(395e) Model for Heterogeneous Feed Vaporization: Effect of Catalyst Particle Porosity

In an FCC riser, liquid feed is injected through nozzles into a flow of hot fluidized catalyst. The feed forms small droplets that interact with their surroundings, exchanging mass and energy. Hot catalyst particles provide thermal energy to heat-up feed droplets and vaporize them. Feed vapors are subject to cracking reactions on the catalyst active sites. The nature of these chemical reactions is to lower the average molecular weight of the hydrocarbon vapor mixture and to increase the number of gas molecules. Because the feed vaporization and chemical reactions are endothermic, the temperature drops along the riser. The increase in number of moles compensates the temperature drop and provides uplift for the catalyst in the riser.

Feed vaporization in the riser is a very complex process since it involves simultaneous heat and mass transfer for a multicomponent hydrocarbon blend with complex physical properties and rheology. There are two major approaches to describe this process: 1) Homogeneous heat transfer between hot catalyst particles and feed droplets through the gas phase conduction and convection mechanisms. 2) Heterogeneous heat transfer involving direct collisions between catalyst particles and feed droplets with accompanying energy transfer. Direct contact between solid catalyst particle and liquid droplet may also involve transfer of a portion of the liquid from the feed droplet onto the catalyst particle surface.

A mathematical model has been formulated to describe the interaction of liquid droplet with a hot porous surface. The model takes into account the convective circulation and diffusion of liquid components inside the droplet and deformation of the outer surface due to contact with the catalyst particle solid interface. Numerical calculations combine Galerkin finite element discretization scheme with implicit Adams-Bashforth second order predictor-corrector time integration algorithm.

This paper addresses some fundamental questions about the heterogeneous mechanism of feed vaporization and interactions between liquid droplets and porous catalyst particles based on first principles by direct solving of mass, momentum, and energy conservation equations.