If the straight line connecting these two points on a plot of log pressure versus reciprocal absolute temperature is drawn, a rough estimation of the vapour pressure at intermediate temperatures can be made, provided the normal boiling point (vapour pressure = 1 atm) and the critical temperature and pressure are known.
Vapour pressure can be expressed as a function of temperature through various equations, and one commonly used equation is the three-term Antoine equation:
ln P = A –[B / (C + T)]
where
P = vapour
pressure, mmHg,
A, B,C =
the Antoine coefficients,
T =
temperature, K.
References provide vapour pressure data in the form of constants in the Antoine equation.Values for several thousand compounds are available in compilations by Ohe(1976), Dreisbach (1952), Hala et al. (1968), and Hirata et al. (1975).Nesmeyanov (1963) gives Antoine vapour pressure coefficients for elements,while hydrocarbon vapour pressure data is found in the four-volume handbook by Yaws.
Ifconstants or other references for a specific fluid are available, use them along with the appropriate equation. However, the Antoine equation should not be applied for temperatures outside the range listed in the reference.
Confidence in extrapolating the Wagner equation to higher temperatures is reasonable. The Wagner equation is applicable down to a reduced temperature of 0.5 or to the value of Tmin listed in the reference.
For reduced temperatures below 0.5, it is preferable to use correlations that incorporate thermal information, such as those in Ambrose and Davies (1980),King and Al-Najjar (1974), Majer et al. (1989), Moelwyn-Hughes (1961), Poling(1996), and Majer (1994).
If constants based on thermal information are unavailable, it is recommended to use the Wagner equation constrained to fit the Watson equation forlow-temperature predictions.
At low temperatures, the Ambrose-Walton method and Riedel methods are preferable over the Clapeyron or Antoine equation.
For polar compounds at reduced temperatures between 0.5 and 1.0, the two-reference-fluidor Riedel method is recommended.
If no data are available for a compound and its normal boiling point is unknown, one of the group-contribution methods mentioned above may be used. However, in such cases, calculated results may not be highly accurate.
In addition to the aforementioned recommendations, there are ways to obtain the required data.
If you gotsome queries, let me ask in comments.
Comments