Gas Engineering, Vol. 2: Composition and Processing of Gas Streams 311069090X, 9783110690903

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James G. Speight Gas Engineering

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James G. Speight

Gas Engineering

Vol. 2: Composition and Processing of Gas Streams

Author Dr. James G. Speight 2476 Overland Road Laramie, WY 82070-4808 USA [email protected] https://www.drjamesspeight.com

ISBN 978-3-11-069090-3 e-ISBN (PDF) 978-3-11-069105-4 e-ISBN (EPUB) 978-3-11-069112-2 Library of Congress Control Number: 2022944254 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2023 Walter de Gruyter GmbH, Berlin/Boston Cover image: nielubieklonu/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface The final three decades of the twentieth century saw not only perturbations of energy supply systems but also changes in attitudes of governments and voters alike toward environmental issues. Thus, environmental issues will be with us as long as there is manufacturing of e-consumer goods and the use of fossil fuels for energy production. And the latter issue is the subject of this text. The continued use of natural gas as combustible fuel is a reality, and gas processing, although generally understandable using chemical and/or physical principles, still requires an attempt to alleviate some of the confusion that arises from uncertainties in the terminology. This three-volume collection of books presents to the reader an understanding of the origin of gases, the properties of gases, and the uses of gases. The primary aim of the first volume is to introduce the reader to the origins of natural gas. This volume also contains chapters dealing with recovery, properties, and composition, including gas production from hydrocarbon-rich deep shale formations, known as shale gas, which is one of the most quickly expanding trends in onshore domestic gas exploration, and presents the development of deep shale formations, typically located many thousands of feet below the surface of the Earth in tight, lowpermeability formations. The basic technology of reservoir engineering is presented using the simplest and most straightforward mathematical techniques. The book focuses on processes and, wherever possible, the advantages, limitations, and ranges of applicability of the processes are discussed so that the selection and integration into the overall gas plant can be fully understood. It is only through having a complete understanding of the technology that the engineer can hope to appreciate and solve complex reservoir engineering problems in a practical manner. Volume 2 deals with the constituents of gas streams and the properties of individual constituents. This volume also presents the chemistry and engineering aspects of the methods and principles by which the gas streams might be cleaned from their noxious constituents. The concept of gas condensate is also introduced and discussed as well as the methods that can be applied to the analysis of gas streams and gas condensate. Thus, Volume 2 also contains references to several other types of gas streams (in addition to the subcategories of natural gas) that need to be presented here and includes the following gas streams that are listed alphabetically rather than by any order of importance: (1) biogas, (2) coalbed methane, (3) coal gas (various types), (4) flue gas, (5) landfill gas, (6) refinery gas, (7) shale gas, and (8) synthesis gas (Table 1.2), and all of which will, more than likely, also require processing for the removal of contaminants. If these gas streams are not already co-processed with natural gas, there is the expectation that as the future unfolds, such gas stream will evolve and become more common to the gas industry. Volume 3 presents a review of the uses of gas streams and their effects on the environment. This volume also introduces the concept of liquefied natural gas and https://doi.org/10.1515/9783110691054-202

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Preface

the concept “gas to liquids.” Also the properties of gas streams relating to corrosion effects are also presented. The relationship of the properties of gas streams as they affect corrosion such as carburization and metal dusting as well as corrosion in steel and other materials used in refinery technology is also presented, and the book summarizes key findings of corrosion processes in gas processing equipment as well as corrosion in offshore structures. Each book contains copious references at the end of the chapter which include information from the open literature and meeting proceedings to give a picture of where the gas processing technology stands as well as indicate some relatively new technologies that could become important in the future. Also, each book contains a comprehensive glossary. The books are written in an easy-to-read style and offer a ready-at-hand (onestop shopping) guide to the many issues that are related to the engineering aspects of the properties and processing of natural gas as well as the effects of natural gas on various ecosystems as well as pollutant mitigation and cleanup. The books present an overview with a considerable degree of the various aspects of natural gas technology in detail. Any chemistry presented in the books is used as a means of explanation of a particular point but is maintained at an elementary level. Dr. James G. Speight, Laramie, Wyoming, USA May 2021

Contents Preface

V

Chapter 1 Types of gases 1 1.1 Introduction 1 1.2 Types and composition 7 1.2.1 Natural gas 7 1.2.1.1 Associated natural gas 14 1.2.1.2 Nonassociated natural gas 14 1.2.1.3 Gas in tight formations 15 1.2.1.4 Geopressurized gas 16 1.2.1.5 Gas hydrates 17 1.2.1.6 Gas condensate 21 1.2.1.7 Liquefied petroleum gas 24 1.2.1.8 Refinery gas 27 1.2.2 Other gases 32 1.2.2.1 Biogas 32 1.2.2.2 Coalbed methane 35 1.2.2.3 Coal gas 37 1.2.2.4 Flue gas 44 1.2.2.5 Landfill gas 46 1.2.3 Other definitions 49 1.2.3.1 Shale gas 51 1.2.3.2 Synthesis gas 51 1.3 Uses 53 References 57 Chapter 2 Constituents of gas streams 61 2.1 Introduction 61 2.2 Constituents of gas streams 66 2.2.1 Methane 68 2.2.2 Ethane 69 2.2.3 Propane 70 2.2.4 Butane 70 2.2.5 Olefins 70 2.2.6 Higher molecular weight hydrocarbons 2.3 Other gas streams 73 2.3.1 Coal gas 73 2.3.2 Biogas 76

71

VIII

2.3.3 2.3.4 2.3.5 2.4 2.4.1 2.4.2 2.5 2.5.1 2.5.2 2.5.3 2.5.4

Contents

Landfill gas 78 Flue gas 81 Refinery gas 84 Chemical and physical properties Chemical properties 85 Physical properties 90 Types and formation of pollutants Primary pollutants 93 Secondary pollutants 93 The Greenhouse effect 95 Global climate change 96 References 98

