Experimentation and thermodynamic representations of binaries containing compounds of low boiling points: Pentane and alkyl methanoates

This work presents experimental mixing properties, hE and vE, at several temperatures and the isobaric vapor–liquid equilibria (iso-p VLE) at 101.32 kPa for four binaries containing pentane and four alkyl (methyl to butyl) methanoates. Particular conditions are established to work with these solutions with highly volatile compounds, especially for the case of methyl methanoate + pentane system, for which a continuous feeding device is designed and constructed for measuring the densities. The mixing processes of the compounds chosen give rise to high values for the excess properties and also for the activity coefficients, since in addition to the known effects that arise in the binaries ester + alkane, in this case the associative effects caused by the HCOO– group, which diminish with increasing methanoate chain length are also relevant. The change in temperature produces slopes of opposite signs for the mixing properties, being (∂vE/∂T)p<0∂vE/∂Tp<0 and (∂hE/∂T)p>0∂hE/∂Tp>0. The systems comprised of methyl and ethyl methanoate with pentane present azeotropes with coordinates (xaz,Taz/K) situated at (0.558, 293.9) and (0.218, 306.5), respectively. The estimation of these coordinates and the iso-p VLE by the UNIFAC method are acceptable but do not give good predictions of the hE even less so for the LLE of the methyl methanoate + pentane system. For this binary, experimental data of all the properties are correlated with a mathematical procedure described using two models, one developed by us, and an extended form of the NRTL model. The latter does not represent the LLE data but shows the properties of isobaric equilibria, with acceptable results, while the proposed model, in addition to representing the binodal curve, gives a good representation of the vE and of these properties derived from excess Gibbs function.

Estimation by UNIFAC of the curves iso-p VLE (––––) and iso-p LLE (– – –) for the binary methyl methanoate + pentane with experimental values from literature (
• ).Figure optionsDownload as PowerPoint slide