Laser Spectrometers for Online

Moisture Measurement in Natural

Gas

Ken Soleyn

GE M&C

ken.soleyn@ge.com

2 GE Title or job number

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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?

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TDLAS Moisture Analyzers - Field Proven

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Triethylene Glycol Gas Dehydration

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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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Schematic of TDL Analyzer

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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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Sample Conditioning

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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?