Gas Treating: Absorption Theory and Practice provides anintroduction to the treatment of natural gas, synthesis gas andflue gas, addressing why it is necessary and the challengesinvolved. The book concentrates in particular on theabsorption-desorption process and mass transfer coupled withchemical reaction. Following a general introduction to gas treatment, the chemistryof CO2, H2S and amine systems is described, and selected topicsfrom physical chemistry with relevance to Gas Treating arepresented. Thereafter the absorption process is discussed indetail, column hardware is explained and the traditional masstransfer model mechanisms are presented together with mass transfercorrelations. This is followed by the central point of the text inwhich mass transfer is combined with chemical reaction,highlighting the associated possibilities and problems.Experimental techniques, data analysis and modelling are covered,and the book concludes with a discussion on various processelements which are important in the absorption-desorptionprocess, but are often neglected in its treatment. These includeheat exchange, solution management, process flowsheet variations,choice of materials and degradation of absorbents. The text isrounded off with an overview of the current state of research inthis field and a discussion of real-world applications. This book is a practical introduction to Gas Treating forpracticing process engineers and chemical engineers working onpurification technologies and gas treatment, in particular, thoseworking on CO2 abatement processes, as well as post-graduatestudents in process engineering, chemical engineering andchemistry.
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List of Abbreviations
Chapter 1: Introduction
1.2 Gas Markets, Gas Applications and Feedstock
1.5 Ambient Conditions
1.6 Objective of This Book
1.7 Example Problems
Chapter 2: Gas Treating in General
2.2 Process Categories
2.3 Sulfur Removal
2.4 Absorption Process
Chapter 3: Rate of Mass Transfer
3.2 The Rate Equation
3.3 Co-absorption and/or Simultaneous Desorption
3.4 Convection and Diffusion
3.5 Heat Balance
3.6 Axially along the Column
3.7 Flowsheet Simulators
3.8 Rate versus Equilibrium Approaches
Chapter 4: Chemistry in Acid Gas Treating
4.3 Acid Character of CO
4.4 The H
S Chemistry with any Alkanolamine
4.5 Chemistry of CO
with Primary and Secondary Alkanolamines
4.6 The Chemistry of Tertiary Amines
4.7 Chemistry of the Minor Sulfur Containing Gases
4.8 Sterically Hindered Amines
4.9 Hot Carbonate Absorbent Systems
4.10 Simultaneous Absorption of H
S and CO
4.11 Reaction Mechanisms and Activators–Final Words
4.12 Review Questions, Problems and Challenges
Chapter 5: Physical Chemistry Topics
5.2 Discussion of Solvents
5.3 Acid–Base Considerations
5.4 The Amine–CO
5.5 Gas Solubilities, Henry's and Raoult's Laws
5.6 Solubilities of Solids
5.8 Partial Molar Properties and Representation
5.9 Hydration and Hydrolysis
Chapter 6: Diffusion
6.1 Dilute Mixtures
6.2 Concentrated Mixtures
6.3 Values of Diffusion Coefficients
6.4 Interacting Species
6.5 Interaction with Surfaces
6.6 Multicomponent Situations
Chapter 7: Absorption Column Mass Transfer Analysis
7.2 The Column
7.3 The Flux Equations
7.4 The Overall Mass Transfer Coefficients and the Interface
7.5 Control Volumes, Mass and Energy – Balances
7.6 Analytical Solution and Its Limitations
7.7 The NTU–HTU Concept
7.8 Operating and Equilibrium Lines – A Graphical Representation
7.9 Other Concentration Units
7.10 Concentrated Mixtures and Simultaneous Absorption
7.11 Liquid or Gas Side Control? A Few Pointers
7.12 The Equilibrium Stage Alternative Approach
7.13 Co-absorption in a Defined Column
7.14 Numerical Examples
Chapter 8: Column Hardware
8.3 Packing Auxiliaries
8.4 Tray Columns and Trays
8.5 Spray Columns
Chapter 9: Rotating Packed Beds
9.2 Flooding and Pressure Drop
9.3 Fluid Flow
9.4 Mass Transfer Correlations
9.5 Application to Gas Treating
9.6 Other Salient Points
9.7 Challenges Associated with Rotating Packed Beds
Chapter 10: Mass Transfer Models
10.1 The Film Model
10.2 Penetration Theory
10.3 Surface Renewal Theory
10.4 Boundary Layer Theory
10.5 Eddy Diffusion, ‘Film-Penetration’ and More
