Conventional extraction technologies

The two-phase vacuum extraction (TPVE) technology allows for the in situ remediation of soils and groundwater contaminated with volatile organic compounds (VOCs). Two-phase vacuum extraction is similar to conventional vapor extraction in the equipment required, with the exception that it is designed to actively remove contaminated groundwater from the extraction well along with the vapor-phase contamination. 

Supercritical fluid extraction (SFE) has been demonstrated as a technique that has eliminated some of the tedious steps of current liquid-liquid and solid-liquid extraction procedures. SFE also offers cleaner extracts, less sample handling and equivalent or better recoveries to conventional technologies. As a technique, it is cost effective, time efficient and low in solvent waste generation. 

The last two decades have seen an increased interest in the use of supercritical fluids in separation science. Supercritical C02 has often been employed as a naturally occurring medium for the separation, purification, and determination of organic substances in environmental samples. However, there are only limited reports on the use of supercritical fluid as solvent in the separation of metal ions from solutions as well as various solid matrices. The supercritical fluid extraction (SFE) technology offers several advantages over conventional solvent-based methods, including the ability to extract radionuclides directly from solids, easy separation of solutes from C02, and minimization of waste generation. It can easily be removed from the extracted substances by degasification under atmospheric pressure and temperature. 

Carrier facilitated transport processes often achieve spectacular separations between closely related species because of the selectivity of the carriers. However, no coupled transport process has advanced to the commercial stage despite a steady stream of papers in the academic literature. The instability of the membranes is a major technical hurdle, but another issue has been the marginal improvements in economics offered by coupled transport processes over conventional technology such as solvent extraction or ion exchange. Major breakthroughs in performance are required to make coupled transport technology commercially competitive. 

Acetic acid is less volatile than water, and the relative volatility is close enough to unity so that simple distillation is not the process of choice for recoveiy of acetic acid, except possibly at extremely high concentrations in aqueous solution.1 For the past 30 or more years, azeotropic distillation has becu the conventional technology for recovering acetic acid from feeds heving greater than 35-43% w/w acetic acid in water, For more dilute feeds Ihe favored process has bean extraction with solvents such as ethyl acetate, mi x lu res of e thy I aoeta and benzene. a nd i sop ropy 1 aceiale,1 These so I vents g ive v a lues o f the equilibriu m... 

In general, three conventional methods were used for the extraction of bioactive compounds such as solvents, steam, and supercritical fluids. On a global level, water extraction is practised while making cofiee or tea. Basically, pretreated plant material is extracted with hot water which takes up the flavor, taste, and color of the components. After filtration, the extract is ready for consumption. In case of the isolation of certain bioactive compounds from plant material by means of liquid extraction, some technological problems needs to be resolved [3]. First the plant material has to be pretreated in order to obtain reasonable extraction yields. Another problem is the need for special solvents to be used in the extraction procedure [4]. More recently, attention has been focussed towards the isolation of specific compounds that can be used in the food industry. Of particular interest is the isolation of bioactive compounds, aromas, and fiiagrances from plants and fruits [5,6]. The sequential extractions of bioactives using nonpolar to polar solvents are depicted in Figure 7.1. Various polarity solvents are reported as follows (1) nonpolar solvents (hexane, heptanes, petroleum ether,... 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

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. ... 

Conventional extraction technologies. . 59 from polymer/additive ... 

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