fundamentos del gas natural

FUNDAMENTALS OF NATURAL GAS

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Module Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

SECTION 1 - THE ORGANIC THEORY OF NATURAL GAS FORMATIONIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3The Organic Theory of Oil & Gas Formation . . . . . . . . . . . . . . . . . . . . . . 4Oil & Gas Deposits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Oil & Gas Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Oil, Gas & Water Locations in a Reservoir . . . . . . . . . . . . . . . . . . . . . . . . 7History of the Development & Utilization of Natural Gas. . . . . . . . . . . . . 8Review 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

SECTION 2 - IDEAL GAS BEHAVIOURIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13States of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Ideal Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Effect of Pressure on Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Effect of Temperature on Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Ideal Gas Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Review 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

SECTION 3 - PROPERTIES & CHARACTERISTICS OF NATURAL GASIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Natural Gas Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Heating Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Ignition Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Molecular Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Review 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

SECTION 4 - BEHAVIOUR OF REAL GASESIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Dalton’s Law of Partial Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Critical Temperature & Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Pseudo-Critical Properties of Gas Mixtures. . . . . . . . . . . . . . . . . . . . . . . . 37Compressibility Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Review 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47ANSWERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

PLEASE NOTEOperations personnel use a combination of skill, knowledge, and technology to accomplish specific goals. A key objective of the GasController Training Program is to promote an understanding of theoreticalbasis for operational decisions used on the job every day. This trainingprogram enhances job-related skills by providing relevant and currentinformation with immediate application for employees.

Information contained in the modules is theoretical. A foundation ofbasic information facilitates an understanding of technology and itsapplication. Every effort has been made to reflect pure scientific principlesin the training program. Nevertheless, in some cases, pure theoryconflicts with the practical realities of daily operations. Usefulness to theemployee is our most important priority during the development of thematerials in the Gas Controller Training Program.

FUNDAMENTALS OF NATURAL GASGas Controller Training Program

© 1998 ENBRIDGE TECHNOLOGY INC.Reproduction Prohibited

ENBRIDGE TECHNOLOGY INC.Suite 601, PO Box 39810201 Jasper AvenueEdmonton, AlbertaCanada T5J 2J9

Telephone +1-780-412-6469Fax +1-780-412-6460

Reference - Gas 1.1 Fund. of Nat. Gas - Jan 2003

STUDY SKILLSEach of the modules in the Gas Controller Training Program is designed in aperformance based self-instructional format. This means that you are responsiblefor your own learning and for ensuring that you are ready to demonstrate yourknowledge and skills. Our focus is on the performance of the necessary skillsand demonstration of the knowledge needed to perform your job.

1. The modules are designed for short but concentrated periods of study fromten to forty-five minutes each. Remember that generally one week of self-study replaces 10 hours of in-class attendance. For example, if you have athree week self-study block, then you have to account for 30 hours of studytime if you want to keep pace with most learning programs.

2. When you are studying the module, look for connections between the infor-mation presented and your responsibilities on the job. The more connectionsyou can make, the better you will be able to recall.

3. There are self-tests at the end of each section in the module. Habituallycompleting these tests will ensure your knowledge of the information. Usethe test to measure your understanding. If you have an incorrect answer,check the information in the section of the module to find out why the errorwas made. Remember, you are responsible for your own performance.

4. Start studying each section of the module by reading the objectives and theintroduction. This provides both the focus for your learning and a preview ofthe test items.

5. Each module is prepared to adapt to a number of different learning styles.Some learners will proceed directly from the introduction and objectives tothe review questions. Then they will study any topic that is missed. Mostlearners, however, work from the introduction through to the end of the textin a systematic way. Whichever way you choose to learn, you are free to usethe materials as you see fit.

6. Every module has a performance based test. Each item in the test is related toan objective for each section. To prepare for the test, you should ensure thatall section reviews are completed and understood. Many learners review thematerial in the module before taking the test.

7. To aid your understanding and enhance your time in the learning activities,new terms, concepts and principles are printed in bold face along with theirdefinition highlighted in italics. These are also listed in the Glossary suppliedat the end of the module.

8. Many learners have had success by reading the module Summary andGlossary. Items in the Glossary are cross-referenced to the place in themodule where they were first introduced. This way, if there is a topic or adefinition that you do not recognize, you can easily find it in the module.

This module focuses on the fundamental concepts and basic lawsassociated with the formation and behaviour of natural gas. Thecontent of this module is presented in a general context that relates toall hydrocarbons and in a specific context that focuses on the uniqueproperties, characteristics, and behaviour of natural gas.

This module provides information on the following goals.• It describes the formation of oil and gas.• It describes the historical development and utilization of natural

gas.• It describes different types of oil and gas deposits.• It describes the properties and characteristics of natural gas.• It explains the effects of pressure and temperature on gas volume.• It explains the behaviour of real gases in mixtures.

None

1


INTRODUCTION

MODULE GOALS

PREREQUISITES


3

SECTION 1

The first section of this module focuses on the formation ofhydrocarbons. The origin of natural gas is explored in the context ofa commonly accepted organic formation theory. A brief chronologyof the development and utilization of natural gas is provided.

This section also includes a discussion of the following topics as theyrelate to natural gas: the hydrocarbon molecule, hydrocarbon depositsand the behaviour of oil, gas and water in a sedimentary reservoir.

After completing this section of the module, you will be able to:• Recognize the organic sources in the theory of oil and gas

formation.• Identify the components and structure of hydrocarbon molecules.• Recognize the migrating behaviour of hydrocarbon deposits.• Differentiate between the types of oil and gas traps.• Relate the locations of oil, gas and water in a reservoir to the

density of the products.• Chronologically order events in the development and utilization of

natural gas.

THE ORGANIC THEORY OFNATURAL GAS FORMATION

INTRODUCTION

OBJECTIVES

GAS CONTROLLER TRAINING PROGRAM

Over the years, many theories have evolved regarding the formationof oil and gas. The most commonly accepted theory is referred to asThe Organic Theory of Oil and Gas Formation.

This theory suggests that millions of years ago, organic plant andanimal material was buried under layers of sand and silt at the bottomof vast primordial oceans. As time passed, the layers of sand and siltchanged into solid, sedimentary rock. The organic material, trappedbetween the layers of sedimentary rock, was gradually transformedinto petroleum by the immense pressure of the ocean and the effectsof geothermal heat.

1. In the primordial oceans, organisms died and fell to the bottom ofthe ocean.

2. At the bottom of the ocean, the remains of the dead organismswere covered up by layers of sand and silt.

3. Over the course of millions of years, the immense pressure fromthe ocean and the effects of geothermal heat converted the organicmaterial into petroleum.

Figure 1Organic Theory of Oil and Gas Formation

4

THE ORGANICTHEORY OF OIL & GAS FORMATION

GasOil

Water

3

1

2

Hydrogen and carbon are the major constituents of all fossil fuels andpetrochemicals. For this reason, fossil fuels such as oil and gas arecommonly referred to as hydrocarbons.

The liquid state of a hydrocarbon is oil. The gaseous state of ahydrocarbon is natural gas. The solid form of a hydrocarbon isasphalt.

All hydrocarbon molecules are made up of various proportions/ratiosof carbon and hydrogen atoms. Some examples of carbon and

hydrogen atom combinations are shown below.

Figure 2Methane MoleculeMethane (CH4) is a simple molecule consisting of one

carbon atom bonded to four hydrogen atoms.

Figure 3Ethane MoleculeEthane (C2H6) is a molecule made up of twocarbon atoms bonded to six hydrogen atoms.

Figure 4 Propane MoleculePropane (C3H8) is a molecule madeup of three carbon atoms bonded toeight hydrogen atoms.

