Thermodynamic Properties Of Natural Gas Research Articles

A rigorous model to compute thermodynamic properties of natural gas without applying full component analysis

Precise computation of thermodynamic properties of natural gas requires applying an accurate Equation of State (EOS) along with component analysis of natural gas. This procedure is a time consuming process which demands expensive apparatuses. In this study, a rigorous model by firstly applying a gross standard EOS called AGA8-GCM is used to predict compressibility factor (Z) and density (ρ). The required input data for this model are operating temperature and pressure, specific gravity at reference conditions, and merely the amount of nitrogen and carbon dioxide in the natural gas. Then, Genetic Algorithm (GA) is employed to estimate component analysis to increase the accuracy of following calculations. Finally, the thermodynamic properties equations, in which partial derivatives were obtained by AGA8-GCM equation is used to calculate other thermodynamic properties: the speed of sound (usound), heat capacity at constant volume and pressure (CV, CP), Joule-Thomson coefficient (μJT), isentropic exponent (κ), internal energy (U), enthalpy (H), and entropy (S). In order to validate the model, the calculated data were compared to experimental ones collected from literature by the Average Absolute Deviation percentage (AAD%). The AAD% amounts for Z, ρ, usound, CV, CP, μJT, U, H, and S were obtained 0.025%, 0.063%, 0.51%, 0.94%, 1.22%, 3.06%, 0.064%, 0.62% and 2.46%, respectively. The pure methane data is utilized for CV, U, H, and S evaluation as for the lack of empirical data. Furthermore, comparing calculated data with that of AGA8-DCM (Detail Characterization Method), requiring detailed composition of natural gas, shows reliability of this model in the custody transfer region. Investigating the model uncertainty showed a figure of about ±0.1% for both the compressibility factor and density. However, as for the other thermodynamic properties, this figure was usually higher (<0.8%).

Prediction of Vapor–Liquid Equilibrium and Thermodynamic Properties of Natural Gas and Gas Condensates

In this work, the performance of the UMR-PRU model, which combines the Peng–Robinson (PR) equation of state (EoS) with the original UNIFAC through the Universal Mixing Rules (UMR), is further impro...

A comprehensive analysis of energy and exergy characteristics for a natural gas city gate station considering seasonal variations

Comprehensive energy and exergy analyses are conducted on a City Gate Station (CGS) having nominal capacity of 20,000 SCMH. For this purpose, thermodynamic properties of Natural Gas (NG) fed into the CGS are firstly determined using American Gas Association Equation of State (AGA-8 EOS). Then, a quantitative analysis is carried out to explore magnitude and exact locations of energy/exergy losses as well as exergy destructions. To this end, four different seasonal strategies are regarded. In all strategies, the largest losses occur within the stack. Although from energy viewpoint, the regulator is a high-efficiency equipment, it is found to be the most exergy destructive component in the CGS. Moreover, maximum and minimum exergy losses occur in the winter (15.33 kW) and summer (1.60 kW), respectively. The best performance based on the second law of thermodynamics for the CGS occurs in the winter with exergy efficiency of 77%, whereas the lowest one happens in the summer with exergy efficiency of 69%. The exergy destruction due to pressure drop in filter and pipes are insignificant. The results obtained from this study can be employed as a guide to reduce exergy destruction in the whole CGS with recognition of the main sources of irreversibility.

