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

9.11 Solids: Modify Size and Shape 321

ant cascade/misaligned sleeve/faulty screw design/screw too hot/extrudate too hot in the coolant bath causing boiling. “Pits on surface”: [contamination]*/moisture/water sprays onto extrudate just after exiting the die/water bath too hot. “Fisheyes in film”: moisture/damp polymer/too many volatiles in polymer. “Gels”: [contamination]*/[degradation of melt in extruder]*/[shear intensity too low]*/[screw tip pressure too low]*/number and density of the screen pack too low/moisture too high/screw speed too low/incompatible blend/[residence time too long]*/ lack of streamlines in extruder/incorrect start up procedures/[melting inadequate]*/[melt too hot]*/for reactive: localized initiator concentration too high. “Shark skin”: [melt fracture]*/die temperature too low at the land end/linear extrusion speed too high/throughput too high/viscosity of polymer too high/ MWD of polymer too narrow/lubricant additive missing/[shear intensity too high]*/die gap too small.

“Polymer build upon die”: melt temperature too low/throughput too high/die gap too small/wrong screw design/low level of antioxidants.

“Porous or bubbles in product”: poor melt quality at vent/plugged vent opening/insufficient vent vacuum/excessive volatiles in feed/screw speed too high/vacuum vent needed. “Spotted, warped or pocked surface”: [mixing of melt inadequate]*/ moisture/roll too cold/contamination/screen size too large/dirty die/[trapped air]*/dirt on rolls/drafty air/wrong tension/boiling on extrudate in cooling bath. “Lines on the product”: surface scratches on tip or die/local buildup/[die swell too high]*/throughput too high/polymer adhesion on channels, tip or die/incorrect contact in the quench tank/melt temperature too cold/throughput too fast/ land length too short. “Indented pock marks on pipe after water cooling”: coolant water spray velocity too high. “Raised pock marks on pipe”: water drops on surface in the air drying zone.

“Discolored material”: temperatures too hot/wrong formulation/discontinuities inside extruder.

x Symptoms

[Contamination]*: contaminated feed/contaminated additives/dirty die/polymer on die lips.

[Degradation of melt in extruder]*: [RTD too wide]*/barrel temperature too high/ screw speed too high (causing overheating and shear damage)/oxygen present/ [oxidation]*/nitrogen purge ineffective/wrong stabilizer/wrong screw/flows not streamlined/stagnation areas present/extruder stopped when temperatures i 200 hC/copolymer not purged with homopolymer before shutdown/[residence time too long]*.

[Degree of fill too high]*: feed rate too high/screw speed too slow.

[Die swell too high]*: tip too short/abrupt change in flow near tip or die/melt temperatures too low in die assembly.

[Draw resistance instability]*: for blown film, fiber spinning, blow molding: draw ratio too high.

[Extrusion instabilities]*: screw speed too high/screw temperature too high/barrel temperature at delivery end too high/channel depth too high in the metering sec-

322 9 Size Enlargement

tion/the length of the compression section too short/read barrel end temperatures too low/diehead pressure is too low.

[Feedrate too high]*: screw speed too fast/feed from hopper too fast.

[Gels]*: [contamination]*/[degradation of melt in extruder]*/[shear intensity too low]*/[screw tip pressure too low]*/number and density of the screen pack too low/moisture too high/screw speed too low/incompatible blend/[residence time too long]*/lack of streamlines in extruder/incorrect start upprocedures/[melting inadequate]*/[melt too hot]*/for reactive: localized initiator concentration too high.

[Low bulk density of feed]*: % regrind too high/grind too coarse. [Low feeding efficiency]* : low friction characteristics.

[“Melt fracture” where the critical shear stress of polymer (about 0.1 to 0.4 MPa) exceeded in the die; excessive shear stress at the wall i 0.1 MPa]*: exit speed at the die is too fast/melt too cold/throughput excessive/die land too short/die opening too small/entrance to die not sufficiently streamlined/screw speed too high/molar mass and melt viscosity too high/cross section area in exit flow channel too small/ external lubricant additive missing.

