Compressibility Factors for Argon

The definition of w makes its value zero for argon, krypton, and xenon, and experimental data yield compressibility factors for all three fluids that are correlated by the same curves when Z is represented as a function of Tr and Pr. Thus the basic premise of the three-parameter theorem of corresponding states is that all fluids having the same value of w have the same value of Z when compared at the same Tr and Pr. 

In this lab, the students determine the compression factor, (9) Z = PV/nRT, for Argon using the hard sphere model, the soft sphere model, and the Lennard-Jones model and compare those results to the compression factor calculated using the van der Waals equation of state and experimental data obtained from the NIST (70) web site. Figure 3 shows representative results from these experiments. The numerical accuracy of the Virtual Substance program is reflected by the mapping of the Lennard-Jones simulation data exactly onto the NIST data as seen in Figure 3. 

Perry s Chemical Engineers Handbook (see footnote 2), pp. 2-140 through 2-150, gives values of z(T, P) for air, argon, COj, CO, H2, CH4, N2, O2, steam, and a limited number of other compounds. Once z is known, it can be substituted in the compressibility factor equation of state, which may in turn be solved for whichever variable is unknown. 

Gosman et have tabulated, for argon, values of density, internal energy, enthalpy, and entropy of the liquid and gas for temperatures from 83.8 to 300 K at pressures from 0.01 to 1000 atm. Diagrams for heat capacities, compression factors, and entropies are included. A vapour-pressure equation covering the temperature range from the triple point to the critical point is also given. 

FIG. 22 Spreading compressibility factor, Z, for argon on graphite at two temperatures [214]. Dots calculated by Glandt et al. [211] from experimental results lines obtained by us [214] from the Reddy-O Shea Eq. (28) and Cuadros-Mulero [Eq. (29)] equations of state. AtT = 158.077 K, we show values of Z + 0.2. 

Figure 8.4 presents a plot of the compressibility factor z versus pressure - at the same temperature - for two pure gases A and B, consisting of small molecules such as argon and methane. The difference in the z values, at a given pressure, is apparent. 

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