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Screening monoethanolamine as solvent to extract phenols from alkane Shoutao Ma, Qiang Yu, Yafei Hou, Jinfang Li, Yuan Li, Zhanhua Ma, and Lanyi Sun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02486 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017
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Screening monoethanolamine as solvent to extract phenols from alkane Shoutao Maa, Qiang Yub, Yafei Houa, Jinfang Lia, Yuan Lia, Zhanhua Maa, Lanyi Sun*a (a State Key Laboratory of Heavy Oil Processing, College of chemical engineering, China University of Petroleum (East China), Qingdao Shandong 266580, China b Do-Fluoride Chemicals Co., Ltd. Jiaozuo Henan 454150, China) ABSTRACT
Coal tar is a byproduct of low temperature coal carbonization. The separation of the compounds has a great significance since its main component is the mixture of phenols and hydrocarbons. In this paper, the separation of specific phenolic compounds from model coal tar (phenols + hexane) was studied. The solvent was screened by empirical analysis, universal quasichemical functional group activity coefficient (UNIFAC) and conductor-like screening model COSMO-SAC (segment activity coefficient) model. The COSMO-SAC was used to calculate the capacity, selectivity and performance index of solvents. Finally, the monoethanolamine (MEA) was selected as the solvent to extract the phenols. The liquid-liquid equilibrium for the ternary mixture of phenols + hexane + MEA was measured at 303.15K and 323.15K under atmospheric pressure, and the results showed that MEA provided high distribution coefficient, efficiency and selectivity for phenols. Meanwhile, the extraction process of phenols was simulated based on Nonrandom Two-Liquid (NRTL) model which binary interaction parameters were obtained through calculations of phase equilibrium. The results showed that the purity of phenols can achieve more than 99.5 wt% using MEA as solvent. Keywords: Phenols, MEA, Liquid-Liquid Equilibrium, NRTL, Simulation 1 INTRODUCTION Coal tar, as a byproduct of coal coking, is an important chemical as a result of containing many useful and valuable components (phenols) in the coal industry.
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Phenols are high value-added chemicals, which are used as the raw materials for the production of bisphenola, phenolic resin, engineering plastics and synthetic fibers, and they have considerable market demand and application value.1,2 Since phenols in coal tar have up to 20%-30%3,4, effective separation for phenols is beneficial to improve the economy of coal processing industry. Firstly, it can reduce the hydrogen consumption in the production of fuel oil during the hydro-treating process, which significantly reduces the cost of production. Secondly, the instability of phenols is not conducive to the storage and transportation of oil, and the separation of phenols from coal tar or liquefied petroleum oil will help improving the stability of product transport and storage process. Therefore, it is significant to separate phenols from coal tar considering high value-added products of them and environmental protection. Some low boiling compounds, and even some high boiling compounds, including BTX，carbolic oil, naphthalene oil, wash oil, anthracite oil and coal-tar pitch, can be separated from coal tar by distillation process.5 However, the energy consumption is high due to the intermediate phase transition process of distillation. In addition, the normal distillation method is not appropriate for some close-boiling or azeotropic mixtures, and it must adopt the special distillation methods, such as extractive distillation or azeotropic distillation.6,7 Therefore, it is difficult and energy-consume to separate phenol from coal tar, and needs to adopt the special distillation process. It is imperative to find a kind of low-energy separation method to obtain phenols from coal tar. For example, the extraction of phenol and phenol derivative can be carried out from coal tar with appropriate solvent. The extraction process is based on the solubility of phenols with alkanes in different solvents, and it is widely adopted because of its absence of phase transition. An optimized planning and the realization of extraction plants in the chemical industry require comprehensively qualitative and quantitative phase equilibrium data.8,9 However, due to the relatively complex composition of coal tar, it is impossible to obtain liquid-liquid equilibrium data for each compound. Therefore, a simple design method was adopted, as referred to that in Catherine A. Peter's report.10 Hexane and phenols were used as mainly representative components in low
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temperature coal tar. According to Zhang's report8, phenol, o-cresol, m-cresol, and p-cresol were the main compounds in coal tar. In order to obtain suitable solvent to extract the phenols from coal tar, the solvents were firstly screened by empirical method11-15 according to the physical properties of hexane and phenolic compounds. Then, the phase diagram of the ternary system of solvent-phenols-hexane was predicted based on UNIFAC model by Aspen Plus. Finally, COSMO-SAC16-18 model was adopted to calculate the activity coefficient, and the liquid-liquid phase equilibrium (LLE) experiment was carried out to select the solvent. In addition, the distribution coefficient and the separation factor were calculated according to the LLE data and used as the criterion to evaluate the separation efficiency. The experimental data were