Solubility in Compressed Gases

At higher temperatures (65-120 °C) elemental sulfur is also soluble in compressed gases hke nitrogen, methane, carbon dioxide, and hydrogen sulfide, a fact which is of tremendous technical importance for the gas industry since many natural gas reservoirs also contain H2S and elemental sulfur. During production of the gas the sulfur is partly transported to the surface and precipitates on decompression and/or cooling of the gas mixture at the well-head [160-165]. Clogging of pipelines may then result [166]. 

Solubility in Compressed Gases.— When an inert gas (1) is added to a system in which a liquid or solid (2) is in equilibrium with its vapour, the vapour pressure of the condensed substance changes. In other words, if the total pressure in the system is p and the mole fraction of (2) in the vapour phase is y, the partial pressure py is not equal to pf, the vapour pressure of the pure substance. If the inert gas is not appreciably soluble in the solid or liquid, the change in vapour pressure is attributable to two effects. (/) The chemical potential of the solid or liquid is increased because of the rise in the applied pressure, the so-called Poynting effect. (k) The chemical potential of the vapour is changed because of interactions between the vapour and the inert gas. 

The correlation of solid solubilities in compressed gases using equations of state is not at present in as satisfactory a condition as the correlation of vapour/liquid equilibrium in comparatively low molecular weight systems. 

Where the reaction mixture is a single compressed fluid phase, the small amounts of initiator used can be expected to be soluble, regardless of the phase in which the pure initiator would exist at the temperature and pressure of the mixture. This will be even so for solids, since thdr equilibrium solubility in compressed gases is usually far greater than that corre nding to their vapor pressure at the pressure exerted by the mixture (2d). 

From the more practical point of view, what is the basis for developing the analytical description of these equilibrium states In previous Chapters, for example, we accepted that when two phases are in equilibrium at a given temperature and pressure, the fugacity of any component in the one phase is equal to that in the other phase. [We used this equality to arrive at a physical interpretation of fugacity in Chapters 9 and 11 to determine vapor pressures of pure fluids in Chapter 10 using equations of state and to calculate gas solubilities in liquids, or solid and liquid solubilities in compressed gases, in Chapter 11.] Where does this equality of fugacities come from ... 

Allnatt). Solubility of Solids in Compressed Gases (Rowlinson and 11 1... 

Despite the importance of mixtures containing steam as a component there is a shortage of thermodynamic data for such systems. At low densities the solubility of water in compressed gases has been used (J, 2 to obtain cross term second virial coefficients Bj2- At high densities the phase boundaries of several water + hydrocarbon systems have been determined (3,4). Data which would be of greatest value, pVT measurements, do not exist. Adsorption on the walls of a pVT apparatus causes such large errors that it has been a difficult task to determine the equation of state of pure steam, particularly at low densities. Flow calorimetric measurements, which are free from adsorption errors, offer an alternative route to thermodynamic information. Flow calorimetric measurements of the isothermal enthalpy-pressure coefficient pressure yield the quantity 4>c = B - TdB/dT where B is the second virial coefficient. From values of obtain values of B without recourse to pVT measurements. 

P. Villard has measured the influence of compressed gases on the vapour press, of liquid bromine, and he found that bromine vapour is fairly soluble in compressed oxygen... 

The enhanced solubility of solids in compressed gases at elevated temperatures and pressures was first noted more than one hundred years ago. Hannay and Hogarth (1) observed that the solubility of salts in compressed ethanol was considerably greater than expected based on the vapor pressure of the salts. Since this early investigation, numerous authors have discussed the phase behavior of solids in dense fluids at elevated temperatures and pressures. 

This equation is analogous to Eq. 5 of Ch. 1 for the solubility of a solid in a SCF. In this equation, the subscript 2 refers to the liquid component. The superscript s refers to saturation conditions at temperature T. Pj refers to the saturation vapor pressure of the liquid at temperature T. The variable uf is the molar volume of the liquid, ( )2 is the fugacity coefficient at saturation pressure and is the fiigacity coefficient in the high pressure gas mbrture. For a detailed derivation of this equation, see Prausnitz. " As is stated in the derivation, it is the escaping tendency of the liquid into the supercritical fluid phase, as described by the fugacity coefficient, ( >2, which is responsible for the enhanced solubility of liquids in compressed gases. 

Zhuze, T. P. Yushkevich, G. N. 1959. Solubility of oil and its heavy fractions in compressed gases. Trudy Institut Nefti Akademia Nauk SSSR, 13, 262-274. 

This process takes advantage of the high solubility of compressed gases (especially SCCO2) in a broad range of solid materials (see Section 6.2.4). The pressure which... 

A. Gas-Liqmd Equilibria.— Because of their great industrial and scientific interest, gas-liquid equilibria have been extensively studied as a function of pressure up to the critical region, and much work has been done for the related phenomena of the solubility of compressed gases in liquids and of liquid or crystalline substances in compressed gases. ... 

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