Apparatus for use below the ice point

Hydrate experimental conditions have been defined in large part by the needs of the natural gas transportation industry, which in turn determined that experiments be done above the ice point. Below 273.15 K there is the danger of ice as a second solid phase (in addition to hydrate) to cause fouling of transmission or processing equipment. However, since the development of the statistical theory, there has been a need to fit the hydrate formation conditions of pure components below the ice point with the objective of predicting mixtures, as suggested in Chapter 5. 

Because most of the upper quadruple points of the hydrocarbon hydrate formers limit the temperature range of simple hydrate formers to a few degrees above the ice point, the region below 273 K was measured to provide more extensive data. However, as recorded in Section 6.3, substantially fewer data below 273 K exist than at higher temperatures. An increasing need for hydrate phase equilibria data below the ice point is evolving as oil/gas exploration move to Arctic conditions, where typically temperatures can be well below 273 K. 

Deaton and Frost (1946) suggested the same apparatus could be used for conditions below the ice point. In these experiments, gas was first bubbled through water above 273 K, to form a honeycomb mass of hydrate. Then free water was drained before the cell was cooled below the ice point. After the temperature was stabilized, gas was removed in small increments until a region of constant pressure was obtained, which indicated dissociation of the hydrate phase. Deaton and Frost used this procedure only for equilibria of simple hydrates, since the hydrated mass of guest mixtures was not constrained to be of uniform composition, and consequently would have decomposed at different pressures. 

The experiments of Hwang et al. (1990) indicated that hydrates from ice are readily formed when the sample temperature is raised just above the ice point. Stern et al. (1996) successfully converted ice to hydrates. They raised the temperature of 200-500 p,m ice grains to 289 K and 31 MPa within the Lw-H-V region (beyond the almost vertical I-H-V line). Ice melted and converted all but 3% of the sample into hydrate within 8 h, as determined by x-ray diffraction. The method of Stern et al. (1996) has been widely adopted by hydrate researchers to achieve complete/near complete conversion of ice to hydrate. 

At temperatures below the ice point more time is required to equilibrate the two solid phases, ice and hydrate. Byk and Fomina (1968) suggested that water molecule rearrangement is very difficult between the ice nonplanar hexagonal 

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