Carroll Natural Gas Hydrates

A Guide for Engineers
3. Auflage 2014
ISBN: 978-0-12-800575-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 340 Seiten

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

John Carroll is currently Director, Geostorage Processing Engineering for Gas Liquids Engineering, Ltd. in Calgary. With more than 20 years of experience, he supports other engineers with software problems and provides information involving fluid properties, hydrates and phase equilibria. Prior to that, he has worked for Honeywell, University of Alberta as a seasonal lecturer, and Amoco Canada as a Petroleum Engineer. John has published a couple of books, sits on three editorial advisory boards, and he has authored/co-authored more than 60 papers. He has trained many engineers on natural gas throughout the world, and is a member of several associations including SPE, AIChE, and GPAC. John earned a Bachelor of Science (with Distinction) and a Doctorate of Philosophy, both in Chemical Engineering from the University of Alberta. He is a registered professional engineer in the province of Alberta and New Brunswick, Canada.

Autoren/Hrsg.

Carroll, John

Leseproben

Chapter 2

Hydrate Types and Formers

Abstract

Hydrates are classified by the crystal structure they form. There are three common structures: type I, type II, and type H. The size of the hydrate former molecule dictates which type of hydrate will form. The smallest guest molecules form type II, the intermediate ones for type I, and the largest form Type II. Type H hydrates are only possible with a large former and a smaller molecule. This chapter discusses the common hydrate formers (also known as guest molecules) and gives the range of temperatures and pressure at which the hydrate will form for the pure components that form hydrates. The common hydrate formers include methane, ethane, propane, carbon dioxide, hydrogen sulfide, and nitrogen. Other hydrate formers are simply listed, particularly those of less interest to the natural gas industry.

Keywords

Crystal structureFormersHydrate typesType HType IType II

Hydrates of natural gas components and other similar compounds are classified by the arrangement of the water molecules in the crystal, and hence the crystal structure. The water molecules align, because of the hydrogen bonding, into three-dimensional sphere-like structures often referred to as a cage. A second molecule resides inside the cage and stabilizes the entire structure.

There are two types of hydrates commonly encountered in the petroleum business. These are called type I and type II, sometimes referred to as structures I and II. A third type of hydrate that also may be encountered is type H (also known as structure H), but it is less commonly encountered.

Table 2.1 provides a quick comparison among type I, type II, and type H hydrates. These hydrates will be reviewed in more detail throughout this chapter.

Figure 2.1 shows the types of polyhedral cages involved in type I and II hydrates. In these diagrams, the water molecule is on the corner of the polyhedral, and the edge of the polyhedral represents the hydrogen bond. The structures for the type H hydrate, being significantly more complex, are not described in such detail, except to note that the small cages are the regular dodecahedron. The information in Table 2.1 and Fig. 2.1 will become clearer as the reader covers the entire chapter.

Type I Hydrates

The simplest of the hydrate structures is the type I. It is made from two types of cages: (1) dodecahedron, a 12-sided polyhedron where each face is a regular pentagon and (2) tetrakaidecahedron, a 14-sided polyhedron with 12 pentagonal faces and 2 hexagonal faces. The dodecahedral cages are smaller than the tetrakaidecahedral cages; thus, the dodecahedra are often referred to as small cages whereas the tetrakaidecahedral cages are referred to as large cages.

Type I hydrates consist of 46 water molecules. If a guest molecule occupies each of the cages then the theoretical formula for the hydrate is ·534H2O, where X is the hydrate former.

