Solubility supercritical pressure

Equation 11 should be the relationship between retention-solubility and pressure at constant temperature for infinitely dilute solutions. The RHS of eq. 11 consists of three terms, the first term will be a constant whose value depends on the partial molar volume of the solute in the stationary phase. The second term is the solubility of the solute in the supercritical fluid mobile phase. 

Extractions or extractive distillations with supercritical solvent need to be performed at as high as possible a solubility of oil in the extract or vapor phase in order to reduce the solvent or carrier gas requirement. From our lemon oil-carbon dioxide phase diagrams, it appears that the highest practical solubility level is 0.9 mole % (2.8 wt ) essential oil. This is obtainable at 313 K. At lower temperature, sensitivity of solubility to pressure requires that solubility be lower (e.g., 0.3 mole % at 308 K). 

The mechanisms of CO2 toxicity at near-atmospheric pressures are amplified at the near-critical and supercritical pressures used to achieve sterilization. For example, Spilimbergo et al. (9) examined the mechanism of inactivation of Pseudomonas aeruginosa and B. subtilis at 38-54°C and 5.8-20 MPa in a batch process. Total inactivation of the bacteria was observed when exposed to SCF CO2 at 38°C and 7.4 MPa for 150 s. The mechanism of inactivation is suggested to involve (a) the diffusion of SCF CO2 into the cells leading to a drop in pH and a subsequent loss of activity of key enzymes and (b) the extraction by SCF CO2 of intracellular substances, including phospholipids. Further analysis determined a high solubility of CO2 in model cell membrane phospholipids, suggesting that the enhanced permeability of the membrane in the presence of CO2 contributed to the inactivation of the cells. 

Figure 12.1-1 The solubility of naphthalene in supercritical carbon dioxide as a function of pressure. The points and are the experimental data of McHugh and Paulaitis [7. Chem. Eng. Data, 25, 326 (1980)] at 7 = 35.0 and 60.4°C, respectively. The lines are the correlations of the data using the Peng-Rpbinsoti equation of state with kcoz-u = 0.103. Note the sharp increase in naphthalene solubility with pressure near the CO2 critic pressure of 73.76 bar.

Supercritical gas extraction Depends upon solubility of a component in a gas variable solubility with pressure permits recovery of component and re-use of gas De-caffeination of coffee using CO2... 

As mentioned earlier, the control of properties in a supercritical fluid can allow separation to be incorporated into a recrystallisation process. This uses the so-called crossover effect, which arises when the isotherms of solubility versus pressure for two compounds at the same temperature can cross, i.e. the solubility of one compound is greater at a lower temperature, whereas that of the other compound is higher at a higher temperature, and they are equal at an intermediate temperature. Chimowitz et al. [103], by expansion of the solution from an extraction vessel down to 1 bar, measured solubility of 1 10 w/w decanediol-benzoic acid mixture in supercritical carbon dioxide. They experimentally defined the crossover behaviour for the two compounds, and subsequently achieved the separation of almost pure benzoic acid [104] from the 1 10 w/w decanediol-benzoic acid mixture. In a subsequent study, Sako et al separated phenanthrene and naphthalene to 97 wt% purity from a 1 1 w/w mixture using a similar technique [105]. 

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubiUties in supercritical fluids approach and sometimes exceed those of Hquid solvents as the SCF density increases. SolubiUties typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubiUties, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubiUty (16). 

In our discussion of the coexistence curve, we tacitly assumed that at least one liquid phase can exist for any composition. However, since component 2 is a supercritical gas, the physically allowable liquid compositions are limited by the solubility of the gas in the binary solvent mixture. As the pressure rises, the solubility of the gas increases, thereby enlarging the composition range which permits the existence of a liquid phase. 

