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 Millersmiller@spectrasensors.com

• 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 Gassmiller@spectrasensors.com(909) 979-4677