Lecture 2

Specific Gravity: (γg) “the ratio of the apparent molecular weight of a natural

gas to that of air, itself a mixture of gases”. The molecular weight of air is usually taken as equal to

28.97 (approximately 79% nitrogen and 21% oxygen). Therefore the gas gravity is

ViscosityGas viscosity is a measure of the resistance to flow exerted by the gas. Dynamic viscosity in centipoises (cp) is usually used in the natural gas engineering.

Properties Of Natural Gas

where the apparent molecular weight of gas can be calculated on the basis of gas composition. Gas composition is reported in mole fractions of components in the gas.

Let yi be the mole fraction of component i, the apparent molecular weight of the gas can be formulated using mixing rule as:

• where MWi is the molecular weight of component i, and Nc is the number of components.

Pseudo-critical & pseudo-reduced properties:Critical temperature of pure gas is the temp above which it cannot be

liquefied, whatever the pressure of the gas.

Critical pressure is defined as pressure above which a liquid and gas cannot coexist, regardless of the temperature. Or  the pressure required to liquefy a gas at its critical temperature.

When the gas consist of mixture of different gases the critical temperature and critical pressure are called the pseudo-critical temp & pseudo-critical pressure.

T pc = ∑ yi Tc & P pc = ∑ yi Pc

Reduced temperature of a gas= abs temp of gas ni °R/ critical temp of pure gas

Reduced pressure of a pure gas = abs pressure of gas/ critical pressure of gas

Pseudo-reduced temp & pseudo-reduced pressure are reduced temp & pressure of mixtures of different gases.Tpr = T/ Tpc & Ppr = P/Ppc

T pr = ∑ yi Tr & P pr = ∑ yi Pr

Pseudo-critical properties from gas gravity:

T pc =170.491+ 307.344 GP pc = 709.604 – 58.718 GWhere:G is the gas gravity

Heating Values of Natural gas• The lower heating value (LHV) or net heating value (NHV) isdefined as “the amount of energy released by burning 1 standardcubic foot of natural gas.” The amount of heat released is measuredin Btu units. The LHV and NHV do not include the heat released bycondensing the water, because it is assumed the water remains avapor. The LHV or NHV term is useful in process engineeringapplications.• The higher heating value (HHV), also known as the gross heatingvalue (GHV), is defined as “the amount of energy released byburning 1 standard cubic foot of natural gas.” The amount of heatreleased is measured in Btu units and includes the heat released bycondensing the water formed as a combustion product.• For natural gas it is approximately in the range of 900 to 1200

Btu/ft3.• Gross heating value of a gas mixture can be calculated as:

» Hm= ∑ (yi Hi)

Flammability limits:• There are two composition limits of flammability

for air and a gaseous fuel under specified conditions. The lower limit corresponds to the minimum concentration of combustible gas that will support combustion, the higher limit to the maximum concentration.

• Flammability limits, also called flammable limits,  give the proportion of combustible gases in a mixture, between which limits this mixture is flammable.

• The lower flammable limit (LFL) describes the leanest mixture that still sustains a flame, i.e. the mixture with the smallest fraction of combustible gas, while the upper flammable limit (UFL) gives the richest flammable mixture. OR

• The lower flammability limit is the minimum concentration by volume of a combustible substance that is capable of propagating a flame under specified conditions.

• The upper flammability limit is the maximum concentration by volume of a combustible substance that is capable of continued propagation of a flame under the specified conditions

Asphyxiation:• Natural gas can also be hazardous to life and property

through an explosion and/or asphyxiation.• Being an asphyxiant, it can kill organisms (including

humans) if it displaces air to the point where the oxygen content of the surrounding atmosphere is insufficient to support life.

• if natural gas leaks go undetected (hence the reason for adding odorants such as thiols).

Wobbe indexIn 1927, Goffredo Wobbe, a physicist in Italy, observed that:• The heat output of a burner is proportional to the flow volume per time (given

constant pressure and constant orifice size)• The flow velocity through a given orifice size at constant pressure is proportional

to the specific gravity of the gas• The calorific value, or heating value, of a gas is proportional to its specific gravity.