Chapter 11: Correlations for Mass Transfer Coefficients
11.2 Packings: Generic Considerations
11.3 Random Packings
11.4 Structured Packings
11.5 Packed Column Correlations
11.6 Tray Columns
Chapter 12: Chemistry and Mass Transfer
12.2 Equilibrium or Kinetics
12.3 Diffusion with Chemical Reaction
12.4 Reaction Regimes Related to Mass Transfer
12.5 Enhancement Factors
12.6 Arbitrary, Reversible Reactions and/or Parallel Reactions
12.8 Numerical Examples
Chapter 13: Selective Absorption of H2S
13.2 Theoretical Discussion of Rate Based Selectivity
13.3 What Fundamental Information is Available in the Literature?
13.4 Process Options and Industrial Practice
13.5 Key Design Points
13.6 Process Intensification
13.7 Numerical Example
Chapter 14: Gas Dehydration
14.2 Dehydration Options
14.3 Glycol Based Processes
14.4 Contaminants and Countermeasures
14.5 Operational Problems
14.6 TEG Equilibrium Data
14.7 Hydrate Inhibition in Pipelines
14.8 Determination of Water
14.9 Example Problems
Chapter 15: Experimental Techniques
15.2 Experimental Design
15.3 Laminar Jet
15.4 Wetted Wall
15.5 Single Sphere
15.6 Stirred Cell
15.7 Stopped Flow
15.8 Other Mass Transfer Methods Less Used
15.9 Other Techniques in Gas–Liquid Mass Transfer
15.10 Equilibrium Measurements
15.11 Data Interpretation and Sub-Models
Chapter 16: Absorption Equilibria
16.2 Fundamental Relations
16.3 Literature Data Reported
16.7 Electrolyte NRTL (Austgen–Bishnoi–Chen–Rochelle)
16.9 Extended UNIQUAC
16.10 EoS – SAFT
16.11 Other Models
Chapter 17: Desorption
17.2 Chemistry of Desorption
17.3 Kinetics of Reaction
17.4 Bubbling Desorption
17.5 Desorption Process Analysis and Modelling
17.6 Unconventional Approaches to Desorption
Chapter 18: Heat Exchangers
18.3 Desorber Overhead Condenser
18.4 Economiser or Lean/Rich Heat Exchanger
18.5 Amine Cooler
18.6 Water Wash Circulation Cooler
18.7 Heat Exchanger Alternatives
Chapter 19: Solution Management
19.2 Contaminant Problem
19.3 Feed Gas Pretreatment
19.4 Rich Absorbent Flash
19.7 Chemicals to Combat Foaming
19.8 Corrosion Inhibitors
19.9 Waste Handling
19.10 Solution Containment
19.11 Water Balance
19.12 Cleaning the Plant Equipment
19.13 Final Words on Solution Management
Chapter 20: Absorption–Desorption Cycle
20.1 The Cycle and the Dimensioning Specifications
20.2 Alternative Cycle Variations
20.3 Other Limitations
20.4 Matching Process and Treating Demands
20.5 Solution Management
20.6 Flowsheet Variations to Save Desorption Energy
Chapter 21: Degradation
21.1 Introduction to Degradation
21.2 Carbamate Polymerisation
21.3 Thermal Degradation
21.4 Oxidative Degradation
21.5 Corrosion and Degradation
21.6 The Effect of Heat Stable Salts (HSSs)
21.7 SOx and NOx in Feed Gas
21.9 Concluding Remarks
Chapter 22: Materials, Corrosion, Inhibitors
22.2 Corrosion Basics
22.3 Gas Phase
22.4 Protective Layers and What Makes Them Break Down (Chemistry)
22.5 Fluid Velocities and Corrosion
22.6 Stress Induced Corrosion
22.7 Effect of Heat Stable Salts (HSS)
22.9 Problem Areas, Observations and Mitigation Actions
Chapter 23: Technological Fronts
23.1 Historical Background
23.2 Fundamental Understanding and Absorbent Trends
23.3 Natural Gas Treating
23.4 Syngas Treating
23.5 Flue Gas Treating
23.6 Where Are We Heading?
Chapter 24: Flue Gas Treating
24.2 Pressure Drop and Size Issues
24.3 Absorbent Degradation
24.4 Treated Gas as Effluent
24.6 Energy Implications
24.7 Cost Issues
24.8 The Greenhouse Gas Problem
Chapter 25: Natural Gas Treating (and Syngas)
25.2 Gas Export Specification
25.3 Natural Gas Contaminants and Foaming
25.4 Hydrogen Sulfide
25.5 Regeneration by Flash
25.6 Choice of Absorbents
Chapter 26: Treating in Various Situations
26.1 Introduction and Environmental Perspective
26.2 End of Pipe Solutions
26.3 Sulfur Dioxide
26.4 Nitrogen Oxides
26.5 Dusts and Aerosols
26.6 New Challenges
End User License Agreement
Table of Contents
DAG A. EIMER
Tel-Tek and Telemark University College, Norway
This edition first published 2014
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Library of Congress Cataloging-in-Publication Data
Eimer, D. (Dag)
Gas treating : absorption theory and practice / Professor D. Eimer.
ISBN 978-1-118-87773-9 (cloth)
