Catalytic extraction using supercritical solvent

CESS, catalytic extraction using supercritical solvent. 

The catalyst can be treated with a solvent to extract hydrocarbon deposits. The most straightforward solvent to use is isobutane, which has been shown to restore catalytic activity only partially. Supercritical solvents have been tested, but they also lead to only partial restoration of the activity. Supercritical alkylation to remove the deposits in situ has been shown in Section III.D.l to be less effective. It is unlikely that this method of operation will lead to a competitive process. 

The process concept involves the extraction of light hydrocarbon oils from asphaltic petroleum supercritical solvents followed by a subsequent fractionation and separation of the oil from the solvent. It is stated that the metal compounds which are present in the asphaltic petroleum do not dissolve in the solvent under the conditions of operation. The primary difference claimed for this new process relative to the old processes is that the solvent is at or above the critical temperature rather than below the critical temperature as is described in prior art. The operation is explained in the patent with the aid of a simple distillation-like extraction vessel. Asphaltic feedstock is heated and introduced into the extraction vessel. The solvent is also heated and introduced into the vessel and the two streams are mixed. The temperature is maintained at or above the critical temperature of the solvent. In the extractor, the non-soluble components of the feed setde and are removed and sent to a stripper to recover and recycle the solvent. Several examples give quantitative information when an asphaltic feedstock containing 28 ppm Ni, 220 ppm V is used. The oil yield and metal content results are given below for two cases where the solvent is catalytic cracker gasoline and propane, resf>ectively. 

Different uses of supercritical fluid (SCF) solvents in chemical separation processes have been of considerable research interest since the 1970s. The basic principles of SCF extraction engineering and a number of applications for this technology are described in several review papers [1,2]. As a new field related to SCF technology, the application of supercritical solvents as reaction media attracts increasing attention, especially for catalytic reactions. In such processes, the SCF may either actively participate in the reaction or function solely as the solvent for the reactants, catalysts, and products. 

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). 

The effects of added C02 on mass transfer properties and solubility were assessed in some detail for the catalytic asymmetric hydrogenation of 2-(6 -meth-oxy-2 -naphthyl) acrylic acid to (Sj-naproxen using Ru-(S)-BINAP-type catalysts in methanolic solution. The catalytic studies showed that a higher reaction rate was observed under a total C02/H2 pressure of ca. 100 bar (pH2 = 50bar) than under a pressure of 50 bar H2 alone. Upon further increase of the C02 pressure, the catalyst could be precipitated and solvent and product were removed, at least partly by supercritical extraction. Unfortunately, attempts to re-use the catalyst were hampered by its deactivation during the recycling process [11]. 

The use of ecologically harmless SCCO2 as solvent and substrate in chemical reactions is a particularly intriguing prospect. Increased governmental and environmental restrictions on solvent emission make this supercritical fluid more and more attractive as a reaction medium because it can be easily separated from the product and recycled more efficiently than conventional liquid solvents. The special properties (miscibility, transport properties, etc.) of sc CO2 require a development of suitably adjusted catalysts. A simple transformation of catalyst properties from conventional solvents to SCCO2 will mostly fail, and will not lead to higher catalytic efficiency. Supported catalysts could perhaps play a particular role in this field as the possibility of product extraction by depressurization of the supercritical phase and subsequent compression of the CO2 (solvent/substrate) should permit the development of a profitable continuous process. 

The supercritical fluid extraction, for example - the use of supercritical CO2 as the extracting solvent in place of organic solvents is becoming popular. The supercritical CO2 has the following advantages non-toxic, environmentally friendly, cheap, non-flammable and applicable on large scales [38]. The intrinsic mechanisms involved in the increased solubility of solute at supercritical conditions [39] as well as the enhancement effect imposed by the catalytic amounts of water[40] known as entrainer effect are studied. 

Saim and Subramaniam [38] and Ginosar and Subramaniam [39] also found that the in situ extraction of the coke compounds by near-critical or supercritical reaction mixtures prevents pore plugging that otherwise occurs at subcritical (gas-like) conditions. Although the coke laydown decreased at supercritical (liquid-like) conditions, the isomerization rates were lower and deactivation rates were higher due to pore diffusion limitations in the liquid-like reaction mixtures. It was therefore concluded that near-critical reaction mixtures provide an optimum combination of solvent and transport properties that is better than either subcritical (gas-like) or dense supercritical (liquid-like) mixtures for maximizing the isomerization rates and for minimizing catalyst deactivation rates. These findings indicate that catalytic reactions which require liquid-like reaction media for coke extraction and heat removal, yet gas-like diffusivities for enhanced reaction rates, can benefit from the use of near-critical reaction media. 

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