(157c) Cfd Modelling of Turbulent Fluidisation in Fcc Unit Strippers

Recently, the BP refinery at Bulwer Island, Brisbane, planned to install new internal baffling in their FCC plant stripper in order to allow higher flow rate of solids. CSIRO Minerals carried out CFD modelling of the fluidised bed to test four stripper designs for various solids and steam stripping rates to determine the design most likely to be optimum, and to generally obtain better understanding of the internals and gas sparge location on stripper operation. Most of the internals tested were of the disk-doughnut design with a hat style disk, and an inclined baffle as the doughnut. The CFD technique used was a two-fluid multi-phase model and the computations were undertaken using the CFX4 software. The simulations show that conventional disk-donut designs operate in a mode where steam rises in a turbulent annulus zig-zagging through the gaps between the hats and wall baffles. Steam accumulates under the hats and wall baffles forming relatively stable voids which periodically ?under-flow' into the turbulent annulus. The central regions are less turbulent ? catalyst flows downward to the next hat, at which point it slides down the upper surfaces until it reaches the edge. Substantial contacting occurs here as the catalyst and steam must pass each other at the ?pinch-points' created by the gaps between the baffle edges. Stripping efficiency, flooding, and entrainment of hydrocarbon into the standpipe were all investigated in the simulations and the four designs compared on the basis of each characteristic.