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New configuration for heteroazeotropic batch distillation:

II. Rigorous simulation results

Ferenc Denesa, Peter Langa, Gabor Modlaa, Xavier Jouliab

aBUTE Dept. of Building Services & Process Engineering, H-1521 Budapest, Muegyetem rkp. 3-5

bLGC-ENSIACET-INPT, 118 route de Narbonne, 31077 Toulouse, France

E-mail: lang@mail.bme.hu Website: www.vegyelgep.bme.hu

Table 3. Optimal parameters and results for the heterogeneous binary mixture (Nα=Nβ=5)

Separation of a binary homoazeotrope by using an entrainer(Table 4)

The charge composition is that of the binary homoazeotrope isopropanol (A)–water (B):

xch,A= 0.674

Production in the BR:

Optimal amount of E by which the duration of Step 1 (determining primarily the duration of the production cycle) is minimal: 4.2 kmol.

- Step 1: A is produced. The duration of this step is much longer than that of Step 2.

Though the amount of distillate (B-rich phase) in Step 1 is not too low but the E-

content of this distillate, which must be removed (together with A) in Step 2, is

very low. Hence E is present as contamination of product A and/or in the

holdup of the column or the decanter.

- Step 2: B is purified not only from E but also A. Because of the very small amount of E

this component leaves the reboiler quickly. The vapour rising from the reboiler

has nearly A-B azeotropic composition whose A-content is relatively high.

Production in the DCS:

For the DCS the optimal division of the total number of plates is rather unequal. (In all binary cases studied the influence of the division of the plates is slight on the results.)

The optimal amount of E: 3.9 kmol. The total amount of E is filled in reboiler α. The heat duty in column α is much higher than in the other column. The A-content of the top vapour of column β is relatively high.

Table 4. Optimal parameters and results for the ternary mixture (Nα=4Nβ)

The evolution of liquid compositions in the reboilers for both configurations is shown in Fig.7.

Fig. 7. The evolution of liquid compositions in the reboiler(s) for the ternary mixture

Conclusion

After the favourable results of the feasibility studies we investigated the new double-column batch heteroazeotropic distillation configuration (suggested by us) also by rigorous simulation.The calculations are performed for a binary (n-butanol – water) and for a ternary heteroazeotropic mixture (isopropanol – water + benzene) by using the dynamic simulator CC-DColumn.

We investigated the effects of the main operational parameters (eg. division of the total heat duty and amount of charge between the two reboilers) and determined their optimal values which result in minimal energy consumption.For the binary mixture DCS gave similar and for the ternary one better performance than the BR.

Acknowledgement

This work was financially supported by the Hungarian Scientific Research Fund (OTKA, T-049184) and the National Office for Research and Technology (NKTH, LYON08DF).

References

- Chemstations (2007). CHEMCAD User Guide.

- Lang P., F. Denes and X. Joulia, (2008). New configuration for hetero-azeotropic batch

distillation: I. Feasibility studies, ESCAPE-18, Lyon, France.

- Modla G., P. Lang and K. Molnar, (2001). Batch Heteroazeotropic Rectification of a

Low Relative Volatility Mixture under Continuous Entrainer Feeding: Feasibility

Studies. 6th World Congress of Chemical Engineering, Melbourne, Australia (10 pages

on CD).

- Modla G., P. Lang , B. Kotai and K. Molnar, (2003). Batch Heteroazeotropic

Rectification of a Low Relative Volatility Mixturre, AIChE Journal, 49 (10), 2533.

- Pommier S., S. Massebeuf, B. Kotai, P. Lang, O. Baudouin , P. Floquet, V. Gerbaud

(2008). Heterogeneous Batch Distillation Processes: Real System Optimisation, Chem.

Eng. Proc.,47 (3), 408-419.

- Rodriguez-Donis I, V. Gerbaud and X. Joulia, (2002). Feasibility of Heterogeneous

Batch Distillation Processes, AIChE Journal, 48 (6), 1168.

- Rodriguez-Donis Y., J. A. Equijarosa, V. Gerbaud and X. Joulia, (2003). Heterogeneous

Batch-extractive Distillation of Minimum Boiling Azeotropic Mixtures, AIChE J., 49

(12), 3074.

- Skouras S., V. Kiva and S. Skogestad, (2005a). Feasible separations and entrainer

selection rules for heteroazeotropic batch distillation, Chem. Eng. Sci., 60, 2895.

- Skouras S., S. Skogestad and V. Kiva, (2005 b). Analysis and Control of

Heteroazeotropic Batch Distillation, AIChE Journal, 51 (4), 1144-1157.

Fig. 3. Steps of the processing of the charge rich in A in the BR

The meaning of the colours in Figs. 3-7:

Product A

Product B

Mixture with binary azeotropic composition

Equilibrium phase rich in A

Equilibrium phase rich in B Homogeneous mixture rich in A

Homogeneous mixture rich in B

(The colouration concerns maximal separation.)

Production in the DCS (Fig. 4):

In column of the DCS the heat duty and the amount of liquid to be distilled is much higher than in the other column due to

- the high content of A of the charge,

- the very low content of A of the B-rich phase to be purified in column β.

