China
Africa
Explore chapters and articles related to this topic
The Basics: Introduction and Thermodynamics Review
Published in R.E. Hayes, Introduction to Chemical Reactor Analysis, 2020
At some temperatures or pressures gases will not behave according to the ideal gas law, because at those conditions the assumptions made in developing the ideal gas law are not valid. In such a case the gas is said to be non-ideal. Many equations have been developed to model the behaviour of non-ideal gases, with the simplest equation using a compressibility factor, Z, to account for the deviation from the ideal. The equation is: PV=ZNRgTIf the compressibility factor equals one the gas behaves as an ideal gas. The computation of Z is relatively straight forward. It is usually calculated using the reduced temperature and pressure, which are in turn defined as the temperature and pressure divided by the critical temperature and pressure respectively. See for example Smith et al (1996) or Sandler (1999).
Off-Spec Natural Gas
Published in Mavis Sika Okyere, Mitigation of Gas Pipeline Integrity Problems, 2020
For an ideal gas the compressibility factor is Z = 1 per definition. In many real-world applications requirements for accuracy demand that deviations from ideal gas behavior, i.e. real gas behavior, be taken into account. The value of Z generally increases with pressure and decreases with temperature. At high pressures molecules are colliding more often. This allows repulsive forces between molecules to have a noticeable effect, making the molar volume of the real gas (Vm) greater than the molar volume of the corresponding ideal gas ((Vm)ideal gas = RT/P), which causes Z to exceed one. When pressures are lower, the molecules are free to move. In this case attractive forces dominate, making Z < 1. The closer the gas is to its critical point or its boiling point, the more Z deviates from the ideal case.
Gases
Published in W. John Rankin, Chemical Thermodynamics, 2019
The compressibility factor is determined experimentally by measurements of p, T and V or estimated using mathematical models. For ideal gases, Z = 1, and Equation 3.8 reduces to the ideal gas law, but for real gases Z is either greater than or less than one. It is difficult to generalise at what pressures or temperatures the deviation of gases from ideal behaviour becomes important. As a rule of thumb, most gases obey the ideal gas law reasonably accurately up to a pressure of about 2 bar and even higher for gases composed of small non-associating molecules. For example, methyl chloride (CH3Cl), with a highly polar molecule and therefore a gas with significant intermolecular forces, has a compressibility factor of 0.9152 at a pressure of 10 bar and temperature of 100°C. For air (consisting of small non-polar molecules) at similar conditions the compressibility factor is 1.0025.
An empirical equation for the gas compressibility factor, Z
Published in Petroleum Science and Technology, 2020
In order to determine the mass flow of gases, the compressibility factor is employed to account for the deviation from ideal gas behavior. Many theorists have developed equations that give greater insight into the effects of acentricity and particle-particle interaction (Çengel and Boles 2001). van der Waals theorized that gases behave similarly when compared at the same reduced temperature and pressure, and this notion inspired the generalized compressibility factor chart (Su 1946). The data we examine is for a large domain of reduced temperature TR (1-5) and reduced pressure PR (0.5-6). Many equations of state have been developed, each with their own strengths and limitations (Horvath 1974; Saleh and Hashim 2009; Lopez-Echeverry, Reif-Acherman, and Araujo-Lopez 2017). The compressibility factor table is notably simple and easy to use, and its appeal therein lies. Since the development of the generalized compressibility chart, there have been great strides in the ability to symbolically regress datasets into manageable expressions. Interested readers can explore the topic at their leisure (Fortin et al. 2012; Kommenda et al. 2013). The following equation is a symbolic regression of the generalized compressibility factor chart.
Modeling natural gas compressibility factor using a hybrid group method of data handling
Published in Engineering Applications of Computational Fluid Mechanics, 2020
Abdolhossein Hemmati-Sarapardeh, Sassan Hajirezaie, Mohamad Reza Soltanian, Amir Mosavi, Narjes Nabipour, Shahaboddin Shamshirband, Kwok-Wing Chau
In this work, GMDH was used to estimate the compressibility factor of natural gas. The GMDH model outperformed other models and provided higher accuracy at the various gas conditions. This was confirmed by measuring the RMSE, average absolute percent relative error, and regression coefficient to be 0.03, 2.88%, and 0.92, respectively. The Hall Yarborough correlation was determined as the second most accurate correlation for estimating the natural gas compressibility factor. Besides, the error distribution curve analysis indicated that the presented model in this study does not have an error trend when predicting very low and very high compressibility factor values. The experimental trend of gas compressibility factor showed that by increasing the pressure, the Z-factor first decreases and then increases. This trend was perfectly shown by the developed GMDH model in this work.