TPG 4140 NTNU
Natural Gas Properties
Professor Jon Steinar Gudmundsson
Department of Petroleum Engineering and Applied Geophysics
Norwegian University of Science and Technology
Trondheim
September 18, 2013
Outline
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Properties used in natural gas calculations
Phase diagrams and terminology (CCP & CCT)
Real gas law, z-factor, density and FVF
Corresponding states
Compressibility factor (z-factor)
Example calculation (GPA gas composition)
Heat capacity and heat capacity ratio
Viscosity of natural gas
Calorific values (GCV & NCV)
Summary
Properties Used
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Density, need z-factor and molecular weight
Flow in wells, need z-factor and viscosity
Pressure drop in pipelines, need density and viscosity
Temperature in pipelines, need heat capacity
Compressor power and exhaust temperature, need molecular
weight and heat capacity ratio
– Molecular weight, need relative density (gravity)
– Reynolds number, need density and viscosity
– Wobbe index, need gross calorific value and relative density
Calculations and Data on Home Page
PHASE DIAGRAM
Pedersen et al. (1989) Properties of Oils and Natural Gases, Gulf Publishing Company
Rojey & Jaffret (1997)
Rojey & Jaffret (1997)
TERMINOLOGY
• Natural gas, C1-C5+, water, inert gases
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NGL (Natural Gas Liquids), under pressure
LPG (Liquefied Petroleum Gas), propan+butan, -42 C
LNG (Liquefied Natural Gas), -162 C, 1 atm
CNG (Compressed Natural Gas), 180-200 bar
• Condensate (liquid), C4-C7, transition gas-to-oil
• Oil, C6 and heavier fractions
TYPICAL SPECIFICATIONS
• Transport Specification
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Hydrocarbon dew point, 5-10 C below ambient
Water dew point, about 5 C below HC dew point
Temperature, 30-50 C
Pressure, depends on receiving terminal
• Sales Specification (in addition to above)
– Heating value (GHV = Gross Heating Value), MJ/Sm3
– Wobbe Index (WI = GHV/(specific density)0,5
– Removal of non-HC gasser (inert gases)
http://www.ipt.ntnu.no/~jsg/undervisning/prosessering/forelesninger/05-Produktspesifikasjoner.pdf
TYPICAL SPECIFICATIONS
COMPOSITION
Generalization
• Non-Associated (dry gas)
• Associated gas (wet gas)
methane > 90 volume %
methane < 90 volume %
• Søtt gass (sweet gas)
• Surt gass (sour gas)
CO2 < 2 volume %
CO2 > 2 volume %
• Søtt gass (sweet gas)
• Surt gass (sour gas)
H2S < 1 volume %
H2S > 1 volume %
Rojey & Jaffret (1997) fra Valais (1983)
Volume Rate at s.c. to Mass Rate
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Compress 7 MSm3/d from 80 to 160 bara
Inlet temperature 30 C
Molecular weight M = 20.43 kg/kmol
Atm. Pressure 1.01325 bara
At s.c., ρ = 0.864 kg/Sm3 (T = 15 C)
Mass flow rate 7∙0.864 = 6.05∙106 kg/d
Per second m = 70 kg/s
M and ρ from Excel sheets
Real Gas Law
pV  znRT
pv  zRT
z sc  1
 p  Tsc  1 
  
