Multiphase behavior

In a supercritical extraction process a solvent is contacted with a solute at conditions near a critical point of the solvent plus solute mixture. The mixture may exhibit multiphase behavior invoving vapor, liquid, and solid phases, depending on the mixture composition and temperature and pressure conditions. 

Segmented polyimide-polydimethylsiloxane copolymers have been successfully synthesized both in laboratory and industrial quantities to produce multiphase siloxane-modified polyimides. The siloxane detracts somewhat from the otherwise excellent thermo oxidative stability of the polyimide, but it does produce a number of important properties. These include multiphase behavior, improved adhesion to many substrates, improvements in fire resistance and enhanced gas and liquid separation membranes, where one wishes not only to maximize the contribution of the siloxane to permeability, but also to utilize the imide to re-... 

A number of ILs are hydrophobic, yet they readily dissolve many organic molecules—with the exception of alkanes and alkylated aromatic compounds (e.g., toluene). Among such ILs we find [bmim][PFg], which forms triphasic mixtures with alkanes and water. This multiphasic behavior has decisive implications for clean synthesis. For example, transition-metal catalysts can be exclusively dissolved in the ionic liquid, thus allowing products and by-products to be separated from the ionic liquid by solvent extraction with either water or an organic solvent. This is advantageous when using expensive metal catalysts, as it enables both the ionic liquid and the catalyst to be recycled and reused. Alternatively, some volatile products can be separated from the IL by distillation, as it has negligible vapor pressure. 

The present results address contributions essential to quasi-chemical descriptions of solvation in more realistic cases. An interesting issue is how these packing questions are affected by multiphasic behavior of the solution. In such cases, the self-consistent molecular field should reflect those multiphase possibilities just as it can in pedagogical treatments of nonmolecular models of phase transitions (Ma, 1985). 

A review of the literature found only a few studies of multiphase behavior for related model surfactant systems. Fleck... 

The full extent and variety of the phase behavior for water-isopropanol-C02 mixtures observed experimentally and calculated with the Peng-Robinson equation of state was not anticipated based on known phase behavior for the constituent binary mixtures or similar ternary mixtures. These results suggest that multiphase behavior for related model surfactant systems could also be complex. Measurements of all the critical endpoint curves, the tricritical points, and secondary critical endpoint for such systems would be tedious and are extremely difficult. However, by coupling limited experimental data with a thermodynamic model based on this cubic equation of state, complex multiphase behavior can be comprehensively described. 

M = Li, Na, K, Cs). Previous literature data (discussed below) suggest that there is scope for a range of structure types to be produced in this system, based both on possible variability in the relative proportions of octahedra and tetrahedra, and a possible diversity in the manners in which they can be interlinked. Broad scope in the possible product structures, however, implies that the simultaneous crystallization of more than one phase might be common. Initial scoping experiments are therefore likely to be complicated by problems associated with product multiphasic behavior. 

In addition to vapor (V), high-density liquid (L2), or low-density liquid (Li) phase behavior, reservoir fluids, oils, and other organic fluids exhibit a variety of multiphase behaviors and critical phenomena as noted in Fig. 1. These include liquid-vapor (LiV or L2V), liquid-liquid (L1L2), and liquid-liquid-vapor (L1L2V) phase behavior and associated critical phenomena. 

Pure solid + fluid phase equilibrium calculations are challenging but can, in principle, be modeled if the triple point of the pure solid and the enthalpy of fusion are known, the physical state of the solid does not change with temperature and pressure, and a chemical potential model (or equivalent), with known coefficients, for solid constituents is available. These conditions are rarely met even for simple mixtures and it is difficult to generalize multiphase behavior prediction results involving even well-defined solids. The presence of polymorphs, solid-solid transitions, and solid compounds provide additional modeling challenges, for example, ice, gas hydrates, and solid hydrocarbons all have multiple forms. 

Raeissi, S. Gauter, K. Peters, C.J. Fluid multiphase behavior in quasi-binary mixtures of carbon dioxide and certain 1-alkanols. Fluid Phase Equilibria 1998, 147, 239-249. 

Multiphase behavior of CO2 with solid aromatics. J. Fluid Phase Equil. 

It is a sobering fact that this chapter introduces only the more common and least complex kinds of multiphase behavior the chapter fails to do full justice to Nature s diversity. Yet, the features described here should provide a structure by which you can effectively analyze and use phase diagrams. 

