Synthetic Natural Gases Research Articles

Hydrocarbon Dew Point Measurement and Model Evaluation of Synthetic and Real Natural Gases.

Hydrocarbon dew point (HCDP) is one of the most important quality specifications of natural gas. Measuring and predicting the HCDP accurately are essential for the natural gas industry. However, the comprehensive experimental HCDP curve data are still rare, and knowledge about adopting proper prediction models remains unclear. In view of this, HCDP determination work by use of an improved test system and model evaluation based on more than 1000 dew points data have been done to improve the aforementioned dilemma. HCDP curve data of three gravimetrically prepared synthetic natural gases (SNGs) and one real gas (RG) are determined first. Then, one set of data containing 712 dew points from 28 SNGs and 334 dew points from 14 RGs is used to evaluate the performance of eight different HCDP prediction models including Soave–Redlich–Kwong (SRK), SRK–Twu, Peng–Robinson (PR), Twu–Sim–Tassone (TST), predictive SRK (PSRK), GERG-2008, PSRK, and perturbed-chain statistical associating fluid theory (PC-SAFT) models. Considerable prediction deviation of these models in the high-pressure region (pressure above 6.0 MPa) is observed compared to that in the low-pressure region (under 6.0 MPa), and the reasons for that difference are discussed. Evaluation results reveal that among the eight prediction models, GERG-2008 has the best performance (overall average absolute deviation (AAD): 1.44 °C) for SNGs, and PSRK and SRK–Twu fits the experimental data best for RGs (overall AAD: 2.50 °C). Therefore, GERG-2008 is recommended for HCDP prediction of relatively lean gas, whereas PSRK and SRK–Twu are recommended for calculating the HCDP of relatively heavy natural gases in low-pressure and high-pressure regions, respectively.

Investigation into the combustion instability of synthetic natural gases using high speed flame images and their proper orthogonal decomposition

In this study, the characteristics of the combustion instability of synthetic natural gases that contained small amount of H2 were investigated to determine the allowable content of H2. Because H2 burns faster and hotter than natural gas, which is the primary fuel of gas turbines, it can cause severe damage to fuel nozzles and contribute to combustion instability. Careful inspection of the combustion instability phenomena in synthetic natural gases containing 0% (as reference), 1%, 3%, and 5% H2 was conducted by measuring pressure fluctuation and high-speed intensified OH chemiluminescence images of oscillating flames and their proper orthogonal decomposition. When the H2 content was changed from 0% to 5%, two representative indicators of combustion instability, pressure fluctuation and heat release oscillation, responded sensitively and nonlinearly. The results of pressure fluctuation and heat release oscillation both showed that synthetic natural gas containing 1% H2 did not change the instability characteristics as much as synthetic natural gases containing 3% H2 and higher did. A box plot showing statistically meaningful values, such as the mean and median, disclosed characteristics regarding the amount of heat release from flame evolution and decay, which could be obtained from neither frequency nor time series plots of pressure fluctuation and heat release oscillation. In particular, quantitative analysis using Pearson correlation coefficients for the time coefficients of proper orthogonal modes and the fast Fourier transform of pressure fluctuation validated a close relationship in synthetic natural gases at 0% and 1%. Based on these results, it was concluded that in synthetic natural gas, 1% H2 is a permissible limit because it did not change the characteristics of combustion instability when natural gas that contained no H2 was substituted into synthetic natural gases in which all H2 could not be eliminated for economic reasons.

Combustion instability characteristics of H2/CO/CH4 syngases and synthetic natural gases in a partially-premixed gas turbine combustor: Part I—Frequency and mode analysis