[Melt too hot]*: screw speed too high/exit barrel zone temperatures too high/degree of fill too low/[shear intensity too high]*/heat zone temperatures set too high/[screw tip pressure too high]*.

[Melting inadequate]*: barrel or die temperature too low/screw speed too fast/ screw design gives insufficient mixing/[shear intensity too low]*/feed rate too high/material too “slippery”/[degree of fill too high]*/additional component has too low a melting point/[residence time too short]*.

[Melting too soon]*: wrong bulk density of feed.

[Melting unstable]*: especially for screws with high compression ratio and short compression length: insufficient melt capacity/too large a channel depth in the metering section/temperature in the metering end of the screw too high/wrong screw design.

[Mixing of melt inadequate]*: [screw tip pressure too low]*/[ feed rate too high]*/ screw speed too high and [residence time too short]*/screw speed too low and [shear intensity too low]*/[degree of fill too high]*/faulty screw design for mixing/temperature set points incorrect/instrument error in temperature sensors/ temperatures too high/loading excessive for one component/no static mixer included/for reactive extrusion: liquid flow rate too high/screw channel under injection not full of polymer.

[Oxidation]*: temperature too hot/screw speed too low/[residence time too long]*/ oxygen present/nitrogen purge ineffective/antioxident stabilizer ineffective/[trapped.air]*/hopper vacuum inadequate.

[Polymer viscosity too high]*: temperature too low/wrong blend/[shear intensity too low]*.

[Residence time too short]*: screw speed too high/too much feed/[degree of fill too high]*/poor screw design.

[Residence time distribution, RTD, too wide]*: [degree of fill too low]*/feedrate too small/screws speed too fast.

9.12 Solids: Solidify Liquid to Solid: Flakers, Belts and Prill Towers 323

[Screw tip pressure too high]*: screens plugged/die or adapter or breaker plates too restrictive and give too much Dp/[polymer viscosity too high]*/temperatures in die assembly too low/barrel temperature too low/screw speed too high/[shear intensity too low]*/lubricant needed/flow restriction/throughput too high/die land too short/cold start/[degradation of melt in extruder]*.

[Shear intensity too low]*: screw speed too low/faulty screw design.

[Solids conveying instability]*: feed hopper fault/internal deformation of the solid bed in the screw channel/insufficient friction against the barrel surface. [Surging]*: 30–90 s : feed particles are not sufficiently softened (usually at the beginning of the second compression zone)/too rapid compression screws/[low feeding efficiency]*/low friction characteristics/low bulk density of feed/[melting too soon]*/adequate early barrel pressure but [melting unstable]*/first barrel heating too high/screw speed too fast/faulty screw design/additional compound slippery/bridging in resin feed hopper/feed zone temperature too high/[screw tip pressure too low]*/compound temperature too high/screw too hot/nonuniform take-off speed/take-off speed too high/throughput too fast/controller fault/feed resin not mixed well/melt temperature too low.

[Trapped air in extruder]*: unvented extruder/wrong screw design/pressure too low/rear-barrel temperature too high/screw speed too high/vacuum too low in feed hopper/powder feed instead of pellets.

9.12

Solids: Solidify Liquid to Solid: Flakers, Belts and Prill Towers

Related topic: dryers, Section 5.6 but use refrigeration (instead of steam) to solidify liquid.

x Area of Application

Flaker: liquid feed. Product: flakes about 1 cm q 1 cm q 1 mm thick; capacity I 10 kg/s. Viscosity of liquid I 105 mPa s. Caustics, resins, resin intermediates, fatty acids and alcohols, detergents, waxes, pharmaceuticals, pesticides, explosives.

Chilled belt: liquid feed. Product: pastilles, flakes, pellets; capacity usually I 10 kg/s per unit. Antioxidants, adhesives, resins, aluminum sulfate, sugar, agar-agar, maleic anhydride, animal fat, phthalic anhydride, chocolate, surfactants, asphalt, stearic acid, waxes, fatty acids and fatty alcohols.