1. Gases—Absorption and adsorption. 2. Gases—Purification. 3. Gases. I. Title.
To the people of Glasgow and Scotland for making me a chemical engineer, and for providing my life-long wife Barbara, and in turn two daughters and grandchildren.
This book came about because of the lack of a suitable text from which to lecture post-graduate students the topic of absorption/desorption mass transfer in combination with chemical reaction. This book would, however, also be suitable to teach mass transfer in a broader way to undergraduates who have a wish to enlarge upon this topic relative to a typical unit operations course. From my industrial experience, I would also say that those engineers who specialise, or who are heavily involved in this field, would benefit from this text.
The book starts by setting gas treating into perspective in Chapter 1 by discussing a few drivers, providing a feel for the size of the problem and the challenges in the form of gas specifications to achieve. In Chapter 2 the absorption process is put into context by discussing alternatives for treating natural gas, synthesis gas and exhaust gas. The latter is a subject that has gained importance in recent years and means that many more engineers work with this process. Chapter 2 also explains a number of other processes that an engineer is likely to come across in this field, and also one or two that are rare but are useful to know. Sulfur is a theme in its own right in the hydrocarbon industry and a quick overview is provided. It should be possible for the interested reader to quickly reel in more information by starting with the references provided.
The text introduces the alternative treatments of rate based or equilibrium based mass transfer analysis as a basis and then goes on to discuss the chemistry of acid gas absorption into alkaline solvents and physical chemistry topics that are important for fundamental understanding and that are somewhat special to this field.
Diffusion is given its own chapter to underline its importance. This is followed by a discussion of the traditional concepts for estimating the relative height requirements of separations in columns in Chapter 7, while the actual mass transfer coefficient estimates are delayed until later in the text. Hardware types are then discussed in Chapters 8 and 9 before correlations of mass transfer coefficients related to these are discussed in Chapter 11. The alternative ideas behind mass transfer coefficients are discussed in Chapter 10.
Chapter 12 develops the concepts and equations for limiting behaviour of mass transfer and the influence of chemical reaction. It is aimed at doing this in more detail than in previous texts to meet the needs of the non-specialists and students who want to gain a proper understanding. This is followed up in Chapter 13 which discusses the particular situation when both CO2 and H2S are being absorbed. The absorption process discussion is rounded off in Chapter 14 with absorption of water in glycol which is a very important process, and is also very different due to the extremely high solubility of water in glycol that renders this process essentially gas side limited.
At this stage it is clear that there is a great need for data to compute solutions. Techniques for measuring these are discussed in Chapter 15. Even when literature data can be found and there is no need for making our own measurements, it is useful to gain insight in these techniques and be able to form an opinion of uncertainties involved, and how data models become integrated when data are interpreted. Chapter 16 discusses absorption equilibria and models to represent them to provide an introduction to this extensive field.
Having discussed the absorption process to a great extent, it is appropriate to discuss the desorption process specifically, and this is dealt with in Chapter 17. There is more to this process than being the reverse of absorption.
Chapters 18–22 discuss various topics that are important in the absorption-desorption process, but are often neglected or overviews missing in related literature. These include heat exchange, solution management, flow sheet variations, degradation of solvent and choice of materials in view of corrosion issues.
