Raman Spectroscopy for LNG

Adam Kurland, Ph.D.

Business Development &Technical Director

Scientific Instruments, Inc.

West Palm Beach, FL

Track D7

May 21, 2015

Who is Scientific Instruments?

• Developing solutions for cryogenic measurements

since 1967

• Temperature measurement & control

• R&D, Industrial, Aerospace, OEM

• 1975: Patented the LTD for LNG/LPG tank gauging

• Model 6280/6290 Level, Temperature, & Density

Gauge

• Rollover prediction

• Nearly 300 systems worldwide

• New Model 7000 in certification stage, Production

expected early 2016

• Emphasis on growing markets (FLNG) and flexible

technologies (communications, sensing, calibration)

• 2006: Partnership with Kaiser Optical Systems

• SI well known in LNG, Kaiser most trusted name in

Raman spectroscopy

Measurements Overview

• Level sensing

• Integrated level, temperature, and

density (LTD) gauging

• CV and composition analysis

techniques

• Raman spectroscopy approach

• Real world performance

Level Sensing for LNG

• Contact:

– Float (magnetic)

– Servo

– Capacitance

– LTD

• Non-contact:

– Microwave (RADAR)

• Requires specialty guide pipe

• Susceptible to echoes and

deflection at low level

– Laser (LIDAR)

• Simple still pipe required

LTD Gauge

Enraf, The Art of Tank Gauging

GIIGNL – LNG Custody Transfer

Handbook, 3 ed. 2011.

Typical accuracies for these types of

gauges ±1-5 mm, <±1 mm possible

Combination Level, Temperature,

Density Gauge

• Probe fully extended until

bottom reference switch

triggered

• Probe is controlled up

liquid level. A stepper

motor accurately

measures traveled

distance.

• Probe tracks liquid level

using interface sensing

Combination Level, Temperature,

Density Gauge

If level rises to touch upper sensor, program logic drives probe up until both

sensors are out of liquid, and then down until lower sensor touches liquid

(hysteresis errors are eliminated)

Accuracy determined by upper-lower sensor offset (±1 mm)

Combination Level, Temperature,

Density Gauge

Frequency of vibration converted to

density through an equation in the

controller and the frequency shift

from the dielectric medium

Connector

PT 100 Temperature Sensor

(Temperature Correction)

Pick-Up

Coil

Driving

Coil

Oscillating Spool

To Controller

Combination Level, Temperature,

Density Gauge

Bottom

Reference

Switch

Assembly

Level

Sensors

270

mm

to

bo

tto

m o

f lo

wer

sen

sor

Oscillating

Spool

Densitometer

2 ¼” DIA (57.15 mm)

Temperature

Sensor Location

• Operation in both LNG and

LPG applications

(cryogenic compatibility)

• Usually installed in

conjunction with primary,

secondary, and high level

RADAR gauges in a

complete tank gauging

system

Beyond Level, Temperature, and

Density: Composition & GHV

• Complete picture for fiscal metering, custody

transfer

– For a single $50M dollar LNG cargo, 1% error equates to

$500K.

• Specific process control

– Blending

– LN2 injection

– LNG quality control

– Inventory management

Current Sampling Standard:

Vaporizer/GC

Vaporizer

Column

Detector

•Must convert to gas

•Composition can change

•Flow rate dependent

•Pressure dependent

•Environment sensitive

•Physical separation

•Water sensitive

•Flow rate dependent

•Pressure dependent

•Ages

•Oxygen damage

•Heat damage

•Consumable

•Universal, univariate

•High background

•ID based on flow, time

•Serial detection

•Drift

•Consumable

A chromatograph physically separates molecules and detects them

serially. There is a lot of opportunity for drift; usually gas standard

necessary for re-calibration daily. Vaporizers and valves are high

maintenance. Sample transport necessary; system can be

temperature sensitive.

N2 CH4 C2H6

Paradigm Shift: 100% Optical

Measurement

• All standards written with LNG sampling in mind:

• ISO 8943 – Refrigerated light hydrocarbon fluids – Sampling of liquefied

natural gas – Continuous and intermittent methods

• GIIGNL – LNG Custody Transfer Handbook

• NBS - National Bureau of Standards: LNG Measurement – A User’s

Manual for Custody Transfer, 1985 ed.

• ISO 10715-2001 – Natural Gas Sampling Guidelines

• API 14.1-2006 – Collecting & handling of natural gas samples for

custody transfer

• BS EN ISO 12838-2001 – Installations & equipment for liquefied natural

gas-suitability testing of LNG sampling systems

• What if physical sampling weren’t necessary?