Vsc  V 
 psc  T  z 
 p  Tsc  1 
  
qsc  q
 psc  T  z 
Density and FVF
pV  znRT
M
V
n
pM

zRT
V  m3 
 3 
B FVF  
Vsc  Sm 
 T  psc 
 z
B   
 Tsc  p 
q  qsc B
Finding Real Gas Factor (z-factor)
– Diagram based on corresponding states, reduced
pressure and temperature for single components
and pseudo-reduced pressure and temperature for
natural gas.
– Empirical equations matched to z-factor diagram for
natural gas. Uses many constants and coefficients
and in some cases iteration.
– Equation Of State (EOS) such as Peng-Robinson,
Redlich-Kwong and Benedict-Webb-Rubin.
Implemented in many different computer programs.
– EOS implemented in many commercial computer
packages such as Hysys, Prosper and PVTsim.
Real Gas Factor vs. Pressure at 25 C
Fletcher (1993)
Real gas factor with reduced p & T
Fletcher (1993)
Reduced p & T
p
pr  p pr  
pc
T
Tr  T pr  
Tc
pc   pci yi
i
Tc   Tci yi
i
Kay’s Rule
Corresponding States
– When pressure and temperature are normalized
using critical pressure and temperature, then all
properties become the same/similar, irrespective of
composition.
– Normalized pressure or temperature are called
reduced pressure or temperature in one
component systems.
– Normalized pressure or temperature are called
pseudo-reduced pressure or temperature in multicomponent systems.
– Commonly used when gas properties (natural gas
and other gases) are to be correlated and/or
presented.
GPA (1998)
Rojey o.a. (1997)
Equations and Gravity
p pc  4,892  0,405 ( MPa )
Tpc  94,72  170,75 ( K )


M gas
Mair
M gas   28,964 (at s.c.)
Thomas o.a. (1970), fra Rojey o.a. (1997)
Physical Constants of Natural Gas
Campbell (1984)
Example Calculation for Natural Gas
GPA (1998)
GPA (1998) Gas Composition (Excel sheet)
Components
Molecular weight
Mole fraction
Tci
Tci
Pci
Pci
g/mole
yi
oR
K
psia
Mpa
Methan, CH4
16,042
0,8319
343
190
667,8
4,61
Ethan, C2H6
30,07
0,0848
549,8
305
708
4,88
Propan, C3H8
44,10
0,0437
665,7
370
616
4,25
i-Butane, C4H10
58,12
0,0076
734,7
408
529
3,65
n-Butane, C4H10
58,12
0,0168
765
425
551
3,80
i-Pentane C5H12
72,15
0,0057
829
460
491
3,39
n-Pentane C5H12
72,15
0,0032
845
469
489
3,37
Hexane C6H14
86,18
0,0063
913
507
437
3,01
Heptane C7H16
100,21
0
972
540
397
2,74
Hydogen, H2
2,02
0
60
33
187
1,29
Nitrogen, N2
28,01
0
227,4
126
492
3,39
Oxygen, O2
32,00
0
277,8
154
731
5,04
Carbon dioxid, CO2
44,01
0
547,6
304
1071
7,38
Hydrogensulfid, H2S
34,08
0
672,4
373
1306
9,01
Dihydrogenoksid, H2O
18,02
0
1165
647
3199
22,06
Σ Mole fraction
Total molecular weight gas
20,43
1,0000
g/mole
Phase Envelope for GPA (1998) Gas
Beggs (1984)
Campbell (1984)
Heat Capacity
C p    T  T 2
Cp
R
 A  BT  CT 2
 Cp 


 1,702  9,08110 3 T  2,164 10 6 T 2
 R CH 4
R  8,314 (kJ / kmol.K )
C 
p CH
4
 0,2047  1,092 10 3  0,2603 10 6 (kJ / kmol.K )
Når per mol bruk molfraksjon for blanding
Når per masse bruk massefraksjon for blanding
Smith o.a. (1996)
Heat Capacity Natural Gas
Temperature, Pressure, Relative Density
a = 0.90
b = 1.014
c = -0.700
d = 2.170
e = 1.015
f = 0.0214
Moshfeghian, M. (2013)
  
