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

7.4 Dry Solids 291

Speed of rotation (rpm) q mixing time (min) = 300.

Double cone: 0.1 to 10 m3 working capacity; 10 kW/m3 reducing to 2 kW/m3 as the working capacity increases.

Ribbon: 0.02 to 15 m3 working capacity; 50 kW/m3 reducing to 4 kW/m3 as the working capacity increases. Typically 12 rpm.

Edge mill: 0.02 to 3 m3 working capacity; 40 to 50 kW/m3 and relatively independent of working volume.

Kneader, double arm: 0.05 to 3 m3 working capacity; 100 kW/m3 and relatively independent of working capacity.

Banbury: 0.05 to 0.6 m3 working capacity; 2500 kW/m3 and relatively independent of working capacity. Processing time = 3 min, rpm = 20–120.

Extruder compounding, pastes and foodstuffs: see Section 9.11.

x Good Practice

We can invest much effort into mixing solids, but we must prevent demixing after the blends are mixed. The four mechanisms of demixing are (i) sifting, (ii) angle of repose, (iii) fluidization and (iv) air-current. Here are the details: (i) Demixing via sifting: occurs if the particles are free flowing with mix of particle size with one size i 3 q the diameter of the other. (ii) Demixing via angle of repose: this occurs with moderately free-flowing particles (AI I 0.18 m) with different angles of repose or two different RAS. For particles characterized in this way, the only blender that seems to prevent demixing is the air pulse blender. (iii) Demixing by fluidization: tends to occur if the blend contains i 20 % fluidizing fines characterized by AI I 0.18 m, RI I 1.5 m, FRI I 0.76 kg/s plus coarser material with AI I 0.012 m, RI I 0.6 m and FRI i 7.6 kg/s. This type of demixing tends to occur if the action of the mixer induces air. (iv) Demixing by air-cur- rent: air carries superfine, easy-flow, nonagglomerating particles into voids. This is a problem if AII 0.18 m, RI I 1.5 m, FRI I 0.4 kg/s. Again try to avoid mixers whose action induces air.

x Trouble Shooting

For polymer blenders of feedstock for extruder:

“Material does not flow”: bridging/see also hoppers, Section 2.6.

“Components do not feed”: jammed valve or auger/solids blockage or bridging/ power fault in feeder. “Inconsistent flowrate”: bridge or block in blender/jammed discharge mechanism/inconsistent feedrates to blender.

“Wrong blend compositions”: calibration error in feeder.

8

Size Reduction

In this Chapter we consider options for creating drops and bubbles. Bubble formation is discussed in the section on bubble reactors, Section 6.13. Here the focus is on stable foams, Section 8.1. Although spray contactors have been described in other sections, here the focus is on the creation of the spray itself, Section 8.2. For liquid–liquid systems, the generation of dispersions of drops was summarized in the section on separating liquid–liquid systems, Section 5.3. Here the focus is on creating smaller size drops – emulsions, Section 8.3. Cell disintegration is featured in Section 8.4. The size reduction of solids by crushing and grinding is discussed in Section 8.5. Although two other “size reduction” options are (i) modifying the shape via extruders, pug mills and molding machines and (ii) modifying a liquid into a solid via prilling and flaking, these topics are discussed in detail in Chapter 9.

8.1

Gas in Liquid (Foams)

Gases are dispersed in liquids for reactions, absorptions, and a variety of separations via gas–liquid contactors. These aspects have been presented in detail elsewhere. Methods of creating bubbles have been described in reactors, Sections 6.13 and 6.27. Such systems are characterized in Section 1.6.1; with the flow characteristics summarized in Section 2.4. However, gases are also dispersed in liquids to create foams. Foams are used as foodstuffs, for foamed plastics, fire extinguishers, foam separations, Section 4.14 and mineral flotation, Section 5.19. Bubbles are used in dissolved air flotations but for DAF, Section 5.16, the bubbles nucleate on the particles present in the liquid.

xArea of Application

Creating a foam.

xGuidelines

Gas–liquid foams are stabilized by having a high disjoining pressure, by flooding the lamella with micelles or charged nonparticles. A high disjoining pressure

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

8.3 Liquid–Liquid 293

arises because the GL surfaces contain ionized surfactants (while the bulk liquid in the thin lamella has a low concentration of electrolyte) or contain bulky polymeric species. Alternatively, flooding the thin lamella film with micelles or with uniform 150 nm diameter highly-charged latex spheres produces structured layers in the lamella that produce a stable foam.

