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

9.3	Size Enlargement: Solid in Liquid: Coagulation/Flocculation	301
through fibers/wrong mix of	fibers/prefiltering ineffective/surface tension
I 1 mN/m for fluoropolymer fibers or I 20 mN/m for usual fibers/wrong de-

sign/included in decanter but should be separate horizontal coalescer promoter unit/faulty design/[stable emulsion formed]*.

[Stable emulsion formed]*: see Section 1.12.

9.3

Size Enlargement: Solid in Liquid: Coagulation/Flocculation

Related topics: flocculants for thickening Section 5.10 and precipitation of proteins from solution, Section 4.7.

x Area of Application

Particle diameter I 1 mm; solids concentration I 0.1 %.

x Guidelines

Add coagulant: usually alum in the sweep floc concentration of 20 to 50 mg/L; adjust pH to 6 to 9.

Rapid mix: 45 s residence time with 1.5 kW/m3 turbine agitation.

Basin: velocity gradient, G, at the inlet 150 to 200 s–1 reduced to 50 s–1 later in the basin. Usually 150, 110 and 70 1/s. Allow residence time for Gt = 105. For choice of mixer see Section 7.3.4.

x Trouble Shooting

Coagulation and flocculation in general:

“Supernatant not clear”: [coagulation doesn’t occur]*/flocculation doesn’t occur]*/ [ floc doesn’t settle out]*/[ floc forms but breaks up]*/faulty design of overflow weirs/short circuiting.

[Coagulation doesn’t occur]*: wrong dosage of coagulant-flocculant/wrong counterion/pH different from expectations/pH far from zpc/faulty mixing in the rapid mix/valence on the counterion too small/charge on the dispersed particles or drops reversed from expectations.

[Flocculation doesn’t occur]*: faulty fluid dynamics into the basin/reel at wrong rpm/residence time too short/mixing not tapered/unexpected turbulence/too short a residence time between coagulant and subsequent flocculant dosage/ wrong flocculant.

[Floc doesn’t settle out]*: floc formed is too loose/see settler design Section 5.8. [Floc forms but breaks up]*: local turbulence i shear strength of floc.

For water treatment:

“Coagulation-flocculation ineffective, supernatant murky”: pH i 10/wrong dosage of alum or coagulant/pH I 4/increase in concentration of particles in feed/rpm of reels in flocculation basin too slow/feed temperature I 12 hC/rpm of reels in flocculation basin too fast.

302 9 Size Enlargement

For latex:

“Exit crumb too small”: brine concentration too high/temperature too low/power input too high/wrong pH.

“Excessive amount of fines in supernatant”: brine concentration too high/wrong pH/ temperature too low. “Strength of the resulting crumb I specifications”: pH too high and brine concentration too high.

9.4

Size Enlargement: Solids: Fluidization

Related topic: dryers, Section 5.6 and reactors, Section 6.30.

x Area of Application

Product diameter: 0.6 to 2.5 mm; batch process.

For spouted bed with feed diameter i 1 mm gives product diameter of 3 to 3.5 mm.

x Guidelines

Fluidized bed: 30 to 50 min for batch of 200 to 700 kg. Shallow bed, 0.3–0.6 m deep; gas velocity 0.1–2.5 m/s or 3–10 q minimum fluidization velocity. Evaporation rates 0.005–1 kg/s. m2 cross sectional area.

9.5

Size Enlargement: Solids: Spherical agglomeration

x Area of Application

Product diameter: 2–3 mm; batch. Tensile strength of agglomerate: 10 to 100 kPa.

x Guidelines

Power 10 to 40 kW/m3. Mixing time 30–300 s.

9.6

Size Enlargement: Solids: Disc Agglomeration

x Area of Application

Product diameter: 10 mm, fertilizer: 1.5–3.5 mm; ore 10–25 mm. Capacity I 25 kg/s. Tensile strength of agglomerate: 10 to 200 kPa depending on the binder. Produces more nearly uniform granules than drum, Section 9.7.

9.8 Size Enlargement: Solids: Briquetting 303

x Guidelines

Rotational speed about 50 % critical speed or 30 rpm decreasing to 6 rpm as diameter increases. Disc area = 01. to 200 m2. L: D = 0.1 to 0.22; angle of inclination with the horizontal 40–70h. Power = 7 to 9 kW s/kg or MJ/Mg.

x Good Practice

Control dust.