“The Wobbe Index is actually the correct representation of the heating value of natural gas arriving, from the gas line, at the orifice where a burner is located.”

• If “Vc” is the higher heating value, or calorific value, and “Gs” is the specific gravity, the Wobbe Index, “Iw” , is defined as:

It is not simply the BTU per cubic foot but rather the BTU per cubic foot divided by the square root of the specific gravity.Wobbe Index = BTU/cubic foot / √SG

• The Wobbe Index (WI) or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas(coal gas) and is frequently defined in the specifications of gas supply and transport utilities.

• The Wobbe Index is used to compare the combustion energy output of different composition fuel gases in an appliance (fire, cooker etc.).

• If two fuels have identical Wobbe Indices then for given pressure and valve settings the energy output will also be identical.

“In almost all gas appliances, the flow of gas is regulated by making it pass through a hole or orifice. The usefulness of the Wobbe number is that for any given orifice, all gas mixtures that have the same Wobbe number will deliver the same amount of heat. Pure methane has a Wobbe number of 1363; natural gas as piped to homes in the United States typically has a Wobbe number between 1310 and 1390.”

Wobbe index for some common fuel gases

Problem:

Calculate the Wobbe index of N.GComp %age Cal.valueCH4 80 % 9500Kcal/m3C2H6 8% 16644Kcal/m3C3H8 2% 23668Kcal/m3C4H10 1% 30714Kcal/m3

The compressibility factor (Z), also known as the compression factor, is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behavior. In general, deviation from ideal behavior becomes more significant the closer a gas is to a phase change, the lower the temperature or the larger the pressure.

Z= PM/ nRTAnother term, the “super-compressibility factor” “Fpv”, which is related to the compressibilty factor Z asFpv = 1/ (Z)1/2

Z = 1/(Fpv)2

There are many methods to calculate compressibility factor, as:

a. Standing-Kartz methodb. Dranchuk, and Robinson methodc. AGA method d. CNGA method

Standing-Katz Chart

Dranchuk, Purvis and Robinson Method

Z = 1+ [ A1+ A2/Tpr +A3/Tpr] + [A4+ A5/Tpr]ρr2 + (A5A6) ρr

5/Tpr +…….

ρr= 0.27 Ppr / Tpr

A1 = 0.31506237A2= - 1.04670990A3= -0.57832729A4= 0.53530771A5= -0.61232032A6= -0.10488813

American Gas Association (AGA) Method:it uses a complicated algorithm and does not lend itself easily to normal calculations. It generally includes a computer programe.

California Natural Gas Association (CNGA) method:

Z= 1/ [1+( Pavg 344,400(10)1.785G/Tf3.825)]

Pavg= avg gas pressureTf = Avg gas tempG = gas gravity

P avg = 2/3[P1+P2 – (P1xP2/P1+P2)]or

P avg = 2/3 (P13 –P2

3/ P12-P2

2)

Impact of Sour Gas & Non-hydrocarbon components on compressibility factor:

• Sour natural gases containing H2S and/or CO2 frequently exhibit different compressibility factor behavior than do sweet natural gases.

• A calculation procedure to account for these differences includes pseudo-critical temperature adjustment factor which is a function of the concentration of CO2 and H2S in the sour gas. This correction factor is then used to adjust the pseudo-critical temperature and pressure according to the following expressions.

ε = 120(A0.9 – A1.6) + 15(B0.5 – B4.0)Where:ε = adjustment factorA = sum of mole fractions of CO2 and H2S B = mole fraction of H2S

The pseudo-critical temperature is modified to get the adjusted pseudo-critical temperature

T’pc = T pc - ε

Similarly the pseudo-critical pressure is adjusted as P’pc = Ppc x T’pc / { Tpc +B(1-B) ε }

Problem:A natural gas mixture consists of the following composition:

a). Calculate the apparent Mol.wt of gas, gas gravity, Pseudo-critical temp and Pseudo-critical pressure

b). Calculate the compressibility factor for gas at 900 °F and 1200 psia.

Component Mole FractionCH4

C2H6

C3H8

N2

CO2

H2S