FCC Troubleshooting Guide | Engineering Library

How to Use This Guide

Use this troubleshooting matrix when diagnosing problems in Fluid Catalytic Cracking (FCC) units. Work through the symptom that matches your observation, review probable causes ranked by likelihood, then execute immediate actions and longer-term fixes in sequence. This guide covers the four major FCC sections: Reactor/Riser, Regenerator, Main Fractionator, and Operations/General. Most FCC problems are interlinked—fixing one symptom often requires diagnosis across multiple sections. Reference your unit’s design basis, operating targets, and recent feedstock/catalyst changes before concluding root cause.

Reactor/Riser Symptoms

Symptom 1: Low Conversion (Feed Not Fully Converted)

Usual Causes (ranked by likelihood): 1. Catalyst activity decline — age, poisoning (Ni, Cu, V), or sintering 2. High inlet riser temperature — thermal cracking competing with catalytic 3. Low catalyst-to-oil (C/O) ratio — insufficient contact time 4. Poor riser hydrodynamics — maldistribution, bypass to regenerator 5. Feedstock change — heavier material, higher sulfur or metals content

Immediate Actions: - Check riser thermocouple readings and compare to target (typically 750–850°F depending on feed) - Verify C/O ratio from material balance (regenerator air flow, spent catalyst stripping) - Review catalyst activity test (MAT or ACE results); if activity < design basis by >5%, consider catalyst replacement - Inspect riser distributor for erosion/plugging; poor oil atomization reduces contact

Longer-Term Fixes: - If catalyst age >5 yrs or activity decayed >10%, plan fresh catalyst addition or replacement - Upgrade riser internals (baffle improvements, distributor nozzles) if hydrodynamics poor - Adjust feed preheat and naphtha reflux to manage inlet temperature within design range - If feedstock contamination suspected, increase catalyst bleed rate; consider feed pretreatment (hydrotreating) for sulfur/metals removal

Symptom 2: Excessive Coke Yield (Catalyst Deactivation, Regenerator Overload)

Usual Causes (ranked by likelihood): 1. Low riser temperature — insufficient thermal energy for cracking, favoring condensation reactions 2. Feedstock aromatic or asphaltene content high — coke precursors 3. Inadequate catalyst activity — promotes secondary coke-forming reactions 4. High C/O ratio — extended residence time, coking reactions 5. Poor mixing in riser — dead zones promote coke formation

Immediate Actions: - Increase riser outlet temperature 5–10°F (within equipment limits) - Review feed assay: aromatic carbon (AC) %, CCR (Conradson Carbon Residue), and metals (Ni, V) - Check regenerator temperature; if >1300°F, coke burn rate is at limit—reduce coke make or increase riser severity - Verify stripping steam flow to riser outlet; inadequate steam can increase apparent coke

Longer-Term Fixes: - Blend heavier feeds (if available) with lighter, less aromatic crudes to reduce coke precursors - Implement feed pretreatment or hydrocracking of the heaviest residue fractions - Upgrade catalyst to higher activity grade; coke yields typically decline 0.5–1 wt% with fresh catalyst - Optimize riser injection point and distributor design to improve mixing and reduce residence time variation

Symptom 3: High Dry Gas Yield (C3– and C4– Production High, Not Desired)

Usual Causes (ranked by likelihood): 1. Riser temperature too high — overcracking to light gases instead of gasoline 2. Excessive catalyst activity — secondary cracking of gasoline into dry gas 3. High C/O ratio — extended gas-phase cracking 4. Poor vapor recovery — inadequate main fractionator stripping 5. Operating feedstock is lighter (lower boiling) than design

Immediate Actions: - Lower riser outlet temperature 5–10°F toward target (typically 750–800°F for residue; 780–820°F for coil oil) - Review regenerator temperature; if >1250°F, catalyst is excessively active and overcracking—consider lower-activity catalyst or higher riser inlet oil temperature - Check main fractionator stripping efficiency; high dry gas product can indicate vapor bypassing fractionator internals

