Elsevier

Fluid Phase Equilibria

Volume 291, Issue 1, 25 April 2010, Pages 18-31
Fluid Phase Equilibria

Excess properties and isobaric vapor–liquid equilibria for four binary systems of alkyl (methyl to butyl) methanoates with decane

https://doi.org/10.1016/j.fluid.2009.12.003Get rights and content

Abstract

This work shows the experimental data of excess properties at several temperatures and the vapor–liquid equilibria (VLE) obtained for four binary systems of alkyl methanoates (methyl to butyl) with decane, all measured at constant pressure of 101.32 kPa. The isobaric VLE data were thermodynamically consistent according to the Fredenslund test and did not present an azeotrope. The experimental data of HmE and VmE showed that the mixing process of the systems studied are clearly endothermic with expansion effects, and this was the case for all the mixtures in which (HmE/T)p>0 and (VmE/T)p>0. The mixing properties data indicate high interactional effects that also produce high values of γi, especially in the system containing methyl methanoate, caused by associative problems that diminish with increased alkanolic chain length of ester. Mixing quantities were correlated with a parametric model with temperature-dependent coefficients. The VLE data and enthalpies were simultaneously correlated with a general model established for excess Gibbs function, yielding an acceptable correlation in all cases. Isobaric VLE quantities were estimated with two known versions of the UNIFAC model, with very similar results. Using the parameters published in the literature for the HCOO/CH2 interaction, the UNIFAC model does not reproduce well the data that characterize VLE for the mixtures studied, although the version of Gmehling et al. [8] using the same set of parameters, makes an acceptable qualitative and quantitative prediction of HmE.

Introduction

In a previous article [1], the results of a study with mixtures of alkyl methanoates + hexane were presented as part of a research project relating to methanoate systems. There are insufficient experimental data in the literature on vapor–liquid equilibria (VLE) of alkyl methanoate + alkane systems. This theoretical-experimental work continues on from a study published previously [1] into four binary systems, with decane as a common compound combined with the first four alkyl methanoates of the series. Isobaric VLE data are found in the literature [2] for only one of the systems chosen here, ethyl methanoate + decane, although experimental data have been recorded for other mixing properties such as HmE and VmE, determined in our laboratory at a temperature of 298.15 K [3], [4], and VmE by other authors [5], [6], also at the same temperature. It is important to evaluate the experimental results in order to determine the behavior of methanoates in solution, because the presence of HCOO– group offers a specific behavior in the mixtures due to hydrogen bonds in the molecule [3]. All experimental information obtained will be used in this work, together with other information about similar systems, to define the behavior of these systems more clearly and to confirm the structural model proposed previously [3]. Alkyl methanoates, and in particular methyl methanoate, are known to have a specific behavior in solution, among other reasons because of existence of autoassociations and possible heteroassociations with molecules of a second active component. Therefore, we have carried out a systematic study elucidating in a first step their behavior in mixtures with an inert component such as alkanes.

Here, we also verify the utility of a model to simultaneously correlate the different thermodynamic quantities, which has produced excellent results in previous works. Different versions of the UNIFAC group contribution model [7], [8] will also be used for analyzing the utility of the model to estimate properties in the set alkyl methanoates + alkanes.

Section snippets

Materials

All pure compounds used in the experimental part of this work, alkyl methanoates and decane, were of the maximum commercial purity supplied by Aldrich. Before use all products were subjected to a preliminary treatment to improve their quality. They were degasified with ultrasound for several hours and then kept in the dark over a 0.3 nm Fluka molecular-sieve before use to reduce the moisture content. The quality of products, verified with a Hewlett-Packard gas chromatograph model HP6890N

Excess properties

The HmE of all the binary systems of this work were determined at 291.15 and 318.15 K, except for the mixture of methyl methanoate which, as mentioned previously, has a very low boiling point. The values of (x, HmE) are shown in Table 2 and the pairs of (x, VmE) obtained from direct measurements of the pairs (x, ρ) for each of the binary mixtures of xalkyl methanoate + (1  x)decane at temperatures of 291.15, 298.15, 308.15 and 318.15 K are shown in Table 3. For reasons mentioned above, the VmE

Treatment and theoretical prediction of VLE data

The isobaric VLE data obtained experimentally and presented in Table 7 were correlated using an expression for the excess Gibbs function, with a very similar form to that expressed by Eqs. (1), (2). In previous works [1], [2], [20], a mathematical model was used to correlate values of excess Gibbs function in the adimensional form Q=GmE/RT=ϕ(x,T), obtained to correlate isobaric equilibria, since the values of Q are determined directly from the weighted sum of the ln γi as indicated in the

Acknowledgements

Financial support from Ministerio de Educación y Ciencia from Spain, through the project No. CTQ2006-12027, is gratefully acknowledged.

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