84

92

Chapter 3 Properties of gases 101 3.1 Introduction 101 3.2 Properties 102 3.2.1 Absorption 102 3.2.2 Adsorption 103 3.2.3 Combustion 104 3.2.4 Compressibility 105 3.2.5 Corrosivity 108 3.2.6 Deviation factor 109 3.2.7 Electrical conductivity and resistivity 110 3.2.8 Flammability and explosive properties 111 3.2.9 Formation volume factor 115 3.2.10 Gas laws 116 3.2.10.1 Avogadro’s law 118 3.2.10.2 Boyle’s law 119 3.2.10.3 Charles’ law 119 3.2.10.4 Dalton’s law of partial pressure 120 3.2.10.5 Gay-Lussac’s law 121 3.2.10.6 Graham’s law of effusion 121 3.2.10.7 Ideal gas law 122 3.2.11 Heat of combustion 122 3.2.12 Helium content 123 3.2.13 Liquefied natural gas 123 3.2.14 Mercury content 124 3.2.15 Methane number 124 3.2.16 Odorization 125 3.2.17 Phase behavior 125 3.2.17.1 Dry gas and wet gas phase behavior 127

Contents

3.2.17.2 3.2.18 3.2.19 3.2.20 3.2.21 3.2.22 3.2.23 3.2.24 3.2.25 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7

Associated gas-phase behavior 128 Residue 128 Specific heat 128 Thermal conductivity 129 Thermal diffusivity 129 Volatility and vapor pressure 130 Viscosity 132 Water content 133 Wobbe index 133 Environmental effects 135 Greenhouse gas emissions 137 Air pollutants 139 Emissions during exploration, production, and delivery Emissions during processing 144 Emissions during combustion 145 Smog and acid rain 146 Emissions reduction 148 References 151

Chapter 4 Analysis of gas streams 153 4.1 Introduction 153 4.2 Types of gases 155 4.2.1 Natural gas 156 4.2.2 Other gases 160 4.3 Analytical equipment 164 4.3.1 Gas chromatography 168 4.3.2 Infrared absorption 168 4.3.3 Ultraviolet absorption 169 4.3.4 Chemiluminescence detector 169 4.3.5 Paramagnetic method 169 4.3.6 Dumbbell analyzer 170 4.3.7 Differential pressure analyzer 170 4.3.8 Thermal conductivity method 171 4.3.9 Catalytic filament method 171 4.3.10 Flame ionization detection 171 4.3.11 Solid-state electrolyte method 171 4.4 Properties of gases 172 4.4.1 Chemical properties 174 4.4.2 Physical properties 183 4.4.2.1 Calorific value 185 4.4.2.2 Composition 187

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X

4.4.2.3 4.4.2.4 4.4.2.5 4.4.2.6 4.4.2.7 4.4.2.8 4.4.2.9

Contents

Density and specific gravity 192 Dew point 196 Higher molecular weight hydrocarbons Mercury content 197 Sulfur content 197 Volatility and vapor pressure 198 Water content 200 References 200

196

Chapter 5 Gas condensate 202 5.1 Introduction 202 5.2 Types of condensate 206 5.2.1 Gas condensate 207 5.2.1.1 Lease condensate 207 5.2.1.2 Plant condensate 208 5.2.2 Low-boiling naphtha 208 5.2.3 Natural gasoline 209 5.3 Production 209 5.4 Condensate stabilization and storage 210 5.4.1 Stabilization 211 5.4.1.1 Stabilization by flash vaporization 211 5.4.1.2 Stabilization by fractionation 212 5.4.2 Storage 212 5.5 Properties 213 5.5.1 Chemical composition 215 5.5.2 Physical properties 217 5.6 Test methods 220 5.6.1 Aniline point and mixed aniline point 225 5.6.2 Benzene and aromatic derivatives 226 5.6.3 Composition 227 5.6.4 Constant composition expansion test 235 5.6.5 Constant volume depletion 235 5.6.6 Correlative methods 236 5.6.7 Density 236 5.6.8 Dew point pressure 238 5.6.9 Distillation 238 5.6.10 Evaporation rate 239 5.6.11 Flammability 240 5.6.12 Flash point 240 5.6.13 Formation volume factor 241 5.6.14 Hydrocarbon analysis 242

Contents

5.6.15 5.6.16 5.6.17 5.6.18 5.6.19 5.6.20 5.6.21 5.6.22 5.6.23 5.6.24 5.6.25

Kauri–butanol value 243 Octane number 243 Odor and color 244 Olefin content 245 Solubility 245 Solvent power 246 Sulfur content 246 Surface tension 246 Vapor pressure and volatility Viscosity 250 Water solubility 250 References 251

247

Chapter 6 Gas processing 255 6.1 Introduction 255 6.2 Gas streams 259 6.3 Glycol-based processes 265 6.4 Olamine-based processes 268 6.4.1 Girdler process 274 6.4.2 Flexsorb process 277 6.4.3 Adip process 278 6.4.4 Purisol process 278 6.5 Physical solvent processes 279 6.5.1 Selexol process 279 6.5.2 Rectisol process 280 6.5.3 Sulfinol process 281 6.6 Metal oxide processes 283 6.6.1 Iron sponge process 284 6.6.2 Adsorption processes 287 6.6.3 Other processes 289 6.7 Methanol-based processes 295 6.8 Alkali washing processes 297 6.8.1 Caustic scrubbing 297 6.8.2 Hot potassium carbonate process 6.8.3 Other processes 300 6.9 Membrane processes 301 6.10 Molecular sieve processes 304 6.11 Sulfur recovery processes 305 6.11.1 Claus process 306 6.11.2 Redox process 309 6.11.3 Wet oxidation processes 310

299

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6.11.4 6.11.5 6.12 6.13

Contents

Tail gas treating processes 311 Hydrogenation and hydrolysis processes 311 Cleaning technologies for other gases 313 Process selection 315 References 316