5


HYDROCARBONMOLECULES

6

Finding oil and gas deposits is not just a matter of finding a bed ofsedimentary rock and drilling a well. Rather, it involves painstakingeffort in analysis and speculative drilling.

Pressure under the earth's surface often causes petroleumhydrocarbon molecules to migrate through pores in the source rock,which is the rock where the organic material was originally trapped.

In some cases, gaseous hydrocarbon molecules are able to escapethrough openings to the surface. When this occurs, the gas moleculesdisperse into the atmosphere and the lighter components of the oilmolecules slowly evaporate. Eventually, a tar deposit known asbitumen is left behind as the viscous liquid cools and solidifies. Thearea known as the "Tar Sands" near Fort McMurray, Canada, is anexample of a bitumen deposit.

Most petroleum hydrocarbons, however, do not migrate all the way tothe surface. More commonly, they migrate through the permeable rocktowards the surface until their path is blocked by an impermeablebarrier. Impermeable barriers that prevent petroleum hydrocarbonmolecules from migrating to the surface are referred to as caprocks.

Hydrocarbons accumulate in the pore spaces of the permeable rockbelow the caprock, and eventually form a reservoir. The hydrocarbontrap is made up of the caprock and resulting reservoir.

There are several types of oil and gas traps:An anticlinal trap is one of the most common traps. It occurs whenthe sedimentary rock layers curve upward. Petroleum hydrocarbonsare able to migrate to the upper part of the formation but areprevented from going further by an impermeable barrier.

A fault trap occurs when a fault in the earth's surface shifts verticallyallowing petroleum hydrocarbons to migrate to a reservoir at the fault.

A stratigraphic trap occurs when the porosity of the rock is greaterin one area than in the surrounding areas, allowing the gas to flowinto the more porous areas.


OIL & GASDEPOSITS

OIL & GAS TRAPS

A salt dome trap occurs when a large amount of molten salt, fromunder the earth's crust, pushes its way toward the surface, creatingbends in the sedimentary rock layers in which petroleum hydrocarbonscan collect if there is an impermeable barrier.

All oil and gas reservoirs must have an impermeable barrier in theform of a caprock that stops oil and gas from migrating to the surface.

As mentioned, migration ceases when petroleum hydrocarbonmolecules are trapped under an impermeable barrier. Trappedmolecules arrange themselves in a specific way within the reservoir.The gaseous hydrocarbon components rise to the top of the reservoir;the liquid hydrocarbon components settle into the middle region of thereservoir; and the water that was mixed with the hydrocarbons settlesto the bottom of the reservoir.

This separation pattern reflects the relative densities of gas, oil andwater. Gaseous hydrocarbons have the lowest density so they rise tothe top. Liquid hydrocarbons have lower density than the water so theoil floats on the water. Separation, however, is not absolute since eachlayer contains some of the ingredients found in the other layers.

Components in the mixture differ in volatility, which is the tendencyof a hydrocarbon mixture to vaporize. The molecules with the leastamount of carbon (those with one, two or three atoms of carbon) havea greater tendency to vaporize. The lower the carbon content of amolecule, the greater its volatility.

Figure 5Distribution of Hydrocarbons in a Reservoir Formation

7


OIL, GAS &WATERLOCATIONS IN ARESERVOIR

MethaneEthane

PropaneHeavy Hydrocarbons (Oil)

HHCH

H

HHCH

HCH

H

HHCH

HCH

HCH

H

Caprock

Water

Since earliest recorded history, there have been accounts of crude oiland gas escaping from the ground. Oil-soaked earth was used tocaulk boats and buildings. It was used to grease wheels or dresswounds, and in places such as the Middle East it was burned as fuel.

Figure 6An Early Oil Lamp

Natural gas fed the perpetual fires of Delphi in Greece, Baku on theCaspian Sea, and many other mystical sites in the ancient world. Inthe third century, the Chinese used bamboo pipes to transport anddistribute gas to light their temples and to crystallize salt from brine.

In 1609, the Flemish chemist, Jean van Helmot invented coal gas.Coal and steam were heated in a closed vessel producing a mixture ofmethane and carbon monoxide. For many years, this manufacturedmethane gas was used to fuel gas lamps.

The supply of manufactured coal gas soon became inadequate,however, as the demand for residential, commercial, and industrialgas increased. Fortunately, discoveries of oil and gas increased in the18th and 19th centuries, as people began to dig deeper wells in searchof water. As a result, new sources of natural gas became available,which could be used to supply the increasing demand.

Figure 7Gas Lamp Used for Street Lighting

8


HISTORY OF THEDEVELOPMENT &

UTILIZATION OFNATURAL GAS

For most of the 1800's, natural gas was used exclusively to light citystreets. Electricity, however, soon became the preferred source oflight energy, and natural gas usage waned. In 1885, Robert Bunseninvented a burner that mixed air with natural gas. This burnerillustrated how gas could be used for cooking and heating purposes.

The construction of long natural gas pipelines didn't begin in earnestuntil after World War II. During the war period, improvements inmetals, welding techniques and pipe making, made pipelineconstruction practical and economically viable.

Once transportation pipelines and distribution networks becameestablished, industry began to use gas in manufacturing plants and forgenerating electricity. For industry, natural gas was an efficient fuelto be used to heat their boilers. For domestic customers, gas becamethe preferred fuel for cooking and heating homes.

The United States has a lot of natural gas, enough to last for at leastanother 60 years and probably a lot longer. Canada also has a lot ofgas.

Figure 8Where Natural Gas is FoundThese are the areas of the United States and Canada where natural gasformations are found.

9


Major Significant

10

Figure 9Regional Distribution of Global Gas Reserves

North America is looking for more ways to use gas, largely becauseit is easy to pipe from one location to another and because it burnsvery cleanly. More and more, gas is being used in power plants togenerate electricity. Factories are using more gas, both as a fuel andas an ingredient for a variety of chemicals.

While natural gas is plentiful, there is still some uncertainty abouthow much it will cost to get it out of the ground in the future. Likeoil, there is "easy" gas that can be produced from undergroundformations, and there is gas that is not so easy. If we can find betterand cheaper ways to find more of the "easy" gas and produce someof the more difficult gas, we can rely increasingly on natural gas inthe future.


Eastern Europe &Russia40.5%

Middle East32.7%

Western Europe - 3.3%Central & South America - 4.2%

North America 4.2%

Far East & Oceania6.5%

Africa6.7%

1. A hydrocarbon trap refers to a(n) __________ .

a) caprock and resulting impermeable barrier b) reservoir and resulting sedimentary basin c) impermeable barrier and associated sedimentary rockd) caprock and resulting reservoir

2. Hydrocarbon molecules are made up of combinations of_________ .a) methane, ethane and propane moleculesb) hydrogen and carbon atomsc) various gases and liquids d) migrating molecules of air, water and organic matter

3. A propane molecule is made up of __________ .a) one carbon atom bonded to four hydrogen atomsb) two carbon atoms bonded to six hydrogen atomsc) seven carbon atoms bonded to three hydrogen atomsd) three carbon atoms bonded to eight hydrogen atoms

4. In 1609, coal and steam were heated together in a closedvessel to form __________ .a) carbon dioxide and oxygenb) methane and carbon monoxidec) propane and carbon dioxided) butane and hydrogen

5. Oil floats on water because the density of oil is ________ . a) lower than the density of waterb) equal to the density of waterc) higher than the density of waterd) lower than the density of gas

11


REVIEW 1

12

6. The migration of hydrocarbon molecules is a result of __________ and is generally caused by __________ .a) atmospheric pressure, porosity of the caprockb) pressure on the reservoir, porosity of the source rockc) the internal pressure of the hydrocarbon molecule,

impermeability of the caprockd) immense pressure of the ocean above, the impermeability of

the source rock

7. The rock where organic material was originally trapped isreferred to as __________ rock.a) faultb) domec) sourced) organic

8. In an oil and gas reservoir, liquid hydrocarbon componentsmove to the __________ of the reservoir.a) top regionb) bottom regionc) top and middle regionsd) middle region

Answers are at the end of this module.


