Riser reactor, catalytic cracking

N V Dewachtere, G F Froment, I Vassalos, N Markatos, N Skandalis. Advanced modeling of Riser-type catalytic cracking reactors. Applied Thermal Engineering, 17, 8-10, 837-844, 1997. 

Fluidized-bed catalytic cracking units (FCCUs) are the most common catalytic cracking units. In the fluidized-bed process, oil and oil vapor preheated to 500 to SOOT is contacted with hot catalyst at about 1,300°F either in the reactor itself or in the feed line (called the riser) to the reactor. The catalyst is in a fine, granular form which, when mixed with the vapor, has many of the properties of a fluid. The fluidized catalyst and the reacted hydrocarbon vapor separate mechanically in the reactor and any oil remaining on the catalyst is removed by steam stripping. 

Figure 7.7b shows the essential features of a refinery catalytic cracker. Large molar mass hydrocarbon molecules are made to crack into smaller hydrocarbon molecules in the presence of a solid catalyst. The liquid hydrocarbon feed is atomized as it enters the catalytic cracking reactor and is mixed with the catalyst particles being carried by a flow of steam or light hydrocarbon gas. The mixture is carried up the riser and the reaction is essentially complete at the top of the riser. However, the reaction is accompanied by the deposition of carbon (coke) on the surface of the catalyst. The catalyst is separated from the gaseous products at the top of the reactor. The gaseous products leave the reactor... 

Figure 1731. Fluidized bed reactor processes for the conversion of petroleum fractions, (a) Exxon Model IV fluid catalytic cracking (FCC) unit sketch and operating parameters. (Hetsroni, Handbook of Multiphase Systems, McGraw-Hill, New York, 1982). (b) A modem FCC unit utilizing active zeolite catalysts the reaction occurs primarily in the riser which can be as high as 45 m. (c) Fluidized bed hydroformer in which straight chain molecules are converted into branched ones in the presence of hydrogen at a pressure of 1500 atm. The process has been largely superseded by fixed bed units employing precious metal catalysts (Hetsroni, loc. cit.). (d) A fluidized bed coking process units have been built with capacities of 400-12,000 tons/day.

Riser pipe the pipe in a fluid catalytic cracking process (q.v.) where catalyst and feedstock arc lifted into the reactor the pipe in which most of the reaction takes place or is initiated. 

A model for the riser reactor of commercial fluid catalytic cracking units (FCCU) and pilot plants is developed This model is for real reactors and feedstocks and for commercial FCC catalysts. It is based on hydrodynamic considerations and on the kinetics of cracking and deactivation. The microkinetic model used has five lumps with eight kinetic constants for cracking and two for the catalyst deactivation. These 10 kinetic constants have to be previously determined in laboratory tests for the feedstock-catalyst considered. The model predicts quite well the product distribution at the riser exit. It allows the study of the effect of several operational parameters and of riser revampings. 

Fluid catalytic cracking (FCC) of heavy oil fractions is a well-known process in oil refineries. Numerous books (e.g., 1—3) and publications about the different aspects of this subject are available. This chapter is concerned with the modeling of the transfer line or riser reactor of an FCC unit (FCCU) or of a pilot plant. The riser reactor in FCCUs is a vertical pipe about 1 m in diameter and 10-30 m in height. The hot catalyst coming from the regenerator at about 710 ° C first comes in contact with steam and is fluidized. Then, at a height of some meters above, the catalyst is mixed with the preheated feedstock at about 300 ° C. 

Krishna, Ashok S. Gasoline octane enhancement in fluid catalytic cracking with split feed injection to a riser reactor, PCT International, WO 8702695 (1987). 

Theologos and Markatos (1992) used the PHOENICS program to model the flow and heat transfer in fluidized catalytic cracking (FCC) riser-type reactors. They did not account for collisional particle-particle and particle-wall interactions and therefore it seems unlikely that this type of simulation will produce the correct flow structure in the riser reactor. Nevertheless it is one of the first attempts to integrate multiphase hydrodynamics and heat transfer. 

Theologos, K. N., and Markatos, N. C., Modelling of flow and heat transfer in fluidized catalytic cracking riser-type reactors. Trans. I Chem. E. 70(Part A), 239 (1992). 

Recentlyr Schockaert and Proment [ref. 41] simulated the catalytic cracking of gasoil in both fluidized bed or riser reactors, connected with a fluidized bed regenerator. The kinetic model for the cracking was based upon the lO- lump loodel of Mobil [ ref 42 ]. Only one deactivation function was used for all the coking reactions and it was exponential in the coke content ... 

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