Phase diagrams, hydrocarbon systems

The example of a binary mixture is used to demonstrate the increased complexity of the phase diagram through the introduction of a second component in the system. Typical reservoir fluids contain hundreds of components, which makes the laboratory measurement or mathematical prediction of the phase behaviour more complex still. However, the principles established above will be useful in understanding the differences in phase behaviour for the main types of hydrocarbon identified. 

Figure A2.5.31. Calculated TIT, 0 2 phase diagram in the vicmity of the tricritical point for binary mixtures of ethane n = 2) witii a higher hydrocarbon of contmuous n. The system is in a sealed tube at fixed tricritical density and composition. The tricritical point is at the confluence of the four lines. Because of the fixing of the density and the composition, the system does not pass tiirough critical end points if the critical end-point lines were shown, the three-phase region would be larger. An experiment increasing the temperature in a closed tube would be represented by a vertical line on this diagram. Reproduced from [40], figure 8, by pennission of the American Institute of Physics.

Carbon disulfide is completely miscible with many hydrocarbons, alcohols, and chlorinated hydrocarbons (9,13). Phosphoms (14) and sulfur are very soluble in carbon disulfide. Sulfur reaches a maximum solubiUty of 63% S at the 60°C atmospheric boiling point of the solution (15). SolubiUty data for carbon disulfide in Hquid sulfur at a CS2 partial pressure of 101 kPa (1 atm) and a phase diagram for the sulfur—carbon disulfide system have been published (16). Vapor—Hquid equiHbrium and freezing point data ate available for several binary mixtures containing carbon disulfide (9). 

The physical chemical behavior of betaine esters of long-chain alcohols shows strong similarities to the common, closely related alkyltrimethylam-monium surfactants both in dilute and concentrated aqueous systems, hi consistence with the findings about CMC s of surfactants containing normal ester bonds (see above) it has been found that the CMC for a betaine ester with a hydrocarbon chain of n carbons is very close to the value for an alkyltrimethylammonium chloride surfactant with a hydrocarbon chain of n + 2 carbons [32], The binary phase diagram of dodecyl betainate-water has an appearance very similar to that of an alkyltrimethylammonium surfactant with a hydrophobic tail of a similar size [30]. 

These compounds differ from other surfactants in the pronounced sensitivity of their association structural organization to temperature. This characteristic feature was noted very early by Shinoda (3) with regard to their micellar association and solubilization. A corresponding sensitivity may also be observed in the strong dependence of the liquid crystalline regions in phase diagrams of the system water, surfactant, and hydrocarbon (4). 

Hydrate phase diagrams for water-hydrocarbon systems provide a convenient overview of the calculation types. These diagrams differ substantially from the normal hydrocarbon phase diagrams primarily due to hydrates and the hydrogen bonds inherent in aqueous systems. The phase diagrams of Section 4.1 provide an overview for the calculation methods in this chapter and the next. 

Hydrate Phase Diagrams for Water + Hydrocarbon Systems... 

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

The phase behavior of multicomponent hydrocarbon systems in the liquid-vapor region is very similar to that of binary systems. However, it is obvious that two-dimensional pressure-composition and temperature-composition diagrams no longer suffice to describe the behavior of multicomponent systems. For a multicomponent system with a given overall composition, the characteristics of the P-T and P-V diagrams are very similar to those of a two-component system. For systems involving crude oils which usually contain appreciable amounts of relatively r on-volatile constituents, the dew points may occur at such low pressures that they are practically unattainable. This fact will modify the behavior of these systems to some extent. 

Within a simple ternary DDAB-water-hydrocarbon system, it is reasonable to expect that the effective surfactant parameter remains approximately constant throughout the triangular phase diagram, just as it does along the upper water limit. (Note however, that the head-group area can change at low water fractions due to the effects of hydration on the polar head.)... 

Figure 1. Phase diagram for high-density polyethylene/hydrocarbon systems.

Figure 12 is a schematic flow diagram for a two column liquid phase TSA drying system where trace water is removed from a hydrocarbon stream [31]. An external regeneration... 

Most perfluoroahphatic solvents are not miscible with hydrocarbon solvents at room temperature. At elevated temperatures, however, a homogeneous system is formed, which separates again on cooling. This strongly temperature-dependent and reversible miscibility gap is well demonstrated by the phase diagram of the perfluoro(methylcyclohexane)/benzene system depicted in Figure 3.1. 

Complete information on phase behavior including tie-lines and on diffusion coefficients is rarely available for oil-water-surfactant systems. Nevertheless, Raney and Miller used plausible phase diagrams for an anionic surfactant-NaCl brine-hydrocarbon system as a function of salinity to calculate diffusion paths that exhibited intermediate phase formation and spontaneous emulsification in agreement with experimental observations made using the vertical cell technique. For example. Figure 9.12 shows a diffusion path for a surfactant-alcohol-brine mixture of composition D in contact with oil for a case when initial salinity is high. An intermediate brine phase is predicted as well as spontaneous emulsification in the oil phase, both of which were, in fact, observed. 

Other investigations on chlorotitanium species include an examination of the phase diagram of the systems TiCl4-MCl c (M = W, Mo, or Re, x = 5 M = W, X = 6), ° a d.t.a. study of the interaction between TiC and a series of metal chlorides, and the gas chromatographic determination of formation constants for the complexes of TiC with chloro- and fluoro-hydrocarbons. ... 

In many applications of low molecular weight hydrocarbon resins, including flooring, adhesives, rubber compounds, inks, and coatings, the best performance is often associated with plasticizers that are marginal solvents rather than perfect ones. The difference between the resin parameter and the plasticizer parameter indicates the place of the system in the Flory-Huggins phase diagram. The separation of phases is responsible for the improved physical properties. While the difference of the parameters readily explains the behavior, the parameters for many industrial materials are not sufficiently well defined, and specific solubility tests must be used to control both resin and plasticizer. 

Fig. 9. Temperature-composition isobaric phase diagram for the fully hydrated dipal-mitoylphosphatidylcholine/dipalmitoyl-phosphatidylethanolamine system constructed using the temperature gradient method. The notation used is that of Luzzati [8] and is as follows Lc, lamellar crystalline (also referred to as the subgel phase) L f, lamellar gel phase with hydrocarbon chains tilted with respect to the bilayer normal P)el, ripple phase L. lamellar liquid crystal phase. Insert bold line in graph as indicated...

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