AGA 2011 PPT Comparison of Dewpoint Methods Natural Gas
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Transcript of AGA 2011 PPT Comparison of Dewpoint Methods Natural Gas
Analytical Solutions for Energy Optimization & Environmental ComplianceThe 54th Annual Symposium of the Analysis DivisionHouston, Texas, USA; 19-23 April 2009
COMPARISON AND ANALYSIS OF MOISTURE DEWPOINT EQUATIONS
Technical Credit: Kurt Klavuhn, PhDProduct Engineering Manager
SpectraSensors 2010
Presenter: Sam [email protected]
• Background
• Dewpoint Equations
• Application
Overview
Moisture Measurement Units
Molar Fraction (%, ppmv, ppbv, pptv)
Weight per volume (lb/mmscf, mg/m3, ppmw) Normalized to Standard T & P (STP)
Dependant on molecular weight
Dewpoint/Frostpoint (°F, °C) Dependant on pressure
Relative Humidity Percent of saturation at a particular T & P
(100% RH is fully saturated)
Units – Molar Ratio
Molar Ratio (%, ppmv, ppbv, pptv) Molecules per molecule
Mole, mass or volume fraction
Independent of reference state
Used in spectroscopic methods
Conversions not necessary with pressure cuts
Units – Weight per Volume
Weight per volume
E.g. lb/mmscf, mg/m3, ppmw
Dependent on reference stateSee next slide
Molecular weight H2O = 18.01528 g/mole
Rules of thumb1 lb/mmscf = 21.1 ppmv*1 mg/m3 = 1.32 ppmv*
* Depends on reference state Definition
Common Reference Standards
Conversions depends on definition of STP
Why is weight per volume important?
Industry standard unit of measurement
In gas processing: Weight of liquid removed per volume of gas processed
Commonly used in glycol contactors and other dehydration processes
Commonly used in standards,e.g. ASTM D1142
Units - Dewpoint
Expressed in °F or °C
Dewpoint changes as pressure changes
Low pressure “suppresses”dewpoint
Dewpoint v. Frost Point?
Dewpoint is slightly higher when ice is present
32°F
Why is dewpoint important?
Dewpoint is the inherent measurement on some analyzers
For example Chilled Mirror
Operational goal: prevent liquid drop out Maintain gas temperature above
dewpoint temperature
Dewpoint Measurements
Dewpoint Analyzer
Pick Dewpoint
Pick Pressure
Lookup Water Content 1
2
3
Reverse Conversion?
Some moisture analyzers inherently measure in molar fraction (ppmv) e.g. TDLAS
Operations may require dewpoint values
Dewpoint reports for corrosion department
Equations can be reversed:
Concentration Dewpoint
Dewpoint Concentration
Pressure Input to Moisture Analyzer
Moisture Analyzer
Signal Converter
Pressure Input from Pipeline Live pressure needed for dewpoint calculation
Analog Modbus input
Static pressure is an alternative
Value from pressure transmitter or RTU/Flow Computer
Conversion Methods
Arden Buck (HCON)
ASTM D1142 equation (1)
ASTM D1142 equation (2)
ISO 18453
Arden Buck (HCON)
Originally published in 1981 with enhancement factor in 1996
Intended for air in atmospheric sciences
Dewpoint and frost point formulas
Many saturation vapor pressure equations exist Goff Gratch equation 1946
Hyland and Wexler 1983
Buck 1981
Sonntag 1994
Magnus Tetens, Bolton, Murphy and Koop, Wagner and Pruß,
Comparison to Goff Gratch
Source: http://cires.colorado.edu/~voemel/vp.html
Comparison at atmospheric pressure
What about natural gas?
Air(Ideal Gas)
Natural Gas(Compressibility factor < 1)
Atmospheric Pressures
Pressures up to4000 psi [275 bar]
Gas composition affects dewpoint calculation
Mostly N2; little change in composition
Buck (HCON) Goff Gratch / Sonntag
ASTM D1142ISO 18453
ASTM D1142, 1995-2006
“This test method covers the determination of the water vapor content of gaseous fuels by measurement of the dewpoint temperature and the calculation therefromof the water vapor content.”
Two equations are given on page 4:
ASTM(1) ASTM(2)
ASTM D1142, 1995-2006 ASTM(1)
Expresses the water content (WC) in terms of the weight of saturated water vapor (at reference conditions
w is the weight of saturated water vapor (lb/ft3); a lookup table is provided for 0-100°F
P is the pressure at which the dew point was determined (psia)
T is the observed dewpoint temperature
Given the water content, the corresponding dew-point temperature can be solved for iteratively.
ASTM D1142, 1995-2006
ASTM(2)
Origination: Bukacek; Research Bulletin 8, Institute of Gas Technology, 1955.