The text is rounded off by discussing technological fronts in general and issues related to flue gas treating and treating of natural and synthesis gas in Chapters 23–26.
A chance happening in my life provided me with the time to write this book. It has been lectured to postgraduate students, and has since been strengthened. I am indebted to my students and colleagues for invaluable discussions on these topics, not the least Professor Klaus Jens, who has elevated my insight to the chemistry aspects and John Arild Svendsen, with whom I have shared joy and despair while modelling. Thanks are also due to Zulkifli Idris who has helped with chemistry figures and proofreading.
Concept checklists, review questions and PowerPoint slides of all figures from this book can be found online at http://booksupport.wiley.com.
American Chemical Society
American Institute of Chemical Engineers
Air Separation Unit
Boiler Feed Water
VOC emissions from glycol plants
Implying ethane (C
) and heavier alkanes
Propane (C3) Mixed Refrigerant process
Combined Cycle Gas Turbine
Carbon Capture and Storage
Controlled Freeze Zone process
Conference Of Parties
Commonwealth Scientific and Industrial Research Organisation.(An Australian research organisation).
Coil-Wound Heat Exchanger
Enhanced Oil Recovery
Gas Processors Association
Gas Processors Suppliers Association
Gas Technology Institute
Height Equivalent of a Theoretical Plate
Height of Transfer Unit
International Energy Agency
International Energy Agency GreenHouse Gas program
International Gas Union
Intergoverntal Panel Climate Change
International Union of Pure and Applied Chemistry
Liquefied Natural Gas
Lliquefied Petroleum Gas
Million Standard Cubic Feet per Day (i.e. 24 hours)
Number of Equivalents of a Theoretical Plate
Natural Gas Liquids
Natural Gas Supply Association
Nuclear Magnetic Resonance
Net Present Value
Nitrogen Rejection Unit
Normal Temperature and Pressure (0 °C and 1.013 bar)
Number of Transfer Units
Organization of the Petroleum Exporting Countries.There are presently 12 member countries.
Printed Circuit Heat Exchanger
Plate & Frame Heat Exchanger
Plate-Fin Heat Exchanger
Pressure Swing Adsorption
Rotating Packed Bed
Statistical Associating Fluid Theory
Standard Cubic Feet
Shell Claus Off-gas Treatment
Standard Temperature and Pressure (1.013 bar and 15 °C
UN Environental Program
Volatile Organic Compounds
Vacuum pressure Swing Adsorption
World Meteorological Organization
Gas treating is featured in many process plants in many contexts. There are almost always unwanted components that need to be removed from a gas stream. These components may need to be removed for a number of reasons like:
Contamination of product
Dew point, unwanted condensation downstream
The challenges are many, and they occur when dealing with natural gas, synthesis gas, air and latterly, the challenge associated with CO2 abatement. Different settings, seemingly different challenges, but for the chemical engineer there is a common denominator as shall become clear by the end of this book.
No matter what the application is, and no matter what the treatment needs are, cost effective solutions are always targeted. Having said that, it must be remembered that operational costs and any lost production are also factors included in this equation. There is always competition and the operator with the best profit margin will be better off in the longer term.
Natural gas is the gas produced from hydrocarbon reservoirs. Some fields are gas fields producing nothing but natural gas, but natural gas is also produced as so-called associated gas where the gas comes from the reservoir along with the oil. The composition of natural gas varies, but is dominated by the presence of methane. It may be contaminated by CO2 and H2S, and there may be more or less of ethane and heavier hydrocarbons.
Most natural gas is transported to its point of use by pipeline, but there are markets that are too far away from the natural gas source. Japan is a case, and is served by liquefied natural gas, LNG, that is shipped in on gas tankers. LNG is mostly methane, it is made to be liquid at atmospheric pressure and requires a temperature down towards 111 K. The low temperature requires that higher boiling components must be removed in order not to precipitate, and water, CO2 and H2S must naturally be removed in order not to freeze out in the condensation process and thus block the flow channels.
Natural gas liquids, or NGLs, is a term that is used to describe the hydrocarbon condensate separated from natural gas on cooling. It is essentially ethane and heavier. In the case of NGL there is no particular refinement of the product such that there can be a tail of heavier hydrocarbons. This is different from liquefied petroleum gas, LPG, which is a tailored product that is mainly ethane and propane and may also contain a little butane, but nothing heavier.
