Solubility behavior of γ-valerolactone + n-tetradecane or diesel mixtures at different temperatures
Introduction
As global population grew over the past decades, demand for petroleum, which is widely employed as feedstock for carbon based chemicals and liquid fuels, has also increased. Unfortunately, oil prices can be severely affected by unexpected political changes since most of the large reserves are geographically concentrated [1]. In this context, biomass has emerged as a viable alternative for fossil fuels. Due to its widespread availability and remarkably diverse product portfolio, biomass has become a subject of general and current interest [[2], [3], [4], [5]]. Among the broad range of chemicals which can be obtained from it, γ-valerolactone (hereafter referred to as GVL) stands out as a potential renewable platform chemical. This compound is most effectively produced via selective hydrogenation of biomass-derived levulinic acid, which generates a mixture of GVL, water, formic acid (used as hydrogen source), and solvent (e.g., methanol, ethanol, and n-propanol) [6,7].
Since the pioneering work which classified GVL as a sustainable liquid [8], researchers endeavored to explore uses for this molecule. The lactone presents lower vapor pressure than its linear oxygenate counterparts [9] and modestly higher energy content than ethanol [8]. Moreover, aqueous GVL mixtures do not form azeotropes, which renders its separation less energy-intensive compared to the ethanol purification process [8,10]. When tested as diesel additive, GVL was proved to decrease exhaust concentration of carbon monoxide, unburned fuel, and smoke, despite having modest effect on energy performance and NOx emission [11]. On the other hand, the molecule exhibited poor solubility in diesel fuel and, for this reason, further experiments should be perfomed to investigate their miscibility threshold. GVL may be further upgraded to other liquid fuels, such as methyl-tetrahydrofuran (MTHF) [12] and liquid alkenes [13,14]. While the latter can be directly used as transportation fuel, GVL and MTHF must be blended to another fuel since their energy density is lower than required [15]. This lactone also stands out as an organic solvent within the context of green chemistry, since it exhibits low vapor pressure, low toxicity, strong and pleasant scent, and high stability [8,16]. GVL-rich solvents have been successfully used to process lignocellulosic biomass [17,18] and are alternatives to enhance cost competitiveness of renewable chemicals [19]. Moreover, GVL can be used as a building block for polymers, and as a food flavoring agent [8,18]. Evidently, there has been growing interest in the synthesis and possible uses of GVL in recent years, as indicates the comprehensive review written by Zhang in 2016 [5].
Design and optimization of processes strongly rely on phase equilibrium data, which are yet scarce for systems containing GVL. Vapor-liquid equilibria (VLE) experiments were performed to investigate GVL + water, GVL + formic acid, and GVL + alcohols systems, mixtures which are found in the lactone production process [7,10,20]. Since GVL has shown appreciable performance as fossil fuel additive, liquid-liquid equilibrium (LLE) of mixtures containing C6 to C12 hydrocarbons and GVL have been in the spotlight recently [21,22]. However, LLE of systems composed of heavier hydrocarbons, which might represent diesel fuel [23], have not been assessed yet. Moreover, quantitative data on the solubility of GVL in commercial diesel itself is not reported in the literature, to the best of our knowledge. In this context, this work aimed to determine binary LLE of GVL + n-tetradecane and GVL solubility in diesel at different temperatures. Phase diagrams were obtained by cloud point detection using the constant-temperature method [24]. The same technique was used to investigate GVL solubility in the fuel. Both the NRTL [25] and UNIQUAC [26] activity coefficient models were used to correlate the measured data. Liquid-liquid flash calculations were perfomed using these models in an attempt to predict the upper critical solution temperature (UCST) [27,28]. Finally, some limitations of widely used group contribution methods [[29], [30], [31]] are explored.
Section snippets
Materials and methods
Chemicals. Characterization of high purity chemicals used in this work are presented in Table 1. Both density and refractive index of chemicals were measured at , using a Mettler Toledo DM45 DeltaRange™ density meter and a Mettler Toledo RM40 refractometer, respectively, and results were compared to previously reported values (Table 1). Water content was determined using a coulometric Karl-Fischer titrator (Metrohm 831). Commercial diesel fuel (B S-10 grade, CAS 68334-30-5) was bought
Results and discussion
Mutual solubility of GVL and n-tetradecane is noticeably low (Table 2), and it does not vary in a considerable extent within the investigated temperature range. Also, the water content in the mixture (Table 1) had a negligible effect in the results (see Appendix A); therefore, water was not considered during calculations. As expected, mutual solubility of mixtures containing acyclic hydrocarbon + GVL tends to decrease as the former's chain is increased (Fig. 2). Moreover, unsaturated and cyclic
Conclusion
Liquid-liquid data equilibrium in the system composed of γ-valerolactone and n-tetradecane were successfully obtained. Results agree with solubility tendency of binary mixtures containing the former and other hydrocarbons. Since GVL is slightly soluble in long-chain, acyclic hydrocarbons, its use as diesel additive might be limited. However, the molecule is still a promising bio-additive for gasoline, in which its solubility is considerably higher. Since γ-valerolactone may react with water at
Acknowledgments
The authors gratefully acknowledge FAPESP - São Paulo Research Foundation (process numbers: 2016/25734-5, 2016/18253-0, and 2014/21252-0), CNPq - National Council for Scientific and Technological Development (process numbers: 302146/2016-4, 454557/2014-0, and 304046/2016-7), and FAEPEX (process number: 2407/16) for financial support and individual grants.
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