• ASTM D7940-14, “Standard Practice for Analysis of Liquefied Natural

Gas (LNG) by Fiber-Coupled Raman Spectroscopy”

Spectroscopy: From Light to Data

• Interaction between radiation and matter as a function of wavelength.

• Any measurement of a quantity as function of either wavelength or frequency.

• Spectrometry is the spectroscopic technique used to assess QUANTITY.

• An instrument that performs such measurements is a spectrometer or spectrograph.

A simple prism splits white light into a spectrum of light of its constituent colors

based on the principle of refraction.

Unique, identifiable, analytical!

Fundamental Spectroscopy

Processes

Emission

Phosphorescence

Absorbance

Fluorescence

Raman

Incr

easi

ng

spec

ific

ity/

sen

siti

vity

Emission Spectroscopy (Atomic)

Heated sample

(usually a gas or

solid)

Specific colors of light

emitted in all directions

Detection

system

H

Fe

White

Light

Emission Spectra

Absorbance Spectroscopy (UV, IR)

Wide-band source

e.g. tungsten bulb

All colors of light go in, only part comes

out. Light travels in one direction only.

Detection system

UV/Vis Spectrum

of Caffeine

IR Spectrum of

Caffeine

Fluorescence Spectroscopy

Detection System

(CCD camera)

Wide-band source

e.g. tungsten bulb

Wavelength

Separator

Wide band of colors

emitted in all

directions

Wavelength

Chooser

Fluorescence

Spectrum of Caffeine

Xu, et. al., Scientific Reports,

3:2255, 2013.

Raman: The Technique

Rayleigh scattering

Elastic scatter

Same wavelength

Raman

Inelastic scatter

Photons shifted from original

wavelength

LASER

1928: Named for Sir Chandrasekhara Venkata (C.V.)

Raman, who received the Nobel Prize in Physics in

1930 for this discovery

Raman is a scattering technique (photons re-emitted in

all directions)

Laser block

(holographic notch filter)

Detection System

(wavelength separator

and CCD camera)

Raman: The Molecular Perspective

• Passive, scattering

interaction with sample

material.

• Measures the fundamental

vibrational modes as in

mid-IR but at any λ of your

choice.

Laser (photons) in

Vibrating Molecule

Scattered laser

Out (no change)

Difference Frequency

(red shift) More likely

Sum Frequency

(blue shift)

Unlikely

Stokes

Anti-stokes

Rayleigh

3551 cm-1

3412 cm-1

1691 cm-1

Raman: Energy Levels

Notch filter to remove this

Raman: Advantages

• All molecules have a Raman response

– Emission, IR, NIR, and UV-Vis do not detect diatomic molecules

(N2, O

2, etc.)

• Very sharp bands

– Fewer interferences

– Simpler calibration

• Can be applied to all three states of matter

– NIR, UV more useful for liquids

– IR more useful for gases (in process environment)

– No need for vaporizers, wide temperature range

• Compatible with fiber-optics

• Inherently in situ – no sample handling

• Multichannel Capability– Optical multiplexing for

multiple measurement points in one instrument

• Optical – No moving parts, fastest acquisition rates

(<30 seconds)

• Direct application to liquefied hydrocarbon gases…

Raman: LNG Spectral Analysis

2800 2900 3000 3100

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

x 106

Raman Shift (1/cm)

Rela

tive Inte

nsity

500 1000 15000.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

x 104

Raman Shift (1/cm)

Rela

tive Inte

nsity

2200 2300 2400 2500 2600 2700

1

1.5

2

2.5

3

x 104

Raman Shift (1/cm)

Rela

tive Inte

nsity

Sharp spectral peaks for qualitative and quantitative analysis

Robust univariate and multivariate prediction models

500 1000 1500 2000 2500 3000

0

2

4

6

x 105

Raman Shift (1/cm)

Rela

tive I

nte

nsity

LNG Raman Spectra

Ethane 2.24-10.24%

Propane 0.39-3.39%

Nitrogen 0.10-0.46%

Methane 85.06-97.06%

i-butane 0.39-3.39%

Implementation of a Raman LNG

Analyzer

Cryogenically compatible,

ATEX certified probe

25.4 mm Diameter

Sealed (IP66), Ex-proof

base unit

Raman LNG Analyzer Components

Imaging Spectrograph

HoloPlex

TE Cooled

CCD

Detector

Slit Notch

Control

Electronics

ProbeHead

Invictus NIR Laser

ProbeHead

Filtering ProbeHead

Immersion and Non-Contact

Sampling Optics

Axial Transmissive Spectrograph

HoloPlex Grating

TE Cooled CCD Detector

Invictus NIR Laser

Raman LNG Analyzer Arrangement

• Four measurement

points per analyzer

•500+ meter fiber

cable runs

• Easy-access fiber

connections

•Analyzer located

where convenient

Junction box Plant control

Transfer pipe

Transfer pipe

Transfer pipe

Transfer pipe

Fiber cables probe

Raman LNG Analyzer Installation

Concept

Optical

Fiber Cable

Probe Head

Cryogenic Extraction

Sleeve

Isolation

valve

Probe

Tip

Preferred

Installation

Area

45º from

vertical

LNG pipe

Getting Useful Information:

