Retrograde phenomena

The pressure at which this dissociation is predicted to occur is called the hydrate pseudo-retrograde pressure at T. Pseudo-retrograde behavior is defined as the disappearance of a dense phase upon pressurization, which is counter-intuitive. This behavior resembles, but is not strictly the same as, vapor-liquid retrograde phenomena (de Loos, 1994). 

Pseudo-retrograde phenomena are predicted to occur between the temperatures of 277.6 and 278.3 K. With a pressure increase of up to 5 bar, sll hydrates will dissociate at any temperature in this range. The lines are model predictions and the circles are experimental observations of hydrate dissociation obtained in the TUD laboratory. As can be seen in Figure 5.19, the TUD hydrate dissociation data do confirm the pseudo-retrograde melting. However, note that the Aq-sII-Lhc predictions deviate 0.2 K from the data. It is usually assumed that hydrates never dissociate with an increase in pressure. However, both measurements and... 

Figure 5.19 P versus T phase diagram for ethane (0.3) + propane (0.7) + water (excess) system with pseudo-retrograde phenomena.

I m. 26. Pressure-temperattire diagram in tlie vioinity of the critical point of a two-component system which exhibits retrograde phenomena. 

Unfortunately, the terminology applied to retrograde phenomena has not yet been standardized in the literature. The phenomena described as retrograde vaporization in the preceding pages are sometimes referred to as retrograde condensation and vice versa. Further-... 

Relative permeability ratio, 168 Reservoir saturation equation, 160 ff. Retrograde phenomena, 60 ff., 75... 

Like gelatinization, retrogradation phenomenon can be studied by DSC. For instance, if gelatinized corn starch is stored in conditions suitable for retrogradation (i.e. 5 °C for several hours or days) and a DSC scan is performed, two endotherms peaks will be detected (Figure 2.3). 

The crumb structure also changes, although at a lower rate. The crumb becomes firm, its elasticity and juiciness are lost, and it crumbles more easily. The so-called staling defect of the crumb is basically a starch retrogradation phenomenon (cf. 4.4.4.14.2) which proceeds at different rates with amylose and amylopectin. On cooling bread, the high-molecular amylose very rapidly forms a three-dimensional network and the crystalline states of order of amylose/lipid complexes increase. These processes stabilize the crumb. 

Neuroanatomists have taken advantage of the phenomenon of fast retrograde transport to locate remote nerve cell bodies in the CNS of an experimental animal that are connected to an identified axonal fiber tract whose origin is uncertain. The tracer material [purified horseradish peroxidase (HRP) enzyme] is injected in the region of the axon terminals, where it is taken up by endocytosis and then is carried by retrograde axonal transport over a period of several hours to days back to the nerve cell body. The animal is sacrificed, and the enzyme tracer is localized by staining thin sections of the brain for peroxidase activity. 

The culprit is the phenomenon of retrograde condensation, which wa6 previously discussed In connection with hydrocarbon dewpoints. This can best be understood by looking at a graph of equilibrium ratios, commonly called K-values, as shown in Figure 2. We have cross-plotted a limited number of curves, to avoid confusion while Illustrating our point. 

Since in the critical point the bubble point curve (l+g—tf) and the dew-point curve (l+g-+g) merge at temperatures between 7C and 7 , an isotherm will intersect the dew-point curve twice. If we lower the pressure on this isotherm we will pass the first dew-point and with decreasing pressure the amount of liquid will increase. Then the amount of liquid will reach a maximum and upon a further decrease of the pressure the amount of liquid will decrease until is becomes zero at the second dew-point. The phenomenon is called retrograde condensation and is of importance for natural gas pipe lines. In supercritical extraction use is made of the opposite effect. With increasing pressure a non-volatile liquid will dissolve in a dense supercritical gas phase at the first dew point. 

Orgasm is a psychosensory phenomenon in which the rhythmic contraction of the pelvic muscles is perceived as pleasurable. Orgasm can occur without either erection or ejaculation or in the presence of retrograde ejaculation (Gingell and Lockyer, 1999). 

Axons transport materials from the cell body to the synaptic terminals (anterograde transport) and in the opposite direction (retrograde transport). Anterograde transport may occur slowly (a few millimeters/ day only) or fast (up to 400 mm/day), whereas retrograde transport is always fast. Diffusion in the perpendicular direction (not along axons) is much more limited as the membranes and the myelin sheath form obstacles. Therefore, diffusion is not isotropic (same in all directions) in the WM. This phenomenon is known as diffusion anisotropy . In contrast, diffusion is rel-... 

Consider the enlarged nose section of a single PT loop shown in Fig. 12.5. The critical point is at C. The points of maximum pressure and maximum temperature are identified as MP and MT. The dashed curves of Fig. 12.5 indicate the fraction of the overall system that is liquid in a two-phase mixture of liquid and vapor. To the left of the critical point C a reduction in pressure along a line such as BD is accompanied by vaporization from the bubble point to the dew Point, as would be expected. However, if the original condition corresponds to Point F, a state of saturated vapor, liquefaction occurs upon reduction of the pressure and reaches a maximum at G, after which vaporization takes place until the dew point is reached at H. This phenomenon is called retrograde condensation. It is of considerable importance in the operation of certain deep natural-gas wells where the pressure and temperature in the underground forma-... 

Starch is likewise physically unstable. Pastes and sols tend to become less soluble on standing, as evidenced by increase in viscosity and opacity and the eventual precipitation of a solid phase. This phenomenon has been termed retrogradation, and is attributed specifically to the A-fraction. Since most separations are based on differential solubility of the several components, retrogradation may seriously disturb the amount and character of the fractions. This is particularly true of those methods where the starch is maintained in solution for extended periods of time. 

Two further interesting points emerge from a study of fig. 16.5. If the system is maintained at a pressure and the temperature raised, initially only liquid is present. When the liquid curve is crossed at G, A aporization begins. Now, in general, a horizontal line will cut the curve again at a point on the vapour branch of the curve, and this will correspond to complete vaporization. In this particular case, however, the line cuts the liquid curve again at D, between L and K, and at this point the vapour disappears. This phenomenon is known as retrograde... 

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