- Email: [email protected]
Azeotropic batch distillation new problems and some solutions
Computers
009%1354(95)00124-7
them. Engng Vol. 19, Suppl., pp. S589-S596, 1995 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0098-1354/95 $9.50 + 0.00
AZEOTROPIC BATCH DISTILLATION New Problems and Some Solutions Stuart Watsontt, Jean-Marc Le Iann*
* t 9 9
Xavier JouliaSI , SMacchiettog,
Gilles Vayrette* and Jean-Jacques
Letoumeauq
E.N.S.I.G.C. , 18 Chemin de la Loge, 31078 TOULOUSE, PRANCE e-mail : [email protected] fax : (+33) 62 25 23 70 tel : (+33) 62 25 23 55 Chemical Engineering Department, Imperial College, London SW7 2BY, UK Centre for Process Systems Engineering, Imperial College, London SW7 2BY, UK e-mail : [email protected] fax : (+44) 71 594 66 06 tel:(+44) 71594 66 08 E.M.A.C. (Ecole des Mines d’A1biCarmaux), 81013 ALBI CT Cedex 09, PRANCE e-mail : [email protected] fax : (+33) 63 49 30 99 tel : (+33) 63 49 30 88
TRACT Two methods of separating a complex quatemsuy industrial mixture of methanol, cyclohexane, ethanol and water using batch distillation are presented. The first method is an extractive batch distillation and the second method is a cyclic operation. Both methods are based on a thermodynamic analysis of the system (which contains four binary azeotropes and a ternary azeotrope, three of which are heterogeneous) and predictions of the product sequences. The effectiveness of these predictions is demonstrated by comparison with detailed simulations. It is found that quantities of products are important in the separation scheme as well as their relative sequence. The column hold-up seems to directly influence the period during which a given product may be obtained at the distillate. Therefore both thermodynamic and dynamic aspects must be considered in the design of optima1 operations.
KEY WORDS
Batch Distillation, Extraction, Heteroazeotrope,
Residue curves, Operating Policy
ODUCTION Batch distillation has been treated most extensively, covering modelling, simulation, optimization and control aspects (Bernot et al., 1990; Macchietto and Mujtaba, 1992; Bossen et al., 1993; Sorensen and Skogestad, 1994). Most works deal with mixtures which present rather simple thermodynamic behaviour. Mixtures with more complex behaviour (e.g. azeotropes), although of very high industrial interest, have been dealt with more sparsely in the research literature. The simulation and design of binary azeotropic batch distillation columns based on modified short-cut methods were discussed by Diwekar (1991). Bernot et al. (1991) present a rather general method for establishing the type and sequence of products which can be- obtained by batch distillation of multicomponent mixtures with azeotropes, based on simple batch distillation curves. They also discuss affecting the separation sequence by choosing suitable entrainers. Their results are based on the limiting behaviour of simple dynamic models which ignore, for example all holdups and apply only at large reflux ratios and a high number of separation stages. A case of extractive batch distillation has been discussed by Yatim et al. (1993). This paper considers the development of operation policies for the separation of a complex quaternary mixture, utilising (i) a priori estimations of product sequences based on the method of Bemot et al. and (ii) ways of modifying such a product sequence by means of solvent addition, to result in the entrainment of the selected species. A simple method is outlined for the a priori calculation of the required entrainer amounts. The effectiveness of these predictions is demonstrated by comparison with detailed simulations using the simulator “ProSimBatch” (Albet et al., 1991; ProSim, 1994). However, it is shown that in some cases, depending on the feed composition and product purity specifications, it is also possible to achieve the desired product purity using an alternative and very much simpler
1 Author to whom all correspondance
should be addressed SS89
European Symposium on Computer Aided Process Engineering-5
s590
cyclic operation. This operating policy cannot be found using the method of Bernot et al. since it does not consider dynamic aspects such as holdups. The results indicate that quantities of products are also important in the choice of the separation scheme, in addition to their relative sequence. These aspects are illustrated with reference to a typical quaternary mixture of industrial interest (with four binary azeotropes and a ternary azeotrope, some of which are heterogeneous) and utilising dynamic simulations with full dynamic models.