9.7

Size Enlargement: Solids: Drum Granulator

x Area of Application

Product diameter 2–5 mm; capacity 0.001 to 30 kg/s. Tensile strength of the agglomerate: 10 to 200 kPa, depending on the binder.

x Guidelines

Drum volume: 5 to 100 m3. L: D = 2 to 4; angle of inclination with the horizontal 10h. For fertilizer: 5 to 7 kW s/kg or MJ/Mg. For iron ore: 2 kW s/kg. 10–20 rpm. Product diameter controlled by speed, residence time and binder.

x Good Practice

Need closed circuit operation to achieve uniformity in product size.

9.8

Size Enlargement: Solids: Briquetting

Use of pressure to create agglomerate.

x Area of Application

Product diameter 15 to 80 mm; capacity 1 to 30 kg/s. Crushing strength of agglomerate 1–10 MPa.

x Guidelines

Constant product volume; operating pressure I 50 MPa. Power 2 to 50 MJ/Mg.

3049 Size Enlargement

9.9

Size Enlargement: Solids: Tabletting

Use of pressure to create agglomerate.

x Area of Application

Product diameter 15 to 100 mm; capacity 0.011 to 1.5 kg/s. Crushing strength of agglomerate: 1.5–10 MPa.

x Guidelines

Operate either as constant volume machine: mechanical tabletter with operating pressure I 50 MPa or as constant mechanical tensile strength (constant pressure) machine: hydraulic tabletter with operating pressure 150 to 250 MPa. Power: 50–60 MJ/Mg.

x Trouble Shooting

“Product tablet weight i design” : sample error/lab error/excessive fines.

9.10

Size Enlargement: Solids: Pelleting

Use of pressure to create agglomerate or extrude molten polymer and shear off pellets.

x Area of Application

Product diameter: 2 to 30 mm; capacity I 6 kg/s. Cylindrical shape product; used primarily for food stuffs. Feed is usually a viscous paste.

For polymer pellets: strand pelletizer is versatile but capacity is limited by the strand handling capacity. Usually I 200 strands per machine. Water ring pelletizer: not for water sensitive polymers or high melt flow polymers and tend to produce nonspherical pellets. Underwater pelletizer: good for PE and PP; produces spherical pellets; not for nylon, PET or melts that solidify rapidly.

x Guidelines

Power 18 to 70 MJ/Mg.

Water ring pelletizer: die-swell allowance is critical.

x Trouble Shooting

For strand pelletizer for polymer resin. “Pellet diameter too small”: hole too small for the desired throughput/extruder output too low. “Pellet diameter too large”: output too high/speed too low/feedroll speed too low/output too high for die-size. “Pellet too short or too long”: mismatch ratio of feedroll speed versus rotor teeth speed. “Strands dropping”: feedroll pressure too small/throughput too low/dieplate has too many holes. “Pellet cuts are angled”: feed not perpendicular to

9.11 Solids: Modify Size and Shape 305

strands/strands overlapping. “Pellet oval shaped”: feedroll pressure too high/inadequate cooling before cutting. “Pellets has tails”: incorrect clearance between rotor and cutters.

For water ring pelletizer for polymer resin: “Pellet diameter too small”: hole too small for the desired throughput/throughput too large. “Pellet diameter too large”: output too low. “Pellet too short or too long”: mismatch throughput versus cutter speed. “Blocked holes”: nonuniform pressure on the die face/throughput too low/die-plate has too many holes. “Pellet oval shaped”: cutter speed too high/inadequate cooling. “Pellet has tails”: incorrect clearance between die and cutters/worn cutter blades.

9.11

Solids: Modify Size and Shape: Extruders, Food Extruders, Pug Mills and Molding Machines

See related topics: dewatering press, Section 5.17, reactive extrusion, Section 6.34 and coating, Section 9.13.

x Area of Application

– For polymers

Batch:

Injection molding machine: Thermoplastics: commodity resins, polyolefins (LDPE, HDPE, PP), styrenics (PS, PMMA, polycarbonates, ABS, PET) and engineered resins for higher impact strength. It is useful to classify thermoplastic polymers according to viscosity, as was done in Section 1.2, item 12.

