Supercritical solvent capacity

Supercritical carbon dioxide was used for bone delipidation. It appeared that this technology is very efficient since supercritical CO2 is able to diffuse into microporous solids much better than liqnids and that it has a good solvent capacity for lipids. Moreover, it is safe since it involves no toxic chemical and is potentially usable with allografts as well as xenografts (Fages et al., 1994). 

A way around this issue may have been found with the use of supercritical fluids. These materials, such as liquid carbon dioxide, have many interesting properties from the point of view of pharmacutical processing since they combine liquid-like solvent properties with gas-like transportation properties. Small changes in the applied pressure or temperature can result in large changes of the fluid density and, correspondingly, the solvent capacity and properties of the resultant particles. 

The gas anti-solvent processes overcome the limited solvent capacity of supercritical gases for substances with high molecular weight by using a classical liquid solvent. 

A test was made with 2,3-dimethylbutane as the supercritical solvent it has a lower critical temperature than 2,2,4-trimethyl-pentane. Operating at a temperature of 508-512 K, a pressure of 4.10-4.37 MPa, a molecular sieve/oil ratio of 6.39, and a solvent/ oil ratio of 21.3, the molecular sieve capacity attained is 5.73 g/100 g of molecular sieves (as compared to 3.2 g/100 g of molecular sieves with 2,2,4-trimethylpentane at 550 K). The n-paraffin content of the wax distillate was reduced by 88% to a level of 2 wt %, giving a pour point of 266 K. The yield of denormal oil was lower (63%) and the n-paraffin content of the desorbate was lower (44%) at this lower temperature level. This is probably due to increased capillary condensation. Conversely, operation at temperatures greater than 550 K should produce less capillary condensation and purer n-paraffin product. It would be interesting to try supercritical solvents with critical temperatures in the 600-670 K range. 

The solvent capacity of supercritical carbon dioxide changes with the variation of density and so it can be easily modulated by the variation of pressure. Adding small amounts of co-solvents (modifiers) changes the chemical and physical properties, like the solvent capacity or the critical point. To ensure supercritical conditions it is thence crucial to know the critical parameters as a function of the modifier concentration. Table 1 lists the critical pressure and temperature as a function of the co-solvent concentration in carbon dioxide/2/. 

Percolation leach particle diameter >700 pm hquid concentration 0.8 to 20% relatively fragile solid (e.g., seeds). Immersion leach particle diameter <700 pm liquid concentration <20% relatively robust solids (e.g., minerals). Combo leach high feed concentration of solute, relatively robust solid. Supercritical solvent usually CO2 for small capacity of high value products, especially for temperature sensitive foods, cosmetics, and pharmaceuticals. 

Combo leach high feed concentration of solute, relatively robust solid. Supercritical solvent usually CO2 for small capacity of high value products especially for temperature sensitive foods, cosmetics and pharmaceuticals. 

Supercritical operation of a reactor means operating it at temperatures and pressures above the critical point of one or more of the substances present - see Figure 5.46. Supercritical fluids exhibit a combination of properties normally associated with both liquids and gases (in particular the solvent capacity of a liquid and viscosity of a gas). 

When the supercritical fluid and drug solution make contact, a volume expansion occurs leading to a reduction in solvent capacity, increase in solute saturation, and then supersaturation with associated nucleation and particle formation. A number of advantages are claimed by using this platform technology (6), such as particle formation from nanometers to tens of micrometers, low residual solvent levels in products, preparation of polymorphic forms of drug, etc. 

Solvent capacity of supercritical fluids does not cover such a broad range as liquid solvents capacity (eg, proteins cannot be processed by PS-SFC). 

This state emphasises its capacity to dissolve chemicals and natural substances of similar way as do different organic solvents such as hexane, acetone or dichloromethane. Therefore, the first applications focused on the extraction of natural substances as an alternative to using organic solvents. Thus, removal of caffeine (coffee or tea) with supercritical C02 is the most mature application at industrial level and is also used in the extraction of hops or cocoa fat. 

Packed-column SFC also is suitable for preparative-scale enatioseparations. Compared with preparative LC, sub- or supercritical fluid chromatography results in easier product and solvent recovery, reduced solvent waste and cost, and higher output per unit time. Because of its reduced sample capacity, SFC usually allows the separation of 10-100 mg samples per run. Chromatographers can compensate for these sample amounts by using shorter analysis times and repetitive injections (Wolf and Pirkle, 1997). 

Water. Water long used mainly for aqueous, generally ionic chemistry, more recently became attractive to replace organic solvents particularly under supercritical conditions (see discussion below). It is abundant, easy to purify, not flammable or toxic, and easily available in large quantities, and, therefore, its use is most economical. Product isolation in many cases can be a simple phase separation, and because of its large heat capacity, water offers an easy temperature control. The... 

Supercriticial Solvents. Although it was known in 1879 that supercritical fluids had solvent properties (180), supercritical extraction was not extensively developed until the early 1980s. This method uses organic or inorganic compounds as solvent, at or usually above their critical temperature and pressure where they are known as supercritical fluids. In a supercritical fluid state, common gases such as carbon dioxide have the properties and extractive capacity of a liquid. The compound most used in supercritical extractions is carbon dioxide. Carbon dioxide can exist as a gas, liquid, or solid, depending on pressure and temperature conditions. However, at or above its critical point. CO2 can only exist as a supercritical fluid. 

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