02 BOPs / Woods D.R 2008 rules-of-thumb-in-Engineering-practice (epdf.tips)

5.9 Hydrocyclones 161

Settlers: “Floc or particulates do not settle”: size different from design/density different from design/unanticipated turbulence or back eddies/liquid residence time too short/feed flowrate i design/incorrect design of inlet/faulty design of solids removal. “Supernatant not clear”: [ floc or particulates do not settle]*/faulty design of overflow weirs/short circuiting.

[Floc or particulates do not settele]*: floc density I design/temperature too low or fluid phase contaminated so that viscosity i design/turbulence/inlet design incorrect/shear breaks up particles/inadequate flocculation and particles to small.

5.9 Hydrocyclones

(see also Section 5.22 for solid–solid separations)

x Area of Application

Particle diameter 4 to 400 mm; feed solids concentration 4 to 30 % solids v/v but usual application is separating particles i 50 mm and concentrations I 10 % v/v with pressure loss of 20 to 100 kPa.

Regular hydrocyclones: particle “settling velocity” 2 mm/s to 5 mm/s and clarified liquid product of 0.1 to 70 L/s.

Miniature hydrocyclones: particle “settling velocity” 0.05 mm/s to 0.04 mm/s and clarified liquid product of 0.02 to 30 L/s.

x Guidelines

Determine diameter of hydrocyclone from D [cm] = 6 q 105 {Feed flowrate [L/s]/ target diameter2 [mm2] Dp [kPa]} {rL, density liquid [Mg/m3], q liquid viscosity [mPa s]/Dr, density difference, [Mg/m3]}.

Typical target diameters are 5 to 100 mm and are the diameters where 50 % reports to the overflow and 50 % reports to the underflow. Typically 3 times target diameter is the diameter below which all particles in distribution are removed. The standard hydrocyclone has an inlet diameter of 0.28D; the overflow exit diameter = 0.34D; the vortex finder length is 0.4D; cylindrical body of height of 0.4D, vertical length of cone = 5D or cone angle about 10h. Underflow diameter adjustable to adjust the volume split between the overflow and underflow.

x Good Practice

Control on pressure drop.

x Trouble Shooting

“Underflow too dilute, underflow appears as smooth inverted cone”: inlet velocity low/ inlet feed pressure low. “Underflow appears as slow, vertical rope of coarse solids”: underflow opening too small/feed concentration of solids higher than design. “No discharge from the underflow”: plugged inlet/plugged underflow.

“Underflow unsteady and variable inlet pressure”: air-gas in the feed.

1625 Heterogeneous Separations

5.10 Thickener

x Area of Application

Particle size 0.1 to 300 mm; try to avoid using a thickener for particle diameters i 200 mm especially if their density is i 2 Mg/m3. Feed solids concentration 0.01 to 20 % v/v; exit liquid contamination in the solids exit 80 to 90 %v/v liquid; Deep cone thickener: exit liquid contamination in the solids exit 30 to 40 % v/v liquid.

x Guidelines

Downward solid flux of 0.2 to 500 g/s m2 with low values of 0.2 to 2 for waste water treatment, pickle liquor and higher values of 40 to 70 for mineral processing. Higher flux rates by factors of 3 to 10 up to fluxes of 800 g/s m2 can be obtained with the addition of flocculants. Fluid residence times 2–4 h; solids residence times 4–24 h.

Torque for rake [Nm] = K( thickener Diameter, m)2 where K = 15 to 30 for solid flux loadings of I 2 g/s m2, 70 to 130 for solid flux loadings of 2 to 7 g/s m2, 150 to 300 for solid flux loadings of 7 to 23 g/s m2 and i 300 for solid flux loadings of i 23 g/s m2.

Operate the rake at 1–20 % of design torque at the drive head. Rake tip speed 15–25 cm/s but for appreciable amounts of particles of diameter i 200 mm, then increase tip speed to 25–40 cm/s. Power 0.06 kW/m2.

