Center cracking

The main components of FCC catalysts are Zeolite Y, e.g., REY orUSY as the major active component (10 to 50%), and a binder that is typically an amorphous alumina, silica-alumina, or clay material. In addition to these main components, other zeolite components, e.g., ZSM-5, and other oxide or salt components are quite frequently used additives in the various FCC catalysts available on the market. The addition of 1 to 5% ZSM-5 increases the octane number of the gasoline. ZSM-5 eliminates feed compounds with low octane numbers because it preferentially center-cracks n-paraffins producing butene and propene [14], These short-chain olefins are then used as alkylation feedstocks... 

Studies have also been reported in which the type of zeolite was varied. ZSM-5 was compared with HY as an acid function in composites with Co and NiMo/Al203 (33, 149). As might be expected, the ZSM-5 additive did little to improve the HDS of 4,6-DMDBT, as the pore size is too small to allow facile entry to 4,6-DMDBT. Mechanistic studies were conducted at higher temperature (360°C), at which cracking was severe. More than 80% of the desulfurization of 4,6-DMDBT was found to occur via center cracking of the C-C bond joining the aromatic rings. Toluene was a major product. Unfortunately, if such a scheme were to be applied to gas oil or diesel fuel, the losses in desired product would be excessive. 

Analogous results can be obtained using stress intensity factors.31,84 For a small center crack in a tensile specimen, Eqns. (30) and (32) give a steady-state result, at large crack lengths,84... 

Fig. 8. Center cracked panel and paths for J integral evaluation.

Fig. 9. Results for the center cracked panel from J integral and crack tip closure.

Figure 13 shows the results for the ratios of platinum-component to acid-component of 25/75, 50/50, and 75/25. It will be noted that the activity for isomerization decreases in that order, i.e., with decreasing acidic component (in spite of increasing Pt-component). Similarly the majority, at least, of cracking to C3 and C4 paraffins (the center-cracking products) decreases similarly with decreasing acidic component. This is consistent with the picture that both reactions are rate controlled by the silica-alumina component. However, the converse is true for the appearance of Ci and C2 products, indicating that a major portion of these products is obtained... 

Figure 4. Fatigue crack growth rates in Ti-6-4 center crack geometry specimens, M(T) in the weld nugget. At stress ratio of 0.05 crack growth rates in the weld nugget are 4 times faster than the parent material, mill annealed Ti-6-4. Crack growth rates at R=0.8 were unaffected. Micrograph shows the weld nugget micro structure.

EXAMPLE 1 - THROUGH-THICKNESS CRACK. The case of a center-cracked plate, subjected to constant-amplitude loading, is considered to provide physical insight. 

The fatigue life of the center-cracked plate is then obtained by straightforward integration of the foregoing rate equation ... 

CSZ-1 is a partially twinned faujasite zeolite [39,40]. On CSZ-1, a trimodal distribution of hydrocracked products from heptadecane is obtained (Fig. 11). The maxima at C5 and Cj2 in the product distribution are interpreted as cage effects occurring in the hypocages, while the maximum at Cg-C9 is the result of center cracking of the feed. The latter is a feature of regular and unobstructed hydrocracking, due to catalysis in hypercages. 

A second fracture parameter is also provided by linear elastic theory, namely, the energy G required to create unit area of fracture surface. Again there is a critical value, of G at which the crack begins to propagate, and a configuration-dependent formula which gives G in terms of measurable quantities. For the center crack mentioned above, the formula for G is... 

FIGURE 3. (a) Cohesive and (b) adhesive cases of the center crack in an infinite sheet. Crack length is c and point coordinates are X, Y. 

FIGURE 6. Center-crack test specimen for adhesive failure energy. 

For an edge crack, the crack length is a, for a center crack the crack length is 2 a. Most of the past research of stress corrosion cracking utilizes the fracture mechanics approach in which the relation between the initially applied stress, stress intensity factor Ki and the time to failure of pre-cracked specimen is observed. The time to failure increases as Ki decreases from the level required for the fracture to occur (Kic, fracture toughness) to a threshold level (Kiscc) below which SCC does not occur. 

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