Natural gas may also be referred to as lean or rich. A rich gas implies that there are significant amounts of ethane and heavier components that may be recovered for extra value. If a gas is lean, no such condensate would be economical to recover and the gas is sold for fuel.
Next there is synthesised gas, often referred to as syngas. This is gas that has been synthetically manufactured. Often natural gas has been the raw material, but it could also be produced as part of the activity in an oil refinery although this is more likely referred to as refinery gas. Ammonia production involves the making of syngas. Here natural gas is heated in the presence of steam and methane is converted to hydrogen, carbon monoxide and carbon dioxide. This gas is further processed with steam to convert monoxide to hydrogen and CO2 and so on.
Flue gas or exhaust gas is the waste stream coming off a power plant. Offgases from syngas plants are usually referred to as bleeds or waste stream. The flue is usually the chimney, or at least the exhaust channel.
In the natural gas industry the Wobbe number is sometimes used. (Geoffredo Wobbe was an Italian Physicist who experimented with combustion of gases.) It is a way of judging if two fuel gases may be interchanged without affecting the performance of the burner. This number is defined as
This sounds simple enough, but specific gravity (note: not ‘specific weight’) is a ratio. For a gas it is the ratio of the density of the gas and that of air. The gas is usually at atmospheric conditions as is the case for the reference air. The related temperatures and pressures must be defined, and in the case of air its water content is also important for its density. Specific gravity is dimensionless. The upper heating value is used but the lower may also be specified. The units should in any case be given, but it has been practised not to do this in order not to get it confused with the gas' volumetric heating value. It is practised to quote the Wobbe number in Btu/ft3, but in Europe it is more common to use MJ/Nm3. Common values of the Wobbe number are 39–45 MJ/Nm3. It is heavily influenced by the gas' content of nitrogen and C2+. If it is specified in a gas sales contract, it is important to understand the implications. Further discussions on this subject may be found in a couple of documents issued by the American Gas Association (Ennis, Botros and Engler, 2009; Halchuk-Harrington and Wilson, 2007).
The natural gas market world-wide is huge. Although there is a need to provide a standardised gas such that all the end users' gas burners will function as intended, there are regional differences in specifications. The US market has this challenge that makes the interchangeability of gases difficult, and the cost and feasibility of standardising has been considered but discarded. In the UK, however, a similar conversion was done area by area in the 1960s and 1970s as the market was converted from ‘town gas’ to ‘North Sea gas’. (Town gas was synthesised by gasification of coal.) Town gas was common in Europe until the advent of gas finds in the North Sea. Pipelines from these and Russian fields serves this market today. North America has had a change of fortune in recent years by technology enabling the production of so-called shale gas. There have also been LNG projects developed, with more coming on stream in the next few years. Gas is challenged by other forms of energy. Although existing users are to an extent ‘sitting ducks’ due to investments made, provision costs of gas must be kept in check to keep its market share. Electricity is the immediate competitor in the retail market, and that in turn could be provided through the combustion of gas, coal or oil, and other sources are nuclear power plants and hydroelectricity. The more alternatives that are available in any one market, the more the focus on provision cost of energy in the market. Deeper discussions of these issues may be found elsewhere (BP, 2011; IGU, 2013a,b; Natural Gas Supply Association, 2005).
Specifications of natural gas as a product is a very interesting topic in many ways and the specifications really determine what treatment a gas eventually needs. There are two dimensions to this. One is the transport system that supplies a market and what treatment the gas needs to uphold flow assurance in the supply chain. The other is the end market with its appliances where gas burners have been fitted with certain gas properties in mind. Interchangeability of gas cannot be taken for granted. There are many stumbling blocks to this (IGU, 2011).
Methane, or natural gas, is less reactive than their heavier analogues like ethane, propane and so on. As feedstock for making hydrogen as in the ammonia process it is the preferred starting point as the ratio of hydrogen to carbon is highest in methane. For this reason, and because of the pricing, natural gas is the feedstock of choice for this purpose.
The C2+ fraction of the natural gas has in the main a higher market value as feedstock than as fuel. Hence the opportunity to separate these components from the gas is often taken. The economics of this has varied over time though.
