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

5.2 Gas–Solid 141

coils/pressure too high (inverted bucket)/orifice too large (inverted bucket)/vent hole plugged (inverted bucket)/defective trap parts (inverted bucket)/clogged orifice (thermodynamic). “Live steam blowing, and inlet and exit temperatures are equal”: bypass open or leaking/worn trap components/scale in orifice/valve fails to seat/trap lost prime (inverted bucket)/sudden drops in pressure/[backpressure too high]* (thermodynamic)/faulty air release (float)/trap too large (thermodynamic). “Continuous discharge when it should be discontinuous”: trap too small/ dirt in trap/high pressure trap installed incorrectly for low pressure service (bucket trap)/valve seat clogged with dirt/excessive water in the steam/bellow overstressed (thermostatic)/one trap serves i one unit/strainer clogged. “OK when discharging to the atmosphere but not when to a backpressure condensate header”: condensate line diameter too small/wrong orifice/interaction with other traps connected to a common header/condensate line partially plugged/[backpressure too high]*. “Slow and uneven heating of upstream equipment”: trap too small/insufficient air handling capacity/short circuiting when units are group trapped. “Inverted bucket trap loses prime:” sudden drop in pressure/faulty seat/faulty valve. “Upstream process cycling”: defective float/multiple sources of condensate to a single trap/trap flooded from condensate header/condensate discharged into the bottom of the condensate header/Dp across the orifice is incorrect for the orifice (inverted bucket). “Discontinuous discharge when it should be continuous”: defective float/load too small.

[Back pressure too high and trap is hot]*: return line too small/other traps blowing steam/obstruction in return line/bypass open/pressure in header too high. [Back pressure too high and trap is cold]*: obstruction in return line/excess vacuum in return line.

5.2

Gas–Solid

x Area of Application

“Fumes” are particles I 1 mm.

In general: use cyclones and settling basins for solids loading i 20 g/m3. Then, select bag filters unless fumes are also present, low temperatures (I 100 hC for natural fibers; I 300 hC for synthetic glass), non-corrosive conditions and not close to the dew point. Select scrubbers if fumes present. For high temperatures consider electrostatic precipitators. Design can be “high efficiency” or “standard” with mass collection efficiency decreasing as size of target particle decreases.

Dry cyclone: dust diameter 10 to 1000 mm; feed concentration 5 to 75 g/m3; temperature I 400 hC; gas phase Dp = 0.2 to 1.6 kPa. mass collection efficiency 50 %; power usage 0.8 to 20 kW/m3/s.

Settling basin: dust diameter i 100 mm; feed concentration 2 to 75 g/m3; temperature I 400 hC; gas phase Dp = 3 to 7 kPa; power usage 0.04 to 0.7 kW/m3/s. Bag filter: very efficient removal of small diameter and small particle loadings; dust diameter 0.5 to 70 mm; feed concentration 0.01 to 100 g/m3; temperature

142 5 Heterogeneous Separations

I 100 hC for natural fibers and I 300 hC; gas phase Dp = 0.5 to 1 kPa; power usage 0.8 to 30 kW/m3/s.

Wet cyclone: dust diameter 0.75 to 10 mm; feed concentration 5 to 75 g/m3; temperature I 100 hC; gas phase Dp = 0.5 to 1 kPa; power usage 0.8 to 20 kW/m3/s. Cross flow scrubber: dust diameter i 3 mm; feed concentration I 10 g/m3; temperature I 100 hC; gas phase Dp = 0.2 to 1.6 kPa.

Wet scrubbers: countercurrent wet packing: dust diameter 0.2 to 3 mm; feed concentration I 0.1 g/m3; temperature I 100 hC; gas phase Dp = 1.25 to 6 kPa. Turbulent bed contactor: [see also Section 4.8 absorber] temperature I 100 hC; OK for heavy sticky particles; allows high gas and liquid flowrates with high mass transfer efficiencies for gas absorption; 1 to 2 mm; 2.5 to 20 kW s/m3.

Venturi scrubbers: dust diameter 0.02 to 3 mm; feed concentration 0.1 to 20 g/m3; temperature I 100 hC; gas phase Dp = 1.25 to 6 kPa; mass collection efficiency 99 %; power usage 3 to 40 kW/m3/s.

