Supercritical fluid separations applications

TABLE 22-11 Commercial Applications of Supercritical Fluid Separations Technology... 

For most applications, it is not possible to treat supercritical fluid separations, for example extraction, as a well-defined unit operation as is the case for simpler proctssts such as distillation. Instead, research is often needed to characterize the important properties for each specific separation or reaction. However, the recent advances in the molecular understanding of SCF solutions provide some general themes that can be utilized in a semi-quantitative manner to evaluate the potential of both research and applications. 

R598 Y.-g. Chen, NMR Spectroscopic Characteristics of Lignans from Plants of Schisandraceae , Bopuxue Zazhi, 2000,17,427 R599 Z.-W. Chen and Z. Chen, Hyphenation of Supercritical Fluid Separation with Nuclear Magnetic Resonance and their Applications , Guangpu Shiyanshi, 2001,18, 139... 

How does one evaluate the process viability of an SCF application We have touched on economics several times in the preceding chapters, but only superficially. As there are no hard and fast answers to questions of viability with traditional processes, so are there none with supercritical fluid extraction. Nevertheless, both of us are asked quite frequently How much will it cost To indicate why there is no single answer, we list below just a few of the parameters that influence the cost of a supercritical fluid separation process ... 

Supercritical extraction (SCE) is a modem separation technique that uses the dramatic increase in solubility of some solutes in supercritical fluids. Important applications have been found in food industry, as the extraction of caffeine from coffee, fats from butter, etc. Ethanol may be also recovered by extraction with COj. We should also mention the use of supercritical water to solve environmental problems, as the destmction of poly-chloro-benzenes (PCBs) by oxidation in supercritical water. 

There have been many useful attempts made to classify separation technologies. Supercritical fluids are applicable with both intra-phase and inter-phase separations. Due to the ease and flexibility in which a new phase can be formed for regeneration of the solvent, inter-phase is the more common. Furthermore, material solubility and swelling problems, particularly with organic-component based membranes, limit inter-phase separations. This is due to the enhanced solubility of these components in supercritical solvents. 

The ability of small molecular fluids under nearcritical conditions to dissolve low-vapour-pressure solid materials was first discovered by Hannay et al. (1). Scheffer and coworkers (2) investigated extensively the solubility of naphthalene in near- and supercritical ethylene. Since then many researchers have started to study the possibilities of supercritical solvents and within the past two decades several research institutes have Investigated and developed the principles and technology of supercritical fluid separations. Commercial application can be found in areas as diverse as spice extraction, monomer purification, coal extraction, nicotine and caffeine extraction, fractionation of (co-) polymers or the extraction of oils from all kinds of natural products. Reviews of most of this work are... 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

Another application of SFC-GC was for the isolation of chrysene, a poly aromatic hydrocarbon, from a complex liquid hydrocarbon industrial sample (24). A 5 p.m octadecyl column (200 cm X 4.6 mm i.d.) was used for the preseparation, followed by GC analysis on an SE-54 column (25 m X 0.2 mm i.d., 0.33 p.m film thickness). The direct analysis of whole samples transferred from the supercritical fluid chromatograph and selective and multi-heart-cutting of a particular region as it elutes from the SFC system was demonstrated. The heart-cutting technique allows the possibility of separating a trace component from a complex mixture (Figure 12.21). 

Supercritical fluid chromatography (SFC) refers to the use of mobile phases at temperatures and pressures above the critical point (supercritical) or just below (sub-critical). SFC shows several features that can be advantageous for its application to large-scale separations [132-135]. One of the most interesting properties of this technique is the low viscosity of the solvents used that, combined with high diffusion coefficients for solutes, leads to a higher efficiency and a shorter analysis time than in HPLC. 

Myths and misconceptions about the characteristics of supercritical fluids have slowed their application to chromatographic separations. While these fluids do have interesting properties, they are not super fluids, and they are not suitable for all types of separations. An understanding of the fundamental behavior of supercritical fluids is key to identifying appropriate applications [10]. 

A third motivation for studying gas solubilities in ILs is the potential to use compressed gases or supercritical fluids to separate species from an IL mixture. As an example, we have shown that it is possible to recover a wide variety of solutes from ILs by supercritical CO2 extraction [9]. An advantage of this technology is that the solutes can be removed quantitatively without any cross-contamination of the CO2 with the IL. Such separations should be possible with a wide variety of other compressed gases, such as C2LL6, C2LL4, and SF. Clearly, the phase behavior of the gas in question with the IL is important for this application. 

The solubilities, discussed above, of the various gases in the ionic liquids have important implications for applications of IFs. The impact of gas solubilities on reactions, gas separations and the use of compressed gases or supercritical fluids to separate solutes from IFs are discussed below. 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

The current state of analytical SPE was critically reviewed and no major changes of the technique have been observed. Overviews of the developments of the extraction technologies of secondary metabolites from plant materials refer to three types of conventional extraction techniques that involve the use of solvents, steam, or supercritical fluids. Each technique is described in detail with respect to typical processing parameters and recent developments. Eollowing the discussion of some technical and economic aspects of conventional and novel separation processes, a few general conclusions about the applicabilities of the different types of extraction techniques are drawn. ... 

High performance liquid chromatography (HPLC) has been by far the most important method for separating chlorophylls. Open column chromatography and thin layer chromatography are still used for clean-up procedures to isolate and separate carotenoids and other lipids from chlorophylls and for preparative applications, but both are losing importance for analytical purposes due to their low resolution and have been replaced by more effective techniques like solid phase, supercritical fluid extraction and counter current chromatography. The whole analysis should be as brief as possible, since each additional step is a potential source of epimers and allomers. 

Supercritical fluid extraction (SEE) is another modern separation technology usually employed to extract lipophilic compounds such as cranberry seed oil, lycopene, coumarins, and other seed oils. Anthocyanins generally and glycosylated anthocyanins in particular were considered unsuitable for SEE due to their hydrophilic properties, since SEE is applicable for non-polar analytes. However, a small amount of methanol was applied as co-solvent to increase CO2 polarity in anthocyanin extraction from grape pomace. New applications of SEE for anthocyanin purification have been reported for cosmetic applications from red fruits. ... 

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