Gases solubility behavior

A third motivation for studying gas solubilities in ILs is the potential to use compressed gases or supercritical fluids to separate species from an IL mixture. As an example, we have shown that it is possible to recover a wide variety of solutes from ILs by supercritical CO2 extraction [9]. An advantage of this technology is that the solutes can be removed quantitatively without any cross-contamination of the CO2 with the IL. Such separations should be possible with a wide variety of other compressed gases, such as C2LL6, C2LL4, and SF. Clearly, the phase behavior of the gas in question with the IL is important for this application. 

Case I. At sufficiently low pressures, the solubility curve does not intersect the coexistence curve. In this case, the gas solubility is too low for liquid-liquid immiscibility, since the coexistence curve describes only liquid-phase behavior. Stated in another way, the points on the coexistence curve are not allowed because the fugacity f2L on this curve exceeds the prescribed vapor-phase value f2v. The ternary phase diagram therefore consists of only the solubility curve, as shown in Fig. 28a where V stands for vapor phase. 

This section is concerned mainly with the approach to modeling the gas> phase behavior of single reactive gases. The basic approach can also apply to sulfur dioxide, ammonia, and other pollutant gases in which water solubility alone controls the rate of uptake. The simpler case of inert gases has been reviewed in a conference report edited by Papper and Kitz. ... 

Pure-component vapor pressure can be used for predicting solubilities for systems in which Raoult s law is valid. For such systems pA = p°Axa, where p°A is the pure-component vapor pressure of the solute and pA is its partial pressure. Extreme care should be exercised when using pure-component vapor pressures to predict gas absorption behavior. Both vapor-phase and liquid-phase nonidealities can cause significant deviations from Raoult s law, and this is often the reason particular solvents are used, i.e., because they have special affinity for particular solutes. The book by Poling, Prausnitz, and O Connell (op. cit.) provides an excellent discussion of the conditions where Raoult s law is valid. Vapor-pressure data are available in Sec. 3 for a variety of materials. 

With regard to miscible polymers, simple blending allows one to develop materials that frequently reveal an intermediate behavior between those of the individual blend components [2, 3], Such systems can be easily exploited for fine-tuning the foam-ability, for example by controlling important foaming parameters such as the melt rheology or the gas solubility. 

Several factors can be identified as being crucial for the foaming of immiscible polymer blends the blend morphology, the phase size of the blend constituents, the interfacial properties between the blend partners, and, last but not least, the properties of the respective blend phases such as the melt-rheological behavior, the glass transition temperature, the gas solubility, as well as the gas diffusion coefficient. Most of these factors also individually influence the melt-rheological behavior of two-phase blends. 

Koros, W. J. (1985). Simplified analysis of gas/polymer selective solubility behavior, J. Polym. Sci. Polym. Phys. Ed. 23, 1611. 

A few reactor models have recently been proposed (30-31) for prediction of integral trickle-bed reactor performance when the gaseous reactant is limiting. Common features or assumptions include i) gas-to-liquid and liquid-to-solid external mass transfer resistances are present, ii) internal particle diffusion resistance is present, iii) catalyst particles are completely externally and internally wetted, iv) gas solubility can be described by Henry s law, v) isothermal operation, vi) the axial-dispersion model can be used to describe deviations from plug-flow, and vii) the intrinsic reaction kinetics exhibit first-order behavior. A few others have used similar assumptions except were developed for nonlinear kinetics (27—28). Only in a couple of instances (7,13, 29) was incomplete external catalyst wetting accounted for. 

It should be pointed out that the gas solubility could be calculated using the laws of solution behavior previously described in Chapter 5, provided sufficient data are available. It is necessary to have not only data for the overall composition of the system hut complete and accurate equilibrium constant data as well. These data are seldom, if ever, available for a crude-oil system and values of gas solubility must be obtained either experimentally or by estimation. However, to illustrate the complexity of computations of this type the method is outlined below for a two-component system of known overall composition. 

A number of models which can estimate density at atmospheric pressure have recently been reported. For example, Rebelo et al. [63, 64] defined the effective molar volumes of ions at 298.15 K and used the assumption of ideal behavior for the determination of the molar volume of ionic liquids. Yang et al. [65] used a theory based on the interstice model which correlated the density and the surface tension of the ionic liquid. Group contribution models have been reported by Kim et al. [66, 67] for the calculation of the density and C02 gas solubility for 1-alkyl-3-methylimidazolium based ionic liquids as a function of the temperature and C02 gas pressure with reasonable accuracy over a 50 K temperature range however, the... 

The equilibrium solubility of a substance is defined as the concentration of solute in its saturated solution, where the saturated solution exists in a state of equilibrium with pure solid solute. As solutes and solvents can be gaseous, liquid, or solid, there are nine possibilities for solutions, although liquid-gas, liquid-liquid, and liquid-solid are of particular interest for pharmaceutical applications. Among these, the most frequently encountered solubility behavior involves solid solutes dissolved in liquid solvent, so systems of this type will constitute the examples of the following discussions. 