Low voltage electrostatic precipitator: dust diameter 1 to 100 mm and conducting particles; feed concentration I 30 g/m3; temperature I 800 hC; gas phase Dp = 0.02 to 0.1 kPa; power usage 0.04 to 0.7 kW/m3/s. Mass collection efficiency 5 % increasing to 90 % as the particle size increase from 0.01 to 5 mm.

High voltage electrostatic precipitator: dust diameter 0.01 to 5 mm and conducting particles; feed concentration I 25 g/m3; with pressures I 7 MPa and temperature I 800 hC; gas phase Dp = 0.02 to 0.1 kPa; mass collection efficiency 99.5 %; power usage 0.04 to 0.7 kW/m3/s. Gas velocity 0.3–5 m/s across the face of the collecting surface. One high voltage system/2500 m2 collecting surface. 30–100 kV DC. High initial capital investment.

Afterburners: dust diameter I 0.1 mm; combustible; feed concentration I 0.1 g/m3 SO2 scrubbers (double alkali, Catox, Tyco): power usage 30 to 40 kW/m3/s

x Guidelines

Start with a cost correlation based on gas flowrate.

Dry cyclone: size based on an inlet gas velocity based on the particle loading: for particle loadings of I 7 g/m3 use inlet gas velocity of 11 to 23 m/s to size

inlet nozzle and then scale configuration from this dimension.

–loadings of 10 g/m3 use 20 m/s

–loadings of 100 g/m3 use 10 m/s

–loadings of 1000 g/m3 use 5 m/s

–loadings of 10 000 g/m3 use 1.8 m/s

Gravity settler: 5 to 7 m/s at inlet baffle; but the gas superficial velocity should be I 3 m/s to avoid re-entrainment.

Bag filter: Batch: load cycle and clean: intermittent shaking, reverse pulse, reverse blow ring or sonic cleaning. Load filter until the gas pressure drop across the filter I 1.5 kPa; then clean. Choice of fabric is critical: static charge on fabric, operating temperature, potential for fumes to absorb with moisture to deteriorate bag and need to select dust removal option to keep the Dp across the bag at 0.5 to 1.5 kPa. Felted material gives higher gas flowrate per unit area than woven, costs 3–4 times more and cannot be cleaned by shaking. Gas to cloth ratio of 25 to

5.2 Gas–Solid 143

150 dm3/s m2 but usually design on I 75 dm3/s m2. Usual range for woven fabric 7.5 to 50 dm3/s m2 ; for felted fabric use 7.5–100 dm3/s m2; for microporous tubes use 9–20 dm3/s m2. Gas loading depends on density of particles, size, inlet dust concentration, type of fabric. Batchwise loading with dust removal by reverse jet or blowring is about 0.04 kW/m2 of bag area. Bag length:diameter I 33:1.

Wet scrubbers: limited to lower temperatures I 100 hC.

Wet cyclone: size on internal superficial gas velocity of 1 m/s; height to diameter of 3:1 and water usage of 0.4 L/m3; water flowrate 1.3 to 2.5 L/m2 s.

Cross flow packed scrubber: Dp = 0.3 kPa/m of width; 1 to 1.5 m width usual; water flowrate 2.7 L/m2 s of horizontal cross-sectional packing; size on actual inlet gas flowrate to the packing face of 1 m3/m2 s.

Countercurrent wet packing: pressure drop 0.3 to 0.5 kPa/m of packing. Liquid loading about 0.4 L/m3 gas or 0.6 to 1 L/s m2; superficial gas velocity 0.5 to 1 m/s; mass loading liquid/gas = 0.7–1.5.

Turbulent bed contactor: Liquid loadings 20 L/m2 s; superficial gas velocity based on actual inlet gas flowrate 2 to 11 m3/s m2 horizontal cross section; mass loading liquid/gas = 4–8. Related topics fluidized bed, drying Section 5.5; reactors, Section 6.30.

