Phase behavior at high pressures

Meilchen, M. A., B. M. Hasch, S.-H. Lee, and M. A. McHugh. 1991. Poly(ethylene-co-methyl acrylate)-solvent-cosolvent phase behavior at high pressures. Polymer. 33 1922-1925. 

Besides the temperature, the pressure influences the value of the enthalpy of reaction. While this effect is negligible in the case of reactions in the liquid phase, considerable effects are observed for gas-phase reactions at high pressures. The deviations from ideal gas behavior can directly be taken into account with the help of equations of state, introduced in Section 2.2.1 using residual enthalpies h - (see Table 2.2), which describe the enthalpy difference between the... 

The extraction of the product from the ILs with SC-CO2 is the most important advantage of the biphasic systems. Typically, the effectiveness of SC-CO2 for extraction depends on the phase behavior of the binary IL-SC-CO2 system. The solubility of CO2 in the IL is important to allow for contact between the CO2 and the products. The dissolved CO2 also decreases the viscosity of the IL and therefore improves mass transfer. It has been reported that CO2 is soluble in every IL, whereas ILs are not soluble in the gaseous CO2 phase, even at high pressures (Blanchard et al., 2001). Such systems have been successfully used in the synthesis of esters. 

In summary, we have demonstrated the phase stabilities of two prototype RTILs, [bmim][BF4] and [bmim][PF6] under high pressure. Interestingly, [bmim][PF6] easily crystallizes upon compression, but [bmim] [BF4] maintains the liquid state up to over 1 GPa. The results indicate that a contribution of the anion is significant to the phase stability at high pressures. These clearly contrast with the results in the low-temperature phase behaviors of both RTILs. 

Shashidhar and Rao [74] performed high pressure X-ray studies on liquid crystals with re-entrant behavior with an opposed diamond anvil cell. They found that the layer spacing of the SmA phase of 4-n-octyloxy-4 -cyanobiphenyl first decreases more or less linearly with increasing pressure up to 140 MPa, then increases at still higher pressures. Since this compound shows re-entrant nematic behavior at high pressures, this result confirms the prediction of Cladis et al. that the occurrence of a re-entrant nematic phase is associated with an expansion of the SmA phase layer spacing. 

When we consider equilibrium between two phases at high pressure, neither phase being dilute with respect to one of the components, we can no longer make the simplifying assumptions made in some of the earlier sections. We must now realistically describe deviations from ideal behavior in both phases for each phase, the effect of both pressure and composition must be seriously taken into account if we wish to describe vapor-liquid equilibria at high pressures for a wide range of conditions, including the critical. 

In the beer industry, foaming behavior is vital to the product. The beer bottle is produced under C02 gas at high pressure. As soon as a beer bottle is opened, the pressure drops and the gas (CO2) is released, which gives rise to foaming. Commonly, the foam stays inside the bottle. Foaming is caused by the presence of different amphiphilic molecules (fatty acids, lipids, and proteins). The foam is very rich as the liquid film is very thick and contains a substantial aqueous phase (such foams are... 

The last ATR cell described here in detail was designed for the study of catalytic reactions at high pressures and in particular in supercritical fluids. A schematic representation of the design is shown in Fig. 17 (76). An important issue in this type of reaction is the phase behavior of the system, which can have a large influence on the catalytic reaction 77,IS). The cell consists of a horizontal stainless-steel cylinder. It is designed to allow monitoring of the phase behavior via a video camera. For this purpose, one end of the cylinder is sealed with a sapphire window, behind... 

The heat of vaporization varies slowly with temperature and eventually becomes zero at the critical temperature. In addition, the assumptions which were used to approximate AV (ideal gas behavior and negligible volume of the condensed phase) are inaccurate at high pressures. However, the assumptions are adequate for data of moderate precision (about 1%) in the 1-760-torr range, and in these cases the heat of vaporization may be determined from the A parameter as indicated in Eq. (3). Of course, different A and B parameters have to be determined for each particular phase (e.g., for the liquid, and for each solid phase, if more than one exists). IfA and if are known for one phase and the heat of transition (fusion or solid-state transition) is known, the heat of vaporization and its equivalent, the A value for the second phase, maybe calculated. The new value of B is then found by taking advantage of the fact that the vapor pressures for the two phases are identical at the transition temperature. 

G. Schneider, Phase Equilibriums in Liquid Systems at High Pressures , Ber. Bunsenges. Physik. Chem., 70, 497-520 (1966). G. M. Schneider, Phase Equilibria in Fluid Mixtures at High Pressures , Adv. Chem. Phys. 17, 1-42 (1970). G. M. Schneider, "Gas-Gas Equilibrium. Fluid Mixtures Under Pressure , Fortschr. Chem. Forsch. 13, 559-600 (1970). G. M. Schneider, Phase Behavior and Critical Phenomena in Fluid Mixtures Under Pressure , Ber. Bunsenges. Physik. Chem. 76, 325-331 (1972). G. M. Schneider in Water A Comprehensive Treatise, Vol II, F. Franks, ed.. Plenum Press, New York, 1973, Ch. 6. 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

A high-pressure micromodel system has been constructed to visually investigate foam formation and flow behavior. This system uses glass plates, or micromodels, with the pattern of a pore network etched into them, to serve as a transparent porous medium. These micromodels can be suspended in a confining fluid in a pressure vessel, allowing them to be operated at high pressure and temperature. Because of this pressure capability, reservoir fluids can be used in the micromodel, and any effects of phase behavior or pressure- and temperature-dependent properties on foam flow can be examined. 

The law of mass action is widely applicable. It correctly describes the equilibrium behavior of all chemical reaction systems whether they occur in solution or in the gas phase. Although, as we will see later, corrections for nonideal behavior must be applied in certain cases, such as for concentrated aqueous solutions and for gases at high pressures, the law of mass action provides a remarkably accurate description of all types of chemical equilibria. For example, consider again the ammonia synthesis reaction. At 500°C the value of K for this reaction is 6.0 X 10 2 F2/mol2. Whenever N2, H2, and NH3 are mixed together at this temperature, the system will always come to an equilibrium position such that... 

At pressures above a few atmospheres, the deviations from ideal behavior in the gas phase will be significant and must be taken into account in process design. The effect of pressure on the liquid-phase activity coefficient must also be considered. A discussion of the methods used to correlate and estimate vapor-liquid equilibrium data at high pressures is beyond the scope of this book. Refer to the texts by Null (1970), Prausnitz et al. (1998), or Prausnitz and Chueh (1968). 

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