C p  a  b T  d  e  p  

 0.60 

T
c
p
f

0.025
Heat Capacity Ratio
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M = 20.43 kg/kmol
T = 30 C = 86 F
p = 80 bara
k = 1.26 (from diagram)
R = 8.314 kJ/kmol.K
z = 0.783
k = 1.26 (from “old” equation = ideal gas)
k = 1.19 (from “new” equation = real gas)
Ratio of Specific Heat Capacity
k
k
Cp
Cv
Cp
C p  zR
k
Cp
Cp  R
H
k
U
Heat Capacity Ratio
Hysys Estimation
GPA (1998) Associated Gas
Comparison of Heat Capacity Ratio
GPA (1998) Gas at 80 bara & 30 C
k
Diagram
Ideal gas
Ideal gas with z-correction
Hysys ideal gas
Hysys
1,26
1,26
1,19
1,158
1,714
Ideal Gas Compression
m (kg/s)
70
M (kg/kmol)
20,43
R (J/kmolK)
8314,34
T innløp C
30
k (Cp/Cv)
1,26
p innløp bara
80
p utløp bara
160
P (MW)
6,44
T utløp C
77
k 1


k
 k   p2 
m
   1
Pideal 
RT1 

M
 k 1   p1 


Real k Gas Compression
m (kg/s)
7
M (kg/kmol)
20,43
R (J/kmolK)
8314,34
T innløp C
30
k (Cp/Cv)
1,19
p innløp bara
80
p utløp bara
160
P (MW)
6,33
T utløp C
65
k 1


k
 k   p2 
m
   1
Pideal 
RT1 

M
 k 1   p1 


Viscosity from Diagram
– Diagram shows viscosity against temperature for gas
components (methane, ethane, propane etc.) at atmospheric
pressure.
– Empirical equation (shown under, based on kinetic theory of
gases) gives estimate of viscosity to natural gas (mixture of
methane, ethane, propane etc.) at atmospheric pressure.
– Diagram gives viscosity ratio to viscosity at atmospheric
pressure against reduced pressure and temperature

1/ 2

y
M
 i i i
1/ 2
y
M
 i i
Katz o.a. (1959), fra Rojey o.a. (1997)
Katz o.a. (1959), fra Rojey o.a. (1997)
Viscosity Correlation
– Several correlations in literature, for example Carr
et al. (1954), Lee et al. (1966) and Pedersen et al.
(1987).
– Lee et al. (1966) correlation has recently been
evaluated by Jeje and Mattar (2004) and has the
form shown below. K, X and y are given by
empirical equations.
– The correlation is available as spreadsheet.
  K exp X
y

Combined Cycle Power Plant
Combustion and Calorific Value
Summary
– Several physical and thermodynamic properties of natural
gas are used in natural gas calculations.
– Real gas law and reduced pressure and temperature used in
diagrams to obtain z-factor.
– Empirical correlations used for transport properties, for
example for viscosity.
– Heat capacity at constant pressure can be obtained from
figures and equations. Also as function of pressure,
temperature and relative density.
– Heat capacity ratio values uncertain.
– Equation Of State (EOS) used in computer programs for pVT
properties (also thermodynamic properties).
References
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Beggs, H.D. (1984): Gas Production Operations, OGCI Publications, Tulsa, Oklahoma
Campbell, J.M. (1994): Gas Conditioning and Processing, Campbell Petroleum Series,
Norman, Oklahoma.
Fletcher, P. (1993): Chemical Thermodynamics, Longman, Harlow, Essex.
Gas Processors Association (1998): Engineering Data Book, Tulsa, Oklahoma.
Gupta, N. (2011): Overview of Ormen Lange Project, Guest Lecture, TPG 4140 Natural
Gas, NTNU.
Jeje. O. & Mattar, L. (2004): Comparison of Correlations for Viscosity of Sour Natural
Gas, 5th Canadian International Petroleum Conference, Calgary, Alberta, June 8-10,
Paper 2004-214.
Moshfeghian, M. (2013): Variation of Natural Gas Heat Capacity with Temperature,
Pressure and Relative Density, J.M. Campbell & Co., (Internett February 2013).
Rojey, A. (1997): Natural Gas, Éditions Technip, Paris.
Smith, J.M., Van Ness, H.C. & Abbott, M.M. (1996): Introduction to Chemical
Engineeering Thermodynamics, McGraw-Hill, New York.