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Section 1Introduction to Natural Gas

Natural gas is a mixture of light hydrocarbon compounds that areproduced from an underground reservoir. A hydrocarbon compound isa molecule consisting of atoms of carbon and hydrogen. The lightesthydrocarbon compound is methane, which is made up of one atom ofcarbon and four atoms of hydrogen. Other hydrocarbon compounds ofincreasing molecular mass, such as ethane, propane, butane, etc., makeup the balance of the natural gas mixture. However, these compoundsare present in much lower and diminishing concentration withincreasing molecular mass. Hydrocarbons of higher molecular massare found in volatile hydrocarbon liquids produced with natural gas,such as condensate and oil.

The molecular structures of hydrocarbons include single and doublebonding between carbon atoms, which have a valence of 4, and thevarious hydrocarbons have been given the following designations:

Paraffin hydrocarbons: general formula is CnH2n+2. The molecularstructure is made up of single bonds between carbon atoms, withoutrings being formed. Hydrogen atoms are attached to the carbon atomsto complete the molecular structure. The single-bonded carbon atomscan form straight chains, referred to as "normal", or branches, in whichcase they are referred to as "isomers". Paraffinic hydrocarbons have thesuffix "ane" in their name, such as methane, ethane, propane, etc.

Olefin hydrocarbons: general formula is CnH2n. The molecularstructure includes one double bond between carbon atoms, withoutrings being formed. Molecular structures of hydrocarbons that includedouble bonds are said to be "unsaturated". These compounds containthe suffix "ene" in their name, such as ethylene, propylene, etc.

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Naphthene hydrocarbons: general formula is CnH2n. The molecularstructure forms rings with the carbon atoms, using single bonds, withtwo hydrogen atoms attached to each carbon atom as well. Thesecompounds are designated with the prefix "cyclo-" in their name, suchas cyclo-butane, cyclo-pentane, etc.

Aromatic hydrocarbons: These compounds are formed with six carbonatoms forming a ring, with alternate single and double bonds betweenthe carbon atoms. Hydrogen atoms attached to the carbon atomscomplete the molecular structure. The best known aromaticcompound is benzene, which has the formula C6H6. If one or morehydrogen atoms are displaced by radicals, other compounds areformed. One such example is the methyl radical displacing onehydrogen atom, with the resulting compound being toluene. Manyother compounds can be formed in this manner with the benzene ringas the basic building block.

Some non-hydrocarbons are also present in the raw natural gasmixture. These compounds are mainly nitrogen, carbon dioxide,hydrogen, helium and water vapour. In addition to thesenon-hydrocarbons, some natural gases contain sulfur compounds, ofwhich the main component is hydrogen sulfide, H2S. A natural gas issaid to be "sour" when the H2S content of the gas mixture exceeds thelimit imposed by the purchaser of the gas, usually a transmissioncompany or the end user. The lowest limit for H2S content in sales gasis for gas sold to California, which is 5.75 mg per standard cubic metre(mg/Sm3), or a volumetric limit of 4 ppm. The H2S limit for gas sold inCanada is 23 mg per Sm3. Sour natural gases can contain H2S inconcentrations from several ppm to over 90 %.

Sour natural gas will also contain other compounds containing sulfurin their molecular structure. The best known compounds in thiscategory are carbonyl sulfide (COS), carbon disulfide (CS2) and themercaptans (RSH).

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Terms Commonly Used

Since natural gas is a mixture of many compounds, it will vary inbehaviour with pressure and temperature, depending on thecomposition. As the concentration of heavier compounds such aspropane, butane, pentane and hexane increases, some hydrocarbonliquid may condense if the gas is cooled, resulting in the recovery ofcondensate. To characterize the gas with increasing content of heavierhydrocarbons, certain terms are commonly used to describe naturalgas:

Dry gas is natural gas that contains mainly methane, with noeconomically recoverable natural gas liquids.

Lean gas is natural gas that contains little or no recoverablenatural gas liquids. The processed gas meeting sales gasspecifications is lean gas or dry gas.

Rich gas is natural gas that is suitable as a feed gas to a gastreating plant for recovery of natural gas liquids. Rich gas doesnot usually meet the hydrocarbon dewpoint specification forsales gas.

Wet gas is natural gas that contains substantial amounts ofcondensate. Wet gas is also used sometimes to indicate thatthe gas contains water in condensable amounts.

Solution gas is natural gas that has been liberated from oil inmoving from reservoir conditions to surface conditions. It isusually a rich gas.

Residue gas is gas that has been processed in a plant to meetthe sales gas specifications.

Condensate is a liquid mixture of hydrocarbons condensedfrom rich or wet gas.

In the natural gas industry, the main reference for physical andchemical properties of natural gas and associated liquid hydrocarbons,as well as for process design data, is the Engineering Data Bookpublished by the Gas Processors Suppliers Association of the UnitedStates, referred to in these notes as the Data Book. In Section 1 of theData Book there are explanations of many terms commonly used in thenatural gas processing industry.

