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

6.29 CSTR: Mechanical Mixer (Backmix) 261

tion/emulsifier post feed too late. “Batch times I design”: too many nucleation sites/lower level of oxygen than design/too much emulsifier/too much initiator. “Batch times i design”: too few initial nucleation sites/too much oxygen in the feed/too little emulsifier/too little initiator.

Agitated bubble reactors:

“Foaming”: mixer tip speed too high/linear gas velocity too high/use of turbine impeller/lack of a gas sparger/and generic causes, Section 1.12.

“Flooded impeller”: too small a diameter impeller/speed too slow.

Gas–liquid–solid bioreactor: Carryover”: [ foaming]*

[Foaming]*: bubble rate too high/liquid downflow velocity through the foam is too low. See Section 1.12 for generic causes of [ foaming]*.

See Trouble shooting: STR, Section 6.27 for more on trouble shooting bioreactors.

6.29

CSTR: Mechanical Mixer (Backmix)

Bd = 0; Pe I 1. Related topics mixing, Section 7.1, solvent extraction, Section 4.10.

x Area of Application

Batch, semibatch and continuous stirred tank reactors: Residence time 600– 15 000 s (10 min to 4 h); heat of reaction: primarily exothermic; reaction rate slow to moderate; High pressure autoclaves I 100 L.

Unique for CSTR: Phases: liquid, gas–liquid, liquid–liquid, liquid–catalytic solid, gas–liquid–catalytic solid, gas–liquid –biosolid. Capacity 0.0001–100 L/s and usually i 0.4 L/s; volumes 1–1 000 000 L. Autothermal reactions. Usually if the concentration of reactants is low, and need low concentration of reactants for selectivity. CSTR is larger and more expensive than PFTR. For multiphase, STR are characterized by high liquid holdups; holdup of the reactive phase is important if the reaction is slow Ha I 1; phase ratio is easy to control. Adiabatic, CSTR usually gives higher productivity for exothermic reactions than for STR batch or PFTR. Use for large capacity, otherwise batch. Heat recovery is easier in CSTR than in a batch STR.

Liquid: I 300 000 mPa s; volume I 75 m3. Use for kinetically controlled reactions that require long residence times.

Liquid–liquid: surface area 400–3500 m2/m3 with area increasing with decreasing surface tension and increasing velocity. Drop diameter 4–5000 mm; for viscosities I 104 mPa s. Phase ratio is easy to control.

Gas–liquid: Surface area 60–500 m2/m3; surface area gas–liquid per volume of reactor: 200–2000 m2/m3 volume reactor; surface area gas–liquid per volume of liquid phase: 220–2500 m2/m3; liquid phase can handle viscous liquids and suspensions. only appropriate for smaller size reactors I 10–20 m3.

Liquid with catalytic solid: catalyst diameter, I 0.1 mm; surface area solid 500 m2/m3.

262 6 Reactors

Gas–liquid with catalyst solid: catalyst diameter, I 0.1 mm; Surface area 50–1200 m2/m3; surface area solid 500 m2/m3; surface area gas–liquid 100–1500 m2/m3.

Gas–liquid–biosolid: Ha I I 0.3 and d+ = 150–800.

Aerobic sludge digesters: reduce the volume of and render biologically stable the sludge from a variety of sources: conventional activated sludge and primary clarifier.

x Guidelines

Reactor size 8 to 32 m3, 1.5 MPa, with jacketed heat transfer surface 1.5– 2.5 m2/m3 volume. Heat transfer coefficient U = 0.06–0.35 kW/m2 K for jacket to inside reactor contents; coil 0.7–0.8 kW/m2 K.

0.2–2 kW/m2 see mixing Section 7.1 and heat transfer, Section 3.3.

Liquid: Power input to promote heat and mass transfer: 1–6 kW/m3 reactor volume.