Chapter 7 Gas processing equipment 321 7.1 Introduction 321 7.2 Process equipment 324 7.2.1 Absorption units 326 7.2.2 Adsorption units 329 7.2.3 Cyclone collectors 331 7.2.4 Electrostatic precipitators 331 7.2.5 Filters 332 7.2.6 Scrubbers 333 7.2.6.1 Dry scrubbers 333 7.2.6.2 Orifice scrubbers 335 7.2.6.3 Packed bed scrubbers 335 7.2.6.4 Venturi scrubbers 336 7.2.6.5 Wet scrubbers 337 7.2.7 Spray towers 339 7.2.8 Materials of construction 340 7.3 Reactor types 342 7.3.1 Batch reactor and semi-batch reactor 343 7.3.2 Continuous reactor and plug flow reactor 344 7.3.3 Flash reactor 345 7.3.4 Fluidized bed reactor 346 7.3.5 Packed bed reactor 347 7.3.6 Slurry reactor 349 7.4 Examples of uses of process units 350 7.4.1 Solids removal 350 7.4.2 Water removal 351 7.4.2.1 Absorption unit 354 7.4.2.2 Adsorption unit 356 7.4.2.3 Molecular sieve 359 7.4.2.4 Membrane 360 7.4.3 Liquids removal 362 7.4.3.1 Gas–liquid separator 362 7.4.3.2 Extractor 363 7.4.3.3 Absorber 363 7.4.3.4 Cryogenic processes 364

Contents

7.4.4 7.4.5 7.4.6 7.4.6.1 7.4.6.2 7.4.6.3 7.4.6.4 7.4.6.5 7.4.7 7.4.8 7.4.8.1 7.4.8.2 7.4.8.3 7.4.9 7.4.9.1 7.4.9.2 7.5 7.5.1 7.5.2 7.5.3

Fractionator 366 Nitrogen removal 370 Acid gas removal 372 Olamine absorber 373 Carbonate washer and water washer 374 Metal oxide reactor 376 Catalytic oxidizer 377 Molecular sieve reactor 378 Enrichment 378 Other process reactors 379 Helium removal 379 Mercury removal 380 Radioactive residue removal 381 Hydrogen sulfide conversion 382 Claus process 382 SCOT process 383 Corrosion of gas processing equipment 384 Corrosion by hydrogen sulfide 385 Corrosion by carbon dioxide 389 Other forms of corrosion 390 References 390

Chapter 8 Principal products of gas processing and uses 8.1 Introduction 394 8.2 Methane 398 8.2.1 Chemical properties 401 8.2.2 Physical properties 405 8.3 Ethane 407 8.3.1 Chemical properties 409 8.3.1.1 Chemical reactions 409 8.3.2 Physical properties 411 8.4 Propane 412 8.4.1 Chemical properties 413 8.4.2 Physical Properties 414 8.5 Butane 416 8.5.1 Chemical properties 418 8.5.2 Physical properties 419 8.6 Olefins and di-olefins 419 8.7 The petrochemical industry 424 8.7.1 Composition and properties 427 8.7.2 Natural gas liquids 437

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8.7.3 8.7.4 8.7.5 8.8 8.8.1 8.8.2 8.8.3

Contents

Gas condensate 438 Gas hydrates 439 Other types of gases 442 Environmental aspects of gas streams and products Emissions 443 Greenhouse gases 444 Climate change 445 References 445

443

Appendices Appendix A ASTM standard test methods for natural gas and low-boiling liquids (gas condensates) 451 Appendix B GPA standard test methods for natural gas Appendix C Conversion factors and constants Appendix D The gas constant Appendix E Key equations Glossary

475

About the author Index

537

535

471 473

461

459

Chapter 1 Types of gases 1.1 Introduction By definition, for the purposes of this text, a gas stream is any gas that is produced either from a well (such recovery of natural gas from a natural gas well or recovery of the gas from a crude oil well) or from one or more processes, such as the gases produced in a refinery or gas that is produced from any other source. Also, gas engineering involves the application of a variety of engineering and technological principles that are necessary to identify the properties, behavior, and uses of the various types of gas streams. Typically, gas streams are produced from a variety of sources, and the technology used to produce a saleable product (or, in the case of the environment, a disposal product) is necessary to understand the source and the composition of the gas stream. An operational gas plant delivers pipeline-quality dry natural gas that can be used as fuel by residential, commercial, and industrial consumers or as a feedstock for chemical synthesis. The prime gas in such cases is natural gas but other possible gas streams that can be used commercially or domestically must also be given consideration. Although the terminology and definitions involved in the area of gas technology are quite succinct, there may be those readers that find the terminology and definitions can be somewhat confusing and, therefore, requires description (Tables 1.1 and 1.2). Briefly, terminology is the means by which various subjects are named so that reference can be made in conversations and in writings so that the meaning is passed on. On the other hand, definitions are the means by which scientists and engineers communicate the nature of a material to each other either through the spoken or through the written word. There are also several types of gas streams (in addtion to the subcategories of natural gas) that need to be presented here and include the following gas streams that are listed alphabetically rather than by any order of importance: (1) biogas, (2) coalbed methane (CBM), (3) coal gas (of which there are various types), (4) flue gas, (5) landfill gas, (6) refinery gas, (7) shale gas, and (8) synthesis gas (Table 1.2). All of these will, more than likely, also require processing for the removal of contaminants (Chapter 6). If these gas streams are not already co-processed with natural gas, there is the expectation that as the future unfolds, such gas stream will evolve and become more common to the gas industry. In some cases, CBM is often included in the natural gas family (as coal-related natural gas) or as coal gas due to lack of standardization of the terminology [1–4]. For clarification, natural gas is different from town gas (which is produced from coal) although the history of natural gas cleaning prior to sales to the consumer has the beginnings of the process in town gas cleaning [2]. https://doi.org/10.1515/9783110691054-001

2

Chapter 1 Types of gases

Table 1.1: Origin of various crude oil-related gas streams. Gas streams

Description

Natural gas

Occurs natural with or without crude oil. A mixture of gaseous (or low-boiling) hydrocarbon derivatives. Predominantly C to C hydrocarbon derivatives. May also contain the constituents (C–C) of low-boiling naphtha.

