Modelling of extractive heterogeneous-azeotropic distillation in dividing wall column

Modelling of extractive heterogeneous-azeotropic distillation in dividing wall column

Anton A. Kiss, Edwin Zondervan, Richard Lakerveld, Leyla Özkan (Eds.) Proceedings of the 29th European Symposium on Computer Aided Process Engineering...

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Anton A. Kiss, Edwin Zondervan, Richard Lakerveld, Leyla Özkan (Eds.) Proceedings of the 29th European Symposium on Computer Aided Process Engineering June 16th to 19th, 2019, Eindhoven, The Netherlands. © 2019 Elsevier B.V. All rights reserved.

Modelling of extractive heterogeneous-azeotropic distillation in dividing wall column Andras Jozsef Toth,*,a Daniel Fozer,a Tibor Nagy,a Eniko Haaz,a Judit Nagy,b Peter Mizsey,a,c a

Department of Chemical and Environmnetal Process Engineering, MĦegyetem rkp. 3., Budapest, 1111, Hungary b

Department of Building Services and Process Engineering, MĦegyetem rkp. 3., H1111, Budapest, Hungary c

Department of Fine Chemicals and Environmental Technology, Egyetemváros C/1 108., Miskolc, 3515, Hungary [email protected]

Abstract The distillation based separation can be extremely complex if highly non-ideal mixtures are to be separated. In spite of different successfully applied unit operations there is always a possible way to improve the distillation technique and widen its toolbar. A novel improvement in this area is the development of the extractive heterogeneous-azeotropic distillation (EHAD). For the sake of the demonstration of the efficient use of EHAD in Dividing wall columns (DWC), two quaternary mixtures are selected for separation: Water – Ethanol – Ethyl acetate – Acetone and Water – Ethanol – Ethyl acetate – Ethylene glycol. There are real waste mixtures from pharmaceutical industry. It must be mentioned, extractive heterogeneous-azeotropic distillation method has never been investigated in dividing wall columns. Conventional distillation column sequences and dividing wall columns are selected for comparison. Rigorous steady-state simulations are carried out using ChemCAD flowsheet simulation software. Number of trays, heat duties and Total Annual Cost (TAC) of the systems are optimized. It can be concluded, the application of the EHAD in DWCs allows also the simplification of the separation schemes and the separation reduces the energy requirements of the distillation and opens new horizons for the separation of non-ideal mixtures saving energy, money and natural resources. Using DWCs the reboiler duties can be reduced with 20% and the appropriate separation can be reached with one less column. Keywords: extractive heterogeneous-azeotropic distillation, dividing wall column, nonideal mixtures, flowsheet simulator

1. Introduction The highly non-ideal mixtures can be quite often found in fine chemical industries, where the separation of the usually azeotropic mixtures are complicated with the high product purity descriptions. Szanyi et al. (2004a, b) introduced a novel kind of distillation, the extractive heterogeneous-azeotropic distillation (EHAD) that has been proved as a powerful and efficient separation method for the separation of highly non-ideal liquid mixtures. The recovery of solvents from industrial aqueous solutions has particular


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interest and moreover the water can be also recycled (Szabados et al., 2018). The EHAD combines the advantages of the heterogeneous-azeotropic and extractive distillations (see Fig.1). Heteroazeotropic distillation exploits the differences in volatility and liquid-liquid phase split by linking a distillation column and a phase separator. Therefore, it may also be interpreted as hybrid separation process. The heterogeneous-azeotropic option assumes that water is present in the mixture and limited immiscibility exists (Toth et al., 2016).

Fig. 1 The extractive heterogeneous-azeotropic distillation (EHAD) (Toth et al., 2016)

The EHAD differs from the heteroextractive distillation since no new azeotrope is formed, namely the extractive agent/entrainer is water and this component is already present in the mixtures to be separated (Toth et al., 2017). Moreover, the extractive and relative volatility changing effect of the autoentrainer/extractive agent is fully utilized and therefore the extractive effect takes place in the whole column (Szanyi et al., 2004b). Dividing wall columns (DWCs) are todays alternative of sustainable distillation technology. Using DWCs in a distillation sequence significantly increases the number of possible arrangements. These options are worth to be considered as DWCs proved to be attractive in terms of energy consumption. Favourable energy consumption of DWCs is caused by the energy integration technique, thermal coupling (Tarjani et al., 2018). There are numerous feasible partition constructions of a DWC in the literature, but it is a common trend to investigate the properties of the original structure introduced by Asprion and Kaibel (2010) with the partition in the middle. As a systematic approach (Rong, 2011) suggests a strong connection between the conventional sequences and the DWCs with upper (DWCU) and lower (DWCL) partitions (see Fig. 2). DWCU can be considered as an alternative of the conventional direct sequence and DWLC as an alternative of the conventional indirect sequence.

