Contributed by Al Kaziunas and Rolf Schlake of Applied Separations
Organic solvents used in chemical processes have enormous environmental and economic costs. They contribute to health problems, adversely impact the environment, are flammable, contribute to smog formation and are eco toxic. The elimination of hazardous organic solvents and the search for useful non hazardous solvents is a prime goal of green chemistry. CO2 as a liquid or supercritical solvent meets many of the characteristics of an ideal green solvent.
CO2 used as a solvent is recovered as a by-product from various industrial processes including fermentation, cement, and fertilizer manufacturing sites. The CO2 generated is purified, compressed and cooled to the liquid state at 20 bar and -20C and stored or transported in insulated bulk containers for reuse in many liquid and supercritical CO2 processes.
The properties of CO2 are well known and can be exploited for many useful scientific purposes and industrial processes. For example, the density of supercritical CO2 can be changed dramatically by small changes in the pressure or temperature around the critical point. CO2 viscosity is very low and the surface tension of supercritical CO2 is nonexistent. Diffusivity is high, which in combination with low viscosity induces significant changes in condensed phases.
Also, supercritical CO2 influences the properties of components with which it is mixed. For example, supercritical CO2 can dissolve many non polar compounds far beyond its vapor pressure. Also, a significant amount of supercritical CO2 can dissolve into condensed phases drastically reducing the surface tension and viscosity of the condensed phases making processing of viscous materials easier.
The following applications cover some of the areas where the properties of CO2 can be exploited and used productively in green applications.
Supercritical fluid extraction (SFE) of a solid material is carried out by pumping supercritical CO2 through a fixed bed of solid substrate. The supercritical CO2 flows through the fixed bed and dissolves soluble components until the substrate is depleted. The loaded solvent is then directed to a separator where the soluble components are precipitated by reducing pressure and temperature. The CO2 is condensed and recirculated.
Extraction of Solid Natural Products
Supercritical CO2 has the ability to diffuse into solid particles and dissolve many valuable non polar molecules. Examples of existing CO2 extractions include:
Countercurrent Separation of Liquid Natural Products
Countercurrent separation of low volatility components of a liquid material can be carried out in a packed column with supercritical CO2. Counter current extraction using supercritical CO2 is an alternative to vacuum distillation and short path distillation. This process is carried out at moderate temperatures and avoids thermal degradation of sensitive components. Some examples are:
Supercritical Drying, Cleaning, and Degreasing Supercritical
CO2 has no surface tension and can easily penetrate micro pores that are not accessible to liquid solvents. This characteristic is useful for drying or cleaning of many porous materials including:
Impregnation with Supercritical Fluids Supercritical
CO2 may also be used to deposit or impregnate materials with soluble components. The low viscosity and high diffusivity of supercritical CO2 allow rapid penetration into solid materials. The small supercritical fluid molecules diffuse into leather, wood, polymers and other porous material carrying dissolved components. The components remain in the porous solid upon depressurization of the CO2. Some examples include:
Carbon dioxide may be used to form nanoparticles by many different techniques. Generally compounds that are soluble in supercritical CO2 may be depressurized through a nozzle or restrictor into an atmospheric chamber. The rapid expansion of the supercritical solution results in the nucleation and formation of many small particles. This technique is known by the acronym of RESS (rapid expansion of a supercritical solution).
Many pharmaceutical compounds are not soluble in supercritical CO2 and can only be dissolved in a polar organic solvent. The ability of supercritical CO2 to dissolve significantly into a polar organic solvent and expand the volume of the solvent 10 fold reduces the solubility of dissolved compounds and initiates precipitation of the previously dissolved particles in the expanded solvent. This CO2 antisolvent technique is generally described as Gas AntiSolvent or GAS. Many variations of the antisolvent technique are available using specific nozzle configurations and solvent introduction techniques.
Supercritical CO2 is a unique solvent that has the characteristics of variable density, low viscosity, and high diffusivity. The manipulation of these characteristics has led to numerous applications of this green solvent in diverse areas including extractions, impregnations, particle formation, and cleaning.
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