PROBLEM STATEMEN.%’ The quaternary system considered is a strongly non-ideal mixture of methanol, cyclohexane, ethanol and water with the initial compositions listed in Table 1. The objective of the distillation is to maximize the production of ethanol meeting the composition and time constraints presented in Table 2. The constraint on the methanol content of the product cut is particularly severe. Note that the mixture contains solid and heavy organic impurities (unmodelled) necessitating the removal of the desired ethanol cut in the distillate. The mixture is to be separated in a column made up of 50 theorical stages (including the reboiler and the condenser), with a hold-up of 30 1 in the condenser and a hold-up of 1.225 1 per plate. The condenser is a total condenser and the heat supplied to the boiler is held constant at 380 kW. Thermodynamic analysis using the NRTL model and the binary interaction coefficients in Table 3. revealed the azeotropes, heteroazeotropes and separatrices shown in Fig 1. There is a two phase region lying to the right hand side of the dotted lines.
Component
1 C2H50H
2 C6H12
3 H20
4 CH30H
Initial Molfraction
0.9208
0.0063
0.0620
0.0109
charge Mass (kg)
3037
38
80
25
Table 1: Composition
before batch distillation.
Component Molfraction
x2 C6H12
X3 H20
X4 CH30H
IO.0202
IO.0944
I 0.0008
Total time
I15 hours
time constraint Table 2 : Product composition
and time constraints.
,......... 65.07
E
1 0.530
1 hetero.
1
1
I
WL is defined as : WL xiheaVy + (l_mL) xilight = xiliquid
I 0.900
I
-
I 0.100
0.383 molfraction bp=78.3 1°C cyclohexane bp = 80.64”C Fig 1 : Batch distillation
regions of the mixture (mole fractions).
-
I
0.617
II
European Symposium on Computer Aided Process Engineering-S
cyclohexane
ethanol
water
ethanol
water
802.840
806.800 -550.610
-81.5990 703.084
1393.80 761.770
2835.00 3125.00
0.2000
0.3060
0.2890
0.4376
0.2740
1125.55
s591
water
-114.840 cal. 1376.34 mol-1 0.2983
Table 3. Binary Interaction Coefficients used in the thermodynamic model, NRTL, 3 parameters
distillate projected curve from point 4 to plane I-2-3 residue curve
1 1
L’
Fig 2 : Distillate and residue curves - 4 hours of total reflux followed by distillation at R=30.
RETICALASPECTS Using the prediction method of Bemot et al. (1991) the residue path is expected to move towards the /$ 1 separatrix along the line that projects from the node with the lowest boiling point in the region, 6, and upon reaching the p-1 separatrix to travel towards node 1, pure ethanol. It should stay at this point until the charge
has been exhausted as the pure ethanol vertex corresponds to the node with the highest boiling point in the region. The distillate path and hence the sequence of cuts obtained is determined by the gradient of this curve. Hence the expected product sequence is 6, E, B, and finally 1. In order to verify this separation sequence a simulation with 4 hours of total reflux followed by more than 60 hours at R=30 was carried out. Such a simulation was chosen in order to attain a very good separation and maximize the purity and duration of the cuts obtained. The distillate and residue curves for this simulation are presented in Fig 2. From Fig 2 it may be seen that the product sequence is the heteroazeotrope 6, the ternary heteroazeotrope E mixed with methanol, the azeotrope j3, and finally pure ethanol. The distillate path clings to the 4-8-a-B face and then to the line p-1.This sequence .is not in agreement with the theory as the cut of heteroazeotrope E is not obtained on its own but instead as a mixture with methanol. From Fig 2 it can be seen that the principal diffkulty is therefore the removal of a sufftcient quantity of methanol from the system in order to meet the methanol composition constraint whilst minimising the ethanol lost with the methanol. The simulation shows that at the end of the period of total reflux the binary methanolcyclohexane heteroazeotrope, 6, is concentrated in the condenser. However, as soon as distillation begins the distillate composition quickly moves away from this unstable point (due to the very small amount of methanol in the initial charge - about 1%) and a mixture of the ethanol-cyclohexane-water ternary heteorazeotrope, a, and methanol is produced. The ethanol-water azeotrope, p, is next produced and finally as the water in the system is exhausted pure ethanol is removed in the distillate. At R=30 it takes 6 hours of distillation before all methanol has been removed from the system and 3 I hours before all water has been removed from the charge (the ethanol
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cut must be removed as distillate due to the heavy impurities contained in the initial charge). At this point the net loss of ethanol amounts to 1200 kg. The next section presents means of obtaining the theoretical sequence of cuts as predicted from the method of Bernot et al. and achieving pure ethanol production more rapidles.
