Abstract

Nucleation and growth of gas hydrate crystals have been Investigated using Photon Correlation Spectroscopy (PCS). Hydrate experiments were performed using distilled and filtered water together with a gas mixture which forms hydrates of structure II (94 mol% methane and 6 mol% propane). Several different pressures and temperatures were investigated and the particle sizes measured during gas hydrate nucleation ranged from 2 nm to 80 nm, approximately, depending on the magnitude of the driving force.

Experiments were performed on both static and moderately stirred systems. The static experiments were difficult to reproduce both with respect to induction time and required driving forces. However, in the stirred experiments the reproducibility was much improved, especially at low temperatures.

Introduction

This paper presents results of laser light scattering studies of natural gas hydrates, initiated in 1989. The ultimate aim of the work was to measure the critical size of gas hydrate nuclei and the size of gas hydrate particles during the early phases of crystal growth. At the time when this work was started, laser light scattering had never before been used for studies of gas hydrates. More recently however, the technique has also been used for screening of kinetic gas hydrate inhibitors, but only as a qualitative tool to detect the onset and growth of hydrate formation.

The nuclei critical size is defined as the size the embryos (clusters) have to reach before macroscopic crystal growth may commence. There are no (other) experimental data of the nuclei critical size available in the literature, and to the authors knowledge only two publications are available, giving a theoretical estimate of the diameter range for critical nuclei. These are 6-34 nm for methane and ethane hydrates, and 140 nm for methane hydrate, at different driving forces.

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