This paper presents the results of combustion instability characteristics of H2/CO/CH4 synthetic gases and synthetic natural gases by testing a scale-downed industrial GE7EA gas turbine that features a partially-premixed swirl-stabilized combustion. H2/CO/CH4 syngases ranging from 0% to 100% and synthetic natural gases containing H2 from 0% to 30% were tested and analyzed in view of H2, which has distinctive combustion characteristics of fast burning and high flame temperature. Interestingly, as features of the partially-premixed flames that were adapted to inhibit flashback from fast-burning H2-containing syngas, high multi-mode combustion instabilities such as 2nd, 3rd, 4th, and over were observed without fundamental longitudinal mode. To investigate these high multi-mode combustion instabilities, combustion properties such as the laminar burning velocity, adiabatic flame temperature and ignition delay time were calculated for the all tested fuels and their dependence within them and dependence on instability frequencies were closely analyzed. Test results demonstrated that H2 contents are the root cause of the high-order instability mode and shifting within modes, since fast burning H2 affects the chemical time and flame temperature of syngas more than that of CO and CH4. The relationships between the combustion properties have also illustrated that the sensitive change of combustion properties are responsible for generating flame oscillations of high multi-mode and sensible frequency and mode shifting. Therefore, the laminar burning velocity, adiabatic flame temperature and ignition delay time should be considered as important parameters when designing a gas turbine combustor; moreover, deciding operating conditions for new fuels and careful management of fuel quality, particularly in view of H2, is required when operating integrated gasification combined cycle plants and synthetic natural gas plants.

Combustion instability characteristics of H2/CO/CH4 syngases and synthetic natural gases in a partially-premixed gas turbine combustor: Part II—Time lag analysis

In continuation of part I, which reported the characteristics of high-frequency multi-mode combustion instability for various compositions of syngases, this study analyzes the instability mechanism using the characteristic length and time scales relating the particular combustion instability of high multi-modes. Here, time lag analysis based on the Rayleigh criterion has been used and the times of convection, acoustic, ignition delay, chemical reaction, and newly developed skewness are considered. The skewness time, which reflects the wave distortion resulting from the addition of a higher harmonic component of combustion instability, is obtained from the analytic derivation of the interval between single-mode and multi-mode waves. The proportion of skewness time in the total lagging time was varied from zero for complete single mode to 11% for high harmonic multi-mode according to the fuel variation. The effectiveness of skewness time on the time lag analysis was verified from the experimental results of 98 cases and it showed remarkable improvement in the prediction accuracy of multi-mode instability from 88.8% to 99.0%. From the analysis results, it can be concluded that skewness time should be considered as an important parameter when calculating characteristic time scales for high multi-mode, and this method can be usefully applied for the development of an optimal design for and safe operation of a syngas-fired gas turbine combustor.

Effect of Gas Composition on sII Hydrate Growth Kinetics

Kinetics of hydrate formation is of paramount importance for hydrate prevention in pipelines and gas storage in a hydrate state. The effect of gas composition on the kinetics of structure II (sII) hydrate growth has been examined by using two different synthetic natural gases, one with two components (SNG2) and the other with seven components (SNG7). The hydrate equilibrium properties of SNG2 and SNG7 were comparable and the initial degree of subcooling was thus approximately the same during all growth experiments. The same PT conditions and stirring rate were also set for both SNGs enabling us to examine the effect of gas composition on the growth behavior of the same crystal structure. The amount of gas consumed and the structure of the growth curves have been investigated to understand the effect of gas composition on the kinetics of sII hydrate formation. In addition, several chemicals (MeOH, PVP, PVCap) have been tested for their effects on the amount of hydrate formation and structure of the growth curves. The results showed that the amount of sII hydrate formed from SNG7 was doubled as compared to SNG2 with and without the additives. This could be explained by the higher amount of large cavity preferring gas components in SNG7 as compared to SNG2. On the other hand, the growth rate of sII hydrate made from SNG7 first increased toward a maximum value after hydrate onset, and then decreased toward zero at the end of experiment. This growth behavior was not observed for sII hydrate structure produced from SNG2. The SNG2 growth rate showed a maximum at onset and then decreased toward zero eventually. The addition of the chemicals did not change the characteristic growth behavior produced by the individual gas mixtures. The chemicals had an effect in shifting the initial growth rates either up or down based on concentration and type of the chemicals used while the structure of growth rate versus time was kept the same. This study confirmed that gas composition alone is an important parameter for sII hydrate growth kinetics, and the same hydrate structure produced from different gas compositions does not necessarily show the same growth behavior.

Calculation of Phase Equilibrium of Natural Gases with the Peng-Robinson and PC-SAFT Equations of State

Cubic equations of state (EOS), such as the Peng-Robinson EOS, are routinely used by the oil and gas industry for the design of recovery and processing operations of gases at high pressures. Nonetheless, many non-cubic equations of state, derived from statistical thermodynamics and with solid theoretical basis, have been proposed in recent years. Among them, EOS of the SAFT-family are possibly those with widest acceptance. In this work, we compare the performance of the Peng-Robinson and PC-SAFT EOS in the calculation of dew points, bubble points, and critical points of natural gases. Binary interaction parameters in both EOS were set equal to zero in all calculations to test the predictive capability of the models. Calculations were performed for 19 synthetic natural gases for which experimental data are available in literature. For most mixtures, predictions of the PC-SAFT EOS are in better agreement with the experimental data.