8.2

Liquid in Gas (Sprays)

Spray scrubbers, absorbers and reactors have been discussed elsewhere. Here the focus is on the actual generation of the spray.

x Area of Application

Pressure nozzle: spray diameter 70 to 1000 mm; capacity 0.03 to 0.3 L/s; low viscosity and clean fluids.

Spinning disc: spray diameter 50 to 250 mm; capacity 0.0015 to 0.4 L/s; for usual fluids and for viscous fluid or for fluid containing solids.

Twin fluid: spray diameter 2 to 80 mm; capacity 0.03 L/s; increasing the ratio of atomizing fluid to liquid from 1 to 10 decreases the spray diameter by a factor of 10.

Rayleigh breakup to produce uniform drops of diameter 1.8 q diameter of orifice. Related topic: prilling, Section 9.12.

Surface aerators: for activated sludge oxidation (instead of diffused air aeration, Section 6.13).

Brush aerators: for oxidation ditches.

Static mixers: spray flow.

x Guidelines

Pressure nozzle: pressure 0.45 to 14 MPa; increasing the pressure increases the capacity.

Spinning disc: increasing the capacity increases the drop size. Twin fluid: high energy input.

Surface aeration: 0.01–0.025 kW/m3 or 0.3–1.2 kg O2/MJ. Brush aeration: 0.015–0.018 kW/m3 or 0.6–0.8 kg O2/MJ.

Static mixers: spray flow: gas superficial velocity 3–25 m/s; liquid superficial velocity 0–0.6 m/s. Turbulent flow.

8.3

Liquid–Liquid

Liquid–liquid contacting for reactions, separations, mixing. See reactions, Section 6.27, separations, Section 4.10 and mixing, Section 7.2, for separate rules of thumb. General characteristics of these systems are given in Section 1.6.2.

294 8 Size Reduction

x Area of Application

Sparger: see Sections 6.13 and 6.27.

Mechanical agitator in tank: drop diameter 4–5000 mm; capacity i 0.05 L/s; for viscosities I 104 mPa s.

Colloid mill: drop diameter 1–8 mm; capacity 0.01–3 L/s; for viscosities I 104 mPa s but usually i 1000 mPa s.

Homogenizer: drop diameter 0.1–2 mm; capacity 0.03–30 L/s; for viscosities I 103 mPa s but usually I 200 mPa s; decrease the drop diameter by increasing the exit pressure.

High shear disperser: for viscosities 103–5 q 106 mPa s. Roller mills: for viscosities i 103 mPa s.

Static mixer: drop diameter 100–1000 mm (about 0.15 times drop diameter for fluid velocity in a pipe without the mixer); capacity 0.3–5 L/s. The densities and flowrates of the two phases should be about equal; viscosities I 50 mPa s. See also Section 6.6.

Ultrasonic: drop diameter 1 to 2 mm; capacity 1 L/s.

Pipeline mixers: narrow residence times and drop size distributions, low holdup.

x Guidelines:

Sparger: see Sections 6.13 and 6.27.

Mixer in tank: surface area 100–80 000 m2/m3 with area increasing with decreasing surface tension and increasing velocity. Power 3 kW/m3.

Colloid mill: surface area 10 000–2 000 000 m2/m3 with areas increasing with decreasing drop diameter and increasing volume fraction of dispersed phase. Power 40 to 200 kW s/L.

Homogenizer: surface area 20 000–10 000 000 m2/m3 with areas increasing with decreasing drop diameter and increasing volume fraction of dispersed phase. Power 25 to 120 kW s/L; 5 kW/m3 for low viscosity. Power increases as exit pressure increase from 3.5 to 55 MPa. At 55 MPa for 0.1 mm drops at 100 kW s/L; 35 MPa for 0.5 mm drops at 45 kW s/L.

Static mixer: surface area 100–20 000 m2/m3 depending on the drop diameter and the concentration of dispersed phase. Velocity 0.25–2.5 m/s. Turbulent flow. Dp is 100 q Dp in pipe without mixer; L/D about 33; power 0.001 to 0.015 kW s/L.

Ultrasonics: 18–30 kHz for 1–2 mm diameter drops.

Pipeline mixers: 1.2 kW s/L. Upstream use at least 3 pipe diameters of straight pipe.

x Trouble Shooting

See Sections 6.29 and 7.2.

8.5 Solids: Crushing and Grinding 295

8.4

Cell Disintegration

x Area of Application

Chemical treatment via acids, bases, solvents or detergents. Physical freeze–thaw or osmotic shock.

Digestion by lytic enzymes.

Mechanical homogenizer cell disintegration while in suspension in a liquid. Wet milling.