For various assessments it is valuable to have a feel for sizes of plants and associated variables. The question being, what is big, what is small, what is a challenge and what is trivial. Plant sizes and complexities will vary widely. Perhaps the simplest gas treating plant to be encountered in this context will the end-of-pipe solution scrubber where some contaminant is to be removed from an effluent gas stream before being released. Maybe this scrubber has a packing height of 3 m and a diameter of 2 m, and furthermore when the absorbent has done its job, it may be returned to the process without further ado. A 400 MW CCGT (Combined Cycle Gas Turbine) power plant that needs CO2 abatement will have a gas stream in the order of 1.8 million m3/h, and the absorber would have a diameter around 17 m if there is one train only.
A large synthesis gas train may have a gas flow in the order of 10 000 kmol/h. This would be 224 000 Nm3/h. However, the pressure could be around 25 bar if this was an ammonia plant, and this would imply a real gas stream in the order of 10 000 m3/h.
In natural gas treating there is a wide range of plants. A fairly small one might be 10 MMSCFD. This is a typical way of specifying plant size in North America. MM stands for ‘mille-mille’, which is Latin inspired, meaning 1000 × 1000 (or a million). SCF is Standard Cubic Feet, and D implies per 24 hours (a Day). In North America ‘Standard’ means the gas volume is at 60°F and an absolute pressure of 14.696 psi (psi = pounds per square inch). Wikipedia points out that the ‘standard’ pressure may also be 14.73 psi, which is based on a pressure of 30 in. of a mercury column. Beware; if you are buying gas the difference in what you get is 0.23%, which is not to be given away easily in negotiations.
A large gas plant could be in the region of 2 million Sm3/day. This is typical of a gas field in the North Sea. This is in metric units, and the ‘standard’ now implies 15°C and 1.013 bar. If this was indeed the gas's temperature and pressure it would be at its ‘standard conditions.’ Note that 15°C and 60°F are not identical. European and American standard conditions are not equal: something to be kept in mind when selling and buying.
An often used specification for H2S allowed in natural gas is 0.25 grain per 100 SCF. This is a US term. One ‘grain’ is 1/7000th of a pound (lb).
LNG plants are usually referred to in million tonnes of LNG per year. A plant of 3 million tonnes per year was considered big less than 10 years ago, but one-train capacities have been stretched to 5–7 and there is a new generation of plants with a third refrigeration loop that could take the capacity to 10 million or more.
A large ammonia plant today would typically be 2000 tonnes per day. This is almost the double of what was usual around 1970. Cryogenic air separation units (ASU) could be as big as 3500 tonne of oxygen per day, but this size of plant is rare. Traditionally they have been built to provide oxygen for steel works. However, they figure in present day studies on oxy-fuel plants. That is, power plants where hydrocarbons, or coal more likely, is combusted with oxygen to make the CO2 resulting more easily accessible for capture and storage.
It is good to develop an intuitive sense for plant sizes and put them into perspective. The ability to distinguish between the various ‘standard’ units of gas quantity is a must. To help in this direction and to summarise the earlier discussion of plant sizes, Table 1.6 is provided at the end of the example problems.
There are a number of units being used in the industry that are not intuitive and will be unfamiliar to newcomers. To fill in the void, this section will go through a number of such units. The reader will undoubtedly come across further units before finishing and these will need to be deciphered using reference works.
Let us start with the measurement of liquid. For most purposes a chemical engineer could use m3 for volume and be done. However, the oil and gas business has a few special quirks when it comes to volumetric units, and oil, in North America in particular, is reported in barrels. Barrels are part of an old system of volumetric units where the sizes have changed over the centuries, and they have also differed between businesses. Today, a barrel as used in the oil industry is 158.9873 l. This is supposed to represent exactly 42 US gal. The reader will no doubt have come across various non-metric units of volume in non-professional context, and Table 1.1 is included to put these volumes into perspective. Oil density varies significantly and there will typically be 6–8 barrels per tonne.
Table 1.1Imperial volumetric relations. (from 1824 onwards in the brewery business.)
