Measuring THT in Natural Gas

Natural gas for direct consumption is odorized for safety reasons. In the odorizing process, a substance with extremely high odor is added to natural gas in a controlled method. The odorized natural gas is then transmitted via pipelines into crowded urban settings and eventually into homes, schools, and workplaces.

In many cases, the smell of the gas is the only mechanism for leak detection and prevention of catastrophic explosions. Mercaptans are often used as odorants due to their low odor threshold. In Europe, tetrahydrothiophene (THT) is commonly used. Since the pipeline material absorbs some of the odorant out of the natural gas stream, the THT level is continuously monitored to ensure the gas is adequately odorized throughout the pipeline.

Case study:

At one border crossing in Western Europe, where custody of a natural gas pipeline is transferred, the operators depend on Applied Analytics technology to continuously validate odorant level at several points. An OMA-300 Process Analyzer is installed at each monitoring point with a dedicated sampling system for handling the high pressure natural gas.

Application:	THT in Natural gas
Location:	Western Europe
Equipment:	OMA-300 Process Analyzer
Span Check:	5 PPM THT in Methane

Figure 1 visualizes how the OMA-300 sees the absorbance spectra of (a) un-odorized natural gas, (b) natural gas odorized with THT, and (c) 5 ppm THT in span gas. Sales-quality natural gas contains mostly methane, which does not absorb in the UV range. The absorbance curve seen in Figure 1 from 245-285 nm is the fingerprint of the aromatic compounds often present in low amounts in natural gas. To isolate THT absorbance, the unit is calibrated to the aromatic background. This procedure for interference-free, reliable odorant measurement is only possible with a multi-wavelength instrument that can properly subtract the aromatic absorbance.

Figure 1: UV absorbance spectra of un-odorized natural gas, odorized natural gas, and THT in span gas.

Each of the measurement checkpoints at this site receives natural gas flowing from a different source, such that each analyzer is being fed a stream with unique gas background matrix.

Figure 2 shows the absorbance spectra measured by the OMA-300 at various checkpoints. Table 1 shows the actual readings of THT in these natural gas streams.

Figure 2: UV absorbance spectra of THT in different natural gas sources.

Table 1: THT readings obtained from the lab and the online OMA-300 Process Analyzer.

Gas Source	THT(PPM)	THT OMA(PPM)
Gas 1	4.78	4.42
Gas 2	1.96	1.81
Gas 3	4.50	4.32
Gas 4	4.60	4.26
Gas 5	2.75	2.83

Conclusion

At this site, the OMA-300 has simplified pipeline operation by providing interference-free, automated odorant monitoring, giving the operators at-a-glance odorant levels at multiple checkpoints. Applied Analytics technology is trusted with the critical task of ensuring gas safety downstream into populated areas.

Download application note.

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