Hydration equilibria

If the composition of the stream is known, the hydrate temperature can be predicted using vapor-solid (hydrate) equilibrium constants. The basic equation for this prediction is ... 

An understanding of covalent hydration is essential for all who work with heteroaromatic compounds containing doubly bonded nitrogen atoms. As chemists become more aware of the circumstances in which hydration occurs, and the means for detecting it, many new examples will probably be discovered and many puzzling discrepancies solved. Many of the values for ionization constants and ultraviolet spectra which are in the literature refer to partly hydrated equilibrium mixtures and should be replaced by values for the pure substances. 

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

T. Y. Makogon, A. P. Mehta, and E. D. Sloan, Jr. Structure H and structure I hydrate equilibrium data for 2,2-dimethylbutane with methane and xenon. J Chem Eng Data, 41(2) 315-318, March-April 1996. 

Our inhibitor design strategy was based on the premise that structural modifications in the base of purine riboside that enhance purine base hydration without impairing the binding of the hydrated species to the ADA binding site would result in purine riboside (PR) analogues with high ADA inhibitory potency. Since the apparent inhibition constant (Kj (app)) is related to the hydration equilibrium constant (Keq) and the inhibitory constant for the hydrated molecule (Kj ) by... 

Mooijer van Heuvel, M.M. Peters. C.J. de Swaan Arons, J. (2000). Influence of water-insoluble organic components on the gas hydrate equilibrium conditions of methane. Fluid Phase Equilibria, 172, 73-91. 

Additional compounds corresponding to anthocyanin dimers in which one of the anthocyanins is in the flavylium form and the other in the hydrated form were detected in the solutions incubated at pH 3.8. Such products arise from nucleophilic addition of the hemi-ketal onto the flavylium, confirming that, at this pH value, anthocyanins exist and react under both forms, as expected from their hydration equilibrium. 

The metastability of the system prevents hydrate forming immediately at Point D (at the hydrate equilibrium temperature and pressure Figure 3.1b). Instead the system pressure continues to decrease linearly with temperature for a number of hours, without hydrate formation occurring (A to B is the induction period, cf. 1 in Figure 3.1a). At Point B, hydrates begin to form. The pressure drops rapidly to Point C (about 1.01 MPa or 10 atm in 0.5 h). B to C is the catastrophic growth period (cf. 2 in Figure 3.1a). 

Hydrate dissociation begins when the cell is heated from Point C in Figure 3.1b, so that the system pressure increases, at first slowly and then sharply along the steep dissociation line (between Points C and D). Finally at Point D, the hydrates are completely dissociated, as confirmed visually through the sight glass. The hydrate equilibrium condition (or hydrate dissociation temperature and pressure) is given by Point D (Section 3.3). 

Figure 3.35 (See color insert following page 390.) X-ray CT imaging shows radial dissociation of a hydrate core. Image number 1 -8 (top number on each image) recorded over 0-245 min (bottom number on each image). The cell pressure was decreased from 4.65 to 3.0 MPa over 248 min. The hydrate core temperature decreased from 277 to 274 K with time, following the three-phase methane hydrate equilibrium line. (From Gupta, A., Methane Hydrate Dissociation Measurements andModeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis Colorado School of Mines, Golden, CO (2007). With permission.)...

Note that the last paragraph contains two-phase regions (H-V, H-Lhc, and I-H) for hydrate equilibrium with a phase that is not liquid water. There is a common misconception that hydrates cannot form without a liquid water phase, a condition clearly possible in these diagrams. Professor Kobayashi s laboratory measured hydrate conditions without a free water phase from vapor or liquid systems from 1973 to 2000. Such equilibria are of interest for gas and gas condensate pipelines without a free water phase. 

Therefore Q2 places an upper temperature limit on sH, while allowing for si hydrate equilibrium at higher temperatures. 

For condensed three-phase (Lw-H-Lhc) hydrate equilibrium, at pressures above the upper quadruple point, the pressure changes extremely rapidly with only a small change in temperature. This is because all three phases are relatively... 

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