Hydrate Formation from CO2 and Sea Water

Abstract Carbon dioxide (CO2), which is one of the most abundant compounds on Earth, is present in rock, the oceans and the atmosphere. CO2 influences many biological and dynamic processes. Carbon dioxide is very soluble in water due to its high dipole moment. Solubility of a gas in water is dependent upon the temperature and pressure. This dependence affects the changing content of gases in the atmosphere, and the variable effect that certain gases have on the climate. Currently, many believe that the effect of CO2 on global warming exceeds 50% of the combined effect of all greenhouse gases. CO2 is present as a gas, or can be dissolved in water, or can be found in a solid state. CO2 can also exist in a solid state as a clathrate hydrate. Gas hydrates are known as metastable minerals, whose composition and properties are determined by the source components, temperature and pressure. Knowledge of the conditions of formation, stability and decomposition of CO2 hydrate with seawater and the mass ratios of gas and water during phase transition appears quite important. In this paper, we present the results of laboratory work concerning the kinetics and morphology of a CO2 -seawater system at pressures up to 50 MPa and temperatures of 268–295 K. Measurements of CO2 solubility in supercooled seawater before and after hydrate formation are discussed. The conditions of CO2 hydrate formation and stability in seawater that is under-saturated with gas are described. The research results reveal a number of details concerning hydrate formation and dissociation, including the limits of dissolved gas in water for stable conditions of gas hydrate crystals in the water volume. Also, the information can be used to interpret the role of gas hydrates in the global changes on our planet. The results of this work permit to determine the most effective technologies to control the content of CO2 in the atmosphere. We cannot include all of our results in this paper, due to the volumes of research data generated on this subject. However, many of our most important findings have been included and discussed. General Information About The CO2 And Seawater Used In This Work The following gas and liquids were used during the experiments. Gas: Carbon dioxide (99.9% CO2 + 0.04% CH4+ 0.06% N2) Liquids: Salt (sea) water - standard laboratory mixture containing 41.953 g/L sea-salt composition (meets American Materials Standard D-1141-52 Formula A) mixed with distilled water. Table 1 presents the composition of the sea salt used to mix the synthetic seawater. The density of the seawater that we mixed in the laboratory was 1.025 g/cm3 at 15 °C. Of course, the density of seawater will depend upon the temperature, pressure and salinity of the liquid. For atmospheric pressure, the density of seawater as a function of temperature and salinity is illustrated in Figure 1 (improved, from Sea Water: Its Composition, Properties and Behavior).