Supercritical carbon dioxide separation/recycling

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

Supercritical carbon dioxide is depressurized through the expansion valves into separator columns 4 and 5, where the product and the unreacted substrates are recovered. The substrates are collected in column 5 and recycled (added to the feed through the pipe connecting column 5 with the feed vessel). The gas phase is finally vented into the atmosphere after flow-rate measurement through a rotameter. The gas can be condensed and recycled on a pilot- or industrial scale. 

It is apparent from the foregoing discussion that both ILs and supercritical carbon dioxide do indeed offer promise as alternative solvents in the reprocessing of spent nuclear fuel and the treatment of nuclear wastes. It is equally apparent, however, that considerable additional work lies ahead before this promise can be fully realized. Of particular importance in this context is the need for an improved understanding of the fundamental aspects of metal ion transfer into ILs, for a thorough evaluation of the desirability of extractant functionalization of ILs, and for the development of new methods for both the recovery of extracted ions (e.g., uranium) and the recycling of extractants in supercritical C02-based systems. Only after such issues have been addressed might these unique solvents reasonably be expected to provide the basis of improved approaches to An or FP separations. 

The separation of the products from the IL catalytic mixture can be performed in various cases by simple decanting and phase separation or by product distillation. In this respect, a continuous-flow process using toluene as extractant has been appHed for the selective Pd-catalyzed dimerization of methyl acrylate in ILs [136]. However, in cases where the products are retained in the IL phase, extraction with supercritical carbon dioxide can be used instead of classical liquid-liquid extractions that necessitate the use of organic solvents, which may result in cross-contamination of products. This process was successfully used in catalyst recycling and product separation for the hydroformylation of olefins employing a continuous-flow process in supercritical carbon dioxide-IL mixtures [137]. Similarly, free and immobilized Candida antarctica lipase B dispersed in ILs were used as catalyst for the continuous kinetic resolution of rac-l-phenylethanol in supercritical carbon dioxide at 120°C and 150°C and 10 Mpa with excellent catalytic activity, enzyme stability and enantioselectivity levels (Fig. 3.5-11). 

Bhise s patent [53] describes a process for preparing and separating ethylene glycol from an aqueous solution of ethylene oxide using supercritical carbon dioxide. Supercritical fluid extraction (T < 100°C p < 295 bar) removed most of the ethylene oxide and a little water into carbon dioxide, which formed directly the feedstock for carbonation (catalysed by methyl triphenyl phosphon-ium iodide at 20-90°C). Sufficient water was then supplied for hydrolysis. Carbon dioxide, both that produced in the hydrolysis step and the original solvent, was partially vented for recycle, liberating the ethylene glycol product and the reaction catalyst. 

A new inverted biphasic catalysis system using supercritical CO2 as the stationary catalyst phase and water as the continuous phase was described for rhodiumotalyzed hydroformylation of polar substrates. Product separation and catalyst recycling was possible without depressurizing the autoclave. Turnover numbers of up to 3560 were obtained in three consecutive runs and rhodium leaching into the aqueous phase was below 0.3 ppm [125]. Hydroformylation of propene was carried out in supercritical carbon dioxide + water and in supercritical propene + water mixtures using Rh(acac) (CO)2 and P(m-C6H4S03Na)3 as catalysts. Compared to traditional hydroformylation technology, the supercritical reactions showed better activity and selectivity [126]. 

Catalytic hydrogenation in supercritical carbou dioxide has been studied. The effects of temperature, pressure, and CO2 concentration on the rate of reaction are important. Hydrogenation rates of the two double bonds of an unsaturated ketone on a commercial alumina-supported palladium catalyst were measured in a continuous gra-dient-less internal-recycle reactor at different temperatures, pressures, and C02-to-feed ratios. The accurate control of the organic, carbon dioxide, and hydrogen feed flow rates and of the temperature and pressure inside the reactor provided reproducible values of the product stream compositions, which were measured on-line after separation of the gaseous components (Bertucco et al., 1997). 

In some cases, green reactions are based on feedstocks derived from renewable resources that produce highly pure compounds. Another green option is the use of supercritical fluids that are more benign substances (e.g., water, carbon dioxide, and light nonhalogenated hydrocarbons) such fluids can be used as solvents for separations or as media for reactions, and can be easily recovered from the product mixture and recycled. We can also include here the use of ionic systems of nonvolatile salts that are molten at ambient temperature, and that act as solvents or even have a dual role (as catalysts and solvents), without the risk of unwanted vapors. These ionic solvents replace the more hazardous, volatile, and expensive organic solvents used traditionally. 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

A Figure 11.21 Diagram of a supercritical fluid extraction process. The material to be processed is placed in the extractor. The desired material dissolves in supercritical CO2 at high pressure, then is precipitated in the separator when the CO2 pressure is reduced. The carbon dioxide is then recycled through the compressor with a fresh batch of material in the extractor. 

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