Supercritical process description

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

Next, we present results obtained by using Aspen Plus [23], Using the Uniquac-RK model with Henry components for supercritical gases ensures correct description of the absorption-desorption process. Table 11.7 shows the composition of streams around the reactor and the first separation step. 

In this paper, an overview of the important phenomena is given. The supercritical combustion process employed is also known to occur in liquid propellant rocket motors (e.g. in LOX/GH2-motors), liquid propellant guns (LPG), advanced aviation gas turbines and, to a lesser extent, in internal combustion engines. Supercritical combustion is characterized by (1) injection of at least one liquid state fuel component into a chamber which is thermodynamically in the supercritical state, (2) density ratios of fuel to oxidizer near one, (3) supercritical phase transitions of fluid-particles due to combustion, (4) non-ideal properties of the fluids. Additionally a short description of pertinent design criteria is given. 

To design large scale supercritical desorption processes is necessary to understand in which way dynamic desorption is influenced by process variables as mass transfer effects and equilibrium considerations. The governing equilibrium in all desorption processes is the adsorption equilibrium and a description of this equilibrium is essential in all desorption models and design equations [3]. 

The van der Waals equation describes the critical phenomena of vapour to supercritical gas or fluid. Below critical temperature Tc gas which coexists with the liquid phase is called a vapour. Vapor has own saturated vapour pressure Pq. Then we can use the relative pressure P/Pq for description of adsorption. Fundamentally, physical adsorption is valid for vapours [10]. As the molecule-surface interaction of physical adsorption is weak, a sufficient intermolecular interaction corresponding to heat of vapourization is necessary for predominant physical adsorption. Micropore filling is a physical adsorption enhanced by overlapping of the molecule-surface interaction potentials from opposite pore walls and the adsorptive force is the strongest in physical adsorption. Nevertheless, micropore filling is a predominant process only for vapour. 

In a first step of the process propane, butane, pentane, or hexane is used to extract vanidyl porphyrins from residual oil. The liquid extract of hydrocarbon and porphyrins is next subjected to supercritical CO2 exU action between 90and 130 °Fand 1,075 to 8,000 psi. At almost all combinations of these pressures and temperatures the C3 through C(, paraffins are miscible in CO2. With a supercritical extraction column we wonder what ratio of C02-lo-extract would result in purified oil and what happens to the vanidyl porphyrins. In the process diagram and description, the supercritical C02-liquid solvent extract is expanded to an unspecified different pressure and temperature. But, it is important to relate that pentane and hexane are miscible with CO2 at room temperature and 800 psi so that the expansion would have to be to conditions higher in temperature and lower in pressure to separate CO2 and hexane. For propane the miscibility conditions extend to much lower pressure and so the separation of propane from carbon dioxide becomes problematic. 

This is the first of the coffee decaffeination patents that describe a continuous, counter-current liquid-liquid extraction. A brief description of the process is provided here. A water extract of roasted coffee beans, called coffee liquor, which contains aromas and caffeine and other water soluble components such as carbohydrate and protein materials is fed to a vacuum suipper. The extract is concentrated to about 30-50% in an evaporator-condenser and is fed to a sieve tray tower. The liquor passes across the hays in the tower downward through downspouts countercurrent to supercritical CO2 which enters the tower at the bottom and passes upward through the holes in the sieve trays. CO2 extracts caffeine from the liquor, and the decaffeinated liquor leaves the near the bottom of tower. The condensate water from the vacuum stripper prior to the tray tower is fed to the sieve trays in the top section of the tower. The water washes the caffeine from the supercritical CO2 passing upward. The caffeine-free CO2 is recycled to the bottom of the column. 

The invention concerns the use of supercritical solvents to extract the cocoa butter from cocoa nibs (comminuted cocoa beans) and cocoa mass (fmely crushed beans). The description of other processes in the prior art section of the patent points out that organic solvent extraction results in the presence of residual solvents. Additionally, some of the newer pressing methods, via expellors, for example, introduce waste bean contaminants into the butter which must be removed with economic and taste penalties. 

Tbe figure in the patent showing tbe process invention is quite detailed, and the complete description of the operation is quite lengthy and involved. There are no data given SO we conclude that this is a thought patent, but it is presented to show the breadth of supercritical fluid applications. 

The extraction of substances from solid substrates with supercritical solvents can be analyzed and modeled in a simple way by considering only the medium values and by determination of unknown coefficients by fitting to the extraction curve and a mass balance [1]. This approach results in simple equations that can represent parts of the extraction curve sufficiently, but fail for others, especially during the first part of the extraction. If the process is to be modeled more accurately, the analysis is far more complex and beyond the scope of this chapter. Nevertheless, some parameters determining the extraction and influencing the result are listed below together with the description of a simplified model that may provide some insight into the applied methods. 

Some advances have been made in mechanistic understanding of SCF particle-formation processes and rigorous descriptions of mass ttansfer and nucleation processes are being developed l The advances in the understanding of the mechanism of supercritical particle formation and SCF mass transfer are forming the basis for efficient scale-up of the laboratory-scale processes. 

In conventional polymerization processes in organic solvents, it is possible to follow the reaction rate by correlating the decrease in pressure of a supply of gaseous monomer to the conversion. Heller describes a method in which the decrease in pressure is correlated to the reaction rate with a virial equation of state [28]. A similar method can be used for reactions in supercritical media, which are often subject to a pressure change upon reaction. In this study, a model was developed to determine the reaction rate indirectly based on the measured pressure during polymerization and based on a description of the phase behavior of the polymer and supercritical fluid phase [9, 29]. 

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. 

Kikic, I., Bertucco, A. and Lora, M. (1997) Thermodynamic Description of Systems Involved in Supercritical Anti-Solvent Processes, The 4 International Symposium on Supercritical Fluids, Sendai, Japan, pp. 39-42. 

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