Mixed fluid solvent systems

The nature of mixed fluid solvent systems and their role in various supercritical fluid applications is complicated by the increased complexity of the phase behavior. Several different types of mixed fluid phase behavior have been identified (17,18), some of... 

Supercritical fluid crystallization (SFC) is a technique for precipitating or crystallizing solutes dissolved in liquid solvents by injecting or mixing the solvent system with a compressed or supercritical fluid antisolvent. SFC is unique in that it uses a compressed gas to trigger the crystallization. Two benefits often associated with SFC include single-step processing of particulate pharmaceuticals with controlled... 

Absorption spectra of 2-nitroanisole in supercritical C02, N20, Freon-13, ammonia and C02-methanol mixtures were obtained on a Cary model 1605 spectrophotometer operated in the dual beam mode. The gases used as supercritical solvents were of the highest purity available from the supplier (Matheson) and were further filtered prior to use. The mixed solvent system of C02-methanol was obtained from Scott Speciality Gases (15.4 wt% methanol), and other mixtures were made in the laboratory. Spectra of 2-nitroanisole in n-pentane, methanol, tetrahydrofuran and acetonitrile (Burdick A Jackson) were obtained using quartz cells with a 1-cm light path and with a pure solvent blank in the reference beam. Vapor phase and supercritical fluid spectra were obtained using an air reference. 

Clarke, M.A. Bishnoi, P.R. Development of a new equation of state for mixed salt and mixed solvent systems, and application to vapour liquid equilibrium and solid (hydrate) vapour liquid equilibrium calculations. Fluid Phase Equilibria 2004, 220, 21-35. 

When chromatographic separations (7) are operated in a batch mode, a portion of the mixture to be separated is introduced at the column inlet. A solute-free carrier fluid is then fed continually through the column, the solutes separating into bands or zones. Some industrial operations such as mixed-vapor solvent recovery and sorption of the less volatile hydrocarbons in natural gas or natural gasoline plants are being carried out on pilot plant and semiworks scales. Continuous countercurrent systems designed along the basic principles of distillation columns have been constructed. 

Fig. 10. Hydraulic fluid-solvent pumping and distribution systems. Black, tubes pressurized liquid. Hatched tubes alternate, a = oil reservoir, b = gear pump, c = pressure limiter, d = bleed check, e = low-pressure accumulator, f = intersifier (1,2,3,4), g = solenoid valves A,B,C,D), h = blending valve, i = pressure transducer, j = mixing chamber, k = to column, 1 = weak solvent, m = strong solvent. Courtesy Micromeritics Instrument Co. Since completion of the manuscript we have been aware of a change In the method for proportioning mobile phase. Now Micromeritics Model 7000 B uses an additional pair of solenoids to blend mobile phase by proportioning the application of hydraulic fluid on the low pressure side of the hydraulic intensifiers.

For systems composed of a pure fluid solvent and an organic solute, standard mixing rules were used for the calculation of the mixture parameters (10,11). Therefore, all the terms in equation 3 and 4 can be evaluated as a function of pressure and density at constant temperature, except for the partial molar volume of the solute in the stationary phase at infinite dilution vi P °. The partial molar volume of the solute in the stationary phase can be estimated, allowing solute retention in the supercritical region to be calculated as a function of pressure. 

Entrainers, modifiers, and co-solvents are basically mixed solvent systems and provide another dimension to supercritical fluid extraction. The entrainers enhance the solubility of the low volatile substance in the solvent, provide selective solubility in multi-solute instances, and enhance the sensitivity of the solubility and selectivity to temperature, pressure, and composition. The entrainers may be reactive and are also useful as slurrying media. Table 21.1.4 shows representative data on the effect of entrainers on vapor liquid systems that has been systematically studied by Brunner s group. Kumik and Reid as well as Johnston s group present data for dense gas-solid systems. 

Because COSMO-RS is a quite general theory for pure or mixed fluid systems, which treats the solvent etnd the solutes on the same footing, it is applicable to a great variety of questions in chemistry. A special advantage of the COSMO-RS formalism is its computational efficiency, since the time-demanding COSMO calculations have only to be performed once per molecule, or once per relevant, electrostatically different conformation, while all variations of solvents, compositions, and temperature only require a recalculation of the a-potential. 

This prompted Flory and Huggins to propose a model specific to polymers, which is an extension of the lattice fluid theory but better considers the specificity of polymers it compares the free energy of polymer-solvent systems before and after mixing. 

An alternative system proved to be both simpler and more user friendly (Unger et al., 2004 Machtejevas et al., 2006). Thus far we have used this configuration to analyze human plasma, sputum, urine, cerebrospinal fluid, and rat plasma. For each particular analysis we set up an analytical system based on a simple but specific strategy (Figure 9.5). The analysis concept is based on an online sample preparation and a two-dimensional LC system preseparating the majority of the matrix components from the analytes that are retained on a RAM-SCX column followed by a solvent switch and transfer of the trapped peptides. The SCX elution used five salt steps created by mixing 20 mM phosphate buffer (pH 2.5) (eluent Al) and 20 mM phosphate buffer with 1.5 M sodium chloride (eluent Bl) in the following proportions 85/15 70/30 65/45 45/55 0/100 with at the constant 0.1 mL/min flow rate. Desorption of the... 

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