PROCESS MOISTURE ANALYZERS - Caltechdownload.caltech.se/.../Michellsem2014/06MIL_Process_Analyzer.pdf · PROCESS MOISTURE ANALYZER Application – Glycol Dehydration of Natural Gas

PROCESS MOISTURE ANALYZER

PROCESS MOISTURE ANALYZERS

Measuring moisture in gas or HC liquids in hazardous areas

Copyright Caltech 2014


EExd ConstructionSafety by containment

Channel 1 gas inlet & outlet

Channel 2 gas inlet & outlet

Cable glands

Through-glass user interface

Channel 1 dew-point &

pressure sensor

Channel 2 dew-point & pressure sensor

EExd enclosure



Control Unit Sensor Assemblies

IS work by using barriers

User interface

Safe area Hazardous area

Optional pressure transmitter

Sensor block

Dew-point transmitter

Electrical connections

Power port and

switch

Built-in I.S. barriers



Gas analysers requireCareful sample system design



Natural Gas Production

Glycol dehydration process monitoring - onshore & offshore



Liquefied Natural Gas Production

• Molecular sieve dehydration prior to liquefaction

• LNG re-gasification at receiving terminals



Refineries and Petrochemical Processing

• Petroleum Refinery– Isomerisation naphtha– Reformer re-cycle H2

• HDPE plant – ethylene

• BTX processes– benzene, toluene and

xylene



Other Applications:

• Pipeline Drying• Offshore export pipeline natural gas• Transmission pipeline monitoring• Fiscal metering of gas• Gas storage facilities• Hydrogen cooling• CNG filling stations• Industrial gases• Welding gases



Measurement Principle – background• Liquids consisting of non-polar molecules such as

hydrocarbon chain or hydrocarbon rings do not mix with water.

• Liquid Hydrocarbons can absorb a certain amount of water vapour (dissolved moisture) and thus have a limited saturation concentration.

• Dissolved moisture acts as a gas, exerting a partial pressure with corresponding dew-point temperature.

• Saturation concentration is specific to the liquid and highly dependant on temperature.

Oil & Water Do Not Mix



Measurement Principle –Henry’s Law

In chemistry, Henry's law is one of the gas laws. It was formulated by William Henry of Manchester, England in 1803.

At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Henry’s law is applicable for organic liquid when the moisture content at saturation does not exceed 1%

S

W

S P

P

C

C



Measurement Principle – Henry’s Law

Henry’s Law is used for determining the Weight Portion of moisture in total Liquid

C = Moisture concentration in ppmWCs = Saturation moisture concentration in ppmW at the temperaturePw = Water vapour pressure from sensor measurementPs = Saturation water vapour pressure at the temperature

K = Cs/Ps (Henry constant)

The impedance sensor measures Pw. To calculate C, the Cs must be known.

WS

S PP

CC



Measurement Principle –Solubility Data (CS value) Example

Calculated saturation concentration of typical LPG propane and butane mixtures

0100200300400500600700

0 10 20 30 40 50

Temperature of liquid, deg. C

Cs,

pp

mW

PropaneButane70%C3-30%C480%C3-20%C490%C3-10%C4



Measurement Principle –Calculation Example

Cyclohexane with measured dew point of –20oC and analysis temperature of +30oC:

Moisture content in ppmW = CS/PS x PW

CS for cyclohexane at 30oC sample temperature = 194 ppmW

PW at –20oC measured dew point = 103 Pa

PS at 30oC sample temperature = 4247 Pa

ppmW = 194 x 103/4247 = 4.7 ppmW



Can be ATEX IS or EExd



Control Unit Sensor Assemblies

I.S. Construction

User interface

Safe area Hazardous area

Temperature transmitter

Sensor block

Dew-point transmitter

Electrical connections

Power port and

switch

Built-in I.S. barriers



EExd and I.S. HC Liquid Sampling System

• Again careful design required.

• Temperature stabilisation needs to be considered

• Sample can return to systems-cost benefit



Petrochemical Plants

• Hydrocarbon feed stock

• Hydrocarbon products



Liquid Fuels

• Aero/jet fuel• Heating oil

• LPG ‘autogas’



Other Applications:

• Prevents acid formation during reactor process• Controls moisture in solvents for polymer production• Liquid fuels to avoid supply line blockage• Lubricant oils for preventative maintenance• Transformer oil to prevent insulation breakdown• Solar power heat transmission media



Quartz Crystal Moisture Analyzers



Quartz Sensor Technology

• A quartz crystal is sensitised with a thin film of hygroscopic material.

