By Jarrod Ford
WITH annual global sales of liquefied natural gas now standing at over 240 billion tonnes, it is important that margins of error on custody transfer be minimised, either at export/ import terminals or ship to ship transfers.
But the fact is that LNG, like natural gas, varies in composition depending on its geographical source, with an associated variation of heating value and other properties.
The standard for compositional analysis of natural gas, gas chromatography, has only a few components present in LNG applications – principally methane, with small amounts of ethane and some traces of others.
The process of LNG chromatography requires a sample of the LNG, a mixture of cryogenically liquefied hydrocarbon gases with vastly different boiling points, to be vaporised without changing the relative concentration of the constituent components in the process.
This is practically impossible in this type of fluid, as there will always be a bias for the lower boiling component to preferentially vaporise; with the methane escaping first, the vapour sample has artificial variation in component concentration compared to that in the liquid phase.
With the inconsistency that the vaporisation process generates, it is already recognised that there will be disorder in component concentration which ultimately generate unseen inaccuracies in the property measurements.
This makes chromatography a completely unsuitable measurement technique for this application.
Therefore it would be much more logical and appropriate – not to mention profitable – to use a device that is specifically tailored to measure these properties of interest, without the expense, time, complexity and deficiencies of obtaining the full composition via chromatography.
Orbital Global Solutions has developed a technique using the GasPT properties transmitter which it has developed to offer real-time measurement of natural gas properties.
The GasPT uses a unique speed-of-sound acoustic resonator and a thermal conductivity sensor, as well as some heavily patented programming to accurately generate the Heating Value (CV), Wobbe Index (WI), Relative Density (RD), Compressibility (Z) and other key properties.
It requires no calibration or carrier gas and is certified intrinsically safe for use in Zone 0 hazardous areas.
The GasPT is much different than a chromatograph; the Gas PT continuously analyses every part of the sample gas stream, not just a slice, with real-time property data reported.
Orbital Global Solutions will be exhibiting the technique, and a variety of other assets, at stand L6 at the Australasian Oil & Gas Exhibition & Conference.