Separation of the mixture (isopropyl alcohol + diisopropyl ether + n-propanol): Entrainer selection, interaction exploration and vapour-liquid equilibrium measurements
Introduction
Isopropyl alcohol (IPA) is a widely applied chemical solvent [1] and can be adopted as the raw material for the production of pharmaceutical intermediates [2], [3], [4]. Generally, IPA is produced by hydration of propylene catalysed by solid or liquid acid [5], with the by-products of diisopropyl ether (DIPE) and n-propanol (NPA). DIPE is not only applied as the gasoline additive, but also used a suitable solvent to extract nicotine from tobaccos [6], [7], [8], [9]. NPA can be used in the production of pharmaceutical and pesticide intermediates of n-propyl amine. However, the separation of DIPE and IPA is a challenging task in chemical industry owing to the formation of its azeotropic mixture at 101.3 kPa that cannot be separated by the conventional distillation. Usually, to separate azeotropic mixtures, special distillation technologies are required, such as azeotropic distillation [10], [11], pressure-swing distillation [12], [13] or extractive distillation (ED) [14], [15], [16], [17]. In this study, the extractive distillation technology was used to separate the mixture (IPA + DIPE + NPA).
For the ED separation, the selection of the potential entrainers and the related vapour-liquid equilibrium (VLE) data is of importance. Generally, the entrainers can be classified into four types in the literature [18], such as the conventional solvents [19], salts [20], ionic liquids [21] and hyperbranched polymers [22]. Luo et al. [23] explored the ED with the entrainer of 2-methoxyethanol for separation of IPA + DIPE and the results showed that both IPA and DIPE products were obtained with high purities. Wang et al. [24] reported the separation of the azeotropic mixture (acetonitrile + methanol + benzene) by ED process using chlorobenzene as an entrainer, and the results indicated that high purity products of acetonitrile, methanol and benzene were obtained. Based on the selectivity criteria used in the above work for the selection of the entrainers, in this work, propylene glycol ethyl ether (PGEE) was adopted as an entrainer to separate the mixture (IPA + DIPE + NPA). Until now, the VLE data for the (DIPE + IPA) system at 101.3 kPa were reported by Lladosa et al. [25] and Arce et al. [26]. The binary VLE data for DIPE and IPA at different pressures have been studied by Villamañãn et al. [27] and Chamorro et al. [28]. However, the VLE data for the systems (IPA + PGEE), (DIPE + PGEE) and (NPA + PGEE) have not been reported.
In our work, the isobaric VLE values for the binary mixtures of (IPA + PGEE), (DIPE + PGEE) and (NPA + PGEE) were determined at pressure of 101.3 kPa. The thermodynamic consistency of the VLE data was checked by the Herington [29] and van Ness tests [30]. In the meantime, the NRTL [31], UNIQUAC [32] and Wilson [33] equations were adopted to fit the measured VLE data.
Section snippets
Selectivity
The selectivity (S) is an important parameter for the extractive distillation separation, which can be defined as the ratio of two infinite dilution activity coefficients () and can be defined as Eq. (1).
In this work, S was calculated based on the UNIFAC model at 298.15 K and 101.3 kPa [24]. The selectivity values of three entrainers, viz. cyclohexanol, furfural and PGEE were calculated and are presented in Table 1. From Table 1, the selectivity value of PGEE is higher than those
Chemicals
The chemicals IPA, DIPE, NPA and PGEE were obtained commercially. All the reagents were checked by GC and used directly [38]. The boiling temperatures of the reagents were determined and compared with the literatures data which are presented in Table 2. As shown in Table 2, the boiling temperatures of the chemicals at pressure of 101.3 kPa are close to the reference values. The detailed information for the reagents is listed in Table 2.
Apparatus and procedure
The VLE values for the systems (IPA + PGEE), (DIPE + PGEE)
Experimental results
In this study, the binary systems VLE data for (IPA + PGEE), (DIPE + PGEE), and (NPA + PGEE) were determined at 101.3 kPa. The results for the determined VLE values are reported in Table 4, Table 5, Table 6 and plotted in Figure 4, Figure 5, Figure 6. Meanwhile, the excess Gibbs energy (GE) for above the systems was also calculated and is shown in Table 4, Table 5, Table 6.
VLE calculation
The thermodynamic relationship for the VLE was expressed as follows [53], [54]:
The Poynting
Conclusions
To separate the mixture (IPA + DIPE + NPA) by extractive distillation, PGEE was selected as the extractive agent based on selectivity. Meanwhile, the charge density surface (σ-profiles) and the interaction energies were calculated by COSMO-SAC model and Dmol3, which was used to explore the interaction mechanism between the entrainer PGEE and the components in the mixture. The results showed that the hydrogen bond formed between PGEE and DIPE was stronger than that formed between IPA and PGEE.
Acknowledgement
The authors are grateful for the support of the National Natural Science Foundation of China (No. 21878178), Shandong Provincial Key Research & Development Project (2018GGX107001), A Project of Shandong Province Higher Educational Science and Technology Program (J18KA072), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents.
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