European Symposium on Computer Aided Process Engineering - 13 A. Kraslawski and I. Turunen (Editors) © 2003 Elsevier Science B.V. All rights reserved.
Batch Extractive Distillation with Intermediate Boiling Entrainer Z. Lelkes, E. Rev, C. Steger, V. Varga, Z. Fonyo, L. Horvath* Chemical Engineering Department, Budapest University of Technology and Economics, H-1521 Budapest, Hungary *Res. Lab. Mat. & Env. Sci., Chem. Res. Center of HAS, H-1525 Budapest, Hungary
Abstract Feasibility of batch extractive distillation in rectifying column with middle boiling entrainer is studied. Separation of methyl acetate and cyclohexane (forming minimum boiling azeotrope) using carbon tetrachloride, and separation of chloroform and ethyl acetate (forming maximum boiling azeotrope) using 2-chlorobutane are theoretically studied based on profile maps and rigorous simulation. Non-extractive distillation with pre-mixing the entrainer to the charge is also studied in both cases. Feasibility of the processes are demonstrated with laboratory-scaled experiments. Versions of BED according to the types of the azeotrope and of the entrainer are compared; and the decisive properties are pointed out.
1. Introduction Extractive distillation is an efficient separation method for non-ideal and azeotropic mixtures; see amongst others Widagdo and Seider (1996), Knapp and Doherty (1994), Wahnschafft and Westerberg (1993). For realisation of the extractive distillation in batch (BED), the simplest configuration is the rectifier with a continuous entrainer feeding into the column, as shown in Fig 1. More sophisticated configurations such as a middle vessel column (e.g. Safrit et al. (1997), Warter and Stichlmair (1999)) could be also used; but so far BED experimental results have been published mainly for the rectifier (Yatim et al., 1993; Lang et al., 1994; Lelkes et al., 1998b; and Milani, 1999). According to Laroche et al. (1991), the entrainer for homoazeotropic distillation can be the lightest, the heaviest, or even the intermediate boiling component in the system. Lang, Lelkes, and co-workers (1998, 1999) analysed feasibility of separating minimumboiling azeotrope applying BED with light and heavy entrainers and maximum-boiling azeotrope applying BED with heavy entrainer. Another possibility for separating mixtures applying entrainers is the so-called solvent enhanced batch distillation (SBD). In the case of SBD the entrainer is not fed continuously into the column; rather it is pre-mixed to the mixture at the beginning of the process, see e. g. Bernot et al. (1991), Rodriguez-Donis et al. (2001a). Even heteroazeotropic batch distillation (Rodriguez-Donis et al (2001b) could also be used. Our test mixtures, selected according to didactic viewpoints, are methyl acetate and cyclohexane with carbon tetrachloride for minimum boiling azeotrope, and chloroform and ethyl acetate with 2-chlorobutane for maximum boiling azeotrope. Operation steps (sequencing), limiting flows, and limiting stage numbers are determined by feasibility study, based on profile maps; the design is validated by rigorous simulation.
198 Comparison of SBD and BED using intermediate entrainer, according to effectiveness, and the use of light intermediate and heavy entrainer are presented. Cyclohexane (B) 1 1
X (rectificatiot section)
0.6 j 0.4 j
Q \ L^J^^J^v VT>^^^\
0 ! 0
Figure 1 Sections of BED.
SN W 0.4
F/V=0.5 R= infinite XD=[0.9; 0.05; 0.05]
1 0.8 1 Methyl acetate (A)
Figure 2 Extractive and rectification profiles at R = oo, F/V = 0.5.