Fig 3 : sm of Addition Method (states considered time invariant), (&$2=Xi,B3=Xi,B4 ; &2=&3=s&. Terminating event Step Total reflux time elapsed since the beginning of the step = 0.39 hr Methanol cut quantity of methanol removed from the charge > 23.4 kg Ethanol cut
time elapsed since the beginning of the simulation 2 15 hrs, or, boiler dry
Objective remove methanol ethanol product cut
Table 4 : Outline of steps for the Addition Method.
I
/start-up
Willate projected :urve from point 4 to plane l-2-3 residue curve
TION OF WUENCE
OF CUTS VIA SOLVENT ADDITION
Based on the evolution of the distillate curves shown in Fig 2 and upon the molar ratio of cyclohexane to (i) methanol in the binary heteroazeotrope, 6, and (ii) ethanol and water in the ternary heteorazeotrope, E, in the initial charge two points are apparent. Firstly, there is insufficient cyclohexane in the system to eliminate all methanol in the form of heteroazeotropes 6 or E. Secondly, because of the relative ratios of ethanol to water in the binary azeotrope, p, the small quantity of water that is contained in the system leads to a sizeable ethanol loss (1200kg at a R=30) before all water has been removed in the form of the azeotrope, p. Cyclohexane is therefore added to the initial charge as an extractive agent to serve two purposes : (i) to remove as much methanol in the initial stages of production as possible (by making up for the cyclohexane deficit in heteroazeotrope, 6) and (ii) to remove as much of the water in the form of the ternary heteroazeotrope, E, as possible and thereby stop the production of the binary azeotrope, @, (by making up for the cyclohexane deficit in the heteroazeotrope, E, and forcing the distillate path towards the binary heteroazeotope, a) The required quantity of the cyclohexane entrainer to be added to the initial charge may be approximately calculated by simple material balances on the the binary azeotrope, 6, and the ternary azeotrope, E. A total of
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14&i km01 or 1231.44 kg of cyclohexane is therefore required by the two azeotropes, 6 et E. However, since the initial charge already contains 38 kg of cyclohexane approximately 1194 kg of cyclohexanxe must be added to the initial charge. Note that for methanol removal only, the tequiered quantity of cyclohexane is about 4 kg. This procedure is represented schematically by the State Task Network, STN (Mujtatba and Macchietto, 1993), in Fig 3 which involves the (instantaneous) addition of the 1194 kg of cyclohexane to the initial charge followed by the heating of the new charge , filling of the plates and condenser, 24 minutes of infinite reflux, the methanol cut and finally the ethanol product cut. Table 4 lists the corresponding step termination events and conditions and the objective of each step. The results of this operating method are presented graphically in Figs 4 and 5 and the amounts of the components in the states defmed in Fig 2 are listed in Table 5 (the reflux ratio profile used is given in Fig 6). The rationale for the use of cyclohexane as an extra&e agent is supported by the distillate and residue curves, produced by simulation, shown in Figs 4 and 5. Initially as production begins after the period of total reflux the binary heteroazeotrope, 6, is produced. However, within 10 minutes the distillate composition progresses through the ternary heteroazeotrope, E, and the binary heteroazeotrope, a, (thedistillation region having been adjusted by the cyclohexane addition) and finally on towards pure ethanol. The ethanol cut begins as soon as most water (about 80%) has been removed. 1
mole fraction
.9
1
.8 .7 .6 .5
-----“”
0
.. ... .. ..
