Compressibility Factors for Nitrogen

The deviation of these gases at high pressure from the perfect gas or Boyle s law is also of Interest and must be taken into account. Figure WfP shows the compressibility factor for pure hydrogen, pure nitrogen and 75 per cent hydrogen plus 25 per cent nitrogen for pressures from atmospheric to 1,000 atmospheres. 

Figure 4.9 Second virial coefficients can be interpreted either as slopes or as intercepts. Top Compressibility factors for pure nitrogen gas at 200 K and 600 K [19]. At each temperature, B(T) is the slope of ffie isotherm as p —> 0. Bottom The data replotted as (Z - l)/p now B(T) is the intercept of an isotherm as p —> 0. Points are data from [19] straight lines are least-squares fits. Note that B may be positive or negative.

Practically, in environmental applications, the temperature ranges from 20 to 400 °C and pressure from 1 to 40 atm. According to available data (Perry and Green, 1999) for air, nitrogen, oxygen, and hydrogen, the compressibility factor is practically unity and only in severe conditions of pressure and temperature varies from 0.98 to 1.02. Thus, the ideal gas law can be safely used in most environmental applications. 

It would be convenient if the compressibility factor at a single temperature and pressure were the same for all gases, so that a single chart or table of z T, P) could be used for all PVT calculations. Nature is not that accommodating, unfortunately for example, z for nitrogen at 0°C and 100 atm is 0.9848 while z for carbon dioxide at the same temperature and pressure is... 

The following data are available for the compressibility factor Z = PV / Kj of nitrogen-butane mixtures at 444.3 K. °... 

Figure 13.3 Isotherms of the compressibility factor (PV/RT) for nitrogen. Temperatures are in kelvins. The inset shows the isotherm for 160 K at higher pressures. Data from Span et al. (2000).

The accurate prediction of thermodynamic properties of natural gas systems is of interest for gas industry. Compressibility factors are used in energy and flow metering. It is also used in calculations of gas pressure gradient in tubing and pipelines. When large volumes of gas are traded between produeers, distributors, and consumers, error in the estimation of the amount of involved are of real economic significance. In gas condensate reservoirs, well-productivity often declines rapidly when pressure drops below the dew point pressure near-wellbore. Therefore, it is very important to accurately determine the dew point pressure. The pressure and temperature of most natural gas mixtures can be found up to 150 MPa and 500 K, respectively (Nasrifar and Boland, 2006). At these eonditions, methane, ethane, and nitrogen are almost always supercritical while other hydroearbons are subcritical. Thus, the equation of state of natural gas mixture must be aeeurate at supercritical and subcritical behavior of methane and heavy hydrocarbons, respectively. 

For detailed information on compressibility factors reference is given to the experimental data by Wiebe, Gaddy [56], by Bartlett et al. [3,4]. These papers give values of the compressibility factors of hydrogen, nitrogen, and 3 1 mix-... 

---