Table 2.1

Comparisons among Type I, Type II, and Type H Hydrates

Water molecules per unit cell

46

136

34

Cages per unit cell

Small

6

16

3

Medium

–

–

2

Large

2

8

1

Theoretical formula1

All cages filled

·534H2O

·523H2O

5X·Y·34H2O

Mole fraction hydrate former

0.1481

0.1500

0.1500

Only large cages filled

·723H2O

X·17H2O

–

Mole fraction hydrate former

0.1154

0.0556

–

Cavity diameter (Å)

Small

7.9

7.8

7.8

Medium

–

–

8.1

Large

8.6

9.5

11.2

Volume of unit cell (m3)

1.728×10?27

5.178×10?27

Typical formers

CH4, C2H6, H2S, CO2

N2, C3H8, i-C4H10

See text

1 X is the hydrate former and Y is a type H former.

Figure 2.1The Polyhedral Cages of Type I and Type II Hydrates.

Often, in the literature, you will find oversimplifications for the hydrate crystal structure. For example, it is common that only the dodecahedron is given as the unit crystal structure. This is incorrect. The correct structures are given here.

One of the reasons why it took a long time to establish the crystal structure of hydrates is because hydrates are nonstoichiometric. That is, a stable hydrate can form without a guest molecule occupying all of the cages. The degree of saturation is a function of the temperature and the pressure. Therefore, the actual composition of the hydrate is not the theoretical composition given in the previous paragraph.

Type I Formers

Some of the common type I hydrate formers include methane, ethane, carbon dioxide, and hydrogen sulfide. In the hydrates of CH4, CO2, and H2S, the guest molecules can occupy both the small and the large cages. On the other hand, the ethane molecule occupies only the large cages.

Type II Hydrates

The structure of the type II hydrates is significantly more complicated than that of type I. The type II hydrates are also constructed from two types of cages. The unit structures of a type II hydrate are: (1) dodecahedron, a 12-sided polyhedron where each face is a regular pentagon, (2) hexakaidecahedron, a 16-sided polyhedron with 12 pentagonal faces and 4 hexagonal faces. The dodecahedral cages are smaller than the hexakaidecahedron cages.

The type II hydrate consists of 136 molecules of water. If a guest molecule occupies all of the cages, then the theoretical composition is ·523H2O, where X is the hydrate former. Alternatively, as is more commonly the case, if the guest occupies only the large cages, then the theoretical composition is X·17H2O.

As with type I hydrates, the type II hydrates are not stoichiometric, so the compositions of the actual hydrates differ from the theoretical values.

Type II Formers

Among the common type II formers in natural gas are nitrogen, propane, and isobutane. It is interesting that nitrogen occupies both the large and small cages of the type II hydrate. On the other hand, propane and isobutane only occupy the large cages.

Type H Hydrates

Type H hydrates are much less common than either type I or II. In order to form this type of hydrate, it requires a small molecule, such as methane, and a type H former. As such, type H hydrates are always double hydrates.

The type H hydrates are constructed from three types of cages: (1) dodecahedron, a 12-sided polyhedron where each face is a regular pentagon, (2) an irregular dodecahedron with 3 square faces, 6 pentagonal faces, and 3 hexagonal faces, and (3) an irregular icosahedron, a 20-sided polyhedron, with 12 pentagonal faces and 8 hexagonal faces.

The unit crystal is made up of three dodecahedral cages (small), two irregular dodecahedral cages (medium), and one icosahedral cage (large). It is composed of 34 water molecules.

Type H hydrates are always double hydrates. Small guest molecules, such as methane, occupy the small, medium, and some of the large cages of the structure whereas a larger molecule, such as those given below, occupies the large cage. The larger molecule, the so-called type H former, is of such a size that it only fits in the large cage of this structure.

Because two formers are required to form a type H hydrate, it is a little difficult to give the theoretical formula. However, if we assume that the small molecule, X, only enters the two smaller cages and we know that the large molecule, Y, only enters the large cages, then the theoretical formula is Y·5X·34H2O.

Type H Formers

Type I and II hydrates can form in the presence of a single hydrate former, but type H requires two formers to be present, a small molecule, such as methane, and a larger type H forming molecule.

Type H formers include the following hydrocarbon species: 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2,2,3-trimethylbutane, 2,2-dimethylpentane, 3,3-dimethylpentane,...