A Highly Accurate Densimeter for Cryogenic Liquid Mixtures - First Results for LNG

Natural gas is an important energy source today and for the future. According to the International Energy Agency the trade with Liquefied Natural Gas (LNG) will become more flexible and will increase significantly in order to secure energy supply. In this context, the accurate determination of LNG densities is gaining importance not only for custody transfer when determining the transferred energy but also for process simulations, e. g., for modelling of economically and ecologically optimised liquefaction and evaporation processes. Today, the calculation models used for natural gas only offer uncertainties of 0.1 % to 0.5 % in the liquid phase. To reduce these relatively large uncertainties new accurate sets of experimental p-r-T-x-data for LNG are essential.

Within the scope of the presentation we will describe a new densimeter specifically designed for measurements of liquid mixtures at cryogenic temperatures. This apparatus allows to measure densities of LNG in the homogeneous liquid phase, along the saturated-liquid line and at supercritical states. Moreover, densities in the homogeneous gas phase can also be measured where required. Measurements can be carried out in a temperature range from 90 K to 290 K at pressures up to 12 MPa. The total measurement uncertainty achievable is supposed to be 0.02 % (k = 2) but does not include the uncertainty associated with the gas analysis, which is required only once for the entire content of a sample gas cylinder. When this influence is taken into account the combined uncertainty is estimated to be smaller than 0.06 % (k = 2) in density. Specific problems involved in the measurement of LNG densities were solved effectively by using state of the art single-sinker densimeter technique in connection with a magnetic suspension coupling and a new kind of VLE-cell. The single-sinker densimeter technique developed at Ruhr-University Bochum has proven successful for the accurate measurement of densities of liquids and gases over large temperature and pressure ranges. This type of instrument applies the Archimedes (buoyancy) principle to provide an absolute determination of density, i. e., a measurement that is independent of calibration fluids.

In addition to the technical description of the new densimeter we will present first results of LNG density measurements. The experimental data will be compared to the relevant thermophysical property models such as the GERG-2008 equation of state for natural gases and the Revised Klosek and McKinley Method commonly used in LNG custody transfer.