Simulating FCC upset operations
Examples are provided of FCC upset event consequences predicted by using FCC performance simulation models.
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More 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 selectively maximising the yield of the gasoline blending components, 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 relative 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 turnaround 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 modifications and to assess upset/deviation cases that could cause harm to people, environment or assets.
Representatives from operators, unit engineers, engineering contractors, and subject matter experts (SME) are generally required to participate in the HAZOP review meetings and are expected to describe the potential consequences 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, consequences or operating impacts from a particular upset event may not be obviously known to the operators, the unit engineer, or the SME, especially if they have not observed, experienced, or analysed the same or similar event in the FCC unit.
The unit engineer or the SME may likely need time to evaluate 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 consequences. Based on the severity levels of the consequences, the required protective measures are discussed and specified in the layers of protection analysis (LOPA).
Various FCC performance models are reported in the literature. However, these models involving numerous parameters 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 simulators, such as Hysys Version 12, include FCC performance models with reasonable details of operating parameters.
The Hysys Version 12 FCC performance model discussed herein is a steady-state model with several options applicable 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 may differ from one FCC unit to another. Applications of the model for predicting the consequences of an upset case need to be consistent with the control schemes of the system being analysed. Figure 1 is an example of an FCC control scheme where the combustion air flows to the regenerator that is on flow control, and the combustion air flow rate to the regenerator will need to be kept constant in the performance model when using the model to assess the impact of different deviating process parameters.
The Hysys Version 12 FCC performance model includes physical dimensions of the reactor and regenerator, feed composition and characteristics, operating parameters including common reaction kinetic parameters, catalyst selection, and details of the reaction. Property data of hydrocarbon feeds, typically ranging from the gasoil fraction of the crude oil to heavier feedstocks, including atmospheric resid, vacuum gasoils, and/or vacuum resids, can be input into the model. The calculated results from the Hysys model include fairly comprehensive parameters similar to those normally provided by the licensor.
With the proper input data, the results from the model can be useful for analysing the upset conditions or off-design operating performances. For further elaboration, examples are provided of upset events from five different deviating parameters: catalyst circulation rate, combustion air flow rate, stripping steam flow rate, steam feed rate, and feed temperature to the riser. These five were chosen as illustrative examples, but other upset events can also be modelled.
The simulated consequences from the upset events discussed herein are intended to show the resulting operational changes at the steady-state condition and have not been checked against the actual field operating data. Moreover, for extreme or severe upset cases such as loss of flow, the model will not be applicable directly to these cases, which typically result in activating the shutdown system. However, the model can be used to generate reference data likely useful for developing a dynamic model, which is typically needed to determine the process safety time available for system shutdown.
Catalyst circulation rate
Catalyst circulation rate is an essential parameter or variable determined by the heat balance between the reactor and the regenerator. An FCC reactor involves both exothermic and endothermic reactions, resulting in a net total endothermic reaction.
The heat required for increasing the sensible heat of the feed, vaporisation, and the net endothermic reaction is supplied by the temperature drop of the circulating catalysts as they pass through the reactor. The resulting catalyst-to-oil ratio (C/O) affects the cracking reaction yield conversions and reactor temperatures. With increasing C/O, active sites increase to cause more cracking and higher conversion of gasoil, and the yield of fuel gases and coke increases. Common parameters such as coke yield and delta coke (wt% difference between the spent catalyst and the regenerated catalyst) can be related to C/O. For a given FCC reactor and regenerator system with the same feed rate and characteristics, the catalyst (regenerator) circulation rate increases with changes in process conditions such as higher reactor riser temperature, lower feed preheat, or others demanding additional heat input to the reactor.
While the Hysys model includes fairly complete input and output parameters commonly used in FCC performance modelling, only some are displayed in this discussion. As an example of upset cases in the normal catalyst circulation rates in Case 1, Table 1 shows the changes or consequences relative to the base case when the catalyst circulation rate in Case 1 reduces by 2.7% (arbitrary, about 120,000 lb/hr reduction). This reduction decreases the C/O, the reactor temperature (1,013-1,003ºF), and, expectedly, the total conversion.
Due to the decreased catalyst circulation, the resulting delta coke in Case 1 increases slightly to satisfy the system heat balance even at the reduced reaction temperature of 1,003ºF, and the resulting coke yield (equal to delta coke x C/O) decreases slightly. For FCC units with control schemes similar to Figure 1, the air flow rate from the main air blower will remain essentially unchanged when a lower set point for reactor temperature reduces the catalyst circulation rate.
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