Gas treatments from beginning to end

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, LNG has become an important topic for buyers and sellers looking to satisfy energy demands. LNG is moved around the world every day, comes from a variety of sources, each with unique characteristics, and is worth billions of dollars.

Tuneable diode laser absorption spectroscopy (TDLAS) can help ensure natural gas is contaminant-free before liquefaction and other analyser technologies can identify when LNG suffers value loss during an extended transit period. New techniques provide greater measurement accuracy and reliability combined with lower lifetime costs and maintenance.

Pretreatment and natural gas liquefaction

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 TDLAS analyser to be implemented at transfer points in each stage. TDLAS analyser can 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 liquefaction process.

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

What challenges or pain points does the industry face during the gas treatment cycle when it comes to natural gas and water measurement?

Water (H₂O) is a critical measurement in LNG production. H₂O can cause many problems from pipeline degradation due to oxidation to a very possible freeze-up of the cold box heat exchanger tubes in the liquefaction trains, which of course will prevent loading and shipment of the LNG.

The first challenge is acquiring accurate and repeatable measurements. Depending on the type of analyser and the measurement technique behind the software and hard-ware, accuracy can range from +/- ppbv (parts per billion by volume) to lbs/million ft³ of a difference. TDLAS technology is a good solution for H₂O contaminant measurements in methane prior to liquefaction as it can provide accuracy of low ppmv. When TDLAS is then paired with differential spectroscopy technique, such as that offered by Endress+Hauser, it will enable detection and quantification of low ppm to sub-ppm concentrations in H₂O in natural gas streams. This combination will provide industry leading accuracy and the user the ease of mind that his liquefaction process will be safe from freeze up due to H2O in the natural gas stream.

Once the proper analyser has been selected, a different challenge can emerge. This is when expertise is very important. An initial challenge post analyser selection is sample transport. When sending the gas sample to the TDLAS analyser, the gas travels from the tip of the probe to the analyser. To stay in the same (gas) phase that is flowing through the pipeline, a few things must be known, including the distance from the tap, environment, gas composition, and analyser location.

Once this data has been collected, an application engineer’s team can advise what is needed for the sample conditioning and the proper analyser that could maximise the customer’s best results.

Ideal hardware for contaminant measurements should be:

• Laser based.
• Proper wavelength.
• Compatible with natural gas and all the different contaminants in the system.
• Easy and convenient to interface with.
• Utilises differential spectroscopy.

LNG transport

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 US$50 million or more, so a change of even 1% is worth US$500 000. Basic contaminant levels and calorific values must be verified. The challenge at hand is finding an analyser technology suited to the application.

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Read the article online at: https://www.lngindustry.com/special-reports/12122023/gas-treatments-from-beginning-to-end/