Longer-Term Fixes: - If fresh catalyst adds activity, implement partial catalyst bleed or lower regeneration temperature target - Adjust feed preheat and feed inlet temperature to the riser to manage heat balance - Consider installing fractionator improvements (more trays, better vapor/liquid distribution) to recover lighter fractions as gasoline rather than fuel gas - Shift feedstock toward heavier material if available (reduces overall dry gas yield per unit conversion)

Symptom 4: Catalyst Loss Rate High (Spent Catalyst in Products or Dust Losses)

Usual Causes (ranked by likelihood): 1. Regenerator cyclone or secondary cyclone erosion/plugging — bypassing catalyst to air grid 2. Riser outlet choking or plugging — surging catalyst carryover to main fractionator 3. Inadequate catalyst circulation — high suspension density causing attrition 4. Poor product separator (main fractionator) cyclone efficiency 5. Catalyst fines generation — attrition in standpipe, circulation legs, or stripper

Immediate Actions: - Measure deltaP across regenerator cyclones and secondary cyclones; higher than normal (>20 inH2O) suggests plugging - Inspect main fractionator cyclones for erosion or dip tube damage (visual inspection during turnaround or camera survey) - Review spent catalyst rate (lb/hr); compare to design bleed rate; significant increase indicates system bypass - Check air grid differential pressure; abnormally low suggests cyclone failure allowing catalyst bypass to grid

Longer-Term Fixes: - Install advanced cyclone designs (high-efficiency or dip-tube erosion-resistant) if losses >5% of circulation rate - Upgrade catalyst to lower-attrition formulation if fines generation is root cause - Implement fines recovery (secondary separator or slurry settler) to return fines to regenerator - Improve standpipe/circulation leg lubrication or add external cooling to reduce attrition temperatures - Periodic regenerator internal inspection and cleaning (every 3–5 yrs) to remove deposits that cause plugging

Symptom 5: Riser Temperature Erratic or Difficult to Control

Usual Causes (ranked by likelihood): 1. Unstable feed flow or preheat temperature — instrument drift or pump cavitation 2. Regenerator temperature swinging — coke burn rate varying, regenerator level oscillating 3. Air flow instability — compressor or combustion air supply fluctuating 4. Catalyst circulation unstable — dense phase aeration or standpipe density surging 5. Riser temperature thermocouple sensor malfunction — reading noise or drift

Immediate Actions: - Verify feed flow stability; plot feed flow chart for ±5 min and check for oscillation - Check feed preheat exchanger outlet temperature; ensure stable within ±2°F - Inspect regenerator level control and air flow control loops; look for hunting or integral windup - Calibrate or replace riser outlet thermocouple if suspected; cross-check with adjacent or redundant sensor if available - Review regenerator temperature and regenerator air (RA) flow for drift; adjust blower outlet pressure or combustion air regulator

Longer-Term Fixes: - Upgrade feedstock supply control or install feed flow loop with tighter tuning - Install redundant riser temperature sensors and voting logic for control - Improve regenerator level control by optimizing standpipe slide valve or external level controller tuning - Inspect and clean riser thermocouple thermowell; remove deposits that slow response - Consider upgrading to advanced process control if oscillations persist (e.g., model predictive control for C/O and temperature balance)

Regenerator Symptoms

Symptom 6: High Regenerator Temperature (Coke Burn Runaway, Over-Combustion)

Usual Causes (ranked by likelihood): 1. Excess air (O2) to regenerator — poor air control or measurement 2. High coke on spent catalyst — riser cracking severity producing excess carbon 3. Regenerator internals (bed, grid) plugged or maldistributed — hot spots forming 4. Catalyst activity high or temperature too high in riser — driving coke formation 5. Air compressor discharge pressure rising — increased air to regenerator

Immediate Actions: - Check regenerator air (RA) flow and inlet O2 content; reduce air by 3–5% if O2 >4% in RA outlet - Verify regenerator thermocouple readings across multiple positions; if stratified, suspect maldistribution - Lower riser outlet temperature 5–10°F to reduce coke make - Check air grid differential pressure; if high, grid may be plugged—consider soot blowing or regenerator shutdown for inspection - Verify oxygen trim control (if automated) is functioning; manual control may be needed temporarily