2
“..“..
I”..
4
. . . . . . . . . . . ..-
. 6
Fig 5. Evolution of the distillate composition. - addition method (Fig 3) -
OPmTING
Table 5. Components amounts. - addition method (Fig 3) -
POLICY. . Cw
An industrial operating policy, based on know-how, used in this case is a cyclic operation. Here use is made of the fact that it is possible to concentrate the binary heteroazeotrope., s, in the condenser at very high reflux ratios. The condenser is then effectively drained and a period of total reflux follows in which the heteroazeotrope, 6, is again concentrated in the condenser. The cyclic nature of this operation gets around the problem linked to the very low concentration of methanol in the initial charge whereby the heteroazeotrope, 6, disappears as soon as production begins at finite reflux (see Fig 2). This procedure is represented schematically by the State Task Network in Fig 6. After heating of the initial charge and filling of the plates and condenser, there follows four successive sets of an infinite reflux step for 2 hours combined with the subsequent total “‘drainage” of the condenser in 4 minutes (to concentrate the methanol in the condenser and then remove it from the system). Since the simulator requires a constant volumetric hold-up in the condenser this so called “drainage” is mimicked by operating at R=0.5. The consequences of this are discussed in the results section of this method. Finally the ethanol is removed from the system in the product cut. Table 6 lists the corresponding step termination events and conditions and the objective of each step. The results of this operating method are presented graphically in Figs 7 and 8 and the amounts of the components in the states defined in Fig 6 are listed in Table 7. Again the rationale for this operating policy is supported by the results of the simulation. As shown in Fig 7, the heteroazeotrope, 6, is concentrated in the condenser after the initial period of total reflux (t = 2.0 hrs) and, after the first and each successive “draining”, as the quantity of heteroazeotrope 6 in the system diminishes,
European Symposium on Computer Aided Process Engineering-S
s594
Fig 6 : STN of Cyclic Method
(~,BO=&Bl=%.B2;
SB&Bl=sB2h(%,B3=&,B4;
@i,BS=Xi.B6;
sBFsB6).
&BFxi,BS;
SB3=sB4), SBFSB8)
drain condenser
Table 6 : Outline of steps for the Cyclic Method
Table 7 : Material balances for STN - Cyclic method -
I =
...-.._.._ ....-..._ ._.. separatrix curve distillate curve
.
t = 6.;3 t= 8.60 hrs Idrainane 4 I I
distillate curve: Total reflux
/
distillate projected curve
Fig 7 : Evolution of distillate mole fractions-Cyclic
Rthll 1 -. 7 0.8 -0.6
ethanol
method -. drainage
Rtotlkl
0.8 -
_
0.4 -. /
+______ .__._ .._ A/-
0
5
__...I
-__-+.
10
Fig 8 : Evolution of the distillate composition - Cyclic method -
0 0
100
200
be 300 @conds)
Fig 9 : Evolution of the molar composition in the condenser during the first “‘drainage” period. - Cyclic method -
European Symposium on Computer Aided Process Engineering-S
s595
more and more ethanol and water are concentrated in the condenser along with the methanol during the period of total reflux. Finally. sufficient methanol (24kg), cyclohexane and water are removed (at t=8.60 hrs) and the desired ethanol product cut can be made. In agreement with the theory of Bemot et al. the distillate path clings to the boundaries of the batch distillation zone in which the initial charge is contained. As already mentioned the simulator requires a constant volumetric hold-up in the condenser. The “drainage” of the condenser can therefore only be simulated by setting the reflux to a low value, R=O.5 during the “drainage” period. During the first “‘drainage”period the variation of the compositions of the four components with time is shown in Fig 9. At the end of the total reflux step the methanol-cyclohexane heteroaxeotrope 8 has been concentrated in the condenser. However, as soon as drainage begins ethanol and water start to dilute 8 and since the condenser is assumed to be perfectly mixed, this ethanol and water leaves with 8 as shown in Figs 8 and 9. This process continues throughout the Yrainage” operation, and tinally axcotrope B becomes established in the condenser. In the period of total reflux that follows the methanol remaining in the system becomes concentrated in the condenser. The simulation is therefore not an entirely accurate representation of the true drainage operation.
