Supercritical crack

Accumulation of cracks at the boundary aj d) and their subsequent merging to generate a supercritical crack. Let the concentration of initial submicrocracks is ac. As a result of cyclic impact they transform into supercritical state for bitumen matrix in characteristic time intervalt. Then they propagate fast in the matrix to the size aj d) and then stop. Concentration c of the stopped cracks varies... 

Outlet Superheater (SH) header of Unit 3 (600 MW. supercritical multi-fuel l of an ENEL power station it consists of 2 twin and independent bodies (22 m length, 488.5 mjn internal diameter, 76.2 mm thickness material SA 430 TP 321H stairdess steel). This header has suffered from relevant cracking problems in assembly welds after 108.000 hours of service and... 

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Keywords thermal and catalytic cracking, endothermic fuel, supercritical conditions... 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

At the lower temperatures (375 and 400 °C), the n-dodecane conversions is higher with a catalyst. Moreover, the products distributions are very different. This is explained by the cracking mechanisms (free radical and carbocation) and maybe by the supercritical conditions. This is no more the case at 425 °C as the catalysts seem to deactivate rapidly by coking. So the formed products come mainly from the thermal cracking. 

Cracking of PS because of capillary forces can be circumvented if one avoids crossing the liquid-vapor boundary in the phase diagram of the solvent. This is the case for supercritical drying [Ca4] or freeze drying [Ami], as shown in the inset of Fig. 6.12. 

Although widely used, solvent extraction procedures have been demonstrated as sensitive to such variables as the content of humic matter and moisture within samples. Supercritical fluid extraction appears to be a more robust procedure. Thermal extraction procedures are sensitive to the size of the soil sample in some cases since the technique can result in cracking higher-molecular-weight... 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

Note Since liquid and vapour are indistinguishable in a supercritical fluid, there is no capillary pressure to cause shrinkage and cracking of the pores formed in the gel. 

Fig. 6.2.9. Indentation /strength model system, (a) Vickers indenter, peak load P, generates median/radial crack, characteristic dimensions (value c0 at completion of contact cycle, with further post-indentation, supercritical extension to c0, if exposed to moisture), (b) Tensile field a in combination with residual ( ghost ) contact field, drives crack system to failure. (After Marshall and Lawn, 1980)...

The microphotographs of Fig. 1 la and b demonstrate the brittle fracture of a film occurring in response to an impact at the point A (the initial state— frame a) with a needle, developing a pressure of 108 Pa for the time 0.1 s. Frame b shows that the impact produces a network of cracks in the grain affected. An analogous effect on a film preirradiated with a supercritical dose of y-rays initiates a traveling wave of chemical conversion (Fig. 17e) similar to those described in Section VIII. 

Figure 7.41 309 Unifuse overlay on 1-1 /4Cr-l /2Mo boiler tube in the wall blower region after service for about 7 yr in a supercritical unit of the coal-fired boiler (a) surface appearance of the weld overlay, weld beads are still clearly visible (b) cross-section of the weld overlay showing no corrosion and no cracking on the weld overlay at the fusion line (Reproduced from COM 1997 with permission from the metallurgy Society of CIM)...

T. Moriya and H. Enomoto, Characteristics of polyethylene cracking in supercritical water compared to thermal cracking. Polymer Degradation and Stability, 65, 373-386 (1999). 

---