The criteria which would be most desirable for industrial application of a separation process involving a supercritical gas may be established by comparing Figs. 3IB, 3ID, and 32. The largest cost in such a process is likely to be that of gas compression. Therefore, the maximum separation possible of the two solvents should occur for the addition of a given amount of gas, and the total pressure required to dissolve this gas should be small. This is the case if the tie lines slope toward the 1-3 binary line and if the gas is readily soluble. In terms of the Margules parameters and Henry s constant, these favorable criteria are ... 

In some cases, the solids themselves are subjected to extraction by a solvent. For example, in one process used to decaffeinate coffee, the coffee beans are mixed with activated charcoal and a high-pressure stream of supercritical carbon dioxide (carbon dioxide at high pressure and above its critical temperature) is passed over them at approximately 90°C. A supercritical solvent is a highly mobile fluid with a very low viscosity. The carbon dioxide removes the soluble caffeine preferentially without extracting the flavoring agents and evaporates without leaving a harmful residue. 

Water in its supercritical state has fascinating properties as a reaction medium and behaves very differently from water under standard conditions [771]. The density of SC-H2O as well as its viscosity, dielectric constant and the solubility of various materials can be changed continuously between gas-like and liquid-like values by varying the pressure over a range of a few bars. At ordinary temperatures this is not possible. For instance, the dielectric constant of water at the critical temperature has a value similar to that of toluene. Under these conditions, apolar compounds such as alkanes may be completely miscible with sc-H2O which behaves almost like a non-aqueous fluid. 

Following this, elastomers can be swollen by some high-pressure gases (especially CO2) as the densities of these gases approach liquid-like levels, at appropriate temperatures they become supercritical fluids which possess a solubility parameter magnitudes that, however, are highly dependent on temperature and pressure... 

Supercritical fluids (SCFs) are compounds that exist at a temperature and pressure that are above their corresponding critical values [70,71]. They exhibit the properties of both gases and Hquids. With gases, they share the properties of low surface tension, low viscosity, and high diffusivity. Their main Hquid-like feature is the density, which results in enhanced solubility of solutes compared with the solubility of gases. Furthermore, the solubility of solutes can be manipulated by changes in pressure and temperature near the critical point [72]. 

The solubilities of adamantane and diamantane in supercritical (dense) methane, ethane, and carbon dioxide gases have been measured by a number of investigators [35-37] at a few temperatures with various pressures and solvent densities. These measurements are reported in Figs. 9-12. 

Experimental data [36] on the effect of temperature and pressure on the supercritical solubility of adamantane in dense (supercritical) carbon dioxide gas is reported in Fig.9. 

The data of Smith [35] is reported graphically in Fig. 11 and shows the effect of pressure on the solubility of adamantane in various supercritical solvents (carbon dioxide, methane, and ethane) at 333 K. 

Trends of solubility enhancement for each diamondoid follow regular behavior like other heavy hydrocarbon solutes in supercritical solvents with respect to variations in pressure and density [38, 39]. Supercritical solubilities of... 

Figure 12. Effect of pressure on solubility (in units of mole fraction) of diamantane in dense (supercritical) gases at 333 K (for carbon dioxide and ethane) and at 353 K (for methane). Data from Ref. [35].

The supercritical fluid and liquid solubilities reported in Figs. 9-12 suggest that diamondoids will preferentially partition themselves into the high-pressure, high-temperature, and rather low-boiling fraction of any mixture including crude oil. 

The tuning of solubility with a relatively small jump or fall in pressure can possibly bestow many benefits with respect to rates, yields, and selectivity. Reaction parameters can be changed over a wide range. Replacement of solvents with high boiling points by supercritical (SC) fluids offers distinct advantages with respect to removal of the solvent. SC fluids like CO2 are cheap and environmentally friendly the critical temperature of CO2 is 31 C and the critical pressure 73.8 atm (Poliakoff and Howdle, 1995). Eckert and Chandler (1998) have given many examples of the use of SC fluids. Alkylation of phenol with tcrt-butanol in near critical water at 275 °C allows 2- erf-butyl phenol to be formed (a major product when the reaction is kinetically controlled 4-rert-butyl phenol is the major product, when the reaction is... 

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