2. Minimum Boiling Azeotrope with Intermediate Boiling Entrainer 2.1. Feasibility of SBD Determination of the separation sequence applying SBD is based on the residue curve map. According to Bernot et al. (1991), the separation with SBD is feasible in a stripper only. In a rectifier the first product is the azeotropic mixture, regardless to the initial position of the still composition. Thus, separation of minimum boiling azeotropes with SBD in a rectifier is infeasible. 2.2. Feasibility of BED Using continuous entrainer feeding into the column the separation becomes feasible (Rev et al., 2003). Rectification and extractive profiles at F/V=0.5 and R = oo are shown in Fig 2. A feasible rectification profile is drawn in the figure by bold line. There is a stable node (SN) of the extractive profiles near the A-E edge. As all the extractive profiles arrive at the neighbourhood of SN, they all cross the specified rectification profile; therefore the rectification profile can be reached by any extractive profile from the still even if the still composition is in the middle of the composition triangle. On the basis of the residue curve map and that of the map of feasible extractive profiles, the operation steps can be established as is shown in Table 1 (column c). Moreover, the locus of SN does not depend on the still composition; thus roughly constant distillate composition can be maintained during the production step of the process. 2.3. Simulation results Simulation of steps 2 and 3 is performed with Nextr=15, Nrect=15, Q=1.5kW, F = 0.085 kmol/h, R = 10.0, Xch = [0.5; 0.5; 0.0], H = 6 litre = 0.0645 kmol. The given feed flow rate and boiling power roughly correspond to a feed ratio of F/V ~ 0.5. The computed recovery ratio varies between 85 % (at high productivity) and 97-98 % (at lower productivity). The results are in good agreement with our expectations.
199 Table 1: Step sequences of BED using different type entrainers. Azeotrope: Entrainer: heat-up run-up V'cui CO
2"^ cut S'^'cut
b Maximum Heavy R = ooF = 0 R = ooF = 0 ( R = oo R = ooF>0 F>0) R0 R0 A A R
c d Minimum Maximum Intermediate R=:ooF = 0 R = ooF = 0 R = ooF>0 R0 A R
R0 EA R
4'*'cut 5^^ cut Main contaminant . in A Bubble point ranking
e Minimum Light R = ooF = 0
AB, A, B, E
A, B, AB, E
AB, A, E, B
A, E, B, AB
3. Maximum Boiling Azeotrope with Intermediate Boiling Entrainer 3.1. Feasibility of SBD and BED Producing Pure A By Bernot et al. (1991), separation of a maximum boiling azeotrope is feasible applying SBD in a rectifier, because the full composition triangle constitutes a single distillation region. Rigorous simulations have been performed (Lelkes et al., 2002) to validate this process. However, even moderately pure chloroform (XAD ^ 0.9) could not be produced with a great number of theoretical stages (N = 100) and unacceptable great reflux ratio (R = 70). We concluded that pure product cannot be achieved in this way. After the study on the feasibility of BED the same results were achieved, thus the separation is not feasible either with continuous entrainer feeding. 3.2. Feasibility of SBD and BED producing AE Mixture in the first production step Although pure A cannot be produced in the first production step, a mixture of A and E (chloroform and 2-chlorobutane) can be produced, and later separated, because A and E do not form any azeotrope in our text mixture. In order to obtain pure A in a later step, reduced molfraction of A, XAR = XA/(XA+XB), in the distillate is specified. With a high enough value, e.g. XAR = 0.98, this assigns a narrow triangle along the A-E edge, as a range of acceptable distillate compositions. The feasible region for R = 9 and the evolution of the still paths for both SBD and BED are shown in Fig 3. The separation of the azeotrope is feasible in both cases, but BED is more advantageous, because the continuous feeding of the entrainer drives the still path in the direction toward edge B-E. Since the feasibility region valid for SBD reaches edge B-E, feeding to the still is sufficient. That is, the feed need not be applied to the
200 column directly, and distillation in step 2 (in terms of Table 1) with the same recovery specification can be started with less amount of pre-mixed entrainer. Ethyl Acetate (B) 1 Still path for SBD
Still path for BED
0 0,2 2-Chlorobutane (E)
0,8 1 Chloroform (A)
Figure 3 Expected still path directions for BED and SBD. 3.3. Simulation Results Simulations of step 2 of SBD and BED are performed. The two simulation runs are specified in a way that they provide the same purity (XAR = 0.98) in the accumulator and consuming the same amount of entrainer (90 mol). According to the results, BED produces the same purity and recovery of component A (= 92 %) in shorter time (10.0 h vs. 11.3 h) and the still hold-up by BED is half of that by SBD (7.1 litre vs. 15.5 litre).