13


SECTION 2

The second section of this module provides a brief explanation of thestates of matter and explains the relevance of these states to the studyof gases. The main focus of this section is the explanation of theeffects of pressure and temperature on volume. More specifically, itfocuses on the effects of pressure and temperature on ideal gasvolume.

In this section we will study three scientific laws that illustrate theeffects of pressure and temperature on volume.• Boyle's Law expresses the effect of pressure on volume.• Charles' Law expresses the effect of temperature on volume.• The Ideal Gas Law describes how the volume occupied by an ideal

gas changes when the pressure and temperature both change.

After completing this section, you will be able to complete thefollowing objectives:• Recognize the three states of matter.• Identify the basic properties of an ideal gas.• Apply Boyle's Law expressing the effect of pressure on volume.• Apply Charles' Law expressing the effect of temperature on

volume.• Apply the Ideal Gas Law as a response to changes in pressure and

temperature.

IDEAL GAS BEHAVIOUR

INTRODUCTION

OBJECTIVES

14


All matter is made up of molecules. All molecules are made up ofatoms. All atoms are made up of tiny particles called electrons,protons, and neutrons.

Each electron in an atom has a negative electrical charge. Each protonhas a positive electrical charge. Neutrons do not have an electricalcharge.

Protons and neutrons are grouped together in a nucleus at the centerof an atom. Electrons are arranged in orbits around the nucleus. Thenumber of electrons in an atom's outermost orbit determines themanner in which the atom will combine with others to formmolecules. Molecules, made up of different combinations of atoms,form the variety of substances found in our world.

All matter exists in one of three states - solids, liquids or gases. Thedifference between the three states of matter can be easily understoodif we visualize the molecules as "ball-bearings".

Solids, the first state presented, have both definite shape and volume.

Figure 10SolidsThe molecules of a solid, shown as ball-bearings in a box,are tightly packed.

The next state of matter is liquids. Liquids have fixed volume but noshape of their own and conform to the shapes of their containers.

Figure 11LiquidsIn a liquid, the molecules are still in contact but move aroundeach other freely.

Gases, which have neither definite shape nor definite volume, assumethe shapes and volumes of their closed containers.

Figure 12GasIn a gas, the molecules are far away from each other andcan move freely through space.

Solid

Liquid

STATES OFMATTER

An ideal gas, sometimes referred to as a perfect gas, is a gas in whichno molecular forces exist. To accurately determine the volume of anideal gas, temperature and pressure must be known.

The behaviour of ideal gases must be kept in mind as we look moreclosely at the effects of pressure and temperature on volume.

These are some of the basic properties of gases.• Gases are affected by changes in temperature and pressure because

of their ability to expand and contract with ease.• If unconfined, gases expand when heated and contract when cooled.• Limited only by the size of their container, gases expand to occupy

the space that they are in.• Gases can be easily compressed into a smaller volume.• Pure gases, which are single component gases, are generally

treated as ideal gases at low pressures (<700 kPa).

Pressure is defined as a force acting over an area. If the pressureexerted on a substance is increased, the molecules of the substance arepressed more closely together. At higher altitudes, atmosphericpressure decreases because the pressure of the earth’s atmosphere islower than it is at sea level.

Pressure is expressed in kilopascals (kPa) or megapascals (MPa). Thetwo ways of stating pressures are:• absolute pressure• gauge pressureAbsolute Pressure (Pabs) is the pressure relative to a perfect vacuum(absolute zero). When atmospheric pressure is discussed, it is alwaysexpressed as an absolute pressure.

Gauge Pressure (Pgauge) is pressure relative to ambient atmosphericpressure. If a vessel has a gauge pressure of 1000 kPa, it means thatthe difference in pressure between the gas in the vessel and thesurrounding atmosphere is 1000 kPa. Most pressure readings used inthe gas industry are gauge pressures expressed as kPa gauge.

15


IDEAL GAS

BASIC PROPERTIES OF GASES

EFFECT OFPRESSURE ONVOLUME

16

There are three additional pressure terms that are commonly used:atmospheric pressure, standard pressure, and vacuum pressure.

Atmospheric Pressure (Patm) is the force exerted on the earth'ssurface by the atmosphere. Atmospheric pressure varies depending onelevation and weather conditions.

Standard Pressure is a standard reference pressure based on theaverage atmospheric pressure at sea level. This pressure is expressedas 101.325 kPa, or one atmosphere (1 atm).

Vacuum (or Negative) Pressure, is any pressure below the localatmospheric pressure.

Figure 13Terms Used in Pressure MeasurementThe figure shows a full range of pressures and how they relate to each other.At the bottom of the scale is absolute zero or a complete vacuum. Halfwayup the scale is the local atmospheric pressure (Patm). Values less than Patmare shown as vacuum or negative pressures. Pressures greater than Patmare positive pressures and can be measured as Pgauge. For the completion ofengineering calculations, Patm and Pgauge are usually converted to absolutepressure values (Pabs).

For all engineering calculations that require pressure as an input,absolute pressures must be used. Pressure gauges indicate only thevalue above or below atmospheric pressure. To obtain absolutepressure, add the gauge pressure and the local atmospheric pressure.If the local atmospheric pressure is not known, use the standardpressure of one atmosphere (101.325 kPa).


Local AtmosphericPressure Readings

PressurePositivePressure

Gauge Pressure

Vacuum NegativePressure

AbsolutePressure

Absolute Zero Pressure or Complete Vacuum

(Patm)(Pabs)

(Pgauge)

17


The following example demonstrates the calculation of absolutepressure. A pressure gauge on a vessel reads 1250 kPa. The localweather service reports that the atmospheric pressure is 98 kPa. Whatis the absolute pressure in the vessel?

The absolute pressure (Pabs) is determined by adding the gaugepressure (Pgauge) and the atmospheric pressure (Patm).

Pabs = Pgauge + Patm

= 1250 + 98

= 1348 kPa

The absolute pressure in the vessel is 1348 kPa.

At higher elevations/altitudes, the earth's atmosphere is thinner than itis at sea level and exerts less gravitational force. The use of absolutepressure, then, compensates for the effects of altitude.

In 1662, Robert Boyle discovered that the pressure of an ideal gas isinversely proportional to its volume at a constant temperature. Thisrelationship is known as Boyle's Law. Boyle’s Law is expressed as:

P ∝ 1 V

where: P = pressureV = volume

This means that if the pressure on a gas sample increases, the volumeof the gas sample decreases. Conversely, if the pressure on the sampledecreases, the volume of the sample increases.

An effective way of expressing Boyle's Law is to compare thepressure and volume of the gas sample at two different pressures andvolumes. The known pressure and volume information can be used tofind the missing or needed value. The following example clarifies therelationship.

A mass of an ideal gas occupies 10 m3 at an absolute pressure of 50 kPa. At the same temperature, what volume will the ideal gasoccupy if the pressure is increased to 500 kPa? (P is absolute).

BOYLE'S LAW

18


To solve the problem, use the equation for Boyle's Law.

P1 x V1 = P2 x V2

Given:

P1 = 50 kPa

V1 = 10 m3

P2 = 500 kPa

Rearrange the equation to solve for the new volume, V2, and substitute theknown values.

