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

4.8 Gas Absorption 111

(surface tension positive), the foam above trays might be unexpectedly stable; stable films on packing.

For amine absorption of sour gas, keep inlet amine solvent temperature at least 5 hC hotter than inlet gas temperature to minimize condensation of volatile hydrocarbons in the inlet gas stream.

x Trouble Shooting

“Product contamination”: leaking valves/crud left in storage tanks/crud left in dead legs in piping/corrosion products/unexpected chemical reactions/sampling fault/ analysis fault/unexpected solubility effects.

“Insufficient absorption or off-specification for exit scrubbed gas”: feed gas concentration off spec/feed gas temperature or pressure outside operating window: for amine absorbers: i 50 hC for H2S and I 24 hC for CO2/feed gas pressure has decreased/[solvent flowrate too low]*; for glycol dehydration: 12.5 to 25 L TEG per kg water removed/[solvent incorrect]*/incorrect feed tray location/[column operation faulty]*/absorber operating conditions differ from design/[absorber malfunction]*.

“Dp across absorber i design”: gas flowrate i design/pressure I design/[ foaming]*/plugged trays/plugged demister pads/collapsed tray or packing.

“Dp on column fluctuating”: [ foaming]*

“Solvent carryover from the top of the column”: [ foaming]* “Liquid level in vessel fluctuates”: [ foaming in column]*

“Change in absorption rate”: for amine absorption: decrease in removal of H2S and increase in removal of CO2/[ foaming]*.

“Overloaded liquid in downstream gaseous processing equipment”: [ foaming in absorber]*.

“Solvent losses high”: [physical losses]*/[entrainment]*/[solubility]*/[vaporization]*/[degradation]*/[loss elsewhere]*/for glycol dehydration typical losses = 0.015 mL/m3 gas treated.

for multitube cocurrent falling film absorber:

“Concentration of product acid I design, inadequate absorption”: liquid maldistribution/gas maldistribution.

“Low heat transfer coefficient”: liquid or gas maldistribution. “Hydraulic instability”: no vent break on the syphon.

[Amine concentration too high or too low]*: if too high, lack of equilibrium driving force/if too low, insufficient moles of amine for the feed concentrations. [Column operation faulty]*: plugged tray or packing/poor distribution for packing/ liquid flowrate I minimum required for loading/[gas velocity too fast]*/collapsed trays or packing/plugged or broken distributors/[ foaming]*/solvent - stripper overhead temperature too low.

see also Section 4.2.

[Degradation]*: chemical reaction; for amine: reacts with CO2 and O2; forms stable salts: for glycol: reacts with O2/thermal decomposition; for amine: surface temperatures i 175 hC; for glycol: surface temperatures i 205 hC.

112 4 Homogeneous Separation

[Entrainment: GL]*: demister plugged, missing, collapsed, incorrectly designed/ [ flooding]*/[ foaming]*/inlet liquid line or distributor undersized or plugged/ poor distribution for packing/liquid flowrate I minimum required for loading/ [gas velocity too fast]*/solvent feed temperature i specifications/[column operation faulty]*/tray spacing I design. see also GL separators Section 5.1 [Entrainment: L–L]*: fluid 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 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]*. see also decanters, Section 5.3 [Foaming]*: a generic listing of causes for foaming is given in Section 1.12. [Flooding]*: see Section 4.2.

[Gas velocity too fast]*: vessel diameter too small for gas flow/column pressure I design/trays or packing damaged or plugged giving excessive vapor velocity/ temperature too hot/upstream flash separator passing liquids: feed contaminated with excessive volatile species/stripping gas fed to column too high/flowmeter error/design error.

[Liquid environment wrong]*: pH far from the zpc/electrolyte concentration too low.

[Physical losses]*: leak to atmosphere/purges for sampling/sampling/heat exchanger leak/pump seal flushes/filter changes/piping, fitting, valve stems, gaskets, pumps.

[Solubility losses]*: liquid-liquid systems: system pressure I design/for amine: concentrations i 40 % w/w/system temperatures too high.

[Solvent contaminated ]*: carryover from upstream equipment; example oil from compressor; brines, corrosion inhibitors, sand, [corrosion products]*/oxygen leaks into storage tank/inadequate corrosion control, example low pH causing corrosion/degradation via overheating, ex hot spots in reboiler tubes or fire tubes/ineffective filters/ineffective cleaning before startup/for amine absorbers: corrosion products/FeS/chemicals used to treat well

[Solvent feed temperature too hot]*: fouled exchanger/undersized heat exchanger/ ambient temperature too hot.