Longer-Term Fixes: - Install oxygen analyzer in regenerator outlet or combustion air inlet to enable closed-loop O2 control - Upgrade air distribution grid (e.g., perforated grid to bubble cap design) if maldistribution suspected - Reduce riser operating temperature by increasing inlet oil temperature and decreasing air blow to regenerator - Implement regenerator soot blowing program (compressed air or nitrogen injection) to prevent carbon laydown on internals - If recurring, plan regenerator internal inspection to check for bridging or grid corrosion

Symptom 7: Low Regenerator Temperature (Incomplete Coke Burn, Catalyst Deactivation)

Usual Causes (ranked by likelihood): 1. Insufficient air to regenerator — low O2 availability for coke oxidation 2. High coke make but low burn rate — air control or grid capacity limiting 3. Catalyst activity low — less heat released per lb of coke burned 4. Regenerator insulation breach — heat loss to surroundings 5. Regenerator outlet valve partially stuck — throttling combustion air, lowering pressure

Immediate Actions: - Increase regenerator air flow 5–10% (if RA outlet O2 <2%); monitor for overshooting - Check spent catalyst coke content via lab analysis (typically 1–2 wt%); if >2.5%, riser cracking severity too high - Lower riser outlet temperature 5–10°F to reduce coke generation; regenerator will recover temperature - Inspect regenerator outlet valve position; if throttled, verify control signal - Verify air compressor discharge pressure and compare to design; low pressure reduces O2 availability

Longer-Term Fixes: - Upgrade air compressor if discharge pressure consistently low (increased air supply head cost ~$50K–200K) - Improve regenerator insulation if suspected heat loss (wrap refractory or upgrade to higher-temp insulation) - Increase riser cracking severity (higher temperature, higher C/O) to increase coke generation, providing more fuel for combustion - Implement feed pretreatment to reduce sulfur and nitrogen compounds that compete for oxygen, lowering coke burn efficiency - Plan regenerator refractory inspection and repair if sections cracked or spalled

Symptom 8: Regenerator Pressure Drop Abnormally High

Usual Causes (ranked by likelihood): 1. Catalyst circulation excessive or dense bed forming — heavy bed weight increases ΔP 2. Regenerator grid or internals plugged — soot, dust, or foreign material blockage 3. Regenerator outlet cyclone clogged or erosion-damaged — backpressure effect 4. Afterburner or waste heat boiler outlet restricted — downstream equipment throttling 5. Catalyst fines accumulation in dense phase — reduced permeability

Immediate Actions: - Check catalyst circulation rate (lb/hr); if elevated >20% above design, reduce air to regenerator or increase riser outlet temperature to slow circulation - Measure air grid differential pressure; if >25 inH2O, grid likely plugged—prepare for soot blowing or shutdown inspection - Inspect regenerator outlet cyclone visually if possible; excessive erosion may require repair or replacement - Check afterburner or boiler inlet/outlet pressures; if elevated, contact downstream equipment operator or plan cleaning - Review regenerator level; if abnormally high, regenerator is full of catalyst—increase riser outlet temperature or reduce feed rate to clear inventory

Longer-Term Fixes: - Implement periodic soot blowing (weekly or monthly nitrogen injection into regenerator bed) to prevent carbon buildup - Upgrade to higher-efficiency regenerator cyclones if pressure drop persistently elevated despite preventive maintenance - Install ΔP transmitter across regenerator to enable early warning of plugging; set alarm at 15 inH2O above baseline - Improve spent catalyst quality by optimizing riser conditions to reduce coke and fines generation - Plan regenerator internal inspection every 2–3 yrs to assess grid condition and refractory wear

Main Fractionator Symptoms

Symptom 9: High Overhead Temperature (Naphtha/LCO Vapors Not Fully Condensed)

Usual Causes (ranked by likelihood): 1. Condenser fouling or cooling water problem — low cooling duty 2. Excessive overhead vapor rate — riser temperature too high or C/O ratio high 3. Inefficient overhead compressor or separator — inadequate cooling capacity 4. Feed inlet temperature too high — sensible heat overwhelming condenser 5. Overhead product specification changes — requiring different condensing temperature

