Phase Equilibrium of CO2 Hydrate

Following the thermodynamics (phase equilibrium) of CO2 hydrate, which is time-independent, it is also important to know the time-dependent phenomenon of hydrate, namely, how hydrates form and dissociate. Note that the study of such phenomena is much more challenging than that of the thermodynamic properties. 

Knowledge of CO2 hydrate formation conditions is very important for rational and design of processes in CO2 capture and sequestration. Several studies have focused on determination of incipient CO2 hydrate formation conditions. The condition at which an infinitesimal amount of hydrate phase is present in equilibrium with liquid phases is referred to as incipient hydrate formation condition [28]. Using different experimental methods, macroscopic hydrate phase equilibrium can be determined... 

Equilibrium properties of the C02/sea-water system have been well researched from an experimental standpoint. In particular, the clathrate hydrate forming conditions T < 285K andP>4MPa) are well established. Several experiments have been performed under conditions mimicking the direct injection process and have attempted to study the dissolution rate of CO2 in seawater. Under direct injection conditions, the injected CO2 is in the form of a liquid droplet and a thin spherical shell of CO2 clathrate hydrate of structure I is observed to form around the CO2 drop, separating it from the sea water. The process of hydrate formation has many similarities with that of crystallization, i.e., it can be divided into a nucleation phase and a growth phase. For CO2 clathrates, the nucleation phase involves the formation of a... 

The analysis of thermo-baric changes in the wet soil samples saturated with CO2 as a function of time under condition of cyclic cooling and heating permits to follow the kinetic and thermo-baric indicators of phase transitions within the pore space of the samples. On cooling of wet gas-saturated soils under gas pressures higher than the three-phase equilibrium line gas - water - CO2 hydrate , conditions for gas hydrates nucleation in pore space of soils are created. Pressure stabilization marks the end of the phase transition of water into hydrate. Upon further cooling below 0°C the remaining, untransformed liquid turns into ice. 

Figure 3 Isothermal phase equilibrium (pressure-composition) relations for the H2 + CO2 + THE + water mixed system in the presence of hydrate phase at 280.1 K at the THE composition of0.030, 0.056 and 0.080 T no additive, A THF 0.030, Ot xthf 0.056, Alt xthf 0.080.

In these equations the asterisk denotes that the concentration of H2CO3 includes dissolved and hydrated CO2 (i.e., CO2 H2O) in addition to true carbonic acid. Pco2 is the partial gas pressure of CO2 in the gas phase in equilibrium with the solution. 

Typical phosphate and borate buffers are listed in Chapter 10, while Table 3.4 gives the concentrations of NaHC03 at 37° C in equiUbrium at various pH values with a gas phase containing 5% CO2. The calculations are based on the Henderson-Hasselbalch equation. For temperatures between 20°C and 37°C the bicarbonate concentrations should be increased by 1.88% per degree. Because the reversible hydration of CO2 is a rather slow process, it may be advantageous to include a trace of the enzyme carbonic anhydrase in systems where bicarbonate/C02 buffers are used, so as to maintain more nearly equilibrium conditions. 

Equilibrium hydrate formation condition for CO2 has been extensively investigated and available in the literature [32-44], A hydrate phase diagram of the CO2 + H2O system is presented in Fig. 10.5. As can be seen in the figure, equilibrium hydrate formation conditions above and below fi-eezing point of water is presented. 

The latter elass of compounds elicited a great interest because of their ability to entrap gas molecules, of which carbon dioxide is a remarkable example. Parieaud and co-workers reported an interesting insight on the dissociation conditions of CO2 from several ammonium salts and tetra-n-butyl phosphonium bromide ([P4444]Br) semiclathrate hydrates. Ye et al deseribed a comparison between phase equilibrium and morphology eharacteristics of hydrates formed by and [P444JCI... 

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