[Solvent flowrate too low]*: flowmeter or sensor error/absorber pressure i design/ plugged strainer, lines or filters/low liquid level in pump feed tank/[cavitation]*/ air locked pump and see Section 2.3 for trouble shooting pumps.

[Solvent incorrect]*: incorrect concentration of active ingredient: for amine absorbers: [amine concentration too high or too low]*; for glycol dehydration: solvent concentration TEG I specifications/[solvent stripping inadequate]*/[solvent feed temperature too hot]*/[solvent contaminated]*.

[Solvent loss elsewhere]*: upstream units, for example for glycol dehydration: glycol dumped with hydrocarbons separated in upstream flash drum/loss in downstream solvent stripper.

[Solvent stripping inadequate]*: not enough steam in stripper/incorrect pressure in stripper/[ foaming]*/[contaminated solvent]*/contaminated feed: for amine strip-

4.9 Gas Desorption/Stripping 113

pers: other sulfur species causing high partial pressure/leak in the feed preheater contaminating feed with stripped solvent.

[Vaporization losses]*: system pressure I design/for amine: concentrations i 40 % w/w/system temperatures too high.

4.9

Gas Desorption/Stripping

Function is to strip species from the liquid to produce a quality bottoms product: for example, a solvent ready to be recycled as in glycol dehydration, amine absorption, extractive distillation or water that has been stripped of contaminants as in Sour Water strippers; deodorize edible oils. Used to regenerate solvent for absorption or extractive or azeotropic distillation. Other equipment that is used for stripping include distillation, Section 4.2, gas–liquid separators, Section 5.1 and gas– liquid–liquid separation in flash drum, Section 5.4. The general characteristics of gas–liquid contacting are described in Section 1.6.1. Other operations that use this type of contactor include gas absorption, Section 4.8; reactors, Sections 6.13–6.16 and 6.19 and direct contact heat exchange Sections 3.7–3.9.

x Area of Application

avp = 2000 to 100 000 and liquid feed concentration of target solute is 0.1 to 5 %; 98 % purity possible.

x Guidelines

Because the target solute usually has low solubility in the liquid usually the desorption is liquid phase controlled.

For packings the goal is high liquid loadings, about 30 L/m2 s and minimum gas flowrates.

superficial density-weighted velocity F-factor of 0.6 m/s (kg/m3)0.5.

Use Kremser or the Colburn equation for the design of dilute units with molar stripping factor, S, = 1.4; m = Henry’s constant/total pressure. See Section 4.8. For stripping of a species from a liquid into a gas, Section 4.9, S i 1 and is in the range 1.15–2 and typically 1.4. For stripping with S = 1.4, the same result occurs

although the driving force ratio is ((xin – (yout/m) )/(xout – (yout/m)). For packing, HETS = 1.83 m.

For stripping ammonia from sour water: stripping steam 1 kg/kg feed. 35 theoretical trays.

Edible oil deodorizing: high vacuum. For I 0.6 kg/s, irregular production, use batch. Requires processing time = 4 h/batch, low heat recovery. For i 0.6 kg/s continuous with processing time = 1 h. Keep liquid films thin to promote mass transfer of volatiles and use astute distribution of sparge steam.

As with absorption, Section 4.8, the numerical values for HTU do not equal HETS.

114 4 Homogeneous Separation

What is the relationship between the theoretical stage and the transfer unit approach?

HETS/HTUOG = ln S/(S – 1) and HETS/HTUOL = (S ln S)/(S – 1) For the usual conditions with S = 1.4 and the above relationships:

HETS/HTUOG = NTUOG/NTS = 0.84

HETS/HTUOL = NTUOL/NTS = 1.17.

xTrouble Shooting

“Solvent or stripped liquid concentration i design”: boilup rate or steam stripping rate too low/feed concentration i expected/feed contamination; for sour water stripper: acid in feed may be chemically bonded with NH3 and prevent adequate stripping of NH3/[ foaming]*/leak in preheater exchanger/[column malfunction]*.

“Overhead from stripper I specifications”: insufficient flowrate of stripping gas/for glycol dehydration: reboiler temperature I 175–200 hC or reboiler too small for required duty or fouling of reboiler tubes/[ foaming]*/dirty or broken packing or plates/[ fouled or scaled internals]*/[ flooding]*/top pressure i design/leak in preheater exchanger/[ feed concentration off specification]*.