• Water molecules are adsorbed into the hygroscopic layer deposited on the surface.

• The change in mass modifies, in a very precise and repeatable manner, the oscillation frequency.

• The moisture concentration is measured as a change in the oscillation frequency



Measurement Principle –Sensor Construction

Sensor Crystal

Reference Crystal

Primary Δf = fReference Crystal – fSensor Crystal



Measurement Principle –Sensor Construction

• Compact design & easy access for serviceability

• Sensor block contains the following elements:

Sensor and reference quartz crystal

3 solenoid valves

Moisture generator

Oscillator electronics



Measurement Principle –T Variation Effect



• Oven contains the following elements:

Sensor block

Mass flow meter

Capillaries

Temperature probe

Heater

Fan

Measurement Principle –Oven Construction



Measurement cycle (phase 1)Dried sample flow

Measurement cycle (phase 2)Sample flow

Dryer

Flow Meter

Measurement Δf = primary Δfsample – primary Δfdry

•30/30 sec measurement cycle•Non-equilibrium for fast response•Self-cancelling effect for contamination

60sec

Measurement Principle – Measurement Cycles



Calibration cycle (phase 1)Dried sample flow

Calibration cycle (phase 2)Calibration flow

Dryer

Flow Meter

Measurement Δf = primary Δfdry – primary Δfcal

•Moisture generator with calibrated value•Calibration adjusted based on measured Δf and the known moisture level

Moisture Generator

Measurement Principle – Auto Calibration Cycles



QMA2020 Construction

Dry Gas inlet

Cable glands

Through-glass user interface

Electronics

Sensor oven assembly

EExd enclosure

Cable glands

Sample inlet

Gas outlet

Mole-sieve dryer


Application – Glycol Dehydration of Natural Gas

Application: Glycol Dehydration / Moisture Monitoring in Natural Gas

Problem: The circulation rate of the glycol must be balanced against flow rate of the natural gas and the amount of water to be removed.

Specification:30-1000ppmVPressure: 200-1500 psig

Installation: Outdoors or in instrument shelter. Div 2 / Zone 1 areas. Offshore Div 1 / Zone 1.

Required Sampling system:- Heated Pressure Regulation- Liquid Separator- Contaminant Trap- Asymmetric timing- Insulated or htd sample lines


Application – Molecular Sieve Dehydration

Application: Mole Sieve dehydration / Moisture Monitoring in Natural Gas

Problem: Time-based bed regenerations cost money, permit process upsets, shorten bed life.



Required Sampling system:

- Heated Pressure Regulation- Contaminant Trap- Asymmetric timing- Insulated or htd sample lines

Sampling System Block diagram, drawing or picture


Application – Custody Transfer of Natural Gas

Application: Moisture Monitoring in pipeline quality Natural Gas

Problem: Gas quality specs must be met before entry into transport pipelines. Excess moisture forms hydrates and causes corrosion.




-Heated Pressure Regulation-Liquid Separator-Contaminant Trap-Asymmetric timing-Insulated heated sample lines



QMA2020 – Refinery Catalytic Reforming

Application: Moisture Monitoring in H2 Recycle Streams

Problem: Excess or insufficient moisture in catalytic reforming impacts catalyst activity and life.

Typical Range: 10-20ppmVTemperature: ambient

Installation: Outdoors, typically in Div 1 / Zone 1 areas

Sampling System Block diagram, drawing or picture

Required Sampling system:-Isolation of Sample System with nitrogen purge for use during catalyst regeneration-Pressure Regulation-Contaminant trap-Asymmetric timing



Application – Propylene

Application: Moisture Monitoring in Propylene Gas Streams

Problem: Water is a contaminant to most processes utilizing propylene; especially reactions used to produce polypropylene.


Installation: Outdoors, typically in Div 1 or 2 / Zone 1 areas


-Pressure Regulation-Super-dryer-Symmetric timing-Heated sample transport lines



EthyleneApplication: Moisture Monitoring in Ethylene Gas Streams

Problem: Water is a contaminant to most processes utilizing Ethylene; especially reactions used to produce plastics.


Installation: Outdoors, typically in Div 1 or 2 / Zone 1 areas


-Pressure Regulation-Super-dryer-Symmetric timing-Heated sample transport lines



Application – LNGApplication: Moisture Monitoring in re-gasification process

Problem: process monitoring and custody transfer / quality assurance purposes

Typical Range: 0-25ppmV

Installation: Outdoors, typically in Div 2 / Zone 1 areas


-Heated Pressure Regulation-Contaminant Trap-Asymmetric timing-Insulated or htd sample lines



Other Applications:

Hazardous area• Industrial gas production• Olefins production• LNG production / Re-vaporization• And many more...

