Concept for a new
accurate LNG-densimeter
M. Richter, R. Kleinrahm, R. Span
1st international workshop “Metrology for LNG“
Stockholm, November 09, 2010
Contents
• Necessity and benefit of new measurements
• Literature study - Density measurements on LNG
• Densimeters for LNG and their uncertainties
• Expertise in density measurement at RUB
• Concept for a new accurate LNG-densimeter
• Outlook
• Conclusion
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Necessity and benefit of new measurements
Detailed studies at RUB have shown:
1. Billing of LNG* (only low pressures)
* According to GIIGNL - LNG Custody Transfer Handbook
E = V · LNG T, p, x · HS,LNG(x) [kWh]
•
Revised Klosek and McKinley EOS for determination of densities
(only liquid region: T = 90K to 115K, p ≈ 1bar,  = 0.1% ???)
•
New Reference EOS: GERG 2004 (entire fluid region: T = 90K to
450K, p  700bar,  = 0.1% to 0.5% for LNG in the liquid region)
 Deviation between Klosek – McK. and GERG 2004 up to 0.2%
 Problem: No model can be clearly recommended!
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Comparison:
GERG-2004 EOS and Revised Klosek-McKinley Method
with saturated-liquid density measurements of LNG
• Uncertainty of the
measurements:
’’ = 0.1% (given by
authors + uncertainty
of the composition =
Total uncertainty ≈ 0.3%)
(estimated by RUB)
• Revised Klosek-McKinley
method was fitted to some
of these experimental data
• GERG 2004 EOS describes these data within
the given uncertainty of
0.1 to 0.5% in this region
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Necessity and benefit of new measurements
Detailed studies at RUB have shown:
2. Simulation of LNG related processes (entire fluid region)
•
Mostly cubic but also empirical EOS are used
(e.g. Peng-Robinson, Redlich-Kwong-Soave, Lee-Kesler-Plöcker)
•
Liquid / supercritical region: large deviations between calculations
with cubic/empirical EOS and GERG 2004
 New reference data is essential to verify thermophysical models!
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Comparison:
GERG-2004 EOS and other equations of state (EOS)
used for process-simulation
LNG composition:
CH4
:
83.2 mol-%
N2
:
0.9 mol-%
C2H6 :
11.8 mol-%
C3H8 :
3.5 mol-%
C4H10 :
0.6 mol-%
• cubic equations of state are
used for process simulation
• they show large deviations
from values calculated with
the GERG 2004 EOS in the
regions of interest
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Literature study – Density measurements on LNG
Fluid
Authors
p,T-range
Number of points
Total
g
l
p>pk
LNG
-
-
-
Result: Poor data
Literature
Research
Hiza + Haynes
T: 105-120K
38
-
LNG
p: 0.04-1.2 MPasituation for
(1980)
LNG
Haynes (1982)
LNG
Klosek +
McKinley (1968)
T: 110-135 K
p: 0.07-0.6 MPa
71
-

62
-
-
-
Method
GERG
2004
database
Magnetically
0.1
38
susp. sinker
0.1
Magnetically
susp. sinker
62
0.5
Pycnometer
71
Summary:
Number of measurements
T: 95-125 K
p:  p u


Haynes et al. (NIST) + Klosek – McK. (Industry): 171 (sat. LNG)
CH4 +
N2
Hiza et al.
(1977)
T: 95-140 K
21
NIST:MPa
ca. 340021(pure -fluids +- mixtures)
p: 0.1-2.1
0.1
Magnetically
susp. sinker
CH4 +
N2
Straty + Diller
(1980)
T: 92-320 KRUB: ca. 3200 (pure fluids)
478
x
x1
x
p: 0.6-35.6 MPa
-
0.15
Pycnometer
CH4 +
CO2
Magee + Ely
(1988)
T: 225-400 K
91
x
x1
x
Industry:
p: 2-35
MPa ca. 350 (pure fluids + mixtures)
0.1
Pycnometer
CH4 +
C2H6
Hiza et al.
(1977)
T: 105-140 K
p: 0.03-0.4 MPa
CH4 +
C2H6
Haynes et al.
(1985)
T: 105-320 K
p: 1.7-35.9 MPa
CH4 +
C2H6
this experimental field
0.1
19
Pycnometer
 No data available for LNG in the homogeneous liquid region
CH4 +
C2H6
Hiza + Haynes
(1978)
T: 125-135 K
p: 0.2-0.4 MPa
Other experimenters: ca. 400 (pure fluids + mixtures)
-
-
20
0.1
Conclusion:
414
x
x1
Magnetically
susp. sinker
x
-
0.1
Pycnometer
20
-
 Only
a 91-116
few groups
work in
Rodosevich
+
T:
K
19
Miller (1973)
p: 0.02-0.9 MPa
5
-
-
Richter et al. | workshop “Metrology for LNG“ | November 2010
-
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0.1
Magnetically
susp. sinker
7
Densimeters for LNG and their uncertainties
Authors
Klosek and
McKinley (1968)
Haynes and
Frederick (1983)
Kleinrahm and
Wagner (1984)
Brachthäuser,
Kleinrahm, Lösch
and Wagner (1993)
Air Products, USA
NIST
RUB
RUB
pycnometer
magnetically
suspended sinker
two sinker
hydrostatic balance
with MSC
single sinker
hydrostatic balance
with MSC
t - range
150°C to 180°C
200°C to 50°C
210°C to 70°C
40°C to 250°C
p - range
p = pa
p  350 bar
p  120 bar
p  300 bar
 