“Overhead temperature on stripper i design”: reflux flowrate too low/[ flooded]*/ [ foaming]*/feed contaminated with light hydrocarbons.

“For sour water strippers or glycol dehydration: Pressure at reboiler i design”: instrument error/top pressure i design/[p across column i design]*/overhead line plugged/[ flooding]*/for stripper for glycol dehydration: slug of hydrocarbon in feed is flash vaporized at reboiler and blows liquid out of stripper.

“For sour water strippers: odor or H2S problems at the storage tank”: 0. 6 to 1 m layer of oil on top of water missing/oil layer exceeds 0.6 to 1 m depth/faulty inert gas operation.

“Plugging of overhead system”: top temperature not within the operating window; for sour water strippers: temperature I 82 hC at which ammonium polysulfides form but temperatures too high give excessive water in overhead vapor causing problems for downstream operation/overhead lines not insulated/insufficient steam tracing on overhead vapor lines.

“Feed flowrate and composition to the stripper varies”: [instrument error]*/sampling error/analysis error/[ faulty separation in flash drum]*/[ foaming in upstream absorber]*/no intermediate storage tank between the flash drum and the stripper/ storage tank faulty operation or design: for SWS: residence time I 3 to 5 days; stratification occurs, bypassing occurs, insufficient mixing in tank/oil layer on top of water in storage tank exceeds 0.6 to 1 m depth.

[Column malfunction]*: [ feed concentration off specification]*/excessive stripping gas or steam velocity/too much cooling or condensation/top temperature i design/insufficient reflux cooling/packing broken, damaged/[ fouled or scaled internals]*/[ foaming]*/[ flooding]*. see also Section 4.2.

[Feed concentration off specification]*: [ foaming in upstream absorber]*/for glycol dehydration: upstream flash separator passing water; for oil or hydrocarbon in feed to SWS”: residence time for sour water in flash drum is I 20 min.

4.10 Solvent Extraction, SX 115

[Fouled or plugged internals]*: for SWS: cooling water leak/pH of feed water too basic/calcium ion concentration too high causing precipitation when temperatures in stripper exceed 122 hC/temperature I 82 hC at which ammonium polysulfides form/overhead lines not insulated.

[Instrument error]*: calibration fault/sensor broken/sensor location faulty/sensor corroded/plugged instrument taps”: for sour water strippers: water or steam purge of taps malfunctioning or local temperatures I 82 hC at which ammonium polysulfides form.

4.10

Solvent Extraction, SX

Related topic size reduction, Section 8.3 and reactive extraction, Section 6.35.

x Area of Application

General: feed concentration 0.03 to 95 % w/w; For minerals typically 0.01 to 2 % w/w; separation factor a = partition coefficient ratio with values 2 to 500 and should be i 5; for bioprocessing of proteins i 3. Separation factor for distillation a I 1.2.

Spray and packed columns, gravity flow (spray, plate, packed column): superficial liquid velocity, 0.001–0.02 m/s; area per unit volume 7 to 75 m2/m3. Product of the density difference with the interfacial tension [Mg/m3, mN/m] i 1 and number of theoretical stages needed I 3; interfacial tension I 10 mN/m.

Gravity flow (Raining bucket, RTL): Product of the density difference with the interfacial tension [Mg/m3, mN/m] i 1 and number of theoretical stages needed I 3; handles dirty liquids and ones that tend to emulsify.

Stirred tanks; mixer settler: (including Lurgi): superficial liquid velocity, 0.00 015– 0.004 m/s; area per unit volume 400 to 10 000 m2/m3. Product of the density difference with the interfacial tension [Mg/m3, mN/m] i 4 and number of theoretical stages needed i 3. Usually about 1 theoretical stage per unit. Rarely build more than 5 stages; can handle high phase ratios.

Stirred or pulsed columns: superficial velocity, 0.002 to 0.02 m/s; area per unit volume 75 to 3000 m2/m3.

Reciprocating plate: Product of the density difference with the interfacial tension [Mg/m3, mN/m] between 1 and 4 and number of theoretical stages needed i 2. Can handle dirty liquids.

Pulsed plate or packed: Product of the density difference with the interfacial tension [Mg/m3, mN/m] between 1 and 4 and number of theoretical stages needed i 2. Sensitive to contamination. Difficult to pulse large columns.

Rotating disk contactor, RDC, ARD contactor; Mixco, Scheibel, Treybal, OldshueRushton, Kuehni : Product of the density difference with the interfacial tension [Mg/m3, mN/m] between 1 and 4 and number of theoretical stages needed i 2. Low HETS, can handle dirty liquids, large throughputs. Needs flow ratios 1:1. Difficulty handling low interfacial tension systems that tend to emulsify.