PARAMETRIC DIAGNOSTICS OF THE CENTRIFUGAL SUPERCHARGER'S TECHNICAL CONDITION DURING OPERATION

Objectives The main aim is to develop a mathematical model of a centrifugal compressor and carry out a parametric diagnostics of a centrifugal supercharger's technical condition during operation. Methods A model is proposed for calculating the thermodynamic properties of natural gas, reducing the parameters of a centrifugal compressor to the initial conditions and to the rotation frequency, as well as the integral indicators of the supercharger's technical state. The technical state of the gas path of the centrifugal supercharger of the compressor unit is determined by the parametric diagnostic method. Results The software implementation of the mathematical model of centrifugal compressor is carried out using a DVIGwT PC. The analysis of calculations indicates that the model is appropriate, with the error being due to taking into account the properties of iso-butane and i-hexane, in contrast with the VNIIGAZ technique. The evaluation studies of a centrifugal compressor's state are indicative of the presence or absence of its defects. Conclusion Among a number of the diagnostic methods for evaluating a centrifugal supercharger, the most effective is vibrodiagnostics. However, the search for malfunctions and nascent defects in the flowing part of the centrifugal compressor cannot be limited only to vibrodiagnostic data, which provides about 60% of the reliable information about the state of the gas-air tract. About 20% of the compressor's malfunctions and approximately half of the dangerous modes of the supercharger's flow-through part is detected using thermogasdynamic parametric analysis (parametric diagnostics). The main difficulty of the control over the technical state of the flow-through part of the centrifugal supercharger is in the complication of the quantitative evaluation of the processes taking place in the supercharger, which leads to problems in providing reliable diagnosis during a reasonable period of time.

REMOVED: Simulation of steady flow of natural gas in a subsea flexible riser with heat exchange

Abstract Flexible riser technology is widely used in today's offshore industry. Evaluating pressure, temperature and velocity profiles in a subsea riser or a flexible pipe is essential in the design of a gas transfer system. Conventional pipeline correlations fail to give an acceptable accuracy for sizing, design and flow assurance applications of flexibles. Hence, availability of a rather simple computational model, which can give results with acceptable accuracy is highly advantageous. The aim of this article is to present a simple numerical method which can be used efficiently to accomplish this goal. A one dimensional finite difference method is used where the riser length is discretized into small segments. Natural gas physical and transport properties are calculated in each segment and flow equations in addition to a state equation are solved simultaneously to find the velocity, pressure and temperature profiles in the flexible riser. The Lee-Kesler corresponding states EOS has been used as the state equation. Mathcad™ has been applied for solving equations. The ability of the model to predict thermodynamic properties of natural gas has been compared with experimental data. Furthermore, friction and heat transfer models for a smooth and rough bore riser have been analyzed. For a rough bore riser with carcass corrugations, selecting a reasonable value for the equivalent sand grain roughness has been found to be crucial. The authors have also found the Joule-Thomson effect to be decisive in temperature change for a high pressure rough bore flexible gas riser subject to high pressure drop.

Thermodynamic DFT analysis of natural gas.

Density functional theory was performed for thermodynamic predictions on natural gas, whose B3LYP/6-311++G(d,p), B3LYP/6-31+G(d), CBS-QB3, G3, and G4 methods were applied. Additionally, we carried out thermodynamic predictions using G3/G4 averaged. The calculations were performed for each major component of seven kinds of natural gas and to their respective air + natural gas mixtures at a thermal equilibrium between room temperature and the initial temperature of a combustion chamber during the injection stage. The following thermodynamic properties were obtained: internal energy, enthalpy, Gibbs free energy and entropy, which enabled us to investigate the thermal resistance of fuels. Also, we estimated an important parameter, namely, the specific heat ratio of each natural gas; this allowed us to compare the results with the empirical functions of these parameters, where the B3LYP/6-311++G(d,p) and G3/G4 methods showed better agreements. In addition, relevant information on the thermal and mechanic resistance of natural gases were investigated, as well as the standard thermodynamic properties for the combustion of natural gas. Thus, we show that density functional theory can be useful for predicting the thermodynamic properties of natural gas, enabling the production of more efficient compositions for the investigated fuels. Graphical abstract Investigation of the thermodynamic properties of natural gas through the canonical ensemble model and the density functional theory.

A fast calculation method of thermodynamic properties of variable-composition natural gas mixtures in the supercritical pressure region based on implicit curve-fitting

Vaporizers are widely used to re-gasify the liquefied natural gas which consists of variable compositions at supercritical pressures. The duration of vaporizer design based on simulations severely depends on the calculation speed of thermodynamic properties of natural gas due to millions of thermodynamic property iterations. This paper presents a fast calculation method of thermodynamic properties of natural gas in supercritical pressure region. In this method, a calculation model for a reference composition is established at first, by dividing the supercritical pressure region into three subsections and regressing each subsection separately based on implicit curve-fitting; and then composition based implicit curve-fitting equations are developed to extend thermodynamic property calculation from the reference composition to any variable compositions. With this method, the equations for typical compositions of natural gas including C1, C2, C3, n-C4, n-C5 and N2 are presented, covering the temperature range from −170 °C to 60 °C and the pressure range from 5 MPa to 10 MPa. The present method is about 104 times faster than GERG-2008, and the mean deviation is less than 0.5%.