Prilling towers: liquid feed. Product diameter: spheres 1–3 mm. Capacity I 5 kg/s per unit.

x Guidelines

Flaker: 20–300 g/s m2; heat transfer coefficient U = 0.35 kW/m2 hC; power: 1–50 MJ/Mg depending on the material; lower values for ammonium nitrate, benzoic acid, tetrachlorobenzene, sodium hydroxide; higher power usages for waxes

324 9 Size Enlargement

and resins. Grooved surface drums for liquids that will not wet the surface. Power: 0.9–1.1 kW/m2.

Chilled steel belt: four options for feeder: (i) heated overflow weir, viscosities I 1000 mPa s, produces flakes 1–3 mm thick; (ii) overhead double roll, viscosities I 108 mPa s, produces flakes; (iii) rotoformer to produce pastilles 1–10 mm diameter; (iv) heated strip former (for brittle products). Capacity 20–300 g/s m2. Heat transfer coefficient U = 0.35 kW/m2 hC; with underbelt water spray 230 W/m2 hC. About half the heat load from the product is removed in the first one third of the length of the cooling belt. Power 1–50 MJ/Mg. depending on the material. Belts are typically 0.5 to 1.5 m wide and 2.4 to 5 m long.

Prilling tower: gas velocities less than the terminal velocity of the prill, I 1–2 m/s; gas to solids ratio 10 kg air/kg solids. Assume solid surface temperature = solidification temperature, volumetric heat transfer coefficient Uv = 0.005 kW/m3 hC. Height I 60 m.

9.13 Coating

Related topics: extruder-coating, Section 9.11.

x Area of Application

Dip, shear stress 10–100 1/s.

Spray, shear stress 1000–10 000 1/s; OK for thick films but not as effective as dip or spin.

Forward roll, (meniscus roll coating), single layer coating, 1–50 mPa s; wet thickness 10–2000 mm; accuracy e 8 %; speed 0.05–1 m/s with maximum speed 2.5 m/s; shear stress 10–1000 1/s. Not as precise as reverse roll. Thickness dependent on roll rpm, viscosity and gap.

Reverse roll, single layer coating, 1–500 000 mPa s; thickness 5–400 mm; accuracy e 5 %; speed 0.02–8 m/s; shear stress 1000–100 000 1/s. More precise than forward roll. Web is usually closed and OK for materials that are easily deformed. Example jersey knits, plastic films. Probably the most versatile.

Roll with knife: single layer coating, 1000–10 000 mPa.s; thickness 50–2500 mm wet coat; web speed 0.05–2 m/s. Simple, low cost. Propensity for streaking. Roll with air knife: single layer coating; 5–500 mPa s; thickness 0.1–120 mm; accuracy e 5 %; maximum speed 8.2 m/s. OK for pigmented surfaces. Usually not used, prefer slide, curtain or extrusion. Examples book, litho, clay-coated sheet. Spin coating: thin films 20 nm; e 10 %.

Reverse gravure, single layer, 1–1500 mPa s; thickness 1–50 mm; accuracy e 2 %; maximum speed 12 m/s; shear stress 40 000–106 1/s; precision for thin coatings, low surface tension, long runs are possible, room temperature, coatings may contain solvents. Example, silicones, adhesives, magnetic tape.

Extruder-slot die, single layer, premetered where the amount applied to web is metered; 50–5 000 000 mPa s; thickness 15–750 mm; accuracy e 5 %; maximum

9.13 Coating 325

speed 11.5 m/s; shear stress 3000–100 000 1/s. Examples jersey knits, plastic films, web is closed.

Slot or blade single layer, premetered where the amount applied to web is metered, 5–20 000 mPa s; thickness 15–250 mm; accuracy e 2 %; maximum web speed 6 m/s; shear stress 20–40 000 1/s.