verified by Othmer-Tobias equation19 and correlated with NRTL activity coefficient model. The binary interaction parameters were required to implement process simulation in Aspen Plus, and the sensitivity analysis of extraction process had been performed in order to determine the main design variables, which provided an important basis for the separation of phenols in coal tar. 2 SOLVENT SCREENING The selection of a suitable solvent is a critical step in the efficient liquid-liquid extraction process. At present, there are two methods of screening solvent for extraction process. One is the empirical analysis11-15 which is based on the physical properties of the extracted material, and it screens out the feasible solvents based on the experience. The other is the computer-aided molecular design method.16-18, 20-22 In recent years, many researchers have begun to use the later as a filter to select solvent for extraction process. In this paper, the solvents were screened combining the empirical analysis and computer-aided design method. 2.1 Solvent Screening Based on the Physical Properties The phenols have the characteristics of benzene ring structure, weak acidity and polarity. Meng et al.23 has elaborated the principle of the solvent extraction process of phenols, and pointed out the relation of the molecular structure of different extraction solvents. The reason that the solvents exhibit selectively extractive capacity for
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phenolic compounds was the molecule of solvents generally forming certain hydrogen-bonded with phenols, such as O-H…N, O-H…O-H and O-H…Cl. So the solvents were initially screened according to the solvent manual and their physical properties were shown in Table S1 of the supporting information. 2.2 Solvent Screening Based on UNIFAC Model According to the solvents which were selected in the section 2.1, the group contribution method (UNIFAC) was adopted for further screening of primary solvents. The UNIFAC model was based on the principle of addictive properties, the sum of the properties of a molecule were equal to the contribution of the various structural elements that make up the molecule.24 And these structural elements were maintained at the same value in different molecules. Thus, two essential points were presumed in UNIFAC model, and the physical properties of the mixture or pure substance were equal to the sum of the contribution of the various groups that make up the mixture or pure substance, and the contribution of the group was same for physical properties. The UNIFAC was used to predict the ternary phase diagrams by Aspen Plus, and the diagrams of hexane-phenols-solvents were shown in Fig. 1 (phenol was selected as the example due to the similar nature of phenols), and the others were shown in Figs. S1-S3 of the supporting information. According to Fig. 1, the area of the two-phase region formed by ethanol, isopropanol, acetamide, butyl acetate, diethanolamine, triethanolamine and phenols was small, and it was not suitable as the extraction solvent of the phenolic compounds. While, ethylene carbonate, 1,2-propylene glycol, triethylene glycol (TEG), and MEA could form big two-phase area with phenolic compounds, which were considered as the appropriate solvents used to extract phenols from hexane.
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k Fig. 1 Phase diagram of ternary mixtures, a: Hexane - Phenol - 1,2-Propanediol, b: Hexane Phenol - Butyl acetate, c: Hexane - Phenol - Diethanolmine, d: Hexane - Phenol - Glycerol, e: Hexane - Phenol - TEG, f: Hexane - Phenol - Triethanolamine, g: Hexane – Phenol - Ethylene carbonate, h: Hexane - Phenol - MEA, i: Hexane – Phenol - Ethanol, j: Hexane – Phenol Acetamide, k: Hexane - Phenol- Isopropanol.
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2.3 Solvents Screening Based on COSMO-SAC Model In the past two decades, a new class of predictive methods had emerged that utilized the results from modern computational chemistry. For example, the COSMO-based methods, such as the COSMO-RS25-29, and its variations COSMO-SAC16-18 and COSMO-RS(Ol)30,31, determine the liquid phase no-ideality using the molecular interactions derived from first-principles solvation calculations.32 COSMO-SAC, which was the improvement of COSMO-RS for vapor–liquid and liquid–liquid equilibrium predictions, was a quantum chemistry model based on statistical thermodynamics for the prediction of thermodynamic properties of fluids. There were generally two steps in the COSMO-SAC prediction procedure33, namely (1) the quantum chemical COSMO computation for the molecular species involved, and (2) the COSMO-SAC for statistical thermodynamic treatment, and a standard COSMO-SAC prediction only requires the screening charge density information of the interested compounds. In this work, the Dmol3 module in Material Studio (MS) was adopted to the COSMO calculation and all the equations were shown in Table S2 of the supporting information. In the COSMO calculation, the surface of a molecule is dissected to small segments and the screening charges are determined for each segment such that the net potential everywhere at the surface is zero (perfect screening). The main steps of calculation are as follows: the structure was drawn in the MS and optimized by density functional theory (DFT)16,34, and then the optimal structures under ideal gas condition and COSMO files were obtained. The optimal structures were shown in Fig. S4 and the σ-profiles were shown in Fig. 2. Fig. 2 showed that the main peak of hexane and 1,2-propanediol was in the -0.0082 e/Å2