Pump the underflow at 1–2.4 m/s. see Section 2.5.

x Good Practice

Consider the use of flocculants or deep cone. Flocculant dosage should be related to feed inlet concentration, see also Section 9.3. Include high pressure water purge lines for both forward and reverse flow at i 1 m/s. Raise and lower the rake once per shift. For startup, pump feed into the empty tank and recycle underflow until the design underflow densities are achieved.

x Trouble Shooting

“Stalled rake”: uneven central feed distribution/excessive flocculant causing island formation/underflow concentration i design/unpumpable underflow/trying to maintain the underflow concentration when the feed contains fines i design/ storing too many solids in the thickener/“sanding out”/particle shape differs from design. “Plugged underflow lines”: insufficient fines/targetting underflow concentration i design/temperature change/pump problems, see Sections 2.3 and 2.5/suction velocity I 0.6–2.5 m/s.

Underflow concentration of solids too low”: removal of too much underflow/flocculation problems that give islands that lead to the feed concentration rat-holing directly to underflow.

“Supernatant cloudy”: feed velocity excessive causing breakup of colloidal flocs/ changes in pH or electrolyte concentration causing floc breakup/excessive feed

5.12 Sedimentation Centrifuges 163

turbulence/excessive vertical drops of feed/insufficient flocculant added or flocculant feedrate constant instead of proportional to solids concentration in the feed. “Sanding out”: too high an underflow concentration/feed concentration of dense particles i 200 mm is i design/power failure.

5.11

CCD: Counter Current Decantation

x Area of Application

When dilute liquid overflow is acceptable because high wash water ratios are used; for high temperature operation, when anticipate changes in feed materials and if the filtration rate is I 140 g/s m2.

x Guidelines

Wash ratio 2–2.2 water to 1 solids. Wash ratio and the number of stages are selected for expected recovery.

x Good Practice

Add one more thickening stage to account for inefficiencies in mixing.

5.12

Sedimentation Centrifuges

Primarily to recover liquid although they can be used to recover solids. Incomplete separation. Gives clear supernatant.

x Area of Application

Liquid contamination in the exit solids: 10 to 15 % v/v liquid.

Batch:

Tubular bowl: batch, very fine particles with settling velocities 5 q 10–8 to 5 q 10–7 m/s; particle diameter 0.1–500 mm, feed concentrations 0.1–20 % v/v; I 5 % w/w. Clarify low concentrations. Not for foaming.

Vertical solid bowl: batch, particle diameter i 30 mm; feed solids concentration 3 to 5 % w/w to prevent frequent cleaning.

Multi-chamber vertical solid bowl, manual batch discharge: particle settling velocity 2 q 10–6–7 q 10–5 m/s; particle diameter 1–50 mm; feed solids concentration I 4 to 5 % v/v. Very dry product and higher capacity.

High speed disc vertical solids retaining: batch, particle settling velocity 8 q

10–8–2 q 10–7 m/s; particle diameter I 1 mm; feed concentration solids I 1 % w/w; I 20 % v/v.

Continuous:

High speed disc, intermittent solids ejecting: particle diameter 1–500 mm; feed concentration solids 2–6 % v/v and I 5 % w/w.

164 5 Heterogeneous Separations

High speed disc, continuous nozzle discharge: particle diameter 0.1–500 mm; feed concentration solids 5–30 % v/v; I 10 % w/w. Usually select when have a large amount of fines.

Horizontal scroll discharge: (solid bowl decanter) 10–6 to 5 q 10–6 m/s; particle diameter 2–5000 mm ; feed solids concentration 0.5 to 50 % v/v. Usually i 10 % v/v; only option when feed concentration i 40 % v/v solids.

x Guidelines

Scaled up based on liquid handling capacity.

