Supercritical fluids mass transfer

The shrinking-core model (SCM) is used in some cases to describe the kinetics of solid and semi-solids-extraction with a supercritical fluid [22,49,53] despite the facts that the seed geometry may be quite irregular, and that internal walls may strongly affect the diffusion. As will be seen with the SCM, the extraction depends on a few parameters. For plug-flow, the transport parameters are the solid-to-fluid mass-transfer coefficient and the intra-particle diffusivity. A third parameter appears when disperse-plug-flow is considered [39,53],... 

Solid—Fluid Mass Transfer in a Packed Bed Under Supercritical Conditions... 

Lim, G. B., G. D. Holder, and Y. T. Shah. 1989. Solid-Fluid Mass Transfer in a Packed Bed under Supercritical Conditions. In Supercritical Eluid Science and Technology, 379-395. American Chemical Society. 

A supercritical fluid exhibits physical-chemical properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those in normal gaseous states. In the vicinity of the critical point the diffusion coefficient is more than 10 times that of a liquid. Carbon dioxide can be compressed readily to form a liquid. Under typical borehole conditions, carbon dioxide is a supercritical fluid. 

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... 

Smith and Udseth [154] first described SFE-MS in 1983. Direct fluid injection (DFT) mass spectrometry (DFT-MS, DFI-MS/MS) utilises supercritical fluids for solvation and transfer of materials to a mass-spectrometer chemical ionisation (Cl) source. Extraction with scC02 is compatible with a variety of Cl reagents, which allow a sensitive and selective means for ionising the solute classes of interest. If the interfering effects of the sample matrix cannot be overcome by selective ionisation, techniques based on tandem mass spectrometry can be used [7]. In these cases, a cheaper and more attractive alternative is often to perform some form of chromatography between extraction and detection. In SFE-MS, on-line fractionation using pressure can be used to control SCF solubility to a limited extent. The main features of on-line SFE-MS are summarised in Table 7.20. It appears that the direct introduction into a mass spectrometer of analytes dissolved in supercritical fluids without on-line chromatography has not actively been pursued. 

Supercritical fluids possess favorable physical properties that result in good behavior for mass transfer of solutes in a column. Some important physical properties of liquids, gases, and supercritical fluids are compared in Table 4.1 [49]. It can be seen that solute diffusion coefficients are greater in a supercritical fluid than in a liquid phase. When compared to HPLC, higher analyte diffusivity leads to lower mass transfer resistance, which results in sharper peaks. Higher diffusivity also results in higher optimum linear velocities, since the optimum linear velocity for a packed column is proportional to the diffusion coefficient of the mobile phase for liquid-like fluids [50, 51]. 

Supercritical fluid chromatography (SFC) is an intermediate chromatographic technique between GC and HPLC. It depends upon the fact that when a fluid becomes supercritical (both the temperature and pressure are at or above its critical point) it develops some of the solvating properties of a liquid whilst retaining the low viscosity of a gas. Hence, mass transfer (essential to efficient chromatography) is more akin to that of GC than HPLC, but many compounds can be chromatographed at temperatures much lower than what would be required by GC, so some thermally labile compounds are amenable to SFC where they would degrade under GC conditions [28]. 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

In many catalytic reactions, solid, liquid, and gas phases are involved, and the phase behavior often has a strong influence on mixing and mass transfer and consequently on the catalytic performance. Supercritical fluids, especially supercritical CO2, have gained considerable attention as environmentally benign solvents (e.g., (94y). The combined use of in situ transmission and ATR-IR spectroscopy together with video monitoring is a promising approach for elucidation of the behavior of a... 

The critical temperature of pure CO2 is 31°C [7]. For the subcritical range of 31-50°C, the fluid entering the extraction cell will consist of two phases - a liquid methanol phase and a supercritical phase. It has been reported that the diffusivity of liquid is about 10-100 times smaller than that of the supercritical fluid [6] and this implies that the difficulty of mass transfer associated with the former is also magnified by the same factor. In an extraction process, mass transfer occurs during 1) the fluid s penetration of the matrix s pores and 2) the subsequent transport of the analyte (solute) from the matrix into the bulk fluid [6]. The presence of entrained liquid methanol droplets will thus greatly increases the amount of mass transfer resistance present in the system. Such resistance is reduced upon an increase in temperature and this accounts for the rise in extraction efficiency observed in the temperature range of 45-50°C. 

The interest in mass transfer in high-pressure systems is related to the extraction of a valuable solute with a compressed gas. This is either a volatile liquid or solid deposited within a porous matrix. The compressed fluid is usually a high-pressure gas, often a supercritical fluid, that is, a gas above its critical state. In this condition the gas density approaches a liquid—like value, so the solubility of the solute in the fluid can be substantially enhanced over its value at low pressure. The retention mechanism of the solute in the solid matrix is only physical (that is, unbound, as with the free moisture), or strongly bound to the solid by some kind of link (as with the so-called bound moisture). Crushed vegetable seeds, for example, have a fraction of free, unbound oil that is readily extracted by the gas, while the rest of the oil is strongly bound to cell walls and structures. This bound solute requires a larger effort to be transferred to the solvent phase. 

In high pressure work, slurry reactors are used when a solid catalyst is suspended in a liquid or supercritical fluid (either reactant or inert) and the second reactant is a high pressure gas or also a supercritical fluid. The slurry catalytic reactor will be used in the laboratory to try different catalyst batches or alternatives. Or to measure the reaction rate under high rotational speeds for assessing intrinsic kinetics. Or even it can be used at different catalyst loadings to assess mass transfer resistances. It can also be used in the laboratory to check the deactivating behaviour. 

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