Supercritical fluid process steps

Since the products and the process steps to which a supercritical fluid process could be successfully applicable are widespread, the demand for high pressure pilot units to check the feasibility arose first of all in the field of Supercritical Fluid Extraction (SFE). 

Bandi, N. Wei, W. Roberts, C. Kotra, L. Kompella, U.B. Preparation of budedonide- and indomethacin- hydroxy-propyl-(3-cylcodextrin (HPpCD) complexes using a single step, organic solvent-free supercritical fluid process. Eur. J. Pharm. Sci. 2004. In press. 

Supercritical fluids have also been used purely as the solvent for polymerization reactions. Supercritical fluids have many advantages over other solvents for both the synthesis and processing of materials (see Chapter 6), and there are a number of factors that make scCCH a desirable solvent for carrying out polymerization reactions. As well as being cheap, nontoxic and nonflammable, separation of the solvent from the product is achieved simply by depressurization. This eliminates the energy-intensive drying steps that are normally required after the reaction. Carbon dioxide is also chemically relatively inert and hence can be used for a wide variety of reactions. For example, CO2 is inert towards free radicals and this can be important in polymerization reactions since there is then no chain transfer to the solvent. This means that solvent incorporation into the polymer does not take place, giving a purer material. 

Current work with supercritical fluids can also illustrate the importance of cosolvents. Cosolvent effects in supercritical fluids can be considerable for systems where the cosolvent interacts strongly with the solute. A correlation suggests that both physical and chemical forces are important in the solvation process in polar cosolvent supercritical CO2 mixtures. The model coupled with the correlation represents a step toward predicting solubilities in cosolvent-modified supercritical fluids using nonthermody-namic data. This method of modeling cosolvent effects allows a more intuitive interpretation of the data than either a purely physical equation of state or ideal chemical theory can provide (Ting et al., 1993). 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

The soil samples which were extracted in our experiments were saturated with methanol prior to the introduction of the mobile phase. Other samples, especially those which have been field aged, are saturated with water and water/methanol mixtures before the supercritical fluid extraction process begins. Our experience indicates that this saturation step is crucial to obtain high recoveries. 

Sample preparation is a key step in all environmental analyses. Two major areas of development in this area have been solid phase extraction and supercritical fluid extraction. Both techniques have made the extraction of pollutants from aqueous and nonaqueous matrices relatively simple, fast, and less expensive. These processes, along with gel permeable chromatography, provide efficient methods of removing interferences. 

The extraction efficiency of supercritical fluids may be enhanced by mixing into it a small amount of a cosolvent such as acetone or methanol. Supercritical fluid extraction offers certain advantages over other extraction processes (1) it is relatively a fast process with greater extraction efficiency (2) sample concentration steps may be eliminated and (3) unlike LLE or Soxhlett extraction, a large amount of organic solvents is not required. 

Supercritical fluid crystallization (SFC) is a technique for precipitating or crystallizing solutes dissolved in liquid solvents by injecting or mixing the solvent system with a compressed or supercritical fluid antisolvent. SFC is unique in that it uses a compressed gas to trigger the crystallization. Two benefits often associated with SFC include single-step processing of particulate pharmaceuticals with controlled... 

Many industrial wastewaters and landfill leachates are contaminated with organic pollutants, which are biologically persistant. Due to the low concentration levels, the removal of such effluents by incineration is not favourable. Therefore, an integrated method is under development at our institute to extract organic pollutants in a first step with supercritical fluids (especially C02) and to further destroy them in a second step by an indirect electrooxidation process. 

Supercritical fluid extraction (SFE) is a suitable process for many separation problems. The regeneration of the supercritical fluid is as important as the extraction step itself Therefore this paper presents a method to do this in a more isobaric way than the customary pressure reduction regeneration. For the example of soil remediation we have investigated the activated carbon regeneration of supercritical carbon dioxide loaded with the low-volatile polycyclic aromatic hydrocarbon (PAH) pyrene. Characteristics of supercritical fluid extraction for soil remediation are elevated temperatures and pressures up to 370 K and 300 bar. For this reason adsorption isotherms of pyrene on activated carbon up to these conditions are measured first. Subsequently this method is used to regenerate carbon dioxide in a closed solvent cycle plant with a 4 1 extractor. An economic analysis using these results indicate that the soil remediation costs will decrease for about 20 - 30 % by means of an activated carbon adsorber. 

The main aspects which determine the module pattern to be applied are the process parameters, the required process steps including product pre-treatment, extraction and separation, the process enhancements and finally the equipment parameters. The corresponding basic modules and some key components are explained hereafter using current examples of SITEC pilot units built for supercritical fluid extraction purposes. 

In general, the extraction process must be streamlined to reduce solvent consumption since removal of solvents can represent a major bottleneck. Additionally, the extraction method chosen should support the first step of fractionation for library generation. Supercritical fluid extraction (SFE) represents an efficient extraction method in terms of low solvent consumption and extraction speed. Supercritical fluids exhibit high diffusivity with low viscosity and low surface tension they can readily permeate biomass matrices and solvate molecules, including drug-like compounds, leading to efficient extractions. The addition of small amounts of organic co-solvents may enhance... 

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