Article pubs.acs.org/jced
Accurate (p, ρ, T, x) Measurements of Hydrogen-Enriched Natural-Gas Mixtures at T = (273.15, 283.15, and 293.15) K with Pressures up to 8 MPa Markus Richter,* Mohamed A. Ben Souissi, and Roland Span Lehrstuhl für Thermodynamik, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany
Peter Schley Energy Systems, E.ON New Build & Technology GmbH, Gladbecker Straße 404, D-45326 Essen, Germany ABSTRACT: Accurate (p, ρ, T, x) measurements of three hydrogenenriched natural-gas mixtures were carried out at T = (273.15, 283.15, and 293.15) K with pressures up to 8 MPa using a two-sinker densimeter. The relative expanded combined uncertainty (k = 2) in density was 0.02 %. To make up the mixtures under study, a 21-component high-calorific natural gas taken from a pipeline in Germany was blended with hydrogen to compositions of approximately (0.05, 0.10, and 0.30) mole fraction hydrogen. Comparisons of the measured densities with values calculated from the GERG-2008 and the AGA8 equations of state for natural gas mixtures are presented. The densities calculated with the equations of state are generally in very good agreement with our measured values and the limited available literature data. For the natural-gas mixtures containing hydrogen mole fractions of up to 0.10, relative deviations in density are essentially smaller than 0.05 %. Relative deviations of up to 0.1 % were observed for the natural-gas mixtures containing a hydrogen mole fraction of 0.30. This result suggests that both equations of state are suitable for hydrogen-enriched natural-gas mixtures in the investigated temperature- and pressure-range.
1. INTRODUCTION The rapid growth of installed wind power capacity requires not only a targeted expansion of the power grids but also possibilities to store the available energy. In this context the blending of hydrogen (generated by electrolysis of water with excess wind energy) into the existing natural gas grids has been proposed to increase the output of renewable energy systems such as large wind farms. This concept is also known as powerto-gas. To investigate the suitability of blending hydrogen into the existing transport infrastructure for natural gas (e.g., pipelines, compressors, etc.), the properties, and in particular the thermophysical properties, of hydrogen-enriched natural gas need to be known. Technical solutions and procedures established for natural gas need to be evaluated for limitations in handling hydrogen-enriched gas. Today, thermophysical properties of natural gas and similar mixtures are calculated in wide temperature- and pressure ranges with the GERG-2008 reference equation of state of Kunz and Wagner.1 To examine the suitability of this reference equation and of the widely used AGA8-DC92 equation of state of Starling and Savidge,2 accurate (p, ρ, T, x) data at typical temperatures and pressures for natural gas transport are necessary. Against this background we have measured the (p, ρ, T, x) properties of three different hydrogen-enriched natural-gas mixtures, filling the data gap in literature for this type of © 2014 American Chemical Society
mixture. To prepare the mixtures under study, a 21-component high-calorific natural gas taken from a pipeline in Germany was blended with hydrogen to compositions of approximately (0.05, 0.10, and 0.30) mole fraction hydrogen. Measurements were carried out along three isotherms at T = (273.15, 283.15, and 293.15) K with pressures up to 8 MPa using an accurate twosinker densimeter.3−5 The temperature range was chosen according to the average pipeline temperature in Europe of approximately T = 283.15 K including a small variation due to regional changes and the change of seasons. The relative expanded combined uncertainty (k = 2) in density was 0.02 %. We show comparisons of the measured densities with the limited available literature data and values calculated from the GERG-2008 equation of state as well as from the widely used AGA8-D92 equation of state.