Software Algorithms

Control, Analysis, & Display

Analyzer

HoloGrams

Grams A/I

BTU

Standard

Core

Model

HoloPro

Local display Process

Computer

Remote display

PLC

LNG

Algorithm

User

Adjusts

Raman Implementation Strengths

• Simple design

– Transmissive holographic grating

– No moving parts in spectrometer

– No mirrors

– Rugged

• High throughput

– Short acquisition times

– Safe, low-power lasers

• Standardized spectrometer

– Intensity axis traceable to NIST

– Wavelength axis standardized to neon lamp

– Laser standardized to diamond

– Automatic re-standardization with Cal-check and Auto-cal

• True process instrument

– Rugged probes, cryogenic compatibility

– ATEX certification-ready

– Communication through industry-standard Modbus or OPC

Calibration and Traceability

Verification, logic, information from spectral data

Spectrum Standardization I

Make data from the spectrometer (the

“spectrum”) independent of the specific

instrument

Physically – two independent

instruments should measure

identically

Over time – an instrument should

be able to reproduce the same

measurement in the same

conditions at any given time

Spectrum Standardization II

• Model is valid as long as the spectral

collection and detection are valid

• Wavelength axis is automatically validated

using primary physics standard (neon,

diamond shift).

• Intensity calibration is performed at

commissioning using a NIST-traceable white

light source

• Intensity calibration is valid as long as probe

does not suffer damage and is at known

temperatures.

• Cryogenic performance is correlated with

ambient performance at factory.

Other Considerations I: Optical

Fiber Length

Different laser λ are attenuated by the fiber optic by different

amounts. A 785 nm laser (in this application) is attenuated 20%

over 300 m.

Not a problem! Update parameters and rate can be optimized to

maintain the same signal strength.

Other Considerations II: Noise

Corrections

•Minimize by cooling detector

•Subtract off a “dark” spectrum

Thermal detector

noise

•All CCD cameras can be affected by cosmic rays

•Normal video cameras, this appears as a single pixel turning white for a fraction of a second

•Visually this is not noticable

•For recorded spectra, shows up as a spike

•Take two spectra to identify spikes

Cosmic ray

correction

Spike

Use this point only

Average

Cosmic Ray Removal Example

Determining Update Rate

Update time is affected by:

• the camera exposure time

• whether cosmic ray correction is used

• the number of exposures averaged together

• the number of channels being used

Corrected Exposure

Time Averages

Total Exposure

This Channel

Cosmic Correction

Exposure

Regular Exposure

Corrected Exposure

Time

Total Exposure

This Channel

Total Exposure

Other Channels

Total Update Time

Developing the Core Method I

Develop

Make samples of known composition

Measure samples

Determine instrument response

Validate

Make samples of known composition

Measure samples

Apply instrument response factors

Compare results to known values

Apply

Measure unknown samples

Apply instrument response factors

Report composition

Phase 1: Developed in-house liquefaction cryostat

Phase 2: Third party validation against traceable standards

Phase 3: Extension of validation range for even greater flexibility

Developing the Core Method II

Preliminary method based on 20 liquefied NG

calibration samples (gravimetric) covering

standard composition ranges (SwRI specified).

Small (0.5%) errors in liquefaction of

components (reverse problem faced by GC

vaporizers) remained

Adjusted during large-scale field trial based on

matching highly averaged GC values.

