Selectivity, cracking

Table 13.12 Effect of framework type on cracking selectivities for n-octane at 500°C [95].

Hydrocarbon Cracking Selectivities with Dual-Function Zeolite Catalysts... 

Thus, the dominant contributor of cracking selectivity of Super D is expected to be the REY zeolite present in the catalyst. Yields of Cs to C paraffin isomers measured during the cracking of the gas oil at 500 C by the steam treated REY catalyst are shown in Table III. The yield of branched paraffins was generally five to ten times higher than the yield of normal paraffins and the major (>50%) portion of the branched paraffin yield consisted of monomethyl paraffins. 

Harding, R. H., Zhao, X., Qian, K., Rajagopalan, K., and Cheng, W.-C. Fluid Catalytic Cracking Selectivities of Gas Oil Boiling Point and Hydrocarbon Fractions. Industrial Engineering Chemistry Research 35 (1996) 2561-69. 

The variation of cracking selectivity in the conversion of alkanes over substituted H-Ga-MFI and H-Al-MFI zeolites has been correlated with the basicity of the C-C bond of the alkane, while the selectivity toward dehydrogenation was found to be related to the attenuation of the acid strength of the zeolite [251]. 

Selectivity as a Function of Alumina Content. At a reaction temperatures of 204° and 343°C, Csicsery (7, 8) found that the major reaction of o-ethyltoluene over H-mordenite was isomerization to the m- and p-isomers. Results of our study, listed in Table I and plotted in Figure 7, show that this is also so at 360°C. Isomers are the major reaction products over both the parent H-mordenite at 360°C and aluminum-deficient H-mordenite at 360°-505°C. At the higher temperatures, H-mordenite cracking selectivity becomes predominant. 

Zeolitic Catalyst—Since the early 1960s. modern cracking catalysts contain a silica-alumina crystalline structured material called zeolite. This zeolite is commonly called a molecular sieve. The admixture of a molecular sieve in with the base clay matrix imparts desirable cracking selectivities. 

Weekman Jr., V.W. and D.M. Nace, Kinetics and Catalytic Cracking Selectivity in Fixed, Moving and Fluid-Bed Reactors., AIChE Joum., 16,397,1970. 

If a larger fraction of the sites are more accessible, the detrimental effect of poisons on the resid cracking selectivity will be less. 

Pitting corrosion Intergranular corrosion Stress-corrosion cracking Selectively leaching Erosion-corrosion... 

From the thermodynamic point of view, the hydroisomerization reaction is not pressure sensitive. However, because the catalyst contains the acid function, hydrocarbon cracking is an unavoidable side reaction. The cracking reaction however should depend on the total pressure. Table 7.4 shows laboratory results obtained in a bench scale reactor at 250°C (482°F), H2/HC = 1 with a synthetic feedstock containing 50wt% of n.Cs, 25 of iCs, 20 of n C6, 5 of methylcyclopentane and no heptane (Feed 1). At a liquid hour space velocity (LHSV) of 2h an increase of the total pressure from 20 bar to 30 bar reduces the cracking selectivity S = Z C4/Z HC from 1.6 to 1.1 wt.-%, whereas at a LHSV of 1 h 1 no effect can be observed. 

Table 7.5 Effect of pressure on cracking selectivity in hydroisomerization of synthetic feedstock on Pt-H-MOR Conditions T=250 °C, Total pressure = 20 bar, LHSV = 2 h 1, Feed 2...

CDTECH Butadiene C4+ from naphtha cracking Selective hydrogenation of C4 acetylenes in a distillation column to produce low acetylene feed for butadiene extraction 2 1998... 

The foregoing can be explained with a simple supply and demand model of cracking [4,10] if a larger fraction of the sites are more accessible, the detrimental effect of poisons on the resid cracking selectivity will be less as both the poisons and the large molecules compete for the most accessible sites. 

Weekman, V. W., and Nace, D. M., Kinetics of catalytic cracking selectivity in fixed, moving and fluid-bed reactors. AIChEJ. 16,397 (1970). 

Figure 3. Effect of O2/LPG ratio on performance of selected catalysts in LPG oxidation at 585°C 1-LPG conversion, 2 - olefins selectivity, 3 - oxygen conversion, 4 - cracking selectivity...

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