CFD modeling of LDPE autoclave reactor to reduce ethylene decomposition: Part 2 identifying and reducing contiguous hot spots

Abstract

CFD was used to study local decompositions in an LDPE autoclave reactor by identifying, characterizing, and tracking the trajectories of contiguous hot spots (CHS). Local decomposition of ethylene occurs in very short time and spatial scales, potentially leading to thermal runaway and global decompositions as temperature and pressure increase beyond a recoverable threshold. The addition of wall baffles proved to increase mixing via local shear which reduced the average temperature throughout two zones. CHS were defined as any contiguous material at a temperature above 296.7 °C. CHS formed in the bulk flow but were quickly dissipating as they encountered strong mixing currents. CHS were found to have a high Biot number, describing that conduction was the limiting mechanism in removing heat. The limiting nature of conduction within a volume implies the importance of minimizing the volume of CHS; a sizeable CHS could allow a local spike in reaction rate that enables a local decomposition to become a global decomposition. Overall, under stable operating conditions the CFD model did not predict conditions which were conducive to global decomposition events resulting from CHS. Corroborating our result is the fact that current plant reactors employing the same operating conditions as CFD also does not report any recent decompositions.