Liquid–liquid: Holdup: volume fraction dispersed liquid 0.01–0.5. Typical drop diameter is 150 mm; Power input 0.2–3 kW/m3.. Related topic: solvent extraction, Sections 4.10 and 1.6.2.

Gas–liquid: Holdup: liquid holdup i 0.7, gas holdup I 0.1; bubble diameter = 2.5 mm regardless of the agitation and has a mean upward velocity of about 0.27 m/s; superficial gas velocity, 0.05 to 1 m/s; Backmix; complete; typical liquid mass transfer coefficient = kLa = 0.02–0.2 1/s; bulk/film volume ratio, d+ i 100; power input 0.1–4 kW/m3.. Height of liquid = tank diameter or use multiple impellers if height of liquid/tank diameter i 2. Impeller diameter 0.3 to 0.5 of tank diameter. See also Section 1.6.1

Liquid with catalytic solid: Holdup: volume fraction catalyst 0.01; volume fraction liquid 0.99; power input to facilitate heat and mass transfer, suspend solids and promote mass transfer: 1–4 kW/m3 reactor volume.

Gas–liquid with catalyst solid: Holdup: volume fraction catalyst 0.01; volume fraction liquid 0.8–0.9; volume fraction gas 0.1–0.2. Power input 0.05–2 kW/m3. Catalyst activity: variable but often able to avoid diffusion limitations because of small diameter catalyst. Catalyst selectivity: OK. Catalyst stability: change between batches.

Heat exchange OK.

Gas–liquid –biosolids: see STR, Sections 6.27 and 1.6.3.

Aerobic sludge digesters: CSTR designed on the basis of VSS reduction. Mixing to keep the solids suspended plus oxygenation. Cell residence time for cells 12–22 d depending on the source of the sludge. Typical organic load 4–26 mg VSS/s m3; dissolved oxygen concentration 1–2 mg/L; air requirement for activated sludge

= 0.25–0.33 dm3/s m3;	mixture of primary plus activated sludge	=	0.4–
0.5 dm3/s m3; 1.42 kg	O2/kg biosolids digested. Oxygen usage 1.4	–11	mg
O2/s kg VSS depending	on the source of the sludge. For diffused air 0.33–
1 dm3/s m3 depending on the sludge; surface aeration = 0.025–0.033		kW/m3.

Power = 0.015–0.02 kW/m3.

6.29 CSTR: Mechanical Mixer (Backmix) 263

x Good Practice

Consider complications because of catalyst deposition and erosion.

x Trouble Shooting

CSTR used for polymerization and, to a lesser extent, nitration, sulfonation, hydrolysis, neutralization and, to a much lesser extent, dehydrogenation, oxidation and esterification can pose potentially unsafe operation. Key indicators of such potential hazards include “Sudden increase in pressure”, “Unexplained increase in temperature”, “Failure of the mixer”, “Power failure”, and “Loss of cooling water”.

For any of these conditions our first question should be: emergency shut down? Our knowledge of the MSDS information for the species and their interaction with each other and with the environment is critical. See semibatch and STR Sections 6.27 and 6.28 for more.

Liquid–liquid: Typically the reactor is a CSTR followed by a decanter to separate the phases and recycle the “catalyst” phase to the reactor. See decanter, Section 5.3.1. For alkylation: “Alkylate is purple”: [stable emulsion formation]*/[density difference decrease]*/[drops don’t settle]*/[acid runaway]*. “Dp across the alkylate cooleri design”: [stable emulsion formation]*/[density difference decrease]*/ [drops don’t settle]*/[acid runaway]*/acid recirculation rate too fast.

“Temperature of the recycled acid is i 1.7 hC hotter than feed entering the reactor”:

[acid runaway]*/alkyl sulfates polymerize in the decanter/acid recirulation rate too fast.

[Acid runaway]*: excessive contaminants in feed to reactor/feed rate too fast/poor contact or mixing between isobutane, olefin and acid/fresh acid makeup feedrate stopped/faulty control/faulty meter/ratio of acid : hydrocarbon outside range 45–60 % v/v/ratio of isobutane : olefin I 8: 1/initial reactor temperature too hot or i 18 hC/poor mechanical design for fresh acid addition.

