Supercritical Fluid Extraction and Drying

The tunable density and compressibility make supercritical fluids adjustable solvents with a continuous transition between “excellent” solvents under supercritical conditions and “poor” solvents in the state of a compressed gas. Most processes are based on these solvent power variations [157].

Supercritical fluid extraction has been traditionally applied in the food and pharmaceutical industries, especially for the nutraceutical area where extraction is done with SCCO2 which does not leave any toxic residues in the final product. Nutraeuticals are natural extracts from plants or natural products that contain physiological or health benefits. The first industrial application of supercritical fluids was a coffee decaffeination plant in 1978, followed by a hops extraction plant in 1982. During the 80’s and 90’s industrial and lab research in supercritical fluids were mainly focused on extraction processes from liquid and solid matrices. Nowadays, Supercritical fluids techniques are being widely used and developed in many applications, such as, remediation of soil, chemical reaction and synthesis of polymers and organic compounds, impregnation and in situ encapsulation, removal of nicotine from tobacco, nucleation and particle size control, cleaning electronic parts and decaffeinating of green coffee beans.

In the method of extraction and drying using supercritical fluids, the selectivity and extractability of the objectives are highly determined by their solubility in supercritical fluids. This is accomplished by tuning the density of the supercritical fluid through minutely adjusting the

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processing temperature or pressure, or by adding co-solvents to enhance the solvating power for specific compounds.

The approaches to drying of wet porous material like alcogel are conventional evaporation, freeze-drying, and supercritical drying. All pore templates and solvents within the porous matrix should be removed before further functionalization or end-use. Supercritical drying methods evolved considerably since the pioneering work of Kistler where excess alcohol was added to the autoclave and then, heat was supplied so that supercritical conditions were reached. Subsequently, supercritical solvent was removed from the autoclave by depressurization above the critical temperature of the solvent [95]. The process of supercritical drying is a specific example of supercritical fluid extraction and it utilizes another property of SCF--low surface tension. Low surface tension accompanied by high diffusivity allows the supercritical fluids to penetrate into a porous matrix very affectively.

When compared to conventional drying for removal of conventional solvents, supercritical drying establishes a sustained single supercritical phase where drying is free from liquid-vapor phase interfacial stresses. This facilitates the production of aerogels that have the same morphological properties as their alcogel precursors. Whereas traditional evaporative extraction and drying uses solvents to separate and extract certain solutes of interest under ambient pressure or vacuum either with or without heating. Traditionally wet gel materials resulting from non-critical evaporative drying methods are defined as xerogels. Conventional evaporative drying uses solvents to separate and extract certain solutes of interest that have a large affinity with interfering matrix materials. As a liquid-vapor interfacial phenomenon, the evaporation of solvent from a sample surface or matrix is associated with severe capillary pressures that could cause permanent collapsing of fragile samples such as sol-gel porous structures. The best approach to minimize the capillary pressure acting on the sol-gel network during drying is to examine the following Equation 3.6

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P =

where P is capillary pressure, σ is surface tension, θ is the contact angle between liquid and solid and r the pore radius. For a given pore size the capillary pressure can be reduced by using a solvent with a lower surface tension than the original solvent in the gel network [98,158] and/or eliminating the liquid-vapor interface with supercritical drying [159,160]. On the other hand, supercritical carbon dioxide offers considerable potential as a replacement for solvents in many extraction and drying processes, such as, extraction of solvents and monomers from polymer solutions.

High purity CO2 is the supercritical fluid medium of choice for most extractions. For reasons already stated, in supercritical drying, the supercritical fluid being used as the drying medium can be tuned to perform different levels of extractions in supercritical fractionation because the supercritical solvating power is tightly tuned and adjustable. One of several available pathways is based upon varying the density of the supercritical fluid through either or both of temperature and pressure during the process, the higher the density of the supercritical fluid, the higher the solvating power of that supercritical fluid and hence the higher drying rate achieved. A very small increase in pressure at or slightly above the critical temperature results in a dramatic increase in the density of the supercritical fluid. This is important when gradual extraction of a certain liquid phase is desired in a fractionation mode where a small incremental change in density is invoked with successive extraction. Supercritical fluid’s density also can be varied via change in temperature when pressure is held constant above the critical pressure. Meanwhile, the value of viscosity and diffusivity are both dependent on temperature and pressure. In the case of gases, as the temperature increases, it leads to an associated increase in the viscosity of the gas, the opposite is observed in the case of the supercritical fluids. Whereas the viscosity of the supercritical fluids decrease with increasing the supercritical fluid

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temperature making the drying process fast and less mass transfer limited. However, in general, drying is best accomplished by modifying supercritical fluid pressure, which is much easier then introducing temperature fluctuation throughout the entire supercritical extraction and drying process.

Another method to tune the solubility of the supercritical fluid is to modify the supercritical fluids to be able to entrain the solute from the solid phase in the drying process. For example, enhanced non-polar supercritical CO2 produced by adding a small amount of miscible polar modifier such as ethanol and/or methanol is able to dissolve certain polar molecules which pure supercritical CO2 has limited solving power to dissolve. The added fluid is referred to in the literatures as an entrainer, co-solvent or modifier. However, it should be noted that while modifiers enhance the solving power of the mixed supercritical fluid, they also interfere with the solute and influence the mass transfer rate. Such phenomenon is a result of the miscibility of the modifier in the liquid solute making the diffusion coefficient for the solute in the supercritical fluid much smaller than without the use of the modifier. In practice, a careful choice of the modifier should be done so that the chemical affinity for the solute in the supercritical fluid is enhanced and at the same time the diffusivity of the solute to the supercritical fluid is not greatly hindered.

In general, selecting a supercritical fluid system for extraction and drying purposes should be chosen based on the following considerations: phase behavior, co-solvent effects, transport properties of the supercritical fluids, molecular structure, polarity, solvent-matrix interaction, porosity of the matrix and solvent location within the matrix.