A fluid catalytic cracker's (FCC) main fractionator heat integration with the gas plant can supply incremental energy needed to meet improved olefins requirements and improve the unit's thermal efficiency. It is important, when optimizing, to understand the fractionator energy recovery limitations, possible pumparound draw temperature options and various temperature levels of the heat sinks. On older units, recoverable energy from the overhead system may be much higher. Including product recoverable heat losses, it may be 50% higher. Main fractionator light cycle oil (LCO) and gasoline product recoveries can be improved by heat balance adjustment associated with optimization. Main fractionator vapor loadings are more than 20% higher in an optimized design.
Integrating the main fractionator heat-removal system with gas-plant-column reboilers is common. However, degree of integration varies. When optimizing the system integration, it is important to examine limitations on recoverable enthalpy and evaluate main fractionator process schemes to match available FCC unit heat sinks. It also helps to compare a "typical" design main fractionator heat-removal system with an optimized design. The focus of the optimization should be incremental energy requirement in the gas plant debutanizer and C3/C4 splitter associated with higher olefin recoveries and potential impact on main fractionator heat balance and internal vapor/liquid loadings.
An FCC unit's main fractionator feed contains a lot of energy that can be recovered by exchange with other plant streams. This heat comes from the reactor where unit charge is exchanged with hot circulating catalyst to provide enthalpy for feed vaporization prior to cracking.
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