Fluid—solid extraction influencing

Principles and Characteristics Supercritical fluid extraction uses the principles of traditional LSE. Recently SFE has become a much studied means of analytical sample preparation, particularly for the removal of analytes of interest from solid matrices prior to chromatography. SFE has also been evaluated for its potential for extraction of in-polymer additives. In SFE three interrelated factors, solubility, diffusion and matrix, influence recovery. For successful extraction, the solute must be sufficiently soluble in the SCF. The timescale for diffusion/transport depends on the shape and dimensions of the matrix particles. Mass transfer from the polymer surface to the SCF extractant is very fast because of the high diffusivity in SCFs and the layer of stagnant SCF around the solid particles is very thin. Therefore, the rate-limiting step in SFE is either... 

The produced fluids and gases are typically directed into separation vessels. Under the influence of gravity, pressure, heat, retention times, and sometimes electrical fields, separation of the various phases of gas, oil, and water occurs so that they can be drawn off in separate streams. Suspended solids such as sediment and salt will also be removed. Deadly hydrogen sulfide (H2S), is sometimes also encountered, which is extracted simultaneously with the petroleum production. Crude oil containing H2S can be shipped by pipeline and used as a refinery feed but it is undesirable for tanker or long pipeline transport. The normal commercial concentration of impurities in crude oil sales is usually less than 0.5% BS W (Basic Sediment and Water) and 10 Ptb (Pounds of salt per 1,000 barrels of oil). The produced liquids and gases are then transported to a gas plant or refinery by truck, railroad tank car, ship, or pipeline. Large oil field areas normally have direct outlets to major, common-carrier pipelines. 

As mentioned above, most MIPs are synthesized in organic solvents to preserve the hydrogen and electrostatic interactions between template and monomer. However, for the application to solid-phase extraction (SPE) where the target is most of the time in water samples or in biological fluids, a lot of studies have been carried out to examine the influence of binding media parameters (solvent polarity and composition, buffer pH, concentration, ionic strength, etc.) with the aim of attenuating non-specific adsorption of the analyte due to hydrophobic interactions which predominate in such media. For a recent review, see Tse Sum Bui and Haupt [95]. 

The morphology of the matrix on which we wish to make a SFE can have an enormous influence on the efficiency of the extraction rate. Generally a rapid and complete extraction depends upon the relative size of the matrix particles, the smaller being the better. This is due principally to the short internal distance that the solute must cover in order to attain the core of the supercritical fluid solution. Some studies have shown that the geometrical form can also have an influence on the rate and efficacity of the extraction. As in the case of an extraction solid-liquid, an increase in the porosity of the matrix will lead to an efficient and rapid extraction. 

During operation of an SMB plant the propagation of components to be separated are influenced by the internal fluid flow rates in the different sections as well as the switching time that simulates movement of the solid. By appropriate choice of operating parameters the movement of the less retained component is focused on the raffinate port while the more retained component is collected in the extract stream. Figure 7.16 shows an optimal axial profile of the liquid concentrations at the end of a switching interval after the process has reached a periodic steady state. The adsorption and desorption fronts of both components have to start or stop at given points to achieve complete separation at maximum productivity. 

In addition to extraction from solids, supercritical fluids can be used to extract aromatic molecules from liquids. Senorans et al. have utilized carbon dioxide to extract high-quality brandy aroma using a countercurrent supercritical fluid extractor. The aroma quality is influenced by the extraction conditions. Medina and Martinez studied alcohol removal from beverages using supercritical carbon dioxide, to produce beverages with low-alcohol content but sufficient flavor, because of three key benefits 1) water and salts are not appreciably removed by the carbon dioxide 2) proteins and carbohydrates are not extracted or denatured and 3) there is a good control in the aroma recovery. The alcohol removal efficiency increases with the extraction pressure raffinate alcohol concentration can be reduced up to 3 wt.% at 250 bar and 40°C, from 6.2 wt.% in the feed. " ... 

The extraction system involves a bulk solid phase and a fluid phase. The fluid phase comprises the supercritical solvent and the dissolved extract. The solid phase remains within the extraction vessel and the fluid phase is passed through the extraction vessel. Mass transport occurs between the two phases. Any of the following parameters that can influence the process may be of interest. 

Supercritical anti-solvent micronization can be performed using different processing methods and equipment [17]. Different acronyms were used by the various authors to indicate the micronization process. It has been referred to as GAS (gas anti-solvent), PCA (precipitation by compressed anti-solvent), ASES (aerosol solvent extraction system), SEDS (solution enhanced dispersion by supercritical fluids), and SAS (supercritical anti-solvent) process [8,17]. Since the resulting solid material can be signiflcantly influenced by the adopted process arrangement, a short description of the various methods is presented below. 

The most important parameters that influence the supercritical fluid extraction process of solid substrates are extraction pressure and temperature, which determine solvent density, solvent ratio and solid substrate pretreatmente and moisture (Brunner, 1994). 

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