Batch: size on cycle time: separate, remove solids, clean. Cleaning interval: hours. Tubular bowl: batch, up to 50 000 rpm; 3 q 103 –6 q 104 g; L/D = 4–8; liquid flowrates 0.03 L/s at high rpm, 0.1–10 L/s at lower rpm. Clarify low concentrations. S 1500–4000 m2

Vertical solid bowl: batch, 450–3500 rpm, 900 to 1100 g; L/D = 0.8; 1.6–2.8 L/s; 70–120 cm diameter bowl; 200 to 500 g/s m2; area 0.5 to 3 m2; 3 to 17 kW. Multi-chamber vertical solid bowl, batch discharge: 4500 to 8500 rpm; 3 q 103– 3 q 104 g; 0.7 to 2.8 L/s; 30 to 60 cm diam. bowl. Clarification efficiency constant until bowl is full.

High speed disc vertical solid bowl: I 12 000 rpm; 3 q 103 –3 q 104 g ; L/D = 1;

I 28 L/s; 15 to 100 cm bowl diameter; cone 35 to 50h. solids retaining: 5 to 20 L/s

Continuous:

High speed disc vertical, intermittent solids ejecting: supernatant clarification; max. 55 L/s; feed 0.01–10 v/v; poor dewatering, can handle foaming, S up to 270 000 m2, cleaning interval: weeks; max. g: 5000–7500.

High speed disc vertical, continuous nozzle discharge: I 85 L/s; feed 1–30 v/v ; poor dewatering; can handle foaming, clarified supernatant; cleaning interval: weeks; max g: 6–9000; S up to 80 000 m2.

Horizontal scroll discharge: 1600 to 6000 rpm; 1500–5000 g; L/D 1.5 to 3.5; 0.1 to 16 L/s;

x Good Practice

Because of high speeds, maintenance tends to be extensive. Capacity sensitive to density and particle size.

V-belt drive tends to isolate vibrations.

5.13 Filtering Centrifuge 165

x Trouble Shooting

Horizontal scroll discharge decanter: “Centrifuge won’t start:” vibration switch triggered/no power/motor or starter failure/overheated motor/[broken shear pin]*/ lubrication oil flowswitch tripped. “Centrifuge shuts down”: blown fuse/overload relays tripped/motor overheated/[broken shear pin]*/lube oil flowswitch tripped. “Excessive vibration”: broken isolators/motor on flexible mounts/motor bolts loose/flexible piping not used/misalignment/bearing failure or damaged/loss of plows/damaged conveyor hub/solid product buildup in conveyor hub/conveyor or bowl not balanced/conveyor flights worn or portion of blade missing/trunions cracked or broken/conveyor bowl cracked or broken/leaking effluent weirs/ plugged solids in the effluent hopper/not level. “High moisture in exit solids”: liquid dams not set alike or set incorrectly/feed temperature too low/feed rate too high/effluent hopper plugged or not vented/conveyor flights worn. “High solids in liquid effluent”: feed rate excessive/effluent dams set wrong/no strip installed/incorrect feed temperature.

[Shear pins breaks]*: Feed rate too high/solids concentration too high/foreign material stuck in bowl. [Solid and screen bowl shear pins break]*: plugged discharge hopper/conveyor blades bent or rough/worn bowl strips/loose or broken trunion bolts/bowl inadequately washed/clearance too large for blade tip to bowl wall/ bowl inside rough/wrong size shear pin.

5.13

Filtering Centrifuge

Table 5.2 summarizes the characteristics of different rates of filtration for different size of particles.

Table 5.2 Characteristics of different rates of filtration.

166 5 Heterogeneous Separations

x Area of Application

Liquid contamination in the exit solids: 4 to 13 % v/v liquid.

Batch:

Vertical basket: batch, particle diameter i 2 mm; feed solids concentration 2–50 % w/w. Minimize damage to crystals. Low capacity: 0.003–5 kg/s. Product dryness 70 % solids. Possible for sterilization between batches.

Vertical basket with bottom plough discharge: batch, particle diameter 5–500 mm; feed solids concentration 9–90 % w/w; capacity 0.1–1.25 kg/s. Product dryness 25–60 % solids. Use with free draining, medium capacity, and where need multiple rinse.