Consider first the schematic P-T and P-x diagrams for the naphthalene-ethylene system. Figure 3.18b depicts the solubility behavior of naphthalene in supercritical ethylene at a temperature greater than the UCEP temperature. Solid-gas equilibria exist at low pressures until the three-phase SLV line is intersected. The equilibrium vapor, liquid, and solid phases are depicted as points on the horizontal tie line at pressure Pj. As the pressure is further increased a vapor-liquid envelope is observed for overall mixture concentrations less than Xl- A mixture critical point is observed for this vapor-liquid envelope, as described earlier. If the overall mixture composition is greater than Xl, then solid-gas equilibria are observed as the pressure is increased above Pj. 

Shown in figure 3.18c is a solubility isotherm at a temperature, Tb, that is less than the previous temperature, Tj, but still higher than the UCEP temperature. The solubility behavior at T is similar to the behavior in figure 3.18b. But at T, the three-phase SLV line is intersected at a higher pressure, closer to the UCEP pressure. Hence, the vapor-liquid envelope has diminished in size and the solid-gas equilibrium curve is shifted toward higher solvent concentrations. As a result, the solid-gas curve is now much closer to the vapor branch of the vapor-liquid envelope. 

Now consider a solubility isotherm at T, slightly less than the UCEP temperature (figure 3.18d). At this temperature, solid-gas equilibria exist at all pressures, since the SLV line is never intersected. As the UCEP pressure is approached the gas phase becomes highly compressible, due to the influence of the vapor-liquid critical point, and the solubility of the solid in the gas phase begins to increase. As the pressure is increased in the immediate vicinity of the UCEP pressure, the T isotherm exhibits a large solubility enhancement. At pressures much higher than the UCEP pressure, the gas is less compressible, therefore the solubility of the solid quickly reaches a limiting value. This solid solubility behavior is similar to the 50°C naphthalene-ethylene isotherm. 

For the naphthalene-ethylene and biphenyl-carbon dioxide systems, the effect of the binary liquid-gas critical point is rapidly diminished as the pressure is increased above the UCEP pressure. For the naphthalene-ethylene system, where the UCEP is at a modest pressure, the solid-fluid equilibrium curve quickly attains a limiting solubility at pressures greater than the UCEP pressure. For the biphenyl-carbon dioxide system, where the UCEP pressure is more than twice that of the naphthalene-ethylene system, the solid-fluid equilibrium curve decreases sharply to lower concentrations of heavy component as the pressure is increased above the UCEP pressure. This solubility behavior is a consequence of a free volume effect that results from the large disparity in size between biphenyl and carbon dioxide (Ranee and Cussler, 1974 von Tapavicza and Prausnitz, 1976). At very high pressures, increasing the pressure further reduces the free volume between carbon dioxide molecules available to the biphenyl molecules and reduces the solubility of biphenyl. Carbon dioxide essentially squeezes out the biphenyl at these high pressures. 

The removal of precipitated polyethylene from the wall is an interesting operation. About once every 2-3 sec the expansion valve is opened more fully than required for the expansion/precipitation function this results in a rapid decrease in pressure in the reactor of as much as 300-600 bar. The concomitant rapid increase in the velocity of the gas phase in the tubular reactor shears the walls and strips off any deposited polyethylene so that a reasonably steady state heat transfer situation exists. This description of the operation of the polymerization process, the polyethylene precipitation step, and the accentuated expansion, which maintains a clean wall and a high heat transfer coefficient, help to illustrate the interesting SCF solubility behavior and they also supply some information on the commercial reality of high-pressure processing in what we consider to be an extreme case. 

Carbon dioxide and other gases dissolve in many, many liquid solvents, Francis (1954) studied the solubility behavior of many classes of liquids, such as hydrocarbons, ketones, and alcohols, with carbon dioxide. He found that at about 25°C and 800 psi, many of the liquids are miscible with COj. In a prelude to the studies of GAS recrystallization, the absorption of CO2 into several liquids was studied using a sight glass (Jerguson gauge). The work of Francis was extended to higher temperatures than 25°C because of yield, particle size, and economic considerations. 

To calculate gas solubility in natural geochemical systems, basic thermodynamic properties such as the Henry s law constant and, in the case of weak electrolytes the dissociation constant, must be combined with a thermodynamic model of aqueous solution behavior. An analogous approach has been used to predict mineral solubilities in concentrated brines (1). Such systems are also relevant to the atmosphere where very concentrated solutions occur as micrometer sized aerosol particles and droplets, which contain very small amounts of water relative to the surrounding gas phase. The ambient relative humidity (RH) controls solute concentrations in the droplets, which will be very dilute near 1(X)% RH, but become supersaturated with respect to soluble constituents (such as NaCl) below about 75% RH. The chemistry of the aerosol is complicated by the non-ideality inherent in concentrated electrolyte solutions. 

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