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Types of Natural Gas Produced

Natural gas originates from underground reservoirs, where it exists inone of the following ways:

• in solution in oil• as associated gas in a gas cap above an oil reservoir• in the vapour phase in the reservoir in the retrograde

condensate region• as non-associated gas (not in contact with a liquid hydrocarbon

phase in the reservoir)

No two reservoirs contain fluids with identical compositions. Once thegas leaves the reservoir, it is in a constant state of flow, from the gas wellto the burner tip. Some natural gas may be temporarily stored inunderground gas storage reservoirs. Gas that has not been subjected toany processing is generally referred to as "raw gas". One characteristicof raw gas is that it is always saturated with water vapour.

Raw gas produced from the reservoir has to be treated to meet certainsales gas specifications concerning purity and heating value. This isaccomplished in gas processing plants. Upon treatment to meet thespecifications, the gas is sold by the producer and enters along-distance gas transmission system. The gas ultimately ends up inlocal gas distribution systems which provide the distribution of naturalgas to the consumers, such as industry, commercial establishments andhomes.

Some natural gas can also contain contaminants such as mercuryvapour and asphaltenes. Mercury has to be removed from natural gasas it will promote corrosion. Asphaltenes are long-chain hydrocarbonmolecules which condense into fine, solid particles, which may hangup in the flow stream and cause plugging, or accumulate in gas treatingsolvents.

Most engineering design calculations for natural gas are based onknowledge of the composition of the natural gas mixture.Compositions are determined in laboratories on samples of gasobtained in plants or in the field. Sampling point data of pressure andtemperature must be recorded on the sample data sheet provided to

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the laboratory. Other information should always include the date of thesample, the formation from which the gas originates, the perforatedinterval, and any data about the flowing or static conditions at the timeof sampling.

The laboratory will generally perform a standard analysis bychromatograph to determine the concentration of 14 components inthe gas sample. Table A lists the 14 compounds analyzed for in acommon industry gas analysis, and indicates some typical gascompositions.

TABLE A - Typical Gas Compositions, Mole Percent

Component Dry Gas Rich GasSolution Gas

Sour Gas

Sales Gas

H2 0.02 0.02 0.01 0.00 0.01

He 0.00 0.01 0.00 0.00 0.00

N2 0.68 0.95 0.51 0.38 0.55

CO2 0.89 0.87 0.82 2.62 0.88

H2S 0.00 0.00 0.00 7.09 0.00

C1 92.32 82.13 77.70 75.41 89.30

C2 3.25 8.25 10.35 5.33 6.25

C3 1.80 4.53 6.85 3.74 2.35

iC4 0.43 1.05 0.77 1.22 0.21

nC4 0.61 1.45 1.73 0.92 0.32

iC5 0.00 0.28 0.36 0.51 0.05

nC5 0.00 0.35 0.41 0.38 0.08

C6 0.00 0.09 0.25 0.19 0.00

C7+ 0.00 0.02 0.24 2.21 0.00

Total 100.00 100.00 100.00 100.00 100.00

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The pressure - temperature (P-T) behaviour of individual hydrocarbonsand non-hydrocarbons has been determined experimentally, and theresults are available in the published literature. Figure A shows the P-Trelationships for five compounds commonly found in sour natural gas,namely methane, ethane, hydrogen sulfide, carbon dioxide, andisobutane. Sour gas mixtures, of course, usually include othercompounds such as propane and normal butanes, pentane, etc. Thepressure - temperature diagram shows vapour pressure curves for theindividual compounds. On the right hand side of the line, eachcompound is in the vapour state. By picking a temperature andpressure on the right hand side of a line in Figure A for example, and bymoving to the left at constant pressure, the temperature is reduced andwill reach the temperature on the line. This is the temperature at whichthe transition from the vapour state to the liquid state occurs. Latentheat has to be removed at constant temperature until all of thecompound is condensed to the liquid state before any furtherreduction in temperature occurs. When all of the compound isliquefied, then further reduction in temperature will occur uponadditional cooling or heat removal.

The heat removed in condensing or heat added in vaporizing purecompounds is the latent heat of vaporization or condensation of thatparticular compound. The amount of this heat energy per unit masswill vary with pressure at which the vaporization or condensation istaking place. The heat of vaporization can be estimated from Molliercharts, or it can be obtained from tabulations in certain books. At apressure of 101.325 kPa (abs), the heats of vaporization for the variouscompounds can be found in Fig. 23-2 of the Data Book, page 23-4,Column L.

Each line in Figure A terminates at an upper pressure and temperature.The terminal temperature for each compound is called the "criticaltemperature". The significance of the critical temperature is that this isthe highest temperature at which this compound, in the pure form, canexist in the liquid state. At any higher temperature, it will always be inthe vapour state or phase. The pressure at the critical temperature iscalled the "critical pressure". The phase at pressures above the criticalpoint is generally referred to as the "dense phase" or the "supercriticalphase".

Fig A

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