laser spectrometers for online moisture measurement in
TRANSCRIPT
Laser Spectrometers for Online
Moisture Measurement in Natural
Gas
Ken Soleyn
GE M&C
2 GE Title or job number
4/1/2013
Introduction • TDLAS (Tunable Diode Lase Absorption Spectroscopy)
Moisture Analyzers have become the technology of choice for natural gas
• Natural gas is dehydrated for storage, transportation, contractual and intercompany requirements (7 lbs/mmscf tariff for interstate transfer in the USA)
• Dehydration is a “trade-off” between costs/energy consumption and gas quality
• Based on Fundamental Beer-Lambert Law
• Non-Contact
• Fast Response
• Field calibration not required
• Higher initial cost; lower cost of ownership and greater precision
• Discussion: Can these instruments be used in closed loop control for optimization of the dehydration process?
5 GE Title or job number
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Desiccant Dryers
Sorbent material such as activated
alumina, molecular sieve etc.
Multiple columns
Of line column is regenerated by
backpurgging with hot lower
pressure dry gas
Capable of drying gas to dew
points of <-100C and sub ppmv
level
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Refrigeration
Gas is cooled below the dew
point (water and Hydrocarbon)
Water & heavy hydrocarbon
condense and are separated
Heavier hydrocarbons may be
separated into liquids
fractionally by temperature
(Inverse of fraction
distillation)
Feed gas must be dried to
prevent ice formation in heat
exchangers
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Beer-Lambert Law
A = Absorption
I = The measurement of beam
intensity tuned to an absorbing
frequency
Io = The reference measurement or
beam intensity when tuned away
from the moisture absorbing
frequency
S = The fundamental absorption line
strength
L = The path length of the beam
through the sample.
N = The mole fraction of water
contained in the beam path passing
through the sample
ln is the natural log
NLS)I
Iln(A 0
The Beer-Lambert law states
that the mole fraction
(concentration) of a gas can
be measured by the intensity
of light at a given wavelength
entering the medium and
comparing the intensity of
light exiting the medium.
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Wavelength Modulated Spectroscopy If the light is “tuned” to a specific wavelength the analyte will absorb
a portion of the photonic energy. The light energy is not lost but is
converted to other types of energy.
Water’s O-H (Oxygen-Hydrogen bond) embodies very unique
stretching and vibro-rotational characteristics resulting in
absorption at very specific wavelengths of light. Water has
particular absorption bands for light in near infrared spectrum (800-
2500 nm).
The laser is typically held to a constant base temperature by thermal
bonding to a thermoelectric heat pump. By ramping the injection
current the laser is swept through a narrow frequency band. The
current ramp is also modulated at high frequency
This technique is known as wavelength modulation spectroscopy
(WMS).
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Wavelength Modulation
• Current ramp applied to tunable laser
• Results in sweep of frequency in the near-infrared
• Laser is modulated at high frequency
• Signal loss (direct absorption) is attributable to water
Current ramp Direct absorption signal
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2F Absorption at Various Concentrations
-0.150000
-0.100000
-0.050000
0.000000
0.050000
0.100000
0.150000
0.200000
1 8
15 22 29 36 43 50 57 64 71 78 85 92 99
106
113
120
127
134
141
148
155
162
169
176
183
2F A
bsop
tion
9 ppmv
25 ppmv
46 ppmv
99 ppmv
246 ppmv
504 ppmv
995 -ppmv
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Psychrometric Relationships Dew Point & Water Vapor Pressure
t97.240
t502.17exp1121.6KP w)water(20H
t55.272
t452.22exp1115.6KP i)Ice(20H
P10x46.30007.1K 6
w
P10x18.40003.1K 6
i
PH20 (water) = Saturation water vapor pressure over water in mBar
PH20 (ice)= Saturation water vapor pressure over ice in mBar
t = Dew/Frost Point Temperature in °C
Kw = Coefficient to correct for deviation from an ideal gas for liquid
water in standard air
Ki = Coefficient to correct for deviation from an ideal gas for ice in
standard air
