Formation 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. 

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Conversely, if the reaction is carried out in a condensed medium such as water, despite the latter causing multiple equilibria with formation of CO2 hydrated forms (4.4) in addition to the forms of dissociated formic acid (4.5), or if a base (ammonia or other bases) is used that produces formate salts instead of free formic acid, the thermodynamics of the process is favored (AG° = —4 kJ mol for (4.6a) and (4.6b) and AG° = —35 kJ mol for (4.7), respectively) [16-21]. 

Consider a CO2 droplet of radius 3 mm injected at 600 m seawater depth with temperature of 5.2°C (Zhang, 2005b). Under these conditions, density and viscosity of seawater are 1026 kg/m and 0.00161 Pa s, and density of liquid CO2 is 916kg/m, or 20.82 mol/L. Because of the formation of hydrate shell, the solubility of CO2 in seawater should be that of CO2 hydrate, which is 1.00 mol/L (CO2 liquid solubility is significantly greater), or Wq = 0.0429. Because solubility of CO2 is small, density of the interface water is similar to the bulk seawater. Hence, the... 

Uchida, T., Ebinuma, T., Kawabata, J., Narita, H., J. Cryst. Growth, 204, 348 (1999a). Uchida, T., Ebinuma, T., Mae, S., Formation Rate Measurements of CO2 Hydrate Film Formed at Liquid CO2 Water Interface, Greenhouse Gas Control Technologies, (Riemer, P., Eliasson, B., Wokaun, A., eds.) Elsevier, 1073 (1999b). 

The direct injection of concentrated CO2 (probably in liquid form) below the thermocline or on the seafloor might sequester carbon for hundreds of years (Herzog et al., 2000). The gas might be dissolved within the water column or held in solid, ice-like CO2 hydrates. The possibility is receiving attention in several national and international experiments (DOE, 1999). Large uncertainties exist in understanding the formation and stability of CO2 hydrates, the effect of the concentrated CO2 on ocean ecosystems, and the permanence of the sequestration. 

Table 2 Supercooling (ATsc, C) ofpore water for the formation of CO2 and CH4 gas hydrates in model samples ...

The analysis of hydrate formation under cooling to sub-zero temperatures shows that there is an activation of the hydrate formation process during the freezing of the remaining pore water. Some 10-25 % of the total amount of CO2 hydrate is formed during the water freezing process. This portion increases in samples with clay particles, in particular for kaolinite. 

For all clathrates formation experiments discussed in this paper, we used ice spheres with a diameter of <350pm formed by quenching sprayed, demineralised water droplets in liq.Ni. The decomposition runs were carried out on powders 250pm, prepared by crushing and sieving of CO2 hydrate through a set of 200 and 300 pm meshes. 

G. Genov, Ph.D. Thesis, Physical processes of CO2 hydrate formation and decomposition at conditions relevant to Mars, 2005, University of Gottingen. 

EFFECTS OF ADDITIVES ON FORMATION RATES OF CO2 HYDRATE FILMS... 

Figure 2 shows the temperature change of typical experimental run with various concentrations of AFP solution. Arrows in this figure indicate the formation times of CO2 hydrate film on the solution droplet. The origin of the time axis (t=0) was defined as the time when the temperature reached Tg at the experimental pressure. Because the concentration of AFP was very small, the value of Tg for this experiment was taken as the same as that for pure CO2 hydrate. From Figure 2, we estimated both rand AT. 

The formation process of C02-hydrate film on the trehalose-solution droplet was almost the same as shown in Figure 1 CO2 hydrate nucleated somewhere on the droplet, and thethin film of CO2 hydrate laterally grew at the interface between CO2 and solution. At higher trehalose-concentration, the nucleation was observed to occur near the top of the droplet for several times as observed on the NaCl solutions. " ... 

Figure 4 Temperature profiles of C02-trehalose solution system at various trehalose concentrations during induction period. Each arrow indicates the formation point of CO2 hydrate film on the droplet.

