Phase equilibria in the system CO2-H2OI: New equilibrium relations at low temperatures

摘要

Graphical analysis of free-energy relationships involving binary quadruple points and their associated univariant equilibria in the system CO2-H2O suggests the presence of at least 2 previously unrecognized quadruple points and a degenerate invariant point involving an azeotrope between CO2 rich gas and liquid. Thermodynamic data extracted from the equilibrium involving Clathrate (hydrate), gas, and ice (H = G+I) are employed along with published data to calculate the P-T range of the 3-ice equilibrium curve, S+I = H, where S is solid CO2 This equilibrium curve intersects the H = G+I curve approximately where the latter curve intersects the S+H = G curve. thus confirming the existence of one of the inferred quadruple points involving the phases S. G. H. and I. Recognition of some binary equilibria probably have been hampered by extremely low mutual solubilities of CO2 and H2O in the fluids phases which, for example, render the S+H = G virtually indistinguishable from the CO2-sublimation curve.To make the published portion of the L(liquid CO2)-G-H equilibrium "connect" with the other new quadruple point involving S, L. G, and H, it is necessary to change the sense of the equilibrium from L G+H at higher pressures to L+H = G at lower pressures by positing a L = G azeotrope at very low concentrations of H2O. At the low-pressure origin of the azeotrope, which is only a few bars above the CO2-triple point, the azeotrope curve intersects the 3-phase curve tangentially, creating a degenerate invariant point at which the 3-phase equilibrium changes from L+H = G at lower pressures to L = G+H at higher pressures. The azeotrope curve is offset at slightly lower temperature from the L = G+H curve until the 3-phase equilibrium terminates at the quadruple point involving G. L. H, and W (water). With further increase in pressure the azeotrope curve tracks the L = G+W equilibrium and apparently terminates at a critical end point in close proximity to critical endpoints for the CO2 saturation curve and the L = G+W curve.Thermodynamic data for clathrate extracted from the slope of the H = G+I curve are consistent with a solid-state phase transformation in CO2-clathrate between 235 and 255 K. Published work shows that the type-I clathrate phase, whose atomic structure is a framework of water molecules with CO2 molecules situated in large "guest" sites within the framework, is variable in composition with similar to1 guest site vacancy per unit cell at the high-temperature limit of its stability; the number of water molecules. however. remains constant. The formula (CO2)(8-y)(.)46H(2)O, where y is the number of vacancies per unit cell, is in keeping with the atomic structure, whereas the traditional formula, CO(2)(.)nH(2)O, where n (hydration number) = 5.75, is misleading.Ambient P-T conditions in the Antarctic and Greenland ice sheets are compatible with sequestering large amounts of carbon as liquid CO2 and/or clathrate. Copyright (C) 2005 Elsevier Ltd.