PTQ Q4 2023 Issue

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.

T he fluid catalytic cracking unit (FCC unit) holds sig - nificant importance within the refinery landscape, cracking heavy oils and residual feedstock using elevated temperatures, low pressure, and a catalyst to pro - duce 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 visbreak - ing gasoils, hydrocracking residues, hydrotreated gasoils and residues, furfural extracts, and demetallised oil (DMO). The practice of adding a higher percentage of atmos - pheric residue to FCC unit feedstock is becoming more common, but this is detrimental to FCC unit performance. Atmospheric residue often contains chloride salts, primar - ily 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 oper - ational changes that align with production goals in produc - ing high-value products that are in demand in the market. This challenge becomes even more complex due to fre - quent 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 oper - ational changes, and validating the actual risks post-imple - mentation 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 accu - mulate 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 con - densation 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:  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  Overhead condensation system : Plugging air-cooling bank tubes causing leaks or necessitating bypassing, or reducing throughput, lower heat transfer rates  Reactor and regenerator : Coke burn reduction, unit feed reduction, lower gasoline octane number • Fouling of TPA circuit:  Reduces heat extraction efficiency  Difficulty in maintaining liquid level in TPA extrac - tion tray reduces maximum extractable flow rate. This increases reflux from overhead condensate accumulator, which enhances cooling but leads to challenges in main - taining 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: The critical importance of factoring in corrosion effects when implementing beneficial modifications to FCC unit operations cannot be underestimated

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PTQ Q4 2023