Table 2 summarizes the results of CO2 hydrate film formation obtained in trehalose-solution experiments. For both r and AT columns, the upper value is that obtained in the first run, while the lower number in brackets is the average for all subsequent runs with its variation. For trehalose-50 wt% samples, we showed the values in brackets are standard deviation> for 8 times repeating experiments. The equilibrium temperature shift of CO2 hydrate in each trehalose solution ATe = Te- tJ where Tf is the dissociation temperature of CO2 hydrate at the same pressure in pure water system is also roughly... 

Another possible mechanism of trehalose molecules as a kinetic inhibitor is mentioned below. In the growth process of CO2 hydrate, trehalose may work as the kinetic inhibitor that prevents the rate-determining process of the crystal formation at the reaction site which might have small dependence on AT. Trehalose has been observed to prevent ice-crystal growth with the reduction of the free-water providing because trehalose strongly hydrated in the solution. This effect is found apparently when the trehalose concentration increases beyond the intrinsic hydration number of trehalose molecules. It is thus reasonable that the kinetic effect of trehalose on the inhibition of the hydrate formation would be resulted from the smaller supplement of free water from the solution of higher trehalose concentrations. 

Clarke, M.A. Bishnoi, P.R. Determination of the intrinsic rate of CO2 hydrate formation using in situ particle size analysis. Chem. Eng. Sci. 2004, 60, 695-709. 

Kinetics. - In the course of developing a general kinetic model of hydrate formation/reaction that can be used to establish/optimize technologies for the exploitation of hydrate reservoirs, two aspects of CO2 hydrate formation have been studied. First, a phase field theory of hydrate nucleation was developed for describing the nucleation of CO2 hydrate in aqueous solutions. It has been shown that the phase field theory is eonsiderably more accurate than the sharp-interfaee droplet model of the classieal nueleation theory. The phase field theory prediets considerably smaller height for the nucleation barrier than the elassieal approach. MR imaging was used to monitor hydrate phase transitions in porous media under realistic conditions, and the transformation rates for the relevant processes (hydrate formation, dissociation and recovery) was presented. 

The heating of aqueous solutions of the rare earth trichloroacetates in acidic medium leads to the formation of carbonate hydrates as described above. In neutral medium the product is hydroxide-containing carbonate, ROHCO3 wHjO (Sklyarenko and Ruzaikina, 1970). The preparation of rare earth carbonates with CO2 does not give hydroxide carbonate, but when alkaU carbonates are used the situation is different. When CO R + is 1.5 with ytterbium, YbOHCOj is formed for europium the ratio must be 2 and for neodymium > 4. 

This chapter provides a comprehensive overview of the fundamentals and applications of CO2 hydrates. Section 10.2 focuses on the microscopic perspective, looking into how gas hydrates form, the three structures of gas hydrates, and the characteristics of CO2 hydrates. From there onwards, the text focuses specifically on CO2 hydrates. The physical properties of CO2 hydrates are considered in Sect. 10.3. Section 10.4 deals with the phase equihbrium of CO2 hydrate. Experimental methods and the phase diagram are showed in this section. The last section covers the applications of CO2 hydrates, including the formation and dissociation of CO2 hydrates, ocean sequestration, the CH4 replacement in hydrates by CO2, and the use of CO2 hydrates in the refrigeration process. 

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... 

In contrast to hydrate formation, hydrate dissociation is an endothermic process in which energy is consumed to break the hydrogen bonds between water molecules and the van der Waals interaction between the guest and water molecules, with the production of water and gas. At around 273 K, the measured dissociation heat of CO2 hydrate varies between 57.66 and 65.22 kJ/mol, with the hydration number of 6.21-7.23 [45 7]. For hydrate with single guest, the amount of heat absorbed by dissociation is equal to that released by formation. As an endothermic process, the hydrate dissociation is t3q)ically dominated by heat transfer but not intrinsic kinetics. 

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