the riser. The combustion of this coke in the FCC unit accounts for approximately 15-20% of the CO₂ emissions from the refinery (Gudde, Larive, & Yugo, 2019) (Jing, et al., 2020). The coke produced in the FCC unit stems from several contributors: Hydrogen redistribution reactions in the catalytic cracking Cracking of heavy feed components Poor feed vaporisation Dehydrogenation reactions catalysed by contaminant metals in the feed Unstripped hydrocarbons entrained in the catalyst entering the regenerator. The coke yield directly contributes to the Scope 1, or direct, CO₂ emissions from the FCC unit. Factors that increase the coke yield (for example, operating at a higher reaction temperature) will increase emissions from the FCC unit. Reducing FCC unit severity (lowering the riser temperature), reducing the catalyst- to-oil ratio (C/O) (for example, by increasing the feed temperature and reducing catalyst cooler duty), or reducing slurry or light cycle oil (LCO) recycle rates will all reduce CO₂ emissions from the FCC unit, accomplishing the objective of reduced Scope 1 CO₂ emissions. However, these moves typically lead to reduced conversion and increased slurry from the FCC unit. While it may be desirable to operate the FCC unit at lower direct emissions by decreasing the coke yield, this is generally not economically attractive. The yield of propylene from the FCC unit can be increased by: Increasing reaction temperature Increasing the C/O ratio Decreasing the hydrocarbon partial pressure (introducing additional steam into the process) Improving feed quality, Reformulating the catalyst to drive propylene yield higher The use of ZSM-5 technology. Options 1-3 will either increase coke yield or require additional energy, thereby increasing emissions from the process, whereas ZSM-5 technology usage has a neutral effect on CO₂ emissions (the catalyst itself can be reformulated to minimise changes in coke yields). The effect of changing operational variables and the impact on CO₂ emissions and propylene production is shown in Table 2 .
Operational
Effect on propylene Effect on CO 2
move
production
emissions
n
p
Increase ZSM-5 additive rate Reduce riser severity Reduce C/O (higher Feed T) Reduce gasoline/ LCO recycle
Table 1 shows that refinery-based propylene has the lowest carbon intensity. The main reason is that, as a catalytic process, the FCC unit typically operates at the lowest temperature and is a more energy-efficient process for the production of olefins. Further, FCC is not equilibrium-limited: while the dehydrogenation of propane in the PDH process takes place in the presence of a catalyst, the reverse reaction is also possible, and higher reaction temperatures are required to drive the equilibrium towards propylene (Stitt, Jackson, & King, 1999). As a result, the FCC has the lowest reaction temperature. CO₂ emissions generated in propylene production processes stem from heating feedstocks to the required reaction temperatures: the higher the temperature, the larger the energy requirement and CO₂ emissions generated. Some of this energy is provided by burning coke produced in these processes (in the case of the FCC unit, most of the energy is supplied this way; more on this below). The implication of Table 1 is striking. If lowering emissions were the sole metric in determining the preferred method for producing propylene, it would be preferred to source propylene from the refinery. In practice, it is not so simple since the refinery and the FCC unit are traditionally geared towards producing fuels. FCC chemistry: Coke generation and propylene production The FCC unit requires the production and combustion of coke to generate the heat required to vaporise the feedstock and drive the endothermic cracking reactions taking place in Table 2 Summary of the effect of different FCC unit operating variables on CO₂ emissions and propylene yield
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