Gas volumes are straightforward in the sense that either metric or well-defined Anglo–American measures are used. The important part here is to be able to distinguish between the various ‘standard’ or ‘normal’ conditions used. These must be defined in any gas sales contract to avoid legal disputes later. This is discussed previously to the necessary extent. However, the reader may well meet further definitions in the future since IUPAC changed their recommendation for standard pressure to 1 bar (100 kPa) in 1982. In other fields of gas processing so-called ‘normal’ conditions are also in use. These are defined as 0°C and 1 atm = 1.01325 bar.
It is prudent to mention that absolute temperatures in the ‘Fahrenheit spirit’ is known as °R (degrees Rankine) and
A final topic worth mentioning is pressure units. They are mostly self-explanatory, but there is a unit called ‘atmosphere.’ If this is spelt ‘atm’, it is ‘one’ when the pressure is 760 mm Hg or 1.013 bar. However, if it is merely spelt ‘at’, we are talking about a ‘technical atmosphere’ (which is an old European tradition). This equals ‘one’ when the pressure is 1 kp/cm2. (kp, or kilopond, is the same as kgf). It is not often used these days, but it may still be found. Varieties are ato (gauge pressure, o = ‘overpressure’) and atü (gauge pressure, German: überdruck). It is slightly higher than 736 mm Hg. The term mm Hg as a pressure unit should strictly be the height of a mercury (Hg) column at 0°C; that is, that mercury has the density it has at 0°C.
To round off this discussion of units a table of conversion factors is provided to enable quick conversion of data discussed in the text to make life easier for those that do not have their reference values in metric units (Table 1.2).
Table 1.2Unit conversion factors.
From unit to unit
From unit to unit: divide by the same number
ft to m
m to ft
lb to kg
kg to lb
lb mol to kmol
kmol to lb mol
°F to °C
Subtract 32, then × 1.8
°C to °F
bar to psi (lb
Psi to bar
1 mm to micron (μ)
micron to mm
kJ to Btu
kJ to kWh
kJ to kWh
Btu/lb to kJ/kg
kJ/kg to Btu/lb
·F·h to W/m
·K to Btu/ft
hp (metric) kW
kW to (metric) hp
bhp to kW
kW to bhp
Imp gallon to m
to imp gallon
US gallon (g) to m3
to US gallon
gpm (per minute) to m
/s to US gpm
a Implies exact conversion factor, otherwise derived.
b This is based on the thermochemical value of BTU, but other definitions range from this value to as high as 1.05987 kJ.
It is also worth mentioning that the unit ton is not necessarily unambiguous. In the metric world the ton is 1000 kg while in the Anglo–American units it is 2240 lbs. Often the metric ton is referred to by ‘tonne’, but if important, this should be verified on a case to case basis. The world of tons is summarised in Table 1.3.
Table 1.3Tons, long tons, short tons and tons, and so on.
Type of ton
1 metric ton (tonne)
1000 kg (approximately 2205 lb)
1 Anglo–American ton (ton, sometimes long ton))
2240 lb (approximately 1016 kg)
1 short ton (in USA and Canada often referred to as ‘ton’)
2000 lb (approximately 907.2 kg)
In the air gases industry it is common to talk about plant capacities in tons per day. Clearly it is essential to know which tons are quoted. Ammonia plants are commonly described in tons per day and LNG plant in million tons per year. It is quick to find yourself short-changed.
A very useful summary of conversions between gas volumes and mole contents are given in Table 1.5. Note that the ‘normal m3’ is defined at the ‘normal conditions’, at NTP (normal temperature and pressure). The ‘standard m3’ are at STP (standard temperature and pressure). The standard temperature is not the same in Europe and the US.
Another useful compilation is a collection of different values of the gas constant R. These will come in useful as the situation arises.
Plants have been built in all sorts of places. Some are hot, some are cold and some are to be found at a high altitude where the air is thin. When comparing plant costs and efficiencies, this must be kept in mind. An LNG plant will of course have a better efficiency if the heat sink is at 5°C compared 35°C. On the other hand winterisation may be costly. Special precautions must be made if it is to be operated for weeks on end at −40°C.
The objective of this book is to give the reader a general background for the world of gas processing. It is also a target to provide specialised teaching with respect to the absorption-desorption process in general, and to mass transfer coupled with chemical reaction in particular.
Some topics in this book are treated cursorily and the only justification for including these chapters is to create a starting point for the reader to dive further into those topics. The book that gives specialist in-depth treatment of all you need to know is still to be written.
