Separation of ternary mixtures by extractive distillation with 1,2-ethandiol and glycerol
Introduction
One of the main concerns, related to major health and environmental issues in the chemical, pharmaceutical and special industry, is solvent waste (Kim and Smith, 2004, Jimenez-Gonzalez et al., 2005), which frequently contains different compounds, e.g. alcohols, ethers, esters, and water.
Separation of such mixtures is complicated by azeotrope phenomenon, which makes conventional processes based on vapor–liquid equilibria (VLE) not feasible. Although interesting membrane techniques not governed by VLE are investigated in literature (Brüschke and Tusel, 1986), they do not allow for separation of large amounts of solvents, and currently present additional technological features which cause regeneration difficulties. Also, in recent years, ionic liquids (Han and Row, 2010, Marciniak, 2010, Lei et al., 2014) have been introduced in the separation field, which, due to their remarkable properties, can be considered as very effective non-volatile entrainers, but in large scale chemistry are not yet widely applied, mostly due to financial and economic reasons. However, it is shown that in some cases high boiling entrainers are of more efficiency than ionic liquids (Navarrete-Contrerasa et al., 2014)
Another interesting solution based on VLE, is represented by extractive distillation process. The process implements adding entrainer, which, combined into the interactions with components of original mixture, selectively changes relative volatility of separated compounds. Correctly chosen entrainer helps to minimize energy consumptions and negative environmental impact on separation processes (Gomez and Gil, 2009).
In this paper we investigate and discuss the separation aspects of three ternary industrial systems, namely tetrahydrofuran–methanol–water, ethyl acetate–ethanol–water and acetonitrile–methanol–water at 101.3 kPa, which contain more than one azeotrope and correspond to diagrams 2.0-2b and 3.1-2 in Serafimov's classification (Kiva et al., 2003) (Fig. 1).
Though binary azeotropes compositions are sensitive to changes in pressure, pressure-swing distillation is not preferable for all compositions of ternary mixtures due to high-energy consumption.
The feasibility of ED process for separation is investigated in this paper. It is well known, that entrainer selection influences a lot on the energy consumptions, so our research work is directed to the establishing connections between physico-chemical characteristics of system in entrainer presence and selective properties of entrainer, which straightforwardly influence the energy consumptions of ED column. Its application is illustrated by separation of three industrial ternary mixtures mentioned above. This approach is suggested for further application for potential entrainer efficiency evaluation when separating systems containing several azeotropes.
Section snippets
Thermodynamic criterion to select entrainers
Excess Gibbs energy is a quantitative and qualitative characteristic of intermolecular interactions in binary and multicomponent solutions. To choose agents for extractive distillation of binary azeotropic mixtures, thermodynamic criterion was proposed, establishing a relationship between excess Gibbs energy (gE) of systems i – entrainer, j – entrainer (i, j – azeotrope forming components) and the relative volatility of substances to be separated. Entrainer efficiency is evident when difference
ED modeling
Conceptual ED flowsheets of tetrahydrofuran–methanol–water and ethyl acetate–ethanol–water using 1,2-ethandiol are presented in Fig. 5, Fig. 6. Conceptual flowsheet for acetonitrile–methanol–water separation with glycerol is equivalent to tetrahydrofuran–methanol–water flowsheet and therefore is not necessary to be presented here. In the case of ethyl acetate–ethanol–water system ED process is performed in two columns.
Calculations of distillation and vapor–liquid equilibria were performed via
Conclusions
The thermodynamic criterion for entrainers selection has been extended to ternary aqueous mixtures of non-electrolyte containing binary and ternary azeotropes. The criterion needs to be applied to all binary azeotropic constituents of ternary system. Separating agent should increase relative volatility of component forming two binary azeotropes in ternary system, in our case those are THF, ACN and EtAc (THF–methanol–water, ACN–methanol–water and EtAc–ethanol–water correspondingly). This
Acknowledgement
The reported study was partially supported by RFBR, research project No. 14-03-00523.
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