Gas hydrate

Formed when methane (and other gaseous hydrocarbon derivatives) is enclosed within a solid lattice (cage) of water molecules that is formed at low temperature and high pressure (as might exit in deep-water oceans). At STP,  m of the gas hydrate can release as much as  m of natural gas. May also decompose explosively.

Refinery gas

Also called process gas. Contains low-boiling hydrocarbon produced by thermal or catalytic processes. Predominantly C to C hydrocarbon derivatives. Also contains C to C olefin derivatives.

Tail gas

Produced from refinery processes after gas treatment. Typically, gas that is not required for further processing. Predominantly C to C hydrocarbon derivatives. May also contain hydrogen carbon monoxide, carbon dioxide, and sulfur compounds. Composition dependent upon the process by which the gas is produced.

The term natural gas has also been extended to gas from shale formations, which may be referred to as tight gas [5–8]. Biogas (i.e., gas produced from biological sources) is produced from biological sources [9–12]. However, for the purposes of this book, the petroliferous natural gas (i.e., crude oil-related natural gas) is placed within the category of conventional gas while petroliferous natural gas from tight formations and non-petroliferous natural gas (such as biogas and landfill gas) are placed under the term nonconventional gas (sometime called unconventional gas) [12]. Thus, the terminology and definitions applied to natural gas (and, for that matter, to other gaseous products) are extremely important and have a profound influence on the manner by which the technical community and the consumers perceive that gaseous fuel. For the purposes of this book, natural gas and those products that are isolated from natural gas during recovery (such as natural gas liquids (NGLs), gas condensate, and natural gasoline) are a necessary part of this text [13]. Thus (listed alphabetically rather than by importance):

1.1 Introduction

3

Table 1.2: Non-crude oil gas streams. Gas stream

Description

Biogas

A mixture of gases primarily consisting of methane, carbon dioxide, and hydrogen sulfide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, and food waste when the source material by microorganisms in the absence of oxygen (referred to as anaerobic digestion). Can occur naturally or as part of an industrial process to intentionally create it as a fuel or a source of petrochemicals.

Coalbed methane

Coalbed methane, coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) which occurs with and can be extracted from coalbeds.

Coal gas

A flammable gaseous fuel that is produced when coal is heated strongly in the absence of air. Town gas is a more general term referring to manufactured gaseous fuels produced for sale to consumers and municipalities. Other types (or subcategories) of coal gas include producer gas, blue gas, and carbureted water gas.

Flue gas

The gas stream that exits from the process by means of a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler, or steam generator. Often, the flue gas refers to the combustion exhaust gas produced at power plants.

Landfill gas

A natural by-product of the decomposition of organic material in landfills that is composed of methane (~% v/v) and carbon dioxide (~%) as well as small amounts of non-methane organic compounds (often referred to as NMOCs).

Shale gas

Natural gas that occurs trapped within shale formations. Not to be confused with the gas stream that is produced by the thermal decomposition of the kerogen in oil shale.

Synthesis gas

A gas stream consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. Produced by the controlled gasification of organic feedstocks. Used as an intermediate for the creation of synthetic natural gas and for producing ammonia or methanol and petrochemicals.

✶

Listed alphabetically.

4

Chapter 1 Types of gases

Conventional gas Associated gas Gas condensate Nonassociated gas Unconventional gas Biogas Coalbed methane Coal gas Flue gas Gas hydrates Gas in geopressurized zones Gas in tight formations Landfill gas Manufactured gas Refinery gas Shale gas Synthesis gas A more meaningful categorization of these gases would include two other categories which are based on the source of the gas or the method of production of the gas which also has some relationship to the composition of the gas and are (1) NGLs, which are product isolated from the natural gas stream and which are liquids at standard temperature and pressure (STP) and (2) manufactured gas, which is gas produced from a variety of nongaseous feedstocks by various manufacturing processes. Thus: Conventional natural gas Associated gas Nonassociated gas Natural gas liquids Gas condensate Low-boiling naphtha Natural gasoline Unconventional gas Gas hydrates Coalbed methane Gas in geopressurized zones Gas in tight formations Shale gas

1.1 Introduction

5

Manufactured gas Biogas Coal gas Flue gas Landfill gas Refinery gas Synthesis gas Finally, another form of categorization (often referred to as a method of classification) involves the following subdivision of gas streams on the basis of origin and chemical composition: Origin Conventional gas Unconventional gas Gas hydrates Coalbed methane Shale gas Chemical composition Natural liquids content Dry gas Wet gas (C2+, 10% v/v) Sulfur content Sweet gas (5 mg/m3) Nevertheless, whatever the method differentiation that is used to categorize gas streams, the most assured method is to define a gas stream by the method used to acquire (or produce) the gas thereby leaving little room for doubt or misinterpretation of the source and the properties of the gas stream [3, 14]. For example: reservoir gas ! produced gas ! wellhead gas ! transported gas transported gas ! stored gas ! sales gas transported gas ! stored gas ! chemicals production (also referred to as petrochemicals) More specifically, the term ‘natural gas’ is the generic term that is applied to the mixture of gaseous hydrocarbon derivatives and low-boiling liquid hydrocarbon derivatives, typically up to and including hydrocarbon derivatives such as n-octane, CH3(CH2)6CH3 (boiling point 125.1 to 126.1 °C and 257.1 to 258.9 °F) that is commonly associated with petroliferous (crude oil-producing, crude oil-containing) geologic formations [3, 4, 15–20].

