Zeolite catalysts riser cracking

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. 

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 The process has been largely superseded by fixed bed units employing precious metal catalysts (Hetsroni, loc.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.

Low yields of butadiene are observed during riser cracking with zeolite Y based catalysts to date, however, there is no evidence for increased diene formation with ZSM-5. 

Table 31 provides some details of zeolite catalyst usage. The best kuowu is that of zeohte Y as a fluidized bed catalyst (FCC) to crack crude oil for gasoliue production. The zeohte is used as a promoter aud comprises up to 50% of a small composite with a clay or silica binder. As such it can remain stable throughout many cycles in the catalyst riser (at 480-520 °C), where it meets the downstream of crude oil followed by a steam blast to release the cracking products, and then a stream of air in a regenerator (590-730 °C). This regeneration process cleans the catalyst of coke and the residual oil products. 

The cracking of PE/LCO and PP/LCO blends over HZSM-5 zeolite catalysts in the riser simulator at 450°C led towards mainly a C5-C12 hydrocarbon fraction of aromatic nature and a low yield of C1-C2 gases and coke. 

Recently, new upflow operation (riser cracking) has become popular, through the development of highly active zeolite catalyst (V15). 

DCC units may be operated in two modes maximum propylene (Type I) or maximum iso-olefins (Type II). Each operational mode utilizes unique catalyst as well as specific reaction conditions. DCC-I uses both riser and bed cracking at more severe reactor conditions, while DCC-II utilizes only riser cracking like a modern FCC unit at milder conditions. The DCC process applies specially designed and patented zeolite catalysts. 

Due to the initial low catalyst activity, no attention was placed on a quick separation of catalyst and hydrocarbons in the early days of FCC. However, after the development of zeolite-based catalysis and riser cracking combined with improved catalyst stability, riser outlet temperatures of 970°F and greater were observed. It was observed that quick disengaging of hydrocarbons reduced dry gas and delta coke yields. 

The development of FCC catalysts can be easily separated into two periods characterized by 1) synthetic catalysts and 2) zeolite based catalysts. The synthetic catalyst era began in 1942 and was made obsolete by Plank and Rosinski s (Mobil Oil) discovery of application of synthetic zeolites in 1964 (33). The inclusion of zeolites into FCC catalysts completely transformed the face of FCC within a relatively short period of time. This benchmark discovery resulted in an uncounted number of unit revamps in order to transition from bed cracking to riser cracking. The history of catalyst manufacturing plants is chronicled in Table IV. 

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