Supercritical carbon dioxide is an attractive alternative in place of It is inert, non -toxic, has a relatively low cost and has moderate . it is a generally recognized as safe (GRAS) solvent that leaves no traces in the product. Ethyl lactate, for example, is a green solvent derived from processing corn. replaced solvents such as toluene, acetone, and xylene, resulting in a much safer workplace. . Supercritical carbon dioxide (scCO2), essentially a 'green' solvent and . Recently a number of chemicals have been used as inexpensive, “green” . Sir, I think there is no real green solvent like water but may be ethanol is next green . in the last three decades that the following are RELATIVELY SAFE " Greenish! hydrogenation carbonylation because of the stability of CO2 and its relative.
Cheaper, Safer, Solvent A – CO2 Greener
Although CO 2 is a greenhouse gas, if it is withdrawn from the environment, used in a process, and then returned to the environment, it does not contribute to the greenhouse effect. There have been an increasing number of commercialized and potential applications for supercritical fluids.
This article summarizes the fundamentals of supercritical CO 2 properties and processing, and presents a number of current and potential applications. Although supercritical fluid has liquid-like density, it exhibits gas-like diffusivity, surface tension and viscosity. Its gas-like viscosity results in high mass transfer. Its low surface tension and viscosity lead to greater penetration into porous solids.
The critical temperatures and pressures of materials vary quite significantly Table 1. Generally, substances that are very polar at room temperature will have high critical temperatures since a large amount of energy is needed to overcome the polar attractive energy. At critical conditions, the molecular attraction in a supercritical fluid is counterbalanced by the kinetic energy. In this region, the fluid density and density-dependent properties are very sensitive to pressure and temperature changes.
The solvent power of a supercritical fluid is approximately proportional to its density. Thus, solvent power can be modified by varying the temperature and pressure. Because their properties are a strong function of temperature and pressure, supercritical fluids are considered tunable solvents.
In contrast, conventional liquid solvents require relatively large pressure changes to affect the density. Unlike many organic solvents, supercritical CO 2 is non-flammable. It is inert, non-toxic, has a relatively low cost and has moderate critical constants. Its solvation strength can be fine-tuned by adjusting the density of the fluid. CO 2 leaves a lower amount of residue in products compared to conventional solvents, and it is available in relatively pure form and in large quantities.
CO 2 as a solvent. Supercritical CO 2 is a good solvent for many nonpolar, and a few polar, low-molecular-weight compounds. It is not a very good solvent for high-molecular-weight compounds and the majority of polar compounds. Uneconomically high process pressure may be required to solvate polar, inorganic or high-molecular-weight material in CO 2. To increase the solubility of such compounds in supercritical CO 2 , small amounts of polar or non-polar co-solvents may be added.
Highly CO 2 -soluble surfactants and CO 2 -phillic ligands have also been developed to improve the solubility of compounds in CO 2. Currently, the widest application of supercritical CO 2 is in extraction. Worldwide, over facilities are estimated to use dense CO 2 for extraction and purification. Large-scale commercial plants using supercritical CO 2 extraction are found in the food industries Table 2.
Conventional processes for extracting various components from food products have limitations regarding the solvent toxicity, flammability and wastefulness. This area is where early commercial applications of supercritical CO 2 were focused.
The relatively low critical temperature and low reactivity of CO 2 allow extraction without altering or damaging the product. Decaffeination of coffee was one of the first processes commercialized using supercritical CO 2. Prior to the use of supercritical CO 2 , several different solvents including methylene chloride, ethyl acetate, methyl acetate, ethylmethylketone and trichloroethane have been used for decaffeination.
Extraction of hops during the beer brewing process is another area where CO 2 is used. Extraction of food and natural products with supercritical CO 2 consists of two steps: The separation of supercritical CO 2 from the extract can be done by either modifying the thermodynamic conditions or by using an external agent. By modifying the thermodynamic conditions via changing the pressure or temperature, the solvent power of CO 2 is changed.
If an external agent is used, separation is carried out by adsorption or absorption. If separation occurs with an external agent, no significant pressure change occurs. Therefore, the operating cost that is associated with pressure requirement is lower. But, an additional step is required, the recovery of the extract from the external agent. In addition, higher losses of the extract can occur during the recovery step.
The feed material is typically ground solid material, which is fed to the extractor. Most commercial operations for supercritical fluid extraction are batch or semi-batch operation especially when the feed material is solid.
For liquid feed material, the extraction occurs in a countercurrent column filled with random or structured packing material. However, for highly viscous liquid feed, the viscous liquid and supercritical fluid may be mixed and sprayed through a nozzle into the extractor vessel.
There has been a great deal of interest in supercritical CO 2 extraction beyond caffeine extraction, particularly in the preparation of high value products, such as flavors and fragrances, food supplements and nutraceuticals. Specialty oils, for example, are high in value and typically low in volume. They have high concentrations of bioactive lipid components that are valued because of various possible health benefits.
Herbal extracts from a wide range of botanical raw materials are used as ingredients to the food-and-flavor, nutraceuticals, pharmaceuticals and the cosmetics industries. Supercritical CO 2 extraction can also be used to purify materials that are used for the production of medical devices.