Slide, or cascade, multi layer, premetered where the amount applied to web is metered, 5–500 mPa s; thickness 15–250 mm; accuracy e 2 %; 0.1–4 m/s with maximum speed 5 m/s; shear stress 3000–120 000 1/s; usual gap width 0.1–0.5 mm with smaller gaps to providing more stability.

Curtain, multilayer, premetered where the amount applied to web is metered, 5–500 mPa s; thickness 2–500 mm; accuracy e 2 %; maximum speed 5 m/s; shear stress 10 000–106 1/s.

x Guidelines

Reverse roll: film thickness is a function of the metering gap, speed of rotation of the applicator and metering rolls and the capillary number. The speed ratio is very important. Example film thickness/metering gap = 0.55; speed ratio of metering to applicator roll = 0.2 for Ca I 2.

Knife and roll coating: self metered, thickness of coat depends primarily on the gap and the geometry of the gap.

Slot or extrusion: edges must be sharp with radius of curvature I 50 mm. Gap width = double the final wet thickness.

Slide: top edge must be sharp with radius of curvature I 10 % of slot opening. Curtain: use for wider gap, usually at higher speeds than slide. Minimum flow i 0.5 cm3/s cm width with design flow usually double minimum (= 1 cm3/s cm width); surface tension usually about 30 mN/m with the dynamic surface tension of the top layer slightly lower than that of the other layers; curtain velocity 100 cm/s The minimum flow rates and minimum speeds = max. speed of a slide coater. Need care in balancing rheologies of the layers. Coat thicker when speed or viscosity increase. Gap tends to have negligible effect. Bottom layer is the most important.

x Good Practice

Ensure the coating speed is such that the dynamic contact angle between the liquid coating and the substrate is I 180h to prevent air entrainment. Web tension must be correct for both unwind and rewind. Web tension/width is in the range 40–300 N/m. For multilayer slide coating, the critical web speed is determined by the local viscosity of the layer that wets the web with low viscosity preferred. To improve operation consider the use of a “carrier layer” with a shearthinning liquid. For curtain coating, use edge guides. Wettable guides give stable curtains but produce a thicker coating near the edge.

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x Trouble Shooting

“Chatter defects” cross web lines, bands, uniform in width and period: edge not sharp/ mechanical vibrations/web speed variation/pulsations in the coating bead/web speed too high/gap too large/bead vacuum insufficient.

“Streaks” downweb lines: gap too small/dirt accumulated/bubbles/nicks in coating head/pulsations in flow rate.

“Tearing of web”: incorrect tension/gap too small.

“Ribbing” evenly spaced lines down the web: Capillary number too high/coating too thin/wet coverage too low/Reynolds number too low/speed too fast/surface tension too high/gap too small.

[Air entrainment]* = [Dynamic contact line approaches 180h]*: web speed too fast/ too little vacuum/gap too large/viscosity too high/Ca too high i 0.1/rough surface/surface tension too low.

Rolls, slot or knife coaters, slide:

“Ribbing” evenly spaced lines down the web: web speed too fast/viscosity too high/ web coat too thin/flow rate too low/too little vacuum/[air entrainment]*/Ca too high/channel divergence angle too small/Re too small. Forward roll coaters are very sensitive to ribbing.

“Chatter defects” or barring ; cross web lines, bands, uniform in width and period: misaligned coating gap/edge not sharp/mechanical vibrations/web speed variation/ pulsations in the coating bead/web speed too high/gap too large/bead vacuum insufficient.

“Streaks” downweb lines: pulsations in flow rate/vibrations at gap/gap too small/ dirt accumulated/bubbles/nicks in coating head.

“Swelling and weeping of upstream meniscus”: web speed too slow/too much vacuum.

[Air entrainment]* = [Dynamic contact line approaches 180h]*: web speed too fast/ too little vacuum/gap too large/viscosity too high/Ca too high i 0.1/rough surface/surface tension too low.

