Mass transfer rates, supercritical fluids

Enhanced Mass Transfer, Diffusivity Supercritical fluids share many of the advantages of gases, including lower viscosities and higher diffusivities relative to liquid solvents, thereby potentially providing the opportunity for faster rates, particularly for diffusion-limited reactions. 

A major factor that affects the dynamic method is the mass-transfer rate between fluid and solute. There is always a possibility that the solute concentration has not reached its equilibrium value. Hence, experiments should be done by further increasing the residence time of the fluid, to ascertain that the equilibrium has been reached. The mass transfer rate can change over time if the supercritical carbon dioxide causes agglomeration of the solute particles, or the overall particle surface area is reduced by dissolution of small particles in early experimental runs. To improve mass transfer by having a high surface area, the solute is typically coated onto silica beads (e.g., 300 pm diameter) before being loaded into the sample vessel. 

One of the benefits of using supercritical fluids as the solvent is the strong dependence of the solubility of the solute on the solvent density. This is a property that could be exploited for facilitating the separation of the solute from the solvent as it leaves the column, by dropping its pressure or raising its temperature, thereby lowering its density and the solubility of the solute. As a result, the extract separates into a liquid solute and a vapor solvent. Another favorable property of supercritical fluids as solvents is the high diffusivity of the solute in these fluids compared to that in liquids. Supercritical fluids also have a substantially lower viscosity than liquids. Because of these properties the mass transfer rate of the solute... 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Although the mass transfer rate in the supercritical fluid is lower than that in the gas phase, the alkene content in the hydrocarbons produced in supercritical... 

Supercritical fluids exhibit gas-like mass transfer rates and yet have liquid-like solvating capability. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from porous solid matrices. From this point of view, SFE is an ideal method to extract uranium and lanthanides from solid wastes. Carbon dioxide (CO2) is most frequently used in SFE because of its moderate critical pressure (Pc) nd temperature (Jc), inertness, low cost, and availability in pure form. Figure 1 illustrates moderate values of Pc and Tc compared with those of water. 

In general, mass transfer is not limited by diffusion rates in the supercritical phase. In fact, significant buoyancy effects in supercritical fluids enhance mass transfer rates with convective mixing (58). Usually, mass transfer limitations will occur in either a liquid or solid phase. 

Supercritical fluid extractions are typically run under conditions of high solvent/feed ratio, high superficial velocity, and low fluid viscosity. Thus, the controlling mass transfer parameter is usually the diffusion rate of the solvent and solute through the botanical substrate into the bulk fluid phase. Therefore, mass transfer rate can be increased by increasing solvent diffusivity, reducing diffusion distance, or elimination of diffusion barriers. 

Among the physical properties of great importance in determining the mass transfer rates in supercritical extraction processes, viscosity affects significantly the efficiency of the extraction system. In fluids, high temperature and pressure directly affect viscosity, for example, at constant pressure, viscosity decreases with an increase in temperature (Brunner, 1994). With respect to the viscosity behavior of the fluids, Brunner (1994) stated that at temperatures above the minimum, the fluid behaves like a gas, and below this minimum, it behaves like a liquid. 

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 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 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. 

Propane or propane/C02 mixtures as liquid, near-critical, or supercritical fluids enhance the solubility of fats and oils (Harrod et al., 2000 Weidner and Richter, 1999). The decrease in viscosity and increase in diffusivity results in a higher hydrogenation rate (Figure 14.4). Harrod et al. (2000) have also demonstrated activity increases by reducing mass-transfer limitations in supercritical propane. 

Process intensification can be considered to be the use of measures to increase the volume-specific rates of reaction, heat transfer, and mass transfer and thus to enable the chemical system or catalyst to realize its full potential (2). Catalysis itself is an example of process intensification in its broadest sense. The use of special reaction media, such as ionic liquids or supercritical fluids, high-density energy sources, such as microwaves or ultrasonics, the exploitation of centrifugal fields, the use of microstructured reactors with very high specific surface areas, and the periodic reactor operation all fall under this definition of process intensification, and the list given is by no means exhaustive. 

Enzymatic reactions in non-aqueous solvents are subjected to a wide interest. A particular class of these solvents is the supercritical fluid (1) such as carbon dioxide that has many advantages over classical organic solvents or water no toxicity, no flammability, critical pressure 7.38 Mpa and temperature 31°C, and allowing high mass transfer and diffusion rates. 

---