Continuous:

Extruder: thermoplastics. Extruders are polymer mixers or reactors that push molten polymer through a die. The die configuration is chosen to produce blown film, coatings, sheets, monofilaments, coatings for wire and cable, cast film or special shapes like siding, downspouts. Three general types: single screw, vented and twin screw.

Single screw preferred for materials not particularly heat sensitive, e.g. PVC. Relatively poor mixing; improve with static mixer inserts or mixing zone. Twin give better mixing at lower temperatures and pressures. Two stage vented excels in devolatization, controlling foam.

Pipe and tube: usually PE, PVC, ABS.

Blown film: LDPE, PP, HDPE: thickness 34 mm. Coating: primarily LDPE.

Casting and sheets: Casting: PP: fine film: 10–50 mm; cast film: 100–400 mm; thermoformable sheet: 200–2500 mm. High impact PS. also HDPE, PVC, ABS. Sheet thickness i 0.25 mm; film I 0.25 mm. (although blown film I 0.5 mm). Coating wire and cable: Usually PE and PVC coating on wire.

Monofilaments: Used for either mono or coextrusion. PP and nylon and most thermoplastics.

306 9 Size Enlargement

Special shapes: for siding, facia, downspouts, eavestrough. ABS, HIPS, RPVC, LDPE, PP, nylon.

–For foodstuffs: extruders used for pasta and cereals; food cooker-extruder used for soup bases, puffed products, dry cereals, textured proteins and pet foods.

–For clay and ceramics: extruders or pug mills: clay. materials for bricks, tiles and

ceramics.

x Guidelines

– For polymers

Batch: cycle: fill, cool, unload.

Injection molding machine: cycle: injection fill, 1/4 cycle or about 3 s; cooling time 3/4 cycles; cool such that a release in pressure does not cause distortion, 17–30 s; machine open close, 7 s. Feed temperature = heat distortion temperature + 55 hC; example temperature 200 hC; mold temperature for commodity resins = 25 hC. viscosity 0.1–100 kPa s; Injection pressure 100 MPa. clamping force 38 MN/ m2 of projected area surface part for polyolefins; 25–30 MN/m2 for styrenics. Cooling time: for polyolefins, 1 s/0.1 mm wall thickness; for styrenics q 1.3– 1.8 longer (F). Cooling time also dependent on type of machine, toggle takes longer than hydraulic takes longer than electric.

The processing temperature ranges are: nylon: 260 to 290 hC; acetal: 185 to 225 hC;

For low viscosity polymers; PA, PE, PP, PS: Acrylic 180 to 250 hC; polypropylene 200 to 300 hC; polystyrene: 180 to 260 hC; polyethylene (LD): 160 to 240 hC; (HD) 200 to 280 hC;

For mid viscosity polymers: ABS, CA, POM, SB: ABS 180 to 240 hC;

For high viscosity polymers: PC, PMMA, PPO, PVC: PC: 280 to 310 hC; PVC rigid: 160 to 180 hC;

Commodity resins, the cheaper resins, include PE, PP, ABS and PMMA. Engineering resins are more expensive and include PC, nylon, acetal and fiber-filled grades.

For the extruder: Barrel and screw design: compression ratio 2.5:1 with a length:diameter ratio of 20:1. Recommended screw speeds are 25 to 75 rpm and depend on the diameter. Avoid the use of high screw rpm because of possible overheating of the melt. N [rpm] = 1910 v/D [cm] where D = screw diameter. v = barrel velocity, m/s and use v = 1 m/s for LDPE, = 0.5 m/s for most polymers and = 0.2 m/s for FPVC.

Select a machine such that the shot size is between 30 to 80 % of machine capacity and for higher temperature resins 60 to 80 % of machine capacity. Try to keep the residence time as short as possible.

Residence time in the barrel = rated capacity of injection cylinder [cm3] q density [g/cm3] q cycle time [s]/shot mass [g]

Select pressure, depending on the thickness of the part, the length of flow and the viscosity of the melt.

Nozzle opening at least 4.75 mm and usually 7.9 mm and 0.8 mm smaller than the orifice dimension of the sprue bushing; land length I 4.75 mm and nozzle

9.11 Solids: Modify Size and Shape 307

bore i 12.7 mm. Avoid the use of an injection nozzle shut-off valve except for structural foams and gas-assist molding.

