Authors: Mohammad Abdur Rakib a, Tommy Firmansyah a, Syed Basheer a, Mohamed Al Musharfy a, and Sam Clark b
a ADNOC Refining Research Centre, Abu Dhabi National Oil Company (ADNOC) Refining, PO Box 3593, Abu Dhabi, UAE
b CPFD LLC, 10899 Montgomery Blvd. NE Suite A, Albuquerque, NM 87111, USA
Source: This paper was published in the 13th International Conference on Fluidized Bed Technology in 2021.
Abstract: The regenerator is typically the largest vessel of the Residue Fluid Catalytic Cracking (RFCC) unit, usually operated in the turbulent fluidization regime. In a typical RFCC unit, catalyst regeneration is conducted in a 2-stage regenerator to minimize hydrothermal deactivation of catalyst, due to high Conradson Carbon Residue (CCR) content of the feed. One of problems faced in this regenerator is afterburn or post combustion in the dilute bed region, resulting in high temperatures in the overhead line of the regenerator.
The present study concerns with observed thermal asymmetry and afterburn in a commercial RFCC regenerator, which resulted in reduced RFCC unit severity. Detailed Computational Fluid Dynamics (CFD) model of RFCC regenerator was developed to study the temperature distribution across the dense and dilute phase of the regenerator. This also included the spatial distribution of partly-regenerated catalyst entering the 2nd Stage Regenerator from the 1st Stage Regenerator, and any maldistribution of air and catalyst in the regenerator.
Uniformity of gas flow is always important for good performance of regenerators, and low uniformity in the dense bed of regenerators is usually associated with afterburn and high levels of NOx emissions. CFD simulation revealed strong bias of higher flow near the axis and the outer wall of the regenerator with very little radial/ lateral velocity components, leading to inefficient mixing of gas and catalyst in the bed, and radial temperature gradient and maldistributions of O2 and CO concentration. The O2-rich air was found streaming near the outer walls and the centre of the dense bed region, and the CO-rich combustion product gases from the dense bed further mixes in the dilute bed region proceeding to complete combustion, leading to further temperature spikes.
Additionally, the distribution of partially regenerated catalyst was revealed to be not optimal. Most of the partially regenerated catalyst goes directly upward from the catalyst distributor without significant lateral distribution, further aggravating the problem.