Laser Spectrometers for Online Moisture Measurement in Natural Gas Ken Soleyn GE M&C ken.soleyn@ge.com
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? 2
TDLAS Moisture Analyzers - Field Proven 3
Triethylene Glycol Gas Dehydration 4
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 5
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 6
Beer-Lambert Law A = Absorption I = The measurement of beam intensity tuned to an absorbing frequency I o = 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 I0 A ln( ) S L N I 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. 7
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). 8
Schematic of TDL Analyzer 9
Wavelength Modulation Current ramp Direct absorption signal 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 10
2F Absorption at Various Concentrations 0.200000 0.150000 2F Absoption 0.100000 0.050000 0.000000-0.050000 9 ppmv 25 ppmv 46 ppmv 99 ppmv 246 ppmv 504 ppmv 995 -ppmv -0.100000-0.150000 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 11
Psychrometric Relationships Dew Point & Water Vapor Pressure P H20(water ) K w 17.502t 6.1121exp 240.97 t K w 1.0007 3.46x10 6 P P H20(Ice) K i 22.452t 6.1115exp 272.55 t K i 1.0003 4.18x10 6 P P H20 (water) = Saturation water vapor pressure over water in mbar P H20 (ice) = Saturation water vapor pressure over ice in mbar t = Dew/Frost Point Temperature in C K w = Coefficient to correct for deviation from an ideal gas for liquid water in standard air K i = Coefficient to correct for deviation from an ideal gas for ice in standard air P = Absolute pressure in mbar 12
Frost Point (F) Dew Point vs. Lbs/MMSCF 40.00 30.00 20.00 10.00 0.00-10.00-20.00-30.00-40.00-50.00-60.00-70.00-80.00-90.00-100.00 0.10 1.00 10.00 100.00 lbs/mmscf 13
Psychrometric Relationships Absolute Humidity & Concentration Dalton s Law P T P 1 P 2... P P T = Total Absolute Pressure P 1, P 2, P n = Component Gases Concentration ppm P H20 6 v 10 PT P H2O = Partial Pressure of H 2 O P T = Total Pressure n Absolute Humidity g 216PH 20 3 m T 273.16 P H20 = Partial Pressure of H 2 O T = Temperature in C From To Multiply By mg/m 3 g/m 3 1000 lbs/mmscf mg/m 3 16.02 lbs/mmscf ppm v 21.07 mg/m 3 ppm v 1.32 14
Sample Conditioning 15
Response Time (Dry to Wet) 1.E+04 1.E+03 1.E+02 Step Change: 11 to 5,375 ppmv Response Time: 40 Seconds 1.E+01 1.E+00 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 16
Response Time (Wet to Dry) 1.E+04 1.E+03 1.E+02 Step Change: 5,375 to 11 ppmv Response Time: 50 Seconds 1.E+01 1.E+00 17:42:43 17:44:10 17:45:36 17:47:02 17:48:29 17:49:55 17:51:22 17:52:48 17
16:28:25 16:28:48 16:29:12 16:29:35 16:29:59 16:30:22 16:30:45 16:31:09 16:31:32 16:31:56 16:32:20 16:32:43 16:33:06 16:33:30 16:33:53 16:34:17 16:34:40 16:35:04 16:35:27 16:35:51 Process Upset Simulation 1.00E+04 1.00E+03 1.00E+02 Flow Rate: 1 SCFH Length of sample Tubing: 10 Ft 1.00E+01 1.00E+00 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 18
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 19
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 20
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 21
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 22
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 23
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? 24