2. EXPERIMENTAL SECTION A two-sinker densimeter, particularly designed for accurate gasdensity measurements of pure gases and natural gases in a temperature range from (273.15 to 323.15) K at pressures up Received: February 21, 2014 Accepted: March 28, 2014 Published: April 17, 2014 2021
dx.doi.org/10.1021/je500181v | J. Chem. Eng. Data 2014, 59, 2021−2029
Journal of Chemical & Engineering Data
Article
to 12 MPa,3−5 was utilized for the measurements reported here. The instrument applied the Archimedes (buoyancy) principle to provide an absolute determination of density, which is a measurement that is independent of calibration fluids. The use of two sinkers, established by Kleinrahm and Wagner,6,7 leads to higher accuracies when measuring densities in the gas phase. The state of the art of this kind of instrument is described by Wagner and Kleinrahm8 as well as by McLinden and LöschWill.9 2.1. Apparatus Description. The densimeter used for the reported measurements was described in detail elsewhere,3−5 and only a brief description is given here. It was made up of a measuring cell, which was placed within a vacuum-insulated thermostat, a magnetic suspension coupling outside the thermostat, and an analytical balance at the top of the apparatus. The thermostat consisted of two layers of temperature-controlled shields and an outer vacuum cylinder. The inner shield was directly attached to the measuring cell and was controlled at the same temperature. The outer shield thermally isolated the measuring cell and the inner shield from variations in ambient temperature. This shield was maintained at a constant temperature about 1.5 K below the measuring cell temperature. In this way, the measuring cell was passively cooled by just a few hundred mW, which enabled a very good temperature control of the cell by means of computerized electrical heating. The remaining small temperature gradients and temperature oscillations over time were less than ± 2 mK. The temperature of the measuring cell was measured with a 25 Ω standard platinum resistance thermometer (SPRT, Tinsley, U.K.) and a resistance bridge (ASL, U.K.) referenced to a thermostated 25 Ω standard resistor (Tinsley, U.K.). To determine the vertical temperature gradient of the measuring cell a second SPRT was installed at the bottom of the cell. The expanded uncertainty (k = 2) in temperature measurement was 5 mK. Pressures from 0.14 to 8 MPa were measured with a piston gauge (Desgranges et Huot, France) used with a differential pressure indicator (range: ± 3 kPa, Rosemount, U.S.A.) to separate the oil-driven piston gauge in the pressure measurement circuit from the measuring cell. A vibratingquartz-crystal-type barometer (Paroscientific, U.S.A.) was used to measure ambient pressure. The relative expanded uncertainty (k = 2) in pressure measurement was 0.007 % of the measured pressure value. To measure the density, two specifically matched stainless steel sinkers, [a hollow sphere (VS ≈ 107 cm3; mS ≈ 123 g; ρS ≈ 1.16 g·cm−3; gold-plated surface) and a solid ring (VR ≈ 15.6 cm3; mR ≈ 123 g; ρR ≈ 7.90 g·cm−3; gold-plated surface)], were weighed separately with an analytical balance (readability: 0.01 mg, Sartorius, Germany) while they were immersed in the gas of unknown density. The two sinkers had nearly the same mass, the same surface area, and were made of the same material but had a considerable difference in volume (VD = VS − VR ≈ 91.4 cm3). Because of the large differential volume the buoyancy effect was quite substantial, and therefore, the low-density range of gases could be measured accurately. The gas density was determined by ρgas (T , p) = (WD,gas − mD)/VD(T , p)
difference in the sinker volumes. The expanded uncertainty (k = 2) in the sinker weighing was the larger of 0.03 mg or 0.005 % of the balance reading. The relative expanded uncertainty (k = 2) in the difference of sinker volumes was 0.01 % (valid for the entire temperature- and pressure range of the densimeter). A magnetic suspension coupling (Rubotherm, Germany) was used to transmit the weight of the sinkers to the balance, thus isolating the gas sample from the balance, which was placed under ambient conditions. The crucial elements of the magnetic suspension coupling were two magnets, one on each side of an almost nonmagnetic, pressure-separating metal disk. The top magnet, which was an electromagnet with a ferrite core, was attached to the under-pan weighing hook of the balance. The bottom (permanent) magnet was immersed in the gas sample, and it was slowly attracted by the top magnet until a stable suspension was achieved. The distance between the two magnets was measured by means of a position sensor and controlled in such a way that the current through the electromagnet was zero on average. Thus, nearly the entire force was provided by the attraction of the permanent magnet to the ferrite core of the electromagnet so that heat dissipation from the electromagnet was largely avoided. The permanent magnet was connected to a sinker support; a sinker changing mechanism was used to alternately put the sinkers on the sinker support. The force transmission error of the magnetic suspension coupling10 was very small due to the use of nonmagnetic materials of the coupling housing and canceled out to first order because of a nearly identical position of the coupling for both sinker weighings (two-sinker principle). However, a small fluid specific effect still existed. This remaining error was covered by the relative expanded combined uncertainty (k = 2) in density measurement, which was estimated to be 0.02 % for densities larger than 3 kg·m−3 (see ref 3 to 5). 