6

Chapter 1 Types of gases

Chemically, natural gas is a mixture of hydrocarbon compounds and nonhydrocarbon compounds. The efficient use of natural gas in the production of energy and chemicals requires the application (and evolution) of up-to-date technology in gas processing since natural gas (and crude oil) will supply the industrialized nations of the world for (at least) the next four-to-five decades until suitable alternative forms of energy and chemicals (such as biogas and other non-petroliferous gas streams) are readily available [21–26]. Any gas sold, however, to an industrial or domestic consumer must (irrespective of the source of the gas) meet the designated specifications that are designed according to the use of the gas and the various environmental issues that may arise [3, 4, 15–20, 27]. Typically, in field operations, the composition of natural gas (which affects the specific gravity of the gas stream), especially of the associated gas (i.e., gas that occurs in an crude oil reservoir) which often contains significant amounts of higher boiling hydrocarbon derivatives (up to and including octane derivatives, C8H18), can vary significantly as the product flowing out of the well can change with variability of the production conditions as well as the change of pressure as gas is removed from the reservoir [28, 29]. Constituents of the gas that were in the liquid phase under the pressure that exists in the reservoir can revert to the gas phase as the reservoir pressure is reduced by removal of the gas and crude oil during production operations. As a result, at each stage of natural gas production such as (1) wellhead treating, (2) transportation, and (3) processing it is necessary to monitor the composition and properties of the gas by standard test methods [30, 31]. Moreover, the data produced from the test methods are the criteria by means of the suitability of the gas for use and the potential for interference with the environment. Some of the contaminants that occur in gas streams which contaminate natural gas have economic value and are further processed or sold. An operational gas plant delivers pipeline-quality dry natural gas that can be used as fuel by residential, commercial, and industrial consumers or as a feedstock for chemical synthesis. The prime gas in such cases is natural gas but other possible gas streams that can be used commercially or domestically must also be given consideration. In this chapter, sources of gas streams, their properties, and potential use are examined. More specifically, it is the purpose of this chapter to identify the gas streams that are produced from a variety of sources and the technology used to produce a saleable product (or, in the case of the environment, a disposal product) it is necessary to understand the source and the composition of the gas stream. In fact, in the context of this book, the term gas streams refer predominantly to natural gas stream as produced from crude oil reservoirs and natural gas reservoirs but also includes other gas streams, such as biogas, CBM, the various types of coal gas, flue gas, landfill gas, shale gas, and synthesis gas all of which will require some degree of contaminant removal before use or disposal. As a consequence, gas streams are highly variable in composition – even natural gas from the same reservoir can vary

1.2 Types and composition

7

in composition over time and/or is dependent upon the placement of the well. Thus, the object of gas processing (gas cleaning, gas refining) is to reduce the effect of these variables and produce a gas stream that meets the designated specifications for transportation, storage, and use (Chapter 6).

1.2 Types and composition In terms of the origin of natural gas, it is commonly accepted that natural gas, like crude oil, has been generated from organic debris that has been deposited in geologic time and has been embedded along with inorganic matter at a considerable depth (following the principles of sedimentology) below the surface of the earth [3, 4, 31–33]. Over time (typically up to several hundred of million years), because of chemical metamorphosis involving (1) compaction, (2) high pressure, and (3) elevated temperature, the organic material gradually evolved in natural gas and/or crude oil.

1.2.1 Natural gas By definition, natural gas (also called marsh gas and swamp gas in older texts) is a naturally occurring gaseous fossil fuel that is found in gas-bearing formations and in crude oil-bearing formations. Natural gas is found to be associated with crude oil in crude oil reservoirs and in reservoirs that contain gas with only minor amounts (if any) of crude oil. While natural gas is commonly grouped in with other fossil fuels and sources of energy, there are many characteristics of natural gas that make it unique. In order for natural gas constituents to accumulate in a geological reservoir, there are three effects that have to be present which are (1) the source rock, which contains the compacted organic debris from which the natural gas is created, (2) compacted organic materials from which the gas is created, (3) the porous formation which becomes the reservoir in which the gas is stored, (4) an impermeable base rock, also known as the basement rock which prevents the gas from migration downward, and (5) an impermeable rock above the formation, also known as the ceiling rock or cap rock. Furthermore, the evolution and the character of gas-containing reservoir (and a crude oil containing reservoir) that are critical to the properties of the gas in a reservoir include (1) the age of the reservoir, that is, the geological age or geological time when the sediment was deposited, (2) the depositional history of the reservoir, which relates to the type of sediment, (3) the temperature history of the reservoir, which typically increases with the depth of the reservoir, and (4) the pressure history of the reservoir, which also increases with the depth of the reservoir. Generally, gas can be in the form of a gas-cap on top of the oil-bearing zone or the gas can be dissolved in crude oil. As the depth of the reservoir increases, the

8

Chapter 1 Types of gases

amount of gas in the reservoir increases and gas reservoirs that are at depths on the order of 10,000 to 12,000 feet beneath the surface are some of the most productive crude oil reservoirs in which the crude oil coexists with substantial quantities of gas. At greater depths (e.g., on the order of and in excess of 17,000 feet), reservoirs may contain (almost exclusively) natural gas. The term crude oil (crude oil-derived gas, crude oil-related gas) that is used in this context is also used to describe the gaseous phase and liquid phase mixtures comprised mainly of methane to butane (C1–C4 hydrocarbons) that are dissolved in the crude oil and/or occur in the gas cap of the crude oil reservoir as well as gases produced during thermal processes in which the crude oil is converted to other products. It is necessary, however, to acknowledge that in addition to the hydrocarbon derivatives, other constituents (such as carbon dioxide (CO2) and hydrogen sulfide (H2S)) also occur in natural gas while for examplemercaptan derivative (RSH) and ammonia (NH3) also occur in ion refinery gas. Olefin derivatives are also present in the gas streams produced by the various crude oil refining processes and are either as contaminants or, if the occasion demands, for use in petrochemical operations [3, 14, 32, 34–36]. Natural gas, while being (predominantly) hydrocarbon in nature, contains substantial amounts of acid gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2) as well as other contaminants such as RSH, carbonyl sulfide (COS), and carbon disulfide (CS2) [3, 4, 15–20]. These non-hydrocarbon constituents can cause damage to natural gas pipelines if not removed. The combustion of sulfur compounds produces serious air pollutants and eventually produces acid rain when combined with water. These sulfur compounds are not only poisonous and floral and faunal species (including humans) but also corrosive to metals and other materials used for gas processing equipment and gas transportation systems. In addition, carbon dioxide is nonflammable, and as a result, large quantities are undesirable in a fuel. Like hydrogen sulfide, carbon dioxide forms a corrosive acid in the presence of water. It is therefore obvious that removal of acid gases along with other sulfur species is a major concern in gas processing (Chapter 6). A gas that is in some way similar to natural gas is the gas which is produced in a crude oil refinery (often referred to as refinery gas, refinery fuel gas, or process gas) and comprises mixtures that vary depending upon the refinery processes that produce the gases [3, 32, 34–36]. The constituents of each type of gas stream may be similar (except for the olefin-type gases produced during thermal processes, i.e., R1CH = CHR2, where R1 and R2 may be hydrogen or the same or different alkyl groups) but the variations of the amounts of these constituents cover wide ranges. Each type of gas may be analyzed by similar methods (Chapter 4) although the presence of high boiling hydrocarbons and non-hydrocarbon species such as carbon dioxide and hydrogen sulfide may require slight modifications to the various analytical test methods (Table 4.2) [30, 31].

