Cracks supercritical liquid

Cracking of PS because of capillary forces can be circumvented if one avoids crossing the liquid-vapor boundary in the phase diagram of the solvent. This is the case for supercritical drying [Ca4] or freeze drying [Ami], as shown in the inset of Fig. 6.12. 

Note Since liquid and vapour are indistinguishable in a supercritical fluid, there is no capillary pressure to cause shrinkage and cracking of the pores formed in the gel. 

Decomposition of methoxynaphthalene In supercritical water at 390 C occurs by proton-catalyzed hydrolysis and results In 2-naphthol and methanol as main reaction products. The rate of hydrolysis Is enhanced by dissolved NaCl. The dielectric constant and the Ionic strength of supercritical water was found to affect the hydrolysis rate constant according to the "secondary salt effect rate law, which commonly describes Ionic reactions In liquid solvents. In subcrltlcal water vapor the decomposition of the ether results In a mixture of cracking products and polycondensates, which Is characteristic for a radical type thermolysis. 

Seafloor temperature and pressure data for venting fluids indicate a wide variability relative to the two-phase (vapor-liquid) boundary for seawater (Fig. 4). Chloride concentrations in vent fluids (Table 1) range from 35 to 1090 mmol/kg due to subcritical boiling and/or supercritical phase-separation. For saline fluids such as seawater, phase-separation can occur above the critical point with condensation of brine droplets from a less saline residual vapor. Obviously, vent fluids decompress on ascent from subsurface reactions zones, which are inferred to be in cracking fronts just above the l-4... 

Cracks can also appear during the pressure release in the autoclave. In the supercritical drying process, the gel is subjected to high temperature and high pressure. When the critical point is reached, the pressure of the autoclave is decreased while the temperature is kept constant. At this instant, the pressure applied to the supercritical fluid is equal to that within the pores. The supercritical fluid has a very low density and viscosity compared with that of the liquid at room temperature however, the low permeability of the gel resists the flow of the supercritical fluid out of the gel. In other words, if the supercritical fluid release is performed too fast a pressure gradient appears. In this case the supercritical fluid within the gel, which is in compression, suddenly expands and the solid part suffers tensile stress. Experiments show that cracking depends on the pressure release rate, on the nature of the gel (basic or neutral), and on its geometrical dimensions. 

Figure 2.7. Typical depressurization crack (perpendicular to the largest surface) experimented by the silica gel during supercritical drying (illustrated here on a 1 cm thick wet silica tile having a liquid permeability between 5 and 10 nm, dried with supercritical CO2 at 313 K and 90 bar, and submitted to an autoclave depressurization of 0.15 bar min ). Courtesy of Rigacci A.

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