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What is the difference between natural gas and LPG analysis by GC?

 

Both natural gas and LPG are analysed by gas chromatography to determine their composition. The analytical goal is the same: separate and measure the individual components present in the sample. But the two applications require different instrument configurations because the samples are physically different and the required outputs are different.

Natural gas

Natural gas is a mixture of methane, higher hydrocarbons (ethane, propane, butane, pentanes), and permanent gases including nitrogen, carbon dioxide, and oxygen. Measuring the full composition requires two detectors: a Thermal Conductivity Detector (TCD) for permanent gases and a Flame Ionisation Detector (FID) for hydrocarbons. The 200 Series GC is configured as a dual-detector system for this application.

The sample is introduced as a gas phase, typically through a gas sampling valve.

The primary outputs from natural gas analysis are calculated properties: calorific value (superior and inferior), Wobbe Index, specific gravity, real density, and compressibility factor. These are calculated automatically from the measured composition using the Clarity data station software in compliance with ISO 6976, ASTM D3588, and GPA 2172.

LPG

LPG is a pressurised liquid consisting primarily of propane, butane, and related hydrocarbons in the C3 to C5 range. Because no permanent gases need measuring, a single FID is sufficient.

Sample introduction is different from natural gas. LPG is stored under pressure as a liquid, so it requires either liquid injection or controlled vapour sampling. The sampling hardware is configured accordingly.

The primary outputs from LPG analysis are compositional percentages of each hydrocarbon present, plus calculated properties including vapour pressure and density. Results comply with ASTM D2163, ASTM D2598, and ASTM D2421.

Which do you need?

If your laboratory tests pipeline gas, natural gas, or any gas stream where calorific value and Wobbe Index are the primary outputs, you need a natural gas configuration. If you test LPG, propane, butane, or related pressurised liquid hydrocarbon products, you need an LPG configuration. Some laboratories test both and run two instruments configured for each application.

Not sure which configuration suits your requirement? Get in touch and we can help.