix To reduce the volume of material to process. CLARIFICATION THICKENING gravity sedimentation gravity sedimentation cyclones cross flow filtration periodic pressure filters To separate the solids from the liquid; to form cakes of dry solids or to produce particulate free liquid. CLARIFICATION FILTRATION CENTRIFUGATION granular bed vacuum sedimenting centrifuge precoat drum gravity filtering centrifuge pressure cyclone expression To remove solubles, remove moisture, reduce cake porosity, or prepare material for downstream processes. PHYSICAL washing repulping drying deliquoring Liquid Table 7-3. Pretreatment techniques. Act Upon Treatment Technique Effect Heating Reduce viscosity (reduce resistance to flow, cake moisture) Dilution with solvent Reduce viscosity Solid particles Solids concentration Solid-liquid interaction Degassing by chemical additive Coagulation by chemical additives Flocculation by shear forces Ageing Increase concentration with a thickener Classify to eliminate files Filter Aid body feed Heat treatment/pressure cooking Freeze/thaw Ultrasonics Ionized radiation Wetting agents Prevents bubbles from forming Destabilize colloidal suspensions, allow particles to agglomerate. Micro-flocs agglomerate into larger flocs. Size of crystals increase. Rate of filtration increased, reduced load on filter Filtration rate increased and reduced moister content. More porous cake, faster filtration rate, but thicker cake Physical conditioning of sludge to induce coagulation and flocculation. Reduce interfacial surface tension, improve drainage and decrease moisture content. 1

2 HANDOUT 7.2 Table 7-2. Effect of concentrating a slurry on the volume. Step Solids Volume % Volume Solids Volume Liquid In Mixture Volume Liquid Removed % Liquid Removed from Feed Feed Volume of Mixture (After Treatment) Thickener /99 = 90.9 Filter /99 = 6.1 Deliquoring /99 = Hence, you can see that even though the thickener only concentrated the solids from 1 to 10%, 91% of the liquid was removed and the mixture yet to be treated was only 10% of the volume of the original feed. Filter Costs Thickener Size Figure 7-1. Relative costs of filters and thickeners as related to size. 2

3 HANDOUT 7.4 Table 7-3. Comparison of Forces Used in SLS. Gravity Vacuum Pressure Centrifugal Other Simple Low Operating Cost Very Bulky Still High Volume of Liquid After Separation Vacuum is Easy to Produce Effective up to P of about 0.8 atm Improved Rates Over Gravity Equipment is Simple, But Bulky and Expensive Compared to Pressure Filters, Output is Low Moisture Content of Cake May Be High Volatile Liquids Difficult to Handle Greater Ouput Per Unit Area Smaller Equipment, Low Cake Moisture Content Difficult to Continuously Discharge Cake Equipment is Expensive, High Operating Costs Maximum Separating Forces (High-Gravity) Construction is simple for Cyclones: -Compact, Low Operating Costs -Efficiently Falls for Particles <10µm Centrifuges Have Longer Residence Time: -More Efficient, Even for Fine Particles -High Throughputs -Low Residual Moisture Sonic, Electrokinetic 3

4 HANDOUT 7.5 Electrical Field Surrounding the particles Low Concentration Slurry, Little Particle-Particle Contact High Concentration Mixture (Low Porosity) With Significant Particle-Particle Contact Figure 7-2. Particle contact as a function of concentration. Surface charges and electrostatic forces can keep particles separated, requiring compressive forces to push particles together. 4

5 HANDOUT 7.6 Zeta Potential versus ph ZP (mv) ph loose-unglued sand run 1 loose-unglued sand run 2 loose-unglued sand run 3 loose-unglued sand run 4 loose-unglued sand run 5 loose-unglued sand run 6 crushed-glued sand run 7 crushed-glued sand run 8 crushed-glued sand run 9 crushed-glued sand run 10 Figure 7-3. Measured zeta potentials for sand particles with and without glue (the glue is used to hold the sand particles together in a sand cartridge). (H. Patel, Characterization of Consolidated Sand Cartridge Filter, Thesis, The University of Akron, 2001) Zeta Potential [mv] ph Kaolinite - 0 M Halloysite - 0 M Illite - 0 M Kaolinite M Halloysite M Illite M Figure 7-4. Zeta potential measurements for Kaolinite, Halloysite and Illite Clayes in 0 M and 0.14 M NaCl solution. (E. A. Stephan, Examination of Deep Bed Filtration, Dissertation, The University of Akron, 1999). 5

6 HANDOUT Porosity Grade E Carbonyl Iron Kaolin Latex Talc Supercel Fine Silica Applied Compressive Stress, kpa Figure 7-3. Compressibility of packed beds of selected materials (H.P. Grace, Resistance and Compressibility of Filter Cakes (Part I and II), Chem. Eng. Prog., 49, , , 1953) THICKENER GRAVITY FILTER Porosity VACUUM FILTER PRESSURE FILTER CENTRIFUGE 0.2 EXPRESSION PARTICLE DEFORMATION Compressive Stress, ATM Figure 7-6. Compressive Stress relation to separation operation. (F. Tiller and W. Li, Theory and Practice of Solid/Liquid Separation, 4 th ed., University of Houston,

7 HANDOUT 7.8 Table 7-5. Filter Selection Chart. (data from D.B. Purchas, Solid/Liquid Separation Technology, Uplands Press, Croydon, 1981). TYPICAL OPERATING RANGE PERFORMANCE Filter SCORE 0 TO 9 (9 IS BEST) % (mass) OF SOLIDS IN THE FEED PARTICLE SIZE OF SOLIDS, MICRONS CAKE DRYNESS WASH PERFORMANCE FILTRATE CLARITY CRYSTAL BREAKAGE LEAF TO PLATE CANDLE/PRECOAT CARTRIDGE FILTER PRESS SHEET FILTER STRAINER VACUUM DRUM VACUUM DRUM WITH PRECOAT VACUUM DISC BAND, PAN, TABLE , WATER SCREEN , DEEP BED

8 HANDOUT 7.9 Table 7-6. Centrifuge Selection Chart (data from D.B. Purchas, Solid/Liquid Separation Technology, Uplands Press, Croydon, 1981). Centrifuge TYPICAL OPERATING RANGE % (mass) OF SOLIDS IN THE FEED PARTICLE SIZE OF SOLIDS, MICRONS CAKE DRYNESS PERFORMANCE SCORE 0 TO 9 (9 IS BEST) WASH PERFORMANCE FILTRATE CLARITY CRYSTAL BREAKAGE PUSHER , PEELER/SCRAPER , WORM SCREEN , OSCILLATING SCREEN , BASKET , CONICAL SCREEN , SCREEN BOWL , DECANTER , TUBULAR BOWL DISC (MANUAL) DISC (SELF CLEANING) NOZZLE BOWL

9 HANDOUT 7.10 Table 7-7. Selection based on cake formation rate (F. Tiller, Chem. Eng. Progr. Oct. 1977, 65-76). Cake Formation Rate (rate at Separation type Equipment which cake height grows in a gravity filter). 0.1 to 10 cm/sec Rapid Filtering Gravity pans; screens; horizontal belt or top-feed drum filter; continuous feed pusher centrifuge 0.1 to 10 cm/min Medium Filtering Vacuum drum, disc, horizontal belt, pan filters, peeler centrifuge 0.1 to 10 cm/hr Slow Filtering Pressure filters, disc and tubular centrifuges, sedimenting centrifuges Negligible cake Clarification Cartridges, granular beds, precoat drums, filter aid admix 9