Select thermodynamic models for process simulation
A Practical Guide to a Three Steps Methodology

Example 3-9: Vapour pressures of di-alcohols

When comparing the vapour pressures of di-alcohols using DIPPR[1], figure 1 is found. It is expected that the lightest component has the highest vapour pressure, and the heaviest component the lowest. Yet, the graph shows that curves that cross each other, clearly indicating that something is wrong.

image Figure 1: Vapour pressure behaviour of the series of diols where the hydroxyl groups are at the end of the chain.


In order to analyze the data, the Clapeyron equation is used: two pieces of information are required in order to fully define the Clapeyron straight line. For example, we can choose normal boiling temperature (one point) and enthalpy of vaporization at this temperature (slope). Doing so, each component is represented by a point on figure 2. For each component, the vaporization enthalpy can be obtained, either using the recommended value from DIPPR, or using the slope of the vapour pressure curve at the normal boiling temperature. These two values do not coincide. Whenever possible, an additional source of data (here CRC) was used

image Figure 2: Vapour pressure behaviour of the series of diols where the alcohols is at the end of the chain.


The plot clearly shows that the data do not follow a regular trend, as it should be expected. Therefore, it is highly recommended to collect new experimental information.


[1] R. L. Rowley, W. V. Wilding, J. L. Oscarson, Y. Yang, N. A. Zundel, T. E. Daubert, R. P. Danner, DIPPR® Data Compilation of Pure Compound Properties, Design Institute for Physical Properties, AIChE, New York, NY (2003).

Ref Type: Grant