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: [email protected]
Website: www.vegyelgep.bme.hu
Introduction
BR
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 closed circuit. 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)) .
Fig. 3. Steps of the processing of the charge rich in A in the BR
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
- ternary mixture: UNIQUAC model
kmol
MJ/min
hour
hour
kmol
kmol
kmol
First product: B
Step 1
Step 2
12
12
0.13
3.20
3.33
25.37
46.21
28.42
Column a Column b
51.8
48.2
11.47
0.53
3.50
26.95
65.45
7.60
Table 3. Optimal parameters and results for the heterogeneous binary mixture (Nα=Nβ=5)
The meaning of the colours in Figs. 3-7:
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
Product A
Product B
Mixture with binary azeotropic composition
Equilibrium phase rich in A
Equilibrium phase rich in B
Simulation method
Division of charge
Heat duty
Duration of steps
Total duration
Product A
Product B
Byproducts
First product: A
Step 1
Step 2
12
12
3.15
0.18
3.33
24.95
65.85
9.20
DCS
Homogeneous mixture rich in A
Homogeneous mixture rich in B
(The colouration concerns maximal separation.)
Production in the DCS (Fig. 4):
In column a 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.
For the solution of the above equations the dynamic simulator of ChemCad 5.6 (program
CC-DCOLUMN) is applied.
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 Econtent 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.
BR
Fig. 4. Processing of the charge rich in A in the DCS
BR
Division of charge
Heat duty
Duration of steps
Total duration
Product A
Product B
Byproducts
kmol
MJ/min
hour
hour
kmol
kmol
kmol
Step 1
10
3.85
Step 2
10
0.04
3.89
81.34
6.78
11.89
DCS
Column a Column b
99.1
0.9
9.4
0.6
4.23
85.52
4.42
10.06
Division of charge
Heat duty
Entrainer
Duration of steps
Total duration
Product A
Product B
Byproducts
kmol
MJ/min
kmol
hour
hour
kmol
kmol
kmol
Step 1
Step 2
12
12
4.2
0.0
35.60
0.85
36.45
55.97
23.59
24.64
DCS
Column a Column b
77.0
23.0
11.06
0.94
3.9
0.0
34.70
57.46
27.54
18.91
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.
Table 1. Optimal parameters and results for the binary charge rich in A (Nα=Nβ=5)
Fig. 1. ChemCad model of the batch rectifier
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.
BR
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 decanter the 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 E in 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 α component A and in column β B is produced, respectively.)
Division of charge
Heat duty
Duration of steps
Total duration
Product A
Product B
Byproducts
kmol
MJ/min
hour
hour
kmol
kmol
kmol
Step 1
16
0.15
Step 2
0.15
97.08
2.92
DCS
Column a Column b
1.5
98.5
0.86
15.14
0.18
4.04
92.48
3.48
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. 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 doublecolumn 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
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
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
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