P = Absolute pressure in mBar
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Dew Point vs. Lbs/MMSCF
-100.00
-90.00
-80.00
-70.00
-60.00
-50.00
-40.00
-30.00
-20.00
-10.00
0.00
10.00
20.00
30.00
40.00
0.10 1.00 10.00 100.00
Fro
st P
oin
t (F
)
lbs/mmscf
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Psychrometric Relationships Absolute Humidity & Concentration
n21T P...PPP
Dalton’s Law
PT = Total Absolute Pressure
P1, P2, …Pn = Component Gases
6
T
20Hv 10
P
Pppm
Concentration
Absolute Humidity
16.273T
P216
m
g 20H
3
PH20 = Partial Pressure of H2O
T = Temperature in C
From To Multiply By
mg/m3 g/m3 1000
lbs/mmscf mg/m3 16.02
lbs/mmscf ppmv 21.07
mg/m3 ppmv 1.32 PH2O = Partial Pressure of H2O
PT = Total Pressure
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Response Time (Dry to Wet)
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
17:28:19 17:29:46 17:31:12 17:32:38 17:34:05 17:35:31 17:36:58 17:38:24 17:39:50 17:41:17
Step Change: 11 to 5,375 ppmv
Response Time: 40 Seconds
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Response Time (Wet to Dry)
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
17:42:43 17:44:10 17:45:36 17:47:02 17:48:29 17:49:55 17:51:22 17:52:48
Step Change: 5,375 to 11 ppmv
Response Time: 50 Seconds
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Process Upset Simulation
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
16:2
8:2
5
16:2
8:4
8
16:2
9:1
2
16:2
9:3
5
16:2
9:5
9
16:3
0:2
2
16:3
0:4
5
16:3
1:0
9
16:3
1:3
2
16:3
1:5
6
16:3
2:2
0
16:3
2:4
3
16:3
3:0
6
16:3
3:3
0
16:3
3:5
3
16:3
4:1
7
16:3
4:4
0
16:3
5:0
4
16:3
5:2
7
16:3
5:5
1
Flow Rate: 1
SCFH
Length of sample
Tubing: 10 Ft
• Water was injected into a 8 ppmv natural gas sample stream.
• The system immediately spiked to 3000 ppmv
• Recovery: Within 4 ppm after 4 minutes.
• Full recovery took 7 minutes
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Pressure & Temperature Compensation
At constant water
concentration and increase in
pressure and temperature
reduces the 2F absorption
peak.
This is due to more
interactions between water as
well other molecules in the
carrier gas (collision
broadening)
By varying the pressure and
temperature calibration data
can be stored in the
analyzer’s memory and used
for compensation
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Gas Composition
As the background gas composition in natural gas changes the dynamics of the
interactions between water and other molecules change the absorption
characteristics. The graph above shows the influence of changing methane
concentration from 98 to 88.6% Methane at constant water concentration of 200
ppmv
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Calibration – Phase 1 N2 Carrier
Two pressure system :
saturates gas at
elevated temperature
and low temperature
6 Point Calibration
stored in analyzer for
linear regression
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Calibration – Phase 2, Methane or Custom Gas Mixture Carrier
A Permeation generator utilizes membrane to
evaporate water into dry gas and controlled rate
Rate is related to temperature
The outflow of permeation cell is diluted with
zero gas to achieve the desired concentration
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Automated Calibration System
• The data is stored in an enterprise server
• Redundant standards: Generator + Chilled Mirror
• “Golden Test Unit” run on every calibration to study long-term drift
characteristics - after 2-1/2 years - no measureable drift
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Conclusion/Discussion: Gas Dehydration Optimization
• Does the potential exists to realize significant cost savings and reduce energy consumption while at the same time precisely adhere to both contractual and intercompany specifications?
• Precision and fast response enables continuous “around the clock” moisture measurement.
• Initial cost greater than other moisture measurement technologies however the cost of ownership is often better due to the minimal maintenance, and reliability.
• Reliable standard to avoid “shut in” due to non-compliant gas caused by high moisture content
• Real-time blending of low moisture gas with wetter gas supplies or bypassing dehydration systems?