THE USE OF MATLAB IN THE DETERMINATION OF THERMODYNAMIC PROPERTIES OF NATURAL GAS

During the preliminary phases of typical operations in the petroleum industry, particularly those involving distribution networks and/or natural gas transmission projects, when complete information is not available, it is sometimes necessary to estimate some thermodynamic properties of natural gas. The behavior of natural gas is affected directly by its chemical composition. It comprises a mixture of volatile hydrocarbons, nitrogen, oxygen, carbon dioxide, and traces of other components. This knowledge is crucial to engineers involved in the areas of equipment design, processing and transportation of such fluids. The design of these systems uses conventional flow equations for both stationary and transient systems, which also include various natural gas fluid properties such as density, gas constant, specific heat, critical temperature and pressure, compressibility factor, and dynamic viscosity, among others. In view of this, a proposition is given to determine the main properties of natural gas from its average chemical composition and related operational conditions (pressure and temperature). This proposal, developed in Matlab, allows the determination of some of the key properties of natural gas, giving users the option to choose the most representative equation of state among the available models, namely, Peng-Robinson, Patel-Teja-Valderrama, Shell Oil Company, Soave-Redlich-Kwong, and van-der-Waals, to determine the compressibility factor of the samples. The results of a particular simulation were compared with actual data from the distribution network COMPAGAS, which is the company responsible for the distribution of piped natural gas in the state of Paraná (Brazil), demonstrating a good convergence between the values obtained and encouraging their use in future.

Evaluation of prediction models for the physical parameters in natural gas liquefaction processes

The natural gas liquefaction process is a complicated and dynamic thermal system. Operation conditions change during the process of compression, throttling and heat transfer, which inevitably leads to changes of the thermodynamic property parameters and the phase state of the natural gas and refrigerants. Performing a simulation of the liquefaction process is one of the main methods to improve the economic efficiency of the process and to reduce the production cost; such a simulation can be conducted using a process simulation software package, such as Aspen HYSYS, Aspen Plus, SIMSCI PRO-II and Honeywell UniSim Design. Accurate prediction of the thermodynamic properties of natural gas and refrigerants with the change of working conditions, such as density, specific heat capacity, enthalpy, and entropy, is of significant importance for the simulation of natural gas liquefaction processes. There are many types of property methods embedded in simulation software. Because each property method achieves good performance in a certain range of working conditions, it is crucial to choose the proper method to conduct the simulation. The Soave–Redlich–Kwong equation, the Peng–Robinson equation and the Lee–Kesler–Plocker equation are the main calculation models for physical parameters in natural gas liquefaction processes. According to a literature review, the GERG-2008 equation shows high precision in calculating the thermodynamic properties and phase equilibrium of natural gas and similar mixtures in a wide range of temperature and pressure. Based on accurate experimental data, a comprehensive comparison and analysis among these equations is conducted in this paper. The GERG-2008 equation is recommended as the basis for the calculation of the physical parameters in natural gas liquefaction processes.

Thermodynamic analysis of natural gas reciprocating compressors based on real and ideal gas models

The accurate modelling and investigating effects of various parameters of the reciprocating compressors are important subjects. In this work, based on first law of thermodynamics, conversation of mass and real and ideal gas assumptions, a theoretical analysis has been constructed to simulate natural gas reciprocating compressors. For computing the thermodynamic properties of natural gas based on real gas model, the AGA8 equation of state has been used. Numerical results validated with previous measured values and showed a good agreement. The effects of important parameters such as: angular speed, clearance and pressure ratio have been studied on the performance of the compressors. The results reveal the in-control volume temperature for ideal gas is more than real gas model but the mass flow rate and work for real gas is higher than ideal gas model. On the other hand, the indicated work that required for compression is greater for ideal gas model.