Venturi scrubbers: size on throat velocity of 15 to 150 m/s selected based on particle size to be removed with 40 m/s (and 25 kPa pressure drop) for 1 mm and 120 m/s (and 25 kPa) for 0.1 mm. Water usage is in the range 0.5 to 5 L/m3 gas with pressure drop increasing as throat velocity increases; mass loading liquid/gas = 1.3–1.6.

Wet impingement baffle (Peabody) scrubbers: height/diameter 1.3:1 to 4.6:1; liquid loading 1 to 2 L/m2 s of horizontal cross-sectional area; mass loading liquid/gas = 0.2–0.7.

Electrostatic precipitator: Batch process: load electrodes then clean: via wet spray or mechanical rapping. For particle conductivity between 10–8 and 0.01 V–1 m. Prefer negatively charged configuration. May need to adjust conditions to get particle conductivity into acceptable range.

Particles i 10 mm use 38 m2 plate area per m3/s gas flow, 1 mm use 100 m2 plate area per m3/s gas flow, 0.4 mm use 120 m2 plate area per m3/s gas flow.

x Good Practice

Bag filters: Replace a complete set of bag filters annually. Install a bypass. Limit the number of parallel rows of bags on either side of the walkway to 3–4 rows for 20 cm diameter bags and 2–3 rows for 30 cm diameter bags. For cleaning, use 0.5–0.7 kPa clean, dry air with an air:cloth ratio of 2:1 for reverse jet and 2.5:1 for shaking.

x Trouble Shooting

Bag filters: “Excessive particle emissions”: cleaning too often/pressure used to clean it too high/bag breaks/gas temperature too high and particles crust on movable blowrings and tear bag. “Dp across bags i design”: faulty cleaning/improper bag tension/excessive moisture causing blinding/poor air distribution/hopper

144 5 Heterogeneous Separations

plugged, see Section 10.3/gas velocity i design. “Short bag life”: excessive cleaning/high inlet gas velocity/fines i design/blinding because of condensation, improper cleaning, excessive dust load or high cake density.

Dry cyclone: “Increase in catalyst losses”: [poor separation in cyclone]*. “Opaque flue gas from the vessel”: [poor separation in cyclone]*. “Particulate carry over that affects operation of downstream equipment”: [poor separation in cyclone]*. [Poor separation in cyclone]*: [stuck or failed trickle valve]*/[plugged dipleg]*/[dipleg unsealed]*/ gas velocity into cyclone too low or too high/faulty design of cyclone/solids concentration in feed too high/cyclone volute plugged/hole in cyclone body/pressure surges/[change in size of particles in feed]*.[Plugged dipleg]*: spalled refractory plug/level of catalyst in bed too high/Dp indicator for catalyst level faulty/Dp indicator for catalyst level OK but bed density incorrect. Air out periods with a lot of water or steam in vessel. [Change in size of particles in the feed]*: [generation of fines]*/[coarse particles]*. [Generation of fines]*: [attrition of the catalyst]*/fines in the new catalyst. [Attrition of the particles]*: local velocities upstream of cyclone i 60 m/s/particle too fragile. [Dipleg unsealed]*: solids level does not cover end of dipleg/Dp indicator for catalyst level faulty/Dp indicator for catalyst level OK but bed density incorrect.

[Stuck or failed trickle valve]*: binding of hinge rings/angle incorrect/wrong material/hinged flapper plate stuck open/flapper plate missing. [Coarse particles (diameter i design)]*: agglomeration of catalyst/[sintered particles]*/wrong specifications for catalyst. [Sintered particles]*: high temperature upstream/[temperature hot spots in the upstream reactor]*. [Plugged grid holes]*: foreign debris entering with fresh catalyst/faulty grid design.

“Temperature hot spots in upstream reactor”: [maldistribution]*/local exothermic reactions.

5.3

Liquid–Liquid

Estimation of dispersed drop size: Usual drop size 200 mm; interfacial tension 30 mN/m. Primary dispersions for drop diameters i 100 mm. Secondary dispersion if the drop diameter I 1 mm.

