Towards in-situ analysis of liquefied natural gas near infrared spectroscopy

A feasibility study into the potential application of alternative analysis methods for monitoring and quantifying LNG in-situ has been performed. Of the various spectroscopic techniques considered it was found that the Near Infrared method of analysis was the most applicable.A comparison of the NIR spectra of the components of LNG, C1 to C7 normal alkanes and odorant components, under standard conditions was completed. This indciated that the optimum region of the NIR for quantitative analysis was across the first and second harmonic regions, i.e. from 5000 to 6200 cm-1 and 6500 to 9000 cm-1. To maximise the efficiency of spectral collection over these regions pathlengths of 2 and 10 mm respectively were required. From the spectral data, a comprehensive assignment for the liquid methane adsorptions was made for the first time.A cryogenic cell was designed and constructed capable of liquefying samples of LGN in-situ. This allowed NIR analysis of the liquids under variable conditions of temperature and pressure that simulated those expected throughout a 30 metre high storage tank. From this simulation, the full temperature and pressure range of conditions were found to have an insignificant effect on the NIR spectra.Statistical methods of analysing the NIR data were investigated in order to quantify liquid mixtures due to the strong similarities of the heavier components of LNG. Of the various "chemometric" methods applied, the CLS method was the most appropriate, and this was successfully applied to the samples of LNG studied.The design and construction of a fibre optic probe, capable of total immersion in cryogenic liquids and the analysis of liquid hydrocarbon mixtures, was completed. The probe is compatible with silica based optical fibres, thus ensuring that the in-situ monitoring could be performed over distances greater than 200m as specified for safer LGN handling operations.