Accurate, precise, and stable

Developing the Core Method III:

Data Obtained

• Methane

• Ethane

• Propane

• Butane and isobutane

• Pentane, isopentane and neo-pentane

• Nitrogen and oxygen

Composition (target molecules)

• Real BTU Dry

• Ideal BTU Dry

• Wobbe Index

• Specific Gravity

BTU Method (calculated from composition)

• Calibration method identification

• BTU Standard identification

• Channel, signal strength

• Date/Time of collection

• Peak areas, peak heights

Other Information

C1-C6+ components

covering range of

validity can be

expanded to LPG

applications

End-user Controls

Slope & Offset

Composition

Accuracy

BTU Calculation

Results

Exposure Time

Precision

Cycle Time

Signal Strength (may require

modified method)

BTU Standard

Real BTU

Ideal BTU

Wobbe

Specific Gravity

Changing:

Affects:

(Force agreement with

a historical standard)

Real-world Performance

Installations & Results

Installations to Date

Analysis at vessel offloading: 4 locations

Process control analysis: 1 location

Nitrogen ballasting: 1 location

First mixed application, multi-site, multi-probe,

installation (June 2015):

CO2 monitoring, process control, LNG tanker

and small truck (fleet) filling station

Real-time, Dynamic Performance

Rapid response to composition changes

Operational Stability:

Disruption in LNG Flow

• Instrument resumes measurement automatically

• No need to reset instrument or perform maintenance/recalibration

• More data available for analysis

Operational Reliability Case Study I:

Distrigas, Everett, MA (2008-2009)

• 10 consecutive months

unattended operation

• Probe at truck transfer terminal

subjected to continuous thermal

cycling

• Maintained repeatability <0.1

BTU

• Accuracy <0.5 BTU compared to

GC data acquired simultaneously

• Publication: Snyder, J.; Capers,

M.; Fairchild, R.; Wiegand, P.

Taking a Closer Look, LNG

Industry, 2009.

Truck Transfer Terminal & Jetty

• 6+ months unattended operation downstream of

GC on Jetty

– Raman analyzer had no failures, no mechanical

maintenance, no user intervention

• Analysis of 60,000+ Raman data points (3X’s GC

capability)

• Significant demonstration of the stability of the

Raman system (optical vs physical sampling

technique)

– Non-ideal behavior of GC resulting from physical process

changes in the pipeline (flow rate, pressure, phase) and

service interruptions (power cycles/shutdowns)

– Non-deal behavior of Raman only from service interruptions

and data communication errors

– Raman still captures useful data during process changes

Operational Reliability Case Study II:

Cameron Parish, LA (2010)

• Nearly two years unattended operation providing consistent

measurement results and maintaining calibration

• Comprehensive trial program demonstrating Raman accuracy

compared to online and offline GC

• Data transfer and system reboots only end-user intervention

1020

1040

1060

1080

1100

1120

1140

Calo

ric V

alu

e [

BT

U]

GC Raman Offline

Operational Reliability Case Study III:

Long-term Performance Trial

Low Operational Maintenance &

Expense

• After commissioning, no standard maintenance schedule

– Recalibration necessary only if change in probe, fiber, or

spectrograph component (return optical path)

– Keep surfaces and cabinet free of dust and particulates

• No consumables (i.e., carrier gases and columns)

• Calibration is validated automatically and regularly

• Updates to model (software) performed on-site or

remotely

• Relative ease of service of major components

– Service contract or user-serviceable

– Example: Laser module damaged during transport, replaced by site

staff; analyzer resumed data acquisition immediately following

• Ideal characteristics for an analyzer in a remote,

inaccessible, or FLNG location

Field Test Results Summary

Repeatability of the Raman measurement on

the GHV is 0.020% relative

10× better than the conventional custody

transfer method

Relative standard deviation for the GHV is

0.20% using the conventional method

No significant deviation in bias compared to

the online GC

Milestones & Future Progress:

Standards and Compliance

ASTM Standard now active

ASTM D7940 – 14

“Standard Practice for Analysis of

Liquefied Natural Gas (LNG) by Fiber-

Coupled Raman Spectroscopy”

Working with LNG industry to

produce ISO standard

Addressing the C6+ Issue

Heat of Formation

property* using

lighter components

to predict > C6.

(Agrees with

Distrigas GC)

*Thanks to

Felix de la Vega

• Raman is not suitable for measuring components below ~200 ppm

• Not much C6+ in LNG.

• Added precision and accuracy of C6- outweighs error of not measuring C6+

• Combination of precise C6- to project C6+ is more accurate in practice than

poorly measuring C6+

Raman Spectroscopy for LPG?

Yes!

Liquefied propane is

particularly easy to

measure with Raman

Large, isolated peak

Concluding Thoughts

Level sensing technologies mature

Radar, Servo, LTD

Raman spectroscopy produces precise

measurements of GHV in LNG

100% optical technique (no moving parts), high

reliability & low maintenance

No sample handling or conditioning, rapid

measurement update rates & error of

vaporization

Low OPEX (no consumables, routine

maintenance, unattended operation,

serviceability)

Thank You!

American Gas Association

Co-Presiding (Track D7):

Christopher Anderson (Southwest Gas Corp.)

Craig Johnson (Consumers Energy Co.)