Avoid excessive back pressure. Usual back pressure is 350 to 700 kPa g line pressure.

Nonreturn or check valve should be sliding check ring type with flow through clearances at least 80 % of the flow area in the metering area of the screw: do not use ball-check type valve; replace the check ring if it has any chips, cracks or damage; use a check ring nonreturn valve that has deep, large radius flutes and has a flow area equal to the screws; prefer a free-flow valve.

Hot or cold runner systems can be used. For cold runners, the sprue should contain a cold-slug well to receive the cold melt emerging first from the nozzle of the extruder. The cold well is typically the largest diameter of the sprue with a depth of 1.5 times this diameter. Runner size 6.3 to 9.5 mm diameter .

Mold materials are usually P-20 with Rockwell hardness of 55 to 57 or aluminum.

Vents 0.05 to 0.07 mm diameter.

Gates should be i 1 mm. The gate land should be short as possible.

The flow path length varies from 40 to 800 mm; the wall thickness ranges from 0.5 to 3 mm with the possible flow path increasing as the wall thickness increases (for 0.5 mm thick walls the flow path ranges from 35 to 130 mm). The required specific cavity pressure varies from 18 MPa for low viscosity melts, thick walls to 200 MPa for high viscosity, thin walls.

Continuous:

Extruder: for processing polymers.

Scale up of extruders:

scaling laws for single screw extruders to give the same melt temperature:

D = diameter of extruder:
metering depth (h)	h /h		2		= (D /D )0.75
	1						1		2
screw speed, N	N /N = (D /D )–0.75
	1			2				1	2
output, F	F	/F		2	= (D /D )2					(L /L )0.5
	1						1		2	2	1
power, P	P1/P2				= (D1/D2)2.5 (L1/L2)0.8
Torque, Md	Md		/Md			2	= (D /D )2.75
		1							1	2

Drive power capacity: kW s/g: rigid PVC, 1.3–2; plasticized PVC, PS, PMMA, 0.7–1.1; PET, HIPS, 0.5–0.8; ABS, PP, 0.8–1.2; LDPE, 0.7–1.1; HDPE, 0.9–1.4; nylon 6, LLDPE, 1.1–1.6; nylon 66, PC, 1–1.5.

Single screw: L/D = 20 to 35; the screw turns as if to unscrew itself backwards out of the barrel. Shear rate z100 1/s in the screw channels; between 100 and 1000 1/s in most die lips and i 1000 1/s in flight-to wall clearances. Volumetric pumps with feed rate determined by the bulk density of the feed. Recommended rpm given previously. Output [kg/h] = 1000 (D/16)2.15.

Two stage vented extruders L/D = 24 to 28. Output = 0.8 q output single stage. Twin screw extruder: L/D = 12 to 16; compounding capacity is proportional to screw speed; conveying capacity = 1⁄2 the volume contained in one turn of the screw per revolution.

308 9 Size Enlargement

The heat to melt the resin comes from feed preheating, barrel and die heaters; frictional heat. The frictional heat because of the rotation of the screw accounts for i 50 % of energy input. Barrel and die heaters: resistance heaters 5 to 6 W/cm2 of inner barrel surface. Power to covey and heat: 0.4 to 1 kW s/g or 400–1800 kJ/kg depending on screw design and polymer processed.

Pipe and tube: PE: front barrel temperature, 160 hC, die temperature, 165 hC, pressure, 10 MPa; rigid PVC: front barrel temperature, 160 hC, die temperature, 170 hC, pressure, 17.5 MPa; ABS: front barrel temperature, 195 hC, die temperature, 200 hC, pressure, 17.5 MPa.

Blown film: draw velocity 0.35 m/s; draw ratio 4:1; extrude at 180 hC; mass flow 0.2 g/s. PE: front barrel temperature, 160 hC, die temperature, 165 hC, pressure, 10 MPa; rigid PVC: front barrel temperature, 160 hC, die temperature, 170 hC, pressure, 17.5 MPa; ABS: front barrel temperature, 195 hC, die temperature, 200 hC, pressure, 17.5 MPa.