[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

264 6 Reactors

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.

[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 1.2]*, 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.

6.30

STR: Fluidized Bed (Backmix)

Related topics: heat transfer, Section 3.4, dryers, Section 5.6 and size enlargement, Section 9.4.

6.30 STR: Fluidized Bed (Backmix) 265

x Area of Application

Phases: GcS, GrS, LcS, GLcS: Residence time: for gas = seconds; for solids = minutes to hours. Primarily for highly exothermic, very fast reactions where the need is for uniform, closely controlled temperature; need fast reaction that occurs at the bottom of the bed. Not good if we have consecutive reactions to produce the product (because of backmixing). Relatively inflexible. Preferred over PFTR for strongly exothermic or endo reactions. Easier than fixed bed for catalyst regeneration, excellent heat transfer for high exothermic reactions; lower pore diffusional resistance because of smaller diameter catalyst particles. Select if catalyst life is I 3 month. Catalyst must withstand attrition.

Gas with catalytic solid: Solid residence time, 300–15 000 s; gas residence time, I 1 s; solid particle diameter 0.005–7 mm.

Gas with inert solid: gasification or incineration: use for homogeneous exothermic reactions where we need to control the heat release or for the creation of biogas from biomaterial (such as bagasse) in a fluidized bed of sand or incineration of wet sludge, again in a fluidized bed of sand.

Gas with reacting solid: Solid residence time, 300–15 000 s; gas residence time, I 1 s; solid particle diameter 0.005–7 mm. For combustion, gasification, incineration and ore roasting or reduction. Advantages: ease in solids handling, uniform temperature, thermal stability even for highly exothermic and endothermic reactions. Cannot be used for solids with partial fusion or softening of particles.

Liquid with catalytic solid: catalyst diameter, 0.1–5 mm; surface area solid 500– 1000 m2/m3.

Gas–liquid with catalytic solid: catalyst diameter, 0.1–5 mm; surface area solid 500–1000 m2/m3; surface area gas–liquid 100–1000 m2/m3.

Gas–liquid with microorganisms (bio): Mainly used for cells immobilized on inert solid. Provides low shear. For gas–liquid kLa = 0.05–0.3 1/s; for liquid solid kLa = 0.1–0.5 1/s; solids holdup = 0.1–0.5 m3/m3 and biofilm area = 2000 m2/m3.

x Guidelines

Fundamentals: Solid particle diameter 60 to 80 mm diameter solid particles with ratio of maximum to minimum diameter about 11 to 25. More generally, particles usually in the range 40–100 mm corresponding to velocities of 0.1–0.3 m/s; and 70–1000 mm corresponding to 0.1–2.5 m/s for minimum for fluidization. Larger diameter particles tend to slug; smaller, tend to bubble.

The fluidization is characterized as “bubbling”, B (aggregate fluidization). Bubbling consists of two phases:

1.Gas bubbles: assume move through in plug flow; superficial gas velocity (0.1 m/s) i i minimum superficial gas velocity to cause fluidization ( 0.01 m/s); mass transfer coefficient between the bubble and the emulsion phase = 0.01 m/s; the fraction of the volume of the fluidized bed occupied by the bubbles is 0.04.

2.Homogeneous emulsion phase: reaction occurs in the emulsion phase. Area between the bubble phase and the

266 6 Reactors

emulsion phase = 10 m2/m3. Diffusivity of the reactant in the emulsion phase = diffusivity in the bubble phase = 10–5 –10–4 m2/s.

