Elsevier

Fluid Phase Equilibria

Volume 527, 1 January 2021, 112827
Fluid Phase Equilibria

Isothermal Vapour-Liquid Equilibrium Measurements for the butan-1-ol + butane-1,4-diol/butane-2,3-diol system within 353.2–388.2 K

https://doi.org/10.1016/j.fluid.2020.112827Get rights and content

Abstract

In this study, the isothermal binary vapour-liquid equilibrium (VLE) data for the butan-1-ol + butane-1,4-diol/butane-2,3-diol system was measured and modelled, for use in the design and rating of separation processes. P-T-x-y equilibrium measurements were performed at four temperatures from approximately 353–388 K. A dynamic-analytical apparatus was utilized to perform the measurements at sub-atmospheric conditions. The measured data was modelled by employing the γ-Φ approach. The liquid-phase correction was accounted for using the Non-Random Two-Liquid and Universal Quasi-Chemical activity coefficient models while the Hayden and O'Connell correlation for the virial equation of state was used to account for the vapour-phase correction. Thermodynamic consistency tests were performed using the point and area tests for the measured experimental data and the data sets passed both tests.

Introduction

The study of renewable biofuels and their development from sucrose crop sources has recently become a key area of research focus. Such biofuels can be used in their pure state without further processing with a few engine modifications or alternatively, can be blended with other fuels and used as “drop-in” fuels [1], [2], [3], [4], [5]. Biofuels such as butane-1,4-diol and butane-2,3-diol have been identified as viable due to their high octane-numbers [2,6]. Butane-1,4-diol has a global market approaching two million tons per year and is also used in the manufacture of automotive plastics, electronics and textiles [7]. Butane-2,3-diol is used as an intermediate in the production of butan-2-one and has a projected global market of approximately 32 million tons per year [4].

A bioconversion process utilizing microorganisms such as Escherichia coli, can produce butane-1,4-diol in a single conversion step [1,6,7] while butane-2,3-diol can be produced by the fermentation of biomass using microorganisms such Bacillus licheniformis[8,9]. These processes produce butane-1,4-diol and butane-2,3-diol in an aqueous solution with smaller concentrations of the desired components. Before the butanediols can be used for blending purposes or as feedstock in other processes, excess water must be removed from the diol products. The dehydration step is regarded as an energy intensive step [4].

While conventional distillation is a suitable separation technique to perform the dehydration step, the process demands a high energy input in the form of high-pressure steam due to the boiling points of the butanediol constituents exceeding 450 K[8,10]. Separation processes such as pervaporation [10,11], reactive distillation [12], liquid-liquid extraction [13] and salting-out extraction [14] have been identified as alternate techniques for the concentration of the required butanediol products.

The literature suggests that the separation process with the greatest techno-economical merit involves the removal of water using a solvent extraction and recovery step by conventional distillation and then concentrating the butanediol products by the removal of trace amounts of water [4,6,8,9,15]. Due to its techno-economic feasibility and relatively low environmental impact, butan-1-ol has been identified as an optimum solvent to achieve the required separation of the butanediol components [4,[16], [17], [18], [19]].

The recovery of the aqueous-extraction solvent for re-use still remains an energy intensive section in the separation process and several processes for this recovery have been proposed in the literature [4,6,8,9,15]. However those process designs that are proposed in the literature were performed with incomplete isothermal vapour-liquid phase equilibrium data for the butan-1-ol and butanediol systems, as the vapour-liquid equilibrium data describing these systems at the optimum process conditions have not been previously measured. In the design of high purity separation processes, experimental isothermal vapour-liquid equilibrium is preferred to perform energy balances across trays by the heat of mixing of the components involved in the separation process.

It is necessary to develop thermodynamic models to accurately perform the design of separation processes. To ensure that correct binary interaction parameters are utilized in thermodynamic models, such as the Non-Random Two-Liquid (NRTL) and Universal Quasi-Chemical (UNIQUAC) activity coefficient models, novel isothermal P-x-y phase equilibria data have been measured for the butan-1-ol + butane-1,4-diol and butan-1-ol + butane-2,3-diol systems at 353.2, 363.2, 373.2 and 388.2 K using a dynamic vapour-liquid equilibrium apparatus for vacuum measurements designed by Raal and Mühlbauer [20]. The experimental VLE data was modelled by employing the combined γ-Φ approach. Thermodynamic consistency tests, such as the area and point test were performed on the measured experimental VLE data.

Section snippets

Modelling Approach

The γ-Φ approach is a commonly used method for the modelling of experimental VLE data at moderate- and low-pressure. The γ-Φ approach has been reviewed extensively by Raal and Mühlbauer [20]. At low pressures, it can be assumed that an ideal solution reference state is applicable to the liquid-phase hence, an activity coefficient model is used to account for the non-ideality present in the liquid-phase. The non-ideality of the vapour-phase is accounted for by the fugacity coefficient in

Materials

In this work, butan-1-ol, butane-1,4-diol and butane-2,3-diol were used to conduct the VLE experimental data measurements and were sourced from Sigma-Aldrich with the supplier mass purities stated as >0.99 mass fraction. To dehydrate the chemicals of trace amounts of water, the components were initially treated with molecular sieve (3Å KnNa12-n[(AlO2)12(SiO2)12]) for 36 hours before use. To validate the purity of the pure component species and the water content, measurements were performed

Results and Discussion

The vapour pressures of the pure butan-1-ol and butane-2,3-diol were determined by the dynamic method. Due to the limitations of the apparatus used, the vapour pressures for pure butane-1,4-diol could not be measured in this work. The measured vapour pressures were compared against vapour pressures determined using parameters found in the literature. Antoine and Wagner parameters are reported in the work of Poling et al. [27] and NIST ThermoData Engine via ASPEN Plus ® V10 [28] respectively,

Conclusion

The vapour-liquid equilibrium phase behaviours of the butan-1-ol + butane-1,4-diol and butan-1-ol + butane-2,3-diol systems were successfully measured using a low-pressure dynamic apparatus. To confirm the accuracy and validity of the experimental VLE data, thermodynamic consistency tests were conducted which showed the data to be consistent. The VLE behaviour of the systems in this work was found to be non-ideal which is attributed to the differences in molecule sizes as well as the

CRediT authorship contribution statement

Shivan Mavalal: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft. Kuveneshan Moodley: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work is based upon research supported by the JW Nelson fund awarded by the University of KwaZulu-Natal.

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