Screening solvents to extract phenol from aqueous solutions by the COSMO-SAC model and extraction process simulation
Introduction
Unreasonable utilization of the coal not only pollutes the environment, but also wastes a large amount of resources. Coal gasification, a high effective and clean technology, is playing a significant role in protecting the environment and supplying energy for the modern society [1]. However, a large amount of highly concentrated phenolic effluents were generated by the Lurgi pressurized coal gasification process [2], [3], which would cause serious damage to human beings and the biosphere. Liquid-liquid extraction, a low energy cost, high throughput, versatile and commercially efficient unit operation, has been reported to treat industrial phenolic effluents in USA [4], China [4], [5] and South Africa [4], etc. A key step in designing a liquid-liquid extraction process is to develop a suitable extractant. Screening the extractant for the extraction process is quite time-consuming, for which computational screening could save a lot of time. Group contribution methods such as UNIFAC [6] are successful computational screening methods to predict thermodynamics properties such as activity coefficient, distribution coefficients and phase diagram [7], [8], etc. However, the accuracy of thermodynamic predictions by UNIFAC depends heavily on the UNIFAC group interaction parameters and the data that were initially used for their fitting. Unfortunately, such data, determined by regressing huge amount of liquid-liquid phase equilibrium data, are often missing. Although the predictions are often close to the experiment results, the accuracy is also determined by the similarity of the environments and interactions between these functional groups to the database used in its parameterization [9]. UNIFAC also shows low accuracies for isomers and compounds with nonalkyl functional groups [10]. Molecular simulation methods such as molecular dynamics (MD) and Monte Carlo (MC) also have been reported to calculate thermodynamic properties [11], [12]. The calculated results were affected by the selection of simulation ensemble and the used empirical force fields describing interactions between atoms. Moreover, molecular simulations are usually quite expensive and time-consuming when flexible or large molecules are involved.
Recently, a novel and efficient method was proposed to predict liquid-liquid phase thermodynamic properties by Klamt's group [13], [14]. In contrast to UNIFAC and other excess Gibbs free energy approaches, Klamt calculated the surface charge densities by the conductor-like screening model (COSMO) to describe molecular interactions and developed a conductor-like screening model for real solvent (COSMO-RS) that can be used to calculate the chemical potential of any substance in any mixture from quantum mechanical calculations [10], [11]. Based on COSMO-RS, Lin and Sandler [10] proposed a new model, known as COSMO segment activity coefficient (COSMO-SAC), to determine the activity coefficient and to overcome the limitations of COSMO-RS, e.g. does not correctly converge to certain boundary conditions and the final expression for the activity coefficient fails to satisfy thermodynamic consistency relations [10]. In the last decade, COSMO-SAC has been widely applied in liquid-liquid equilibrium prediction and solvent screening. Mateusz [15] determined the solubility curve between acetonitrile and six C8 aliphatic ethers, and then characterized the influence of sigma-profile of different ethers on their properties by using COSMO-SAC. Hsieh [16] predicted 1-octanol-water partition coefficient and infinite dilution activity coefficient in water for alcohols and amines. Mitesh [17] studied the liquid-liquid equilibrium of ionic liquid extracting biodiesel and bio-alcohols by COSMO-SAC, and results agreed with the experimental data very well. It has been proven as an excellent combination approach to screen solvents and design an extraction process with the COSMO-SAC model and then validate the calculation results by experiments.
In this work, COSMO-SAC model was used to screen solvents to extract phenol (from aqueous solution) from common organic solvents, including: alkanes, arenes, alcohols, ethers, esters and ketones. The selectivity and distribution coefficients of these solvents were calculated from the activity coefficient at infinite dilution by analyzing the sigma-profiles. Then, liquid-liquid equilibrium experiments and extraction process simulation were performed to further study the most promising solvents predicted by the COSMO-SAC calculation.
Section snippets
The COSMO-SAC model
COSMO-SAC was first proposed by Lin in 2002, and then an improvement on the definition of hydrogen bonding was developed by Wang [18] in 2007. Later Hsieh [19] proposed a refinement COSMO-SAC (2010) which consider the electrostatic interaction parameter as a temperature-dependent parameter. In 2013, Xiong [9] made a refinement on calculating activity coefficients by including the dispersive interaction contribution. There are two steps to determine the thermodynamic activity coefficients by
Prediction analysis
The activity coefficients at infinite dilution of phenol and water in all the discussed solvents were calculated by COSMO-SAC (2007) model at 298.2 K and 333.2 K and are listed in Table 1. A promising solvent should have a low activity coefficient of phenol and a high activity coefficient of water. Seen from this table, all the alkanes and arenes studied in this work show quite higher activity coefficient of phenol than ketones, ethers, esters and alcohols. Higher activity coefficient indicates
Materials and experimental procedure
The chemicals used in this work are listed in Table 3. They were of high purity (>99 wt.%) and were all used without any further purification. The purity of all the used reagents were verified by using a gas chromatography. Deionized water was used through this work.
The tie lines measurement for the ternary systems, MIPK + phenol + water, was carried out at 333.2 and 343.15 K, MTBK + phenol + water and mesityl oxide + phenol + water, were carried out at 333.2 and 353.15 K, under 101 kPa.
Tie-line data
The experimental tie-line data of the systems, MIPK + phenol + water at 333.2 K and 343.2 K, MTBK + phenol + water and mesityl oxide + phenol + water at 333.2 K and 353.2 K, under 101 kPa, were listed in Table 4, Table 5, Table 6, and shown as ternary diagram in Fig. 4, Fig. 5, Fig. 6. Seen from those figures, the slopes of all the tie lines are positive, indicating that the distribution coefficients of all the three extractants to phenol are larger than 1. The distribution coefficient (D) and
Conclusion
In this work, COSMO-SAC model was used to screen solvents for extracting phenol from water. Activity coefficient at infinite dilution calculated by COSMO-SAC and the sigma-profiles of solvents was used to analyze the interaction of solvents with phenol. The theoretical studies revealed that, ketones are quite promising to extract phenol from water according to the predicted selectivity and solvent power. MIPK, MTBK and mesityl oxide were then chosen for further experimental liquid-liquid
Acknowledgements
Financial support from the National Science Foundation of China (21506066), State Key Laboratory of Pulp and Paper Engineering (201708), the Guangdong Science Foundation (2014A030310260), the Guangzhou Technology Project (20181002SF0525), and the Fundamental Research Funds for the Central Universities SCUT (2017ZD069) are gratefully acknowledged.
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