Modified Raoult’s law approach where water content of sweet gas is calculated using the ideal expression supplemented by a deviation factor
Coefficients A and B listed as a function of temperature in Table 2 for dew-point temperatures ranging from –40°F to 440°F
Given the water content, the corresponding dew-point temperature can be solved for iteratively
ASTM D1142, 1995-2006
Overall Assessment: ASTM methods are convenient and simple
ASTM methods do not take gas composition into account
The range of data made available for the specific volume of saturated water vapor (ASTM1) or for the coefficients A and B (ASTM2) is somewhat limited
ISO 18453:2006 Based on study conducted by Groupe Europeen de
Recherches Gazieres; GERG TM14
Title: Relationship between Water Content and Water Dew Point keeping in consideration the Gas Composition in the Field of Natural Gas
Uses an equation of state (EOS) approach
Mole fractions of the components of the gaseous mixture are required - or choose a default mixture – see next slide
Iterative process – complex procedure of equalizing mixture equations; best handled with software
Widely accepted in Europe and Middle East
ISO 18453 Uncertainty Analysis
72.5 to 1450 psig
5 to 41°F
±~0.5-1.0 lb/mmscfdepending on pressure
Excerpts from ISO 18453 Pages:
ISO 18453:2006
Experimental gas compositions from GERG TM14
Dewpoint Comparison
Method Comparison using NG1 Composition
1450 psi
72.5 psi
~3 lb/mmscf
32°F
Dewpoint Comparison
Method Comparison using NG3 Composition
1450 psi
72.5 psi
1450 psi
32°F
~3 lb/mmscf
Dewpoint Comparison
Method Comparison using NG4 Composition
1450 psi
72.5 psi
32°F
~3 lb/mmscf
Dewpoint Comparison
Method Comparison using NG7 Composition
1450 psi
72.5 psi
23°F
~3 lb/mmscf
Dewpoint Comparison
Comparison of NG1, NG3, NG4 and NG7 Composition
870 psi
~3 lb/mmscf
Δ36°F
Dewpoint ComparisonIncluding Buck (HCON)
(1)(-9°F , 3 lb/mmscf)
Dewpoint Example
>10°C disagreementat 1000psi
22 lb/mmscf(459 ppmv)
ASTM eq(1)
1.5lb/mmscf & 1000 psig
Using IGT-8, the dewpoint is -10°F
Using ISO, the dewpoint is +10°F
Low Dewpoint estimates can cause condensation
1.5 lb/mmscf(31 ppmv)
IGT-8ASTM eq(2)
ISO
Conclusions
Buck Research method and ASTM1 agree very well with each other over typical pressure ranges and concentrations
The ASTM1 method agrees well with the experimental data at low pressure (5 bar) but deviates significantly at higher pressure (100 bar)
ASTM2 method exhibits similar behavior to ASTM1 but with even less agreement with experimental data especially at lower temperatures with the exception of the CO2-rich NG7 mixture at high pressure.
Of the three methods discussed, only the ISO method takes into account the actual gas composition.
The deviations in dewpoint between NG4 and NG7 at illustrate theimportance of accounting for the gas composition, especially when performing calculations with low water content at moderate to high pressure.
Dew-point temperatures calculated from water contents were validated to be generally within ±2C for pressures 0.5≤P≤10MPa and dew-point temperatures 258.15≤T≤278.15K
Conclusions
Use inherent analyzer readings if possible TDLAS ppmv Chilled Mirror dewpoint
If using dewpoint, consider changes to dewpoint value when pressures are increased or decreased
If converting to weight per volume, consider STP standard If converting from dewpoint to concentration or vice versa
Consider equation choices Take equation uncertainty into consideration Use conservative method, or Use the most appropriate method for the scenario
Final
In summary, for moderate to high water contents at low pressures, all three correlations produce acceptable results.
Although somewhat more difficult to implement, the ISO method is arguably the more accurate of the methods (especially for low water contents and high pressures) and provides a great deal more range and flexibility.
References
Buck, A. L. (1981), "New equations for computing vapor pressure and enhancement factor"
Buck (1996), Buck Research Manual ISO-18453 (2004) Standard. “Natural Gas – Correlation between Water
Content and Water Dew Point ASTM-D1142 (1995, Reapproved 2006). “Standard Test Method for
Water Vapor Content of Gaseous Fuels by Measurement of Dew PointTemperature”
Bukacek, R. F., “Equilibrium Moisture Content of Natural Gases,”Research Bulletin 8, Institute of Gas Technology, 1955.
Oellrich, L. R. and Althaus, K., “Relationship between Water Content and Water Dew Point keeping in consideration the Gas Composition in the Field of Natural Gas,” GERG Technical Monograph TM14
Questions?
For more questions, please contact the Product Line Manager at SpectraSensors Inc.
Sam MillerProduct Line Manager – Natural [email protected](909) 979-4677