The mass of ideal gas will occupy a volume of 1 m3 if the pressure is increasedto 500 kPa.

Figure 14The Same Mass of Ideal Gas Occupies Less Volume at Increased Pressure

P1 x V1

P2V2 =

50 x 10

500V2 =

V2 = 1 m3

V1 = 10 m3

P2 = 500 kPa

P 1 = 50 kPa

V2 = 1 m3

The Système International (SI) method of reporting temperature isexpressed in degrees Celsius (°C). The Celsius scale uses the boilingand freezing points of water, at 1 atmosphere, as reference points. Ingas calculations, an absolute temperature scale is used. Zero, on theabsolute temperature scale (absolute zero), is the temperature at whichmolecules of all matter reach a motionless state.

The absolute temperature scale is known as the Kelvin Scale. On thisscale:

• One Kelvin (K) is the same size as a Celsius degree

• 0°C is the same temperature as 273.15 K

To convert a temperature in degrees Celsius to Kelvin, add 273.15.

K = °C + 273.15

Standard temperature is a standard reference temperature. In SI unitsthis temperature is expressed as 15°C or 288.15 K.

In 1802, J. Charles and J. Guy-Lussac discovered that the volume ofan ideal gas is directly proportional to its absolute temperature at aconstant pressure. This relationship is known as Charles' Law and isexpressed as:

V ∝ T

where: V = volumeT = temperature

For example, if the temperature of a gas sample increases and thepressure on the sample remains constant, the volume taken up by thegas will increase.

An effective expression of Charles' Law compares the pressure andtemperature of a gas sample (at constant pressure) at two differentconditions of volume and temperature.

19


V1 V2

T1 T2=

EFFECT OFTEMPERATUREON VOLUME

CHARLES' LAW

20


To make the relationship between pressure, temperature, and volumeclearer, consider a mass of natural gas which occupies 10 m3 at atemperature of 40°C. The problem is to find the volume that the gaswill occupy if the temperature is decreased to 10°C.

To solve the problem, first rearrange the formula for Charles Law asfollows:

Now, by substituting the known values, the problem can be solved.

V1 = 10 m3

T2 = 10°C or 283 KT1 = 40°C or 313 K

The gas will occupy a volume of 9.04 m3 if the temperature isdecreased to 10°C.

Figure 15Pistons and Cylinders at Constant PressureAs stated by Charles’ Law, the volume of an ideal gas is directly proportionalto its temperature.

The preceding example shows the direct proportionality of the volumeof an ideal gas to its absolute temperature at a constant pressure.

Notice that this calculation,as with all gas calculationsneeding temperature, usesthe absolute temperature (K).

The value of T2 at 10°Cbecomes 283 K and 40°Cbecomes 313 K.

V1 x T2

T1V2 =

V2 = 9.04 m3

10m3 x 283 K

313 KV2 =

GAS GAS

FrictionlessPiston

V1= 10 m3

T1= 313 K T2= 283 K

V2= 9.04 m3

21


As we learned earlier in this section, an ideal gas is a gas in which nomolecular forces exist. At pressures up to a few atmospheres, most gasesare considered ideal.

The Ideal Gas Law combines the principles of Boyle's Law, whichexpresses the effects of pressure on volume, and Charles' Law, whichexpresses the effects of temperature on volume.

The Ideal Gas Law is expressed as:

P x V = n x R x T

where: P = absolute pressure (measured in kPa)V = volume (measured in m3)n = the amount of the substance (measured in moles)R = universal gas constant (measured in KJ per kg mol K)T = temperature (measured in absolute degrees K)

A mole is the amount of a substance expressed in terms of the numberof atoms that are present in the substance. It is usually expressed interms of the kilogram mole (kg mol), which is the atomic numbermultiplied by the weight of the substance in kilograms. For example,the kg mol of methane (CH4) is 16.04 and a kg mol of propane (C3H8)is 44.

The Universal Gas Constant (R) is a constant value for a givenquantity of an ideal gas at a combination of pressure, temperature andvolume. The units of the constant combine the units of energy (KJ),temperature (K), and quantity (kg mol).

To show how the Ideal Gas Law works, we will use it to calculate thevolume occupied by 1 kg mol of an ideal gas at 1 atmosphere pressure(101.325 kPa) and 0°C (273.15 K).

In this case, the value for the Universal Gas Constant (R) is 8.314 KJ per kg mol K.

IDEAL GAS LAW

22


From the Ideal Gas Law we know:

P x V = n x R x T

Rearranging the formula to isolate the unknown value, we find:

V =n x R x T

P

Substituting values from above:

V =1 x 8.314 x 273.15

101.325

V = 22.41

The Ideal Gas law shows, therefore, that 1 kg mol of any ideal gas at1 atmosphere and 0°C occupies 22.41 m3.

Figure 16The Ideal Gas LawThe volume occupied by 1kg mol of an ideal gas changes according tovariations in temperature and pressure.

Knowing the Ideal Gas Law is fundamental in understanding thebehaviour of real gas, and real gas mixtures. These are the topics inthe next sections of the module.

Pressure 1 atm(101.325 kPa)

Volume is 22.41 m3Pressure 1 atm

Temperature 0 ºCTemperature 0 ºC

(273.15 K)

1. Absolute pressure is pressure relative to ___________ .a) a perfect vacuum b) a gaugec) ambient pressured) standard pressure

2. The force exerted by the air on the earth's surface isreferred to as ____________ .a) molecular pressureb) absolute pressurec) gauge pressured) atmospheric pressure

3. Standard pressure is expressed as ___________ .a) 101.325 kPab) 101.325 Kc) 10°Cd) 288.15 kPa

4. PV = nRT is an expression of ___________ and applies tothe effects of ___________ on volume.a) Dalton's Law, pressureb) Ideal Gas Law, pressure and temperaturec) Boyle's Law, temperatured) Charles' Law, pressure and temperature

5. The state of matter that has a definite shape and volume is ___________ .a) a solidb) a liquidc) an ideal gasd) a real gas

23


REVIEW 2

24


6. By adding 273.15 to a temperature in degrees Celsius, thetemperature is converted to a(n)______________ .a) absolute temperatureb) absolute pressurec) standard temperatured) standard pressure

7. Standard temperature has been established at _________ .a) 288.15 K or 15°Cb) 10°C or 288.15 Kc) 273.15 K or 0°Cd) 101.325 kPa or 1 atm


25


SECTION 3

This section of this module focuses on the properties andcharacteristics of natural gas, including heating value, ignitiontemperature, flammability limits, molecular weight, and composition.

After completing this section of the module, you will be able to:• Identify the properties of natural gas.• Relate the heating value of natural gas to the volume measurement.• Distinguish between the ignition temperature and the flame

temperature of natural gas.• Recognize the flammability limits of natural gas.• Identify general characteristics of natural gas in terms of molecular

weight and specific gravity.• Identify the components of natural gas.

PROPERTIES & CHARACTERISTICSOF NATURAL GAS

INTRODUCTION

OBJECTIVES

26


As distributed, natural gas is colourless, odourless, flammable, andnon-toxic. It will, however, displace air in a closed environment.Natural gas is an excellent fuel because it burns easily andcompletely and produces very little pollution.

Natural gas is treated after it comes out of the well. The compositionof the treated gas, as sold to consumers, is not the same as the gasfound at the wellhead. The three parameters used to describe aquantity of natural gas are pressure, temperature, and volume. Gas isfrequently sold based on its heating value or energy content.

The heating or energy value of natural gas is the energy releasedwhen a standard volume of gas is burned and is expressed in Joulesper cubic metre (J/m3). Because the Joule is a very small unit ofenergy, the heating value of gas is normally expressed in megajoules(one million Joules) per standard cubic metre (MJ/m3) of gas burned.