Knife coaters:

“Ribbing”: knife angle of inclination is too divergent/viscoelasticity is too high/ knife angle of inclination is convergent and too high.

“Flooding”: feed film thickness is too thick. “Starvation”: feed flow to knife is too low.

“Streaks”: unable to pin the static contact line to the downstream corner of the knife/contamination trapped in gap.

Slide: “Waves”: surface elasticity effects.

“Ribbing” with wavelength about 2 mm: pressure difference over the bead is too high.

Roll with air knife: usually coating flows are unstable because of the action of the air knife.

“Streaks and chatter” downweb lines: air flow at knife causes oscillations in the upstream bead/distance between the bottom of the air knife and the coating pan edge is too large/inadequate contact angle between the liquid and the web.

9.13 Coating 327

Curtain:

“Heel formation”: web speed too low/curtain Re too high/high flow rates.

“Air entrainment and heel formation”: impinging curtain velocity too high/curtain Re too high/high flow rates/high web speeds.

“Disintegration of curtain”: curtain Re too low.

“Pulled film”: curtain Re too low/web speed too high relative to curtain velocity i 4.3/low flow rates and low coating speed/high surface tension/low curtain height.

“Air entrainment”: web speed too high relative to curtain velocity/curtain Re too high/high web speed and relatively low flow rates.

Extrusion coating “Poor adhesion”: a variety of apparently contrary causes related to polymer viscosity, degradation, oxidation, tackiness, temperature: melt temperature too low or high/air gap too small/chill roll temperature too cold or hot/line speed too fast/poor match between coating and substrate.

“Rough wavy surface (applesauce)”: wrong resin/temperature too low or high. “Edge tear”: draw ratio too high/die end temperature too low/temperature too low or high.

“Oxidation”: temperature too hot/screw speed too low/flows not streamlined/ extruder stopped when temperatures i 200 hC/copolymer not purged with homopolymer before shutdown. “Pinholes in coating”: substrate too rough/coating thickness too thin.

“Surging”: bridging in resin feed hopper/feed zone temperature too high/wrong screw design. “Voids”: moisture/leaks in resin handling system/inadequate drying and storage/[thermal degradation]*/[gels]*.

“Die lines”: nicks in die/dirty lips/particles in die.

“Pin holes and breaks”: coating too thin/contamination/decomposition/compound temperature too hot/moisture.

“Web tears”: compound temperature too cold/too much drawdown/die lip opening too large.

“Poor adhesion”: compound temperature too low/substrate problems.

“Excessive neck-in”: die-to-roll gap too large/material temperature too high/die-lip opening too large/throughput too low/use resin with lower Melt Index/die land length too long.

See Section 9.11 for the following [Contamination]*; [Degradation of melt in extruder]*; [Screw tip pressure too low]*; and [Shear intensity too low]*.

[Gels]*: [contamination]*/[degradation of melt in extruder]*/[shear intensity too low]*/[screw tip pressure too low]*/number and density of the screen pack too low/moisture too high/screw speed too low/incompatible blend/[residence time too long]*/lack of streamlines in extruder/incorrect start upprocedures/[melting inadequate]*/[melt too hot]*/for reactive: localized initiator concentration too high.

[Surging]*: screw speed too high/take off speed too high/back pressure too low/ compound temperature too high.

[Thermal degradation/crosslinking]*: polymer temperature too hot/screw speed too low.

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Curtain coaters: “Entrained air”: web speed too high/coat too thin and Reynolds number too low.

“Waves”: surface elasticity effects.

Related to downstream drying:

“Dryer bands”: width of bands = hot air nozzle diameter: nonuniform air flow/velocity too high/coating too thick/coating viscosity too low.

“Mottle”: air motion too vigorous/coating viscosity too low.

“Fat edges” or “picture framing”: Marangoni effects. “Nonuniform distribution of binder”: Marangoni effects. “Hexagonal pattern on surface”: Marangoni effects.