The options include:

– single bubbling bed (BFB), gas velocity about 3–5 q the sedimentation velocity of the particles; usually 1–2 m/s. Height of the fluidized bed: shallow beds 0.15–0.2 m deep; deep beds 0.3–15 m but usually 0.5–1.9 m deep, regardless of the diameter. Usually try to have the height: diameter about 1–1.5: 1. For BFB, Archimedes no. ranges from 1–106; particle Froude no. (3/4 Frp (rG/(rs – rG)) in the range 10–4 –0.4 and the particle Reynolds no. 0.1–400.

–BFB/BFB combo with catalyst as the oxygen carrier,

–Circulating fluidized bed CFB, CFB are superceding BFB for many applications although one of their major limitations is the erosion from the particles. Used for short gas contact times, plug flow gas; for rapidly decaying catalyst or solids that must transport a lot of heat. Gas velocity is 4–8 m/s; the solids flux of minerals or catalyst is typically 100–1000 kg solids/s m2 . For example, typically 500 kg catalyst solids/s m2 for an fluid cat cracker, FCC.

For CFB, Archimedes no. ranges from 1–102; particle Froude no. in the range 10–2–0.4 and the particle Reynolds no. 0.1–8.

–CFB/BFB combo with catalyst as the oxygen carrier,

–Multistage BFB,

–Multistage BFB with split air flow and temperature programming. Incineration: BFB, gas velocity 0.5–1.5 m/s and use 1 m/s on heated air and

0.6 m/s on the freeboard; 650–980 hC; heat load 200–350 kW/m3 with the usual design capacity about 6 MW. Operate 3 to 5 q minimum fluidizing velocity and usually 0.5–1.5 m/s, solid flux waste sludge = 0.06–0.07 kg wet feed/s m2, coal = 0.01 to 0.015 kg coal/s m2; for biomass gasifiers = 0.22 kg biomass feed/s m2; gas residence time = 3 –4 s. Height: diameter usually about 1 to 1.2:1. Volumetric loading 5 % solids; temperatures 650–980 hC; heat release from combustion 200–340 kW/m3; superficial gas velocity 60–75 g/s m2 of cross sectional area for wet solids; 15–20 g/s m2 of cross sectional area for dry solids combustion. CFB: gas velocity 6–8 m/s; used for coal combustion.

Gas plus catalyst solid: Usually BFB. For fast reactions, gas film diffusion may control and catalyst pore diffusion mass transfer may control if catalyst diameter i 1.5 mm. Heat transfer: heat transfer coefficient wall to fluidized bed is 20– 40 q gas-wall at the same superficial velocity, h = 0.15–0.3 kW/m2 K. Nu = 0.5–2. Heat transfer from the bed to the walls: U = 0.45 to 1.1 kW/m2 hC.; from bed to immersed tubes: U = 0.2 to 0.4 kW/m2 hC; from solids to gas in the bed U = 0.017 to 0.055 kW/m2 hC. Fluidized bed usually expands 10–25 %. Backmix type reactor which increases the volume of the reactor and usually gives a loss in selectivity. Usually characterized as backmix operation or more realistically as a series of CSTR if the height/diameter i 2; Usually 1 CSTR for each H/D = 1. If the reactor operates in the bubble region, then much of the gas short circuits the catalyst so the overall apparent rate constant is lower by a factor of 10.

6.30 STR: Fluidized Bed (Backmix) 267

Minimum gas fluidization velocity of 0.5 mm/s to 15 mm/s or about a factor of 0.01 to 0.1 % of the pneumatic conveying velocity. Ratio of height to diameter j1. Bed depth usually 0.3 to 15 m. When used for heat transfer: particles and gas tend to leave the bed at the same temperature. Here are some guidelines from the few data that are available in the open literature:

