Hydrates, decomposition

Pyrimido[5,4-e][l,2,4]triazinediones in aqueous acidic media were hydrated at the N4-C4a bond to give covalent hydrates the structures of which were confirmed by X-ray analysis. The products of hydrate decomposition in acidic media were formic acid, 5-diazo-3-methylbarbituric acid, and methylparabanic acid (88KGS1654). 

The combination of C02 injection and methane production over specific PT regimes allows the heat effects of C02 hydrate formation and methane hydrate decomposition to nullify each other resulting in a sustainable delivery process which both reduces C02 emissions to combat global warming and recovers methane to supplement the declining reserves of conventional natural gas (Fig. 4). This gas hydrate phase-behaviour in response to the dissociation and formation processes clearly demonstrates the potential of C02 enhanced CH4 recovery from the Mallik gas hydrate deposit. 

Table 2. Structural, stability properties and heat of hydrate decomposition at 0°C of most technologically relevant hydrate formers...

It is also of practical interests to control hydrate decomposition. The early studies were intended to understand the mechanism of hydrate decomposition when plugging in pipeline was encountered. The hydrate crystals generally decompose by de-pressurization (Kelkar et al., 1998, Peters et al., 2000 Hong et al., 2006). Thermal stimulation has also been considered in order to provide strategies for methane recovery from the natural hydrate by thermal stimulation (Selim and Sloan, 1989 Ji et al., 2001 Hong et al., 2003 Hong and Pooladi-Darvish, 2005). Hydrate decomposition studies may also be applicable to gas storage in hydrates. 

Koh, C.A. (2005). Search for memory effects in methane hydrate Structure of water before hydrate formation and after hydrate decomposition. J. Chem. Phys. 123 (16), Art. No. 164507. 

Kvenvolden and Claypool estimate that hydrate decomposition does not contribute to... 

Kim, H.C., A Kinetic Study of Methane Hydrate Decomposition, Ph.D. Thesis, University of Calgary, Alberta (1985). 

Core temperatures upon recovery on the catwalk were variable. Small areas of low temperatures (6-8°C versus other parts of the core at 11-13°C) were interpreted as indicating areas where endothermic hydrate decomposition decreased the core temperature. Cores evolved large amounts of gas, which was considered responsible for low core recovery—from a norm of > 80% to 20-60% in the hydrate region. 

This work presents experimental data on the CO2 hydrate formation in gas saturated wet samples under cooling conditions as well as on the hydrate decomposition kinetics in frozen C02-hydrate saturated samples. 

Temperature acts as the most important factor influencing the process of self-preservation of gas hydrates in pore space. The study of sand sample with 7% (Wjn=10%) carried out at different temperature conditions shows that the time of CO2 pore hydrate decomposition in frozen samples varies from 5 hours at -2 C to 60 hours at -13 C. According to our observations, the pore hydrate dissociation process at -20 C stopped in one hour with no further dissociation in the following 40 hours. At the end of the experiment (about 100 hours) the CO2 hydrate content was about 7% in volume. 

The frozen hydrate-saturated media formed during these experiments were used for a study of the CO2 hydrate decomposition kinetics in the pore space. The influence of soil mineral composition, ice content and temperature on the CO2 hydrate self-preservation effect was established. It is revealed, that a temperature decrease slows down the CO2 hydrate dissociation low negative temperatures (below -13 C) cause a complete stop of the CO2 hydrate dissociation process. It is also shown that ice forming in the remaining pore space from freezing of unreacted water enhances the CO2 hydrate self-preservation effect. 

The decomposition temperatures of hydrates were measured by means of differential thermal analysis (DTA) under the conditions of excess gas in a stainless steel flask that was developed specially for the investigation of hydrate formation with a gaseous guest at high hydrostatic pressure. The hydrate decomposition temperature was measured with a chromel-alumel thermocouple to the accuracy of 0.3 K. The thermocouple was calibrated with the use of temperature standards. Pressure was measured with a Bourdon-tube pressure gauge. The error of the pressure measurements did not exceed 0.5 %. This procedure was described in more detail previously.The gases used in the investigation... 

Figure 4 Hydrate decomposition rate at 253K and 1 atm for NH and MCH system (left) followed by reformation at 253K and 4.5MPa for the NH system (right)...

Hydrate crystal decomposition, like the hydrate growth, is a deterministic process. Hence, the process is amenable to study experimentally and modeling. Although some studies have been undertaken at the molecular level, most of them on the hydrate decomposition kinetics are based on a macroscopic approach. The hydrate decomposition is a heterogeneous process where liquid water and gas are released as the solid... 

The intrinsic rate constant, ]Q, and activation energies, AE, for CH4, C2H6, and CO2 hydrate decomposition are given in Table 4. For the decomposition of hydrates formed from mixtures of methane and ethane, Clarke and Bishnoi showed that the total rate of decomposition is equal to the sum of the decomposition rates of the individual species. 

Table 4 Intrinsic rate constants, and activation energies, A , for methane, ethane, and CO2 hydrate decomposition...

K Intrinsic rate constant for hydrate formation Ka Intrinsic rate constant for hydrate decomposition n Moles... 

Ershov, E.D. Yakushev, V.S. Experimental research on gas hydrate decomposition in frozen rocks. Cold Regions Sci. Technol. 1992,20,147-156. 

Ahmadi, G. Chuang, J. Smith, D. A simple model for natural gas production from hydrate decomposition. Proceedings of the Third International Conference on Gas Hydrates, Salt Lake City, Utah, July 21-24 Holder, G., Bishnoi, P.R., Eds. Annals of the New York Academy of Science, 1999 42(M127. 

Clarke, M.A. Bishnoi, P.R. Determination of the activation energy and intrinsic rate constant of methane gas hydrate decomposition. Can. J. Chem. Eng. 2001, 79, 143-147. 

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