Process Design Package for Heat Recovery at SCR Outlet in FCC CO Heaters (CCU-1/CCU-2)
Furnace Improvements (FIS) was employed by a refinery located in Illinois to develop a Process Design Package of a new economizer to be located downstream of the Selective Catalytic Reduction (SCR) unit of the FCC CO Heaters. Refinery wanted to recover the heat by installing a waste heat recovery unit at the SCR outlet.
FIS studied the existing SCR reactor housing arrangement. FIS proposed to install a new economizer in the space provided for future installation of catalyst. The design case absorbed heat duty of the economizer is 26.7 MMBtu.hr.
SCR Unit Description:
The FCC CO heaters were used to burn flue gas generated in the catalyst regenerator from catalytic cracking unit. The heaters are fuel gas fired and operated at 70% excess air. The heat generated by the combustion was used to preheat the FCC charge feed and generate steam in the convection section. The flue gas temperature leaving the convection section was 690°F. Ammonia Injection Grid (AIG) was used to inject air/ammonia mixture in the flue gas stream. The flue gas was then routed to a SCR reactor housing for NOx reduction. The SCR reactor has four catalysts modules. Top three modules are filled with catalyst and bottom module is provided for future installation of catalyst.
The reactor housing was provided with structural support/column at the center. Removable doors were also provided for replacing the catalyst. The flue gas temperature leaving the last module was 680°F and contains 0.18 vol% of SOx.
The client wanted to recover the heat by installing a waste heat recovery unit in the bottom module.
The proposed economizer is designed to cool the flue gases from 680°F to 502°F. The flue gases flow downwards, and water flows upwards respectively in the counter current flow pattern. Internal baffles were provided to avoid bypass of flue gas and to prevent noise vibration.
FIS also analyzed the possibility of vibration and noise by estimating vortex shedding frequency, acoustic frequency and natural frequency. The potential for tube damage due to fluid elastic instability was also assessed and was found to be negligible.
The economizer was successfully commissioned in February 2018.