0.5%
 0.1% (plus unc.
of composition)
Total: ~ 0.3%
0.02% to 0.01%
 0.02%
Main
Problems
VLE-Measurement
VLE-Measurement
⇒ demixing of the
liquid
⇒ only saturated
liquid densities
⇒ demixing of the
liquid
⇒ only saturated
liquid densities
Until today:
mixtures measured
only in gas phase
Until today:
mixtures measured
only in gas phase
Technique
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Expertise in density measurement at RUB
Kleinrahm (~1980)
… and other research institutes as well as industrial
companies (RWTH, TUDO, Max Planck institute,
Fraunhofer institute, Bayer, BASF, Daimler …)
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High-Accuracy Two-Sinker Densimeter
for Natural Gas
(Developed for E.ON Ruhrgas, 1991)
Expertise in density measurement at RUB
Two-sinker Reference Densimeter
for Natural Gas at Standard Conditions
(Developed for E.ON Ruhrgas, 2004)
Portable Reference Densimeter
for Checking the Density
Determination in Natural gas
Meter Runs
(Developed for E.ON Ruhrgas, 2001)
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Specifications for a new LNG-Densimeter
• Quantity to be measured: Density of LNG:
• in the homogeneous liquid region
• on the saturated-liquid line, including vapour pressure
• in the homogeneous gas region
• Temperature Range:
• Pressure Range:
• Density Range:
• Total uncertainty :
90 K to 290 K
0.05 MPa to 12 MPa
10 kg/m3 to 1000 kg/m3
0.02 %
(plus uncertainty resulting from gas analysis)
• Measuring principle:
Single-Sinker method
(Silicon sinker: V = 25.7 cm3, m = 60 g)
• Temp. measurement:
• Pressure measurement:
• Density measurement:
• Gas analysis:
• Thermostating:
• Insulation:
Uncertainty approx. 0.015 K
Uncertainty approx. 0.01 %
Uncertainty approx. 0.01 % (for liquids)
Only one analysis of the 0.050 m3 gas cylinder is required
Liquid nitrogen (LN2)
Outer cylinder, filled with Argon at about 2 kPa
as insulation gas
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p,T – diagram of a typical natural gas
Illustration of the measurement points
(Calculated with the GERG-2004 equation of state)
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Choosen for new
LNG density
measurements:
Single-sinker
densimeter
technique
by Brachthäuser,
Kleinrahm, Lösch
and Wagner (1993)
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Difficulties regarding the density
measurement of LNG
[see e.g. Klosek and McKinley (1968) or Haynes and Frederick (1981)]
The three main problems
Solutions
• Decomposition of the fluid
• during the filling process
• Filling of the densimeter at ambient
• temperature Ta up to a pressure p > pCP
• and then isobaric cooling (e.g. at 12 MPa)
• Accurate pressure measurement
• (or differential pressure measurement)
• at low temperatures is not possible
• Pressure measurement at Ta and
• integration of a special reference cell
• Decomposition of the fluid in the measuring
• cell resulting from the existing VLE state
• (A gas phase is not avoidable because the pressure
• sensor is located at ambient temperature)
• ⇒ Controlled phase transition liquid-gas (VLE)
• ⇒ in the reference cell and specific adjustment
• ⇒ of the fluid pressure
• Only homegenous liquid phase in the
• measusring cell (due to the integration
• of a refenrence cell)
• Max. 1% - 3% vapour volume in the
• measuring cell during the measurement
• of saturated-liquid densities
Richter et al. | workshop “Metrology for LNG“ | November 2010
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p,T – diagram of a typical natural gas
Illustration of the filling and measurement procedure
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Schematic of the
density system
and thermostat
of the new
LNG-densimeter
Richter et al. | workshop “Metrology for LNG“ | November 2010
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Setup of the new LNG-densimeter
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Outlook
• Set-up of new densimeter completed by end of 2011
• Comprehensive LNG-density measurements on 4 LNG
qualities completed by the beginning of 2013
• Review of relevant equations of state done by April 2013
 Depending on the results of the measurements:
Further measurements and remodelling of equations of state
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Conclusion
• New reference data is definitely needed!
• Expertise at RUB
accurate density measurement
techniques successfully proven at low temperatures since
30 years
self-help capacity for unexpected challanges
• Main problems with LNG density measurement identified
and solved (innovation: special reference cell)
• Concept for a new accurate LNG-densimeter is done
• Detail-engineering + planning for first orders are in progress
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Project 1.67: Density of LNG
Thank you for your attention!
www.lngmetrology.info
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