Carbon dioxide critical parameters

The most common and widely used supercritical fluid in SFC is carbon dioxide. It is inert, in that it is non-toxic and non-flammable, it also has mild critical parameters, a low critical temperature of 31.3°C and a critical pressure of 72.8 atm [1], Using pure, supercritical carbon dioxide eliminates organic solvent waste and with it waste disposal costs and concerns. This is extremely practical advantage in the industrial environment where the generation of waste requires special handling and significant cost. 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

Critical points vary widely. Table 6.1 shows a representative sample of critical parameters and it is immediately obvious why carbon dioxide is widely used. With a critical temperature just above room temperature and a critical pressure that is relatively low, the amount of energy needed to render carbon dioxide supercritical is comparatively small. Fluoroform (CHF3) and difluoromethane also have easily attainable critical parameters, but they are much more expensive than carbon dioxide. Despite its high critical temperature and pressure, supercritical water (SCH2O) is used widely as a destructive medium since it is highly acidic. 

The diffusion coefficient as defined by Fick s law, Eqn. (3.4-3), is a molecular parameter and is usually reported as an infinite-dilution, binary-diffusion coefficient. In mass-transfer work, it appears in the Schmidt- and in the Sherwood numbers. These two quantities, Sc and Sh, are strongly affected by pressure and whether the conditions are near the critical state of the solvent or not. As we saw before, the Schmidt and Prandtl numbers theoretically take large values as the critical point of the solvent is approached. Mass-transfer in high-pressure operations is done by extraction or leaching with a dense gas, neat or modified with an entrainer. In dense-gas extraction, the fluid of choice is carbon dioxide, hence many diffusional data relate to carbon dioxide at conditions above its critical point (73.8 bar, 31°C) In general, the order of magnitude of the diffusivity depends on the type of solvent in which diffusion occurs. Middleman [18] reports some of the following data for diffusion. 

Pure fluids. Carbon dioxide is often the mobile phase of choice for SFC, since it has relatively mild critical parameters, is nontoxic and inexpensive, chemically inert, and is compatible with a wide variety of detectors including the flame ionization detector (FID) used widely in GC and the UV absorbance detector employed frequently in HPLC (7). The usefulness of carbon dioxide as a mobile phase in many instances is somewhat limited, however, because of its nonpolarity (8), and many polar compounds appear to be insoluble in it. For a sample containing polar compounds, pure carbon dioxide may not be the proper mobile phase. The elution of polar compounds is often difficult and the peak shapes for these polar compounds are sometimes poor. This latter difficulty is commonly observed with nonpolar supercritical fluids and may be due to active sites on the stationary phase rather than any inherent deficiency in the fluid itself. 

Figure 11 illustrates the parameter space defined by the equilateral triangle. The initial pressure and conditions for the 3 vertices of the pressure gradient/ temperature triangle were determined arbitrarily from the critical conditions of the supercritical fluid (carbon dioxide), the retention characteristics of nitroaromatic compounds, and the following criteria (i) the first analyte should not co-elute with the sample solvent and (ii) the retention factor of the last analyte should not exceed 30. 

The solubility of TB 1 in supercritic carbon dioxide over the pressure range from 8 to 19 MPa and from 308 to 328 K has been measured using a flow system. Models based on chemical association, which did not require the critical parameters of the solute, were used to correlate the experimental data (00JCED464). Addition of methanol dramatically enhances the solubility (00MI823). 

A substance with favorable critical parameter values and that best matches the other aforementioned criteria is carbon dioxide (C02). The critical temperature of C02 is +31.3°C, which is especially important for thermally unstable analytes, and its critical pressure of 72.9 bar (1 bar = 105 Pa) is easy to obtain under laboratory conditions. Moreover, C02 is nonflammable, nontoxic, does not pose any additional, serious threat to the environment, and is relatively inexpensive. For on-line solutions, it is important that C02 be compatible with most chromatographic detectors. 

The carbon di oxi de/lemon oil P-x behavior shown in Figures 4, 5, and 6 is typical of binary carbon dioxide hydrocarbon systems, such as those containing heptane (Im and Kurata, VO, decane (Kulkarni et al., 1 2), or benzene (Gupta et al., 1 3). Our lemon oil samples contained in excess of 64 mole % limonene so we modeled our data as a reduced binary of limonene and carbon dioxide. The Peng-Robinson (6) equation was used, with critical temperatures, critical pressures, and acentric factors obtained from Daubert and Danner (J 4), and Reid et al. (J 5). For carbon dioxide, u> - 0.225 for limonene, u - 0.327, Tc = 656.4 K, Pc = 2.75 MPa. It was necessary to vary the interaction parameter with temperature in order to correlate the data satisfactorily. The values of d 1 2 are 0.1135 at 303 K, 0.1129 at 308 K, and 0.1013 at 313 K. Comparisons of calculated and experimental results are given in Figures 4, 5, and 6. 

The results of an experimental Investigation are presented for the separation of mixtures of 1,3-butadiene and 1-butene at near critical conditions with mixed and single solvent gases. Ammonia was used as an entrainer to enhance the separation. Several non-polar solvents were used which included ethylene, ethane and carbon dioxide, as well as mixtures of each of these gases with ammonia in concentrations of 2, 5, 8 and 10% by volume. Each solvent and solvent mixture was studied with respect to its ability to remove 1-butene from an equimolar mixture of 1,3-butadiene/ 1-butene. Maximum selectivities of 1.4 to 1.8 were measured at a pressure of 600 psia and a temperature of 20 C in mixtures containing 5%-8% by volume of ammonia in ethylene. All other solvents showed little or no success in promoting separation of the mixture. The experimental results are reported for ethylene/ ammonia mixtures and are shown to be in fair agreement with VLE flash calculations predicted independently by a modified two parameter R-K type of equation of state. 