The fundamentals are as follows. For immiscible liquids flowing in turbulent flow in a pipe of diameter, D, the dispersed phase breaks up into drops with the diameter of the maximum (or 95th percentile) size drop predicted as follows: (Dp, 95/D) = 4 [1/We]0.6 where We = Weber number. Since most drop size distributions are geometrically distributed and since the geometric standard deviation is about 2, the geometric mass average is about 30 % of the Dp, 95. Thus, the average size drop would be 300 mm if the predicted Dp, 95 = 1000 mm.

When drops encounter shear or turbulence different from turbulent flow in a pipe, such as flow across a valve, or a rotating impeller in a pump, then (for interfacial surface tension of 30 mN/m), the following are typical drop size distributions: from a reciprocating pump pumping an oil water mixture will produce

5.3 Liquid–Liquid 145

a dispersion with geometric mass average = 1000 mm with geometric std. deviation of 3.5; from a centrifugal pump, 200 mm [3.5]; Dp across a valve, 400 mm [2.2]; and in-line mixer, 200 mm [1.75].

The general characteristics of dispersions from different sources:

static mixers, flash drums: 100–100 mm; solvent extraction units, mechanically agitated systems: 20–300 mm; high Dp across a valve, steam stripper bottoms, caustic wash drums: 10–400 mm; systems with interfacial tension I 10 mN/m or containing surfactant systems or secondary coalescence: 0.1–25 mm.

x Area of Application

For primary dispersions.

Decanter: drop diameter i 100 mm; feed concentration i 2 % v/v. Related topics where decanters are used include three phase separation, see gas–liquid–liquid separation, see Section 5.4; solvent extraction, see Section 4.10; azeotropic and extractive distillation, see Section 4.2; liquid–liquid CSTR reactions (such as alkylation), see Section 6.29.

Hydrocyclone: drop diameter i 20 mm; feed concentration 6 to 60 % v/v. Interfacial tension must be i 10 mN/m to prevent drop breakup.

Sedimentation centrifuge: Disc type: drop diameter i 20 and I 200 mm; feed concentration 6 to 60 % v/v; suited for low surface tension, density differences i 0.02 Mg/m3. Solids contamination I 0.1 % v/v. Use differential type for drop diameter i 200 mm; feed concentration 6 to 60 % v/v; suited for low surface tension, density differences i 0.05 Mg/m3. Solids contamination I 0.1 % v/v.

Electrodecanter: drop diameter 9 to 500 mm; feed concentration 0.8 to 8 % v/v. Fibrous bed coalescer: drop diameter 3 to 75 mm; feed concentration I 3 % v/v. see size enlargement Section 9.2.

API separator: drop diameter i 75 mm; feed concentration 0.015 to 3 % v/v. Dissolved air flotation: drop diameter i 8 mm; feed concentration 0.005 to 0.015 % v/v and drop diameter I 8 mm; feed concentration 0.0075 to 0.1 % v/v. see Section 5.16.

Coagulation/flocculation: drop diameter i 8 mm; feed concentration 0.005 to 0.015 % v/v and drop diameter I 8 mm; feed concentration 0.0001 to 20 % v/v. See size enlargement, Section 9.3.

Deep bed filtration: drop diameter i 8 mm; feed concentration 0.0002 to 0.005 % v/v and drop diameter I 8 mm; feed concentration 0.002 to 0.05 % v/v. see Filters, Section 5.14.

Solvent extraction: drop diameter 0.1 to 1 mm; feed concentration 0.001 to 10 % v/v. see Section 4.10.

1465 Heterogeneous Separations

5.3.1

Decanter

x Guidelines

Ill-behaved dispersions usually drift with time; are sensitive to incoming drop-dia- meter distribution and to upstream energy input. Examples include most systems with kerosene-based immiscible systems.

First approximation: allow 20 min residence time or total overflow velocity of 0.35 L/s m2.

Feed concentration I 10 % v/v. Size as sedimentation-controlled provided surfactants and contamination negligible and mixture is not “ill-behaved.” Use overflow total flowrate velocity of 0.5 to 3 L/s m2 based on horizontal cross-sectional area with a usual value of 1.4 L/s m2. This is for a horizontal cylinder with length to diameter ratios of 3.5. Allow both phases to have i 20 % of the diameter and no less than 0.2 m to ensure that the exit phases do not become cross-contaminated. For process control the minimum distance between the high and low levels of the interface should be 0.36 m or at least 2 min residence time.

