Water Dew Point Measurement in Natural Gas and its ... · PDF fileWater dew-point measurement...

Water Dew Point Measurement in

Natural Gas and its Traceability

Andrea Peruzzi - VSL

Pag. 2

Water dew-point measurement

in natural gas and its traceability

Andrea Peruzzi and Rien Bosma

EUROPEAN FLOW MEASUREMENT WORKSHOP

Noordwijk, 18 March 2015

Pag. 3

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 Al2O3 sensors

o Response time

o Stability on time



Pag. 4

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. 5

Given a sample of moist natural gas:

TDP = f(xW, P) (xW = mole fraction of water in the sample, P = pressure)

• remove water → lower xW → lower TDP

• reduce P → lower TDP (but same xW)

TDP after pressure reduction (after expansion) requires the knowledge of the

enhancement factor f(P, TDP):

Transforming TDP → xW requires the knowledge of the pressure P and the enhancement

factor f(P, TDP):

Water dew-point of natural gas

P

TeTPfx DPWDP

W

)(),(



2

222

1

111 )(),()(),(

P

TeTPf

P

TeTPfx DPWDPDPWDP

W

Pag. 6

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



Natural gas chain

Pag. 7

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


networks through medium

pressure pipeline


- Retailers (25%)


- Storage



Natural gas chain

Pag. 8

Natural gas processing plant



Extracted gas is not suitable for high-pressure transmission and consumption

Dehydration: H2O must be removed at a level below a specified contractual value

• Safety: remove H2O to avoid corrosion, condensation and hydrates formation in

the pipeline → lower TDP

• Removing H2O costs money (each °C of TDP means M€ in plant operation costs)

Find the optimal balance between satisfying contractual value and cost effective

operation of the plant

Accurate measurement of TDP is crucial

Acid gases removal (CO2 and H2S)

Dehydration Mercury removal

Nitrogen rejection

NGLs recovery

Pag. 9

Measurement at gas processing

plant: glycol dehydration

Measurement point: Glycol contactor

Typical conditions: TDP < -30 °C at 5 to 8 MPa

Requirements:

• fast wet-up response

• protected against glycol or other liquid

contaminants

• immune to chemical attack from H2S,

mercaptans and other sulphides

Accuracy: 1°C TDP (declared by manufacturer)



Pag.

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


networks through medium

pressure pipeline


- Retailers (25%)


- Storage



Natural gas chain

10

Pag. 11 EUROPEAN FLOW MEASUREMENT WORKSHOP


Long distance transportation of

natural gas?

Pag.

Natural gas transport

12 EUROPEAN FLOW MEASUREMENT WORKSHOP


Transport:

• High-pressure pipeline (80%)

• LNG (20%)

In both cases:

• Pipeline custody transfer contractual requirements

o Pipeline: TDP < -10 °C at 6.5 MPa

EU (EASEE – Madrid Forum: TDP < -8 °C at 7 MPa)

• LNG liquefaction plant: TDP < -80 °C at 7 MPa

Measurement requirement:

• Fast wet-up response

http://www.futurity.org/earth-environment/is-natural-gas-the-bridge-to-cleaner-energy/

http://maritime-connector.com/image/arctic-princess-9271248/215/

Pag. 13

Measurement of trace moisture

before LNG liquefaction

Measurement point: molecular sieve columns

Typical requirement: TDP < -80 °C at 7 MPa (0.1 ppmV)

Sample point: middle bed of each tower and outlet of each tower



Pag. 14

LNG chain



Pag.

Measurement at re-gasification

terminal

Typical requirement: TDP < -10 °C at 7 MPa (leakage indicator in heat-exchangers)





Sensor technologies

Different measurement principles:

• Impedance sensors: polymeric and metal oxyde sensors (Al2O3

(Easydew, Michell), P2O5 (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

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://cosaxentaur.com/page/516/dew-point-transmitter-hybrid-hdt&ei=07r-VKjPGsa-PPnxgaAK&bvm=bv.87920726,d.ZWU&psig=AFQjCNFunDkKq-RZxOkau-C1kVFCE90NEQ&ust=1426066510594764

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=https://www.instrumart.com/products/31157/ge-panametrics-aurora-moisture-analyzer&ei=U73-VKTsNoP0OtqNgPAN&bvm=bv.87920726,d.ZWU&psig=AFQjCNHsAQ2tQb1F0lWiZcWcp03TEdN0Pg&ust=1426067110423049



