Supercritical fluid transport

Supercritical fluid transport involves dissolving the precursor(s) in a supercritical fluid and spraying this liquid into a CVD reactor.43-45 The rapid expansion of the supercritical fluid enhances evaporation of the solute and so aids transport of the precursor to the substrate. This method has the potential advantages of the other methods, but is more limited in the choice of solvent, e.g., C02. It should be noted that this method may not require the evaporation of the precursors and under these conditions would not be classified as a CVD method. 

Hansen BN, Hybertson BM, Barkley RM, Sievers RE. Supercritical fluid transport-chemical deposition of Aims. Chem Mater 1992 4 749-752. 

Hybertson, B. M., Hansen, B. N., Barkley, R. M., and Sievers, R. E. (1991). Deposition of palladium films by a novel supercritical fluid transport-chemical deposition process. Mater. Res. Bull. 26, 1127. 

Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

A paiticularly attiactive and useful feature of supeicritical fluids is that these materials can have properties somewhere between those of a gas and a hquid (Table 2). A supercritical fluid has more hquid-hke densities, and subsequent solvation strengths, while possessiag transport properties, ie, viscosities and diffusivities, that are more like gases. Thus, an SCF may diffuse iato a matrix more quickly than a Hquid solvent, yet still possess a Hquid-like solvent strength for extracting a component from the matrix. 

Transport Properties Although the densities of supercritical fluids approach those of conventional hquids, their transport properties are closer to those of gases, as shown for a typical SCF such as CO9 in Table 22-12. For example, the viscosity is several orders of magnitude lower than at liquidlike conditions. The self-diffusion coefficient ranges between 10" and 10" em /s, and binaiy-diffusiou coefficients are similar [Liong, Wells, and Foster, J. Supercritical Fluids 4, 91 (1991) Catchpole and King, Ind. Eng. Chem. Research, 33,... 

TABLE 22-12 Density and Transport Properties of a GaS/ Supercritical Fluid/ and a Liquid... 

Although critical pressures are many times greater than atmospheric pressure, supercritical fluids have important commercial applications. The most important of these is the use of supercritical carbon dioxide as a solvent. Supercritical CO2 diffuses through a solid matrix rapidly, and it transports materials well because it has a lower... 

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 same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

Supercritical fluids (SCFs) offer several advantages as reaction media for catalytic reactions. These advantages include the ability to manipulate the reaction environment through simple changes in pressure to enhance solubility of reactants and products, to eliminate interphase transport limitations, and to integrate reaction and separation unit operations. Benefits derived from the SCF phase Fischer-Tropsch synthesis (SCF-FTS) involve the gas-like diffusivities and liquid-like solubilities, which together combine the desirable features of the gas- and liquid-phase FT synthesis routes. 

Supercritical fluids are benign alternatives to conventional organic solvents that may offer improvements in reaction rate, product selectivity, and product separation. We reported the first use of SCFs for phase-transfer catalysis (PTC), where these benign alternatives also offer greatly improved transport, product separation, catalyst recycle, and facile solvent removal (26-29). 

Small changes in the temperature or pressure of a supercritical fluid may result in great changes in its viscosity and in the diffusivity and solubility of compounds dissolved within it. In such systems, the bioconversion rate is increased thanks to the high diffusion rates which facilitate transport phenomena. In some cases a high diffusion rate can also facilitate product separation. 

A way around this issue may have been found with the use of supercritical fluids. These materials, such as liquid carbon dioxide, have many interesting properties from the point of view of pharmacutical processing since they combine liquid-like solvent properties with gas-like transportation properties. Small changes in the applied pressure or temperature can result in large changes of the fluid density and, correspondingly, the solvent capacity and properties of the resultant particles. 

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 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],... 

In order to overcome the main limitations of the impregnation processes, connected to the limited solubility of the compounds in the supercritical fluids, Perman [68] proposed an alternative method. A supercritical impregnation process was coupled with a liquid solvent (preferentially water) to enhance the drug solubilization. The system composed of a liquid drug solution and the polymeric support was pressurized with the supercritical fluid. Consequently, the swelled polymer allows rapid diffusional transport of the solute into the polymeric substrate. In different examples, bovine serum albumin microspheres were impregnated with insulin, trypsin and gentamicin (see Table 9.9-5). 

Other low-temperature studies have been motivated by the desire to characterize and understand processes occurring in unusual media. For example, the use of liquid ammonia [8-10] and liquid sulfur dioxide [11-13] naturally requires reduced temperatures unless high pressures are used, as is done for electrochemistry in supercritical fluids [14]. Frozen media are interesting systems in terms of mass transport phenomena and microstructural effects. Examples include glasses of acetonitrile and acetone [15], frozen dimethyl sulfoxide solutions [16,17], and the solid electrolyte HC104 5.5 H20 [18-20]. 

A gaseous pure component can be defined as supercritical when its state is determined by values of temperature T and pressure P that are above its critical parameters (Tc and Pc). In the proximity of its critical point, a pure supercritical fluid (or a dense gas as it is alternatively known) has a very high isothermal compressibility, and this makes possible to change significantly the density of the fluid with relatively limited modifications of T and P. On the other hand, it has been shown that the thermodynamic and transport properties of supercritical fluids can be tuned simply by changing the density of the medium. This is particularly interesting for... 

Because of their unique characteristics, supercritical fluids have received a great deal of attention in a number of important scientific fields (1-14). Several reasons are given for choosing a supercritical fluid over another solvating system, but choice is governed generally by 1) the unique solvation and favorable mass transport properties (5) and 2) the ease with which the chemical potential can be varied simply by adjustment of the system pressure and/or temperature (13). 

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