• Energy Research

Abstract The boil-off gas (BOG) produced from liquefied natural gas (LNG) mixtures in cryogenic storage tanks must be predicted reliably as a function of tank shape, heat ingress, thermal stratification, pressure, and liquid volume fraction. However, current methods of estimating BOG rates for large-scale tanks are entirely empirical and based on limited available data, with no models available for reliable predictions. This affects the ability of LNG carriers to optimise BOG compressor sizing. A new apparatus was developed to explore the effects of heat flux, liquid stratification, volume, and mixture composition on the measured boil-off rate. The apparatus is demonstrated using liquid nitrogen with BOG rates quantified as a function of various heat fluxes, pressures, and initial liquid volume fractions. Three distinct periods of boil-off were observed: the pressurisation, transient, and steady-state stages. The data are compared with the available literature and the predictions of a new dynamic model accounting for heat transfer from the super-heated vapour. Excellent agreement is observed between model predictions and the data measured during the pressurisation and steady-state stages. However, the model does not capture the BOG rate observed in the transient stage, suggesting liquid thermal stratification should be considered in future models for LNG boil-off.

Abstract The freeze-out of impurities in LNG production can pose significant operational risks and lead to costly blockage-induced plant shutdowns. Study of ternary and higher-order mixtures, which are more analogous to LNG, present the critical tests of thermodynamic models in terms of their utility and predictive accuracy. In this work, a visual CryoSolids apparatus was upgraded to allow analytical measurements of solvent composition in multi-component systems where a solid phase is present at equilibrium. The analytical system, which included a ROLSI sampling valve and capillary together with a gas chromatograph, was successfully commissioned and used to measure melting temperatures and solvent compositions of a ternary mixture containing methane, ethane, and benzene at temperatures down to 125 K and pressures up to 6 MPa. The effect on the solubility of benzene by adding ethane to the solvent was investigated by varying the ethane mole fraction from 0 to 0.96. The resulting temperature at which benzene melted into the liquid solvent decreased from 246 K at 4.7 MPa to 125 K at 5.7 MPa. The comparison of results with the ThermoFAST model showed that it could describe the new data with a deviation less than 1 K for ethane liquid phase mole fractions from 0 x C 2 x C 2

Abstract Current methods of estimating boil-off gas (BOG) rates for large-scale liquefied natural gas (LNG) storage tanks are largely empirical and based on limited available experimental data. More accurate models would be extremely valuable for estimating the potential for excessive BOG generation during LNG storage and transportation scenarios as well as providing critical inputs into the design of BOG re-liquefaction systems. This study reports a series of experiments that have been conducted for LNG-like binary mixtures of methane and ethane to measure the BOG production and resultant pressure change under various industrially relevant conditions. Experimental data and observations made in this work are compared with both the available literature and with the predictions of a new non-equilibrium model that uses the GERG-2008 equation of state to calculate relevant LNG and BOG properties. The data reveal three distinct stages of BOG evolution, here labelled as self-pressurisation, transient, and homogenous. It is observed that, in the self-pressurisation stage, the thickness of a thermally stratified layer adjacent to the liquid–vapor interface increases with time. The transient stage is defined to commence when the system reaches the specified relief pressure and the homogeneous stage is reached upon the effective elimination of thermal stratification in the LNG. Good agreement exists between this new model and the experimental and literature data acquired during the self-pressurisation and homogeneous stages. In the transient stage, the model does not accurately quantify the BOG rate indicating a need to incorporate the effects liquid thermal stratification in future model development.

Abstract In this work, a visual high pressure sapphire cell was employed to measure the solid-fluid equilibrium (SFE) of methane + neopentane binary systems at temperatures down to 90 K and pressures up to 11 MPa. First, the conditions of solid-liquid-vapour equilibrium (SLVE) and solid-liquid equilibrium (SLE) were measured for synthetic mixtures of neopentane in methane. The SFE conditions were then investigated at a constant neopentane mole fraction of 2036 ppm. The SFE data showed a variety of phase transitions starting at higher temperatures (SVE conditions) where neopentane solid formed. Further cooling at pressures below 2 MPa resulted in a reduction of the amount of solid neopentane present, which eventually melted entirely after passing through a solid-liquid retrograde boundary. A model implemented in the software tool ThermoFAST was tuned to SFE experimental data obtained in this work and taken from the literature, which led to an optimized binary interaction parameter, capable of describing the experimental data within an R.M.S deviation of 2.7 K. Overall, the data measured suggest that neopentane is quite soluble in methane (0.10 mol fraction) at typical LNG conditions, with limited freeze-out risk during production given the small concentrations of neopentane typically present in natural gas.

