Supercritical gas separations

However, we need a more physical basis on the concept of the saturated vapour pressure for the adsorptive of the supercritical gas. When we use the molecular statistical approach to description of adsorption for the supercritical gas, the saturated vapour pressure is not necessarily indispensable. The distinction between vapour and supercritical gas can be taken into account through the intermolecular and molecule surface interaction energies [12-15]. Nevertheless, it is better to discuss the micropore filling mechanisms for vapour and supercritical gas separately in order to obtain each physical picture. 

In the remaining sections of this overview, polymer materials design strategies to achieve high performance in supercritical gas separation based on diffusivity selectivity and in vapor separation based on high solubility selectivity will be discussed. 

As shown in Figure 4, diffusion coefficients decrease with increasing penetrant size. Therefore, for an organic vapor/supercritical gas separation, diffusivity selectivity is always less than one ... 

Due to possible environmental problems with acetone, new technologies are being developed for the production of deoiled lecithins involving treatment of Hpid mixtures with supercritical gases or supercritical gas mixtures (10—12). In this process highly viscous cmde lecithin is fed into a separation column at several levels. The supercritical extraction solvent flows through the column upward at a pressure of 8 MPa (80 bar) and temperature between 40 and 55°C. The soy oil dissolves together with a small amount of lecithin. 

The solubilities, discussed above, of the various gases in the ionic liquids have important implications for applications of IFs. The impact of gas solubilities on reactions, gas separations and the use of compressed gases or supercritical fluids to separate solutes from IFs are discussed below. 

The criteria which would be most desirable for industrial application of a separation process involving a supercritical gas may be established by comparing Figs. 3IB, 3ID, and 32. The largest cost in such a process is likely to be that of gas compression. Therefore, the maximum separation possible of the two solvents should occur for the addition of a given amount of gas, and the total pressure required to dissolve this gas should be small. This is the case if the tie lines slope toward the 1-3 binary line and if the gas is readily soluble. In terms of the Margules parameters and Henry s constant, these favorable criteria are ... 

To get an idea about the relative volatilities of components we proceed with a simple flash of the outlet reactor mixture at 33 °C and 9 bar. The selection of the thermodynamic method is important since the mixture contains both supercritical and condensable components, some highly polar. From the gas-separation viewpoint an equation of state with capabilities for polar species should be the first choice, as SR-Polar in Aspen Plus [16]. From the liquid-separation viewpoint liquid-activity models are recommended, such as Wilson, NRTL or Uniquac, with the Hayden O Connell option for handling the vapor-phase dimerization of the acetic acid [3]. Note that SR-Polar makes use of interaction parameters for C2H4, C2H6 and C02, but neglects the others, while the liquid-activity models account only for the interactions among vinyl acetate, acetic acid and water. To overcome this problem a mixed manner is selected, in which the condensable components are treated by a liquid-activity model and the gaseous species by the Henry law. 

Gas flow processes through microporous materials are important to many industrial applications involving membrane gas separations. Permeability measurements through mesoporous media have been published exhibiting a maximum at some relative pressure, a fact that has been attributed to the occurrence of capillary condensation and the menisci formed at the gas-liquid interface [1,2]. Although, similar results, implying a transition in the adsorbed phase, have been reported for microporous media [3] and several theoretical studies [4-6] have been carried out, a comprehensive explanation of the static and dynamic behavior of fluids in micropores is yet to be given, especially when supercritical conditions are considered. Supercritical fluids attract, nowadays, both industrial and scientific interest, due to their unique thermodynamic properties at the vicinity of the critical point. For example supercritical CO2 is widely used in industry as an extraction solvent as well as for chemical... 

The adsorption of gases at room temperature and up to pressures in the region of 50 bars is of interest from several points of view. From an industrial standpoint, both gas separation (by pressure-swing adsorption (PSA)) and gas storage operate in this pressure range [1]. From a fundamental point of view however, the nature of the adsorbate-adsorhent interactions close to and within supercritical regions merit detailed study [2],... 

It is a precondition for both operations that the density of the liquid pumped onto the top of the column is higher than the density of the supercritical gas. The difference should be large enough in order to give a proper phase separation. If, at very high pressures, the gas density surpasses the liquid density a so-called barotropic phase inversion can be observed at which the liquid swims on the dense gas. 

The phase-split block can be a single flash, a series of flashes, or a combination of flash and absorption/stripping columns. Flash temperature and pressure are design variable that may be optimised to fulfil a separation objective, as sharp gas/liquid split or recovery of some components. For water-driven condensers the recommended condensation temperature is of about 35 °C. Vapour components can be condensed and sent to the liquid separation system. The supercritical components carried in the liquid phase can be recovered in a stabiliser column (see later in this section). Further, these can be sent to the gas separation system, used as fuel, or purged. 

The application of and N.M.R. spectroscopy, gas chromatography (G.C.) and mass spectrometry (M.S.) in the separation and identification of alkanes extracted from fossil fuels is illustrated with three Turkish lignites (including one extracted by supercritical gas), coal tar and petroleum crude. Elution of hydrocarbons from a silica-gel column may be monitored by N.M.R. and molecular-sieve sub-fractionation into normals and branched/cyclics by G.C., together with... 

National Coal Board supercritical extraction In separate hydrogenation reactor 350-450 100-200 H2 in separate reactor Supercritical gas extraction of portion of coal with PhMe as solvent... 

Precipitation of the extract is achieved by pressure reduction and/or absorption of the extracted compounds in an absorbing liquid. For the extraction of xanthines (caffeine, theobromine and theophylline) the absorbing liquid is water. The supercritical gas. loaded with the extract, enters the precipitator at the lower end and flows upwards against the absorbing water, which is sprayed from the top of the vessel. The regenerated gas leaves the vessel at the top after passing a liquid/gas separator for removing droplets. 

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