Feed concentration i 10 % v/v, or contamination present or ill-behaved. Size as coalescencecontrolled. For vertical decanters, allow a total residence time that depends on density difference and interfacial surface tension. For a typical 0.5 m height of coalescent band (or a decanter of 0.7 m height), use an overflow total flowrate velocity of 1.5 (Dr/0.1)0.5 L/s m2 where the density difference is in units of Mg/m3 to determine the horizontal cross-sectional area. For horizontal configurations, use half of the vertical overflow velocity.

Surface area versus volume: for rectangular vessels of length, depth and width, L:D:W = 10:0.5:1 the area = 40 m2 for a volume = 40 m3 with n = 0.65 for volumes 5–40 and n = 0.51 for volumes 40–1500 m3. L:D:W = 2:1:1 the area = 15 m2 for a volume = 40 m3 with n = 0.65 for volumes 1–300 m3. Factor, L:D:W = 1:1:1, q 0.75. Where area1 = area2 (volume1/volume2)n.

Surface area versus volume: for vertical cylindrical vessels of length, and diameter: L:D = 5:1, the area = 3 m2 for a volume = 30 m3 with n = 0.65 for volumes 1–400 m3. Factors, L:D = 5:1, q 1.00; L:D = 3:1, q 1.4; L:D = 1:1, q 3.2. For horizontal cylindrical vessels of length, and diameter: L:D = 5:1 the area at the centerline = 14 m2 for a volume = 30 m3 with n = 0.49 for volumes 2–200 m3. Factors, L:D = 5:1, q 1.00; L:D = 3.5:1, q 0.82.

Can add parallel plates or high and low energy combination coalescer promoters (see Section 9.2).

x Good Practice

Contamination can interfere with the operation. Traditionally this contamination is surfactants, or particulates. The particulates can be corrosion products, amphoteric precipitates of aluminum or iron. Try changing the pH of the water to alter the surface charge on the dispersed drops. The separation capacity of a settler/ decanter doubles for every 20 hC increase in temperature. Caution, if, to ease this separation, the temperature is increased, such an increase in temperature

5.3 Liquid–Liquid 147

will increase the bulk phase contamination because of the increased cross-con- tamination by the mutual solubility.

x Trouble Shooting

“Entrained droplets in liquid effluent”: sensor error/sampling error (immiscible drops are not being entrained)/faulty design of separator/improper cleaning of vessel after shutdown, e.g., rust left in vessel/pressure fluctuation/pressure too low causing flashing/[inaccurate sensing of interface]*/[drop doesn’t settle]*/ [drop settles and coalesces but is re-entrained]*/[drop settles but doesn’t coalesce]*/[stable emulsion formation]*.

“Fluctuation in liquid level”: no vacuum break on syphon line for bottoms/level sensor error/poorly tuned controller/surges in feed.

[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/flowrate too slow 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 design/included in decanter but should be separate horizontal coalescer promoter unit/faulty design. See Section 9.2.

[Density difference decrease]*: dilution of the dense phase/reactions that dilute the dense phase; for sulfuric acid alkylation: if acid strength I 85 % w/w the olefins polymerize with subsequent oxidation of the polymers by sulfuric acid as a selfperpetuating continuing decrease in acid strength. Alkylate–acid separation is extremely difficult when acid concentration is 40 % w/w.

[Drop doesn’t settle]*: [density difference decrease]*/[viscosity of the continuous phase increases]*/[drop size decreases]*/[residence time for settling too short]*/ [phase inversion or wrong liquid is the continuous phase]*/pressure too low causing flashing and bubble formation.

[Drop settles and coalesces but is re-entrained]*: faulty location of exit nozzles for liquid phases/distance between exit nozzle and interface is I 0.2 m/overflow baffle corroded and failure/interface level at the wrong location/faulty control of interface/liquid exit velocities too high/vortex breaker missing or faulty on underflow line/no syphon break on underflow line/liquid exit velocities too high.

