A Preliminary Study of the Morphology of Gas Hydrates
Authors: Yousif M.H. 
Published: December 2007

This work is an attempt to characterize the morphology of the hydrate particles under mixing conditions. We measured the hydrate particle size, rc in the presence of excess water while mixing and calculated the critical particle size of both structure I and II hydrates. For structure I hydrate, rc is determined to be 165 m and for structure II, it was 119 m. The shapes, sizes, and number of hydrate particles have been characterized at the early stages (10–15 seconds) of the hydrate formation process of structures I and II. The effects of pressure, temperature, degree of supercooling, and mixing rate on these parameters were discussed. The results showed that the geometry of the hydrate particles depends on the shear force caused by the mixing and the thermodynamic driving force. The hydrate particles break off the gas-water interface as two-dimensional flakes that gradually degrade to a spherical shape. The particles remained suspended in the water without bridging.

Introduction

At favorable conditions of pressure and temperature, water in contact with natural gas crystallizes and physically entraps certain molecules of the hydrocarbon stream. Natural gas components like N2, H2S, CO2, C1, C2, C3, and iC4 are well known hydrate formers. Larger molecules like n-butane, cyclopentane, and cyclohexane form hydrate only in the presence of a helper gas such as methane or ethane. Hydrates can form from a hydrocarbon stream that is a single phase vapor or liquid, or from a two phase stream. Three crystal structures have been identified for gas hydrates. These are structure I, structure II, and structure H. The hydrate structure is kept thermodynamically stable through the interaction between the water molecules forming the cavities and the gas molecules occupying them.

For over 160 years hydrates remained a mere scientific curiosity. Their importance to the oil and gas industry was realized in the early 30's when Hammerschmidt discovered that the solid compounds that frequently plugged the gas transmission lines were not ice but hydrates.

In this paper, we attempted to investigate how the hydrate particles form and grow under mixing conditions. The shapes, sizes, and number of hydrate particles have been characterized at the early stages (10–15 seconds) of the hydrate formation process of structures I and II. The effects of pressure, temperature, degree of supercooling, and mixing rate on these parameters were discussed. A total of 14 hydrate formation runs were carried out under isothermal and isobaric conditions. The results showed that the shape and size of the hydrate particles depends on the shear force caused by the mixing and the thermodynamic driving force. The hydrate particles break off the gas-water interface as two-dimensional flakes that gradually degrade to a spherical shape. The particles remained suspended in the water without bridging. The equilibrium critical radii for structure I and II hydrates were also determined in this study.

Other studies mainly focused on determining the critical particle size, rc either experimentally or theoretically. These studies reported critical particle (crystal) sizes of 0.017 to 0.048 m. In another work critical particle sizes of 0.14 to 14.2 m were calculated. In this study, we measured the hydrate particle size in the presence of excess water while mixing, and calculated the critical particle size of both structure I and II hydrates.

Hydrate Formation and Growth Under Static Conditions

It has been observed that under static conditions, gas hydrates form and grow as a film covering the gas-water interface. To shed some light on the mechanisms of hydrate formation and growth under static conditions, the gas-water interface is represented as a region of finite thickness as shown in Fig. 1. Inside the interface region, the mole fraction of gas rapidly drops from approximately 1.0 in the gas to the solubility limit in the bulk water (approximately 0.002 to 0.003), while the mole fraction of gas in the hydrate structure at full occupancy of the cavities is 0.1481.

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