Impact of Local Fluidized Bed Hydrodynamics on the Distribution of Liquid Sprayed into the Bed
Abstract
In Fluid Coking, heavy oil is sprayed into a fluidized bed of hot coke particles that provide heat for the reaction and vaporization of the products, using nozzles located at different radial and axial locations of the reactor. In industrial units, imperfect initial liquid-solid contact results in the formation of wet agglomerates, which have been shown to cause fouling of the stripper sheds.
This thesis aims to reduce agglomerate formation by modifying the bed hydrodynamics in the spray region by adjusting the initial fluidization gas distribution or using a baffle.
Experimental methods were developed to study local gas flow, solids and liquid mixing, and initial liquid-solid contact. In each case, several measurement methods were applied to get reliable results.
Modifying the local bed hydrodynamics improves the initial liquid-solid contact. In industrial cokers, nozzles cannot be extended too far from the wall. The initial liquid-solid contact can be enhanced by directing gas bubbles near the nozzle tip, where they keep the bed fluidized to facilitate solids suction into the jet, and to the first half of the spray jet cavity, where they add solids to mix with the injected liquid.
Adjusting the initial fluidization gas distribution can reduce the proportion of the injected liquid initially trapped within wet agglomerates by 25 %, at a fluidization velocity of 1 m/s (typical of the upper rector sections). Adding a baffle reduces this proportion by 80 % and is even more effective at the lower fluidization velocities corresponding to the lower injection nozzles in cokers. Baffle performance is affected by the baffle geometry: vertical "flux tubes" located in some industrial baffles reduce the beneficial impact of the baffle on the initial liquid distribution. In cokers, baffles are applied to prevent wet solids from reaching the stripper at the bottom of the reactor. With the best baffle for initial liquid-solid contact, the wet solids took 5 to 10 % longer to travel from the spray region to the bed bottom. The baffle did not hinder the rapid mixing of wet solids from the spray region with the hot bed particles, which is required for rapid vaporization.