CANDIDATES FOR SUPERCRITICAL FLUIDS

Water is also included in the table to make one point— the solvent that we are all most familiar with is a poor candidate from both engineering and safety standpoint. The critical temperature and pressure are among the highest for common solvents. Ammonia is very unpleasant to work with since a fume hood or other venting precautions are needed to keep it out of the laboratory atmosphere. One of the alternative fluids of potential interest is nitrous oxide. It is attractive since it has molecular weight and critical parameters similar to carbon dioxide, yet has a permanent dipole moment and is a better solvent than carbon dioxide for many solutes. There are evidences of violent explosive reactions of nitrous oxide in contact with oils and fats. For this reason, nitrous oxide should be used with great care and is not suitable as a general purpose extraction fluid. 

Fluid Molecular weight, (gm/mol) Density of Liquid (gm/mol) Temp, of liquid ( K) Dipole moment (debyes) Critical Temp. Factor, (Tytc) C K/ C) Critical Pressure (F e) (bar) (atm) (psi) Critical Volume, (cmVmol) Critical Compress. (Z)c  

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Tables. Candidates for supercritical fluids and their various physical properties. The critical pressure is represented in three different units. (Constructed from Refs. 21-24.)... 

Typical materials that might be used for supercritical fluid cleaning or extraction are given in Ch. 1 by Manivannan and Sawan. The only fluid that will be discussed in this chapter, however, is carbon dioxide. The combination of low critical temperature and pressure, 31.7°C and 72 atm, nontoxicity, and low cost make it an ideal candidate for cleaning applications. 

White recently illustrated the use of fast supercritical fluid and EFLC for drug discovery and purification [46]. The optimized isocratic separations used to scale up to preparative-scale separations were often EFL mixtures. For example, Figure 9.13 shows the optimized conditions for the separation of a drug candidate included 30% methanol (with 0.2% isopropyl amine)/C02 on a Chiralcel OJ-H column at 5 mL/min [46]. His work also illustrates by using gradients that start in supercritical conditions and then move into EFL mixture conditions provides efficient and fast separations. 

One promising way to proceed seems to be the extraction of the halogenated components by supercritical fluids. This technique is called supercritical fluid extraction (SFE). Because of their special properties supercritical fluids show solubilities like organic solvents and transport properties like gases. Especially carbon dioxide with it s low critical data (Tc = 31,3 °C, pc = 7,28 MPa) was found to be a good candidate for the extraction of organic substances. There are several applications of the SFE-technique with SC-CO2 for instance in the field of ... 

Once it is determined that a particular material is a candidate for CO2 extraction, selection of appropriate temperatures and pressures must be made. As pointed out by Krukonis and McHugh, the solubilities of most materials in CO2 or any other supercritical fluid follow a curve shown in Fig. 2.1" Only the numerical values along the X and Y axis change wdth each solute and solvent. The foundation for this curve is contained in Eq. 1, where the different components of this equation have different temperature dependencies. 

Reverse Mieelles. Reverse Micelles in supercritical fluids are currently being studied for several distinct applications (15-18). Normal micelles and microemulsions in aqueous solutions are known to be capable of increasing solution viscosity in several applications including the surfactant flooding of petroleum reservoirs.(19) If reverse micelles or microemulsions can be formed in C02> an increase in solution viscosity could possibly occur. The surfactants chosen as candidates for CO2 flooding application should be characterized by low water solubility and a strong CO2 solubilityi minimal adsorption onto the porous media and stability at reservoir conditions. (20)... 

Since the nitrogen and sulfur contained in oil shale and tar sands are similar to these model compounds, supercritical fluids are therefore good candidates for removing nitrogen from shale oil and sulfur from tar sands-derived crude. 

Table 1.3 Potentially Important candidates for RD with supercritical fluids.

Chapter 2 discussed microemulsion structure. These organized media are stable and transparent. They are possible candidates for mobile phases in chromatography. Bile salt solutions are another kind of special micelles with chiral properties that can be used in MLC as well. Supercritical fluids (SF) were also used as surfactant solvents to perform micellar SFC, a variation of MLC. 

Solubility. Solubility in the primary process solvent is a mandatory criteria which is easily met in most extraction processes conducted in water. But, new applications such as polymeric viscosifiers for supercritical carbon dioxide flooding in enhanced oil recovery present enormous solubility problems for candidate polymers. If the polymer must remain soluble in secondary fluids encountered during the extraction process, then solubility requirements on the polymer will span large ranges of solvent pH, solvent-and-other-solute activity, and solvent polarity. 

Supercritical fluids (SCFs) offer the potential for a controlled solution environment because of the tunability of their properties by small changes in temperature and pressure. Indeed, near-critical water and supercritical water are obvious candidates as solvents in nanoparticle formation because water is the most commonly used solvent in conventional synthesis of inorganic particles. However, other solvents, such as carbon dioxide, can also be used. Several methods that take advantage of SCF behavior are described below. Not all have been employed in the production of magnetic nanoparticles. However, they represent a natural bridge between methods that are carried out mainly in the liquid state and those that are carried out in the gaseous state. 

Loading test experiment was performed on a methylcarbamate CF6 CSP with truns-l-amino-2-indanol in the polar organic mode. As shown in Fig. 15, 3.37 mg of the primary amine can be baseline separated in 20 min on an analytical column (250x4.6 mm). Furthermore, cyclofructan-type CSPs operates best in organic solvents and supercritical fluid solvents as well, which makes the sample recovery much easier than with aqueous mobile phases. These characteristics make the cyclofructan-type CSPs viable candidates for the preparative separations of primary amines. In addition, high loading of N-blocked amino acids were also reported on aromatic-derivatized CF6 CSPs [39]. 

As for preparative applications, the choice of technique is closely dependent on the amount of compound to be purified. The use of SFC will continue increasing, in combination with SMB mode or even in single-column mode, given the environmentally friendly character of the technique and the low cost in solvents. SMB, either with liquid or supercritical fluid mobile phases, allows for an improved use of the expensive stationary phase and it is the technique of choice if the purification of important amounts of enantiomers is required. However, one main disadvantage of SMB is the considerable inversion in equipment required, which is only recoverable in a reasonable period of time for companies dealing with a number of analytes or candidates in production. 

Comparison of the supercritical temperature and pressure conditions of some candidate fluids for industrial exploitation Q igure 3.1) may exclude those requiring extreme conditions, such as water, and others on environmental (SFg) or cost grounds (xenon). 

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