Immediate Actions: - Check cooling water inlet/outlet temperatures and flow; if ΔT <3°F, water side may be fouled—clean tubes or increase water rate - Lower riser outlet temperature 5–10°F to reduce overhead vapor rate - Verify overhead compressor discharge pressure and aftercooler duty; if aftercooler outlet >110°F, fouling or water side plugging likely - Check condenser outlet subcooling; if <5°F, may be approaching dew point—check refrigerant pressure/level if used - Review main fractionator level and tray liquid distribution; if high level, stripping efficiency low—reduce overhead rate

Longer-Term Fixes: - Schedule condenser tube cleaning (chemically or mechanically) every 12–18 months if overhead temperature drifts upward - Upgrade condenser to higher duty (larger area, enhanced tubes) if cooling capacity systemically insufficient (~$100K–300K) - Optimize cooling water supply (install dedicated loop if shared) or upgrade to enhanced refrigeration (propane, ammonia) if water cooling insufficient - Implement feed preheat temperature control tighter than ±5°F to prevent transient overhead temperature spikes - Install overhead temperature monitoring with trend analysis to predict fouling before operating upset

Symptom 10: High Main Fractionator Bottoms Temperature (Slurry Oil Too Hot)

Usual Causes (ranked by likelihood): 1. High fractionator feed temperature — riser outlet temperature too high or hot feed entering fractionator 2. Inadequate reboiler duty — underperforming or fouled reboiler 3. Bottoms pump or line restriction — residence time too long, temperature buildup 4. Fractionator bottoms level too low — reduced residence time, insufficient cooling 5. Inadequate stripping steam to bottom section — poor heavy component removal

Immediate Actions: - Lower riser outlet temperature 5–10°F; this reduces feed sensible heat to fractionator - Check reboiler inlet/outlet temperatures; if ΔT <10°F, reboiler may be fouled—flush or plan tube cleaning - Verify bottoms product pump discharge pressure; if elevated, line may be restricted—check strainers or heat exchanger - Increase bottoms circulation rate if possible, or adjust fractionator level to increase bottoms holdup - Inspect stripping steam flow to fractionator bottoms section; if low, increase steam rate 5–10%

Longer-Term Fixes: - Install temperature transmitter in reboiler outlet; set alarm at design limit (typically <700°F slurry oil) to prevent fouling and coking - Schedule reboiler cleaning every 12–24 months if bottoms temperature chronically elevated - Upgrade reboiler to higher duty (larger steam inlet, enhanced internal design) if fouling rate high (~$50K–150K) - Improve feed inlet distribution to fractionator (install baffle or distributor internals) to ensure uniform cooling across cross-section - Optimize fractionator pressure and vacuum bottoms product target to manage viscosity and fouling tendency

Symptom 11: Low Gasoline Yield (Insufficient Heavy Naphtha Recovery)

Usual Causes (ranked by likelihood): 1. Heavy naphtha (LCO) stripping inadequate — naphtha carryover into slurry oil product 2. Fractionator vapor distribution poor — uneven tray flooding, light material bypassing 3. Fractionator inlet nozzle erosion or plugging — maldistribution of riser feed 4. Light cycle oil (LCO) boil point specification changed — product lighter/heavier than designed 5. Main fractionator level control poor — variable residence time reducing separation efficiency

Immediate Actions: - Check fractionator internal temperatures (every 2–3 trays in heavy section); if erratic, suspect vapor or liquid maldistribution - Increase stripping steam to the heavy naphtha (HN) cut section; 5–10% increase can improve LCO/HN separation - Verify fractionator inlet feed position (at riser termination); if off-spec, reroute inlet or reinstall distributor - Review LCO product boiling range (IBP, end-point); compare to design specification—target is typically IBP ~330°F, end-point ~560°F - Check fractionator level control tuning; if oscillating >0.5 ft, tighten PID loop or install external level control

