Elsevier

Fluid Phase Equilibria

Volume 370, 25 May 2014, Pages 34-42
Fluid Phase Equilibria

Vapor–liquid equilibrium for the binary mixtures of dipropylene glycol with aromatic hydrocarbons: Experimental and regression

https://doi.org/10.1016/j.fluid.2014.03.003Get rights and content

Abstract

Vapor–liquid equilibrium data was determined by using a static method for the binary mixtures of dipropylene glycol (4-oxa-2,6-heptanediol) with benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene at temperatures within 293.15 K–481.15 K. The pTx experimental data obtained was regressed with NRTL and UNIQUAC thermodynamic models in order to obtain the binary interaction parameters of the models, specific to each mixture. Furthermore, the Txy diagrams were determined based on these parameters and then compared with the diagrams calculated using the UNIFAC predictive model. We observed differences between the Tx curves calculated with the two models mentioned above and the UNIFAC predictive model.

Introduction

Aromatic hydrocarbon extraction from mixtures is a process with significant importance for industry and for researchers. Throughout the years a large number of solvents for the liquid–liquid extraction were researched and used, such as ethylene glycols, sulfolane, n-methyl-2-pyrrolidone, formyl-morpholine or dimethyl-sulfoxide. Ethylene glycols, have widespread applicability in petrochemical installations. Although the extraction processes are widespread in the industry, there is limited documented data set related to the liquid–liquid and vapor–liquid equilibrium between aliphatic and aromatic hydrocarbons and the solvents mentioned above. During our research process we identified a number of articles on this subject, such as:

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    Vapor–liquid equilibrium and the density measurement for mixtures formed by sulfolane and aromatics hydrocarbons were reported by Wei-Kuan et al. [1], by Rappel et al. [2] and for mixtures of aromatics and NMF vapor–liquid equilibrium and density data was reported by Wei-Kuan [3].

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    Equilibrium data between n-methyl-2-pyrrolidone and aromatics was reported by Al-Zayied et al. [4] and by Gupta et al. [5].

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    Dimethyl sulphoxide and aromatic hydrocarbon systems vapor–liquid equilibrium data was reported by Al-Sahhaf, Kapetanovic [6].

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    For ethylene glycols, limited data about the liquid–liquid and vapor–liquid equilibrium for the mixtures ethylene glycols–hydrocarbons is available, although the ethylene glycols are still used for the extraction of aromatics from gasoline. Vapor–liquid equilibria for mixtures of triethylene glycol and aromatics hydrocarbons are presented by Gupta et al. [5] and by Ng et al. [7].

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    The mixtures of tetraethylene glycols and aromatics data concerning phase equilibria are mentioned by Al-Sahhaf and Kapetanovic [8], and by Yu et al. [9].

The propylene glycols have an increased potential to be used as solvents for the extractions of aromatics from mixtures, due to their similarities in terms of chemical structure and properties with the ethylene glycols. Taking into account this assertion and the results from previous works [10], [11] related to the utilization of propylene glycols as solvents, dipropylene glycol (which is similar to diethylene glycol) could be used successfully and with good results as solvent for aromatics extraction. As we mentioned in a previous paper [12], equilibrium data for the systems formed by dipropylene glycol and inferior aromatics (BTX) or aliphatic hydrocarbons (with six, seven and eight atoms of carbon) was not previously documented. In order to design a new process for the extraction of aromatics hydrocarbons from mixtures, using as solvent dipropylene glycol, it is required to have consistent equilibrium data of mixtures. These data will be used further for the liquid–liquid extraction or/and extractive distillation processes design using computer simulated chemical processes.

In this study, vapor–liquid equilibrium experimental data was measured for binary mixtures formed by dipropylene glycol and inferior aromatics: benzene, toluene, ethylbenzene, o-xylene, m-xylene and p-xylene. Vapor–liquid equilibrium for the binaries mentioned above was determined by measuring the vapor pressure of the mixtures with a static apparatus described in detail in [13]. The pTx experimental data acquired was regressed with the NRTL [14] and UNIQUAC [15] models to obtain the binary interaction parameters of the thermodynamic model, specific to each binary. Parameters obtained on this way could be used further in the liquid–liquid extraction process or extractive distillation process design, using computer simulated chemical processes. Using the binary parameters of the NRTL and UNIQUAC models obtained for each binary and PRO II simulation software [16] were calculated the Txy diagrams and then were plotted and compared with the Txy diagrams for the same binaries calculated with the UNIFAC predictive model.

Section snippets

Materials

The suppliers and the purity levels of the chemical substances utilized in this work are reported in Table 1. Purity was verified through gas chromatography analysis per the ASTM D 6370 method using a Clarus 500 instrument from Perkin Elmer equipped with a split/splitless injector, a flame ionization detector, and using a capillary column coated with methyl silicon in liquid phase. No impurities were detected.

Apparatus and procedures

We used a static apparatus in our laboratory, purpose-built to determine pTx

Experimental results

The experimental method was validated by measuring the vapor pressure of the mixture 1,2-propanediol (1) + dipropylene glycol (2), mixture for which accurate data can be calculated using simulation software [16]. The experimental results (T, x1, and p) are listed in Table 4. Fig. 1 charts the experimental results against the data calculated with simulation software [16] using PRO II 9.2 database. As expected, the two data sets are similar. After the experimental method was validated, we

Binary interaction parameters of NRTL model and VLE calculation

The experimental pTx data was regressed using the PRO II regress module and the NRTL and UNIQUAC thermodynamic models. Eqs. (1), (2), (3), (4), (5) describe the NRTL binary interaction parameters specific for each binary.lnγi=jτjiGjixjikGkixk+jxjGijGkjxkτijkxkτkjGkjGkjxkτij=aij+bijT+cijT2(unit is K)τij=aij+bijRT+cijR2T2(unit is kcal or kj)Gij=exp(αjiτij)αji=αji+βjiT

Eqs. (6), (7), (8), (9), (10), (11), (12), (13) describe the UNIQUAC parameters:lnγi=lnγi(c)+lnγi(r)lnγi(r)=qiln

Conclusion

We obtained experimental vapor liquid equilibrium data using a static apparatus for the following binary mixtures: benzene + dipropylene glycol, toluene + dipropylene glycol, ethylbenzene + dipropylene glycol, o-xylene + dipropylene glycol, m-xylene + dipropylene glycol, and p-xylene + dipropylene glycol.

We applied regression analysis for this data using the NRTL and UNIQUAC thermodynamic models to obtain the binary interaction parameters, specific for each binary. Furthermore, we used the resulting

Acknowledgements

The authors would like to thank Mr. Alexandru Pană, Head of Quality Control Department, S. C. Oltchim S. A. Râmnicu Vâlcea, România, for assistance and fruitful discussions, S. C. Oltchim S. A. Râmnicu Vâlcea, România, for assistance and Dow Chemical, Germany, for materials.

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