Modeling of Feed Vaporization in Fluid Catalytic Cracking

Abstract

Feed vaporization in the fluid catalytic cracking (FCC) process affects the yield of valuable products in risers and downers. Feedstock droplets sprayed into a fluid cracker undergo both homogeneous vaporization in the gas phase and heterogeneous vaporization as they collide with catalyst particles. This paper models both processes. Homogeneous vaporization, with both convection and radiation heat transfer, cannot completely vaporize oil droplets with diameters larger than 10 μm. Spraying droplets into a dilute cloud of catalyst particles forms a “tunnel” of hydrocarbon vapor that minimizes droplet−catalyst contact and vaporization. Alternately, spraying the droplets onto a dense jet of catalyst particles does not provide proper heat transfer and vaporization. Modeling indicated that the best way of ensuring fast vaporization of oil droplets in FCC risers and downers is to spray the droplets onto a jet of catalyst particles with a porosity ranging between 70 and 90%.

Modeling of vaporization and cracking of liquid oil injected in a gas–solid riser

“…This model predicted order of magnitude faster evaporation rates. Mirgain et al (2000) used the concept but distinguished three different types of collisions. The first particle to be hit by the droplet gets covered with liquid.…”

“…This type of collision is referred to as impact with transfer. Mirgain et al (2000) applied the approach of Buchanan (1994). The vaporization time was then estimated from the time needed for the liquid to impact enough particles.…”

Comparison of vaporization models for feed droplet in fluid catalytic cracking risers

“…On contrary to the theory proposed by Martin (1990) and Buchanan (1994), Mirgain et al (2000) argued that Leidenfrost effect possibly does not occur during droplet-particle collision in FCC riser primarily because of high Weber number (We > 5000) of the injected droplets. They reasoned since critical We number for droplets is reported to be 80 in Leidenfrost regime (Wachters and Westerling, 1966), droplet must break under this condition.…”

“…They reasoned since critical We number for droplets is reported to be 80 in Leidenfrost regime (Wachters and Westerling, 1966), droplet must break under this condition. Mirgain et al (2000) also critically analyzed the effect of homogeneous and heterogeneous heat transfer modes on the vaporization time in FCC riser and showed that within the typical residence time in a riser, complete vaporization of the feedstock is not possible if only the homogeneous mode is considered. The residence time in an industrial scale FCC riser is usually defined as the time since the feed is injected into the bottom of the riser reactor until the product vapours comes out at the top of the riser.…”

“…It was further shown in the studies of Mirgain et al (2000) that when the heterogeneous mode (presence of catalyst particles) is included, heat transferred to the droplet is enhanced resulting in reduction of vaporization time which is able to completely vaporize the feedstock in the given residence time. Three different droplet-particle collision mechanisms were proposed between a liquid-covered and a liquid free catalyst particle: (i) two particle stay bonded by the liquid film (ii) liquid film is shared between the two particles and (iii) liquid film is completely transferred to the other particle.…”