Riser cracker

In the riser cracker, the feed oil is sprayed into the fast upflowing stream of regenerated catalyst. The cracking reaction occurs entirely in this nearly plug flow riser, and the selectivity of desired hydrocarbon fractions is thereby markedly improved. Catalyst circulates smoothly between the regenerator and the riser reactor. The regenerator can be an ordinary fluidized bed or a combination of risers with ordinary fluidized beds. 

It should be noted that, at this point, it is unlikely that pillared clays will replace zeolites for fluidized catalytic cracking, the reason being the hydrothermal instability of the clays at the conditions typically used in a modem riser cracker. Nevertheless, there is ongoing interest in clays and pillared clays as shape-selective catalysts for other, more specific reactions or separations. 

DuPont has commercialized a transport bed process for the production of maleic anhydride. The transport bed reactor is a special fluidized bed reactor similar to a riser cracker reactor used in the petroleum refining industry. In this process, maleic anhydride is converted to maleic acid which is hydrogenation to tetrahydrofuran. 

The understanding of fluidized-beds is far from satisfactory, particularly regarding the fluid mechanics. The value of various models for fluidized-beds, however, lies in providing the framework within which each specific application can be considered. This chapter examines fluidization characteristics, the role that gas bubbles play, and the rationale behind the two-phase theory, which naturally leads to the models based on the two-phase theory. A full section will be devoted to the catalytic cracker, in particular the riser cracker, since it represents the most important application of fluidized-beds to catalytic reactions. This chapter starts with the understanding that the intrinsic rate is essentially the same as the global rate in fluidized-beds, unless the catalyst is deactivated. 

As a last example I want to discuss the catalytic cracking of oil in a riser cracker. Weekman (49. 50) proposed a simple scheme ... 

For a given catalyst and feedstock, catalytic coke yield is a direct function of conversion. However, an optimum riser temperature will minimize coke yield. For a typical cat cracker, this temperature is... 

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... 

In the 1970s more-active zeolite catalysts were developed so that the cracking reaction could be conducted in the transport riser. Recently, heavier crude feedstocks have resulted in higher coke production in the cracker. The extra coke causes higher temperatures in the regenerator than are desired. This has resulted in the addition of catalyst cooling to the regeneration step, as shown in Fig. 17-25. 

In a typical fluid catalytic cracker, catalyst particles are continuously circulated from one portion of the operation to another. Figure 9 shows a schematic flow diagram of a typical unit W. Hot gas oil feed (500 -700°F) is mixed with 1250 F catalyst at the base of the riser in which the oil and catalyst residence times (from a few seconds to 1 min.) and the ratio of catalyst to the amount of oil is controlled to obtain the desired level of conversion for the product slate demand. The products are then removed from the separator while the catalyst drops back into the stripper. In the stripper adsorbed liquid hydrocarbons are steam stripped from the catalyst particles before the catalyst particles are transferred to the regenerator. 

A number of different types of laboratory scale units have been developed to simulate commercial catalytic crackers. These include fixed bed (MAT), fluidized bed, and riser units.(1,2,3) In particular, for simulating commercial riser FCC units which process residue, a riser pilot plant is the preferred choice. 

The original fluidized-bed reactor was the Winkler coal gasifier (patented 1922), followed in 1940 by the Esso cracker that has now been replaced by riser reactors with zeolite catalysts. 

Fig. 4. Quick contact reactor, concurrent downflow cracker. Modified from P. K. Niccum and D. P. Bunn, U. S. Pat. 4,514,284 (1985). 1, catalyst storage 2, recirculating pipe 3, quick contact reactor 4, steam stripper 5, riser regenerator.

Riser Cat Cracker, FCU Piug flow (reactor) mixed flow (regenerator)... 

Total Resid Cracker Side-by-side configuration External straight vertical riser with two sets of multi-directional feed nozzles Two stage regeneration with external cyclones in the second stage... 

---