2.2. Measurement Procedure. Measurements were carried out along isotherms. With the temperature at steady state conditions, the gas mixture under test was filled into the (evacuated) measuring cell and the pressure measurement circuit at the highest desired pressure. Further (T, p) state points at lower pressures followed. After filling the measuring cell with the gas mixture, enough equilibration time was allowed before starting the density measurement. The measuring cell was flushed with “fresh” gas from the sample bottle at every single state point. For this purpose the densimeter was intentionally designed with a separate gas-inlet and a gas-outlet. The gas flow was conducted through the measuring cell at constant conditions (isobaric and isothermal) to not distort the equilibrium. For gas mixtures containing heavier hydrocarbons and carbon dioxide, adsorption- and desorption-effects will unavoidably occur because of physical reasons. Due to preferential adsorption of heavier hydrocarbons and carbon dioxide on the internal surfaces of the measuring cell, the composition of the gas mixture could change, and, therefore, the density of the gas could decrease. With desorption the reverse effect would occur. The influence of adsorption and desorption on accurate density measurements of gas mixtures at a substantial distance to the maximum dewpoint temperature (ΔT ≥ 25 K) was recently described by Richter and Kleinrahm.11 Furthermore, adsorption effects were studied by May et al.,12 who quantified adsorption and its impact on gas phase densities in binary and ternary mixtures as the dew-point curve was approached and crossed. By flushing the measuring cell in an appropriate way, systematic errors
(1)
where WD,gas = (WR,gas − WS,gas) is the difference in balance readings between the two sinkers when they are immersed in the sample gas, mD = (mR − mS) ≈ 0.00078 g is the difference in the masses of the two sinkers, and VD corresponds to the 2022
dx.doi.org/10.1021/je500181v | J. Chem. Eng. Data 2014, 59, 2021−2029
Journal of Chemical & Engineering Data
Article
could be avoided. The duration for flushing was in general 15 min. After flushing the measuring cell, enough time was allowed for thermal equilibration. When temperature and pressure were constant within the acceptable limits, an additional 30 min were allowed before a density measurement was started. The sinkers were alternately weighed 15 times each resulting in a measurement period of about 20 min. Density measurements with high-purity (99.9995 %) methane were undertaken before starting the measurements with each gas mixture and upon completion of an isotherm. This was done to ensure that no change/damage of the densimeter occurred due to the higher hydrogen fractions in the investigated gas mixtures. Moreover, before and after the density measurements with each gas mixture, measurements in vacuum were carried out to check for a possible change in the difference of sinker masses. Corresponding concerns were related to possible diffusion of hydrogen into the permanent magnet of the magnetic suspension coupling or into the sinkers. 2.3. Sample Gas-Mixtures. The three hydrogen-enriched natural-gas mixtures studied here started with a sample taken from a pipeline in Germany. The original 21-component gas sample contained a hydrogen mole fraction of only 0.000149. The original sample was blended with different amounts of hydrogen to achieve mixtures with hydrogen mole fractions of approximately 0.05, 0.10, and 0.30. The blending and the gas analysis were carried out in the accredited testing laboratory for gas chromatography of Open Grid Europe GmbH (Essen, Germany). The natural-gas mixtures were analyzed by gas chromatography according to the accepted standards ISO 697413 and ISO 6975.14 The compositions and the calculated molar masses are listed in Table 1, and they are designated as NG1 through NG3. Most important to mention is that NG1 (hydrogen mole fraction ∼0.05) and NG2 (hydrogen mole fraction ∼0.10) contained a methane mole fraction of more than 0.80 after blending, whereas NG3 (hydrogen mole fraction ∼0.30) contained a methane mole fraction of only 0.63. The mixtures contained higher hydrocarbons up to decane and a significant amount of carbon dioxide. A p,T-diagram containing the phase boundaries of the three studied hydrogen-enriched natural-gas mixtures is depicted in Figure 1. The phase boundaries were calculated with the GERG-2008 equation of state and give an impression about the location of the data measured within this work relative to the maximum dew-point temperature. 2.4. Effect of Hydrogen on Densimeter Sinkers. Prior to the density measurements, tests were carried out to determine any possible effect of hydrogen exposure on the mass or density of the densimeter sinkers. A disk (o.d. = 26 mm, h = 9 mm) was fabricated of the same type of stainless steel used to fabricate the densimeter sinkers. The mass of the disk (m = 37.931373 g, with an expanded uncertainty (k = 2) of 0.000050 g) was determined with a double-substitution weighing design (“SOP No. 4” in Harris and Torres15) using class E2 standard masses. The density of the disk was determined in a hydrostatic comparator by use of the techniques outlined by McLinden and Splett.16 The density reference in the comparator consisted of two cylinders (m ≈ 100 g) of single-crystal silicon; the densities of the silicon cylinders at T = 273.15 K and p = 101.325 kPa (ρ = 2329.08884 kg·m−3 and ρ = 2329.08694 kg·m−3 with an uncertainty of 0.00053 kg·m−3 [0.2·10−6]) were certified by PTB (Germany). The density of the disk at T = 273.15 K and p = 82 kPa was 7916.905 kg·m−3 with an expanded uncertainty of
Table 1. Compositions (mole Fraction) and Molar Mass M of the Studied Hydrogen-Enriched Natural-Gas Mixtures (NG1, NG2, and NG3) composition component
NG1
NG2
NG3
CH4 C2H6 C3H8 n-C4H10 iso-C4H10 n-C5H12 iso-C5H12 neo-C5H12 n-C6H14 n-C7H16 n-C8H18 n-C9H20 n-C10H22 C6H6 C7H8 o-C8H10 CO2 N2 O2 He H2 Ma/g·mol−1 Mb/g·mol−1
0.853 810 0.054 534 0.010 347 0.001 479 0.001 355 0.000 256 0.000 355 0.000 015 0.000 214 0.000 087 0.000 008 0.000 001
