Pseudo-retrograde

For example, at 278.2 K, hydrates form at a pressure of approximately 5 bar and dissociate upon pressurization at approximately 600 bar. A more detailed explanation of the pseudo-retrograde hydrate phenomena can be found in the binary hydrates section which follows. Note that the hydrate formation pressure of propane hydrates along the Aq-sII-V line at 277.6 K is predicted to be 4.3 bar. 

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.

Concealed extrasystoles causing pseudo-AV block. (Look for associated unexpected sudden PR prolongation, combination of what appears to be type I and type II and isolated retrograde P-waves from retrograde conduction of the concealed extrasystole)... 

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