PTQ Q1 2024 Issue

Simulating FCC upset operations

Examples are provided of FCC upset event consequences predicted by using FCC performance simulation models

Tek Sutikno Fluor Enterprises

M ore than half of the world’s petroleum refineries include a fluidised (or fluid) catalytic cracking (FCC) operation generating 30-50% of the gasoline product pool of a refinery. The FCC is typically one of the most productive and profitable processing units among the refining processes. Primarily by utilising the proper catalyst type, product yield distribution can be modified for selec- tively maximising the yield of the gasoline blending compo- nents, light ends high in olefins, high-grade petrochemical feedstocks, or LCO (feedstock for diesel). Light ends from an FCC unit include propylene and olefins that are alkylated to produce high-octane gasoline. Likely due to its role in refinery profitability, an FCC unit often operates at much higher throughput capacities rel- ative to the original design capacity and yields a product distribution significantly different from that of the original yield target. Field implementation of the associated revamp projects is typically completed during the scheduled turn- around period. A hazard and operability (HAZOP) review is normally necessary in each of these revamp projects to examine the design and engineering of the revamp mod- ifications and to assess upset/deviation cases that could cause harm to people, environment or assets. Representatives from operators, unit engineers, engi- neering contractors, and subject matter experts (SME) are generally required to participate in the HAZOP review meetings and are expected to describe the potential con- sequences of an upset event where a process parameter deviates from the normal or regulated level. Due to the complexity of the FCC reactor and regenerator involving several essential operating parameters, conse- quences or operating impacts from a particular upset event may not be obviously known to the operators, the unit engi- neer, or the SME, especially if they have not observed, expe- rienced, or analysed the same or similar event in the FCC unit. The unit engineer or the SME may likely need time to eval- uate the upset consequences using an FCC performance or simulation model. However, the performance or simulation model for the FCC reactor and regenerator in a revamp or new project is commonly developed by the licensor or/and the catalyst vendor and not accessible to the engineering contractor. In these cases, the unit engineer or SME will need to work with the licensor to assess the upset conse- quences. Based on the severity levels of the consequences, the required protective measures are discussed and speci- fied in the layers of protection analysis (LOPA).

Various FCC performance models are reported in the liter- ature. However, these models involving numerous parame- ters are mathematically complex and likely time-consuming to utilise for a particular FCC system, in addition to the likely absence of a specific deviating parameter in the accessible models. However, recent versions of commercial simula- tors, such as Hysys Version 12, include FCC performance models with reasonable details of operating parameters. Performance model The Hysys Version 12 FCC performance model discussed herein is a steady-state model with several options appli- cable to common FCC designs. The default parameter input values in the Hysys model template are used and defined as the base case. By changing one of the input variables in the model, the operating consequences can be checked from the calculation results. The results are the steady-state operation and do not predict any actual or probable time-dependent deviation or response before reaching the steady state. Transient responses from a deviating parameter in an upset event will depend on the control schemes, which

Reactor euent to main fractionator

To ue gas pressure control valve

PDC

To ue gas system

Reactor

Regenerator

Stripping steam

Regen. Cat. slide valve

Feed steam

Spent Cat. slide valve

Main air blower/ compressor

Lift steam/gas

Figure 1 FCC control scheme

PTQ Q1 2024