These high-value-product applications typically involve small volumes. Flexible, medium-capacity plants for supercritical CO 2 extraction offer toll processing for these smaller volume products. The most important driving force for using supercritical CO 2 in this application area is that it is a generally recognized as safe GRAS solvent that leaves no traces in the product. The relatively low temperature of the process and the stability of CO 2 also allows most compounds to be extracted with little damage or denaturing.
In addition, the solubility of many extracted compounds in CO 2 varies with pressure,  permitting selective extractions. Carbon dioxide is gaining popularity among coffee manufacturers looking to move away from classic decaffeinating solvents , because of real or perceived dangers related to their use in food preparation.
The caffeine can then be isolated for resale e. Supercritical carbon dioxide is used to remove organochloride pesticides and metals from agricultural crops without adulterating the desired constituents from the plant matter in the herbal supplement industry. Supercritical carbon dioxide can be used as a more environmentally friendly solvent for dry cleaning over traditional solvents such as hydrocarbons, including perchloroethylene.
Supercritical carbon dioxide is used as the extraction solvent for creation of essential oils and other herbal distillates.
Furthermore, separation of the reaction components from the starting material is much simpler than with traditional organic solvents. The CO 2 can evaporate into the air or be recycled by condensation into a cold recovery vessel. Its advantage over steam distillation is that it operates at a lower temperature, which can separate the plant waxes from the oils. In laboratories , s CO 2 is used as an extraction solvent, for example for determining total recoverable hydrocarbons from soils, sediments, fly-ash and other media,  and determination of polycyclic aromatic hydrocarbons in soil and solid wastes.
Processes that use s CO 2 to produce micro and nano scale particles, often for pharmaceutical uses, are under development. The gas antisolvent process, rapid expansion of supercritical solutions and supercritical antisolvent precipitation as well as several related methods process a variety of substances into particles.
Due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes, s CO 2 has been suggested as a potential solvent to support biological activity on Venus - or super-Earth -type planets. Environmentally beneficial, low-cost substitutes for rigid thermoplastic and fired ceramic are made using s CO 2 as a chemical reagent.
The s CO 2 in these processes is reacted with the alkaline components of fully hardened hydraulic cement or gypsum plaster to form various carbonates. Supercritical carbon dioxide is used in the foaming of polymers. Supercritical carbon dioxide can saturate the polymer with solvent. Upon depressurization and heating the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.
Research is also ongoing at many universities in the production of microcellular foams using s CO 2. An electrochemical carboxylation of a para- isobutyl benzyl chloride to ibuprofen is promoted under s CO 2. Supercritical CO 2 is chemically stable, reliable, low-cost, non-toxic, non-flammable and readily available, making it a desirable candidate working fluid.
The unique properties of s CO 2 present advantages for closed-loop power generation and can be applied to various power generation applications. Power generation systems that use traditional air Brayton and steam Rankine cycles can be upgraded to s CO 2 to increase efficiency and power output. The relatively new Allam power cycle uses sCO2 as the working fluid in combination with fuel and pure oxygen. The CO2 produced by combustion mixes with the sCO2 working fluid and a corresponding amount of pure CO2 must be removed from the process for industrial use or sequestration.
This process reduces atmospheric emissions to zero. It presents interesting properties that promise substantial improvements in system efficiency. Due to its high fluid density, s CO 2 enables extremely compact and highly efficient turbomachinery. It can use simpler, single casing body designs while steam turbines require multiple turbine stages and associated casings, as well as additional inlet and outlet piping. The high density allows for highly compact, microchannel-based heat exchanger technology.
It requires less compression and allows heat transfer. It reaches full power in 2 minutes, whereas steam turbines need at least 30 minutes. Further, due to its superior thermal stability and non-flammability, direct heat exchange from high temperature sources is possible, permitting higher working fluid temperatures and therefore higher cycle efficiency. Despite the promise of substantially higher efficiency and lower capital costs, the use of s CO 2 presents material selection and design issues.
Materials in power generation components must display resistance to damage caused by high-temperature , oxidation and creep. Candidate materials that meet these property and performance goals include incumbent alloys in power generation, such as nickel-based superalloys for turbomachinery components and austenitic stainless steels for piping.
Components within s CO 2 Brayton loops suffer from corrosion and erosion, specifically erosion in turbomachinery and recuperative heat exchanger components and intergranular corrosion and pitting in the piping.
Testing has been conducted on candidate Ni-based alloys, austenitic steels, ferritic steels and ceramics for corrosion resistance in s CO 2 cycles.
Supercritical carbon dioxide
CO2 used as a solvent is recovered as a by-product from various of CO2 can be exploited and used productively in green applications. Carbon dioxide is often promoted as a green solvent, and its use in this role has . and K with a highly flammable component and hence, safe operation of a. implementation of such green solvents. Solvents, and .. Density versus pressure isotherms for liquid and supercritical carbon dioxide. cially because it is plentiful and inexpensive, . S. C. DeVito, in Designing Safer Chemicals: Green.