1PROPERTIES OF NATURAL GASJn Steinar Gumundsson

TPG4140 NaturgassSeptember 16, 2008

Introduction Equations and properties used in course Real gas law, z-factor, density and FVF Corresponding states Viscosity of natural gas Heat capacity and heat capacity ratio Water vapour in natural gas Summary

2Properties Used in Equations

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 calorific value and relative density Hydrates and water, need water vapour in natural gas

(diagram or PVT package)

3Real Gas Law

=

=

==

zTT

ppqq

zTT

ppVV

z

zRTpvznRTpV

sc

scsc

sc

scsc

sc

1

1

1

4Density and FVF

( )

Bqq

zpp

TTB

Smm

VVFVFB

zRTpMnVM

znRTpV

sc

sc

sc

sc

=

=

=

=

==

3

3

5Finding 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.

6Real gas factor with pressure at 25 C

Fletcher (1993)

7Real gas factor with reduced p & T

Fletcher (1993)

8Reduced p & T

( )( )

ii

cic

iicic

cprr

cprr

yTT

yppTTTT

pppp

==

==

==

Kays Rule

9Corresponding 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 multi-component systems.

Commonly used when gas properties (natural gas and other gases) are to be correlated and/or presented.

10

GPA (1998)

11

Rojey o.a. (1997)

12

Equations and Gravity

.).(964,28

)(75,17072,94

)(405,0892,4

csvedMMM

KT

MPap

gass

luft

gass

pc

pc

==

==

o

Thomas o.a. (1970), fra Rojey o.a. (1997)

13

Physical Constants of Natural Gas

Campbell (1984)

14

Example Calculation for Natural Gas

GPA (1998)

15

Viscosity from Diagram

Diagram shows viscosity against temperature for gas components (methane, ethane, propane etc.) at atmospheric pressure.

Empirical equation (shown under) 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

= 2/1

2/1

ii

iii

MyMy

16

Katz o.a. (1959), fra Rojey o.a. (1997)

17

Katz o.a. (1959), fra Rojey o.a. (1997)

18

Viscosity from 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 on home page to course, link shown below.

www.ipt.ntnu.no/~jsg/undervisning/naturgass/BeregningerDiverse.xls

( )yXK exp=

19

Campbell (1984)

20

Beggs (1984)

21

Heat Capacity

( )

blandingforjonmassefraksbrukmasseperNrblandingfornmolfraksjobrukmolperNr

KkmolkJC

KkmolkJR

TTRC

CTBTARC

TTC

CHp

CH

p

p

p

)./(102603,010092,12047,0

)./(314,8

10164,210081,9702,1

63

263

2

2

4

4

+=

=

+=

++=++=

22

Smith o.a. (1996)

23

GPA (1998)

24

Tools, Data & HysysBEREGNINGER DIVERSE (EXCEL) - Calculations Miscellaneous(Excel)GASS-SAMMENSETNING EKSEMPLER - Gas CompositionExamplesGASS-SAMMENSETNING I EKSPORTRRLEDNINGER - Gas Composition in Export PipelinesFLUIDEGENSKAPER - Fluid Properties

OM KOMME I GANG MED HYSYS (fra N. Nesse)LREPROGRAM - TutorialINSTALLASJON AV PROGRAMPAKKE - Installation of Program Package

25

Summary

Several physical and thermodynamic properties of natural gas are used in course.

Real gas law and reduced pressure and temperature used in diagrams.

Empirical correlations used for transport properties, for example for viscosity.

Heat capacity can be obtained from figures. Equation Of State (EOS) used in computer programs for

pVT properties (also thermodynamic properties). Hysys available at NTNU (see info. on home page).

26

References Beggs, H.D. (1984): Gas Production Operations, OGCI Publications, Tulsa, Oklahoma Campbell, J.S. (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. 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. Rojey, A. (1997): Natural Gas, ditions Technip, Paris. Smith, J.M., Van Ness, H.C. & Abbott, M.M. (1996): Introduction to Chemical Engineeering

Thermodynamics, McGraw-Hill, New York.