Cracking of naphthenes

Naphthenes are desirable FCC feedstocks because they produce high-octane gasoline. The gasoline derived from the cracking of naphthenes has more aromatics and is heavier than the gasoline produced from the cracking of paraffins. 

The kinetics of hydrocracking reactions has been studied with real feedstocks and apparent kinetic equations have been proposed. First-order kinetics with activation energy close to 50 kcal/gmol was derived for VGO. The reactions declines as metal removal > olefin saturation > sulfur removal > nitrogen removal > saturation of rings > cracking of naphthenes > cracking of paraffins [102],... 

Whereas over the dual-bed catalyst system, namely Pt/Z12(80) HB(20), a significant improvement in benzene purity up to 94.60% was observed. This is ascribed due to selective cracking of naphthenes over acidic zeolite H-Beta at the bottom bed. 

R1 represents a paraffinic side chain. This means that R1 can be cracked in the same way as was shown in reaction (9.1). At 425°C, the cracking of naphthenic and/or aromatic rings is not possible as this reaction requires a temperature of over 550°C. However, it is especially interesting that hydrogen transfer and hydrogenation of aromatic cores of asphaltenes take place by the mechanism described in reactions (9.4-6) [9]. 

The fact that at the low temperature no cracking of naphthenic rings occurs (which could lead to the nascence of olefins) permits us to say that the quality of the product formed is better if crude oil is thermally processed at a low tempera-... 

Greensfelder, B.S. and Voge H.H., "Catalytic Cracking of Pure Hydrocarbons. Cracking of Naphthenes", Ind. Eng. Chem. 37(11), 1038-1043 (1945c). 

The effect of conversion on the structure of an asphaltene molecule has been reported to depend on the operating conditions and on the presence or not of a catalyst. The effect of thermal processing reaction of a vacuum residue resulted in the selective cracking of the aliphatic or naphthenic side chains of the molecule, leaving the highly condensed aromatic core structure almost intact (see Fig. 16) [116]. 

A naphthene is used for this illustration as we believe that the relative amounts of naphthene cracking versus hydrogen transfer control product distributions and qualities in octane catalyst systems. Gasoline selective catalysts favor hydrogen transfer reactions with these molecules with consequent formation of coke. 

Tables V and VI show that the distribution of molecular types in the liquid products after cracking at 500° and 560°C shifts towards more aromatics, particulary monoaromatics, and polars with increasing boiling point range of the feed. The yields of monoaromatics increase at the higher temperature since they are favoured thermodynamically as dehydrogenation products of naphthenes. Polyaromatics are rejected to coke. The conversion of polars decreases with increasing boiling point of the feed at 500°C, but increases...

Other reactions may also occur. These include carbon formation, hydrocracking or thermal cracking, dehydrocyclization of paraffins to naphthenes, and dehydrogenation of naphthenes to aromatics. These have been discussed in the deactivation of reforming catalysts, in Section 2. 

In any of these cases, an analogy of the initiatory mechanism to that encountered in olefin cracking is clear thus, association with a proton, rather than hydride ion removal (as required for paraffins and naphthenes), normally constitutes the first step in the cracking of both aromatics and olefins. 

Cracking involves aromatic dealkylation and cracking of paraffins, methylene linkages, and naphthenic rings. Aromatic dealkylation is rather easy under current liquefaction conditions (below 450°C) however, the cracking reactions are not facile. Competitive reactions of various species should be carefully considered in catalyst design. 

Like the paraffins, naphthenes do not appear to isomerize before cracking. However, the naphthenic hydrocarbons (from C9 upward) produce considerable amounts of aromatic hydrocarbons during catalytic cracking. Reaction schemes similar to that outlined here (page 131) provide possible routes for the conversion of naphthenes to aromatics. 

Up to 27% benzene, toluene, xylenes (BTX), and 24% ethylene were obtained by cracking a highly hydrogenated coal extract, compared to less than 4% of each from unhydrogenated coal, coal extract, and anthracene oil. The importance of naphthenes as BTX and ethylene precursors was confirmed. 

In a previous paper (7), we have illustrated that diffusion in FCC takes place in the non-steady regime and that this explains the failure of several attempts to relate laboratory measurements on FCC catalysts to theories on steady state diffusion. Apart from the diffusion aspects, Nace (13) has also indicated the limited accessibility of the zeolite portal surface area by comparing the cracking rates of various model compounds with an increasing number of naphthenic rings on zeolite and amorphous FCC catalysts, figure 2. 

With an increase in the number of naphthenic rings, the cracka-bility of the hydrocarbon molecule increases (vide data with SiO -Al O catalyst), while the relative cracking rate by zeolites drops off due to the limited accessibility of the acid sites in the zeolite. 

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