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Simulation of pressure drop and coke deposition in the grid of a scrubber Subudhi, Nirmalkumar

Abstract

Hot vapours from bitumen coking are contacted with bitumen fractions and gas oils in scrubber systems in order to cool the vapour and remove the heavy components. In fluid coking, the scrubber contains a grid of layers of structured packing as well as a section of sheds. The pressure drop in the scrubber grid increases over time due to the formation of coke on the grid surface. This build up eventually results in the shutdown of the fluid coker. The objective of this study is to understand the pressure drop change in the scrubber grid due to the formation of coke. HYSYS was used to calculate physical and thermodynamic properties in the system and to model the contact and separator processes. The Bravo-Rocha-Fair (BRF) model was used to calculate pressure drop for Koch-Glitsch Flexigrid #2 packing in counter-current flow. Liquid droplets of high molecular mass species from the coker are present in the vapour which enters the grid. Transport and attachment models for the droplets are used to calculate the total mass of droplets which stick to the surface per unit time. The +524°C heavy components in the droplets are assumed to undergo reactions to form volatiles and toluene insolubles (coke). The mass of carbonaceous solids formed per unit time is calculated using the available coking kinetics. As the coke layer grows, the voidage in the packing decreases, and the pressure drop increases. Therefore the pressure drop build-up calculation involves using HYSYS to calculate the contribution of droplets in the vapour and all fluid properties, the BRF model to calculate pressure drop in counter-current multiphase flow, a mass transfer model to evaluate transport of fine droplets to both dry and wetted portions of the grid, an attachment model to determine the quantity of droplets which adhere to the surface, and a coking kinetics model to calculate the solids build-up. The attachment coefficient was set for a base case to give an increase in pressure drop of 1" H₂0 over a one year operating time. Two extreme cases were considered - a highly temperature sensitive (high activation energy) adhesion process, and a nearly temperature independent (low activation energy) adhesion process. Calculations were undertaken to illustrate the roles of wetting of the packing, droplet size, and wash oils flow and temperature on the deposit build-up with time. This model can, in principle, be extended to other scrubbers for multi-component hydrocarbon mixtures.

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