Since the breaking of the bonds between hydrogen and carbongenerates heat, the more hydrogen and carbon atoms in eachmolecule of a hydrocarbon gas, the higher the heating value of thegas. Since natural gas is a mixture of gases, its heating value dependsupon its composition. If nitrogen is present in natural gas, the heatingvalue is less because the nitrogen adds to the volume, but not to theenergy released.

Natural gas, as generally consumed, has a heating value from 35.4 to42.8 MJ/m3 (Mega or million Joules). Before processing, however,its heating value is usually higher. This is because reservoir naturalgas contains more heavy hydrocarbons with higher heating values.Once the gas has been processed and most of the heavier

hydrocarbons like propane and butanehave been extracted, the heating value isless. Methane of the primary componentsof natural gas, has a heating value of 37.6 MJ/m3.

Figure 17Heating ValuesIt would take approximately 4200 J to raise thetemperature of 1 gram of water by 1°C.

1°C change in temperature

1 gram H2O

NATURAL GASPROPERTIES

HEATING VALUE

27


The ignition temperature of a gas is the lowest temperature at whichself-sustaining combustion occurs. The ignition temperature of naturalgas is lower than pure methane because it contains otherhydrocarbons with lower ignition temperatures.

The ignition temperature is an important value to consider. At theignition temperature, the gas ignites in a suitable mixture of gas to air.This possibility of ignition has both safety and utility implications.

It is also important to distinguish between the ignition temperatureand the flame temperature. The flame temperature is the temperaturethat can be measured in a burning substance. For example, methaneignites at 705°C. However, the flame that results will quickly achievea temperature of 1918°C. This is the flame temperature of methane.

The flammability limits of a gas define the range over which it willsupport combustion.

Lower Flammability Limit (LFL) is also called the Lower ExplosiveLimit (LEL) and is the lowest concentration of a gas in air at whichsustained combustion will occur.

Upper Flammability Limit (UFL) is also called the Upper ExplosiveLimit (UEL) and is the highest concentration of a gas in air that willsupport combustion.

Figure 18Combustible Gas IndicatorThe upper scale shows the percentage of gas present in the air. The left handmargin of this scale (from 0-15) is expanded on the lower scale to show theexplosive range of the mixture. Measurements can be displayed on eitherscale, depending on the conditions.

The flammability limits for natural gas are approximately 5% - 15%,depending on composition. If the concentration of natural gas in air isbetween 5% and 15%, the gas will ignite if exposed to an ignitionsource.

0 100% gas in air

0 155 10

Explosive RangeLEL

UEL

IGNITIONTEMPERATURE

FLAMMABILITY LIMITS

28

The behaviour of a gas mixture is influenced by the composition ofthe gas mixture. Compositions are described in terms of themolecular proportions of each component in the mixture.

Because the individual molecules in a gas mixture are microscopic,mass quantities are expressed in terms of the molecular weight inmass units. These units are referred to as moles.

Molecular weight, M, refers to the mass of a molecule of a substancerelative to an atom of hydrogen.

In the previous section, we defined a kilogram mole (kg mol) of anysubstance as the quantity of that substance whose mass in kilogramsis equal to the molecular weight of the substance. To illustrate theserelationships, follow the calculations of the molecular weight ofmethane gas.

One molecule of methane (CH4) is formed from one carbon atom (M = 12.0) and four hydrogen atoms [M = 4(1.01)]. The molecule ofmethane, therefore, has a molar mass of 16.04. [12.0 + 4(1.01)].

This means that 1 kg mol of methane would weigh 16.04 kg.

In the same way, 1 kg mol of hydrogen (H2) is equivalent to 2.02 kg.

Molecular weight is central to the calculation of the specific gravityof the gas mixture.

The specific gravity of gas (SG) is defined as the ratio of the density ofthe gas to the density of dry air at the same temperature and pressure:

The density of air at standard conditions (101.325 kPa and 15°C) is 1.225 kg/m3.

To calculate the specific gravity of a gas, the density of air at thesame temperature and pressure as the gas sample must be known. Ifboth air and the gas are at the same temperature and the pressure islow, the specific gravity of the gas will depend only on its molecularweight (the molecular weight of air is 28.97). The formula becomes:


SG = Density of the gasDensity of the air

MOLECULARWEIGHT

SPECIFIC GRAVITY

SG = Mgas

Mair

29


Where Mgas is the molecular weight of the gas and Mair is themolecular weight of the air.

It is important to remember that the Ideal Gas Law is valid only atlow pressures. At standard temperature (15°C), natural gas isgenerally assumed to be ideal if the pressure is less than 700 kPaabsolute.

The following example shows the calculation of specific gravity ofmethane. The molecular weight of methane gas was found to be16.04 kg.

The specific gravity is equal to the molecular weight of the gasdivided by the molecular weight of air.

SG = Mgas

Mair

The specific gravity of the methane gas is 0.554.

The specific gravity of natural gas ranges between 0.55 and 0.90,depending on the amount of heavier hydrocarbons in the gas. Naturalgas is lighter than air. Therefore, if unconfined, natural gas will riseand dissipate. The specific gravity of a gas is important because it isused in calculating flow in a pipeline.

As we learned in an earlier section, all substances are made up ofmolecules which, in turn, are made up of atoms.

Some substances are made up of single types of atoms joined to eachother to form molecules. For example, two oxygen atoms join to formone oxygen molecule (O2).

Some substances are made up of various types of atoms in varyingnumbers and ratios. For example, methane (CH4) is the majorcomponent of natural gas and consists of one carbon atom and fourhydrogen atoms. Nitrogen (N2), carbon dioxide, (CO2) and water(H2O) are also frequently found as components of natural gas.

Figure 19Molecular structure ofcarbon dioxide, water, andnitrogen.

= = 0.554

COMPOSITION

0

HH

C N00 N

16.0428.97

30


MOLE FRACTIONS OFCOMPONENTS

The composition of natural gas varies. Natural gas can contain 80%to 95% methane, depending on the source of the gas and the degreeof processing. In discussing the components of natural gas, it is usualto describe them in terms of the mole fraction. (The mole fraction issimply the number of moles of a component divided by the totalnumber of moles in the mixture.)

The following table illustrates the composition of three typicalsamples of natural gas. Because natural gas is composed mainly ofmethane, the properties of natural gas are very similar to those ofmethane.

Component Wellhead Partially Processed Marketed SalesGas Gas Gas

Helium (He) 0.0002 0.0002 0.0002

Methane (CH4) 0.7268 0.9297 0.9687

Ethane (C2H6) 0.0800 0.0068 0.0003

Propane (C3H8) 0.0750 0.0220 0.0002

Heavy hydrocarbons (C4+) 0.0450 0.0050 0.0004

Carbon dioxide (CO2) 0.0310 0.0283 0.0278

Hydrogen sulphide (H2S) 0.0200 0.0000 0.0000

Nitrogen (N2) 0.0220 0.0080 0.0024

TOTAL 1.0000 1.0000 1.0000

The fractions are proportions of each component in the total gasmixture.

In the table, the Marketed Sales Gas column shows a fraction of0.9687 of methane. This is the mole fraction of methane in theMarketed Gas. The total of all the mole fractions is 1. In the sameway, the mole fractions of each component in the Marketed Gascolumn can be compared at each stage from wellhead to burner. Thetable shows a mole fraction for methane from the wellhead as 0.7268.This figure changes to 0.868 after processing due to the removal ofsome components and reduction of others.

