Commercial supercritical gas extraction

Design, Construction, and Operation of a Multipurpose Plant for Commercial Supercritical Gas Extraction... 

The extraction of metals based on a membrane contactor system with conventional solvents is a process widely studied using different configurations, extractants, and extraction solvents. One of the upcoming applications of membrane contactors is supercritical extraction. This process is called porocritical extraction. Porocritical process or porocritical extraction is a commercial supercritical fluid extraction (SFE) technique that utilizes an hollow fiber membrane contactor (HFMC) to contact two phases for the purpose of separation. As an improvement, the extraction of Cu + from aqueous solutions by means of dense gas extraction was achieved by using a hollow fiber membrane contactor device [7]. The authors... 

The main part of a laboratory plant for countercurrent gas extraction is the separation column, where the countercurrent contact of liquid and gaseous phase takes place. Equally important is the equipment for establishing the flow of the phases through the column. Other components depend on the size of the column, whether the supercritical solvent is recycled or not, and on the degree of automation. In the following, an example of a laboratory installation is described. It is a non-commercial installation used at our laboratory for separation experiments on edible oils, fats and related compounds, and on mineral oil fractions. 

This ability of compressed gases to display solvent powers akin to liquids is termed the gas extraction effect. The existence of this effect has been known for over 100 years (a brief historical survey is included as an appendix to this chapter). However it was not until about 1978 that a commercially viable extraction process based entirely on the gas extraction effect came into operation. The process, developed by HAG.AG in Germany, was for the decaffeination of coffee using compressed supercritical carbon dioxide and is described in chapter 5. Today this process and variants of it have been applied also to the decaffeination of tea and to the extraction of the bitter components from hops (chapters 4 and 8). 

The term supercritical fluid is used to describe any substance above its critical temperature and pressure. The discovery of the supercritical phase is attributed to Baron Cagniard de la Tour in 1822 [3], He observed that the boundary between a gas and a liquid disappeared for certain substances when the temperature was increased in a sealed glass container. While some further work was carried out on supercritical fluids, the subject remained essentially dormant until 1964 when a patent was filed for using supercritical carbon dioxide to decaffeinate coffee. Subsequent major developments by food manufacturers have led to the commercialization of this approach in coffee production. The use of supercritical fluids in the laboratory was initially focused on their use in chromatography, particularly capillary supercritical fluid chromatography (SFC). However, it was not until the mid-1980s that the use of SFE for extraction was commercialized. 

For safe and long-term storage, carbon dioxide must be injected more than 800 m below the Earth s surface. At that depth, the gas becomes a supercritical fluid. Such fluids have the gas-like characteristic of low viscosity and the liquidlike characteristic of high density. Supercritical behaviour exists only when temperature and pressure both reach, or exceed, their respective values at the so-called critical point . Every substance has its own critical temperature, above which the gas cannot be liquefied no matter how high the pressure. For carbon dioxide, the critical point lies at 31 °C and 7.4 MPa. Supercritical fluids have properties similar to those of liquid solvents and are employed commercially to extract soluble substances. For example, supercritical carbon dioxide is used to remove caffeine from coffee. 

Several processes utilizing supercritical fluids for materials processing have been reported in the literature although their commercial use is not well documented. Among the well-known processes are rapid expansion of supercritical solutions (RESS) (Phillips and Stella, 1993), the gas antisolvent process (GAS) (Yeo et al., 1993), aerosol solvent extraction system (ASES) (Bleich and Muller, 1996), a precipitation with compressed antisolvent process (PCA) (Brennecke, 1996), and solution-enhanced dispersion by supercritical fluids (SEDS) (Samp et al., 2000). The first four processes are for products that are soluble in the supercritical fluid or in an organic solvent. Biomolecules such as proteins or nucleic acids cannot be dissolved, and for such processes... 

Recent developments in the preparative supercritical fluid chromatography and extraction area include the following commercially available instruments and iimova-tions. Suprex have introduced the Prepmaster and Accutrap, two new products for automated quantitative sample preparation. The Prepmaster is a dedicated supercritical fluid ctraction system designed for analytical and semi-preparative extractions. It can be used on-line, directly interfaced to a gas chromatograph or SFC or off-line with the Accutrap cryogenic collection module. The Prepmaster can accommodate extraction vessels with capacities from 0.5 to 8 mL. 

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