Non hazardous area• High purity gas production• Industrial gas production• Electronic gases• Air separation plants• Catalyst bed protection• Semiconductor grade gases• Cylinder gases• Speciality gases• Process dryers• Hydrocarbon gases• And many more...



What is the Hydrocarbon Dew point?

The dew point is the temperature at which hydrocarbon condensates firstbegin to form when natural gas is cooled, at constant pressure.

Similar of course for water, but water has no retrograde curve

-40 -30 -20 -10 0 +10

Water and HC liquidspresent in gas

HC liquidspresent ingas

Wholly gaseous

Temperature, oC

Pres

sure

, Ba

ra

H2O Dewpoint

HC Dewpoint

0

10

20

30

40

50

60

70

80

Retrograde Phase Envelope

Hydrocarbon and Water Dewpoint in Natural Gas



Hydrate formations



Compressor damage



Hydrocarbon dew point control in gas turbines

•A major industrial application for natural gas is as the feed energy source for combined cycle gas turbine power plant, producing electricity.

• Often 50ºF of superheat is required to prevent HCL’s forming



Hydrocarbon Dew point Reduction Plant

Well HeadNGL

Separator

Glycol Contactor

10-15MPa

J.T. Valve

P= 3 to 5 MPa

L.T.S.

Condensate

Compressor

Transmission Pipeline

7 to 15 MPa

Water dew point reduction

Hydrocarbon Dewpoint reduction

After extraction from the gas field, natural gas is processed to remove water and heavy hydrocarbon components, before transmission through the pipeline network. This diagram shows a typical plant for the removal of both excess moisture and heavy hydrocarbons



What the processing achieves



How can hydrocarbon dew point be measured?



Equation of state calculation from extended GC analysis: Sample, analyse, calculate

Mol%C1 Methane CH4 88C2 Ethane C2H6 5C3 Propane C3H8 2C4 Butane C4H12 1C5 Pentane C5H14 0.3C6 Hexane C6H16 0.1C7 Heptane C7H16 0.03C8 Octane C8H18 0.01C9 Nonane C9H20 0.001C10 Decane C10H22 0.0003C11 Undecane C11H24 0.0000C12 Dodecane C12H26 0.0000

CO2 1.5H2O 0.005 (50ppm)H2S 0.0001N2 2.0O2 0.08



3 calculations 3 results

In addition the presence ofaromatics such as toluene and benzene can affect the result



Optical Condensation Dew Point Meter

Temperature, oC

mg/

Nm

3

0

20

40

60

80

100

120

140

160

-30 -20 -10 0 +10 +20

Liquid to gas ratio

Temperature,

mg/

Nm

3

0

20

40

60

80

100

120

140

160

-30 -20 -10 0 +10 +20

Temperature. C

LGR

mg/m3

AB C

Hydrocarbon tail forms B~C

C is the lowest theoretical pointcould be 20ºC higher than firstvisible quantity.

It is accepted practice to regress the Line to point A to eliminate the tailAnd arrive at an over all DP



The DARK SPOT Technique

Wetted withCondensate

Light Source Detector

Dry

Light Source Detector



The DARK SPOT Technique

The dark spot forming

- - - -

- - -



CONDUMAX Hydrocarbon Dewpoint Analyser

Confronting the issues:• High sensitivity - DARK SPOT

Technique• Facility to produce an ‘LGR plot’

- sensitivity calibration routine• Decoupling the sample flow

rate/cooling rate inter-relationship - fixed volume sample analysis

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

10 15 20 25 30 35 40 45 50

Optical Surface Temperature, deg.F

Sig

nal

Ch

ang

e, m

V

Trip point setting at 275mV (Factory Default)

Optical Surface Temperature, °F

Sign

al C

hang

e, m

V



CONDUMAX II Hydrocarbon Dewpoint Analyser

Control Unit

Remote interface

Sampling System

Sensor Assembly



Inside the optical sensor cell



The End

Thanks


Documents

PROCESS MOISTURE ANALYZERS - Caltechdownload.caltech.se/.../Michellsem2014/06MIL_Process_Analyzer.pdf · PROCESS MOISTURE ANALYZER Application – Glycol Dehydration of Natural Gas