COMPARISON AND ANALYSIS OF MOISTURE DEWPOINT EQUATIONS Presenter: Sam Miller
[email protected] Technical Credit: Kurt Klavuhn, PhD Product Engineering Manager SpectraSensors 2010 Analytical Solutions for Energy Optimization & Environmental Compliance The 54th Annual Symposium of the Analysis Division Houston, Texas, USA; 19-23 April 2009
Overview
• Background • Dewpoint Eq Equations • Application
Overview
• Background • Dewpoint Eq Equations • Application
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
Dewpoi Dewpoint/ nt/Fro Frostp stpoin ointt (°F, (°F, °C) °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 state See next slide
Molecular weight H2O = 18.01528 g/mole
Rules of thumb 1 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
Operational goal: prevent liquid drop out Maintain gas temperature above dewpoint temperature
Chilled Mirror
Dewpoint Measurements
Dewpoint Analyzer 3 Lookup Water Content
1 Pick Dewpoint
2 Pick Pressure
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 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
Moisture Analyzer
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
Comparison at atmospheric pressure Source: http://cires.colorado.edu/~voemel/vp.html
What about natural gas? Air (Ideal Gas)
Natural Gas (Compressibility factor < 1)
Atmospheric Pressures
Pressures up to 4000 psi [275 bar]
Mostly N2; little change in composition
Gas composition affects dewpoint calculation
Buck (HCON)
ASTM D1142
Goff Gratch / Sonntag
ISO 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 therefrom of 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 Excerpts from ISO 18453 Pages: 72.5 to 1450 psig
5 to 41°F
±~0.5-1.0 lb/mmscf depending on pressure
ISO 18453:2006 Experimental gas compositions from GERG TM14
Dewpoint Comparison Method Comparison using NG1 Composition
72.5 psi
1450 psi
~3 lb/mmscf
32°F
Dewpoint Comparison Method Comparison using NG3 Composition
72.5 psi
1450 psi
~3 lb/mmscf
32°F
Dewpoint Comparison Method Comparison using NG4 Composition
72.5 psi
1450 psi
~3 lb/mmscf
32°F
Dewpoint Comparison Method Comparison using NG7 Composition
72.5 psi
1450 psi
~3 lb/mmscf
23°F
Dewpoint Comparison Comparison of NG1, NG3, NG4 and NG7 Composition 870 psi
~3 lb/mmscf
Δ36°F
Dewpoint Comparison Including Buck (HCON)
(1) (-9°F , 3 lb/mmscf)
Dewpoint Example IGT-8
ISO
ASTM eq(1)
ASTM eq(2)
1.5 lb/mmscf (31 ppmv)
>10°C disagreement at 1000psi
22 lb/mmscf (459 ppmv)
1.5lb/mmscf & 10 psig
Using IGT-8, the dewpoint is -10°
Using ISO, the dewpoint is +10°
Low Dewpoint estimates can ca condensation
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 CO 2-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 the importance 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 Point Temperature” 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