4. Experimental Results Feasibility of the novel processes was demonstrated with experiments in a laboratory scale packed column (Rev et al., 2002). The column was made of glass with a height of 1.5 m and inside diameter of 5 cm. The initial still hold-up was 1 litre in all of the experiments. For demonstrating the feasibility of separating the minimum boiling azeotrope with middle boiling entrainer, equimolar binary mixture was initially charged into the still. The distillation path in step 2 crossed the isovolatility curve. It means that the azeotrope can be broken in this way; and the separation of a minimum boiling azeotrope with intermediate boiling entrainer is possible by applying BED in a rectifier. In the case of a maximum boiling azeotrope, the distillation path can be driven along the AE side of the triangle by applying BED with intermediate boiling entrainer. For demonstrating this possibility, two experiments were performed with identical specifications. The distillation path in the SBD experiment turned sharply inside of the triangle when the still path reached the boundary of the feasible region. On the contrary, the distillation path in the BED experiment ran along the AE side. This latter result can be considered as an effect of the continuous entrainer feeding into the still. It can be
201 concluded that BED is more advantageous separating a maximum boiling azeotrope with middle boiling entrainer than SBD.
5. Comparison of the Different Cases Separation of minimum boiling azeotropes with light and heavy entrainers, and that of maximum boiling azeotrope with heavy entrainer, all applying BED, have earlier been investigated. The study on separating minimum and maximum boiling azeotropes with intermediate boiling entrainer makes possible to conclude viewpoints of designing an effective separation process. Table 1 presents the separation steps for the different cases. Table 2 summarises the main limiting parameters of the separating processes. On the basis of comparing the corresponding columns of these two tables, the most important information for designing a BED process is the relative position of the entrainer to the azeotrope according to the series of the characteristic bubble points in the studied system. Table 2: The essential limiting parameters of the studied separation process. Azeotrope Entrainer ^min
N • rect A^min, N ^^max, rect N • extr ^^min,
b Maximum Heavy
+ + + +
c d Minimum Maximum Intermediate
+ + + +
e Minimum Light
Envelope of the rectification profiles (This can be extended marginally by extractive profiles.)
Envelope of the rectification profiles (This can be extended marginally by extractive profiles)
• *F/V ' ' mm (at R = 00)
Separatrix of the extractive profiles
Specified rectification profile and the corresponding extractive profile
If the bubble point of the entrainer is higher than that of the azeotrope then pure products can be maintained, and the operation steps and limiting parameters are almost the same for all the three systems (Table 1 column a-c). There are some differences for the maximum boiling azeotrope with heavy entrainer. The run-up step (Table 1 column b), that serves for the purification of the first cut product, can be omitted if Xch,A is greater than XAZ,A (see Lang et al, 2000). The other difference is the non-existence of the limiting parameters (Table 2 column b) Nmax, rect and F/Vmin at R = ©o. These parameters do not appear here because the 1^^ cut is an unstable node of the residue curve map in the case of separating a maximum boiling azeotrope, and is a saddle point in the case of separating a minimum boiling azeotrope. If the bubble point of the entrainer is lower than that of the azeotrope then pure products cannot be maintained at the beginning of the process, but binary mixtures, without any azeotrope, can be produced. The separation schemes for minimum boiling azeotrope with light entrainer and for maximum boiling azeotrope with intermediate entrainer are
202 similar, and even their other characteristics are identical. SBD is feasible in both cases; BED is applied just for increasing the effectiveness of the process. The entrainer (component E), instead of B, becomes the main contaminator of A if the entrainer is not the heaviest component. This can be an important viewpoint if A should be produced as free of B as possible.
6. Conclusion Feasibility of batch extractive distillation in rectifying column with middle boiling entrainer was studied for both minimum and maximum boiling azeotropes. Separation of a minimum boiling azeotrope with intermediate boiling entrainer is feasible in rectifier applying BED. Separation of a maximum boiling azeotrope with intermediate boiling entrainer is feasible in rectifier applying either SBD or BED; but application of BED is more advantageous. The results of the feasibility method were validated by rigorous simulations. The main steps of the separations were justified by laboratoryscaled experiments. The results of the studies on separating minimum and maximum boiling azeotropes with light, intermediate, or heavy entrainers are compared according to their operation steps and feasibility domains. The decisive property for designing an effective BED process, separating azeotropes, is the relative position of the entrainer to the azeotrope in the bubble point series. But the type of the azeotrope (minimum or maximum) can modify the existence of some limiting parameters (F/Vmin, Nmax, rect)-
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8. Acknowledgement: This study has been supported by OTKA T037191, F035085, T030176, and AKP 2001112.