Liquid–liquid equilibrium for the system water + 1,4-dioxane + cyclohexanol over the temperature range of 313.2–343.2 K
Highlights
► We investigated The LLE data for the system water + 1,4-dioxane + cyclohexanol. ► There are no data in the literature for the mixtures discussed in this paper. ► Both NRTL and improved UNIQUAC methods fitted satisfactorily to the experimental data.
Introduction
Cyclohexanol is used as an intermediate to produce adipic acid and caprolactam which are needed for the production of Nylon 6,6 an Nylon 6. Since Nylon is an important bulk chemical and extensively used, cyclohexanol is being produced on a very large scale nowadays.
Conventionally, there are three routes to produce cyclohexanol: the oxidation of cyclohexane, the hydrogenation of phenol, and the direct hydration of cyclohexene. At the present time, being comparatively economical and safe, cyclohexene hydration process catalyzed by solid acid catalysts is widely researched and used. But this process still suffers from the very low reaction rates and the fairly low equilibrium conversion. Since the cyclohexene hydration is a slightly exothermic and equilibrium limited reaction, Steyer et al. conjectured that using the hydration reaction of cyclohexene to form cyclohexanol in a reactive distillation process could solve all the potential problems [1]. Residue curve maps were used to analyze the feasibility and applicability of reactive distillation for cyclohexene hydration system [2]. Even though there are a number of evident advantages of integrating direct hydration reaction into a reactive distillation, such as reducing both investment and operational costs, it still suffers from the high amount and particle size of catalysts. In order to solve the above difficulties, they proposed a novel reactive distillation process for indirect hydration of cyclohexene to cyclohexanol using formic acid as a reactive entrainer [3]. During the reaction process, formic acid which is recycled can be seen as a reactive entrainer. The reaction rates catalyzed by typical acidic ion-exchange resin such as Amberlyst 15 in this two steps are high enough. The residue curve maps were still used to analyze the feasibility of this new reactive distillation process [4]. The simulation study of this new process has proved that it was possible to achieve almost complete conversion of cyclohexene to cyclohexanol with a comparatively low amount of catalysts in a suitable coupled-column system [5].
There are many other ways for solving the difficulties which exist in the hydration of cyclohexene. It is obvious that the low mutual solubility between cyclohexene and water is the main reason for low reaction rate which leads to large amounts of catalysts needed in the direct hydration process. So increasing the solubility of cyclohexene in water will overcome the difficulties of the process integrating the direct hydration reaction into a reactive distillation which proposed by Steyer et al. [1]. In order to increase the solubility of cyclohexene in water, the use of a cosolvent, which will also change the reaction equilibrium and kinetic behavior is required. A new route, using a cosolvent for cyclohexanol production by catalytic hydration of cyclohexene in a reactive column, is put forward in our present study. It is evident that a proper cosolvent should be helpful and effective to increase the solubility of cyclohexene in water and the hydration reaction. Meanwhile, the cosolvent must be inert during the reaction and easily separated from the reaction mixture. In the process of olefin hydration, 1,4-dioxane is a well cosolvent for its stability. And it has been used as a solvent for the synthesis of 2,6-dimethyloct-7-en-2-ol and α-terpineol [6], [7], [8]. So, 1,4-dioxane was chosen as a cosolvent for the cyclohexanol production in our study. To be able to investigate such a reactive distillation process, a complete set of thermodynamic data such as vapor–liquid and liquid–liquid equilibria is necessary. In this work, the liquid–liquid equilibrium data of water + 1,4-dioxane + cyclohexanol over the temperature range of (313.2–343.2) K at 1 atm were measured and a suggested set of NRTL and improved UNIQUAC parameters to describe this system was presented.
Section snippets
Chemicals
The chemicals used in this work are listed in Table 1. All the chemicals were purchased from Sinopharm Group Co. Ltd. The purities of these materials were checked by gas chromatography and they were used without further purification due to high purities. Deionized water was prepared in our laboratory. The densities and refractive indices of all chemicals used in this study are presented in Table 2. The densities were measured using a temperature controlled Anton Paar DMA 4500 density meter with
Experimental data
The LLE compositions for water + 1,4-dioxane + cyclohexanol system at 313.2 K, 323.2 K, 333.2 K, and 343.2 K are listed in Table 3, and the experimental data of the ternary system at 313.2 K and 343.2 K are plotted in Fig. 1. All compositions are expressed as mole fraction.
As can be seen from Fig. 1, the studied ternary system exhibits type-I behavior of LLE. The mutual solubility of cyclohexanol and water increased obviously with the addition of 1,4-dioxane but increased slightly with the temperature
Conclusions
An experimental investigation of liquid–liquid equilibrium behavior of the system composed of water + 1,4-dioxane + cyclohexanol was carried out at different temperatures of 313.2 K, 323.2 K, 333.2 K, and 343.2 K. The reliability of experimentally measured tie-line data was correlated by Othmer–Tobias correlation. The studied ternary system exhibits type-I behavior of LLE. The ternary diagram indicated that the solubility of the organic phase in aqueous phase increased obviously with the addition of
Acknowledgements
The authors would like to thank the National Natural Science Foundation of China (No. 21176049) and Natural Science Foundation of Fujian Province (No. 2011J01038) for the financial support.
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