Processing using supercritical fluids

When a fluid substance such as carbon dioxide or water is heated under pressure, the density of the liquid phase will decrease as the temperature is increased, whereas the density of the gas phase will increase with increased pressure. If a temperature-pressure phase diagram is constructed, and if we move upward along the curve separating the liquid phase from the gas phase, a temperature and pressure will be reached where the densities of the liquid and gas phases become identical and there will be no distinction between the gas and liquid phases. This point on this phase diagram is known as the critical point, and a supercritical fluid is any fluid that is at a temperature and pressure greater than those at the critical point. The critical temperature and pressure will vary according to the chemical structure of the fluid. For example, carbon dioxide (CO ) has a critical temperature of 304 K and a critical pressure of 74 bar, whereas the critical temperature and pressure of water are 647 K and 221 bar, respectively (Williams et al, 2002). Above the critical temperature, a gas cannot be liquefied by pressure. 

The solubihty and the diffusion coefficients of CO in starch-water mixtures were determined by Chen and Rizvi (2006a) using native and pregelatinized com starches for their experiments. Starch mixtures containing 50 50,75 25, and 100 0ratios of pregelatinized 

Starch to native com starch were prepared, and sufficient water at 50 °C was then added to obtain a water content of 40%. The starch-water mixtures were then placed in a diffusion cell and the solubility and diffusion coefficients of CO were determined at pressures up tol6 MPa at 50 °C. The experimentally-measured solubility accounted for the estimated swollen volume the solubility increased with pressure. The solubility of CO showed no dependence on the degree of gelatinization of the starch. The diffusion coefficient of CO increased with the concentration of CO (which was pressure dependent) and decreased with increasing degrees of starch gelatinization. 

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Palakodaty, S. and York, P. (1999) Phase behavioral effects on particle formation processes using supercritical fluids. Pharmacological Research, 16 (7), 976-985. 

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

In extraction processes using supercritical fluids it is of interest to predict the heat-transfer processes that take place in heat exchangers in forced flow. 

State of the art of material processing using supercritical fluids... 

Supercritical fluids display attractive solvent characteristics which can be manipulated by either the pressure or temperature. Using supercritical fluids as reaction media, simultaneous reaction and separation are also achievable. This methodology has recently been applied to the reactive separation of wood constituents, especially lignin, by supercritical fluids (1-4). Delignification processes using supercritical fluids are of potential Industrial Importance (5,6) and there Is a need for the development of kinetic models which could permit a priori prediction of the rate of lignin removal. The present paper discusses such a model. 

Adshiri, T. Arai, K. Kitamura, M. Masuoka, H. Sako, Y. Takishima, S. Material processing using supercritical fluids. In Supercritical Fluids Arai, Y., Sako, T., Takebayashi, Y., Eds. Materials Processing, Springer, 2001 280-345. 

Palakodaty S and York P. Phase Behavioral Effects on Particle Formation Processes using Supercritical Fluids. PharmRes 1999 16 976-985. 

Mandel FS. Mixing system for processes using supercritical fluids. US patent 5 993 747, 1997. 

Shine AD, Gelb J Jr. Microencapsulation process using supercritical fluids. US patent 5766637, 1998. 

H. Wakayama, H. Itahara, N. Tatsuda, S. Inagaki and Y. Fukushima (2001) Chemistry of Materials, vol. 13, p. 2392 - Nanoporous metal oxides synthesized by the nanoscale casting process using supercritical fluids . 

Tomasko, D. L, Hay, K. J, Leman, G. W., and Eckert, C. A. (1993) Pilot scale study and design of a granular activated carbon regeneration process using supercritical fluids. Environ Progress 12, 208-217. 

Montes, A., Gordillo, M.D., Pereyra, C., de la Rosa-Fox, N., and Martmez de la Ossa, E.J. Silica microparticles precipitation by two processes using supercritical fluids. The Journal of Supercritical Fluids 75 (2013) 88-93. 

The interaction of CO2 and polymers can be divided into three application areas processing of swollen or dissolved polymers and applications where carbon dioxide does not interact with the polymer. An extensive review on polymer processing using supercritical fluids has been written by Kazarian [55], including possible applications based on the specific interaction of CO2 and the polymer system involved. 

It was also reported that PPy could be polymerized using nontoxic supercritical fluids as solvents. In conventional chemical polymerization, incorporation of the oxidant into the polymerization process and/or washing to remove unwanted byproducts generates a large amount of environmentally hazardous solvent. In the process using supercritical fluid, which is nontoxic, nonflammable, and environmentally acceptable, PPy was polymerized within preformed polyurethane (PU) foam using supercritical carbon dioxide as solvent [41]. 

An extensive review on polymer processing using supercritical fluids by Kazarian [21] includes the applications mentioned above. To illustrate the possibilities of polymer processing with SCCO2 from an engineering point of view, two important applications will be discussed in more detail, namely extraction (see Section 21.2.3.1) and impregnation (Section 21.2.3.2). 

These processes using supercritical fluids are of increasing importance for the... 

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