Modelling solubility of solid sulphur in natural gas

Abstract The purpose of this work is to propose a model able to estimate the solubility of sulphur in natural gas transmission and distribution networks. Thus, we developed a model to predict the equilibrium solubility of elemental sulphur in natural gas containing low amount of H 2 S, at pressure and temperature ranges lower than those encountered in production conditions. We established a reactive flash model. The constitutive equations are classically the chemical and physical equilibriums and partial mass balances. We chose the Peng–Robinson equation of state to describe the phase behaviour coupled with the van der Waals one-fluid mixing rules with one binary interaction coefficient. This approach needed critical parameters for each component. We determined the set of critical parameters for S 8 specie from three thermodynamic intrinsic properties including the description of sulphur dissociation reactions. Binary interaction parameters between CH 4 , CO 2 , H 2 S and sulphur are obtained by optimisation from binary and ternary solubility data. Comparisons with experimental values of solid sulphur solubility show that the model is able to describe the sulphur solubility in synthetic natural gases. The application of our model to different natural gases indicated that under transport conditions sulphur solubility is less than 0.005 mg per normal cubic meter.

Measurement and modeling of hydrocarbon dew points for five synthetic natural gas mixtures

The dew points of five synthetic natural gas (SNG) mixtures were measured using a custom made chilled mirror apparatus. The chilled mirror apparatus was designed to detect hydrocarbon dew points from low pressures up to the cricondenbar. The experimental temperature range was from 235 to 280 K and the pressure range from 0.3 to 10 MPa. The synthetic natural gases were comprised of methane and gravimetrically prepared fractions of ethane, propane, i-butane, n-butane and n-pentane. The experimental data were compared to calculations with the Soave–Redlich–Kwong (SRK) equation of state with classical mixing rule. However, considerable and increasing deviations between calculated and experimental dew points were observed as the pressure approached the cricondenbar. Therefore, a model was utilized based on the Redlich–Kwong (RK) equation of state. The Mathias and Copeman (MC) function was used to express the temperature dependence of the attractive term for all components. For methane, different sets of MC coefficients were used below and above the critical point. An optimization procedure was employed to fit the coefficients of supercritical methane to both pure component fugacity and experimental dew points. There is good agreement between experimental data and modeling results. For pressures higher than the pressure corresponding to the cricondentherm, the proposed model is better than the standard SRK equation of state. Good predictions with the model were obtained when comparing to bubble and dew points data from literature.

Predicting Natural Gas Dew Points from 15 Equations of State

This paper is a comparative study of 15 cubic equations of state (EoSs) for predicting natural gas dew points. Two-, three-, and four-parameter EoSs are used to predict the natural gas dew points. Natural gases contain a large amount of supercritical methane; therefore, the fugacity of methane, as a test of suitability of the EoSs, is predicted and compared to the recommended values in International Union of Pure and Applied Chemistry (IUPAC) tables. The vapor pressures of components that are normally present in natural gas mixtures, as another test of the suitability of the EoSs, are also predicted and compared with experimental data. The dew points of several natural gas mixtures then are predicted, using the EoSs, and compared to experimental values. Results reveal that the dew points of lean synthetic natural gases are predicted best by the Redlich-Kwong family of EoSs, whereas rich natural gases dew points are described significantly better by the Patel-Teja family of EoSs.

Thermodynamic Properties of Synthetic Natural Gases. 5. Dew Point Curves of Synthetic Natural Gases and Their Mixtures with Water and with Water and Methanol: Measurement and Correlation

Dew points for two synthetic natural gas (SNG) mixtures between 1.2 × 105 and 81.8 × 105 Pa in the temperature range from 213.6 to 261.4 K, four SNG + water mixtures between 1.1 × 105 and 41.0 × 105 Pa and temperatures from 244.7 to 288.1 K, and four SNG + water + methanol mixtures between 1.1 × 105 and 20.7 × 105 Pa and temperatures from 247.6 to 288.6 K were experimentally determined. The experimental results obtained on the multicomponent systems were analyzed in terms of a predictive excess function−equation of state (EF−EOS) method, which reproduced experimental dew point temperature data with absolute average deviation (AAD) between 0.9 and 3.1 K for the dry systems, from 0.0 to 1.6 K for the systems with water, and from 0.0 to 3.0 K for the systems with water and methanol. The experimental results obtained for synthetic natural gas (SNG) + water mixtures at pressure values higher than 5 × 105 Pa were also compared to a predictive equation of state (EOS) model. It reproduced experimental dew point temperature data within AAD between 1.8 and 5.3 K.