[Drop settles but doesn’t coalesce]*: [phase inversion]*/pH far from zpc/surfactants, particulates or polymers present/electrolyte concentration in the continuous phase I expected/[coalescer pads ineffective]*/[drop size decrease]*/[secondary haze forms]*/[stable emulsion formation]*/[interfacial tension too low]*/[Marangoni effect]*.

[Drop size decrease]*: feed distributor plugged/feed velocity i expected/feed flows puncture interface/local turbulence/distributor orifice velocity i design; for amine units: for amine i 0.8 m/s; for hydrocarbon i 0.4 m/s/[Marangoni effects]*/upstream pump generates small drops/[secondary haze forms]*/poor design of feed distributor.

[Inaccurate sensing of the interface]*: instrument fault/plugged site glass.

148 5 Heterogeneous Separations

[Interfacial tension too small]*: temperature too high/[surfactants present]* at interface.

[Marangoni effects]*: nonequilibrated phases/local mass transfer leads to local changes in surface tension and stability analysis yields stable interfacial movement.

[Phase inversion]*: faulty startup/walls and internals preferentially wetted by the dispersed phase.

[Rag buildup]*: collection of material at the interface: [surfactants present]*/particulates: example, products of [corrosion see Section 1.3]*, amphoteric precipitates of aluminum/naturally occurring or synthetic polymers.

[Residence time for settling too short]*: interface height of the continuous phase decreases/[inaccurate sensing of interface]*/turbulence in the continuous phase/ flowrate in continuous phase i expected; for example i 3 L/s m2/sludge settles and reduces effective height of continuous phase/[phase inversion]*/inlet conditions faulty.

[Secondary haze forms]*: small secondary drops are left behind when larger drop coalesces, need coalescer promoter, see Section 9.2.

[Stable emulsion formation]*: [surfactants present]*/contamination by particulates: example, products of [corrosion products. see Section 1.3]*, amphoteric precipitates of aluminum or iron/pH far from the zpc/contamination by polymers/temperature change/decrease in electrolyte concentration/the dispersed phase does not preferentially wet the materials of construction/coalescence -promoter malfunctioning/improper cleaning during shutdown/[rag buildup]*.

[Surfactants present]*: formed by reactions/enter with feed, example oils, hydrocarbons i C10, asphaltenes/left over from shutdown, example soaps and detergents/enter with the water, example natural biological species, trace detergents.

[Viscosity of the continuous phase increases]*: temperature too low, for alkylate–acid separation, temperature I 4.4 hC/[phase inversion]*/contamination in the continuous phase/unexpected reaction in the continuous phase causing viscosity increase.

5.3.2

Hydrocyclone

Design using same principles as liquid-solid hydrocyclone, Section 5.9. For flooded underflow, the pressure drop is about 2 to 7 times greater than air-core operation.

5.3.3

Sedimentation Centrifuge

Disc type: (Westfalia, Alfa-Laval, Robatel) continuous: Centrifugal field about 104 g and 100 rps with residence times of 1 to 10 s. Power 3 to 10 kW s/L of feed. Differential type: (Podbielniak; Quadronic) continuous: Centrifugal field about 500 g and 25 rps with about 10 to 75 s residence time. Power 1 kW s/L.

5.4 Gas–Liquid–Liquid Separators 149

5.4

Gas–Liquid–Liquid Separators

x Area of Application

Horizontal drum: Separates gas, oil and water; as for example as an early separation of natural gas upstream of drying or to handle sour water. Typically a relatively small load of hydrocarbon. Often called a “flash drum”.

Often follow the flash drum with a storage tank to allow further separation of water and hydrocarbon.

x Guidelines

Size of dispersed liquid phase is given in Section 5.3. Horizontal cylindrical vessel with allowance for 20 min residence time for the water phase. Keep liquid velocity I 10 L/s m2. Carefully size the inlet distributor for the liquid so that inlet velocity is I 0.4 m/s.

If some hydrocarbon is heavier than water, then include a boot. For more, see Section 5.3.1, decanter design, and Section 5.1, horizontal knockout pots. Include a vortex breaker and demister.

