Supercritical fluid separations transport

Supercritical fluids (SCFs) offer several advantages as reaction media for catalytic reactions. These advantages include the ability to manipulate the reaction environment through simple changes in pressure to enhance solubility of reactants and products, to eliminate interphase transport limitations, and to integrate reaction and separation unit operations. Benefits derived from the SCF phase Fischer-Tropsch synthesis (SCF-FTS) involve the gas-like diffusivities and liquid-like solubilities, which together combine the desirable features of the gas- and liquid-phase FT synthesis routes. 

Supercritical fluids are benign alternatives to conventional organic solvents that may offer improvements in reaction rate, product selectivity, and product separation. We reported the first use of SCFs for phase-transfer catalysis (PTC), where these benign alternatives also offer greatly improved transport, product separation, catalyst recycle, and facile solvent removal (26-29). 

Small changes in the temperature or pressure of a supercritical fluid may result in great changes in its viscosity and in the diffusivity and solubility of compounds dissolved within it. In such systems, the bioconversion rate is increased thanks to the high diffusion rates which facilitate transport phenomena. In some cases a high diffusion rate can also facilitate product separation. 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

Operation of the FDU with coal-derived residuum was preceded by tests on pure compounds and distillable coal liquids using n-pentane as the supercritical fluid ( ). The results of tests with the coal-derived distillate showed that the FDU was basically performing as expected. Liquid reflux was generated by means of the hot finger when the device was operated at a temperature slightly above the critical temperature of the transport fluid. When reflux was established, fractionation based upon volatility was observed. Poorer separation was achieved in the absence of reflux. 

The first work on the T102 bottoms involved operation in the non-reflux mode to obtain base-line data on the transport of this residuum in various hydrocarbon solvents. Using n-pentane, cyclohexane, and toluene at a Tg of 1.02 and at a Pg of 2, the residuum brought overhead was 23, 54, and 67 percent of that charged, respectively. Based on this information, further studies were performed using cyclohexane as the supercritical fluid, since a large portion of the residuum could be destracted at a temperature similar to or less than that in the T102 separator. 

The general properties of supercritical fluids make them an attractive alternative to liquid solvents in column operations where transport effects come into play. If supercritical CO2 is employed as the solvent, this advantage is further supplemented by the non-flammable, non-toxic nature of the fluid, and the relative ease of solvent recovery. Supercritical solvents also offer the potential to greatly enhance thermally driven separations through dramatic changes in component solubility, adsorptive characteristics, and thermal conductivity near the critical region. 

Supercritical fluids are attractive solvents as they exhibit physicochemical properties intermediate between those of hquids and gases (Table 2). The density, thus the solvating power, of a SCF approaches that of a liquid, whereas the diffusivity and viscosity are intermediate between gas-Uke and liquid-like values, resulting in faster mass transport capacity (5). As a result of the large compressibility near their critical points, SCFs densities/solvent power can be varied by changing operating conditions (temperature and pressure), resulting in operational flexibility, which can be exploited to achieve the required separation. 

Dynamic factors are among the key variables to be optimized in an SFE process. In addition to extracting the analytes, the primary function of the supercritical fluid is to transport the solutes to the collecting vessel or to an on-line coupled chromatograph or detector. Ensuring efficient transportation of the analytes following separation from the matrix entails optimizing three mutually related variables, namely the flow-rate of the supercritical fluid, the characteristics of the extraction cell and the extraction time. These factors must be carefully combined in order to allow the flow-cell to be vented as many times as required. 

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 special properties of supercritical fluid (SCF) solvents [7,8] for PTC reactions bring substantial environmental and econonric advantages. First, they permit the use of totally benign solvents, especially CO2, and the solvent separation from product becomes quite facile. Moreover, since PTC processes always involve mass transfer, the lower viscosity and higher diffusivity of SCFs significantly enhance transport. 

Compared with the samples typically separated by supercritical fluid chromatography most solvents are significantly more volatile and can be removed from the sample by evaporation in a short open tube or packed precolumn [174-178]. Solventless injection requires only minor modification to a standard rotary injector, either addition of a second valve or tee piece and a precolumn. This allows sequential programming of the independent processes of solvent removal and sample deposition in the precolumn followed by dissolution of the sample in the mobile phase and its transport to the column for refocusing. Quite large solvent volumes (hundreds of p,l) can be handled in this way, but typically smaller volumes are used. The only real limitation to solventless injection is that the volatility difference between the sample and solvent must be sufficient for effective removal of the solvent without loss of sample. In addition, effective sample focusing at the head of the column is required to maintain an acceptable separation... 

Table I. lists a few possible advantages for conducting chemical synthesis in a supercritical environment. For example, supercritical fluids might provide a means to manipulate reaction environments by altering density and temperature to influence both reaction rate and selectivity. Futhermore, in a homogeneous supercritical phase, one can, in principle, eliminate interfacial transport limitations. Effectively, temperature and pressure are used to alter density in a way to influence both solvation dynamics and equilibrium solubility. With supercritical fluid solvents, there is the possibility of integrating both reaction and separation processes, which could lead to economic advantages over conventional synthetic processes carried out in liquid solvents.

Supercritical fluids are effective at much lower temperatures than distillation, and their application in separation avoids degradation and decomposition of heat-labile compounds. Attractiveness of supercritical extraction processes are due to the sensitivity of responses to process variables, promise of complete and versatile regeneration of solvents, energy savings, enhanced solute volatilities, solvent selectivities, favorable transport properties for solvents, and state governed effectiveness of solvents which enables the use of low cost, non-toxic, environmentally acceptable solvents. The impact of inherent characteristics of supercritical fluids on separations is summarized in Table 21.1.5. 

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