“Curling and cracking”: coating too thick/lack of plasticizer additives/air humidity too low or air too dry/temperature too low/pigment volume too high/Deborah number too high.

“Holes”: dirt/oil/grease/particulates in air/Marangoni effects. “Delamination”: faults in the surfactant system.

“Blushing” milky opalescence: temperature drops below dew point of air.

10

Process Vessels and Facilities

This chapter considers process vessels (such as reflux drums, KO pots, columns) in Section 10.1, and storage tanks in Section 10.2. Bins and hoppers for the storage and delivery of solids are considered in Section 10.3. Bagging machines are discussed in Section 10.4.

10.1

Process Vessels

x Area of Application

Used for a wide range of temperatures, pressures and applications. Can be decanters, reflux drums, intermediate storage, columns, KO pots, reactors.

x Guidelines

Operating pressure and temperature constrain practical size of vessel. Design codes for pressure vessels vary slightly with the country. In general, for operating pressures i 10 MPa, vessel volume usually I 1 m3. Pressure decreases as temperatures exceed 250 hC. For temperatures above 350 hC consider carbon/molybdenum and for temperatures i 500 hC consider austenitic steels. See Fig. 6.2.

Corrosion allowance: 1.5 mm for corrosion rates 0.08 mm/a; 3 mm for rates 0.09–0.3 mm/a; 4.5 mm for 0.31–0.4 mm/a; 6 mm for i 0.4 mm/a. If pressure I 400 kPa use L/D of 2–3:1; for pressures i 400 kPa use L/D of 4–5:1.

Relating vessel mass to dimensions: product of (vessel height, m) (diameter, m)1.5 = 100 corresponds to a vessel mass of 30 Mg for vessel designed for pressure of 0.8 MPa with n = 1.0 for the product range 1–500. Pressure corrections: 0.8 MPa q 1.0; 2.7 MPa q 1.9.

For surge vessels, allow 2 min liquid residence time; for drawoff, use 15 min; for reflux use 5 min provided this allows sufficient time for controllers to function. Total volume = 1.3 q holdup if the holdup volume is i 3 m3.

For equalization basins, allow a detention time of 14 h.

Rules of Thumb in Engineering Practice. Donald R. Woods

Copyright c 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31220-7

33010 Process Vessels and Facilities

10.2

Storage Vessels for Gases and Liquids

x Guidelines

For liquids, consider floating head vertical cylinders for pressures I 100 kPa; spheres, vertical cylinders with dome ends for pressures I 250 kPa; small spheres and horizontal cylindrical tanks for pressures I 800 kPa.

For gases, consider pressure cylinders and small horizontal cylindrical tanks for pressures i 800 kPa.

10.3

Bins and Hoppers for Bulk Solids

Johanson’s definitions of terms used to characterize solid particles are given in Section 1.6.4. Related topics include mixing of solids, Section 7.4 and transportation of solids, Section 2.6. The important terms are AI, RI, HI, FRI, FDI, BDI, CI, RAS and SBI.

x Guidelines

Promote mass flow with cone angle to the vertical related to the wall friction angle of the solid.

Wall friction angle (h)	Cone angle with the vertical (h)	Wedge angle with the vertical (h)
	less than	less than
30	10	20
20	25	38
10	35	48
0	45	60

Both bottom angle and surface smoothness are important. For a circular cone the outlet should be at least 6–8 times the diameter of the largest particle. For wedge hoppers the discharge opening should be at least 3–4 times the diameter of the largest particle. The length of the slot opening should be at least 3 q the width. The desired mass flow requires the entire discharge opening to be active by the use of: a tapered interface belt conveyor, a tapered shaft screw conveyor, a screw conveyor with an increasing pitch (providing the length: screw diameter I 3:1) or a combination of screw conveyor with a tapered shaft plus increased pitch provided the length: screw diameter I 6:1.

Design hopper: prevent ratholing with hopper angle (with the vertical) I HI + 3h. Diamondback hopper: tends to prevent bridging and ratholing.