Crystal Growth of Structure-H Hydrate with Water-Soluble Large Molecule Guest Compound: 1-Methylpiperidine as a Case Study

This paper presents visual observations of formation and growth of structure-H hydrate formed with methane and 1-methylpiperidine (MPD). MPD is one of the water-miscible guest compounds which form structure-H hydrates with a help gas. This guest promotes hydrate formation of the help gas and has an advantage of omitting the dissolution process in water during potential industrial utilization. The observed crystal growth dynamics were categorized into two types depending on the subcooling temperature ΔTsub, the difference between the phase equilibrium and experimental temperatures. At ΔTsub = 2.8 K, the gas/liquid interface was covered by an agglomeration of hydrate grains. No remarkable hydrate growth was recognized after coverage of the gas/liquid interface. At ΔTsub ≥ 4.9 K, hydrate crystals continued forming and growing even after the gas/liquid interface was covered by hydrates, which increased the amount of formed hydrate crystals compared to lower ΔTsub. The crystal shape changed from polygonal or platelike into pyramidal, and the size of individual crystals increased with the rise of ΔTsub, opposite the common trend where crystals become smaller at higher supercooling. Implications of the results of crystal morphology on suitable conditions for industrial utilization of this hydrate for gas capture applications are discussed.