31


1. The three main parameters used to describe a quantity ofnatural gas are __________ .a) density, odour and volumeb) volume, temperature and pressurec) pressure, volume and colourd) colour, density and odour

2. The lowest temperature at which self-sustaining combustionoccurs is referred to as _______ temperature.a) UFLb) ignitionc) LFLd) flame

3. The ratio of the density of the gas to the density of dry air,at the same temperature and pressure, refers to the__________ of gas.a) specific gravityb) molecular weightc) per cent volumed) quantity

4. Heating value is expressed in __________ and depends onthe __________ of the gas.a) °C, compositionb) K, hydrocarbon contentc) J/m3, compositiond) kPa, hydrocarbon

REVIEW 3

32

5. Molecular weight refers to the mass of a molecule of asubstance relative to an atom of __________. a) propaneb) carbonc) methaned) hydrogen

6. The composition of natural gas varies, normally containing__________ methane.a) 50-65%b) 60-75%c) 70-85%d) 80-95%

7. Combustion of natural gas occurs __________ .a) above the UELb) below the LELc) between the UEL and LELd) above the UEL and below the LEL

8. The number of moles of a component divided by the totalnumber of moles of gas in a mixture is referred to as a __________ .a) gigajouleb) mole fractionc) Jd) K



33


SECTION 4

In this section, the behaviour of real gases are explained in terms ofDalton’s Law of Partial Pressure. This is one way of describing thedifference between the behaviour of ideal gases and real gasmixtures.

Real gases can also be analysed in the same way as ideal gases byusing the concepts of critical temperature and pressure.

A further refinement of the application of the Ideal Gas Law to realgas mixtures is accomplished by considering pseudo-criticalproperties.

All these factors contribute to the determination of thecompressibility of the mixture.

After completing this section of the module, you will be able to:• Relate total pressure exerted by a gas mixture to the individual

pressures of the components, as expressed in Dalton's Law ofPartial Pressures.

• Relate critical temperature and pressure to the compressibility ofreal gases.

• Identify the pseudo-critical properties of gas mixtures and relatethem to the compressibility factor and volume measurement.

• Recognize the compressibility factor and relate it to gas mixtures.

BEHAVIOUR OF REAL GASES

INTRODUCTION

OBJECTIVES

34


Dalton's Law of Partial Pressures states that the total pressureexerted by a mixture of gases is equal to the sum of the pressuresexerted by the individual components. The law governs the behaviourof mixtures of real gases. It was developed to help calculate the totalpressure and total volume of real gas mixtures. This is particularlyimportant for natural gas. As one component of a real gas, natural gasis composed of a number of different components.

The individual pressure exerted by a component in the gas mixture isknown as partial pressure. Using the ideal gas law, the partial pressureexerted by a component in a gas mixture can be calculated. For amixture consisting of three gases, A, B, and C:

According to Dalton’s Law of Partial Pressures:

P = PA + PB + PCand

Where: P = the total pressure (kPa) of the mixture

PA,B,C = the partial pressure (kPa) of each component

nA,B,C = the number of moles of the component

R = the universal gas constant

T = the absolute temperature of the gas

V = the volume of the gas mixture

It should be noted that the ratio of the partial pressure of anycomponent in a real gas mixture to the pressure of the entire mixtureis equal to that component's mole fraction.

PA = R x T

VnA x PB =

R x T

VnB x

PC = R x T

VnC x

DALTON'S LAW OFPARTIAL

PRESSURES

35


The partial pressure is equal to the mole fraction of the componentmultiplied by the total pressure. This represents the contribution ofthis component to the total pressure. This can be shown as:

Where: YA = the mole fraction for component A in the gas mixture nA = the number of moles of component A

n = the total of all moles of the component (nA + nB + nC)

In the same way, the mole fractions for each component in themixture can be found. In order to see the calculation of the partialpressures, consider a gas sample that contains a methane componentA (mole fraction of 0.85) and an ethane component B (mole fractionof 0.15) at a total pressure of 1000 kPa.

Dalton’s Law of Partial Pressures can be used to find the partialpressure of the ethane component B in the mixture. Restating theformula to solve the problem:

PB = YBP

PB = YB x PPB = 0.15 x 1000 kPaPB = 150 kPa

The partial pressure of ethane in the mixture is 150 kPa. That is, thecontribution of ethane to the mixture of ethane and methane is 150 kPa at a total pressure of 1000 kPa. The percentage of methane

in the mixture followingdirectly from the calculationis 85%. Similarly, thecontribution of methane willbe 850 kPa.

Figure 20Dalton’s Law of PartialPressuresIdentical sample bottlescontaining gas at a constanttemperature and different pressures.

PA

P

nA

n= = YA

+ =MethaneGas

850 kPa

EthaneGas

150 kPa

Mixture1000 kPa

36


Every substance has a critical temperature and pressure. Liquidcannot exist at temperatures above the critical temperature, nomatter how great the pressure. The critical temperature is shown bythe symbol Tc. The critical pressure is the minimum pressure neededto liquefy a gas at its critical temperature. If the pressure is greaterthan the critical pressure, it is not possible to distinguish betweenliquid and gas. The gas is so dense that it resembles a liquid. Thecritical pressure is shown symbolically as Pc.

Every pure substance has a different critical temperature and pressure.For example, the critical temperature (Tc) of pure methane is 191.1 K,(-82ºC); the critical pressure (Pc) is 4640 kPa. This means that tokeep methane as a liquid, the temperature must be kept below -82ºC.

The combination of the highest pressure and temperature at whichboth the vapour and liquid phases of a pure material can co-exist isknown as the critical point. Critical points can only be established forpure substances. A single critical point cannot be established for airbecause it is a mixture of different gases. The critical point isimportant in calculations such as the compressibility factor. In thesecalculations, quantities called reduced temperature (Tr) and reducedpressure (Pr) are used. These two quantities relate the actualtemperature and pressure of a substance as fractions of its criticaltemperature and pressure.

To illustrate, consider the calculation of the reduced temperature andreduced pressure of methane at standard conditions. (Remember thatstandard pressure is 101.325 kPa and standard temperature is 15ºC or288.15 K.) The critical points of methane are 4640 kPa and 191.1 K.Therefore, the reduced temperature and pressure of methane are:

Tr = 1.51 and Pr = 0.022

The reduced temperature of methane is 1.51 and the reduced pressureis 0.022. Note that reduced temperature and pressure are ratios, andtherefore do not have units associated with them.

Tr = T

Tc

Pr = P

Pcand

Tr = 288.15 K

191.1 KPr =

101.325 kPa

4640 kPaand

CRITICALTEMPERATURE

AND PRESSURE

37


So far in this module, we have learned that critical points have beendefined for pure components. Natural gas is a mixture of pure gascomponents. It is useful to calculate reduced pressures andtemperatures for gas mixtures so that a compressibility factor can befound for that mixture. For this reason, a gas mixture is said to havepseudo-critical properties. Pseudo-critical properties are thecalculated equivalents of a real gas mixture treated as a purecomponent.

In the same way that the molecular weight for natural gas wascalculated based on the composition of the gas, so can the pseudo-critical properties of the gas mixture be calculated. The pseudo-critical pressure and temperature (P'c & T'c) can be used to calculate apseudo-reduced pressure and temperature. These values are then usedto correct volume measurements.

The pseudo-critical temperature and pressure of a gas mixture are aweighted summation of the critical temperatures and pressures, usingthe mole fractions as a weighting factor for each component of themixture.

P'c = ∑ Y x Pc and T'c = ∑ Y x Tc

Where P'c = the pseudo-critical pressure of the mixture

∑ = the mathematical symbol for summation

Y = the mole fraction for each component

Pc = the critical pressure of each component

T'c = the pseudo-critical temperature of the mixture

Tc = the critical temperature of each component

PSEUDO-CRITICALPROPERTIES OFGAS MIXTURES

38


The application of the formula can be illustrated by calculating thepseudo-critical temperature and pressure of a natural gas mixture asshown in the following table.