For gas reactions with solid catalyst, BFB: gas velocity 0.5–1.5 m/s. propylene to acrylonitrile, the liquid product loading = 0.14 kg/s m2 of grid area; the H:D = 1.7:1 and the gas phase residence time = 12 s. For allyl chloride, the gas residence time is 0.4 s. For the Unipol production of LDPE polyethylene from ethylene, the H:D = 2.6:1 with residence time for the solid = 3–5 h until the particles are 500 mm; for HDPE via the Union Carbide process the residence time for the solid is 2–3 h with 10 % per pass with recycle. For cat cracking, the liquid feed loading is 1.0 kg/s m2 of grid area; the H:D = 2:1 and the gas phase residence time = 11–75 s; solids residence time 5–10 min. For phthalic anhydride, gas phase residence time = 10–20 s. CFB: gas velocity 4–8 m/s; solid mass flux 100–1000 kg solids/s m2 (usually 500) with the slip velocity (ratio of interstitial gas velocity to the solids velocity) usually 2. Used for cat cracking and Fischer Tropsch synthesis to produce hydrocarbons from hydrogen and carbon monoxide over an iron catalyst.

For gas–solid reactions (as in ore roasting or reduction and calcination), BFB or series of four BFB. gas velocity 0.5–1.5 m/s. feed solids flux 1 kg/s m2; gas residence time per stage about 0.25 s; solid residence time per stage 3–5 h. Solids flux = 1 kg pyrites feed/s m2; For multistage ore reduction, solids flux = 0.025 feed ore/s m2 grid. For calciner solids feed flux = I 1 kg/s m2. CFB gas velocity 4–8 m/s; solid mass flux 100–1000 kg solids/s m2 (usually 500) with the slip velocity (ratio of interstitial gas velocity to the solids velocity) usually 2. Solids residence time 40 min. H:D = 3–20:1.

Liquid with catalytic solid: Particulate fluidization: homogeneous with no bubbles. For highly exothermic reactions, consider external recirculation through a heat exchanger.

Gas–liquid with catalytic solid: Holdup volume fraction catalyst 0.1–0.5; volume fraction liquid 0.2–0.8; volume fraction gas 0.05–0.02. Catalyst activity: variable but often reduced because of mass transfer limitation; backmixing is unfavorable. Catalyst selectivity: often reduced because of mass transfer limitation; backmixing is unfavorable. Catalyst stability: must withstand attrition; can be removed for regeneration. Heat exchange; good heat transfer. Isothermal efficiency may be 1 but with larger diameter particles this can decrease. May have lower isothermal efficiency because of diffusion into the pellet and the larger size pellet.

Gas–liquid with microorganisms (bio): For gas–liquid kLa = 0.05–0.3 1/s; for liquid–solid kLa = 0.1–0.5 1/s; solids holdup = 0.1–0.5 m3/m3 and biofilm area = 2000 m2/m3.

x Good Practice

Design of gas distributor to ensure uniform dispersion of the gas across the bed: select conditions to achieve the highest values from among the following

2686 Reactors

1.Dp across the distributor = 0.1 q Dp across the bed (latter = apparent weight of the bed)

2.0.35 kPa

3.100 q Dp for fluid in an empty tube.

x Trouble Shooting

First we consider fluidized bed reactors in general, then fluidized combustors or regenerators and then provide specifics for a fluid catalyst cracking unit, FCCU, which consists of a riser or fluidized bed reactor, cyclone separator, steam stripper, spend catalyst transport, air-oxidizing regenerator, cyclone separator and a regenerated catalyst return.1)