1.2 Types and composition

9

However, at the onset of the study of gas streams (from the variety of sources), it is necessary that the analyst determines the type of gas to be analyzed. Mixtures of the various constituents of gas streams are commonly encountered in gas stream testing, and the composition of each stream varies depending upon the source. The nonhydrocarbon constituents of the gas stream are important constituents insofar as they may be recognized as useful products or may be recognized undesirable as a source of problems that can (and will) occur during gas cleaning processes (Chapter 6). Thus, as a result of the variation in composition of natural gas and other gas streams, it is essential to give consideration of the selection of suitable gas processing options streams as well as the properties of the individual constituents and the effects composition on gas behavior, even when considering the hydrocarbon constituents only. If not, the properties of the gas may render it unsuitable for the desired use insofar as the ability of the gas to be used for the desired purpose will be seriously affected. While there is always the possibility of carbon dioxide (CO2) and hydrogen sulfide (H2S) to occur in raw (untreated, unprocessed, unrefined) natural gas streams, other constituents such as helium, hydrogen, argon, oxygen, nitrogen, carbon monoxide, and organic sulfur-containing derivatives (such as mercaptan derivatives (RSH), where R is an alky moiety such as methyl (CH3) or ethyl (CH3CH2)), and higher molecular weight hydrocarbon derivatives as well as (in refinery gas) a variety of nitrogen-containing, oxygen-containing, and other sulfur-containing compounds [3, 32, 34–36]. Thus, the desired testing of the various gas streams (from the various sources materials) hydrocarbon mixtures involves (1) identification of the type of gas for component identification and the amount present in the stream and (2) the influence of the composition not only on the bulk physical or chemical properties of the gas stream but also more important, (3) the influence of the composition of the gas stream on the performance, and (4) the limits of accuracy of the test method [30, 31]. Furthermore, there is no single composition of components which might be termed “typical natural gas.” Methane (CH4) and ethane (C2H6) often constitute the bulk of the combustible components whereas carbon dioxide (CO2) and nitrogen (N2) are the major noncombustible (inert) components of the stream. In addition, the so-called sour gas is (in this immediate context) a natural gas stream that has relatively high levels of sulfur compounds (such as hydrogen sulfide, H2S, and mercaptans, RSH, also called thiols) and which constitutes a corrosive gas [31, 37]. Typically, in the current context, a sour gas steam is a natural gas that contains significant amounts of hydrogen sulfide. Natural gas is usually considered sour if there are more than 5.7 mg of hydrogen sulfide/m3 (>5.7 mg/m3) of natural gas, which is equivalent to approximately 4 ppm by volume (4% v/v) under STP. Any other gas streams that match this content of hydrogen sulfide are also designated sour gas.

10

Chapter 1 Types of gases

Carbon dioxide and hydrogen sulfide are commonly referred to as acid gases and are often (incorrectly) included in the definition of sour gas since they form corrosive compounds in the presence of water. Nitrogen, helium, and carbon dioxide are also referred to as diluents since none of these burn, and thus they have no heating value. Mercury can also be present either as a metal in vapor phase or as an organo-metallic compound in liquid fractions. Concentration levels are generally very small, but even at very small concentration levels, mercury can be detrimental due its toxicity and its corrosive properties (reaction with aluminum alloys). Thus, a sour gas stream will require additional processing for the removal of the hydrgoen sulfide (as well as any other excessive amounts of carbon dioxide and other contaminants) [3, 4, 15–20]. Olefins are also present in the gas streams from various refinery processes and are not included in natural gas designated for sales to the consumers but are removed as valuable feedstocks for use in various petrochemical operations [3, 14, 32, 34–36]. The analysis of any gas streams, compared to liquid streams, is relatively simple because bulk characterization of a single phase is implicit. However, when gas condensate is present, the analysis is more complicated [30, 31]. In the case of gas condensate, besides bulk analysis, there may be interest in surface-composition of the condensate which may be distinct to the composition of the bulk (gaseous) phase. Compositional analysis in which the components of the mixture are identified may be achieved by (1) physical means, which is the measurement of physical properties, (2) pure chemical means, which is the measurement of chemical properties, or more commonly (3) by physico-chemical means. Gas analysis is even more difficult if the composition of the gas is completely unknown and needs pre-processing definition. This pre-processing definition is important when water vapor is present in the gas stream which may condense on the instruments or when the molecular behavior of the water (and any other contaminants) may complicate any of the analytical methods (Chapter 4). While not typically an issue with natural gas streams, there is also the need to consider the presence of a known dangerous constituent (of which hydrogen cyanide is the most dangerous) that may occur in certain gas streams produced during production of the gas from the various nitrogen-containing sources (such as biomass). This type of dangerous constituent should be identified and removed at the earliest possible opportunity. In addition, the gas streams produced during crude oil refining and upgrading (i.e., process gas streams) contain a variety of saturated and unsaturated gaseous hydrocarbon derivatives, predominantly in the C1–C6 carbon number (methane to hexane derivatives) range. Some refinery gas streams may also contain inorganic compounds, such as hydrogen, nitrogen, hydrogen sulfide, carbon monoxide, and carbon dioxide. As such, crude oil-related gas stream and refinery gas streams (unless produced as a salable product that must meet specifications prior to sale) are often of variable composition and/or unknown composition, even to the point of being toxic [38, 39]. The site-restricted crude oil-related gas and refinery gas (i.e.,