116 4 Homogeneous Separation

Centrifugal extractor: Product of the density difference with the interfacial tension [Mg/m3, mN/m] I 1 and number of theoretical stages needed I 6. Cannot handle dirty systems or high phase ratios. For continuous differential type machines (density differences i 0.05 kg/L; drop diameter i 200 mm): 6 to 8 stages per machine. For discrete, disc type machines (density differences i 0.02 kg/L and handle drops I 200 mm) 2 to 3 stages per machine.

For bioprocessing, the solvent is usually polyethylene glycol plus electrolyte or dextron.

x Guidelines

Design flowrate about 50 to 90 % of flooding with the superficial velocity selected varies directly with the density difference and interfacial tension. Solvent/feed = 0.5–1/1 m (volumetric flowrates of solvent/feed) = 1.5. HETS increase slightly with increase in interfacial tension.

Gravity:

–spray: HETS increases exponentially with diameter. 10 to 20 m at 1 m diameter;

superficial velocity usually 5.5 L/s m2. kL = 0.001–0.01 m/s; area = 100–1000 m2/ m3; kLa = 0.1–10 1/s; dispersed phase holdup = 0.05–0.1.

–packed: HETS increases exponentially with diameter. 2.5 m at 1 m diameter; prefer diameter I 0.6 m; superficial velocity of combined dispersed and continuous phase 3 to 8, usually 5.5 L/s m2, 2.5 cm Pall rings. Redistribute the dispersed phase every 1.5 to 2 m. kL = 0.003–0.01 m/s; area = 100–1000 m2/m3; kLa = 0.3– 10 1/s; dispersed phase holdup = 0.05–0.1.

–sieve tray: HETS increases exponentially with diameter. 1 m at 1 m diameter; usually diameteri 0.6 m; superficial velocity about 5.5 L/s m2 (although some authors cite values in the range 7.5 to 16). Efficiency inversely proportional to the interfacial tension: 40 % at 5 mN/m.

–RTL: HETS increases exponentially with diameter. 0.5 m at 1 m diameter; superficial velocity based on combined flow of both phases 0.3–0.55 although some designed at 2 L/s m2.

Static mixer-settler: kL = 0.001–0.01 m/s; area = 100–2500 m2/m3; kLa = 0.1–

25 1/s; dispersed phase holdup = 0.05–0.2.

Mixer settler: 1 theoretical stage per unit; input energy 1 kW/m3; 2 min residence time for mixer. For settler: superficial velocity for the settler 0.5 to 7 L/s m2 with 1.4 L/s m2 being typical for liquid–liquid extraction. Low values for small density differences. For leaching: 0.4 L/s m2 for liquid–liquid–solids (slimes) leaching; 0.2 L/s m2 for slurries with 50–60 % solids in aqueous phase. Height increases with increasing total feed flowrate with 1 m height for 10 L/s. Typical drop diameter is 150 mm; extraction efficiencies about 80 %. Settler design is also described in decanters, Section 5.3.1. kL = 0.003–0.01 m/s; area = 100– 80 000 m2/m3; kLa = 0.3–800 1/s; dispersed phase holdup = 0.05–0.4.

Pulsed packed column: HETS increases exponentially with diameter. 0.7 m at 1 m diameter; max. diameter 2.5 m; superficial velocity of the combined flow of continuous and discontinuous phases 5 to 6.4 with usual value of 5.5 L/s m2.

4.10 Solvent Extraction, SX 117

Pulsed sieve plate column: HETS increases exponentially with diameter. 0.4 m at 1 m diameter; max. diameter 3 m; superficial velocity of the combined flow of continuous and discontinuous phases 7.5 to 16 with usual value about 5.5 L/s m2; sieve holes 3 to 8 mm; velocities through the holes I 0.2 m/s to minimize the formation of small drops. Tray efficiencies about 20 to 30 %.

Reciprocating plate: HETS increases exponentially with diameter. 0.35 m at 1 m diameter; max. diameter 1.5 m; superficial velocity of the combined flow of continuous and discontinuous phases 8 to 11 with usual value about 11 L/s m2.

Lurgi contactor: HETS increases exponentially with diameter. 0.7 m at 1 m diameter; diameters I 8 m; superficial velocity about 5.5 L/s m2.

