Phase behavior hydrocarbon mixtures

Aughel and coworkers [63] studied the phase behavior of hydrocarbon-water mixtures in the presence of alkyl(aryl)polyoxyethylene carboxylates for enhanced oil recovery and found good salt tolerance with an alkyl ether carboxy-late (C13-C15) with 7 mol EO and a good microemulsion forming effect with the 3 EO type. 

From phase behavior studies of hydrocarbon-water mixtures in the presence of ether carboxylates it was concluded that C13-C15 ether carboxylic acids with 3 and 7 mol EO were more suitable than the nonylphenol ether carboxylates with 5.7 and 10 mol EO and the tridecyl ether carboxylic acids with 6.5 mol EO. However, with the use of cosolvents these types were also acceptable [191]. 

The Benedict (4) equation of state has opened a new avenue to the correlation of the phase behavior of hydrocarbons. However, the extent of the iterative calculations required to determine the properties of the coexisting phases has prevented its widespread adoption. Recently the concept of convergence pressures (58, 76) has gained acceptance in the industry. This pressure is presumed to be that at which the equilibrium ratios approach unity. However, for a given mixture a true convergence pressure exists only at the critical temperature of the mixture. 

Physical chemistry and related sciences have played an increasingly important role in the explanation and prediction of physical phenomena which are useful in the production and processing of petroleum. Knowledge of the volumetric and phase behavior of hydrocarbons has so developed that such properties may be predicted with reasonable accuracy at most of the states of interest except those near retrograde dew point. The inability to describe with certainty the composition of many hydrocarbon mixtures in terms of their components places a severe limitation on the prediction of the volumetric and phase behavior of petroleum and of mixtures of its components. 

Fig. 2-20. Pressure-volume diagram of a mixture of 47.6 weight percent n-pentane and 52.4 weight percent n-heptane. (From Volumetric and Phase Behavior of Hydrocarbons, Bruce H. Sage and William N. Lacey. Copyright 1949, Gulf Publishing Co., Houston. Used with permission.)...

In this chapter we will consider methods of calculating the behavior of hydrocarbon mixtures in this two-phase region. Three types of calculations will be examined ... 

The phase behavior of a synthetic lecithin, dipalmitoyllecithin, as analyzed by Chapman and co-workers (5), is diagrammed in Figure 3. The main features are the same as in the phase diagram of egg lecithin a mixture of numerous homologs. As a consequence of the variation in fatty acid chain length, the chain melting point is lowered which means that the critical temperature for formation of liquid crystalline phases is reduced. This temperature is about 42 °C for dipalmitoyllecithin, and, if the lamellar liquid crystal is cooled below this temperature, a so-called gel phase is formed. The hydrocarbon chains in the lipid bilayers of this phase are extended, and they can be regarded as crystalline. The gel phase and the transitions between ordered and disordered chains are considered separately. 

The phase behavior of hydrocarbon + water mixtures differs significantly from that of normal hydrocarbon mixtures. Differences arise from two effects, both of which have their basis in hydrogen bonding. First, the hydrate phase is a significant part of all hydrocarbon + water phase diagrams for hydrocarbons with a molecular size lower than 9 A. Second, water and hydrocarbon molecules are so different that, in the condensed state, two distinct liquid phases form, each with a very low solubility in the other. 

The PT diagram of Fig. 12.6 is typical for mixtures of nonpolar substance such as hydrocarbons. An example of a diagram for a highly nonideal syster methanol/benzene, is shown in Fig. 12.8. The nature of the curves in this figu suggests how difficult it can be to predict phase behavior, particularly for sped so dissimilar as methanol and benzene. 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

The qualitative phase behavior of hydrocarbon systems was described in the previous chapter. The quantitative treatment of these systems mil now be discussed and tire methods for calculating their phase behavior presented. It will became apparent that the liquid and vapor phases of mixtures of two or more hydrocarbons are in reality solutions (see below), so that it will be necessary to discuss the laws of solution behavior. Analogous to the treatment of gases, the behavior of a hypothetical fluid known as a perfect, or ideal, solution will be described. This will be followed by a description of actual solutions and tlie deviations from ideal solution behavior that occur. 

Standing, M. B., A Pressure-Volume-Temperature Correlation for Mixtures of California Oils and Gases, API Drilling and Production Practice, p. 275, 1947. Standing, M. B., Volumetric and Phase Behavior oj Oil Field Hydrocarbon Systems, Reinhold Publishing Corp., New York (1952). 

Consequently, it can be concluded for the mixtures of LLL-MMM, LLL-PPP, LLL-SSS, MMM-PPP, and PPP-SSS that the TAG binary mixtures are miscible in metastable polymorphs of a and p forms when the difference in the number of carbon atoms of the fatty acid moieties. An, equals 2, whereas immiscible mixtures are found in all polymorphic forms when An is larger than 2. Results obtained for these mixture systems may indicate a relationship between polymorphism and phase behavior of the binary mixtures of the saturated-acid TAGs in such a way that rotational freedom of hydrocarbon chains and entropy of methyl-end stacking are crucial factors determining the polymorph-dependent phase behavior. 

Fig. 1 Multiphase, fluid-only behaviors exhibited by hydrocarbon mixtures. Dashed phase boundaries indicate critically identical phases.

Measurement and prediction of transitions from one type of phase behavior to another are key to our understanding of the physical properties of hydrocarbon mixtures. One can examine phase behavior type transitions for binary mixtures from the perspective of the solute or the solvent while varying the other. Both approaches are found in the literature. The solvent fixed approach is shown in Fig. 5 for carbon dioxide + n-alkane binary mixtures. The anthracene... 

---