General fluidized bed reactor: “Gradual change in yield”: [carbon buildup]*. “Poor yield”: [Loss of catalyst activity]*/[maldistribution]*/[unacceptable temperature profiles]*/[inadequate heat transfer]*/wrong locations of feed, discharge or recycle lines/faulty design of feed and discharge ports/[inadequate mixing]*/[excessive backmixing]*/wrong internal baffles and internals/[poor bubbling hydrodynamics]*/[inadequate solids circulation rates in reactor]*. “Change in product distribution”: [maldistribution]*/poisoned catalyst/feed contaminants/change in feed/change in temperature settings.“Temperature hot spots”: [maldistribution]*/ local exothermic reactions. “Temperature runaways”: temperature hot spots. “Pressure and bed temperature and reactor unsteady”: water in feed/reactor grid hole erosion/[maldistribution]*/for FCCU: surging regenerator holdup/unsteady reactorregenerator Differential Pressure controller operation/rough circulation/incorrect aeration of U-bend/incorrect aeration of standpipe/sticky stack slide valves/sensor control performance for stack slide valve unsatisfactory.“Particulate carry over that affects operation of downstream equipment”: [poor separation in cyclone]*. “Shifts in yield distribution”: [Feed contaminated with light hydrocarbons]*/[sintered catalyst]*/coarse particles. “Dp increase across the grid”: [plugged grid holes]*/fluid flow i usual. “Dp across grid I expected”: air flowrate I design/[eroded grid holes]*/for FCCU: [Failure of internal seals in regenerator]*. “Erratic or cycling pressures”: [surging of the catalyst bed]*. “Catalyst losses increase”: [poor separation in cyclone]*/insufficient head space above bed/fluidization velocity too high/increase in volume of product through unexpected side reactions/change in feed flowrate/flowrate instrument error/velocity through reactor too high/pressure surges/attrition of catalyst.

[Attrition of the catalyst]*: steam flowrate i expected/air flowrate i expected/local velocities into the dense phase i 60 m/s/catalyst too fragile.

[Carbon buildup]*: [inadequate regeneration]*/[excessive carbon formed]*.

1)Based on Luckenbach, E.C. et al. Encyclopedia of Processing and Design, Marcel Dekker, 1981, p. 89; Dutta, S., Gualy R., Overhaul process reactors, Hydrocarbon Process., 1999 Sept.

pp. 43–50; Lieberman, N. P., Troubleshooting Process Operations, 2nd edn., 1985, Pennwell Books.

6.30 STR: Fluidized Bed (Backmix) 269

[Coarse particles (diameter i design)]*: [generation of fines]*/[loss of catalyst fines]*/[poor separation in cyclone]*/agglomeration of catalyst/[sintered catalyst]*/wrong specifications for catalyst.

[Eroded grid holes]*: hole velocity too high/materials of construction/contaminants in fluid. [Excessive backmixing]*: [maldistribution]*/[poor bubbling hydrodynamics]*.

[Excessive carbon formed in cracker]*: cracker operating intensity above usual; for FCCU excess aromatics in feed/changes in feed/poor catalyst stripping/heavier recycle/leakage of fractionator bottoms into the feed/[sintered catalyst]*/[ feed contaminated with metals]*/[ feed contaminated with heavy hydrocarbons, especially aromatics]*.

[Failure of internal seals in regenerator for FCCU]*: pressure bump during startup/ regenerator pressure too high/velocity through the grid too low/low flow of air to the grid/stresses too high/erosion/abnormal conditions with the auxiliary burner on startup.

[Gas bubbles too big]*: particles heavier than design/particles larger than design/ sintered particles/single fluidized bed too deep.

[Gas bypassing in fluidized bed]*: particles heavier than design/particles larger than design/agglomerated particles/single fluidized bed too deep instead of multiple beds in series.

[Gas velocity too high]*: [increase in production of light ends in reactor]*. [Generation of fines]*: [attrition of the catalyst]*/fines in the new catalyst. [Inadequate heat transfer]*: [maldistribution]*/insufficient heat exchanger area/ design error/fouled exchanger. See Section 3.3.

[Inadequate mixing]*: [maldistribution]*/[poor bubbling hydrodynamics]*. [Inadequate regeneration]*: [regenerator doesn’t remove all carbon from the catalyst]*/excessive temperature during regeneration/coarse particles.

[Loss of catalyst activity]*: [carbon buildup]*/[inadequate regeneration]*/[sintered catalyst]*/excessive regeneration temperature/[poisoned catalyst]*/[loss of surface area]*.

[Loss of catalyst fines]*: insufficient disengaging space above the top of the bed/ agglomeration of catalyst/[poor separation in cyclone]*/Dp indicator for catalyst level faulty/Dp indicator for catalyst level OK but bed density incorrect.