Primary dew-point generator

Source of traceability for water dew-point temperature measurement:

primary dew-point generator

Thermodynamically-based: • A gas stream (air, N2, CH4, …) …

• … is saturated with water vapour…

• … by flowing the gas over a plane surface of water (or ice)…

• … at known temperature TS and pressure PS

• If the gas is fully saturated in the saturator, the dew-point temperature of

the moist gas drawn from the saturator is:

Dry gas Moist gas

Saturator at

{TS, PS}

Water

SDP TT

Pag. 18

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

Saturator design

Pag. 19

HPDP generator construction

Saturator

Ethanol

reservoir

Ethanol

bath

Precooler



Pag. 20 20

Pressure

controller

Syringe

pump

Presat

Saturator

Precooler

Ethanol

bath

SPRT

Pressure

CMH

MBW

373 LX

CMH

MBW

373 HPX

CMH

MBW

973

N2

CH4

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

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

Pag. 21

Pressure

controller

Syringe

pump

Presat

Saturator

Precooler

Ethanol

bath

SPRT

Pressure

CMH

MBW

373 LX

CMH

MBW

373 HPX

CMH

MBW

973

N2

CH4

PPC = 6.000 MPa P973 = 1078 hPa

PAtm = 1024 hPa

PS = 6.000 MPa

TS = 14.97 ⁰C

PHPX = 6.000 MPa

TREF1 = 14.97 ⁰C

PAtm = 1024 hPa

PAtm = 1024 hPa

PAtm = 1024 hPa

PLX = 1037 hPa

TREF2 = -30.72 ⁰C

ΦS = ΦHPX + ΦLX+ ΦX

ΦHPX = 2 l/min

ΦLX = 0.5 l/min

ΦX

Validation of HPDP generator (CH4, TDP = +15⁰C, P = 6 MPa)



Pag. 22

Validation of HPDP generator (CH4, TDP = +15⁰C, P = 6 MPa)



Pag. 23

Pressure

controller

Syringe

pump

Presat

Saturator

Precooler

Ethanol

bath

SPRT

Pressure

CMH

MBW

373 LX

CMH

MBW

373 HPX

CMH

MBW

973

N2

CH4

PPC = 5.997 MPa P973 = 1066 hPa

PAtm = 1013 hPa

PS = 5.997 MPa

TS = -14.95 ⁰C

PHPX = 5.997 MPa

TREF1 = -14.95 ⁰C

PAtm = 1013 hPa

PAtm = 1013 hPa

PAtm = 1013 hPa

PLX = 1028 hPa

TREF2 = -50.71 ⁰C

ΦS = ΦHPX + ΦLX+ ΦX

ΦHPX = 2 l/min

ΦLX = 0.5 l/min

ΦX

Validation of HPDP generator (CH4, TDP = -15⁰C, P = 6 MPa)



Pag. 24

Validation of HPDP generator (CH4, TDP = -15⁰C, P = 6 MPa)

Measurement cycle:

N2 → CH4 → N2

CMHs start drifting

after switching to CH4

Hydrate Formation!



http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=&url=http://www.pet.hw.ac.uk/research/hydrate/hydrates_why.cfm&ei=m6MGVabmHcvePdCLgOAP&bvm=bv.88198703,d.ZWU&psig=AFQjCNGEllq445vl3f_YBd-ZQNXFt6frgg&ust=1426584859920234

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://www.youtube.com/watch?v=hZgYUlp88l4&ei=66MGVeaoCsTbPK2cgYAN&bvm=bv.88198703,d.ZWU&psig=AFQjCNGEllq445vl3f_YBd-ZQNXFt6frgg&ust=1426584859920234



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. 26

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 TDP (declared by manufacturer)

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://www.findtheneedle.co.uk/companies/michell-instruments-ltd/news/interactive-timeline-celebrates-40-years-of-innovation&ei=1SD4VMkDhrw62o-ByAE&bvm=bv.87519884,d.ZWU&psig=AFQjCNHrkCpRGL_ZTO99_iZAma9-NDSTLw&ust=1425633805369689



Investigation of Al2O3 dew-point

sensors

Dry-to-wet response time of Al2O3 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



Investigation of Al2O3 dew-point

sensors (time drift)



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 Td 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

Date post:	01-Feb-2018
Category:	Documents
Upload:	hahanh
View:	219 times
Download:	1 times

Water Dew Point Measurement in Natural Gas and its ... · PDF fileWater dew-point measurement...

Documents