Retrograde behavior

The retrograde behavior of Case 5 has vanished, and Case 6 has become worse than the zero-order fit of Case 1. The recommended fit for the reaction rate at this point in the analysis, = 0.516n, is very similar to the original recommendation, but confidence in it has increased. 

An understanding of the phase behavior of a particular system of interest is important because complex results can sometimes occur. A dramatic example, which occurs frequently for solubilities in supercritical systems, is the retrograde behavior. Figure 3 clearly shows the presence of a retrograde region. For an isobaric system at some pressure, such as 12.7 MPa (1841.5 psi), an increase in temperature of a solution of ethylene and naphthalene from 300 to 320 K results in an increase in the equilibrium solubility of naphthalene. This behavior is typical of liquid solvent systems. For the same increase in temperature (300 to 320 K) but at a pressure of 8.1 MPa (1174.5 psi), the solubility of naphthalene decreases by nearly an order of magnitude. Because this latter behavior is the opposite of typical liquid solvents, it is termed retrograde solubility. 

A petroleum reservoir is discovered at 3000 psia and 130°F. If the reservoir fluid is represented by the phase diagram of Figure 2-32, will the reservoir contain liquid or gas What about the fluid of Figure 2-33 Figure 2—35 Figure 2-36 Which of the four fluids will exhibit retrograde behavior at reservoir conditions ... 

Retrograde gases exhibit a dew point when pressure is reduced at reservoir temperature. The heptanes plus fraction is less than 12.5 mole percent. Retrograde behavior will occur at reservoir conditions for gases with less than one percent heptanes plus, but for these gases the quantity of retrograde liquid is negligible. 

Retrograde gases are also called retrograde gas-condensates, retrograde condensate gases, gas condensates, or condensates.1,2 The use of the word condensate in the name of this reservoir fluid leads to much confusion. Initially, the fluid is gas in tire reservoir and exhibits retrograde behavior. Hence, the correct name is retrograde gas. 

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). 

Nawaby AV, Handa YP, Liao X, Yoshitaka Y, Tomohiro M (2007) Polymer-C02 systems exhibiting retrograde behavior and formation of nanofoams. Polym Int 56 67-73... 

The equilibrium solubility of Lovastatin in carbon dioxide at 55 C and 75 C and for pressures up to 400 bar was obtained using the HPLC apparatus and reported in Table II and Figure 9. The data exhibit both the abrupt change in solubility above the solvent s critical point, as well as the retrograde behavior, both of which characterize supercritical extraction processes. The solubilities were reproducible to within 5%. 

Gregorowicz, J. de Loos, Th.W. de Swaan Arons, J. Liquid-liquid-vapour phase equilibria in the system ethane J- propane J- eicosane retrograde behavior of the heavy liquid phase. Fluid Phase Equilibria 1993, 84, 225-250. 

Another gxample of retrograde behavior occurs when one starts with the.vapor at... 

Equations of state do predict retrograde behavior. However, since simple cubic equations, such as the Peng-Robinson equation used for illustration in this section, are not accurate in the critical region, retrograde predictions may not be of high accuracy. More complicated, multiterm equations of state are needed for reasonably accurate description of the critical regioitand retrograde behavior. 

The solubility isotherms cross one another. Namely, at pressures roughly ranging from 40 bar to 170-180 bar (zone B of the plot) a retrograde behavior is seen, where the solubility is decreased by a temperature increase, whereas above the crossover pressure the usual dependence is encountered (zone C of the plot). 

Both adsorption from a supercritical fluid to an adsorbent and desorption from an adsorbent find applications in supercritical fluid processing.The extrapolation of classical sorption theory to supercritical conditions has merits. The supercritical conditions are believed to necessitate monolayer coverage and density dependent isotherms. Considerable success has been observed by flic authors in working with an equation of state based upon the Tofli isoterm. It is also important to note that the retrograde behavior observed for vapor-hquid phase equilibrium is experimentally observed and predicted for sorptive systems. 

Figure 5 shows the effect of pressure on the absorption of each BTEX component at 95°F (35°C) at 0.25 US GPM TEG/MMSCFD of gas (2.04 m /h TEG per 10 SmVd of gas). Be reminded that these results have not been experimentally validated at pressures above 1000 psia (6895 kPa). Notice that for pressures below 500-700 psia (3450-4830 kPa), increasing pressure increases absorption but at higher pressures increasing pressure decreases absorption. This decreasing solubility of BTEX components with increasing pressure is similar to isothermal retrograde behavior in gas condensate systems. 

These assumptions are violated at high pressures when the vapor phase becomes nonideal and in the cases where the multicomponent nature of the mixture cannot be neglected. The extreme case of non-Kelvin behavior is the behavior of hydrocarbon mixtures in od-gas-condensate reservoirs. Although the Kelvin equation is apphed to tests of the porous media of the reservoirs [74,75], it cannot be used for modeling of equilibria in such reservoirs, not only because of the high pressure and the multicomponent composition of the mixture but also because this mixture exhibits retrograde behavior when the liquid phase precipitates as the pressure decreases. Such a behavior cannot, in principle, be described in terms of a single-component model. 

The behavior of the mixing volume is illustrated in Fig. 13 for the mixture of methane-n-butane. The phase diagram for this mixture at 300 K, calculated on the basis of the Peng-Robinson equation of state, is shown in Fig. 7b. The critical point corresponds to a pressure of 137 bar and to a molar fraction of methane of 0.77. If the molar fraction of methane exceeds this value, the mixture shows retrograde behavior. The partial volume of butane, Vg 2, is negative in the region of retrograde condensation. 

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