Numerical simulation of filling process of natural gas onboard vehicle cylinder

The accurate modeling of the filling compressed natural gas fueled vehicle storage cylinders is a complex process and should be studied deeply. The minimum filling time has positive impact on commercialization of natural gas vehicles. In other hand, very fast filling may be resulted to unexpected temperature rise and violating the safety standards. This study investigates flow and heat transfer in natural gas vehicle’s onboard cylinder during filling. The cylinder is assumed to be a type III onboard storage cylinder. An axisymmetric computational model for unsteady, compressible turbulent flow has been built. A computational fluid dynamics has been developed for predicting the temperature and pressure change during the fill based on using commercial software Fluent. The natural gas (NG) as working fluid is treated as a real gas. The Redlich–Kwong equation of state has been employed to compute the thermodynamic properties of NG. The computation results have been compared with previous measured values and show good agreement. The results show that the temperature rise for NG is about 35 K. The most of heat dissipation from the in-cylinder gas is stored in the cylinder wall during the fill and the heat lost to the ambient is small.

Numerical procedures for natural gas accurate thermodynamic properties calculation

Natural gas (NG) is a mixture of 21 elements and widely used in the industries and domestics. Knowledge of its thermodynamic properties is essential for designing appropriate process and equipments. In this study, the detailed numerical procedures for computing most thermodynamic properties of natural gas are discussed based on the AGA8 equation of state (EOS) and thermodynamics relationships. To validate the procedures, the numerical values are compared with available measured values. The validations show that the average absolute percent deviation (AAPD) for density calculations is 0.0831%, for heat capacity at the constant pressure is 0.87%, for heat capacity at the constant volume is 1.13%, for Joule-Thomson coefficient is 1.93%, for speed of sound is 0.133%, and for enthalpy is 1.06%. Furthermore, in this work, the new procedures are presented for computing the entropy and internal energy. Due to lack of experimental data for these properties, the validation is done for pure methane. The validation shows that AAPD is 0.078% and 0.0133% for internal energy and entropy, respectively.

Developing novel correlations for calculating natural gas thermodynamic properties

Developing novel correlations for calculating natural gas thermodynamic properties Natural gas is a mixture of 21 components and it is widely used in industries and homes. Knowledge of its thermodynamic properties is essential for designing appropriate processes and equipment. This paper presents simple but precise correlations of how to compute important thermodynamic properties of natural gas. As measuring natural gas composition is costly and may not be effective for real time process, the correlations are developed based on measurable real time properties. The real time properties are temperature, pressure and specific gravity of the natural gas. Calculations with these correlations are compared with measured values. The validations show that the average absolute percent deviation (AAPD) for compressibility factor calculations is 0.674%, for density is 2.55%, for Joule-Thomson coefficient is 4.16%. Furthermore, in this work, new correlations are presented for computing thermal properties of natural gas such as enthalpy, internal energy and entropy. Due to the lack of experimental data for these properties, the validation is done for pure methane. The validation shows that AAPD is 1.31%, 1.56% and 0.4% for enthalpy, internal energy and entropy respectively. The comparisons show that the correlations could predict natural gas properties with an error that is acceptable for most engineering applications.

OPTIMIZATION OF THE C3MR CYCLE WITH GENETIC ALGORITHM

The aim of this paper is thermodynamic simulation and optimization of the C3MR system with Genetic Algorithm. For this purpose, in the first step, the Peng Robinson equation of state is simulated with a code in MATLAB and then used for simulating thermodynamic properties of natural gas and refrigerants that are used in the cycle. Following that the cycle is thermodynamically simulated and composite curves for subcooling and liquefaction heat exchangers are plotted. If composite curves in heat exchangers approach, the total power will decrease. Then, the total power used by the compressors is calculated. In the next step, the thermodynamic modeling is linked with Genetic Algorithm and the total power consumed by compressors is defined as objective function. The best value resulted from optimization has 23% lower power than the base design. In addition, heat exchange curves closed together.

Thermodynamic Properties of Natural Gas, Petroleum Gas and Related Mixtures. Part I: Mixed Fluid Densities

AbstractThe present paper describes a method for very accurate correlation of mixed fluid densities of natural gas, petroleum gas and related mixtures, including low molecular weight hydrocarbons, nitrogen, carbon dioxide, carbon monoxide and hydrogen sulphide. The method is applicable for mixed fluid densities both on and off the saturation surface. The method is based on a statistical mechanical formulation of the principle of corresponding states using methane as the reference fluid. The overall agreement with known experimental densities is 0.1% for LNG and 0.2% for LPG.