Horizontal basket, plough discharge: particle diameter 200–20 000 mm; feed solids concentration 10–70 % w/w. free – slow draining, multiple rinses, low – medium capacity: 0.5–5.5 kg/s. Product dryness 87–96 %. Keep feed consistent with high solids concentration.

Semicontinuous:

Horizontal basket, pusher discharge: particle diameter 50–5000 mm; feed solids concentration 15–70 % w/w. capacity 0.3–7 kg/s. Product dryness 85–98 %. Free draining. Main advantage is to rinse the solids.

Continuous:

Horizontal combo solid-screen, scroll discharge: particle diameter i 150 mm; feed solids concentration 10–75 % w/w. Solvent-based reactions.

Vertical or horizontal cone: slip discharge: continuous, particle diameter i 200 mm; feed solids concentration 10–80 % w/w, capacity 0.25 kg/s. Dewater crystals and free draining fibers.

Vertical cone, scroll discharge: continuous, particle diameter 200–5000 mm; feed solids concentration 5–60 % w/w. capacity 0.02–8 kg/s.

Horizontal cone oscillating/torsional vibration discharge: particle diameter

I 6 mm; feed solids concentration 40–80 % w/w. capacity 7–40 kg/s.

x Guidelines

In general, for most centrifugal filters, the filtering area, m2 = 1.75 (basket diameter, m)2.

Batch: size on cycle times: accelerate to load speed, load and cake formation, accelerate to wash, wash, accelerate to full speed, spin dry, decelerate to unload speed, unload. Cake volume in the centrifuge is key. During load and cake formation the particles migrate to the periphery to form a cake, the supernatant liquid moves through the bed to the cake surface and then the liquid drains out of the interstices in the wet cake. The latter two times usually control.

Vertical basket: batch with cycle: cake formation, wash, discharge, clean. Total cycle i 10 min e.g. 180 min. Cake buildup rate 200–1000 g/s m2; area per unit: 2.8 m2; cake volume 0.009–0.5 m3; cake thickness 25–150 mm with smaller thickness in smaller diameter centrifuges. 600–2100 rpm; 300–800 G. Basket dia-

5.13 Filtering Centrifuge 167

meter 0.3–1.5 m; L/D = 0.5–0.6/1. Power 3–6 kW/m2 filter area. Temperature I 175 hC.

Vertical basket, bottom discharge: batch with cycles: accelerate to load speed, load and cake formation, accelerate to wash, wash, accelerate to full speed, spin dry, decelerate to unload speed, unload: 3–120 s. Cycle time for free draining 2–6 min; slower draining materials cycle time 20–30 min with some i 60 min. area per unit 0.4–4.5 m2; cake volume 0.018–0.53 m3; about 67 % of feed volume; cake thickness 50–150 mm with smaller thickness in smaller diameter centrifuges. 900–1800 rpm. Basket diameter 1–1.2 m; L/D = 0.5.

Horizontal basket, plough discharge: batch with cycles: load 7–25 s; wash 5–25 s, spin dry 12–30 s, unload cake 3–12 s; total cycle time 20 s–15 min but usually I 3 min. Cake buildup rate 500–3000 g/s m2; area per unit, 0.1–3.8 m2; cake volume 0.03–0.17 m3. Bowl diam. 0.3–1.2 m. 1000–2500 rpm; I 1250 G. Power 25–40 kW/m2 filter area.

Semibatch: particles move through the cycle continually.

Horizontal basket, pusher discharge: 20–120 cycles/min: cake formation, wash, discharge. Cake buildup rate 1000–10 000 g/s m2; area per unit: 0.1–1.8 m2; cake thickness I 75 mm. Basket diameter 0.2–1.2 m; 300–600 G. Power 40–60 kW/m2 filter area or 7–15 kJ/kg “salt” (about 3000 g/s m2). Keep feed solids

2.2 kg/s m2; ammonium sulfate 2.36 kg/s m2; and sodium chloride 3.1 kg/s m2.

