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COMPRESSIBILITY FACTOR

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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, vreal = Z vid is used to calculate the actual volume, vreal, 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, pr = p/pc, and the reduced temperature, Tτ = T/Tc where pr and Tr are the reduced variables and the subscript 'c' refers to the critical point. (See Corresponding States, Principle of.)

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(Pr, Tr, ω), where ω = acentric factor = −log10 pr (Tr = 0.7) −1, involves the use of two compressibility factor charts so that Z = Z0(pr,Tr) + wZl(pr, Tr). [See, e.g., Sonntag, R. E. and van Wylen, G. J. (1991).]

Generalized compressibility factor chart.

Figure 1. Generalized compressibility factor chart.

REFERENCES

Nelson, L. C. and Obert, E. F. (1955) Trans. A.S.M.E., 76 (10), 1057-1066 (1954); Laws of corresponding states, A.I. Ch. E. J. 1 (1), 74-77.

Liley, P. E. (1987) Chemical Engineering (NY), 94 (10), 123-126.

Sonntag, R. E. and van Wylen, G. J. (1991) Introduction to Thermodynamics, Classical and Statistical, Wiley, NY, 800 pp.

References

  1. Nelson, L. C. and Obert, E. F. (1955) Trans. A.S.M.E., 76 (10), 1057-1066 (1954); Laws of corresponding states, A.I. Ch. E. J. 1 (1), 74-77. DOI: 10.1002/aic.690010111
  2. Liley, P. E. (1987) Chemical Engineering (NY), 94 (10), 123-126.
  3. Sonntag, R. E. and van Wylen, G. J. (1991) Introduction to Thermodynamics, Classical and Statistical, Wiley, NY, 800 pp.

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