Methanol critical point

On the basis of this approach, a triblock copolymer of ethylene oxide (EO) and propylene oxide (PO), HO(EO) (PO)m(EO) OH was analyzed with respect to the PPO inner block and the PEO outer blocks by LCCC and SEC (Adrian et al., 1998). For the selective separation of the block copolymer with respect to the PPO block, experiments were conducted using chromatographic conditions, corresponding to the critical point of PEO. These could be established on a RP-18 stationary phase when an eluent of methanol-water 86 14 (v/v) is used. The separation of the triblock copolymer at the critical point of PEO is shown in Fig. 17.15. 

Figure 6.2—Density for carbon dioxide as a function ofpressure at four different temperatures. At the critical point, the density of CO2 is 0.46 g/cm3. The figure on the right represents the variation of the retention factor k for three alkaloids analysed under identical conditions and pressure, fixed downstream from the column by a restrictor (T = 4"C, modifier 5% water and 15% methanol 1 codeine, 2 thebaine, 3 papaverine). As the pressure increases, k decreases.

Mixtures of C02 and methanol were selected for the initial investigation of the solvatochromic behavior in supercritical fluid systems. This combination is of interest as it combines the low critical temperature and pressure of carbon dioxide with a polar, less volatile modifier. This system exhibits relatively simple Type I phase behavior and several groups have published measurements of mixture critical points (19-21). At intermediate compositions the critical pressure for this fluid is much higher than that of either pure C02 or pure methanol, reaching a maximum of approximately 2400 psi (20). 

Supercritical fluid chromatography (SFC) is very similar in principle to, and is as convenient as, high-performance liquid chromatography, but it uses as the high-pressure eluant fluid CO2 (or other fluid) above its critical point (for CO2 Tc = 31.3°C, Pq = 7.38 MPa, pc = 0.448 g cm-3). SPC can separate relatively small and/or thermally labile molecules. The analyte is introduced as a solution in methanol. Small amounts of organic solvents can be added as "modifiers." Any C02 brought out with the analyte to ambient atmosphere will evaporate harmlessly. 

Pressure-temperature diagrams offer a useful way to depict the phase behaviour of multicomponent systems in a very condensed form. Here, they will be used to classify the phase behaviour of systems carbon dioxide-water-polar solvent, when the solvent is completely miscible with water. Unfortunately, pressure-temperature data on ternary critical points of these systems are scarcely published. Efremova and Shvarts [6,7] reported on results for such systems with methanol and ethanol as polar solvent, Wendland et al. [2,3] investigated such systems with acetone and isopropanol and Adrian et al. [4] measured critical points and phase equilibria of carbon dioxide-water-propionic acid. In addition, this work reports on the system with 1-propanol. The results can be classified into two groups. In systems behaving as described by pattern I, no four-phase equilibria are observed, whereas systems showing four-phase equilibria are designated by pattern II (cf. Figure 3). 

FIGURE 5.1. Liquid/vapour coexistence curves of SPC water (solid line) and OPLS methanol (dashed line) following hrom the RISM-KH theory. Molecular simulation results for water [45] and methanol [46] (open circles and squares, respectively) and critical point extrapolations (filled symbols). 

FIGURE 5.2. LiquidA apour coexistence curves of hydrogen fluoride (solid line), methanol (short-dashed line), water (long-dashed line) and dimethylsulfoxide (dash-dotted line) following from the RISM-KH theory. Experimental data for their critical points (filled triangle, square, circle and rhomb, respectively), and at ambient conditions (corresponding open symbols). 

All calculations were carried out at T = 313.15 K. The vapor-liquid equilibrium (VLB) data for the ternary mixture and the corresponding binaries were taken from [32]. The excess volume data for the ternary mixture A,A-dimethylformamide-methanol-water and binary mixtures A, A-dimethylformamide-methanol and methanol-water were taken from [33], and the excess volume data for the binary mixture A,A-dimethylformamide-water from [34]. There are no isothermal compressibility data for the ternary mixture, but the contribution of compressibility to the binary KBls is almost negligible far from the critical point [6]. For this reason, the compressibilities in binary and ternary mixtures were taken to be equal to the ideal compressibilities, and were calculated from the isothermal compressibilities of the pure components as follows ... 

Consequently, the vast majority of SCF applications are based on CO2 near the GL critical point, with a possible admixture to support the ability for solvating dipolar components. The extraction of carcinogenic aromatic hydrocarbons and their nitro derivatives from diesel particulates by CO2 + toluene or methanol SCF can serve as an example. CO2 based SCF also helps in cleaning polyethylene from undesired polymer additives. In a similar way one can consider technologies focused on so called h q)er-coal, an extremely pure and environment friendly fuel for turbines in power plants. Recently, the first power plants based on this idea are being constructed in China. The removal of pesticides from meat, decaffeinated coffee and denicotinized cigarettes are the next society-relevant applications. Noteworthy is the h q)er-oxidation with supercritical water and bitumens extraction based on supercritical toluene. The latter system is also used for the liquefaction of coal. ... 

The basic advantages of using binary mixtures with the critical CP instead of the GL critical point can be found in the history of critical phenomena, namely establishing the basic universal parameters appeared to be much simpler for binary mixtures with CP than for GL systems. Firstly, CP investigations can be carried out under atmospheric pressure. Secondly, one can select a binary mixture with CP close to room temperature. " Finally, it is possible to select a mixture which emphasizes the desired specific feature, for instance (a) methanol - cyclohexane mixture can simulate weightless conditions since densities of both components are almost equal (b) there are almost no critical opalescence for isooctane - cyclohexane mixture since their refractive indices are almost the same (c) one can considerably change the concentration of the dipole component of the mixture. The latter feature can strongly influence both dielectric properties and solvency. 

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