Macroscopic defects upon decomposition of CO2 clathrate hydrate crystals.

Stefan Arzbacher,Bernhard Massani,Jörg Petrasch,Alexander Ostermann,Thomas Loerting,Nima Rahmatian

doi:10.1039/c8cp07871h

Abstract

Micrometer- and submicrometer-sized pores and macroscopic defects like cracks and tubular channels can be found in a variety of clathrate hydrates (hydrates for short) during formation and decomposition. Their origin, their evolution in time, and their effect on hydrate decomposition kinetics are unclear. We used time-lapse micro computed tomography (μCT) in combination with temperature control and pressure monitoring to study the formation and evolution of pores and macroscopic defects in decomposing CO2 hydrates at subzero (Celsius) temperature. Our results suggest that the decomposition of hydrates is always accompanied by the formation of pores and an increase of the apparent hydrate volume by more than 3%. Hydrate decomposition often starts with the formation of cracks inside the hydrate and not necessarily at the free surface of the hydrate, which frequently remains intact for a long period and seems to be self-preserved in some regions. Decomposition spreads out from these cracks in a uniform fashion yielding a variety of macroscopic features. In some cases, the propagating decomposition front seems to get blocked by planar barriers inside the hydrate yielding regions with high resistance against decomposition. This, together with a generally heterogeneous distribution of decomposition resistant regions, challenges the shrinking core model of hydrate decomposition as well as the popular ice-rind theory used to explain the effect of self-preservation.

Highlights

Hydrates are generally non-stoichiometric compounds, meaning that not all polyhedra have to be occupied by a guest molecule.in the ideal stoichiometric case all cavities of a CO2 hydrate are occupied by exactly one guest molecule
At standard temperature and pressure (STP) conditions 174 volumes of gaseous CO2 are stored in one volume of CO2 hydrate
Establishing a predictive model for the decomposition of clathrate hydrates is a worthwhile but difficult task. It is worthwhile because such a model will eventually help with the estimation of the applicability of hydrate technologies. It is difficult because such a model has to combine thermodynamics, decomposition kinetics, heat and mass transport mechanisms as well as micro- and macroscopic features of the bulk and surface

Summary

Introduction

Hydrates are generally non-stoichiometric compounds, meaning that not all polyhedra have to be occupied by a guest molecule. In the ideal stoichiometric case all cavities of a CO2 hydrate are occupied by exactly one guest molecule. That gives a ratio of 8 guest molecules per 46 water molecules or CO2Á5.75H2O. Combined with a theoretical stoichiometric density[1] of 1.13 g cmÀ3 this yields 7.7 kilomoles of CO2 per cubic meter of hydrate. At standard temperature and pressure (STP) conditions 174 volumes of gaseous CO2 are stored in one volume of CO2 hydrate. Dry ice accumulates approximately 796 volumes of gas (STP) per volume of solid

Methods

Results

Conclusion