Non-Aqueous Phase Equilibrium

An important aspect of the design of an acid gas injection scheme is the non-aqueous phase equilibrium. Fluid phase equilibrium involving water, which is also very important, will be discussed in chapter 4 and hydrates in chapter 5. 

It is important to identify the conditions for the liquefying of acid gas. Therefore, the construction of a phase envelope is the first step in analyzing an acid gas injection scheme. The state of the mixture has a dramatic effect on all aspects of the acid gas injection design. For example, the injection pressure, as discussed in chapter 8, is critically related to the phase of the fluid being injected. 

Several experimental investigations of phase equilibrium that are relevant to acid gas injection were summarized by Carroll (1999, 2002). 

The binary mixture hydrogen sulfide + carbon dioxide is the most important non-aqueous system involved in acid gas injection, since acid gas is composed almost exclusively of these components. 

Two early studies of the phase equilibrium in the system hydrogen sulfide + carbon dioxide were Bierlein and Kay (1953) and Sobocinski and Kurata (1959). Bierlein and Kay (1953) measured vapor-liquid equilibrium (VLE) in the range of temperature from 0° to 100°C and pressures to 9 MPa, and they established the critical locus for the binary mixture. For this binary system, the critical locus is continuous between the two pure component critical points. Sobocinski and Kurata (1959) confirmed much of the work of Bierlein and Kay (1953) and extended it to temperatures as low as -95°C, the temperature at which solids are formed. Furthermore, liquid phase immiscibility was not observed in this system. Liquid H2S and C02 are completely miscible. 

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It assumes that there are no significant solute-solute interactions and no strong solute-solvent interactions which would influence the distribution process. Concentrations are expressed as mass/unit volume, and usually C1 refers to an aqueous phase and C2 to a non-aqueous phase. The equilibrium constant (P or K) defining this system is referred to as the partition coefficient or distribution ratio. The thermodynamic partition coefficient (P ) is given by the ratio of the respective mole fractions as follows ... 

Schmidt, T. C P. Kleinert, C. Stengel, K.-U. Goss, and S. B. Haderlein, Polar fuel constituents - Compound identification and equilibrium partitioning between non-aqueous phase liquids and water , Environ. Sci. Technol., in press (2002). 

Experiments on mixed electrolytes were carried out using chloride + picrate mixtures. 20 ml of a solution containing tetraethylammonium chloride and picrate was shaken with 20 ml of pure di-/jopropyI ketone, and the e.m.f. of the corresponding cell and the equilibrium concentration of the picrate were measured as described above. As very little of the chloride passed into the non-aqueous phase, the initial value of the aqueous concentration was used to calculate the chloride fraction x. 

The unilateral hydrodynamic equilibrium [the two solvent phases are unilaterally distributed along the length of the coil, one phase (head phase) entirely occupying the head side and the other phase (tail phase) the tail side of the coil]. The head phase can be the lighter or the heavier phase and also can be the aqueous or the non-aqueous phase, depending on the physical properties of the liquid system and the applied experimental conditions. This type of equilibrium may also be called bilateral, indicating the distribution of the one phase on the head side and the other phase on the tail side [3],... 

A partition coefficient measurement involves the equilibration of a solute between an aqueous and an organic phase. A compound that is ionizable will be present a combination of ionic and neutral species, determined by its pifa values(s) and the pH of the aqueous phase. Hence, the equilibrium of a solute between the aqueous and organic phases will depend on pH in a manner that is dependent on the structure (and hence properties) of the solute. The measurement of such an equilibrium results in a distribution coefficient, which is defined as the ratio of the total concentrations of all species of the compound in the non-polar to the polar phase. The log of this is the logZ) value. Because it is usually assumed that charged species do not partition to the non-aqueous phase, logZ) values are lower than logP values for the same compound. This is depicted in Figure 3.6. 

Non-equilibrium sorption/desorption. Can also be used for non-aqueous phase liquid dissolution). 

The two concurrent processes, the transfer from the water to the non-aqueous phase and the transfer from the non-aqueous to the water phase, are related by an equilibrium constant, which is expressed as the unitless partition coefficient (F) ... 

Attempts are being made to determine non-aqueous phase liquid (NAPL) mole fraction and micelle-aqueous partition coefficients with rhamnolipids to understand equilibrium solubilization behaviour in surfactant-enhanced soil remediation situations [51]. A modification of Raoult s law was used for surfactant-enhanced solubilization but deviation from this ideal behaviour depended on the hydrophobicity of the compounds and the NAPL-phase mole fraction. Micelle-water partition coefficients were non-Unear in relation to the NAPL-phase mole fraction. Also enhancements by the surfactant were... 

Unlike the previous kinetics imposed by the sink condition, steady-state transport kinetics under non-sink conditions will lead to equilibrium partitioning between the aqueous phase of the donor and receiver compartments and the cell mono-layer. In contrast to the sink condition wherein CR 0 at any time, under nonsink conditions CR increases throughout time until equilibrium is attained. As previously stated in Eqs. (1) and (3), the rate of mass disappearing from the donor solution is... 

This type of sensor often does not have a membrane it instead utilizes the properties of a water-oil interface, a boundary between an aqueous and a non-aqueous (organic) phase. Traditionally, sensors based on non-equilibrium ion-selective transport phenomena were distinguished as a separate group and considered as the electrochemistry of the ion transfer between two immiscible electrolyte solutions (IT1ES). Here, we will not distinguish polymeric membrane electrodes and ITIES-based electrodes due to the similarity in the theoretical consideration. 

It has been proposed to define a reduced temperature Tr for a solution of a single electrolyte as the ratio of kgT to the work required to separate a contact +- ion pair, and the reduced density pr as the fraction of the space occupied by the ions. (M+ ) The principal feature on the Tr,pr corresponding states diagram is a coexistence curve for two phases, with an upper critical point as for the liquid-vapor equilibrium of a simple fluid, but with a markedly lower reduced temperature at the critical point than for a simple fluid (with the corresponding definition of the reduced temperature, i.e. the ratio of kjjT to the work required to separate a van der Waals pair.) In the case of a plasma, an ionic fluid without a solvent, the coexistence curve is for the liquid-vapor equilibrium, while for solutions it corresponds to two solution phases of different concentrations in equilibrium. Some non-aqueous solutions are known which do unmix to form two liquid phases of slightly different concentrations. While no examples in aqueous solution are known, the corresponding... 

The thermodynamics of the extraction mechanism is extremely complex. In the initial equilibration of the ion pairs (Scheme 1.6) account has to be taken not only of the relative stabilities of the ion-pairs but also of the relative hydration of the anionic species. Assuming the complete non-solvation of the ion-pairs, the formation of the ion-pair [Q+Y] will generally be favoured when the relative hydration of X- is greater than that of Y. However, in many cases, the anion of the ion-pair is hydrated [8-11] (Table 1.1) and this has a significant effect both on equilibrium between the ion-pairs in the aqueous phase and the relative values of the partition coefficients of the two ion-pairs [Q+X ] and [Q+Y ] between the two phases. 

At pH 9.05, the phenol-phenolate equilibrium favours the phenol by a factor of 7 1, and the amine-ammonium ion equilibrium favours the amine by a factor of 7 1. In other words, the non-ionized morphine predominates, and this can thus be extracted into the organic phase. What about the amounts in ionized form are these not extractable By solvent extraction of the non-ionized morphine, we shall set up a new equilibrium in the aqueous phase, so that more non-ionized morphine is produced at the expense of the two ionized forms. A second solvent extraction will remove this, and we shall effectively recover almost all the morphine content. A third extraction would make certain that only traces of morphine were left as ionized forms. 

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