The diagram is very complex with at least 17 phases. The main reason is the fact that the pure COP already shows a multiphase behavior [50]. According to Yoon and coworkers [105] there are two mesophases in the pure COP, SmE and SmB, which might also be observable in the blend. However, in the liquid state, especially at temperatures higher than 260" C, transesterification and/or degradation processes are observable, so that all phases claimed in this area are in some sense hypothetical because they could not be observed unequivocally due to the reasons outlined above, although their existence can be predicted from theoretical arguments. 

The fluid multiphase behavior of various ternary systems and one series of quasi-binary systems has been investigated, with carbon dioxide (CO2) as the near-critical solvent. The ternary systems examined can be grouped as follows ... 

Although the fluid multiphase behavior of various series of ternary systems containing CO2 as the near-critical solvent were investigated (see above), only the series of CO2 + tetradecane + 1-alkanol with as the alkanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol and 1-decanol will be discussed here. 

L Earlier Observations on Unexpected Fluid Phase Behavior Patton et al. [4] found unexpected fluid multiphase behavior for the system CO2 + 1-decanol + tetradecane. The two binary border systems CO2 + 1-decanol and CO2 + tetradecane both show type-III fluid phase behavior, see [41] and [10,43], respectively, in the classification of Scott and Van Konynenburg [14,30] (section 2), with its characteristic UCEP For the ternary system, the three-phase surface Ug is... 

In this chapter we discussed the occurrence of holes as one of the pecularities in fluid multiphase behavior that may occur in ternary mixtures of near-critical carbon dioxide and two low-volatile solutes, e.g., certain combinations of tetradecane with, 1-alkanols. 

According to Schneider (see elsewhere in this volume) the physical origin of the holes has to be ascribed to the so-called co-solvency effect. In particular, when the two low-volatile components are chemically different, the critical pressures of the critical loci of the two binary border systems do not differ too much and when no specific interactions like, for instance, hydrogen bonding play a role, co-solvency is likely to have a strong influence on the multi-phase behavior in the ternary systems. For a detailed discussion on the relationschip between co-solvency and multiphase behavior, one is referred to else> bere in this volume and to [48] as well. 

Gauter, K., Florusse, L J., Smits, J.C., Peters, C J. and de Swaan Aarons, J., Fluid multiphase behavior of various ternary systems carbon dioxide + 1-alkanol + n-tridecane, J. Chem. Therm. 30,1617-1631. Gauter, K., Florusse, LJ. and Peters, CJ. (1998), Experimental results on the fluid multiphase behavior of various ternary systems of near-critical carbon dioxide, certain 1-alkanols and o-nitrophenol. Fluid Phase Equilibria 150-151, 501-514. 

Gauter, K., (1999) Fluid multiphase behavior in ternary systems of near-critical CO2 Measurements, Modeling and Computation, Ph.D, thesis. Delft University of Technology, Delft, The Netherlands. Creek, J.L., Knobler, C.M. and Scott, R.L. (1981) Tricritical phenomena in quasibinary mixtures of hydrocarbons I. Methane systems, J. Chem. Phys., 74, 3489-3499. 

Phase Equilibria. From recent research (Schneider and Peters) it became apparent that in the near-critical region of certain ternary carbon dioxide mixtures, due to co-solvency effects of the two solutes relative to each other, the fluid multiphase behavior can be quite complex. Phenomena like immiscibility windows and holes are not unusual, which have their consequences for separations in near-critical processing. Peters stressed that for many applications in supercritical technology carbon dioxide is not an appropriate choice since for many solutes it is a poor solvent that would require the use of a cosolvents. If safety and environmental constraints permit, it is certainly worthwhile to consider alternatives for carbon dioxide. Gulari, Schneider and Peters emphasized the importance of studying representative model systems in order to obtain insight into the systematic variations of the complex phase behavior that may occur in near-critical multicomponent mixtures. Debenedetti stressed the importance of focusing on complex fluids like emulsions. 

Examples of the multiphase behavior using a reservoir crude oil are shown in Figures 10 and II. Thus, in both cases (temperature and... 

The development of multiphase behavior was observed to be a function of composition and block molecular weights. The generalized synthesis is shown in Equation 2. Certain important bulk properties of these copolymers, such as... 

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