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Raman Spectroscopy vs Gas Chromatography (GC) for LNG transport

Based on the recently published article coauthored by Alan Garza, Sam Miller, and Scott Sutherland, Discussing LNG: Adapting to a more strategic role calls for effective quality analysis, this blog references the main points of discussion relating to Hydrocarbon Processing and the use of analyzer technology for LNG transport.  

Natural gas must be treated before liquefaction to remove containments that can condense at cryogenic temperatures or damage equipment.
Natural gas must be treated before liquefaction to remove containments that can condense at cryogenic temperatures or damage equipment.

As the disruption of the global hydrocarbon supply chain increases, liquefied natural gas (LNG) has become an important topic for buyers and sellers looking to satisfy energy needs. LNG is moved around the world every day, comes from a variety of sources, each with unique characteristics, and is worth billions of dollars.   

Analyzer technologies ensure natural gas is fully processed before liquefaction and can identify when it suffers value loss during an extended transit period. New techniques provide greater measurement accuracy and reliability combined with lower lifetime costs and maintenance. This blog will compare how traditional Gas Chromatography and Raman Spectroscopy differ in evaluating LNG through its production chain and verifying its suitability for pipeline use after pretreatment. 

Pretreatment and natural gas liquification  

Variability with the sample vaporizer results in erratic BTU readings from the GC
Variability with the sample vaporizer results in erratic BTU readings from the GC

Compared to locally produced gas, LNG has fewer contaminants due to them being removed during the extensive pretreatment process. 

The contaminant levels involved in this pretreatment stage call for tunable diode laser absorption spectroscopy (TDLAS) analyzers to be implemented at transfer points in each stage. These TDLAS analyzers will then verify the quality of the catalyst and the overall pretreatment process by the level of contaminants in the gas before reaching further into the liquification process.  

Once pretreatment is complete, the gas moves to liquefaction and onto the transport mechanism. 

LNG Transport  

Next, the natural gas is liquefied and transferred onto a ship. Arriving shoreside, the LNG is then transferred to a storage tank for regasification before entering the pipelines.  

LNG changes while in transport, it boils off its light components, causing them to escape into the tank. The typical LNG tank load can have a value of up to $50 MM or more, a change of even 1% is worth $500,000. Basic contaminant levels and calorific values must be verified. The challenge at hand is finding an analyzer technology suited to the application. 

Traditional Gas Chromatography (GC) 

The most traditional method when it comes to analyzing natural gas after the pretreatment stage is Gas Chromatography (GC). A typical GC analysis takes a sample of the natural gas, mixes it with an inert carrier gas, and pushes it through a packed column enclosed in an oven.  

Gas Chromatography tends to be more on the complex side. The analyzer is often near the source because it depends on a sampling system to deliver gas to the analyzer. Due to the need for an ongoing supply of carrier gases and test gases for calibration, this can also start to become costly.  

An LNG sample must pass through a vaporizer to phase change the sample into a gaseous form suitable for GC analysis. The vaporizer stage is usually more problematic than the analyzer itself and must ensure that it does not lose the lighter fractions or stop the process while some of the heavier fractions remain partially liquefied. 

The basic challenges while using GC analyzers are:  

  • It can be operationally complex.  
  • Requires daily calibration performed by feeding a premeasured test gas into the analyzer.
  • Requires consumables and taking analyzer offline.
  • Can maintain measurement accuracy, but it does not solve the larger issue at hand.
  • Can provide highly precise analysis, but if a sample is not representative of the LNG composition, then the underlying problem remains unresolved.  
  • Requires you to convert LNG into gas for analysis. 
  • Analyzer must be near the source due to the tubing and valving at a limited distance. 

Raman Spectroscopy  

Various gas and liquid analysis technologies have been developed around the ability of lasers to produce highly specific wavelengths of light. Raman Spectroscopy uses a laser to produce light of visible or near-infrared wavelengths. When various molecules pass through this light, the light’s energy excites the vibrations of molecular bonds. This vibration creates a scattering effect, casting the laser light into different wavelengths.  

Visual describing the ability of probe to be inserted directly into LNG stream during LNG transport
The ability to insert a probe directly into the LNG stream eliminates the need for a traditional handling system.

A Raman analyzer looks for these specific wavelengths and intensities to create a chemical profile of the sample. When this concept is applied to LNG analysis, a probe is inserted into the pipe to analyze the flowing liquid or gas. Laser light is emitted from the end of the probe into the LNG sample, and the Raman light is collected back through the same probe tip. The collected Raman light travels through a second fiber-optic cable and then enters a detector in the analyzer, where the resulting individual wavelengths are identified and quantified. 

Using this approach results in several critical advantages when compared to GC analysis, including:  

  • The probe inserts directly into the LNG stream, so that it takes the reading in-situ, with the moving liquid constantly refreshing the sample.  
  • Measuring LNG in-situ means there is no vaporizer and no need for sample transport lines, valves, heaters, or regulators.  
  • A single analyzer can be connected with up to four probes, so readings can be taken at multiple locations in the process stream. 
  • Output from the probe changes in real-time, and the analyzer can take a snapshot of the composition in less than 10 seconds with no delay between readings.
  • Measures stream up to 500 meters from the analyzer, with no lag time, using cryogenic Raman optical immersion probes and industrial fiber optic cables. 
  • Operates virtually maintenance-free, ensuring that the analyzer is ready any time measurements are needed.
  • Can operate for up to 2 years without needing to be calibrated.

Click Here to read the full article, Discussing LNG: Adapting to a more strategic role calls for effective quality analysis, in the August 2022 edition of Hydrocarbon Processing magazine