ONOFThe cyclic policy, simple to implement (requiring the reflux valve to be totally open or totally closed) offers tbe most advantages at an industrial level for this particular case where the constraint on water is not severe (see table 2). The addition policy, while effective, requires adding a large amount of entrainer in this case. The methodology behind the addition policy does however present a series of possibilities for the separation of other systems of mixtures requiring the addition of a smaller amount of an extractive agent and/or a high recovery of the agent in another column. The addition of a given component as entrainer (here: cyclohexane) to the initial charge before distillation may be used to steer the residue and hence the distillate paths by changing the distillation region. This effect is clearly shown by comparing the residue paths followed in Figs 2 and 7 with those in Fig 4. Without the addition of cyclohexane the residue path follows the p-1 separatrix whilst in the case of the addition the path is closer to the a-l separatrix and therefore as expected from the evolution of the gradient of this line the distillate path is forced towards heteroaxeotrope a. The effect of the three-phase nature of the system at certain compositions (see Fig 1) upon the separatrices and upon the operational policies is uncertain and requires further investigation (all simulations were performed on the basis of a liquid-vapor model). In the case of the cyclic operation with a period of total reflux followed by a period of zero reflux it is felt that the effect would be negligible (assuming liquid phases are well mixed). However, in the case of the addition method this is not so clear cut. Also worthy of note is the short lived production period of the methanol-cyclohexane heteroaxeotrope, 8, in the distillate. In both operational policies this hetereoaxeotrope, which has the lowest boiling point in the system, is concentrated in the condenser in the period of total reflux but as soon as production begins the distillate composition moves away from this point within a few minutes. The methanol and cyclohexane contained in the initial charge amounts to 25 kg and 38 kg respectively. By comparison of the ratio of methanol to cyclohexane it is found that only 23.3kg of this methanol can combine with the cyclohexane in the form of heteroazeotrope 8 if no cyclohexane is added to the system. The holdup in the system, especially in the condenser, is therefore very significant when compared to the quantity of 8 and it is for this reason that the period of production of 8 is so short. It seems that it is not only the distillation region within which the separation is being carried out that is important but the quantity of material that is to be separated. Plate and condenser holdups seem to directly influence the period over which a given cut is obtained.
CONCLUSION A way of modifying the sequence of cuts for a batch separation of a quaternary mixture of ethanol, cyclohexane, water and methanol has been presented together with an industrial cyclic policy. Both methods offer a wide range of possible solutions for other systems. The best operational policy could be a combination of these two methods. Further investigations are necessary. With the cyclic method, operation might be improved by for example using a variable condenser holdup. The addition method becomes more favourable if the component to be added is needed in only small quantities and/or if this component can be recovered (e.g. via distillation or even decantation if the presence of heteroaxeotropes allows for it) and then recycled.
European Symposium on Computer Aided Process Engineering-5
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The results also show that it is not only the distillation region within which the separation is being carried out that is important but the quantity of material that is to be separated. A given cut may only be obtained for a significant period of time if it is in a large enough quantity within the batch distillation region. If the quantity of plate and condenser hold-ups is significant next to the amount of the desired cut within the initial charge then the production of this product at the distillate is likely to be very short lived if not negligible. Column hold-ups therefore have a direct influence on the product sequence and the choice of an overall best operation requires careful optimisation of a variety of options and conditions.
We thank M. Marxullo of the Sociktk Chimique ROCHE (France) for the information they supply us.