Coating: air gap is more critical for LDPE than for copolymers. Extruder: compression ratio i 4:1 and long metering sections; use of static mixers as inserts in the downspout; keep temperature variation across the polymer fed to the die of I 5 hC. 180 m/s to 360 m/s. L/D = 24. with 0.4 to 0.7 g/s cm of die width. PE: front barrel temperature, 315 hC, die temperature, 325 hC, pressure, 7.5 MPa. Cast film and sheet extrusion: sheet 0.6 m q 50 mm; extrude at 200 hC; drum diameter 0.45 m. Film velocity 1 m/s. Take-off of sheet 2 to 3 m/s increasing to 15 to 20 m/s as the thickness decreases. Higher extrusion temperature at the die gives a more permanent gloss. Surface finish is very dependent on roll temperature: rolls should be run as hot as possible and just below the sticking point. Cooling rate is very important especially for thicker sheets; surface cooled too fast causes wrinkles. Edge trim is cost control issue. Film: PE: front barrel temperature, 160 hC, die temperature, 165 hC, pressure, 10 MPa; rigid PVC: front barrel temperature, 160 hC, die temperature, 170 hC, pressure, 15 MPa; plasticized PVC: front barrel temperature, 170 hC, die temperature, 175 hC, pressure, 10 MPa. Sheet: PE: front barrel temperature, 205 hC, die temperature, 200 hC, pressure, 17.5 MPa; PS: front barrel temperature, 200 hC, die temperature, 210 hC, pressure, 7 MPa. Coating wire and cable: ID of the tip for the wire must be closely matched to the diameter of the wire to prevent polymer back-flow, especially during start up. Usually keep the extruder speed constant and adjust the speed of the wire. Usually rate of cooling is the limitation. Air gap between the die and cooling trough = 0.1 to 0.45 m for PE so that it enters cooling water 60 to 80 C; increase to up to 1 m for polyolefins. Polysulphone, = 0.05 m. Rear of barrel is run hotter to reduce the load on the extruder drive. PE: front barrel temperature, 240 hC, die temperature, 240 hC, pressure, 20 MPa; plasticized PVC: front barrel temperature, 160 hC, die temperature, 170 hC, pressure, 10 MPa; nylon: front barrel temperature, 295 hC, die temperature, 300 hC, pressure, 6 MPa.

Monofilament: Extruder-melt pump-spinneret-quench-stretch-“heat-set”-wind. For Newtonian fluids the critical draw-down ratio is about 20 (take up velocity/ die exit velocity). Extrusion speeds 1 to 8 m/s. Feed must be dry. 20 to 200 filaments per spinneret. Operating limit is linear speed filaments can be drawn.

9.11 Solids: Modify Size and Shape 309

Die holes 2 to 4 q target filament diameter depending on the polymer, PVC is 2. PP: front barrel temperature, 240 hC, die temperature, 250 hC; nylon: front barrel temperature, 290 hC, die temperature, 300 hC; PS: front barrel temperature, 205 hC, die temperature, 210 hC, pressure, 10 MPa.

Special shapes: Extruder-cooling-puller-cutoff; usually twin screw extruder (although single screw used in the past). For each resin the final die lip plate dimensions must be correct (length of die/wall thickness) and die land length dependent on type of polymer. PS: front barrel temperature, 200 hC, die temperature, 205 hC, pressure, 10 MPa, land length ratio 15–25; plasticized PVC: front barrel temperature, 165 hC, die temperature, 165 hC, pressure, 7.5 MPa, land length ratio 10–25; rigid PVC: front barrel temperature, 165 hC, die temperature, 170 hC, pressure, 17.5 MPa.

– For dewatering:

Extruders for dewatering: 1 to 4 kW s/g or 1000–4000 kJ/kg. Related to rotary press, Section 5.17.

– For foodstuffs:

Extruders for foodstuffs: rpm 30–40; shear 5–10 1/s; power 200 kJ/kg or 0.1 to 0.4 kW s/g with values increasing with shear. For pasta, cookies, pastries and dough: rpm 30; shear 5 1/s; power 200 kJ/kg or 0.2 kW s/g with values increasing with shear and maximum temperature 50 hC. For RTE cereals, rpm 40; shear 10 1/s; power 200 kJ/kg or 0.2 kW s/g and temperature I 80 hC.