Longer-Term Fixes: - Install bypass or redistribution trays in fractionator if inlet feed distribution poor (cost ~$30K–80K) - Upgrade fractionator stripping section with additional trays or internal packing if gasoline yield consistently below design - Implement product yield monitoring via online simulation or lab assay; trend gasoline % to track separation efficiency degradation - Plan fractionator internal inspection every 2–3 yrs to check for tray damage, scaling, or erosion - Optimize riser outlet distributors and fractionator inlet nozzle design to improve momentum and distribution

Operations/General Symptoms

Symptom 12: Sudden Feed Reduction Capability (Can’t Process Design Rate)

Usual Causes (ranked by likelihood): 1. Regenerator or reactor coke burning capacity saturated — can’t handle design feed rate 2. Fractionator or vacuum bottoms capacity limiting — product takeaway bottleneck 3. Main air compressor capacity degraded — insufficient air to sustain regeneration 4. Catalyst circulation pathway obstructed — standpipe, stripper, or dense phase restriction 5. Product specifications tightened — lower yield targets (e.g., lower sulfur, lower coke) reducing throughput

Immediate Actions: - Attempt to increase riser outlet temperature 10°F (if equipment/safety allows) to increase cracking severity and coke generation; if coke burn rate doesn’t increase, regenerator air-limited - Check air compressor discharge pressure and flow; if discharge pressure 2 psig, compressor degraded or fouled - Verify product specifications (sulfur, coke, aromatic carbon); if tightened, calculate revised maximum throughput - Monitor regenerator and reactor inventory (pounds of catalyst); if inventory low, increase bleed rate from regenerator - Review fractionator throughput bottleneck: measure slurry oil, light cycle oil, and gasoline rates; if one is limiting, fractionator capacity constraint

Longer-Term Fixes: - Plan air compressor maintenance (valve cleaning, intercooler tube cleaning) or upgrade if pressure/flow consistently low - Increase regenerator coke burning capacity by upgrading internals (grid, distributors) or operating regenerator at higher design temperature - Expand fractionator capacity (additional trays, vapor/liquid redistribution) or install parallel slurry settler if bottoms takeaway limiting - Implement dynamic fractionator modeling to identify bottleneck section and optimize reflux and stripping rates - Consider partial feed bypass to hydrocracker or downstream hydrotreater if FCC unit saturated and full throughput not achievable

Symptom 13: Catalyst Selectivity Change (Gasoline Selectivity Down, Dry Gas/Coke Up)

Usual Causes (ranked by likelihood): 1. Catalyst deactivation (age, metals, thermal) — loss of shape selectivity 2. Riser temperature elevated above design — secondary cracking of gasoline and distillate 3. Catalyst composition drift — metal contamination (Ni, Cu, V, Fe) from feed or attrition 4. Operating feedstock has changed — heavier, higher metals, higher aromatic carbon 5. Regenerator temperature too high — excessive regeneration reducing catalyst hydrothermal stability

Immediate Actions: - Reduce riser outlet temperature 5–10°F; improved selectivity should follow within 30 min - Obtain catalyst activity test (MAT or ACE) and metal analysis (Ni, V, Cu ppm); compare to baseline - Review feedstock assay (metals content, aromatic carbon); if feed quality declined, implement blending or pretreatment - Lower regenerator temperature 10–20°F if possible; over-regeneration (>1200°F) can degrade catalyst structure - Increase fresh catalyst addition rate if metals accumulation suspected; partial regenerator purge (bleed) may improve selectivity

Longer-Term Fixes: - Plan catalyst change (complete regenerator purge and reload) if metals >2–3 ppm above design or activity declining >10% annually - Implement feed hydrotreating to remove metals and sulfur if contamination from crude source unavoidable - Upgrade to more metal-tolerant catalyst if feedstock consistently high in contaminants - Optimize regenerator calcination temperature via continuous O2/CO monitoring to ensure complete combustion without over-heating catalyst - Establish baseline catalyst selectivity curve (gasoline % vs. conversion) and monthly monitoring protocol to detect degradation early

