Secondary crack

Stabilized by hydrogen transfer. The stabilized free radicals undergo secondary cracking reactions as they come in contact with the hot coke. 

Close examinations of the internal surface under a low-power stereoscopic microscope revealed— in addition to the open cracks illustrated—numerous tight secondary cracks. Tube surfaces were smooth, uncorroded, and free of deposits. 

Figure 9.12 Discontinuous, transverse cracks close examination of the photograph will disclose at least five tight, secondary cracks.

Close visual inspection of the blunt, fractured ends revealed small secondary cracks along the internal surface. Internal surfaces were covered with a layer of deposit that was highly alkaline (Fig. 9.18). 

In a cat cracker, a portion of the feed, mostly from secondary cracking and polymerization reactions, is deposited on the catalyst as coke. Coke formation is a necessary byproduct of the FCC operation the heat released from burning coke in the regenerator supplies the heat for the reaction. 

Behar, F., Audibert, A., Villalba, M., "Secondary Cracking of Crude oils Experimental Study" - Submitted to Energy and Fuel (1988). 

In the hydrocracking process, this phenomenon is exploited to shift catalyst selectivity from the naphtha to the distillate products. Here the wide separation of sites is exploited to minimize the potential for secondary cracking in initial products and intermediates. This, along with the introduction of escape routes for the primary product tends to preserve the higher molecular weight hydrocarbons, thereby producing more dishllates [49, 61, 62]. 

The desired primary liquid products of biomass pyrolysis react readily in secondary cracking reactions, shifting the product yield from liquids to gases. Those cracking reactions are assisted by contact of the volatiles to the remaining solids, which can be reduced significantly by running the fast pyrolysis in a reaction cyclone [79, 80]. A simplified flow sheet of such a laboratory plant for the fast pyrolysis of biomass is shown in Fig. 15.9. 

Wet fluorescent magnetic particle inspection of the remaining journal fillets on this crankshaft revealed the presence of a number of secondary cracks in the same orientation and position as the failure initiation site. A metallographic specimen, including the matching halves of the fracture from the initiation site in the fillet of main journal, was cut, mounted, polished and etched in 2% nital, in preparation for examination with an optical microscope. The polished and etched specimen is shown in Figure 7.44. 

Figure 7.52 Secondary cracks. (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)...

This is called primary cracking. By secondary cracking reacdons (reactions II and m), the entities thus formed give rise, at various points of their hydrocarbon chain, to a number of light produce, rich in olefins, whose composition and yidd depend on the operating conditions sdected. 

---