Density Measurements of (0.99 Methane + 0.01 Butane) and (0.98 Methane + 0.02 Isopentane) over the Temperature Range from (100 to 160) K at Pressures up to 10.8 MPa

Abstract

Densities of two methane-rich binary mixtures were measured in the homogeneous liquid and the supercritical region at temperatures between (100 and 160) K using a low-temperature single-sinker magnetic-suspension densimeter. For each mixture, four isotherms were studied over the pressure range from (0.3 to 10.8) MPa. Molar compositions of the gravimetrically prepared methane-rich binary mixtures were approximately 0.01 butane and 0.02 isopentane, respectively, with the balance being methane. The relative expanded combined uncertainty (k = 2) of the experimental densities was estimated to be in the range of (0.02 to 0.06) %. Due to a supercritical liquefaction procedure and the integration of a special VLE-cell, it was possible to measure densities in the homogeneous liquid phase without changing the composition of the liquefied mixture. Based on the supercritical liquefaction procedure, a new time-saving measurement procedure was developed and applied. Moreover, saturated-liquid densities were determined by extrapolation of the experimental single-phase liquid densities to the vapor pressure calculated with an equation of state (EOS); here, the relative expanded combined uncertainty (k = 2) is less than 0.05 % in most cases. The new experimental results were compared with the GERG-2008 equation of state, the EOS-LNG and the enhanced revised Klosek and McKinley (ERKM) method.

Laboratory‐scale liquefiers for natural gas: A design and assessment study

“…As can be seen in Figure 5, the relative deviations of the experimental densities ρ exp from the interpolated reference densities ρ ref reveal that the condensation of sample into the precooled and evacuated measuring cell resulted in a slight decrease in density, which implies an enrichment of methane in the liquid phase. Nevertheless, the shift in experimental density is within the relative expanded uncertainty ( k = 2) of the experimental reference densities of less than U ( ρ )/ ρ = 0.015% 23 . We note that the uncertainty in composition of the reference gas mixture is excluded in this combined uncertainty because exactly the same mixture sample was used for the condensation experiments within the present work as for the reference measurements by Eckmann et al 23 Nevertheless, due to the consistency of the data, neither a change in composition caused by the condensation procedure nor a dependency on the filling pressure can be observed, which supports the findings of Walker et al 10 …”

“…The experimental densities were compared with reference densities of the same mixtures that were measured using a reference densimeter. 8,23 It was found that the change in composition of the liquid phase was only small when the sample was filled into the precooled and evacuated system. Here, the deviations from the reference data were within the experimental uncertainty of the reference densimeter.…”

“…Nevertheless, the shift in experimental density is within the relative expanded uncertainty (k = 2) of the experimental reference densities of less than U(ρ)/ρ = 0.015%. 23 We note that the uncertainty in composition of the reference gas mixture is excluded in this combined uncertainty because exactly the same mixture sample was used for the F I G U R E 5 Relative deviations of experimental densities ρ exp at T = 110 K measured after condensation of the sample from reference densities ρ ref interpolated using experimental densities at T = (100 and 120) K measured after supercritical liquefaction of a binary (0.98017 methane +0.01983 iso-pentane) mixture. , filling to p = 1.0 MPa prior to cooling; , filling to p = 0. condensation experiments within the present work as for the reference measurements by Eckmann et al 23 Nevertheless, due to the consistency of the data, neither a change in composition caused by the condensation procedure nor a dependency on the filling pressure can be observed, which supports the findings of Walker et al 10 The condensation procedure where the system was pressurized at ambient temperature, however, resulted in a stronger decrease in density clearly exceeding the uncertainty of the reference densities.…”

“…The different condensation procedures, four in total, were each carried out at a measuring cell temperature of 110 K. The temperature of the densimeter's VLE‐cell was at 250 K and, thus, well above the saturated vapor temperature so that the phase transition from vapor to liquid is located in the connection line between the measuring cell and the VLE‐cell. At a temperature of 110 K, no direct reference densities are available; therefore, the data measured by Eckmann et al at 100 and 120 K 23 were fitted to a quadratic polynomial and then interpolated to obtain the densities at T = 110 K. To consider the nonlinear relationship between density and temperature, the interpolation was not carried out using absolute density values but relative deviations of the reference densities from the EOS‐LNG equation of state, 22 which is capable of representing the temperature dependency of density with reasonable accuracy 23 . For each of the four fillings carried out in this work, density measurements at different pressures were performed.…”

“…Hence, the EOS-LNG equation of state22 as an enhancement of the GERG-2008 equation of state12,14 with improvements regarding butanes and pentanes is used for the determination of the change in composition. The required reference densities23 were determined using the supercritical liquefaction technique. In order to gain a more comprehensive understanding of the impact of condensation on the liquid composition, different approaches of condensation were studied.Concerning the filling and cooling sequence, two scenarios were experimentally assessed.…”