Fig. 4. Processing of the charge rich in A in the DCS

Table 1. Optimal parameters and results for the binary charge rich in A (Nα=Nβ=5)

Homogeneous charge rich in B(Table 2)

Production in the BR:

- Step 1: B is produced as bottom residue. Since in the charge the amount A is very

low the bottom residue reached the prescribed purity of B before filling up the

decanter. (The majority of A appears in the column hold-up.)

- Step 2: Since the amount of distillate in Step 1 is zero there is no need for Step 2.

Production in the DCS:

In the DCS (similarly to the BR) A of prescribed purity can not be produced at all, A accumulates in the holdup. In column α the heat duty and the amount of liquid to be distilled is much lower than in the other column due to the low content of A in the charge.

Table 2. Optimal parameters and results for the binary charge rich in B (Nα=Nβ=5)

Heterogeneous charge (Table 3)

Before the distillation the charge is separated into two liquid phases:

A-rich phase: Ubα = 51.8 kmol ; xA = 0.568

B-rich phase: Ubβ = 48.2 kmol ; xA = 0.012

Production in the BR (Fig. 5):

- Step 1: In this step both components could be produced. If A is produced first we get

better results.

- Step 2: This step is very short since the amount of distillate (B-rich phase) in Step 1 is low

and the A-content of this distillate, which must be removed in Step 2, is very low.

Fig. 5. Steps of the processing of the heterogeneous charge in the BR

Production in the DCS (Fig. 6):

The heat duty of column α is much higher than in the other column due to the high content of B whilst the B-rich phase purified in column β hardly contains A.

Fig. 6. Steps of the processing of the heterogeneous charge in the DCS

Introduction

In the pharmaceutical and fine chemical industries the batch heteroazeotropic distillation (BHD) is frequently applied for the recovery of organic solvents. If the components of a mixture form a heteroazeotrope or by the addition of an entrainer a heteroazeotrope can be formed, then it is possible to cross the azeotropic composition by decantation. To our best knowledge the process is exclusively applied in the industry in open operation mode in batch rectifiers (BR) equipped with a decanter.

We suggested a new double-column system (DCS) for BHD on the basis of the results of feasibility studies based on a very simplified model (e.g. assumption of maximal separation, negligible plate and decanter holdups etc.). The double-column system is operated in closedcircuit. This new configuration was found competitive with the BR and worthy of further, more accurate investigation.

Our objectives:

- to study the new double column system by rigorous simulation ,

- to compare its performance with that of the batch rectifier,

- to verify the conclusions of the feasibility studies.

The calculations are performed for a binary (n-butanol (A) – water (B)) and for a ternary heteroazeotropic mixture (isopropanol (A) – water (B) + benzene as entrainer (E)) .

Simulation method

Simplifying assumptions:

- theoretical trays,

- constant volumetric liquid holdup on the trays and in the decanter,

- negligible vapour holdup,

- negligible duration of pumping between the two steps of the BR.

The model equations to be solved:

- Non-linear differential equations (material balances, heat balances)

- Algebraic equations (VLE, LLE relationships, summation equations, hold-up and

physical property models).

Describing of the phase equilibria: - binary mixture: NRTL model

- ternarymixture: UNIQUACmodel

For the solution of the above equations the dynamic simulator of ChemCad 5.6 (program CC-DCOLUMN) is applied.

Fig. 1. ChemCad model of the batch rectifier

Fig. 2. ChemCad model of the new double-column system

Simulation results

Operational parameters :

- In each case the total number of theoretical stages (N) is 10 for both configurations.

- The separation is performed at atmospheric pressure.

- Both reflux and distillate (BR) are homogeneous.

- In the decanterthe volume of liquid phases are prescribed constant (after the start-up).

- In each case the amount of charge (Uch) is 100 kmol.

-The prescribed purity of both products is 99.5 mol%.

Parameters influencing the total duration of the process:

-The amount of Ein the case of ternary mixture,

- In case of the double-column system (DCS) the division of the total

- number of plates (Na/N),

- heat duty (Qa/Q),

- amount of charge (Uba/Uch).

(In column αcomponent A and in column βB is produced, respectively.)

For the DCS the prescribed purity is reached in both columns at the same time.

For each configuration we show the results only for the optimal case with minimal duration.

Separation of a binary heteroazeotropic mixture

Three different charge compositions are studied:

1. Homogeneous charge rich in n-BuOH (A), xch,A = 0.90

2. Homogeneous charge rich in water (B), xch,A = 0.01

3. Heterogeneous charge, xch,A = 0.30

Homogeneous charge rich in A

Production in the BR (Fig. 3):

- Step 1: A is produced as bottom residue .

- Step 2: It is very short since the amount of distillate (B-rich phase) in Step 1 is very low

and the A-content of this distillate, which must be removed in Step 2, is also

very low

18TH EUROPEAN SYMPOSIUM ON COMPUTER AIDED PROCESS ENGINEERING, LYON, 1-4 JUNE 2008

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