Biosensors. Sensors are required to adequately monitor bioreactor performance. Ideally, one would like to have online sensors to minimize the number of samples to be taken from the bioreactor and to automate the bioreactor process. Most bioreactors have autoclavable pH and dissolved oxygen (D.O.) electrodes as online sensors, and use offline detectors to measure other critical parameters such as glucose and glutamine concentration, cell density, and carbon dioxide partial pressure (pC02). An online fiber-optic-based pC02 sensor is commercially available and appears to be robust.37 Probes are also commercially available that determine viable cell density by measuring the capacitance of a cell suspension. Data from perfusion and batch cultures indicate that these probes are reasonably accurate at cell concentrations greater than 0.5 X 106 cells/mL.38,39... 

Figure 5. Pressure-temperature diagram for carbon dioxide-water-1-propanol — critical lines calculated with Peng-Robinson EOS using the mixing rule of Panagiotopoulos-Reid, parameters fitted to ternary three-phase equilibria at temperatures between 303 and 333 K...

As pressure and temperature increase, the properties of the gas and liquid phase, for example density or dielectricity, get to be more and more identical. The critical point is defined as the temperature (Tc critical temperature) and pressure (pc critical pressure) at which the properties of the phases become identical and so merge into a single, supercritical phase. The critical point is a specific parameter for a substance and for carbon dioxide it is at Tc = 31.3 °C and pc = 73.8 bar/ /. 

The solvent capacity of supercritical carbon dioxide changes with the variation of density and so it can be easily modulated by the variation of pressure. Adding small amounts of co-solvents (modifiers) changes the chemical and physical properties, like the solvent capacity or the critical point. To ensure supercritical conditions it is thence crucial to know the critical parameters as a function of the modifier concentration. Table 1 lists the critical pressure and temperature as a function of the co-solvent concentration in carbon dioxide/2/. 

Table 2 shows critical parameters of the fluids most used for SFE. When it comes to choosing a supercritical fluid, the critical pressure and the critical temperature are two important parameters. The critical pressure determines, from a first approximation, the importance of the solvent power of the fluid. Ethane, for example, which has a lower critical pressure than carbon dioxide, will not dissolve a moderately polar soluble in the same way as carbon dioxide. Similarly, fluids with a higher critical pressure are more able to dissolve polar compounds. The critical temperature has practical implications. Indeed, one should always consider the influence of the extraction temperature on the stability of the component to extract. 

One other parameter critical to product properties is the size distribution of the bubbles in the expanded product. Comparable bulk densities will be measured either with a few large bubbles or a large number of small ones. However, the rehydration and textural properties of the two structures will be markedly different. The distribution of bubble sizes relates to nucleation rather than growth. Frequently, the presence of insoluble particles in the melt is sufficient to cause multisite nucleation as shown in the above figure, but when this is not the case, small amounts of finely divided powder can be added to the formulation. Calcium carbonate is frequently used, acting as a weak point in the continuous melt, and also releasing gaseous carbon dioxide (personal communication, Charles Chessari, Food Science Australia, N. Ryde, Australia). 

SCFs have a tunable density that may offer further advantages in reaction and processing applications. This tunability is illustrated in Fig. 1 for carbon dioxide. Near the critical point, even small changes in the temperature or pressure of carbon dioxide dramatically affect its density. Similarly, the viscosity, dielectric constant, and diffusivity are also tunable parameters, which allows specific control of systems involving supercritical fluids. 

Water is also included in the table to make one point— the solvent that we are all most familiar with is a poor candidate from both engineering and safety standpoint. The critical temperature and pressure are among the highest for common solvents. Ammonia is very unpleasant to work with since a fume hood or other venting precautions are needed to keep it out of the laboratory atmosphere. One of the alternative fluids of potential interest is nitrous oxide. It is attractive since it has molecular weight and critical parameters similar to carbon dioxide, yet has a permanent dipole moment and is a better solvent than carbon dioxide for many solutes. There are evidences of violent explosive reactions of nitrous oxide in contact with oils and fats. For this reason, nitrous oxide should be used with great care and is not suitable as a general purpose extraction fluid. 

For the prediction of the mixed-gas solubilities from the solubilities of the pure individual gases, the pressure dependence of the binary parameters ku is needed. The Peng—Robinson EOS was used to determine the binary parameters ku. The binary interaction parameter qi2 in the van der Waals mixing rule was taken from ref 28, where it was evaluated for the water-rich phases of water—hydrocarbon and water—carbon dioxide binary mixtures. The calculated binary parameters ku are listed in Table 1. One should note that, as expected for a liquid phase, the above parameters are almost independent of pressure, in contrast to their dependence on pressure in the gaseous phase near the critical point,... 

The porous structure of chars from a high volatile bituminous coal from mine Pumarabule in Spain, initial and preoxidized, then steam activated, was characterized by carbon dioxide and benzene adsorption measurements, as well as by immersion calorimetry molecular probes with increasing critical dimensions were used. The influence of preoxidation of the coal on the values of parameters describing the pore size distribution, with particular attention to micropores, evaluated according to each of the applied methods, is discussed. 

The pure component intermolecular potential parameters used in this study are shown in Table I. They were obtained as follows for carbon dioxide, we fitted the experimental critical temperature and pressure (12) using data from ( ) for the critical constants of the Lennard-Jones (U) system (T - 1.31, - 0.13). For acetone, a... 

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