Symptom 14: Unexpected Increase in Regenerator Frequency or Severity of Upsets

Usual Causes (ranked by likelihood): 1. Feed quality degradation (metals, sulfur, asphaltenes) — catalyst poisoning accelerating 2. Regenerator control loop tuning poor — oscillation or hunting in temperature or level control 3. Air supply intermittency — compressor surge, pressure fluctuation, or instrumentation drift 4. Catalyst fines in dense phase — density and pressure spikes during circulation 5. Process operating envelope drifting — cumulative changes in feed, temperature, or C/O pushing system instability

Immediate Actions: - Check feed metals content (Ni, V, Cu) and compare to previous assay; if elevated, implement feed blending or pretreatment - Review regenerator control loops (level, temperature); observe for oscillation or slow response—retune PID or add filtering to noisy signals - Inspect air compressor discharge line for pulsation or pressure spikes; if present, compressor inlet filter or output valve may be failing - Check regenerator dense phase aeration; if density > design limit, dense phase may be unstable—adjust air or increase stripping steam - Run mass balance and compare current C/O, riser ΔT, and regenerator temps to design envelope; if drifted, incrementally return to nominal

Longer-Term Fixes: - Implement continuous feed metals monitoring (online analyzer or frequent lab assay) with alert thresholds for immediate feed switching - Upgrade regenerator control to model-predictive control (MPC) if oscillations persist after PID tuning - Plan air compressor recertification or upgrade if discharge pressure/flow unstable despite maintenance - Install catalyst screening system (advanced cyclone + fines settler) if fines accumulation recurring - Develop operating envelope documentation with maximum allowable values for key parameters (feed rate, temperature, C/O); train operations to monitor envelope compliance daily

Symptom 15: Poor Sulfur Removal (Sulfur in Products Higher Than Designed)

Usual Causes (ranked by likelihood): 1. Riser temperature too low — insufficient thermal cracking to break C–S bonds 2. Catalyst activity low or metals-poisoned — reduced desulfurization capability 3. Feed sulfur content increased — crude or residue input heavier or different source 4. Inadequate riser residence time — C/O ratio too low for complete desulfurization 5. Regenerator temperature low — poor catalyst oxidative regeneration, activity decline

Immediate Actions: - Increase riser outlet temperature 10°F to promote thermal desulfurization - Obtain catalyst activity and metal content; if metals (esp. V, Ni) elevated, implement catalyst bleed/replacement - Verify C/O ratio from material balance; if below design, increase air to regenerator or reduce feed rate - Review feedstock sulfur content; if >2 wt% (for residue), consider blending with lighter, low-sulfur crudes - Ensure regenerator temperature at or above 1150°F; insufficient regeneration reduces desulfurization activity

Longer-Term Fixes: - Implement upstream hydrotreating of feed to remove sulfur before FCC (capital ~$50M–100M but enables sulfur product targets <0.5 wt%) - Upgrade catalyst to higher desulfurization activity type if feed sulfur trending upward - Optimize riser inlet temperature and feed preheat; higher inlet temp (relative to riser outlet) improves cracking severity and S-removal - Increase stripping steam to riser outlet to enhance H2S and sulfur-bearing gas removal - Install online sulfur analyzer in gasoline, LCO, and slurry oil to enable dynamic optimization of riser conditions

Symptom 16: Cyclone Erosion or Plugging (Pressure Drop Rising, Catalyst Bypass)

Usual Causes (ranked by likelihood): 1. Catalyst fines generation high — attrition from riser acceleration or standpipe 2. Cyclone design inadequate for current operating rate — velocity too high, erosion accelerating 3. Corrosive environment (H2S, steam, abrasive catalyst) — material degradation 4. Thermal stress or vibration — dip tube erosion or cyclone shell cracking 5. Foreign material in feed or air — sand, corrosion particles, or equipment debris

Immediate Actions: - Measure deltaP across affected cyclone; if >25 inH2O, plugging imminent—reduce feed rate 5–10% to lower velocity - Inspect cyclone dip tube and inner surfaces visually if possible (borescope, camera); if dip tube heavily eroded, replacement needed - Reduce riser outlet temperature 5°F to lower suspension density and cyclone inlet velocity - Increase nitrogen or steam soot blow through cyclone for 5–10 min to clear loose deposits - Monitor spent catalyst fines content (cyclone catch); if >15% of circulation rate, fines generation excessive