Extruders and cookers: If the moisture content is I 20 %, high shear is used with the power usage typically 0.36 kW s/g. If the moisture content is i 28 %, low shear is used with the power I 0.075 kW s/g. For low shear cookers (for soup bases): rpm 60–200; shear 20–100 1/s; power 75–200 kJ/kg or 0.075–0.2 kW s/ g and temperature I 150 hC; L:D typically 12:1. For intermediate shear cookers (puffed products, relatively dry materials 20 to 28 % moisture) rpm 300; shear 140 1/s; power 450 kJ/kg or 0.45 kW s/g and temperature I 200 hC; L:D typically 3:1. For high shear cookers (dry cereals, textured proteins, pet foods): rpm 350– 500; shear 120–180 1/s; power 500 kJ/kg or 0.5 kW s/g and temperature I 180 hC; L:D typically 15 to 20:1.

– For clays and ceramics:

Pug mills for clays, thick pastes and fertilizers: 0.004 kW s/g or 3–12 kJ/kg. For a combination pugmill plus extruder for clay: 0.012 kW s/g.

x Good Practice

Consider first injection molding machines, then extruders.

Injection molding machines: Resin should be dried I 0.02 %. Do not use resin that has been out of the dryer for i 20 min. Cold molds are difficult to fill and require higher injection pressures. Hot molds, generally, give better finish and less molded-in stress. Melt temperature is very sensitive to very small changes in rpm or back pressure despite sensor or controller set point. Measure with hand held pyrometer or laser sensor. Need slower fill rate for sprue-gated parts to prevent blush, splash or jetting. If the walls are i 5 mm, then slow fill helps reduce sinks and voids.

310 9 Size Enlargement

Back pressures of 0.35 to 0.7 MPa help ensure homogeneous melt and consistent shot size. As back pressure increases, melt temperature increases. Holding or pack pressures that are 0.4 to 0.8 of injection pressures are typical. To purge a machine, acrylic is recommended.

x Trouble Shooting

Injection molding: Basically the cause can be with the material, the machine, the operator, the operating conditions, the mold or the part design. To check on the material, try material from another supplier; to check the machine, use same material, conditions, mold on another machine; if the trouble is random, then it is probably the machine; try a different operator on the machine; trouble appears same location in the product, then flow conditions and look for problems from the front of the piston to the gate. The symptom–cause information is presented as issues related to appearance (color, surface finish and transparency), strength and shape defects and operation and symptoms.

x Appearance

Color, surface finish, transparency

– Color. “Discoloration (typically appears before burn marks appear; location appears at the weld line or where air is trapped in the mold)”: [contamination in heating cylinder]*/sensor error/control error/[degradation, mechanical]*/[degradation, thermal]*/[melt too hot]*/[melt not homogeneous]*/overall cycle too long/[contamination in hopper and feed zones]*/incorrect cooling of ram and feed zone/[venting in mold insufficient]*/[residence time too long]*/cooling time too short/dryer residence time too long/excessive clearance between screw and barrel/clamp pressure too high/injection forward time too long/gate too small/runner-sprue-nozzle too small. “Black specks inside transparent product”: faulty cleanout of machine from previous molding operations/failure to purge when not running for extended times/nozzle too hot/barrel temperature in the feed area is too low combined with high screw speed or high back pressure/sensor error/sensor located too far from heater bands/hangup in nozzle tip/nozzle adapter and endcap. “Brown streaks/burning”: wet feed/[melt too hot]*/[shear heating in the nozzle]*/[degradation, mechanical]*/loose nozzle/wrong nozzle/dead spots in hot manifold/mold should be cold runner system/gate or runner too small/[contamination]*/injection speed too fast/booster time too long/injection pressure too high/mold design lacks vents at burn location/gate size too small or at wrong location/plunger has insufficient tolerance to allow air to escape back around the plunger/poor part design/[venting of mold insufficient]*/[residence time too long]*. “Brown streaks at the weld lines or at the end of flow paths, black or charred marks”: [air trapped in mold]*. “Brown streaks at the same location”: nozzle loose, wrong, too hot/[shear heating (at gate, runner, cavity restrictions)]*. “Brown streaks dispersed throughout”: material fault at the hopper: wet material. “Weld burns”: [melt too hot]*/injection speed too fast/[mold too cold]*/injection hold time too long/injection pressure too high/faulty nozzle heating bands/[air trapped in mold]*.