Component Mole Critical CriticalFraction Temperature (K) Pressure (kPa)

1. Methane 0.7268 191.1 4640

2. Ethane 0.0800 305.5 4880

3. Propane 0.0750 369.8 4260

4. Isobutane 0.0200 408.1 3650

5. n-Butane 0.0185 425.2 3800

6. n-Pentane 0.0065 469.7 3370

7. Carbon Dioxide 0.0310 304.2 7390

8. Hydrogen Sulphide 0.0200 373.7 9000

9. Helium 0.0002 5.3 230

10. Nitrogen 0.0220 126.2 3390

The formula shows:

P'c = (y1 PC1) + (y2 PC2) + … + (y10 PC10)

= (0.7268 x 4640) + (0.0800 x 4880) + ... + (0.0220 x 3390)

= 3372 + 390 + … + 75

= 4731 kPaand:

T'C = (y1 TC1) = (y2 TC2) + … + (y10 TC10)

= (0.7268 x 191.1) + (0.0800 x 305.5)+ … +(0.0220 x 126.2)

= 138.89 + 24.44 + … + 2.7764

= 229.9 K

39


The pseudo-critical pressure of this natural gas mixture is 4731 kPa.The pseudo-critical temperature of the mixture is 229.9K. With thecalculation of these pseudo-critical properties, the mixture of gascomponents can be mathematically treated like a pure component ofa real gas. In the same way that the reduced temperatures andpressures were calculated, pseudo-reduced pressures and pseudo-reduced temperatures can also be calculated.

In calculating the volume of real gases at elevated pressures, acompressibility factor, (Z), is applied to compensate for non-idealbehaviour. The compressibility factor is the ratio of the volumeactually occupied by a gas at a given pressure and temperature(Vactual) to the volume the gas would occupy at the same pressureand temperature if it were an ideal gas (Videal). The compressibilityfactor can be expressed as:

For an ideal gas, the compressibility factor will always equal 1.00.For real gases, however, Z is not constant. The value varies withchanges in gas composition, temperature and pressure. The value ofZ is determined experimentally for each set of conditions. Earlier inthis module, the gas law for ideal gases was stated as:

P x V = n x R x T

For real gases, the gas law is restated as:

P x V = Z x n x R x T

The compressibility factor for each gas component has beenexperimentally determined. The factors are frequently placed on agraph. The Principle of Corresponding States shows that real gaseshave approximately the same value of Z at the same values ofreduced temperature and reduced pressure. In other words, by usingpseudo-reduced temperature and pseudo-reduced pressure, thecompressibility factor (Z) for all real gases behaves in practically thesame manner. This behaviour is illustrated by the following chart.

Vactual

VidealZ =

COMPRESSIBILITYFACTOR

40


Figure 21Compressibility Chart of Natural GasTo determine the compressibility factor, first calculate the pseudo-reduced pressureand temperature. Start at the horizontal axis and find the pseudo-reducedpressure. Draw a vertical line from this point to the appropriate line for the pseudo-reduced temperature. From the intersection point, draw a line horizontally acrossto the vertical axis. Read the compressibility factor at this point.

3.02.82.4

2.0

1.8

1.6

1.45

1.9

1.7

1.5

1.4

1.3

1.25

1.2

1.05

1.35

2.2

1 1.5 2 3 4 50

0.25

0.3

0.4

0.5

0.60.63

0.7

0.8

0.9

1.0

1.1

py

Pseudo-Reduced Pressure,

Pseudo-reducedtemperature

2.6

1.1

1.15

T'r

P'r

41


To demonstrate the use of the chart, we can use the natural gasmixture that was given in the previous example. Assume the mixture isat a pressure of 7100 kPa and temperature of 273.15K. First, to findthe pseudo-reduced temperature, we use the formula:

T'r = pseudo-reduced temperature of the mixtureT = actual temperature = 273.15K

T'C = pseudo-critical temperature of the mixture = 229.9K

Substituting the values, we find:

Next, to find the pseudo-reduced pressure, use the formula:

P' = pseudo-reduced pressure of the mixtureP = actual pressure = 7100 kPaP'C = pseudo-critical pressure of the mixture = 4731 kPa

Substituting the values, we find:

For this mixture under these conditions, the compressibility factorcan be obtained from the chart in Figure 21. Using the chart, Z = 0.63

Next, use compressibility factor to find a volume. From the formula:

P x V = Z x n x R x T

Rearrange to find volume as follows:

V = Z x n x R x TP

Where: Z = 0.63n = 1R = 8.3143T = 273.15P = 7100

273.15229.9

P'rPP'c

71004731 = 1.5

=

0.63 x 1 x 8.3143x 273.157100

V =

T'rTT'c

=

or 1.19

= 0.2 m3

42


If the sample had been treated as an ideal gas, the volume would notinclude the Z factor and would be calculated as follows:

V = 0.32m3

It is obvious that there is a difference between the volume of the realgas example to the ideal gas calculation. The real gas withcompressibility was 0.2m3 while the calculation of the ideal gas gavea value of 0.32m3. The difference of 0.12m3, while exaggerated fromwhat one would normally expect in a typical gas application,illustrates why compressibility is important when considering real gasmixtures.

Huge volumes are involved in the transmission and distribution ofnatural gas. Even a small Z factor can result in significant errors inmeasurement if the compressibility of the mixture is not considered.

n x R x TP

V =

1 x 8.3143 x 273.157100

=

43


1. Dalton's Law of Partial Pressure states that the totalpressure exerted by a mixture of gases is ___________ thesum of the pressures exerted by the individual components.a) greater thanb) less thanc) equal tod) relative to

2. The combination of the highest pressure and temperature,at which vapour and liquid phases of a pure material canco-exist, is referred to as the ___________ . a) ignition temperatureb) critical pointc) compressibility factord) percentage volume

3. In calculating the volume of real gases at elevatedpressures, a _______________ is applied to compensatefor non-ideal behaviour.a) compressibility factorb) critical temperaturec) critical pressured) mole fraction

4. The ratio of the partial pressure of any component in anideal gas mixture to the pressure of the entire mixture, isequal to that component's __________ .a) compressibility factorb) critical temperaturec) critical pressured) mole fraction

REVIEW 4

44


5. The pseudo-critical temperatures and pressures of a gasmixture are ____________ .a) real gases that have approximately same value of Z b) temperature and pressure that reflect ideal behavioursc) standard temperatures and pressuresd) weighted summation of the reduced temperatures and

pressures

6. The expression stating that real gases have approximatelythe same value of Z at the same values of reducedtemperature and pressure is _______________ . a) The Principle of Corresponding Statesb) Dalton's Law of Partial Pressurec) Boyle's Lawd) Charles' Law

7. At elevated pressures, volume calculations for real gaseshave to be adjusted for _______.a) flammabilityb) shrinkagec) heating valued) non-ideal states


45


SECTION 1 - THE ORGANIC THEORY OF NATURAL GASFORMATION

• The most commonly accepted theory of oil and gas formation isthe organic theory. This theory suggests that millions of years ago,organic plant and animal material was buried under layers of sandand silt at the bottom of vast oceans. As time passed, the layers ofsand and silt changed into solid, sedimentary rock. Organicmaterial, trapped between the layers of sedimentary rock, wasgradually transformed into hydrocarbons by the immense pressureof the ocean and the effects of geothermal heat.

• Fossil fuels are composed of molecules of hydrogen and carbon.For this reason, they are referred to as hydrocarbons.