For downstream buffer tank, design for 3–5 day residence time and provide gentle mixing to prevent stratification and fresh feed bypassing directly to the exit nozzle. Example horizontal cross-sectional area versus volume is given in decanters, Section 5.3.1.

x Good Practice

Contamination from naturally occurring or synthetic surfactants or polymers, or corrosion products from upstream processing can cause stable foam or emulsion formation.

x Trouble Shooting

“Entrained liquid in overhead gas”: sensor error/[entrainment: GL]*. “Incomplete separation of oil from water”: faulty design of separator/residence time of liquid phases too short/liquid velocity in the decant phases too fast/Marangoni instabilities/liquid feed velocity too fast/poor distribution of liquid feeds/faulty location of exit nozzles for liquid phases/overflow baffle corroded and failure/interface level at the wrong location/faulty control of interface/no vortex breaker at water and heavy oil exit nozzles/liquid exit velocities too high/[emulsification]*.

“Poor separation”: level control fault/phase velocities too high/contaminant gives stable dispersion/smaller drop size than design/rag formation/temperature change/pH change/decrease in electrolyte concentration. See Sections 5.1 and 5.3.1 for more details.

[Entrainment: GL]*: vessel diameter too small for gas flow/no demister or demister malfunctioning/vessel pressure I design/[ foaming]*/inlet liquid line or distributor undersized or plugged.

[Entrainment: L–L]*: liquid velocity too high; example i 10 L/s m2/liquid distributor orifice velocity i design; for amine: for amine i 0.8 m/s; for hydrocarbon

1505 Heterogeneous Separations

i 0.4 m/s/faulty location of exit nozzles/interface level wrong location/faulty control of interface/no vortex breaker/exit fluid velocities i design/insufficient residence time/[stable emulsion formation]*.

[Foaming]*: see Section 1.12 for generic causes.

[Stable emulsion formation]*: see Section 1.12 for generic causes; Section 5.3.1 for more specific causes.

The dispersed phase should not preferentially wet the materials of construction. If unexpected rapid coalescence occurs, suspect Marangoni effects and change the dispersed phase. Treat the buildup of the “rag” at the interfaces based on the cause: corrosion products or stabilizing particulates, surfactants, or amphoteric precipitates of aluminum or iron. Consider adjusting the pH. Solid particles tend to accumulate at the liquid–liquid interface.

For column extractors: “Decrease in extraction efficiency”: agitator speed to fast/ excessive backmixing/[ flooding]*.

[Flooding]*: agitator speed too fast/feed sparging velocity too high/drop diameter smaller than design.

5.5

Liquid–Solid: General Selection

Effect of particle diameter and solid concentration on the choice: For particle diameter greater than 1000 mm and solid concentration i 3 %, use screens, Section 5.7.

For particle diameter less than 2 cm and i 5 mm and solid concentration 1 to 50 %, consider settlers, filters or centrifuges.

For particle diameter less than 300 mm and solid feed concentration 0.01 to 20 %, consider thickeners, Section 5.10.

For particle diameter greater than 20 mm and solid feed concentration greater than 50 %, consider dryers, Section 5.6.

For particle diameter 0.01 to 150 mm, consider deep bed filter, Section 5.14 or dissolved air flotation, Section 5.16.

For particle diameter 0.6 to 40 mm and solids concentration I 0.1 %, consider homogeneous separation via ultrafiltration, Section 4.22.

For particle diameters from 0.8 to 20 mm consider using a filter aid to precoat on the filter medium. For example, use diatomous earth or perlite. A fine filter aid is 8 to 20 mm diameter to give a precoat bed of permeability 0.05 to 0.5 mm2; a medium filter aid is 30 to 60 mm diameter to give a precoat bed of permeability 1 to 2 mm2; a coarse filter aid is 70 to 100 mm diameter to give a precoat bed of permeability 4 to 5 mm2.

For particle diameter less 1 mm, consider size increase via coagulation/flocculation, Section 9.3. An example coagulant is starch.

Effect of recovery on the choice: To recover liquid: in the order of preference of filters, Section 5.14 and filtering centrifuges, Section 5.13: deep bed, horizontal vacuum, pressure leaf, gravity flat table; cartridge, precoat drum and plate and