Pressure extrusion. Sonification.

x Guidelines

Homogenizer: first order process; ln (Rmax/( Rmax – R) / p2.9 where Rmax = maximum amount of released protein/unit mass; R = amount of protein released/

unit mass at time t; p = pressure.

Temperature increases 2.2–2.4 hC/10 MPa. The power required is 0.35 kW/ 10 MPa.

x Good Practice

Homogenizer: Temperature control is difficult. Usually operated at constant throughput.

8.5

Solids: Crushing and Grinding

Capacities are expressed for open-circuit operation. For closed circuit with the same size reduction ratio, power, reduce the capacity by a factor of 2. Rod mill product is usually larger diameter than from a ball mill. Selection depends on the size of feed, the reduction ratio, the target diameter, the capacity, and the hardness, toughness, fibrous, and sticky nature and whether wet grinding is OK. Ball and rod mills can be used for most hardnesses including fibrous, friability and stickiness. Use jaw crushers for hard materials but shift to cone crushers when hardness Mohs I 8 but not for sticky materials.

Size reduction is about 1 to 5 % efficient; most of the energy generates heat. Very ductile materials are difficult to break mechanically; use cold temperature to make brittle.

Impact mills give products with less area per unit mass than ball or rod mills.

x Area of Application

Jaw crusher: feed diameter 0.1 to 1.5 m; reduction ratio 5:1 to 10:1; capacity 1 to 300 kg/s; Mohs hardness I 9 [reduction by compression].

296 8 Size Reduction

Gyratory crusher: feed diameter 0.75 to 1.5 m; reduction ratio 5:1 to 10:1, usually 8:1; capacity 140 to 1000 kg/s; Mohs hardness I 9. More suitable for slabby feeds than jaw crusher. [reduction by compression].

Roll crusher: feed diameter 1 cm; reduction ratio 5:1 to 10:1; capacity 0.3 to 20 kg/s; Mohs hardness I 7.5. Suitable for softer, friable and nonabrasive materials. Ok for wet and sticky materials.

Cone crusher and short head cone: feed diameter I 25 cm; reduction ratio 5:1 to 10:1 usually 7:1; capacity 5 to 300 kg/s; Mohs hardness I 8. Usually secondary or tertiary crusher.

Impact crusher: pulverizers, shredders or smooth roll: feed diameter 1 cm with a reduction ratio of 7:1 to 10:1; capacity 0.3 to 50 kg/s.

Mills, hammer, feed diameter 10 mm, reduction ratio 10:1 to 50:1, capacity 0.01 to 5 kg/s; Mohs hardness I 4.5. maximum fines; feed not hard nor abrasive. Mills, ball and rod: feed diameter 0.5 mm with a reduction ratio of 10:1 to 50:1; Mohs hardness I 9.

Mills, autogenous, semi-autogenous; feed diameter 200 mm, reduction ratio 10:1 to 50:1; capacity 0.1 to 100 kg/s; Mohs hardness I 6.

Mills, fluid energy: feed diameter 50 mm; reduction ratio 10:1 to 50:1; capacity I 2 kg/s; Mohs hardness I 4.5.

Comminutor: to reduce the size of solid material in waste water.

x Guidelines

Two-stage grinding has lower capital cost but higher operating cost than singlestage primary grinding. Ball mills have lower capital costs but higher operating costs than pebble mills. Semi-autogenous usually have lower capital and operating costs than fully autogenous.

Breakage by compression the power needed increases with increase in the hardness of the solid being processed.

Breakage by tumbling, the power increases with increase in the reduction ratio and is relatively independent of the hardness.

Jaw crushers: power 0.5 to 5 MJ/Mg; rpm 300–100; Maximum capacity occurs under choke feed; minimum of fines. Breakup by compression, product diameter determined by the adjustment clearance between compressing plates.

Gyratory crusher: power 3 to 10 MJ/Mg; rpm 450–110; minimum of fines. Breakup by compression, product diameter determined by the adjustment clearance between compressing plates.

Cone crusher: selected as secondary and tertiary reducers; power 0.9 to 5 MJ/Mg; rpm 290–220. Breakup by compression, product diameter determined by the adjustment clearance between compressing plates.

Short head cone crusher: power 3 to 12 MJ/Mg. Breakup by compression, product diameter determined by the adjustment clearance between compressing plates. Often choke fed as tertiary crusher.

Roll crusher: power 3 to 15 MJ/Mg. Breakup by compression, product diameter determined by the space between the rolls. Speed determines the capacity.

8.5 Solids: Crushing and Grinding 297

Roller mill: 50 to 500 kPa. Breakup by compression, product diameter determined by the space between the rollers. Speed determines the capacity.

