COMPRESSIBILITY FACTOR

Compressibility factor, usually defined as Z = pV/RT, is unity for an ideal gas. It should not be confused with the isothermal compressibility coefficient. In most engineering work, the compressibility factor is used as a correction factor to ideal behavior. Thus, v_real = Z v_id is used to calculate the actual volume, v_real, as the product of the compressibility factor and the ideal gas volume, all at the same pressure and temperature. Z is most commonly found from a generalized compressibility factor chart as a function of the reduced pressure, p_r = p/p_c, and the reduced temperature, T_τ = T/T_c where p_r and T_r are the reduced variables and the subscript ‘c’ refers to the critical point.

Figure 1 shows the essential features of a generalized compressibility factor chart. The most widely-used compressibility factor charts are apparently those of Nelson and Obert (1954, 1955). These have been extended [see, e.g., Liley (1987)] to include the saturated liquid. A three-parameter correlation Z = f(P_r, T_r, ω), where ω = acentric factor = −log₁₀ p_r (T_r = 0.7) −1, involves the use of two compressibility factor charts so that Z = Z₀(p_r,T_r) + wZ_l(p_r, T_r). [See, e.g., Sonntag, R. E. and van Wylen, G. J. (1991).]

Figure 1. Generalized compressibility factor chart.