Abstract The liquefaction of natural gas is an energy intensive process, requiring at least 5% of the energy associated with methane's lower heating value. Key to estimating and optimizing these energy requirements are process simulations which rely upon calculated thermophysical properties of the natural gas. In particular, the prediction of thermophysical properties of natural gas mixtures at pressure-temperature conditions close to the mixture’s critical point or cricondenbar is challenging but important as often natural gas processes operate close to these conditions. In this work, we present a comprehensive study of two natural gas related systems: (CH4 + C3H8 + CO2) and (CH4 + C3H8 + C7H16) with n-heptane fractions up to 15 mol%. High accuracy measurements of densities, at temperatures from 200 K to 423 K and pressures up to 35 MPa are presented. The extensive experimental data collected for these mixtures were compared with the GERG-2008 equation of state, as implemented in the NIST software REFPROP. The relative deviations of the measured densities from those calculated using the GERG-2008 model range between (−2 to 4)% for all mixtures, presenting a systematic dependent on mixture density and n-heptane content. Finally, a case study is presented that probes the impact of the accuracy of density on the pinch point in a simulated LNG heat exchanger. An uncertainty in the density of 1% is shown to cause significant 30% reduction in the minimum approach temperature difference, suggesting that accurate thermophysical property calculations are key to reducing over-design of processing plant.

Abstract A stirred, high-pressure, visual microscopy-based apparatus was used to investigate solid-formation kinetics in LNG-relevant binary mixtures. The apparatus can detect solid crystals as small as 20 µm in a 3.5 mm field of view and is capable of measurements at temperatures as low as 90 K and pressures up to 20 MPa. The apparatus is described in detail and is presented alongside experimental results for two benzene + methane mixtures. Measurements of the solid–fluid equilibrium temperatures for the benzene + methane mixtures at 10 MPa were determined by raising the temperature of the mixture in (0.5 to 1) K steps and observing whether the crystals were still present after 2 h. The equilibrium temperatures were consistent with those predicted using the software package ThermoFAST within the combined experimental and model uncertainties. Formation measurements were carried out using a 100 ppm benzene-in-methane sample. Isobaric constant-cooling experiments at pressures of 8 and 10 MPa were used to construct subcooling formation-probability distributions for this mixture by identifying the temperature at which solid crystals were first observed to form on a copper substrate. The subcooling values at formation ranged from (4.4 to 11.0) K and were used to generate a cumulative probability of formation. The measured formation-probability distribution was fit to a model based on Classical Nucleation Theory to yield an estimate of 5 mJ/m2 for the effective surface free energy of solid benzene in liquid methane on the copper substrate. These results pave the way to probabilistic estimates of risk for solids formation in cryogenic heat exchangers.

Accurate thermodynamic property data and models are demanded to reduce the design margins of industrial processes based on these fluids. In this work, measurements of density (ρ), heat capacity (cp), and vapour-liquid equilibrium (VLE) for CO2 + difluoromethane (R32) have been performed by the vibrating tube densimeter and the magnetic suspension balance densimeter, the visual cell and gas chromatograph, and the differential scanning calorimeter, respectively. Experiments were conducted over the temperatures ranging from (208.3 to 334.3) K and pressures reaching 10.04 MPa at CO2 concentrations of 0.7, 0.8 and 0.9. The measured data, together with the results reported in literature where applicable, were subsequently applied to regress the binary interaction parameters utilised in the mixture functions of Helmholtz energy model from REFPROP 10 software package. Noticeable improvements have been achieved for the model's ability to represent thermodynamic property data. The most significant achievement exists in density description: compared with the default parameters, the root-mean-square (RMS) deviation has been decreased by half.

Hydrogen is emerging as one of the most promising energy carriers for a decarbonised global energy system.

Abstract Information about the solubility of benzene in light hydrocarbons is particularly important for the prediction of freeze-out risk in LNG production. Engineering models developed to predict this risk need to be tested against high quality experimental data covering a range of conditions to assess their validity. A visual high pressure sapphire cell, housed in a specialized cryogenic environmental chamber, was employed to measure the melting temperature of methane + benzene binary systems at temperatures from 120 K, pressures up to 22 MPa, and benzene concentrations ranging from 120 to 1012 parts per million (ppm) by mole. The results obtained were compared with literature data and the predictions of the thermodynamic model implemented in the software package ThermoFAST. These comparisons reveal that the literature data are in fact consistent with each other, and with the measurements and predictions made in this work, within their experimental scatter. ThermoFAST was able to represent the melting temperatures obtained for benzene concentrations of 1012 and 199 ppm with r.m.s deviations of 0.7 and 3.4 K, respectively. At 120 ppm and 6.3 MPa, the measured solid-liquid equilibrium (SLE) temperature deviated from the ThermoFAST prediction by less than 2 K. However, at the higher temperature conditions representative of solid vapour equilibrium (SVE), the data measured for mixtures with concentrations at 199 and 750 ppm benzene deviated from the model predictions by up to 5 K.