• The hydrocarbon liquid state is oil; the gaseous state is naturalgas; the solid form is asphalt.

• Oil and gas accumulate behind impermeable barriers calledcaprocks. A caprock, and associated reservoir, are referred to as atrap.

• The separation patterns of oil, gas and water in a reservoir reflecttheir densities.

• Natural gas has been used as a fuel throughout recorded history.

SECTION 2 - IDEAL GAS BEHAVIOUR• Matter exists in the form of one of three states: solid, liquid or

gas.

• Boyle’s Law expresses the effect of pressure on volume.

• Charles’ Law relates the effect of temperature on volume.

• The volume of an ideal gas is affected by changes in temperatureand pressure.

SUMMARY

46


SECTION 3 - PROPERTIES & CHARACTERISTICS OF NATURAL GAS

• Natural gas is a gas mixture. Therefore, its behaviour isinfluenced by its composition. Its molecular weight, specificgravity, heating value, ignition temperature, and flammabilitylimits are all influenced by its composition.

• Natural gas is colourless, odourless, non-toxic and flammable.

• Pressure, temperature and volume are parameters used to describea quantity of natural gas.

SECTION 4 - BEHAVIOUR OF REAL GASES• The total pressure exerted by a mixture of gases is equal to the

sum of the pressures exerted by the individual components of themixture.

• The highest pressure and temperature at which both vapour andliquid phases of a pure material can co-exist is the critical point.

• The pseudo-critical temperature and pressure of a gas mixture areweighted summations of the reduced temperature and pressure foreach component of the mixture, using mole fractions as theweighting factor.

• A compressibility factor is applied to compensate for the non-ideal behaviour of a real gas.

47


absolute zerothe temperature at which molecules of all matter reach a motionlessstate. (p. 19)

absolute pressure (Pabs)the pressure relative to a perfect vacuum (absolute zero). (p. 15)

absolute temperature scaleknown as the Kelvin Scale. (p. 19)

anticlinal trapan oil and gas trap that occurs when sedimentary rock layers curveupward. (p. 6)

atmospheric pressure (Patm)the force exerted on the earth's surface by the atmosphere.Atmospheric pressure varies depending on elevation and weatherconditions. (p. 16)

Boyle’s Lawa law of physics that states that the pressure of an ideal gas isinversely proportional to its volume at a constant temperature. (p. 17)

caprocksimpermeable underground barriers that prevent petroleumhydrocarbon molecules from migrating to the earth’s surface. (p. 6)

Charles' Lawa law of physics that states that the volume of an ideal gas is directlyproportional to its absolute temperature at a constant pressure. (p. 19)

compressibility factor (Z)the ratio of the volume actually occupied by a gas at a given pressureand temperature to the volume the gas would occupy at the samepressure and temperature if it were an ideal gas. (p. 39)

critical pointthe combination of the highest pressure and temperature at whichboth the vapour and liquid phases of a pure material can co-exist. (p. 36)

GLOSSARY

48


critical pressure, PCthe minimum pressure needed to liquefy a gas at its criticaltemperature. (p. 36)

critical temperature, TCthe temperature above which a liquid cannot exist, no matter howgreat the pressure. (p. 36)

Dalton's Law of Partial Pressuresstates that the total pressure exerted by a mixture of gases is equal tothe sum of the pressures exerted by the individual components. (p. 34)

ethane (C2H6)a molecule made up of two carbon atoms bonded to six hydrogenatoms. (p. 5 Figure 3)

fault trapan oil and gas trap that occurs when a fault in the earth’s surfaceshifts vertically, allowing petroleum hydrocarbons to migrate to areservoir at the fault.(p. 6)

flame temperaturethe temperature that can be measured in a burning substance. (p. 27)

flammability limitsthe range over which gas will support combustion. (p. 27)

gasa state of matter which has neither shape nor definite volume. A gasassumes the shape and volume of a closed container. (p. 14)

gauge pressure (Pgauge)pressure relative to ambient atmospheric pressure. (p. 15)

heating valuethe energy released when a standard volume of gas is burned,expressed in Joules per cubic metre (J/m3). Also known as energyvalue. (p. 26)

hydrocarbonsmolecules containing both hydrogen and carbon; includes fossil fuelssuch as oil and gas. (p. 5)

49


hydrocarbon trapthe caprock and resulting gas reservoir. (p. 6)

ideal gasa gas in which no molecular forces exist. (p. 15)

Ideal Gas Lawcombines the principles of Boyle's Law, which expresses the effectsof pressure on volume, and Charles' Law, which expresses the effectsof temperature on volume. (p. 21)

ignition temperaturethe lowest temperature at which self-sustaining combustion occurs.(p. 27)

Kelvin Scaleone Kelvin (K) is the same size as a Celsius degree: 0 K = 273.15°C;0°C is the same temperature as 273.15 K. (p. 19)

kilogram mole (kg mol)the atomic number multiplied by the weight of the substance inkilograms. (p. 21)

liquida state of matter that has no shape of its own; rather, a liquid has afixed volume and conforms to the shape of its container. (p. 14)

Lower Flammability Limit (LFL) or Lower Explosive Limit (LEL)the lowest concentration of a gas in air at which sustainedcombustion will occur. (p. 27)

methane (CH4)a simple molecule consisting of one carbon atom bonded to fourhydrogen atoms. (p. 5 Figure 2)

molethe amount of a substance expressed in terms of the number of atomspresent. (p. 21)

mole fractionthe number of moles of a component divided by the total number ofmoles in the mixture. (p. 30)

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molecular weight (M)the mass of a molecule of a substance relative to an atom of hydrogen.(p. 28)

Principle of Corresponding Statesstates that real gases have approximately the same value of Z at thesame values of reduced temperature and reduced pressure. (p. 39)

propane (C3H8)a molecule made up of three carbon atoms bonded to eight hydrogenatoms. (p. 5 Figure 4)

pseudo-critical propertiesthe calculated equivalents of a real gas mixture treated as a purecomponent. (p. 37)

pure gasa single component gas, treated as ideal gas at low pressures. (p. 15)

real gasis composed of a number of different gas components. (p. 34)

salt dome trapan oil and gas trap that occurs when molten salt creates bends in thesedimentary rock layers, forming reservoirs. (p. 7)

solida state of matter that has definite shape and volume. (p. 14)

source rockthe rock where the organic material was originally trapped. (p. 6)

specific gravity (SG)the ratio of the density of a gas to the density of dry air at the sametemperature and pressure. (p. 28)

standard pressurea standard reference pressure based on the average atmosphericpressure at sea level. This pressure in SI units is expressed as 101.325 kPa, or one atmosphere (1 atm). (p. 16)

51


standard temperaturea standard reference temperature. This temperature in SI units isexpressed as 15°C or 288.15 K. (p. 19)

stratigraphic trapan oil and gas trap that occurs when the porosity of the rock isgreater in one area than another. (p. 6)

Universal Gas Constant (R)a constant value for a given quantity of an ideal gas at a combinationof pressure, temperature and volume. (p. 21)

Upper Flammability Limit (UFL) or Upper Explosive Limit (UEL)the highest concentration of a gas in air that will support combustion.(p. 27)

vacuum or negative pressureany pressure below the local atmospheric pressure. (p. 16)

52

REVIEW 1 REVIEW 2 REVIEW 3 REVIEW 41. d 1. a 1. b 1. c

2. b 2. d 2. b 2. b

3. d 3. a 3. a 3. a

4. b 4. b 4. c 4. d

5. a 5. a 5. d 5. d

6. b 6. a 6. d 6. a

7. c 7. a 7. c 7. d

8. d 8. b


ANSWERS

fundamentos del gas natural

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