ANALYSIS OF HEATING AND VAPORIZATION OF FEED DROPLETS IN FLUIDIZED CATALYTIC CRACKING RISERS

An analysis of the factors involved in heating and vaporizing feed droplets in a fluidized catalytic cracking (FCC) riser shows the following: heat transfer between gas and catalyst is so fast that the temperature of the catalyst can be considered equal to that of the gas in its immediate surroundings; heat transfer by radiation is much slower than for convection; and the droplet temperature will be equal to the wet-bulb temperature based on the droplet surface composition. An expression for the heat transfer coefficient to an FCC feed droplet is developed which takes into account vapor effusing away from the droplet and also the presence of catalyst particles in the gas phase. Estimates of feed droplet vaporization times with this correlation were made using a number of simplifying assumptions about riser conditions. Vaporization times were proportional to the 1.1-1.5 power of initial droplet diameter. The results suggest that the distribution of droplet sizes is especially important in evaluating FCC feed nozzles; a relatively few large droplets may comprise a large volume fraction of the feed, and these will be much slower to vaporize. For nozzles with good atomization, the major limitation on feed vaporization is probably the macroscopic mixing of feed into the catalyst-rich phase.