Rotating disk contactor, RDC, ARD contactor; Mixco, Scheibel, Treybal, OldshueRushton, Kuehni: HETS is sensitive to rotor speed. HETS increases slightly with diameter. 0.5 m at 1 m diameter; superficial velocity about 5.5 L/s m2 with Kuehni 9.7 L/s m2 . Mixco, Scheibel, Treybal, Oldshue-Rushton, diameter I 2.5 m; RDC diameter I 9 m. More specific recommendations are Scheibel, HETS 0.1–0. 2 m with combined superficial velocity of 3–4 L/s m2; RDC: HETS 0.3–0.4 m with combined superficial velocity of 4–8 L/s m2; Kuehni: HETS 0.1–0. 2 m with combined superficial velocity of 2–3 L/s m2. Total flow through a column = 10 L/s m2 with a density difference of 0.2 Mg/m3.

Centrifugal: 2 to 6 units per machine depending on the machine.

x Good Practice

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 centrifugal extractors for bioprocessing/proteins: partition coefficient sensitive to pH, electrolyte type and concentration.

x Trouble Shooting

“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.

[Rapid coalescence]*: wrong phase is the continuous phase/[Marangoni instabilities]*/pH at the zpc/high electrolyte concentration in the continuous phase.

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.

“Product contamination”: leaking valves/crud left in storage tanks/crud left in dead legs in piping/corrosion products/unexpected chemical reactions/sampling fault/ analysis fault/unexpected solubility effects.

118 4 Homogeneous Separation

[Marangoni effects]*: non-equilibrated phases/local mass transfer leads to local changes in surface tension that give stable interfacial movement. see Marangoni number, Appendix B.

Other suggestions for trouble shooting decanters are given in Section 5.3.1. More about stable emulsion formation is given in Section 1.12.

4.11 Adsorption: Gas

x Area of Application

Use when feed concentration of the more volatile species is small, 0.15 to 10 % and when aads i 2; when the target species is difficult to condense.

x Guidelines

Select adsorbent based on pore size related to the target species.

alumina: surface area: 210 to 350 m2/g; pore volume 0.21 cm3/g, temperature I 320 hC; superficial gas velocity 125 to 500 dm3/dm2 s; usually adsorb 800 kg/ m3 or 0.14 to 0.22 kg organics/kg dry solid; 0.15 kg water/kg dry solid. Lifetime: 150 cycles.

silica: surface area: 750 to 830 m2/g; pore volume 0.4 to 0.45 cm3/g, temperature I 230 hC; superficial gas velocity 125 to 500 dm3/dm2 s: usually adsorb 720 kg/m3; 0.3 to 06 kg organics/kg dry solid; 0.4 kg water/kg dry solid.

4 molecular sieve: surface area: 640 to 80 m2/g; pore volume 0.27 cm3/g, temperature I 300 hC: superficial gas velocity 150 to 250 dm3/dm2 s; usually adsorb 480 to 720 kg/m3; 0.05 kg nitrogen/kg dry solid; 0.22 to 0.36 kg water/kg dry solid. Lifetime: 400 cycles.

activated carbon: surface area: 1000 to 1500 m2/g; pore volume 0.6 to 0.8 cm3/g, temperature I 540 hC: superficial gas velocity 100 to 600 dm3/dm2 s ; capacity depends on organic; range 0.06 to 0.2 kg organics/kg dry solid adsorbent.

For fixed bed: batch: size on cycle time: load, swing out of service, regenerate, swing back into service. Loading time: 100–3000 bed volumes (BV)/h with time based on the ratio of the adsorption isotherm to the feed concentration of the target species (usual range 50–300 corresponding to load times of 0.2–2 h). Regenerate with steam (at 3 to 5 kg steam per kg organic removed), solvent, reduced pressure, combustion or via vacuum/pressure shift. Use superficial gas velocity of 60 to 600 dm3/dm2 s or recommended value for the adsorbent to determine cross sectional area. Residence time 0.03 to 0.8 BV/s: (F/V) with depth i 0.33 diameter.

Heat required for thermal regeneration = 2.5 q (enthalpy to heat the adsorbent bed + enthalpy of desorption); for organics = enough to heat the system to 30–50 hC above the boiling temperature of the highest boiling component in the mixture.

4.12 Adsorption: Liquid 119

x Trouble Shooting

“Wet gas”: steam leak/leaky valves/inadequate regeneration/wrong adsorbent/ adsorbent damaged by excessive regeneration temperature/adsorption cycle too long/[early breakthrough]*.