[Loss of surface area]*: [sintered catalyst]*/[carbon buildup]*.

[Maldistribution]*: faulty feed distributor design/plugging of fluid distributors with fine solids, sticky byproducts or trace polymers/[temperature hot spots]*/ [sintered catalyst particles]*/[poor bubbling hydrodynamics]*/[poor circulation]*. [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.

[Plugged grid holes]*/foreign debris entering with fresh catalyst/faulty grid design/ lumps of coke or refractory in catalyst/failure of grid hole inserts/[sintered catalyst]*/bits of refractory.

[Poisoned catalyst]*: poisons in feed/flowrate of “counterpoison” insufficient/ poison formed from unwanted reactions.

270 6 Reactors

[Poisons in feed]*: depends on reaction: for FCCU poisons in the feed include nickel, vanadium and sodium; the counterpoison is a solution of antimony. [Poor bubbling hydrodynamics]*: [Gas bypassing in fluidized bed]*/[gas bubbles too big]* particles heavier than design/[particles larger than design]*/[sintered catalyst]*/fluid feed velocity too high/too deep a bed of catalyst/[maldistribution]*. [Poor circulation]*: coarse particles/[maldistribution]*.

[Poor separation in cyclone]*: [stuck or failed trickle valve]*/[plugged dipleg]*/dipleg unsealed/solids level does not cover end of dipleg/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/Dp indicator for catalyst level faulty/Dp indicator for catalyst level OK but bed density incorrect/pressure surges. “Dp i design”: fines in packed beds/fines in distributors/fines in exit nozzles/ crud left in from construction or revamp.

[Reactor instability]*: control fault/poor controller tuning/wrong type of control/ insufficient heat transfer area.

[Regenerator doesn’t remove all carbon from the catalyst]*: damaged air grid/insufficient air/excessive regenerator velocity/poor spent catalyst initial distribution/ coarse particles.

[Sintered catalyst]*: local high temperatures/[maldistribution]*/for FCCU [afterburn in regenerator]*/[Feed contaminated]*/high temperature in the regenerator/[temperature hot spots in the reactor]*.

[Solids conveying lines flow capacity I design]*: sticky fines buildup in lines/wrong

Dp across line.

[Stuck or failed trickle valve]*: binding of hinge rings/angle incorrect/wrong material/hinged flapper plate stuck open/flapper plate missing.

[Surging of the catalyst bed]*: water in the feed/[plugged grid holes]*/faulty grid design/[grid holes eroded]*/[ for FCCU: failure of internal seals in regenerator]*/for FCCU: [seal failures]*/hole in the overflow well/[reactor instability]*/control fault in Dp between cracker and regenerator.

[Unacceptable temperature profiles]*: fluctuating temperature/unsteady bed temperatures.

Specific for a fluidized bed combustion/catalyst regenerator: “Increase in catalyst losses”: [poor separation in cyclone]*/[ failure in regenerator plenum]*/for FCCU [ failure of internal seals in regenerator]*.

[Failure in regenerator plenum]*: faulty cyclone design/catalyst feed too high/regenerator velocity too high/faulty spray nozzles causing impingement of plenum sprays/temperatures too high causing failure in plenum.

Specific for Fluid Cat Cracker Unit - including regenerator system: “Overloaded wet gas compressor”: for FCCU high hydrogen production/increase in production of light ends. “Gas compressor flow reversal”: [poisoned catalyst]*. “Gas compressor surge”: [poisoned catalyst (that causes production of lower MM species)]*. “Gas compressor flow reversal”: [poisoned catalyst]*. “Wet gas compressor surge”: [poisoned catalyst (that causes production of lower MM species)]*.

“Hydrogen concentration in wet gas increases”: [poisoned catalyst, especially with nickel and vanadium]*/[ feed contaminated with metals, especially nickel and