Continuous:

Horizontal combo solid-screen, scroll discharge: area per unit, 1.2 m2; power 12–15 kW/m2 filter area.

Vertical or horizontal cone: slip discharge: cake buildup rate 3000–20 000 g/s m2; area per unit, 4 m2; Cone angle 20–35h and i angle of repose of the solids. 2500 G. Rinsing efficiency: fair.

Vertical cone, scroll discharge: diameter 0.5–1 m. 2500–3800 rpm; 1800–2500 G. Product solids 24 %.

Horizontal cone oscillating/torsional vibration discharge: cone angle 13–18h; cycles i 20 cycles/min. 300–500 rpm; I 500 G. Diameter 0.5–1 m; L/D = 0.5– 1/1. Product 92 % solids. Limited liquid handling capacity. Power 0.2 kW/Mg solids.

x Good Practice

Monitor pH and temperature.

x Trouble Shooting

Pusher: “Machine floods”: feed concentration I design/feedrate i design/irregular feedrate/change in size distribution or particle diameter. “Unstable cake formation”: feedrate i design.

1685 Heterogeneous Separations

5.14 Filter

Table 5.3 relates filtration rate to process operation.

For incompressible cakes, the filtration rate is directly proportional to the specific cake resistance, the pressure/vacuum and inversely proportional to the viscosity and the cake thickness.

The filtration rate is inversely proportional to the ratio of solids to filtrate while the rate of cake formation is directly related to this ratio.

For compressible cakes, the filtration rate is relatively independent of pressure. The more flocculated the solids, the more compressible will be the filter cake.

x Area of Application

Prefer Batch, when cake formation rate is I 0. 01 mm/s (low concentrations of small diameter particles). OK for higher liquid viscosities and higher temperatures. Batch can be vacuum or pressure operation. Prefer vacuum operation for particle diameter i 30 mm; if there is a small mass of fines (I 50 % with diameter I 5 mm) and bed has permeability between 0. 1–1000 (mm)2. Prefer pressure filters for particle diameter 1–70 mm, if there is a large mass of fines (with i 50 % w/w with diameter I 10 mm) and bed has permeability between 0.001 and 0.1 (mm)2.

Prefer Continuous for cake formation i 0.01 mm/s (high concentrations of larger diameter particles); viscosity I 50 mPa s. Continuous is usually gravity or vacuum operation. For gravity operation particle diameter i 1 mm; and bed has permeability i 1000 (mm)2. For vacuum, usually particle diameter i 30 mm and bed has permeability between 0. 1 and 1000 (mm)2.

Table 5.3 How process operation relates to filtration rates.

5.14 Filter 169

For compressible cakes, prefer mechanical compression via diaphragm plates or belt presses. Compressed volume = 0.6–0.75 volume before compression. Compression cycle 0.33–0.4 h for diaphragm plate and frame press.

Highly compressible: latex, highly flocculated materials i silica, talc, attapulgite i kaolin i i barite, diatomaceous earth i i incompressible = polystyrene, carbonyl iron.

Batch:

Leaf, pressure vertical: particle diameter 1–120 mm; feed solid concentration 0.08–0.5 % w/w. Recover liquid.

Leaf, vacuum: particle diameter 1–500 mm; feed solid concentration 0.07–2 % w/w; solids capacity I 5.5 kg/s.

Leaf, horizontal pressure: particle diameter 1–100 mm; feed solid concentration 0.003–0.05 % w/w. Recover wet solid with good washing (rank 2).

Plate, horizontal vacuum: Recover liquid (rank 1) Recover wet solid with good washing (rank 2); Liquid contamination in exit solids: 15–50 % v/v liquid. Dry solids capacity I 0.0027 kg/s.

Plate and frame: particle diameter 1–100 mm; feed solid concentration 0.003–25 % w/w. Recover liquid (rank 5); use for liquids with viscosity i 50 mPa s. Recover wet solid with good washing (rank 6); with good washing and fragile crystals (rank 3). Relatively incompressible solid bed. Liquid contamination in exit solids: 30 to 70 % v/v liquid. Solid capacity I 55 kg/s.