“Dp high”: fine particulates in feed/breakdown of adsorbent/high gas feedrate. “Product contamination”: leaking valves/crud left in storage tanks/crud left in dead legs in piping/[corrosion]* products/unexpected chemical reactions/sampling fault/analysis fault/unexpected solubility effects.

[Corrosion]*: see Section 1.3.

[Early breakthrough]*: gas short circuiting bed/faulty regeneration/increased concentration in feed/other contaminants in feed.

4.12

Adsorption: Liquid

Related topics include ion exchange, Section 4.13.

x Area of Application

Prime option for dilute concentrations with a need for greater selectivity than solvent extraction can provide.

x Guidelines

Select adsorbent:

Activated carbon: surface area: 1000 to 1500 m2/g; pore volume 0.6 to 0.8 cm3/g, temperature I 540 hC; loading very dependent on molar mass of target solute, solubility in the carrier liquid and pH. Example loading 0.01 kg organic molar mass 100/kg dry solid. The value varies with the molar mass3.5. Carbon usage expressed as kg carbon required/m3 liquid increases with increase in the TOC in the feed and depends on the type of species present. A gross approximation is that 1 kg/m3 is required for 300 mg TOC/L with n = 1.0 for the range 200–30 000 TOC, mg/L. acid treated clay: surface area: 225 to 300 m2/g.

Fuller’s earth: surface area: 130 to 250 m2/g.

Batch:

Fixed bed: Batch: size on cycle time: load, backwash/clean, regenerate, swing onstream. Load: typical flowrate 2–3.5 BV/h with the load time based on the ratio of the adsorption isotherm to the feed concentration of the target species (usual range varies with the application: 18–100 min while for water treatment: 70–100 days). Backwash with velocity to fluidize the bed; velocity 0.8 BV/h. Time such that I 5 % feedrate used in backwash. Usually carbon is removed and regenerated about four times per annum. Try to match the loading cycle to the regeneration cycle.

Use fixed bed if I 20 L/s and carbon usage i 180 kg/day. Superficial velocity 1 to 15 L/m2 s; use 2.6 to 5 L/m2 s to estimate cross-sectional area. Too low a superficial velocity (I 3 L/m2 s) gives poor feed distribution. Use 2 to 3.5 BV/hour; to

120 4 Homogeneous Separation

give the required residence time of 18 to 100 min. Bed depths in the range 3 to 10 m but keep the pressure drop I 75 kPa. Height/diameter 1:1 to 4:1. Maximum size is based on carbon usage I 9 Mg dry carbon per day.

Active carbon volume/area = 1.6 m3/cross-sectional area, m2.

Continuous:

Moving bed: Use if i 20 L/s. Related topic transfer line reactor, Section 6.7. Fluidized bed: Use if slimes or fine particles in feed, Use superficial velocities of 8 to 14 L/m2 s. Feed contacts bed for 30 min or 0.8 BV/h. Related topics include reactors, Section 6.30, and drying, Section 5.6.

Slurry approach: Use if the carbon usage is I 180 kg/day. Mix and suspend powdered adsorbent and then filter exit line. Often use up to three stages of countercurrent contacting. Used for continuous bleaching of edible oils. Batch process is simple, flexible and easy to change feedstocks. Continuous operation o ers better protection against oxidation, provides shorter holdup and has the potential of heat recovery. Bleach time 25 min. Related topic transfer line reactor, Section 6.7.

Loading times and elution-regeneration times should be approximately equal.

x Good Practice

For edible oils, prevent contact with air. Carbon regeneration by multiple hearth furnaces, see Section 6.21.

x Trouble Shooting

“Early breakthrough”: liquid short circuiting bed/faulty carbon regeneration/ increased concentration in feed/other contaminants in feed.

“Pressure drop high”: fine particulates in feed/breakdown of carbon/high liquid feedrate.

4.13

Ion Exchange

Related topics include the use of ion exchange resins as catalysts in reactors, Section 6.35, liquid–solid fixed bed reactors, Sections 6.9-6.12, and adsorption-liquid, Section 4.12.

x Area of Application

High valence ionic species in liquid phase with aIX = G+ (1 – c+)/c+ (1 – G+) = 1.01 to 1.04 where G+ = surface concentration of cations, c+ = bulk concentration of cations, and feed concentration 0.02 % to 2 % w/w.

x Guidelines

Select the ion exchange resin based on the pH of the environment and the valence of the target ion. For high efficiency, try to use weak electrolyte resin.