Pressure-Temperature Diagrams for Structure H Systems

The pressure-temperature diagram for structure H has both similarities and differences from those diagrams shown above. Because the new hydrate was discovered 

Calculations of Methane Solubility in Water and Seawater, at Conditions Above and Below the Hydrate Point 

Source From ffanda, Y.P., J. Phys. Chem., 94, 2652, 1990. With permission. 

The temperatures and pressures for the sH system are very similar to those found in si and sll diagrams. If ahydrate forms in a pipeline with both gas and condensate/oil phases present, examining the pressure-temperature conditions may be insufficient to determine the hydrate crystal structure. 

The lower quintuple point (Qi) is the location of five coexisting phases—namely, I-Lw-sH-V-Lhc- The position of Qi is determined by the intersection of the I-sH-V-Lhc fine and the Lw-sH-V-Lhc line, close to the ice point temperature (273.15 K). However, the pressure of Qi is a function of the hydrate guests. 

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