03/02207 Thermodynamic properties of synthetic natural gases. Part 3. Dew point curves of synthetic natural gases and their mixtures with water. Measurement and correlation.: Avila, S. et al. Energy & Fuels, 2003, 17, (2), 338–343

03/02207 Thermodynamic properties of synthetic natural gases. Part 3. Dew point curves of synthetic natural gases and their mixtures with water. Measurement and correlation.: Avila, S. et al. Energy & Fuels, 2003, 17, (2), 338–343

Thermodynamic Properties of Synthetic Natural Gases. Part 3. Dew Point Curves of Synthetic Natural Gases and Their Mixtures with Water. Measurement and Correlation

Experimental measurements of dew points for eight synthetic natural gases (SNG) + water mixtures were carried out between (1.2 and 99.3) × 105 Pa in the temperature range from 226.4 to 287.6 K. The experimental results were analyzed in terms of both an equation of state model and an excess function-equation of state method, which reproduced the experimental data within AAD from 1.4 to 4.1 K and from 0.6 to 4.3 K, respectively.

Thermodynamic Properties of Synthetic Natural Gases. 1. Dew-Point Curves of Synthetic Natural Gases and Their Mixtures with Water and Methanol. Measurement and Correlation

Experimental measurements of dew points for five synthetic natural gas (SNG) mixtures between 1.9 × 105 and 106.0 × 105 Pa in the temperature range from 195.5 to 277.3 K and nine SNG + water + methanol mixtures between 1.4 × 105 and 73.1 × 105 Pa and at temperatures from 237.3 to 288.2 K were determined. The experimental results obtained on the multicomponent systems were analyzed in terms of an equation of state chemical reticular method, which reproduced experimental dew-point temperature data within an absolute average deviation between 0.8 and 3.7 K for the dry systems and from 0.5 to 1.9 K for the systems with water and methanol.

Thermodynamic properties of synthetic natural gases: Part 4. Dew point curves of synthetic natural gases and their mixtures with water: measurement and correlation

Experimental measurements of dew points for five synthetic natural gases (SNG)+water mixtures were carried out between 2.1×10 5 and 73.2×10 5 Pa in the temperature range from 224.3 to 270.2 K. The experimental results were analysed in terms of both an equation of state (EOS) model and an excess function–EOS method, which reproduced the experimental data with an AAD from 2.1 to 3.4 K and from 1.9 to 3.0 K, respectively.

Thermodynamic Properties of Synthetic Natural Gases. 2. Dew Point Curves of Synthetic Natural Gases and Their Mixtures with Water and Methanol. Measurement and Correlation

Experimental measurements of dew points for four synthetic natural gases (SNG) and seven SNG + water + methanol mixtures were carried out between (1.4 and 77.7) × 105 Pa and the temperature range from 199.9 to 284.1 K. An EOS−CR method (equation of state−chemical reticular) reproduces quite accurately the experimental dew point temperature data within average absolute deviation (AAD) from 0.7 to 2.7 K.

Calorimetric measurements for modeling thermodynamic properties of pure fluids and fluid mixtures

Recent progresses in calorimetric techniques are reviewed and some typical measurements are used to illustrate the importance of such data to further develop either equations of state for pure fluids, or group contribution based models for fluid mixtures. Heat capacities of gases, pure fluids and fluid mixtures can be easily measured from ambient temperature to almost 600 K at pressures up to 30 MPa. Similarly, heats of mixing are obtained in the same ranges of temperatures and pressures. In addition calorimetric measurements of pressure effects, up to 300 MPa, by the newly established technique, transitiometry, yield thermal expansion coefficients of gases and fluids up to critical points. Synthetic natural gases and binary as well as ternary nonelectrolyte mixtures serve to show how such data constitute severe tests for comparison with theoretical models.