Validation of spectroscopic instruments for the direct measurement of multi-component cryogenic liquid hydrocarbons

Opportunities exist for completely revising the way liquefied natural gas (LNG) is measured with the aim of achieving a meaningful reduction in uncertainty to below 1%, thereby providing confidence to both parties involved in custody transfer agreements. In addition, much attention has been directed toward improving the efficiency of LNG production processes by putting in place advanced control and monitoring technology. The demand for shifting to direct measurement technology stems from the inherent problems associated with sampling and vaporisation of LNG and measurement response time for mixed-refrigerant (MR) compositions. To this point, there has been no successfully developed system that allows direct measuring instruments to be validated. For the first time, the research set out in this thesis systematically evaluates the application of Raman and Infrared spectroscopy for directly measuring LNG and MR composition using a bespoke cryostat liquefier. The Raman spectrometer investigated showed acceptable performance for measuring a range of MR compositions, which was determined by comparing the differences in calculated heat capacity between the measured and reference liquid compositions. Validating the Raman spectrometer for LNG measurement was not possible due to the unavailability of the model for testing. However, an intermediate evaluation of a non-refined model showed that the Raman spectrometer was not fit for purpose for custody transfer applications. Moreover, valuable information was gained pertaining to the behaviour of multi-component mixtures at cryogenic conditions and instrument linearity, repeatability and sensitivity. In contrast, The IR spectrometer evaluated showed acceptable performance for a narrow range of compositions as indicated by an ISO 10723 evaluation, however did not meet current custody transfer benchmarks when tested over the full composition range. The data collected will contribute heavily to further developing the model and to extend its current measurement range. A rigorous testing method has been established to validate direct measuring instruments which have the capacity to considerably reduce the uncertainty in LNG composition measurement and improve the thermodynamic efficiency of LNG production processes.