Water Dew Point Measurement in Natural Gas and its Traceability Andrea Peruzzi - VSL
Water dew-point measurement in natural gas and its traceability Andrea Peruzzi and Rien Bosma Pag. 2
Outline The measurement of the water dew-point of natural gas: The water dew-point temperature of a moist gas The natural gas chain The measurement of the water dew-point of natural gas along the chain Sensor technologies Traceability of water dew-point measurements: Primary standard: high-pressure dew-point generator o Principle of operation o Design o Construction o Use Investigation performances of Al 2 O 3 sensors o Response time o Stability on time Pag. 3
Water dew-point of a moist gas Dew-point (frost-point) temperature of a moist gas: the temperature at which a sample of a moist gas must be cooled, at constant pressure, for water vapour to condense into water (ice) Why does the water vapour start condensing? Because, when cooling a moist gas, the water vapour contained in it eventually reaches saturation with respect to liquid water (ice) Dew Frost (In this case the moist gas is atmospheric air) Pag. 4
Water dew-point of natural gas Given a sample of moist natural gas: T DP = f(x W, P) (x W = mole fraction of water in the sample, P = pressure) remove water lower x W lower T DP reduce P lower T DP (but same x W ) T DP after pressure reduction (after expansion) requires the knowledge of the enhancement factor f(p, T DP ): x W f ( P 1, TDP 1) ew ( TDP 1) f ( P2, TDP2) ew ( TDP2) Transforming T DP x W requires the knowledge of the pressure P and the enhancement factor f(p, T DP ): x W P 1 f ( P, TDP ) e P W ( T DP ) P 2 Pag. 5
Natural gas chain Extraction Processing Transport Distribution Users Offshore or on-shore gas field Gas processing plant: dehydration and removal/separation of various components High-pressure pipeline or LNG Distribution to local networks through medium pressure pipeline - Power plants (35%) - Large customers (10%) - Retailers (25%) - Storage Pag. 6
Natural gas chain Extraction/ Production Processing Transport Distribution Users Offshore or on-shore gas field Gas plant: dehydration and removal/separation of various components High-pressure pipeline or LNG Distribution to local networks through medium pressure pipeline - Power plants (35%) - Retailers (25%) - Large customers (10%) - Storage Pag. 7
Natural gas processing plant Acid gases removal (CO 2 and H 2 S) Dehydration Mercury removal Nitrogen rejection NGLs recovery Extracted gas is not suitable for high-pressure transmission and consumption Dehydration: H 2 O must be removed at a level below a specified contractual value Safety: remove H 2 O to avoid corrosion, condensation and hydrates formation in the pipeline lower T DP Removing H 2 O costs money (each C of T DP means M in plant operation costs) Find the optimal balance between satisfying contractual value and cost effective operation of the plant Accurate measurement of T DP is crucial Pag. 8
Measurement at gas processing plant: glycol dehydration Measurement point: Glycol contactor Typical conditions: T DP < -30 C at 5 to 8 MPa Requirements: fast wet-up response protected against glycol or other liquid contaminants immune to chemical attack from H 2 S, mercaptans and other sulphides Accuracy: 1 C T DP (declared by manufacturer) Pag. 9
Natural gas chain Extraction/ Production Processing Transport Distribution Users Offshore or on-shore gas field Gas plant: dehydration and removal/separation of various components High-pressure pipeline or LNG Distribution to local networks through medium pressure pipeline - Power plants (35%) - Retailers (25%) - Large customers (10%) - Storage Pag. 10
Long distance transportation of natural gas? Pag. 11
Natural gas transport Transport: High-pressure pipeline (80%) LNG (20%) In both cases: Pipeline custody transfer contractual requirements o Pipeline: T DP < -10 C at 6.5 MPa EU (EASEE Madrid Forum: T DP < -8 C at 7 MPa) LNG liquefaction plant: T DP < -80 C at 7 MPa Measurement requirement: Fast wet-up response Pag. 12
Measurement of trace moisture before LNG liquefaction Measurement point: molecular sieve columns Typical requirement: T DP < -80 C at 7 MPa (0.1 ppmv) Sample point: middle bed of each tower and outlet of each tower Pag. 13
LNG chain Pag. 14
Measurement at re-gasification terminal Typical requirement: T DP < -10 C at 7 MPa (leakage indicator in heat-exchangers) Pag.
