Supercritical solids separations

Figure 15.9 Schematic diagram of a simplified supercritical water oxidation plant. It consists of pumps for the water/effluent mixture (A) and for oxygen or air (B), a reaction vessel (C), a solids separator (D) and a pressure control valve (E), to maintain the desired pressure of the...

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. 

Figure 15.14 Separation of explosives exnacted from water by using SPE-SFE-GC at several SEE trapping temperatures, peak identification is as follows NG, nitroglycerin 2,6-DNT, 2,6-dinitrotoluene 2,4-DNT, 2,4-dinitrotoluene TNT, triniti otoluene IS, 1,3-tiichloroben-zene. Adapted Journal of High Resolution Chromatography, 16, G. C. Slack et al., Coupled solid phase extraction supercritical fluid extraction-on-line gas cliromatography of explosives from water , pp. 473-478, 1993, with permission from Wiley-VCH.

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 dense fluid that exists above the critical temperature and pressure of a substance is called a supercritical fluid. It may be so dense that, although it is formally a gas, it is as dense as a liquid phase and can act as a solvent for liquids and solids. Supercritical carbon dioxide, for instance, can dissolve organic compounds. It is used to remove caffeine from coffee beans, to separate drugs from biological fluids for later analysis, and to extract perfumes from flowers and phytochemicals from herbs. The use of supercritical carbon dioxide avoids contamination with potentially harmful solvents and allows rapid extraction on account of the high mobility of the molecules through the fluid. Supercritical hydrocarbons are used to dissolve coal and separate it from ash, and they have been proposed for extracting oil from oil-rich tar sands. 

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (the stationary phase), while the other (the mobile phase) moves in a definite direction. A mobile phase is described as a fluid which percolates through or along the stationary bed in a definite direction . It may be a liquid, a gas or a supercritical fluid, while the stationary phase may be a solid, a gel or a liquid. If a liquid, it may be distributed on a solid, which may or may not contribute to the separation process. ... 

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. 

To date most of the work which has been done with supercritical fluid extraction has concentrated on the extraction of analytes from solid matrices or liquids supported on an inert solid carrier matrix. The extraction of aqueous matrices presents particular problems [276-278]. The co-extraction of water causes problems with restrictor plugging, column deterioration, and phase separation if a nonpolar solvent is used for sample collection. Also, carbon dioxide isay have limited extraction efficiency for many water soluble compounds. 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

SFE has been used extensively in the analysis of solid polymers. Supercritical fluid extraction of liquid samples is undertaken less widely because dissolution or entrainment of the matrix can occur. As illustrated elsewhere SFE has also been applied for the analysis of liquid poly(alkylene glycol) (PAG) lubricants and sorbitan ester formulations [370]. The analysis of PAG additives (antioxidants, biocides and anticorrosion, antiwear and antifoaming agents) is hindered by the presence of the low molecular weight PAG matrix (liquid) and therefore a method for the selective separation of additives from PAG is required. The PAG... 

The discovery of supercritical fluids occurred in 1879, when Thomas Andrews actually described the supercritical state and used the term critical point. A supercritical fluid is a material above its critical point. It is not a gas, or a liquid, although it is sometimes referred to as a dense gas. It is a separate state of matter defined as all matter by both its temperature and pressure. Designation of common states in liquids, solids and gases, assume standard pressure and temperature conditions, or STP, which is atmospheric pressure and 0°C. Supercritical fluids generally exist at conditions above atmospheric pressure and at an elevated temperature. Figure 16.1 shows the typical phase diagram for carbon dioxide, the most commonly used supercritical fluid [1]. 

The separation and estimation of diloxanide furoate and metronidazole in solid dosage forms was reported by Bhoir et al., using packed column supercritical fluid chromatography [38], A JASCO Cig colunm (10 pm particle size, 25 cm x 4 mm) was used at 40°C, with an injection volume of 20 pL. The mobile phase consisted of 26% methanol in CO2 (flow rate of 2 mL/min), and operated at a pressure of 17.6 MPa. When detected on the basis of its ultraviolet absorbance at 230 nm, the retention time for the drug was 1.6 minutes. The linear region of the calibration graph was reported to be 20-70 pg/mL. 

Classical sample preparation methods such as distillation, soxhlet extraction are still used [839, 840], but specific techniques such as supercritical fluid extraction (SFE) [841], and increasingly in recent years, adsorption techniques such as solid phase micro-extraction (SPME) [841a] are also being used for isolation, separation, and identification of flavor and fragrance materials. 

Supercritical fluid extraction is a new separation technique that finds a number of applications in the natural products, biochemicals, food, pharmaceuticals, petroleum, fuel, and polymer industries (1-8). There is now an interest in applying this technology in the pulp and paper industry (9,10). In a recent comprehensive study on the interaction of supercritical fluids with lignocellulosic materials, it has been shown that lignin can be not only extracted from wood by reactive supercritical fluids but also separated as solid products in solvent-free form by reducing the extraction fluid pressure from a supercritical to sub critical level (11,12). 

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