Fig. 2 DWC with Upper partition (DWCU, left) and DWC with Lower partition (DWCL, right) (Tarjani et al., 2018)

Modelling of extractive heterogeneous-azeotropic distillation method in dividing-wall column


The most significant advantage of a DWC is the potential cost saving. Using a DWC can lead to cost savings up to 30% (Sangal et al., 2012) as it only requires one distillation column instead of two ones considering the separation of a three component mixture. On the other hand, thermodynamic efficiency can be increased by avoiding the remixing effect (Kiss et al., 2012). In such a way both investment and operational costs can be reduced compared to the conventional sequences (Tarjani et al., 2018). Recent studies investigate these complex arrangements and the interest in DWCs begins to increase. Several design methods (Ramírez-Corona et al., 2015; Van Duc Long and Lee, 2012) and applications for azeotropic, extractive and reactive distillation are also developed (Yildirim et al., 2011). It has to be mentioned, EHAD has never been examined in DWC. Therefore, the aim of this work is to investigate the applicability of DWC construction in the case of extractive heterogeneous-azeotropic distillation method considering the separation of quaternary mixtures.

2. Material and methods Two quaternary mixtures are selected for comparison: Water (Component 1) – Ethanol (2) – Ethyl acetate (3) – Acetone (4) as Mixture I and Water (1) – Ethanol (2) – Ethyl acetate (3) – Ethylene glycol (4) as Mixture II. Table 1 shows the binary and ternary azeotropes of the selected mixtures. The target of this study to create a separation process based on extractive heterogeneous-azeotropic distillation to split the compounds of these mixtures. The limit value for the composition is determined, which is min. 99.5 m/m%, except Ethanol (96 m/m%) and Ethyl acetate (96 m/m%). Table 1 Binary and ternary azeotropes of the investigated mixtures (Szanyi, 2005) Azeotrope type Binary Ternary

Mixture I

Mixture II

(1) - (2) (1) - (3) (2) - (3) (1) - (2) - (3)

Boil. T [°C] 78 71.3 71.5 70.3

Azeotropic composition [m/m%] 4-96 8.3-91.7 29.6-70.4 8.0-8.7-83.3

ChemCAD flowsheet operator program is applied for comparison of column types. Trayed SCDS columns and UNIQUAC method is used for the calculation of the vaporliquid equilibrium (VLE) as an equilibrium model. The optimal number of trays and heating requirements can be determined (Toth et al., 2017). First, the optimal number of trays are calculated in conventional case and then this value is used in simulation of DWC case.

3. Results and discussion Fig. 3 and Fig. 4 show the separation alternatives of Mixture I. The separation of Mixture II in conventional and in dividing wall columns can be compared in the case of Fig. 5 and Fig. 6. The achieved product purities can be seen in flowsheet figures. First column can separate the non-azeotropic compounds of the mixtures (Acetone or Ethylene glycol) and the EHAD separation is carried out in the second column in conventional column cases. It can be seen DWCL construction is used in non-conventional case. Column II, III and IV symbolize the DWC construction in Fig. 4 and Fig. 6.


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Fig. 3 Separation of Mixture I in conventional column

Fig. 4 Separation of Mixture I in dividing wall column (CII, CIII, CIV)

Fig. 5 Separation of Mixture II in conventional column

Modelling of extractive heterogeneous-azeotropic distillation method in dividing-wall column


Fig. 6 Separation of Mixture II in dividing wall column (CII, CIII, CIV)

Total Annual Cost (TAC) of the optimized process is calculated according to the cost correlations of Douglas (1988) with current M&S index (1605). Investment costs of the distillation depend on different parameters, for e.g.: heat duty, the sizes of the column, and purity of the products. The operating costs contain the annual costs of the steam and water consumption. 8000 hours/year continuous operation is selected for the calculation of the operating cost. 10-year amortization of capital cost is assumed for the total cost estimation. The mixtures are selected for the comparison for 1000 kg/h feed stream. Valve trays are used in flowsheet simulator (Toth et al., 2017). Table 2 shows the comparison of two column constructions. Table 2 Comparison of column constructions

Feed water [kg/h] Total number of trays [-] Reboiler duty [MJ/h] Total Annual Cost (TAC)

Mixture I Conv. DWC 600 70 3343 2740 3206 2565

Mixture II Conv. DWC 800 80 3837 3252 3540 2761

It can be seen the reboiler duty values are in accuracy with TAC and the DWC constructions have lower heat duties.

4. Conclusions The applicability of extractive heterogeneous-azeotropic distillation is tested on two nonideal mixtures in flowsheet simulator environment. Two column constructions are investigated and compared: (i) conventional columns, (ii) dividing wall column. It can be concluded both structures are capable for separation of quaternary mixtures. The application of dividing wall column results in cheaper solutions and less energy consumption then conventional column case, which means that DWC can be efficacious


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tool for separation of highly non-ideal mixtures. Average 17% reduction can be reached in reboiler duty values and the Total Annual Cost can be reduced by up to 21%.

Acknowledgments This paper was supported by the János Bolyai Research Scholarship of Hungarian Academy of Sciences, ÚNKP-18-4-BME-209 New National Excellence Program of Ministry of Human Capacities, NTP-NFTÖ-18-B-0154, OTKA 112699 and 128543. This research was supported by the European Union and the Hungarian State, co-financed by the European Regional Development Fund in the framework of the GINOP-2.3.4-152016-00004 project, aimed to promote the cooperation between the higher education and the industry.

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