Shredders: power 25 to 250 MJ/Mg.

Hammer mill: power 2 to 80 MJ/Mg. Breakup by impact against a plate traveling at 20 to 60 m/s, product diameter determined by exit screen size.

Cage mill: Breakup by impact against a plate traveling at 20 to 30 m/s, product diameter determined by exit screen size. Handles amorphous materials. Pin-disc mill: Breakup by impact against a plate traveling at 200 m/s, product diameter determined by feed flowrate and speed of the pins. Ideal for soft material. Impact mill: Breakup by impact against a plate traveling at 50 to 110 m/s, product diameter determined by exit screen size.

Autogenous mill: length:diameter 0.2 to 0.5 with 0.33 usual. Breakup by impact among particles.

Rod mill: power 5 to 80 MJ/Mg; length:diameter 1.4:1 to 1.6:1 with length I 6.8 m; 35 to 40 % v/v rod charge to give total charge of 45 % v/v. Breakup by variety of mechanisms with revolving media.

Ball mill: power 30 to 10 000 MJ/Mg; length:diameter 1:1 to 2:1; 50 % v/v charge of balls. Breakup by variety of mechanisms with revolving media, large balls give coarse particles, small balls give fine particles.

Fluid energy mill; about 6 to 9 kg air/kg of solid or 1 to 4 kg steam/kg solid. Breakup by impact with other particles traveling at 100 to 300 m/s, product size determined by the feedrate. Power 700 to 1000 MJ/Mg.

Comminutor: used for reducing the size of the particulates in waste water.

x Good Practice

For more brittle materials, consider an impact crusher provided very abrasive (such as silica) or sticky materials (such as clay) are not present.

For gyratory crushers, no feed control is needed.

Rod mills can have spout feeder with head i 1.5 m above mill center line with the length of rods at least 15 cm shorter than the working length of the mill.

Ball mills usually have double scoop feeder especially with higher loadings, closed circuit operations with a rake or spiral classifier. Makeup balls can be fed directly in if a spout feeder is used but makeup balls fed into the scoop box cause jamming. Use a central makeup ball feed pipe.

Measure the experimental Work Index at or near the target product size.

9

Size Enlargement

This chapter considers equipment to coalesce drops in gas, Section 9.1; coalesce liquid drops in a liquid environment, Section 9.2, and create solid aggregates or flocs in a liquid environment, Section 9.3. Then we consider the creation of larger size particle clusters fluidization, Section 9.4; spherical agglomeration, Section 9.5; disc agglomeration, Section 9.6; drum granulation, Section 9.7; briquetting, Section 9.8; tabletting, Section 9.9, and pelleting, Section 9.10. The last three processes considered in this chapter focus on change in shape by extrusion, Section 9.11; flaking, Section 9.12; and coating, Section 9.13. Other options for increasing the size of particles (such as crystallization, Section 4.6, spray or flash drying, Section 5.6 and sintering/pelletizing, Section 6.22) are discussed elsewhere.

The fundamentals upon which most of these operations are based are surface phenomena. Surfaces are attracted to each other by van der Waals forces; surfaces are repelled by the electrochemical double layer or by steric hindrance. Surface energies, contact angles, and wetting are important.

9.1

Size Enlargement: Liquid–Gas: Demisters

x Area of Application

In general, liquid should not bind to the inserts or walls; they should flow as rivulets or drops along the surface of the insert.

Vane separators: droplet diameter i 20 mm; droplet concentration i 0.1 mg/m3. Mesh pads: droplet diameter droplet diameter 3–20 mm; droplet concentration 0.01–0.1 mg/m3.

Fiber beds designed for impaction (usually cylindrical or “candles”): droplet diameter 0.2–3 mm; droplet concentration 0.001–0.01 mg/m3.

Fiber beds designed for Brownian motion (usually cylindrical or “candles”):

I 0.1 mm; droplet concentration I 10–3 mg/m3.

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

9.2 Size Enlargement: Liquid–Liquid: Coalescers 299

x Guidelines

For all devices inlet gas flow rates 80 to 120 % of design.

Vane separators: collecting fiber i 300 mm; gas velocity 2.5 to 5 m/s; Dp 0.03– 0.25 kPa.

Mesh pads: collecting fiber 100–300 mm; gas velocity 2 to 4 m/s with smaller values used as liquid loading increases; Dp 0.1–0.75 kPa. For the suction side of compressors, the density weighted velocity through the mist eliminator pad: k = 0.106 m/s for operating pressure 0.7 MPa and k decreases by 0.003 for each increase in pressure by 0.7 MPa. Wire mesh demister removes droplets down to 10 mm and Dp about 0.25 kPa.

