Supercritical fluid extraction phase equilibria

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

Equilibrium phase diagrams for one or more solutes in supercritical CO2 and other solvents can be very complicated. Relatively little equilibrium information has been published this lack coupled with the cost of high-pressure equipment and the difficulties of scaleup have to date limited the commercial applications of supercritical fluid extraction. 

To date little or no thermodynamic modeling of the phase behavior of the ligand/C02 or metal chelate/C02 systems has been conducted. However, in order for supercritical fluid extraction to be considered as a possible replacement for organic solvent extraction, accurate models must be developed to predict the phase behavior of these systems to allow for both equipment and process design. Equation of state (EOS) modeling was chosen here to model the vapor-liquid equilibrium of the P-diketone/C02 systems studied. Cubic EOSs are the most widely used in modeling high pressure and supercritical fluid systems. This is... 

In conclusion, it is possible to concentrate the flavor fraction of cold-pressed citrus oils with supercritical fluid technology by selectively extracting the terpenes from the oil. During continuous extractions, the amount of extract followed a linear trend with time over the first 5 hours of extraction and it increased five times when the flow rate was increased ten times. Since the design of supercritical fluid extraction and solvent regeneration processes for the concentration of citrus oils require accurate calculation of phase equilibria, more research must be done to determine the equilibrium solubility data, the thermod3mamic model to represent the system, and the economic feasibility of the process. 

The basic for developing a high pressure liquid extraction unit is the phase equilibrium for the (at least) ternary system, made up of compound A and compound B, which have to be separated by the supercritical fluid C. Changing pressure and temperature influences on one hand the area of the two phase region, where extraction takes place, and on the other hand the connodes, representing the equilibrium between extract and raffinate phase. 

We refer to Fig. 6.7-1. Reaching once equilibrium between the supercritical fluid SCF1 and the feed in the extractor El is enough for separation. By changing pressure and temperature the produced extract EX1 and raffinate R1 concentrations can be varied following the ternary phase equilibrium. The supercritical solvent-to-feed flow rate ratio affects the amounts of products obtained from a given feed. The apparatus required to apply this method are a normal stirred reactor, where contact of the two phases takes place, followed by a separator eliminating the extract from the extraction gas, which is recycled back to the extractor. 

For any pure chemical species, there exists a critical temperature (Tc) and pressure (Pc) immediately below which an equilibrium exists between the liquid and vapor phases (1). Above these critical points a two-phase system coalesces into a single phase referred to as a supercritical fluid. Supercritical fluids have received a great deal of attention in a number of important scientific fields. Interest is primarily a result of the ease with which the chemical potential of a supercritical fluid can be varied simply by adjustment of the system pressure. That is, one can cover an enormous range of, for example, diffusivities, viscosities, and dielectric constants while maintaining simultaneously the inherent chemical structure of the solvent (1-6). As a consequence of their unique solvating character, supercritical fluids have been used extensively for extractions, chromatographic separations, chemical reaction processes, and enhanced oil recovery (2-6). 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

The system carbon dioxide - acetone - water was investigated at 313 and 333 K. The system demonstrates several of the general characteristics of phase equilibrium behavior for ternary aqueous systems with a supercritical fluid. These include an extensive LLV region that appears at relatively low pressures. Carbon dioxide exhibits a high selectivity for acetone over water and can be used to extract acetone from dilute aqueous solutions. 

Which extraction mode is the better remains a controversial issue. While the static mode provides longer contact between the sample and solvent, swells the matrix and facilitates penetration of the extractant in its interstices — thereby increasing its efficiency — the dynamic mode allows the analyte to be continuously exposed to the pure (clean) solvent, thus favouring displacement of the analyte s partitioning equilibrium to the mobile phase. Most SFE methods use both modes a static step is employed to ensure close contact between the sample and supercritical fluid without consuming much extractant that is followed by a dynamic step where the extracted analytes are driven to the restrictor and equilibrium is allowed to complete. 

PHASE EQUILIBRIA. A useful solvent for supercritical extraction, especially in food processing, is carbon dioxide, which has a critical point of 31.06 C and 73.8 bars (1070 Iby/in. ). The phase diagram for pure COj (Fig. 20.16) shows the equilibrium regions of solid, liquid, and gas and the conditions under which a supercritical fluid exists. In the supercritical region there is no distinction between liquid and gas and no phase transition from one to the other the supercritical fluid acts like a very dense gas or a light, mobile liquid. 

We now want to consider the extent to which a solid is soluble in a liquid, a gas. or a supercritical fluid. (This last case is of interest for supercritical extraction, a new separation method.) To analyze these phenomena we again start with the equality of the species fugacities in each phase. However, since the fluid (either liquid, gas, or supercritical fluid) is not present in the solid, two simplifications arise. First, the equilibrium criterion applies only to the solid solute, which we denote by the subscript 1 and second, the solid phase fugacity of the solute is that of the pure solid. Thus we have the single equilibrium relation... 

Solubility calculations are merely phase-equilibrium calculations applied to supercritical gases in liquids, solids in liquids, and solutes in near-critical fluids. The last application has drawn substantial attention, for near-critical extraction processes are being applied, not only in the chemical and energy industries, but also in food processing, purification of biological products, and clean-up of hazardous wastes. 

For the design of extraction processes of mostly high boiling liquid or solid compounds with the help of supercritical fluids, for example, the extraction of caffeine from coffee beans using carbon dioxide the phase equilibrium behavior as a function, of the pressure without or in the presence of co-solvents is required. As in the case of all other phase equilibria, the isofugacity condition has to be fulfilled ... 

First, the distribution of the extract component between the feed and the contacting supercritical fluid is given by the phase equilibrium. The distribution coefficient relates the extract concentration in the supercritical phase to the extract concentration in the corresponding liquid phase at equilibrium conditions. Distribution coefficients have been evaluated for ternary systems in a broad range of... 

Kalra H, Chung SYK, Chen CJ. 1987. Phase Equilibrium Data for Supercritical Extraction of Lemon Flavors and Palm Oils with CO. Fluid Phase Equil. 36 263-278. 

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