Longer-Term Fixes: - Install erosion-resistant cyclone liners (95% alumina, sintered bauxite) if operating at high suspension densities; typical cost $20K–50K per cyclone - Upgrade to advanced cyclone designs (e.g., dip-tube less, high-efficiency inlets) to reduce erosion and improve separation - Implement fines recovery system (secondary separator or slurry settler) to capture and return fines to regenerator, reducing bypass - Optimize riser outlet design and standpipe aeration to minimize fines generation at source - Plan cyclone replacement during next planned turnaround; preventive replacement cheaper than emergency shutdown due to failure

Symptom 17: Temperature or Pressure Instrumentation Unreliable (Readings Inconsistent or Drifting)

Usual Causes (ranked by likelihood): 1. Thermocouple or sensor fouling — deposit buildup on sensing element 2. Transmitter zero or span calibration drift — due to temperature, age, or vibration 3. Reference junction compensation failure (thermocouples) — cold-junction cold-block temp sensor malfunction 4. Pressure tap plugged — coke or particle blockage in impulse line 5. Instrument cable or connector corrosion — signal attenuation or intermittency

Immediate Actions: - Perform manual temperature spot-check with portable pyrometer or handheld IR camera at thermocouple location; compare to control room reading - Check pressure gauge (direct reading, if available) against transmitter output; significant deviation indicates transmitter drift - Inspect thermocouple thermowell; if black/heavy deposit coating visible, thermowell requires cleaning or replacement - Verify transmitter 4–20 mA output against handheld calibrator; if out of range, retransmitter calibration (zero/span) needed - Check electrical connections at transmitter and in control room; look for corrosion or loose terminals causing intermittent signal

Longer-Term Fixes: - Establish instrument maintenance schedule: thermocouple calibration every 12 months, transmitter verification every 6 months, pressure tap cleaning quarterly - Replace aging transmitters (>5–10 yrs) with modern digital units offering self-diagnostics and improved stability - Upgrade to thermocouple wells with improved design (better thermal response, easier cleaning, reduced lag) - Install redundant instruments (dual thermocouples, dual pressure transmitters) for critical process points; implement voting or cross-check logic - Implement preventive transmitter recalibration before drift becomes significant; trending of calibration errors helps predict maintenance intervals

Symptom 18: Corrosion or Material Degradation (Equipment Wall Thinning, Pitting, or Cracking)

Usual Causes (ranked by likelihood): 1. H2S corrosion (regenerator, reactor) — acid attack on carbon steel at high temperature 2. Oxygen corrosion (near air inlet) — oxidation of steel, especially if stressed or creviced 3. Thermal fatigue (cyclones, elbows) — cycling stress from temperature swings 4. Erosion-corrosion (riser, outlet elbows) — particle impact + corrosive environment 5. Galvanic corrosion (if mixed metallurgy) — dissimilar metal coupling in wet environment

Immediate Actions: - Cease operation if corrosion rate rapid or crack detected; schedule ultrasonics (UT) thickness survey to quantify remaining wall thickness - Review regenerator H2S concentration (from flue gas); if >100 ppm, increase air flow or reduce coke to lower H2S generation - Inspect visually for active corrosion (shiny/wet surface) vs. passive (dry oxide film); active corrosion requires immediate process change - If cracking detected, cease operation and conduct non-destructive testing (UT, eddy-current) to assess structural integrity - Reduce regenerator temperature 20–50°F if H2S corrosion suspected; lower temperature reduces H2S formation and corrosion rate

Longer-Term Fixes: - Upgrade equipment metallurgy (316 stainless or 5% Cr alloy steel) in high-corrosion zones (reactor bottom, regenerator bed, hot cyclone) - Implement corrosion inhibitor injection (amines, filming amines) to passivate corroding surfaces; typical cost $5K–20K/yr for inhibitor supply - Increase stripping steam to reactor and regenerator to inert oxygen and reduce air-side corrosion - Install anodic or cathodic protection (sacrificial anode or impressed current) if galvanic couples unavoidable - Establish regular ultrasonics (UT) wall thickness survey program (every 2–3 yrs) to track remaining equipment life and plan replacement