1.2 Types and composition

11

those not produced as saleable products) often serve as (1) fuel gases to be consumed on-site, (2) intermediate streams for purification and recovery of various gaseous products, or (3) feedstock streams for isomerization and alkylation processes within a facility [3, 14, 32, 34–36]. As with crude oil natural gas from different wells varies widely in composition and analyses [3, 4, 15–20], and the proportion of non-hydrocarbon constituents can vary over a very wide range. The non-hydrocarbon constituents of natural gas can be classified as two types of materials which are (1) diluents, such as nitrogen, carbon dioxide, and water vapors and (2) contaminants, such as hydrogen sulfide and/ or other sulfur compounds. Thus, a particular natural gas field could require production, processing, and handling protocols that are different (or modified) from those used for gas produced from another field. The diluents in natural gas streams are the noncombustible constituents that reduce the heating value of the gas and are on occasion used as fillers when it is necessary to reduce the heat content of the gas. On the other hand, the contaminants are detrimental to production and transportation equipment in addition to being obnoxious pollutants. Thus, the primary reason for gas refining is to remove the unwanted constituents of natural gas and to separate the gas into its various constituents. The processes are analogous to the distillation unit in a refinery where the feedstock is separated into the various constituent fractions before use as feedstocks for the production of more valuable products. The major diluents or contaminants of natural gas are (1) acid gas, which is predominantly hydrogen sulfide although carbon dioxide does occur to a lesser extent, (2) water, which includes all entrained free water or water in condensed forms, (3) liquids in the gas, such as higher boiling hydrocarbons as well as pump lubricating oil, scrubber oil, and on occasion, methanol, and (4) any solid matter that may be present such as fine silica (sand) and scaling from the pipe. Like any other refinery product, natural gas (or, for that matter, any refinery gas destined for sales and/or use) must be processed to prepare it for final use and to ascertain the extent of contaminants that could cause environmental damage. Furthermore, gas processing is a complex industrial process designed to clean raw (dirty, contaminated) gas by separating impurities and various non-methane organic compounds and any other undesirable constituents to produce what is known as pipeline quality dry natural gas (Chapter 6) [3, 4, 15–20]. However, there are many variables to be assessed and considered in treating gas streams that are dependent upon the constituents and the properties of the individual gas streams. These properties are either temperature-independent or values of some basic properties at a fixed temperature. However, the composition of natural gas is not universally constant, as it is normally drawn from several production fields. Variations in the composition of gas delivered by pipelines can be caused by (1) the nature of the source from which the gas was produced during the metamorphosis of the various and variable components of the source material,

12

Chapter 1 Types of gases

(2) variations in the proportion of the contribution from various sources at a given supply location, and (3) time variations within a given supply source. In addition, the composition of a gas stream gas from a source or from a specific reservoir can vary over production time which can cause difficulties in resolving the data from the application of standard test methods [31]. Raw (unrefined) natural gas comes from three types of wells: (1) wells that produce crude oil, (2) wells that produce only, or predominantly, natural gas, and (3) wells that produce gas condensate, condensate wells. Natural gas that comes from oil wells is typically termed as associated gas. This gas can exist separately from oil in the formation (free gas) or dissolved in the crude oil (dissolved gas). Natural gas from gas and condensate wells, in which there is little or no crude oil, is termed as nonassociated gas. Gas wells typically produce raw natural gas while condensate wells produce natural gas along with a semi-liquid hydrocarbon condensate. Whatever the source of the natural gas, once separated from crude oil (if present) it commonly exists in mixtures with other hydrocarbon derivatives, principally ethane, propane, butane, and pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide (H2S), carbon dioxide (CO2), helium (He), nitrogen (N2), and other compounds. In fact, associated hydrocarbons, known as NGLs, can be very valuable by-products of natural gas processing. NGLs include ethane, propane, butane, iso-butane, and natural gasoline that are sold separately and have a variety of different uses, including enhancing oil recovery in oil wells, providing raw materials for oil refineries or petrochemical plants, and as sources of energy [4, 19]. Thus, raw natural gas varies in composition and the constituents can be several of a group of saturated hydrocarbons, typically from methane to butane (Table 1.3), as well as non-hydrocarbon derivatives. Table 1.3: Range of composition of natural gas. Constituent

Formula

% v/v

Methane Ethane Propane Butane Pentane (CH)+ Benzene + aromatics Carbon dioxide Hydrogen sulfide Mercaptans Helium Water

CH CH CH CH CH CH+ CO HS RSH He HO

– – – – – – – – – – –

1.2 Types and composition

13

Briefly, natural gas contains hydrocarbons and non-hydrocarbon gases of which the former (the hydrocarbon gases) are methane (CH4), ethane (C2H6), propane (C3H8), butanes (C4H10), pentanes (C5H12), hexane (C6H14), heptane (C7H16), and sometimes trace amounts of octane (C8H18) and higher molecular weight hydrocarbons. Aromatic derivatives (such as the BTX group – benzene (C6H6), toluene (C6H5CH3), and the xylene isomers (CH3C6H4CH3)) can also be present, raising safety issues due to their toxicity.