Fiber beds/Impact: collecting fiber 10–40 mm; gas velocity 1.25 to 2.5 m/s; Dp 1–2.5 kPa.

Fiber beds/Brownian motion: collecting fiber 8–10 mm; gas velocity 0.05 to 0.25 m/s; Dp 1–4.5 kPa.

x Good Practice

Consider using mesh pads upstream of impact filter beds to reduce the load on the filter bed. Consider installing mesh pads vertically to facilitate drainage and minimize re-entrainment. For noncorrosive and nonfouling, consider installing vane separators downstream of mesh pads to collect larger drops sheared off from the mesh pad.

Cannot be used for up to 25 % turnup capacity; avoid the use of inertial devices for up to 25 % turndown capacity.

x Trouble Shooting

“Demisters ineffective”: temperature too hot/fibers have the same charge as the droplets/wetting properties of fibers changed/fibers “weathered” and need to be replaced/flow rate too slow through fibers/wrong mix of fibers/prefiltering ineffective/[ foaming]*/wrong design/re-entrainment.

[Foaming]*: see Section 1.12.

9.2

Size Enlargement: Liquid–Liquid: Coalescers

For typical drop sizes produced from different sources, see Section 5.3. Related topic: decanters, Section 5.3.1.

Fundamentals, assume that all surfaces have a surface charge cause by the preferential adsorption of the charge determining ions (usually called the potential determining ions, PDI). The PDI is usually hydrogen ions; change the pH and change the surface charge. This surface charge is small but for small drops and bubbles with large surface area/volume the charge is enough to control behavior.

The solid surface should be preferentially wet by the dispersed phase, and the drops and the surface should have opposite surface charges so that they will attract.

300 9 Size Enlargement

x Area of Application

Stacked trays or parallel plates: feed droplet diameter i 10 mm and usually in the range 40–1000 mm; concentration I 15 % v/v.

Packed bed: feed droplet diameter 1 to 10 mm.

Mesh, wire or wool: feed droplet diameter 50 to 500 mm. and interfacial tension i 20 mN/m.

Co-knits of wire plus polymer: feed droplet diameter 10–200 mm.

Glass mats or co-knits of wire plus fiberglas: feed drop diameter 1–25 mm.

Fibrous bed: droplet diameter I 20 mm; usually 1 to 9 mm; concentration I 1 % v/v. Deep bed: droplet diameter I 1 mm. See Section 5.14.

Ultrafiltration: see Section 4.22.

x Guidelines

For direct interception, the diameter of the fiber is usually chosen to be approximately the diameter of the inlet drops. The exit drop is two to four times larger than the feed diameter. The thickness of the bed is about 5 cm. The cross section of the coalescer is selected to give an approach velocity of the total liquid stream of about 5 L/s m2 although some authors recommend 10–50 L/s m2

Stacked trays: 1 to 25 L/s per pack of trays on 2 cm spacing with 75 m2 area per pack. Diameter of captured drop increases as the flow rate increases and the density difference and interfacial tension decrease.

Packed beds: at lower capacities the exit drop diameter proportional to void diameter in the packing.

Mesh: use mix of high and low energy materials in mesh about 100 to 500 mm; flood velocities for mixed high and low energy fiber mesh I 1.2 m/s; usual design velocity 10–50 L/s m2 Pressure drop 6 to 140 kPa.

Mesh, wire or wool: usual design velocity 5.6–12.5 L/s m2.

Co-knits of wire plus polymer: usual design velocity 5.6–12.5 L/s m2.

Glass mats or co-knits of wire plus fiberglas: usual design velocity 5.6–12.5 L/s m2.

Fibrous bed: select fiber diameter that is about the diameter of droplets, fibers about 10 to 40 mm; typical exit drop diameter 2 to 4 times the inlet diameter; flood velocities 1 cm/s; usually design for 0.5 cm/s or 5 L/s m2 with velocity decreasing as surface tension decreases. Try for surface tension i 20 mN/m.

x Good Practice

Consider decreasing the temperature to decrease the solubility and increase the surface tension.

Adjust pH for water flowing through fibrous and mesh beds so that drop and fiber have opposite surface charge. Promote coalescence in solvent extraction systems by using surface tension positive configurations.

x Trouble Shooting

“Coalescer pads ineffective”: temperature too hot/pH incorrect/fibers have the same charge as the droplets/surface tension negative system/wetting properties of fibers changed/fibers “weathered” and need to be replaced/flow rate too slow