Systems at high pressure

Consider what happens when the system point is at point a in Fig. 13.13 and the pressure is then increased by isothermal compression along line a-b. The system point moves from the area for a gas phase into the two-phase gas-liquid area and then out into the gas-phase area again. This curious phenomenon, condensation followed by vaporization, is called retrograde condensation. 

Under some conditions, an isobaric increase of T can result in vaporization followed by condensation this is retrograde vaporization. 

If the pressure of a system is increased isothermally, eventually solid phases will appear these are not shown in Figs. 13.13 and Fig. 13.14. 

Thermodynamics and Chemistry, second edition, version 3 2011 by Howard DeVoe, Latest version www. chem.umd.edu/themobook 

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As is implied by the name, a unimolecular reaction is one in which a single molecule of reactant decomposes or rearranges to give rise to product molecules. Ordinary thermal reactions can be modeled by a process which considers the reactant to be in thermal equilibrium with a transition state which then decomposes (rearranges) to give products. One can theoretically describe the process and its isotope effects using transition state theory. For unimolecular reactions, on the other hand, while there is still a transition state, it is not in thermal equilibrium with the reactant except for systems at high pressure. Consequently, a more elaborate theoretical framework is required to understand unimolecular reactions and their isotope effects. 

We generally distinguish between two methods when the determination of the composition of the equilibrium phases is taking place. In the first method, known amounts of the pure substances are introduced into the cell, so that the overall composition of the mixture contained in the cell is known. The compositions of the co-existing equilibrium phases may be recalculated by an iterative procedure from the predetermined overall composition, and equilibrium temperature and pressure data It is necessary to know the pressure volume temperature (PVT) behaviour, for all the phases present at the experimental conditions, as a function of the composition in the form of a mathematical model (EOS) with a sufficient accuracy. This is very difficult to achieve when dealing with systems at high pressures. Here, the need arises for additional experimentally determined information. One possibility involves the determination of the bubble- or dew point, either optically or by studying the pressure volume relationships of the system. The main problem associated with this method is the preparation of the mixture of known composition in the cell. 

G. Schneider, Phase Equilibriums in Liquid Systems at High Pressures , Ber. Bunsenges. Physik. Chem., 70, 497-520 (1966). G. M. Schneider, Phase Equilibria in Fluid Mixtures at High Pressures , Adv. Chem. Phys. 17, 1-42 (1970). G. M. Schneider, "Gas-Gas Equilibrium. Fluid Mixtures Under Pressure , Fortschr. Chem. Forsch. 13, 559-600 (1970). G. M. Schneider, Phase Behavior and Critical Phenomena in Fluid Mixtures Under Pressure , Ber. Bunsenges. Physik. Chem. 76, 325-331 (1972). G. M. Schneider in Water A Comprehensive Treatise, Vol II, F. Franks, ed.. Plenum Press, New York, 1973, Ch. 6. 

Fei, Y., Mao, H.-K. Mysen, B. O. (1991) Experimental determination of element partitioning and calculation of phase relations in the MgO-FeO-Si02 system at high pressure and high temperature. J. Geophys. Res., 96,2157-69. 

Avdeev, V. V., V. A. Nalimova, and K. N. Semenenko. 1990. Sodium-graphite system at high pressures. 

The investigations of platinum pyrochlores have demonstrated the effectiveness of high pressure techniques in the synthesis of anhydrous oxides when one or both reactants have limited thermal stability. The bulk of the work reported here represents a continuation of an exploration of metal oxide-platinum oxide systems at high pressure. 

Tiltscher H., Schelchshom J., Wolf H., Dialer K., Differential Recycle Reactors for Investigation of Heterogeneous Systems at High Pressures and Temperature Ger. Chem. Eng. 313-320. 

There is considerable difference in the system behavior at high pressure batch and continuous performance. In the continuous system the decrease in conversion between 80 bar and 150 bar is about 30% at 50°C and not only some percent as in the batch system. The difference may be that in the continuous system at high pressure the water is removed from the enzyme and in the batch system, although it is solubilized in SC CO2, it remains in the system and the activity of the lipase only slightly decreases. 

While phase equilibria for the a-tocopherol/carbon dioxide system at high pressures have been studied by several authors [1-6], only a few measurements of dynamic viscosity [7], thermal conductivity [7] and mass transfer coefficients [3] were carried out. The present study of the interfacial tension in the a-tocopherol/carbon dioxide system at temperatures between 313 and 402 K and pressures from 10 to 37 MPa aims on the one hand at completing characterisation of this system and on the other at contributing to understanding interfacial phenomena in mass transfer processes. 

Ehrlich, P., "Phase Equilibria of Polymer-Solvent Systems at High Pressures Near Their Critical Loci. II. Polyethylene-Ethylene," J. Polym. Sci. Part A., 3, 131 (1965). 

Ohgaki, K. and Katayama, T. 1977. "Isothermal Vapor-Liquid Equilibrium Data for the Ethane-Carbon Dioxide System at High Pressure" Fluid Phase Equil., 1 27-32. 

Pytkowicz, R.M. The carbon dioxide-carbonate system at high pressure in the oceans, p. 83-135, Barnes, H., ed. "Oceanogr. Mar. Biol. Ann. Rev. 6," George Allen and Unwin., London (1968). ... 

Artioli G., Fumagalli P., and Poli S. (1999) The crystal structure of Mg8(Mg2Al2)AlgSii2(0,0H)56 pumpeUyite and its relevance in ultramafic systems at high pressure. Am. Mineral. 84, 1906-1914. 

Asahara Y. and Ohtani E. (2001) Melting relations of the hydrous primitive mantle in the CMAS-H2O system at high pressures and temperatures, and imphcations for generation of komatiites. Pkys. Earth Planet. Inter. 125, 31-44. 

Boehler R. (1992) Melting of the Fe-FeO and the Fe-FeS systems at high pressure constraints on core temperatures. Earth Planet. Sci. Lett. Ill, 217—227. 

Shterenberg L. E., Slesarev V. N., Korsunskaya I. A., and Kamenetskaya D. S. (1975) The experimental study of the interaction between the melt carbides and diamond in the iron-carbon system at high pressures. High Temp.-High Press. 7, 517-522. 

The product gas of the methanation section contains mainly CHi, Hj, HjO, and CO2. Removing H1O from this stream results in SNG as the final product, which leaves the system at high pressure. The heat released from the hydrogasifier product gas, and the heat generated in the methanation reactors, are used to generate superheated steam (40 bar and 540°C), which enters a steam turbine. A fraction of partly expanded steam is used to dry the biomass, while the remaining part of the steam is used for power generation. 

M. I. Ravich and V. Y. Borovaya, Phase Equilibria in the Sodium-Sulphate Water System at High Pressures and Temperatures, Russ. J. of Inorg. Chem., 9(4), 520-532 (1964). 

Kordikowski A, Siddiqi M, Palakodaty S. Phase equilibria for the CO2 + methanol + sulfathiazole system at high pressure. Fluid Phase Equilibria 2001 4829 1-13. 

H. J. (1996) Synthesis of cubic diamond in the graphite-magnesium carbonate and graphite - K2Mg(C03)2 systems at high pressure of 9-10 GPa region. J. Mater. Res. 11, 2622-2632. 

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