Sensor technologies Different measurement principles: Impedance sensors: polymeric and metal oxyde sensors (Al 2 O 3 (Easydew, Michell), P 2 O 5 (Accupoint LP 2, Meeco), polymer (E+E)) Spectroscopic analyzers (Aurora, GE) Calibrated at atmospheric pressure and in nitrogen or air gas (conditions completely different from field conditions) Investigation of the performances of sensors and analyzers: Response time Stability on time (drift) Pressure dependence See: An investigation of the comparative performance of diverse humidity sensing techniques in natural gas, J.G. Gallegos et al., Journal of Natural Gas Science and Engineering, 23 (2015) 407-416 Pag. 16
Primary dew-point generator Source of traceability for water dew-point temperature measurement: primary dew-point generator Thermodynamically-based: A gas stream (air, N 2, CH 4, ) is saturated with water vapour by flowing the gas over a plane surface of water (or ice) at known temperature T S and pressure P S Dry gas Moist gas Water Saturator at {T S, P S } If the gas is fully saturated in the saturator, the dew-point temperature of the moist gas drawn from the saturator is: T DP T S Pag. 17
Saturator design Design criteria: Maximize turbulent flow Reduced tilt of water surface Acceptable pressure drop Design features: Channel Segments and bends (180 each) Dams and barriers Choice of design parameters: Depth of the channel: 30 mm Number of segments and bends: 20 Number of dams and barriers: 20 Water depth: 7 mm Pag. 18
HPDP generator construction Ethanol reservoir Saturator Ethanol bath Precooler Pag. 19
Validation of HPDP generator N 2 CH 4 Pressure controller Presat CMH MBW 973 Syringe pump Pressure SPRT Ethanol bath Precooler Saturator CMH MBW 373 HPX CMH MBW 373 LX CMH Pressure Range Temperature range MBW 973 0.1 MPa -60 C to +20 C MBW 373 HPX 0.1 MPa to 10 MPa -80 C to +20 C MBW 373 LX 0.1 MPa to 0.25 MPa -95 C to +20 C Pag. 20 Validation Monitor differential response of CMHs when: changing saturator flow rate while keeping fixed flow rate at CMHs changing dew-point of inlet gas
Validation of HPDP generator (CH 4, T DP = +15⁰C, P = 6 MPa) N 2 CH 4 P PC = 6.000 MPa Pressure controller Presat P 973 = 1078 hpa CMH MBW 973 P Atm = 1024 hpa Syringe pump Ethanol bath Precooler Φ S = Φ HPX + Φ LX + Φ X Pressure SPRT Saturator P S = 6.000 MPa T S = 14.97 ⁰C P HPX = 6.000 MPa T REF1 = 14.97 ⁰C CMH MBW 373 HPX P LX = 1037 hpa T REF2 = -30.72 ⁰C CMH MBW 373 LX Φ HPX = 2 l/min Φ LX = 0.5 l/min P Atm = 1024 hpa P Atm = 1024 hpa P Atm = 1024 hpa Φ X Pag. 21
Validation of HPDP generator (CH 4, T DP = +15⁰C, P = 6 MPa) Pag. 22
Validation of HPDP generator (CH 4, T DP = -15⁰C, P = 6 MPa) N 2 CH 4 P PC = 5.997 MPa Pressure controller Presat P 973 = 1066 hpa CMH MBW 973 P Atm = 1013 hpa Syringe pump Ethanol bath Precooler Φ S = Φ HPX + Φ LX + Φ X Pressure SPRT Saturator P S = 5.997 MPa T S = -14.95 ⁰C P HPX = 5.997 MPa T REF1 = -14.95 ⁰C CMH MBW 373 HPX P LX = 1028 hpa T REF2 = -50.71 ⁰C CMH MBW 373 LX Φ HPX = 2 l/min Φ LX = 0.5 l/min P Atm = 1013 hpa P Atm = 1013 hpa P Atm = 1013 hpa Φ X Pag. 23
Validation of HPDP generator (CH 4, T DP = -15⁰C, P = 6 MPa) Measurement cycle: N 2 CH 4 N 2 CMHs start drifting after switching to CH 4 Hydrate Formation! Pag. 24
Hydrate formation line Stable response CMH s : No hydrate formation Drift in response CMH s : Hydrate formation Setup can be used at low temperatures, but only for low pressures (< 2 MPa) Pag. 25
Impedance sensor technology Hygroscopic non-conductive layer (< 1 µm) Two conductive layers (porous top layer 0.1 µm) Base ceramic substrate Absorption of water vapour Fast wet-up response Pressure rating: 30 MPa Accuracy: 1 C T DP (declared by manufacturer) Pag. 26
Investigation of Al 2 O 3 dew-point sensors Dry-to-wet response time of Al 2 O 3 sensors versus the water mole fraction in nitrogen. The graph is obtained from dew-point temperatures in the range -60 ºC to +3 ºC and pressure up to 6 MPa Pag. 27
Investigation of Al 2 O 3 dew-point sensors (time drift) Pag. 28
Conclusions The measurement of the water dew-point of natural gas is required at different points along the natural gas chain with an accuracy of 1 C The water dew-point T d of natural gas is measured on a continuous on-line basis at high pressure with humidity sensors and analyzers Sensors and analyzers are calibrated only at atmospheric pressure and in air or nitrogen The extrapolation of the calibration to field conditions (line pressure of 8 MPa and natural gas) translates into uncertainties of up to 10 C Calibration of sensors at field conditions is strongly recommended Pag. 29