Abstract
Chemical equilibrium constants for the methanol from CO/H2 reaction calculated from literature thermodynamic parameters are too high as compared to experimental results. Therefore, both the entropy value and the enthalpy of formation value of methanol were reviewed. A rigorous analysis of the association behaviour of methanol vapour confirms that dimers and cyclic tetramers play an important role, but physical non-ideal gas behaviour must also be taken into account. Accurate relationships for the association equilibrium constants and the physical contribution to the second virial coefficient were derived with the use of a combined fit of multiple experimental data sources including heat capacity, speed of sound, thermal conductivity, excess molar enthalpy of methanol and nitrogen and heat of vaporization. Additionally, high temperature second virial coefficients and measures for the consistent temperature dependencies of the entropy and enthalpy of formation were included in the parameter optimization to support the accuracy of the model. All experimental results and supporting data were taken or derived from the literature. The resulting virial equation of state was used to calculate new ideal-gas entropy and enthalpy of formation values of methanol, which now turn out to be consistent with values derived from experimental chemical equilibrium data. Furthermore, the new third-law entropy value turns out to be consistent with the current literature value. A new enthalpy of formation value is recommended and an improved chemical equilibrium relationship for the methanol from CO/H2 reaction is presented.
Highlights
We published a review on the chemical equilibria in methanol synthesis [1]
The most important publication in this respect is from Weltner & Pitzer [6], who concluded from heat capacity measurements that methanol vapour is a mixture of monomers, dimers and tetramers
An improved third law entropy and enthalpy of formation analysis is presented based on modelling the temperature and pressure dependency of several experimental data, including heat capacity, heat of mixing, thermal conductivity, speed of sound and enthalpy of vaporization
Summary
We published a review on the chemical equilibria in methanol synthesis [1]. It turned out that accurate ideal-gas equilibrium constant relationships could be derived from thermochemical basic data taken from the literature in combination with small corrections for the Gibbs energy of CO and CH3OH to match these. Using the literature entropy value, the calculated equilibrium constants of the methanol from CO/H2 reaction are approximately 1.2 times too high For this purpose, we will carefully review the third-law entropy analysis including the influence of the formation of associated methanol clusters in the vapour phase. We will review the enthalpy of formation value of methanol, because the observed difference might be caused by this parameter as well It will allow us to confirm or further improve the chemical equilibrium relationship of the methanol from CO/H2 reaction, which is important for the accurate design of methanol synthesis processes
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