Nov-2023

Enabling safe FCC unit operational changes through ionic modelling

How ionic modelling assisted in the management of change in FCC unit operations.

Cristian Spica
OLI Systems Inc

Article Summary

The fluid catalytic cracking unit (FCC unit) holds significant importance within the refinery landscape, cracking heavy oils and residual feedstock using elevated temperatures, low pressure, and a catalyst to produce valuable lighter products.

This process is highly efficient and contributes to refinery profitability and flexibility by expanding the gasoline pool composition, supplying valuable propylene and butylene building blocks to petrochemical plants, and positively impacting the overall energy balance of the refinery.

FCC unit feedstock typically includes atmospheric and vacuum gasoils, atmospheric residues, coker and visbreaking gasoils, hydrocracking residues, hydrotreated gasoils and residues, furfural extracts, and demetallised oil (DMO).

The practice of adding a higher percentage of atmospheric residue to FCC unit feedstock is becoming more common, but this is detrimental to FCC unit performance. Atmospheric residue often contains chloride salts, primarily NaCl, which hydrolyses in the riser. The sodium reduces catalyst activity and increases the fouling rate, while the chlorides trigger the corrosion risk. Imported residues and vacuum gasoils (VGOs) can be detrimental, as they may contain a significant concentration of inorganic salts from seawater.

Furthermore, it is common for this unit to undergo operational changes that align with production goals in producing high-value products that are in demand in the market. This challenge becomes even more complex due to frequent changes in feedstock, both in the crude slate, FCC feed, and catalyst additives.

In such a scenario, having access to a robust tool that aids in defining the unit’s integrity operating window, accurately predicting corrosion risks well ahead of implementing operational changes, and validating the actual risks post-implementation becomes an invaluable asset.

FCC unit main fractionator overhead condensation system and top pumparound corrosion
Corrosion and fouling within the FCC unit primarily stem from the formation of ammonium chloride (NH4Cl) and ammonium bisulphide (NH4HS) salts. These salts accumulate on fractionators’ internals, overhead condensation systems, and pumparounds.

Ammonium salts are soluble in water but relatively insoluble in hydrocarbons. Their formation, deposition, and accumulation within the FCC unit occur in various locations based on reactant levels and process conditions. Consequences of ammonium chloride salts include:
• Wet gas compressor suction drum pressure reduction: Decreased compressor volumetric rate, reduction in unit feed rate or riser top temperature    
• Corrosion-related leaks and damage to tower internals, dome, top pumparound (TPA) circuit, and overhead condensation system due to under-deposit pitting corrosion
• Fractionation efficiency reduction caused by differential pressure (DP) increase, tray active area reduction due to plugging, jet flooding
• Increased delta pressure:
 ν Tower top: Flash zone pressure drops in tower bottom increase – higher bottom temperature to recover the same LCO rate – hence more fouling in the slurry loop circuit
 ν Overhead condensation system: Plugging air-cooling bank tubes causing leaks or necessitating bypassing, or reducing throughput, lower heat transfer rates
 ν Reactor and regenerator: Coke burn reduction, unit feed reduction, lower gasoline octane number
• Fouling of TPA circuit:
 ν Reduces heat extraction efficiency
 ν Difficulty in maintaining liquid level in TPA extraction tray reduces maximum extractable flow rate. This increases reflux from overhead condensate accumulator, which enhances cooling but leads to challenges in maintaining gasoline endpoint and increases overhead flow rate. Overloading the condensation system raises column head pressure, requiring plant capacity reduction and posing additional erosion-corrosion issues.

Mitigation strategies may encompass actions that affect capital and operational expenditures:
• Control the quality of crude slate and FCC feedstock:
 ν Decrease the proportion of atmospheric residue (ATMRES) or imported ATMRES and imported gasoil in the FCC feed
 ν Monitor the sodium content in the residue and FCC feed and in the exhausted catalyst
• Enhance efficiency of the CDU desalting stage
• Implement a dedicated desalting stage for FCC operations
• Increase the top temperature of the main fractionator
• Inject corrosion inhibitor
• Inject wash water and/or salt dispersant.

These mitigation strategies could yield partial or complete effectiveness based on the understanding and resolution of the extent of corrosion-related phenomena. This is where an ionic modelling tool is valuable in pinpointing areas most susceptible to corrosion and in devising appropriate mitigation plans.

Ionic modelling
Electrolyte (or ionic) models provide insights into corrosion and fouling risks in the FCC unit.

Thermodynamic analysis can be used to evaluate the scaling (fouling) and corrosion potential of FCC unit streams with varying water qualities and operating conditions (temperature, pressure, pH, and composition). These insights allow the engineer to adjust operating conditions, which reduces inefficiencies and process bottlenecks, and avoids potential leaks and unplanned shutdowns.

Similarly, corrosion risks change with changing vapour/liquid/liquid composition and temperatures and pressures in the main fractionator. The corrosivity of individual streams can be analysed at various operating conditions to aid in the selection of materials for tower internals, heat exchangers, and overhead systems. This can further optimise the tower design and establish operating envelopes that avoid highly corrosive conditions, extending the life and reliability of the units.