Vapor-liquid behavior types

High pressure vapor-liquid behavior is typically classified into one of five basic types illustrated in Figure 11. (Classification numbering of the systems varies and one additional classification is sometimes added (8,10,17)). If the only area of interest is above the critical point of the lighter component, then the only temperatures of interest are above Cj. As an illustration of type III behavior, a P-x-y diagram of the ethylene-n-propanol... 

There are many types of phase diagrams in addition to the two cases presented here these are summarized in detail by Zief and Wilcox (op. cit., p. 21). Solid-liquid phase equilibria must be determined experimentally for most binaiy and multicomponent systems. Predictive methods are based mostly on ideal phase behavior and have limited accuracy near eutectics. A predic tive technique based on extracting liquid-phase activity coefficients from vapor-liquid equilib-... 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

There are three independent variables in coexisting equilibrium vapor/liquid systems, namely temperature, pressure, and fraction liquid (or vapor). If two of these are specified in a problem, the third is determined by the phase behavior of the system. There are seven types of vapor/liquid equilibria calculations in our program, as in Figure 1 under "Single Stage Calculation."... 

Chapter 14 describes the phase behavior of binary mixtures. It begins with a discussion of (vapor + liquid) phase equilibria, followed by a description of (liquid + liquid) phase equilibria. (Fluid + fluid) phase equilibria extends this description into the supercritical region, where the five fundamental types of (fluid + fluid) phase diagrams are described. Examples of (solid + liquid) phase diagrams are presented that demonstrate the wide variety of systems that are observed. Of interest is the combination of (liquid + liquid) and (solid + liquid) equilibria into a single phase diagram, where a quadruple point is described. 

The phase behavior for the polymer-solvent systems can be described using two classes of binary P-T diagrams, which originate from P—T diagrams for small molecule systems. Figure 3.24A shows the schematic P-T diagram for a type-III system where the vapor-liquid equilibrium curves for two pure components end in their respective critical points, Ci and C2. The steep dashed line in figure 3.24A at the lower temperatures is the P-T trace of the UCST... 

The essential features of vapor-liquid equilibrium (VLE) behavior are demonstrated by the simplest case isothermal VLE of a binary system at a temperature below the critical temperatures ofboth pure components. Forthis case ( subcritkaT VLE), each pure component has a well-defined vapor-liquid saturation pressure ff, and VLE Is possible for the foil range of liquid and vapor compositions xt and y,. Figure 1.5-1 ffiustrates several types of behavior shown by such systems. In each case, (he upper solid curve ( bubble curve ) represents states of saturated liquid (he lower solid curve ( dew curve ) represents states of saturated vtqtor. 

In this chapter we continue the discussion of fluid phase equilibria by considering examples other than vapor-liquid equilibria. These other types of phase behavior include the solubility of a gas (a substance above its critical temperature) in a liquid, liquid-liquid and vapor-liquid-liquid equilibria, osmotic equilibria, and the distribution of a liquid solute between two liquids (the basis for liquid extraction). In each of these cases the starting point is the same the equality of fugacities of each species in all the phases in which it appears. 

All the experimental studies [18-24] for methane hydrates are in agreement that for a constant value of pressure, the methane solubility, under H-Lw equilibrium, decreases with decreasing temperature, whereas the trend is reversed under vapor-liquid water equilibrium. Similarly, for a constant value of temperature the methane solubihty, under H-L equilibrium, decreases with increasing pressure. This behavior is clearly depicted in the schematic of Figure 1. Shown with the black dashed line is the three-phase (H-Lw-V) equilibrium curve, as calculated with the CSMGem simulator [1]. The colored solid lines correspond to the two-phase (H-Lw) equilibrium calculations using the correlation reported by Lu et al., [18], for six isotherms. Lu et al., reported a correlation for their experimental data of the type ... 

The cubic form of an equation of state is the simplest form which enables the description of the PvT behavior of gases and liquids and thus the representation of the vapor-liquid equilibrium with only one model. At constant temperature and aL a given pressure this equation has three solutions. These solutions may be - depending on the values of temperature and pressure - all of real type or of mixed real and complex type. Figure 2.14 shows an isotherm in the Pv-diagram, calculated with the Soave-Redlich-Kwong equation for ethanol at 473.15 K. The cho.. en temperature is lower than the critical temperature of ethanol (T = 516.2 K),... 

In Fig. 11, we draw schematically the case of fluid-solid phase behavior for the Type-I fluid mixture water-NaCl. For critical temperatures this far apart, the three-phase line Sb-L-V from the low-temperature quadruple point (where four three-phase lines meet) to the solutes triple point develops a high maximum that reaches above water s critical pressure and temperature. If a salt solution is heated at a pressure above the critical pressure of water, the vapor-liquid critical line is crossed first, and a two-phase L-V region entered. At high enough temperature the three-phase line Sb-L-V may be crossed, and solid salt will form. Thus supercritical water, fully miscible with air constituents and hydrocarbons, becomes a poor solvent for salts. 

However, the available experimental data show that the above-mentioned seven types of phase diagrams do not describe some versions of fluid phase behavior in the highly asymmetric binary systems where the melting temperature of nonvolatile component is significantly greater than the vapor-liquid critical point of the volatile one. 

It is interesting to note that it is possible to observe a tricritical point+ in binary polar/nonpolar (or quadrupolar/non-polar) systems of the type considered above. Thus if the polar component is a and is Increased, the tricritical point is observed as an Intermediary stage in the transition from class II to class III behavior. This is shown in Figure 6, the tricritical point occurring where the vapor-liquid critical curve, liquid-liquid critical curve, and the liquid-liquid-gas curve meet. (It should be noted that the formation of a tricritical point in this binary mixture does not violate the phase rule, since acts as an additional degree of freedom). 

The concept of universality classes, mentioned in the previous section, can be extended so as to be applicable to the characterization of the asymptotic critical behavior of dynamic properties (Hohenberg Halperin 1977). Two systems belong to the same dynamic universality class when they have the same number and types of relevant hydro-dynamic modes. Thus the asymptotical critical behavior of the mutual mass diffusivity D 2 and of the viscosity rj of liquid mixtures near a consolute point will be the same as that of the thermal diffusivity a and the viscosity j of one-component fluids near the vapor-liquid critical point (Sengers 1985). Hence, in analogy with equation (6.16) for liquid mixtures near a consolute point it can be written... 

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