Symptom 19: Unexpected Shift in Product Mix or Quality (Gasoline, LCO, Slurry Oil Yield/Properties Drifting)

Usual Causes (ranked by likelihood): 1. Feed composition changed — crude source, blending ratio, or residue feedstock shift 2. Operating conditions drifted — temperature, C/O, or riser residence time no longer at design 3. Catalyst composition or activity changed — metals accumulation or selectivity degradation 4. Fractionator separation efficiency degraded — internal fouling, tray damage, or level control drift 5. Downstream product processing (hydrotreater, hydrocracker) conditions changed — affecting blending pools

Immediate Actions: - Obtain current feedstock assay and compare to last known baseline; if significantly different (metals, boiling range, sulfur), adjust process expectations - Review operating targets (riser temperature, C/O, regenerator temp) against design envelope; if drifted >5% from target, incrementally restore setpoint - Run material balance across FCC; calculate actual C/O, conversion, and selectivity; compare to historical trend - Inspect fractionator internal temperatures (thermocouples) across trays; if profile changed significantly, suspect internal fouling or damage - Pull product samples (gasoline, LCO, slurry oil) for immediate property analysis (boiling range, sulfur, flash point); compare to spec

Longer-Term Fixes: - Establish product spec monitoring program with automated lab analysis or online property analyzers (refractometer, NIR, etc.) - Implement daily mass balance calculation and yield tracking with automated alert for >2% deviation from rolling average - Create feedstock quality dashboard (metals, sulfur, aromatic carbon, boiling range) with monthly updates; flag changes requiring process adjustment - Upgrade fractionator control to dynamic simulation (soft sensor) linking feed properties, operating conditions, and product yields for real-time optimization - Establish quarterly product quality review with downstream (hydrotreating, polymerization) to identify shift causes and agree on corrective actions

Symptom 20: Unit Trips Unexpectedly (Shutdown Due to High Temperature, Pressure, or Safety Alarms)

Usual Causes (ranked by likelihood): 1. Process instrumentation false alarm — sensor malfunction, drift, or noise causing false high reading 2. Regenerator temperature spike — sudden coke ignition or air surge event 3. Riser pressure excursion — dense phase fluidization upset or standpipe aeration loss 4. Cooling/condensing system failure — loss of cooling water or compressor shutdown 5. Feed or air supply interruption — pump trip, compressor shutdown, or block valve closure

Immediate Actions: - Immediately after trip, verify reported cause from distributed control system (DCS) event log and confirm with manual instrument checks - Check cooling water flow/temperature to condenser and reboiler; if low/off, restart cooling system - Verify air compressor status and discharge pressure; if offline or low, compressor startup may be blocked - Check feed pump and air supply block valves; if closed or failing, restart pump and verify valve position - Review thermocouples and pressure transmitters for obvious sensor failure (zero reading, out-of-range, inconsistency)

Longer-Term Fixes: - Retune high-alarm deadbands and rate-of-change thresholds if false alarms frequent; ensure alarm thresholds account for normal process oscillation bandwidth - Implement instrumentation verification logic (cross-check or voting) for critical interlocks; avoid single-point sensor failures causing shutdown - Install backup cooling water pump or dual condensers if cooling system single-point failure causes frequent shutdown - Upgrade to larger capacity air compressor or install air receiver storage to buffer minor air supply transients - Conduct hazard and operability (HAZOP) study to identify and mitigate other potential false-alarm or shutdown scenarios - Establish restart procedure documentation with pre-startup checklist to prevent repeated trips from root cause - If trips become frequent (>5/yr), plan comprehensive process control system upgrade with redundancy and improved fault tolerance (cost $200K–500K)

Sources

Wilson, K. (1997). Advances in Fluid Catalytic Cracking. Editions Technip, Paris.
Sadeghbeigi, R. (2020). Fluid Catalytic Cracking Handbook (Fourth Edition). Gulf Publishing Company, Houston, TX.