The BTX group On occasion, if it is present, ethyl benzene is also included in this aromatics group and the group is then given the name the BTEX group:

Ethylbenzene

The non-hydrocarbon gas portion of the natural gas contains nitrogen (N2), carbon dioxide (CO2), helium (He), hydrogen sulfide (H2S), water vapor (H2O), and other sulfur compounds (such as carbonyl sulfide (COS) and mercaptans (e.g., methyl mercaptan, CH3SH)) and trace amounts of other nonhydrocarbon on gases. If the natural gas stream has a high proportions of higher boiling hydrocarbon derivatives (referred to as NGLs and include hydrocarbon derivatives such as ethane, propane, butane, and pentanes as well as higher molecular weight hydrocarbon derivative up to and including the C8 derivatives) it is referred to as rich gas. Any higher molecular weight constituents (i.e., the C5+ derivatives) are commonly referred to as gas condensate or natural gasoline (sometimes, on occasion, erroneously called casinghead gas). Briefly, when referring to natural gas condensate or NGLs, the term gallons per thousand cubic feet (g/1,000 ft3) is used as a measure of the content of high-molecular-weight hydrocarbon derivatives in the gas stream.

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Chapter 1 Types of gases

Many sources of natural gas and crude oil-related gas streams contain sulfur compounds that are not only odorous but also corrosive and poisonous to refinery catalysts. In fact, sulfur odorants are added (in the ppm range, i.e., 1 to 4 ppm v/v) to natural gas for safety purposes. However, careful choice of the odorant is necessary since some odorants are unstable and react to form compounds having lower odor detection thresholds. By way of definition, the odor detection threshold is the lowest concentration of a certain odor compound that is perceivable by the human sense of smell. The threshold of a chemical compound is determined in part by its shape, polarity, partial charges, and molecular mass. 1.2.1.1 Associated natural gas Associated natural gas (also called associated gas or associated petroleum gas) is a type of natural gas which is found in reservoirs that contain crude oil either dissolved in the crude oil or as a free gas cap above the crude oil in the reservoir. Natural gas is found in crude oil reservoirs as free gas (associated gas) or in solution with crude oil in the reservoir (dissolved gas) or in reservoirs that contain only gaseous constituents and no (or little) crude oil (unassociated gas) [3, 32, 34–36]. Thus, by convention, associated gas contains natural gas plant liquids such as ethane (C2H6, CH3CH3), propane (C3H8 CH3CH2CH3), normal butane (C4H10, CH3CH2CH2CH3), isobutane [C4H10, (CH3)2CHCH3], and natural gasoline (also called low-boiling naphtha, n-C5H12, CH3CH2CH2CH2CH3) and higher molecular weight hydrocarbon derivatives up to C8H18. Associated gas is, by definition, often characterized as wet gas because it must be treated at natural gas processing plants to remove higher molecular weight hydrocarbon derivatives (such as ethane, propane, and butane as well as any C%+ derivatives) before it can be marketed as natural gas. The hydrocarbon content varies from mixtures of methane and ethane with very few other constituents (dry gas) to mixtures containing all of the hydrocarbons from methane to pentane and even hexane (C6H14) and heptane (C7H16) (wet gas). In both cases some carbon dioxide (CO2) and inert gases, including helium (He), are present together with hydrogen sulfide (H2S) and a small quantity of organic sulfur. 1.2.1.2 Nonassociated natural gas Nonassociated natural gas (also referred to as free gas or dry gas) is a naturally occurring natural gas that is not associated with crude oil (i.e., not dissolved in crude oil) in a reservoir. On occasion, this type of natural gas is found above the crude oil or under the crude oil present in the crude oil reservoir but in a state of equilibrium. Thus, nonassociated natural gas, which is found in reservoirs in which there is no crude oil or, at best, only minimal amounts of, crude oil [3, 4]. Nonassociated gas is usually richer in methane but is markedly leaner in terms of the higher

1.2 Types and composition

15

molecular weight hydrocarbons and condensate. Conversely there is also associated natural gas (dissolved natural gas) that occurs either as free gas or as gas in solution in the crude oil. Gas that occurs as a solution with the crude oil is dissolved gas, whereas the gas that exists in contact with the crude oil (gas cap) is associated gas [3, 32, 34–36]. Associated gas is usually leaner in methane than the nonassociated gas but is richer in the higher molecular weight constituents. The most preferred type of natural gas is the nonassociated gas. Such gas can be produced at high pressure, whereas associated (i.e., or dissolved) gas must be separated from crude oil at lower separator pressures, which usually involves increased expenditure for compression. Thus, it is not surprising that such gas (under conditions that are not economically favorable) is often flared or vented. 1.2.1.3 Gas in tight formations A tight formation refers to a formation (or a reservoir) in which the reservoir rock is extremely impermeable thereby trapping the gas. Such formations can be composed of sandstone (sometime known as tight sand formations) or limestone which are typically impermeable or nonporous. Thus, gas from tight formations, also called tight gas and shale gas (Section 1.2.2.6, below), occurs in low-permeability reservoir rocks (such as shale) which prohibit natural movement of the gas to a well [3]. Tight formations are relatively low permeability sedimentary formations that can contain (in the current context) natural gas. Furthermore, a tight gas reservoir is one that cannot be produced at economic flow rates or recover economic volumes of gas unless the well is stimulated by a large hydraulic fracture treatment and/or produced using horizontal wellbores. This definition also applies to CBM and tight (low permeability) carbonate reservoirs – shale gas reservoirs are also included in this definition by some observers [40]. Typically, tight formations which formed under marine conditions contain less clay and are more brittle and thus more suitable for hydraulic fracturing than formations formed in fresh water which may contain more clay. The formations become more brittle with an increase in quartz content (SiO2) and carbonate content (such as calcium carbonate (CaCO3) or dolomite, which is a calcium carbonatemagnesium carbonate (CaCO3.MgCO3), mineral). By way of explanation and comparison, in a conventional sandstone reservoir the pores are interconnected so that natural gas and crude oil can flow easily through the reservoir and to the production well. Conventional gas typically is found in reservoirs with permeability >1 mD and can be extracted via traditional techniques. However, in tight sandstone formations, the pores are smaller and are poorly connected (if at all) by very narrow capillaries which results in low permeability and immobility of the natural gas. Such sediments typically have an effective permeability of