Natural Gas Hydrates

A Guide for Engineers

John Carroll

• Publisher: Gulf Professional Publishing
• Year: 2014
• eBook ISBN: 9780128005750
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Rarely covered in formal engineering courses, natural gas hydrates are a common problem and real-life danger for engineers worldwide. Updated and more practical than ever, Natural Gas Hydrates, Third Edition helps managers and engineers get up to speed on all the most common hydrate types, how to forecast when they will appear, and safely mitigate their removal. Known for being highly flammable, gas hydrates are a preventable threat that can costs millions of dollars in damage, as well as take the lives of workers and engineers on the rig. The third edition of Natural Gas Hydrates is enhanced with today's more complex yet practical utilization needs including: - New hydrate types and formers, including mercaptans and other sulfur compounds- Vital information on how to handle hydrate formation in the wellbore, useful information in light of the Macondo explosion and resulting oil spill- More detailed phase diagrams, such as ternary systems, as well as more relevant multicomponent mixtures- Quantifiably measure the conditions that make hydrates possible and mitigate the right equipment correctly- Predict and examine the conditions at which hydrates form with simple and complex calculation exercises- Gain knowledge and review lessons learned from new real-world case studies and examples, covering capital costs, dehydration, and new computer methods

Chapter 1

Introduction

Abstract

This chapter provides an introduction to gas hydrates—why are they important, some history, and a brief overview of what they are. To begin, some of the unusual properties of water are presented: high boiling point, high enthalpy of vaporization, unusual density, etc. This leads to the structure of the water molecule and a brief discussion of hydrogen bonding. It is because water forms such strong hydrogen bonds that gas hydrates are possible. Also given in this chapter are the criteria for hydrate formation: water, a hydrate former, and the right combination of temperature and pressure. Although water is essential for hydrate formation, free water (i.e., water in the liquid phase) is not. Some secondary conditions that enhance hydrate formation are also discussed.

Keywords

Hydrates; Hydrogen bond; Water; Water molecule structure

This chapter is an attempt to introduce hydrates, without much background material. Many of the words and principles will be better defined in subsequent chapters of this book. However, they are needed here to present the basic introductory concepts. If you are a little confused as you read this chapter, hopefully things will become clearer as you progress through the book.

In its most general sense, a hydrate is a compound containing water. For example, there is a class of inorganic compounds called “solid hydrates.” These are ionic solids where the ions are surrounded by water molecules and form crystalline solids. However, as used in this book, and commonly in the natural gas industry, hydrates are solid phase composed of a combination of certain small molecules and water.

So hydrates are crystalline solid compounds formed from water and small molecules—without water there are no hydrates and without the small molecules that stabilize the structure there are no hydrates. They are a subset of compounds known as clathrates or inclusion compounds. A clathrate compound is one where a molecule of one substance is enclosed in a structure built up from molecules of another substance. Here, water builds up the structure and the other molecule resides within. The size of the other molecule must be such that it can fit within the water structures. More details of the nature of these structures formed by water and the molecules within are presented in Chapter 2 of this book.

Even though the clathrates of water, the so-called hydrates, are the focus of this work, they are not the only clathrate compounds. For example, urea forms interesting inclusion compounds as well.

Although hydrates were probably encountered by others earlier, credit for their discovery is usually given to the famous English chemist, Sir Humphrey Davy. He reported of the hydrate of chlorine in the early nineteenth century. In particular, he noted (1) that the ice-like solid formed at temperatures greater than the freezing point of water, and (2) that the solid was composed of more than just water. When melted, the hydrate of chlorine released chlorine gas.

Davy's equally famous assistant, Michael Faraday, also studied the hydrate of chlorine. In 1823, Faraday reported the composition of the chlorine hydrate. Although his result was inaccurate, it was the first time that the composition of a hydrate was measured.

Throughout the nineteenth century, hydrates remained basically an intellectual curiosity. Early efforts focused on finding which compounds formed hydrates and under what temperatures and pressures they would form. Many of the important hydrate formers were discovered during this era.

Amongst the nineteenth century hydrate researches who deserve mention are the French chemists Villard and de Forcrand. They measured the hydrate conditions for a wide range of substances, including hydrogen sulfide.

However, it would not be until the twentieth century that the industrial importance of gas hydrates would be established.

Over the years there have been many, many experimental studies of hydrate formation. These include the hydrates for single components, binary mixtures, as well as multicomponent mixtures. Some of these studies are discussed in the chapters that follow. If the reader has doubts about methods used in the work, they should consult the literature. They may not find the exact data for their situation, but they may find data that are useful for testing the models they chose to employ.

1.1. Natural Gas

Even though all terrestrial gases (air, volcanic emissions, swamp gas, etc.) are natural, the term “natural gas” is customarily reserved for the mineral gases found in subsurface rock reservoirs. These gases are often associated with crude oil. Natural gas is a mixture of hydrocarbons (such as methane, ethane, propane, etc.) and a few nonhydrocarbons (hydrogen sulfide, carbon dioxide, nitrogen, etc., and water).

The light hydrocarbons in natural gas have value as fuels and as feedstocks for petrochemical plants. As a fuel, they are used for heating and cooking in private homes, to generate electricity, and increasingly as fuel for motor vehicles. In the chemical plants, they are converted a host of consumer products; everything from industrial chemicals, such as methanol, to plastics, such as polyethylene.

The nonhydrocarbons tend to be less valuable. However, depending upon the market situation, hydrogen sulfide has some value as a precursor to sulfur. Sulfur in turn has several applications, the most important of which is probably the production of chemical ferti...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Acknowledgment
• Preface to the Third Edition
• Preface to the Second Edition
• Preface to the First Edition
• Chapter 1. Introduction
• Chapter 2. Hydrate Types and Formers
• Chapter 3. Hand Calculation Methods
• Chapter 4. Computer Methods
• Chapter 5. Inhibiting Hydrate Formation with Chemicals
• Chapter 6. Dehydration of Natural Gas
• Chapter 7. Combating Hydrates Using Heat and Pressure
• Chapter 8. Physical Properties of Hydrates
• Chapter 9. Phase Diagrams
• Chapter 10. Water Content of Natural Gas
• Chapter 11. Additional Topics
• Index