Phase equilibrium data and thermodynamic modelling of the system (propane + DMF + methanol) at high pressures

doi:10.1016/j.jct.2010.10.014

The Journal of Chemical Thermodynamics

Volume 43, Issue 3, March 2011, Pages 413-419

https://doi.org/10.1016/j.jct.2010.10.014 Get rights and content

Abstract

Reported in this work are phase equilibrium data at high pressures for the binary and ternary systems formed by {propane + N,N-dimethylformamide (DMF) + methanol}. Phase equilibrium measurements were performed in a high-pressure, variable-volume view cell, following the static synthetic method for obtaining the experimental bubble and dew points transition data over the temperature range of (363 to 393) K, pressures up to 11.5 MPa and overall mole fraction of the lighter component varying from 0.1 to 0.995. For the systems investigated, vapour–liquid (VLE), liquid–liquid (LLE) and vapour–liquid–liquid (VLLE) phase transitions were visually recorded. Results show that the systems investigated present UCST (upper critical solution temperature) phase transition curves with an UCEP (upper critical end point) at a temperature higher than the propane critical temperature. The experimental data were modelled using the Peng–Robinson equation of state with the Wong–Sandler and the classical quadratic mixing rules, affording a satisfactory representation of the experimental data.

Introduction

The presence of nitrogen compounds in crude petroleum and in its fractions may give rise of serious problems in the refining process, such as catalyst poisoning, corrosion and gum or colour formation in final products [1], [2], [3]. Besides, high toxicity and carcinogenic activity have been attributed to several basic and even neutral nitrogen compounds [4]. It is well known that the presence of nitrogen compounds has a negative effect on hydro-desulfurization treatment (HDS), the most widely used method for the pollutants removal from fossil fuels [5], [6], [7], [8]. As such a process alone is ineffective and expensive [9], it is important to study the removal of nitrogen compounds prior to the HDS treatment [10], [11].

The nitrogen content, usually present in low levels in crude oil (0.1 to 2)%, generally increases with increasing boiling temperature of oil fraction and thus, with most parts concentrated in the heavy fractions and residues [12]. In the petroleum segment, several organic solvents have been pointed as potential for extractive removal of nitrogen compounds in oil, which is the case of N,N-dimethylformamide (DMF), a relatively low boiling temperature solvent [13], [14].

The progressive increasing amounts of heavy oil fractions leads to the need of adapting and expanding actual installations. Thus, the study and understanding of phase behaviour of the heavy oil fractions in proper solvents constitute a fundamental step for optimization and production purposes, the backbone of re-evaluation of industrial install capacity.

Recently, the use of compressed fluids has received increasing attention towards performing selective extractions/separations with high selectivity and process yields. Additionally, the use of a compressed fluid, such as propane can help to break the azeotropes formation. Furthermore, phase equilibria data are useful for designing the removal of nitrogen compounds present in crude oil fractions [1], [2], [3]. In this scenario, the application of thermodynamic models may be relevant to describe the phase behaviour of multicomponent mixtures and hence to elucidate the phenomena involved in the fractionation/separation step, in the process design and optimization and to establish the process flow sheet of a chemical plant.

Undoubtedly, the general class of cubic equations of state may be advantageously used to represent the experimental phase equilibrium data, thus allowing process simulation so as to select appropriate operational conditions. Many cubic equations of state (EoS) have been proposed in the literature through the years to improve the prediction and correlation of thermodynamic and phase equilibrium properties [15], but certainly the cubic EoS due to Peng and Robinson (P–R EoS) [16] with a variety of mixing rules has been one of the most successful [17].

In this context, the aim of this work is to report new experimental data for the binary (propane + DMF) and ternary (propane + DMF + methanol) systems. In addition, the P–R EoS with two different mixing rules, classical quadratic van der Waals and Wong–Sandler, is employed to represent the experimental data.

Section snippets

Materials

Propane (minimum mass fraction purity of 0.995 in the liquid phase) was purchased from White Martins S.A. (Brazil), methanol (0.998) and DMF (0.998) were supplied by Vetec S.A., Brazil. All chemicals were used without further purification.

Phase equilibrium apparatus and experimental procedure

Phase equilibrium experiments were conducted employing the static synthetic method in a high-pressure variable-volume view cell. The experimental apparatus and procedure have been used in a variety of studies [18], [19], [20], [21], [22], [23], [24], [25] and

Results and discussion

TABLE 2, TABLE 3, TABLE 4 contain the experimental phase transition data for the binary system (propane + DMF) and for the ternaries (propane + DMF + methanol) at the two DMF to methanol mole ratios investigated in this work, 1:1 and 2:1. It can be observed from these tables the occurrence of the biphasic LLE equilibrium {(liquid + liquid) equilibrium}, VLE-BP/DP {(vapour + liquid) equilibrium bubble and dew points} and also three-phase VLLE {(vapour + liquid + liquid) equilibrium}. These tables express the

Conclusions

Reported in this work are phase equilibrium data for the system (propane + DMF) over the temperature range of 363.15 K to 393.15 K and propane composition ranging from 0.1 to 0.995, affording phase transition pressures up to 11.5 MPa. Results show that the system presents high asymmetry and UCST transitions type and an UCEP. According to the experimental data obtained, it is reasonable to suggest that the system (propane + DMF) presents type III phase diagram. It was experimentally observed that the

Acknowledgements

The authors thank CNPq, CAPES, and MEC/REUNI for scholarships and financial support.

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View full text

Phase equilibrium data and thermodynamic modelling of the system (propane + DMF + methanol) at high pressures

Abstract

Introduction

Section snippets

Materials

Phase equilibrium apparatus and experimental procedure

Results and discussion

Conclusions

Acknowledgements

Fuel Process. Technol.

Fuel

Fuel

Fuel

Fuel

J. Supercrit. Fluids

Fluid Phase Equilib.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Supercrit. Fluids

J. Chem. Thermodyn.

Fluid Phase Equilib.

Fluid Phase Equilib.

Chem. Phys. Lett.

Fluid Phase Equilib.

J. Supercrit. Fluids

J. Supercrit. Fluids

Anal. Chem.

J. Chromatogr. A

J. Mol. Catal. A

Fuel