Supercritical fluid separations mass transfer

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

Microwave induced plasma mass spectrometry has also been used as a detector for supercritical fluid chromatography (SFC) [113] for the separation of halogenated hydrocarbons. The design of an SFC-MIP interface must ensure that the frit restrictor temperature remains at a high temperature to prevent condensation of analytes. Stainless steel transfer lines may be used. The frit restrictor should be connected to a length of deactivated fused silica capillary, inserted through the transfer line, and positioned flush with the aluminum MIP torch inset (Fig. 10.21). 

To meet the supercritical conditions more suitable a mixer-settler unit has been developed by Schaffner [2], A regenerative pump was used as mixer to achieve an intense contact between the two phases and to create a large mass transfer surface area. This type of pump is capable of conveying gases as well as liquids and creates a high pressure difference which is used to separate the phases in the following fluid cyclone (fig. 1). 

The prospect of using enzymes as heterogeneous catalysts in scC02 media has created significant interest. Their low viscosity and high diffusion rates offer the possibility of increasing the rate of mass-transfer controlled reactions. Also, because enzymes are not soluble in supercritical fluids, dispersion of the free enzymes potentially allows simple separations without the need for immobilization. 

The use of supercritical fluids in separation processes has received considerable attention in the past several years and the fundamentals of supercritical fluid (SCF) extraction and potential applications have been described in a recent review article (p. It is generally known that supercritical conditions enhance the dissolution of solid particles. In comparison with liquid solvents, supercritical fluids have a high diffusivity, a low density and a low viscosity, thus allowing rapid extraction and phase separation. Little information is available in the literature however, on mass transfer coefficients between supercritical fluids and solids. 

If the polarity is considered equivalent to hexane and polar modifiers are added to the supercritical fluid, then the separation may be considered similar to normal-phase HPLC. However, the viscosity and mass transfer properties of supercritical fluids are more favorable and can lead to increased separation efficiencies and decreased analysis times. Berger and Wilson,for example, have demonstrated that separations with up to 260,000 theoretical plates can be achieved by serially coupling 10 HPLC columns without the deleterious pressure effects that would be encountered in separations using a liquid mobile phase. For applications that are not limited by polar matrices, SFC is, therefore, a viable option. 

The effect of dissolved CO2 on the miscibility of polymer blends and on phase transitions of block copolymers has been measured with spectroscopy and scattering (40). The shifts in phase diagrams with CO2 pressure can be pronounced. Polymer blends may be trapped kinetically in metastable states before they have time to phase separate. Metastable polymer blends of polycarbonate (PC) and poly(styrene-cn-acrylonitiile) were formed with liquid and supercritical fluid CO2 in the PCA process, without the need for a surfactant. Because of the rapid mass transfer between the CO2 phase and the solution phase, the blends were trapped in a metastable state before they... 

Revercon E. Fractional separation of of SCF extracts from marjorum leaves mass transfer and optimization. J Supercrit Fluids 1992 5 256-261. 

One of the benefits of using supercritical fluids as the solvent is the strong dependence of the solubility of the solute on the solvent density. This is a property that could be exploited for facilitating the separation of the solute from the solvent as it leaves the column, by dropping its pressure or raising its temperature, thereby lowering its density and the solubility of the solute. As a result, the extract separates into a liquid solute and a vapor solvent. Another favorable property of supercritical fluids as solvents is the high diffusivity of the solute in these fluids compared to that in liquids. Supercritical fluids also have a substantially lower viscosity than liquids. Because of these properties the mass transfer rate of the solute... 

Compared with LC, SFC can provide rapid separations without the use of organic solvents. SFC separations can be achieved faster than LC separations because the diffusion of solutes in supercritical fluids is about ten times greater than that in liquids (and about three times less than in gases). This results in a decrease in resistance to mass transfer in the column and allows for fast, efficient separations. 

The special properties of supercritical fluid (SCF) solvents [7,8] for PTC reactions bring substantial environmental and econonric advantages. First, they permit the use of totally benign solvents, especially CO2, and the solvent separation from product becomes quite facile. Moreover, since PTC processes always involve mass transfer, the lower viscosity and higher diffusivity of SCFs significantly enhance transport. 

Besides environmental and separation advantages, the use of supercritical fluids associated with PTC provides the extra benefit of an increased mass transfer. Viscosities and solute diffusivities of supercritical fluids are gas-like. 

Supercritical solvents have generated an increased interest in the last few decades. One reason is that their solvent properties vary considerable with temperature and density. They are tunable solvents [1] and for each purpose - separations or reactions - the optimal properties can be adjusted (see, for example, [1-8]). Usually, supercritical fluids are used as a tool to get homogeneous mixtures. In a homogeneous phase, for example, oxidations are extraordinarily fast and complete. The usually improved heat and mass transfer is a further advantage. Supercritical fluids show their good solvent properties only in the supercritical state. Therefore separation after reaction or extraction is very simply achieved by reducing temperature and pressure. This enables very sustainable processes (for example [1, 9]). Here supercritical carbon dioxide and water are of special interest, because they are cheap, nontoxic or of very low toxicity, in the case of carbon dioxide and nonexplosive. 

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