Diaphragm plate and frame: compressible cake.

Deep bed: particle diameter 0.01–50 mm; feed solid concentration 0.002–0.02 % w/w. Recover liquid (rank 1); 50–90 % removal efficiency. 10–300 mg/L feed solids concentration but usually I 40 mg/L. Dry solids capacity I 0.5 mg/s.

Cartridge: particle diameter 0.8–50 mm; feed solid concentration 0.003–0.02 % w/w. Recover liquid (ranks 3). Dry solids capacity, 0.0014 kg/s.

Continuous

Drum, gravity: particle diameter 40–5000 mm; feed solid concentration 0.08–0.8 % w/w. Recover wet solid with good washing and fragile crystals (rank 2).

Drum, pressure: particle diameter 5–200 mm; feed solid concentration 0.75–5 % w/w. Recover wet solid with good washing and fragile crystals (rank 2).

Drum, rotary vacuum: particle diameter 1 to 700 mm; feed solid concentration 5–60 % w/w. Recover wet solid with good washing (rank 4) Recover wet solid with good washing and relatively compressible cake (rank 1). Recover wet solid with good washing and fragile crystals (rank 2). Dry solids capacity I 50 kg/s.

Drum, precoat rotary vacuum: particle diameter 0.5–80 mm; feed solid concentration 0.02–0.1 % w/w. Recover liquid (rank 4).

Disk, rotary vacuum: Particle diameter 15–500 mm; feed fraction solids 0.02–0.70. Free flowing.

Table/pan gravity: recover liquid (rank 1) Recover wet solid with good washing (rank 1) Recover wet solid with good washing and fragile crystals (rank 1). Particle diameter 40–50 mm; feed fraction solids 0.02–0.20. Dry solids capacity, 280 kg/s. Table/pan vacuum: Particle diameter 40–50 mm; feed fraction solids 0.02–0.20. Dry solids capacity: 280 kg/s.

170 5 Heterogeneous Separations

Belt, gravity: Recover wet solid with good washing (rank 3); Liquid contamination in exit solids 20 to 30 % v/v liquid.

Belt, vacuum: Particle diameter 20–70 mm; feed fraction solids 0.05–0.6. Dry solids capacity: I 30 kg/s.

Belt press: dewater and handle flocculated solids.

Rotary press: dewater and handle flocculated solids. Particle size 1–50 mm; feed solids 0.1–25 % w/w. Related to extruders, Section 9.11 and expellers, Section 5.17.

Ultrafiltration: see Section 4.22. Particle diameter 0.4–200 nm; feed solids concentration I 20 % w/w.

Microfiltration: see Section 4.23. Particle diameter 0.05–800 mm; feed solids concentration I 75 % w/w.

Dissolved air flotation, DAF: see Section 5.16. Particle diameter 0.1–50 mm with typical target diameter 2 mm; feed solid concentration 0.002–0.08 % w/w; i 80 % removal efficiency.

x Guidelines

General porosity of cake over the usual range of pressure difference for filtration: 0.9: silica, polystyrene; 0.8–0.85: talc, zinc sulfide; 0.7: calcium carbonate; 0.6 titanium dioxide; ignition plug; 0.5 kaolin; 0.42 iron carbonyl.

Table 5.4 relates particle diameter to parameters important in filtration.

For filter press, nominal press volume within the frames = 1 m3 corresponds to a filter area of 40 m2 for a 5 cm cake and 80 m2 for a 2.5 cm cake (or chamber depth).

Batch:

Usually size unit on cycle and based on volume of cake removed. Select required cake volume in batch unit. 2/3 of the cake forms in 1/3 of filtration time: consolidation of the last 1/3 of the cake during the last 2/3 of the filtration time. Typically filtration cycle stops when the filtrate flux I 0.01 L/s m2. Cycle: filter, drain,

Table 5.4 Particle diameter and cake permeability and resistance.