Length scale internal modes

Inelastic neutron scattering experiments have shown that strong structural similarities exist between P-cristobalite and amorphous silica, but also HP-tridymite over a length scale up to 1.0 nm. This has been interpreted in terms of the internal flexibility of the Si04 tetrahedra, as deduced from the rigid unit mode (RUM) model (see Appendix B). This internal flexibility will distort the local structure of the crystalline phase such as P-cristobalite to achieve a disorder similar to that found in the amorphous phases, silica gel, or silica glass. 

On the other hand, the existence of a concentration-independent crossover probe diameter d (R, R is consistent with polymer solution models based on an assumed dominance of hydt ynatnic interactions in nondilute solution. Models such as the hydrodynamic scaling model (6.7) identify the chain radius as the primary solution length scale at all concentrations at which the model applies. With this identification, a crossover from small-probe to large-probe behavior, perhaps correlated with differential ability to interact with internal chain modes, would at all concentrations occur over the same range of d/R, or d/R, precisely as observed experimentally. 

The feed used for all experiments was straight-run gas oil obtained from a heavy crude oil, whose properties are presented in Table 7.3. The commercial catalyst used for the experiments was a presulfided cobalt-molybdenum supported on y-alumina. The reactor was loaded with 99.43 g (100 mL) of powdered catalyst previously crushed and sieved, the properties of which are also given in Table 7.3. The bench-scale reactor is operated in downflow and isothermal mode provided with independent temperature control of a three-zone electric furnace. The internal diameter of the reactor is 2.54cm, and at the center, a thermowell of external diameter of 0.635 cm was placed. Catalytic length was of 25.2 cm (Figure 7.14). 

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