Throughout this book we shall need relevant case studies to illustrate the use of the tools and theories developed. The development of these case studies starts here, and they will be based on the problems outlined when discussing typical plant sizes. To a degree reverse engineering will be applied to extract the problems relevant for discussion in this book. Immediate question: Which is the bigger gas processing plant of the following: Flue gas from a 400 MW CCGT, 600 MW coal power, 2000 tonnes per day ammonia plant, 30 MMSCFD natural gas, 3 million Sm3 per day natural gas plant, or a 7 million tonnes per year LNG plant? After working the example problems, you will know. When gas concentrations are given, they are on a molar (or volumetric) basis unless otherwise specifically stated. Ideal gas is assumed throughout these examples.
A good and well-defined example is an ammonia plant. Here the synthetic gas is eventually converted to ammonia (NH3). Such a plant is in most situations fed by natural gas at pressure. The gas needs to be treated for sulfur compounds to avoid poisoning of catalysts before processing can proceed. This is followed by ‘reforming’ the natural gas to H2 and CO in the first sections of the plant before the CO is converted to H2 by the help of steam.
Thereafter the CO2 must be removed. At this stage we ask ourselves, how much gas must be treated if the ammonia plant has a capacity of 2000 tonnes per day. The eventual reaction is:
Now, 2000 tonnes per day work out at:
and since the molecular weight of ammonia is 17, it follows that the ammonia production is:
Next it is observed that in the ammonia synthesis reaction 0.5 + 1.5 = 2 mol of N2 and H2 gas are converted to 1.0 mol of ammonia. Hence, the net stream of treated gas after CO2 removal will be:
Let us assume that the conversion efficiency is 99% and call that 9804/0.99 = 9903 kmol/h.
It may be worked out from analysis of the ammonia train from the start, but we shall take it as read that the CO2 content of the gas prior to CO2 removal is 20% (mol) with a gas pressure of 25 bar and a temperature of 40°C. On this basis the feed to the CO2 removal unit is:
CO2 to be removed is thus: 12 378 − 9903 = 2475 kmol/h.
In this industry it is also quite common to quote flows in Nm3/h and that works out at:
With the temperature and pressure given, this means that the actual flow of gas at operating conditions is:
Characterising a natural gas treatment plant as small or large is not an exact science. The following example could, for what it is worth, be described as mid-range. A plant is needed to process a stream of 30 MMSCFD.
Using the conversion factor available from Table 1.4, this stream becomes:
This is turn is:
With temperature and pressure given as 40°C and 35 bar, the actual gas flow at operating conditions are:
If 8% of this feed is CO2, then there are (0.08)(4902) = 392 kmol CO2/h in the feed.
Table 1.4Various often quoted volumes of gas of given mole mass.
Table 1.5Values of the gas constant, R. (psi is lbf/square in.)
.psi/(lb mol R)
LNG plants are complex and as such their economics thrives on economics of scale. Plant sizes in excess of 10 million tons per year are possible, but we shall look at the implications of a 7 Mton/year capacity.
We shall assume that this capacity is reached by being on-stream for 8600 hours per year. Furthermore, it will be assumed that the average molecular weight of the LNG is 17. Capacity may then be rated as:
If there is 12% CO2 in the feed, its CO2 removal plant will receive:
Given a temperature of 40°C and a pressure of 50 bar, this implies a flow at operating conditions equal to:
The abbreviation CCGT stands for combined cycle gas turbine (power plant). These plants are often described in terms of CO2 emission, but we shall approach this from its power rating. A state of the art CCGT will be as big as 440 MW rated power output, and its power efficiency is 58% or more. In this plant gas is burnt under pressure, expanded in the gas turbine and the heat in the hot exhaust is recovered to make steam that is in turn used in steam turbines to boost energy efficiency.
We shall assume a fuel gas feed of 83% (mol) CH4, 9% C2H6, 4% C3H8, 1% C4H10, 2.5% CO2 and 0.5% N2. There is also expected to be 3 ppm of H2S, but this is neglected for the present considerations. Based on heat of combustion data from Perry and Green (1984), the average upper heat of combustion for this gas is 997.06 kJ/mol. The ‘upper’ value is used since the power process is expected to use a condensing steam turbine at the end. Average molecular weight is estimated to be 18.84. The need for fuel gas is accordingly:
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