The batch distillation operation is schematically represented as a State task Network @TN) where a state (denoted by a circle) represents a specified material, and a task (rectangular box) represents the operational step which transforms the input state(s) into the output states (Mujtaba and Macchietto, 1993) - see Figs 3 and 6. In these figures the letter B referres to the boiler and the letter D referres to the distillate. States are characterized by the following time invariant vectors and scalars : = mole fraction of species i in state s xi,s x0i.S
=
initial mole fraction of species i in state s at time 0 (initial)
= quantity of material in state s Ss Tasks are character&d by operational attributes such as the reflux ratio profile used during the step etc. R : corresponds to the external reflux ratio.
Albet J., Le Lann J.M., Joulia X. and Koehret B., Rigorous simulation of multicomponent multisequence batch reactive distillation. European Symposium on Computer Applications in Chemical Engineering, Process technology proceeding 10. pp. 75-80, COPE’9 1 Barcelona Spain October 14- 16, ( 199 1). Bemot C., Doherty M.F. and Malone M.F., Patterns of composition change in multicomponent batch distillation. Chem. Engng. Sci. 45. 1207-1221 (1990). Bemot C., Doherty M.F. and Malone M.F., Feasibility and separation sequencing in multicomponent batch distill&on. Chem. Engng. Sci. 46, NO 516, 1311-1326 (1991). Bossen B.S., Jorgensen S.B. and Gani R., Simulation, Design and Analysis of Azeotropic Distillation Operations. I&EC Research, 32, (1993). Diwekar U.M., An Eficient Design Method for Binary, Azeotropic, Batch Distillation Columns. AIChE Journal, 37, No. 10, (1991). Macchietto S., and Mujtaba LM., The Design of operations for batch distillation, proceedings, NATO ASI on Batch Processing, Antalya, Turkey (1992) [in press]. Mujtaba I.M. and Macchietto S., Optimal Operation of Multicomponent Batch Distillation - Multiperiod Formulation andSolution. Computers Chem. Engng., 17, No. 12, pp. 1191-1207, (1993). Pham H.N. and Doherty M.F., Design and synthesis of heterogeneous azeotropic distillations. I. Heterogeneous phase diagram II. Residue curve maps III. Columns sequences Chem. Engng. Sci. 45, 1823-1854, (1990). ProSim Batch, User guide (1994), ProSim S.A. , Toulouse FRANCE. Serensen E., Skogestad S., Optimal Operating Policies of Batch Distillation with Emphasis on the Cyclic operating Policy. Proceedings of the 4th International Symposium on Process Systems Engineering, 1, pp.449-456, (1994). Yatim H., Moszkowicx, P., Otterbein M. and Lang P., Dynamic Simulation of a Batch Extractive Distillation Process. Comput. Chem. Engng., 17S, pp. S57-62. (1993).
009%1354(95)00124-7
them. Engng Vol. 19, Suppl., pp. S589-S596, 1995 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0098-1354/95 $9.50 + 0.00
AZEOTROPIC BATCH DISTILLATION New Problems and Some Solutions Stuart Watsontt, Jean-Marc Le Iann*
* t 9 9
Xavier JouliaSI , SMacchiettog,
Gilles Vayrette* and Jean-Jacques
Letoumeauq
E.N.S.I.G.C. , 18 Chemin de la Loge, 31078 TOULOUSE, PRANCE e-mail : [email protected] fax : (+33) 62 25 23 70 tel : (+33) 62 25 23 55 Chemical Engineering Department, Imperial College, London SW7 2BY, UK Centre for Process Systems Engineering, Imperial College, London SW7 2BY, UK e-mail : [email protected] fax : (+44) 71 594 66 06 tel:(+44) 71594 66 08 E.M.A.C. (Ecole des Mines d’A1biCarmaux), 81013 ALBI CT Cedex 09, PRANCE e-mail : [email protected] fax : (+33) 63 49 30 99 tel : (+33) 63 49 30 88
TRACT Two methods of separating a complex quatemsuy industrial mixture of methanol, cyclohexane, ethanol and water using batch distillation are presented. The first method is an extractive batch distillation and the second method is a cyclic operation. Both methods are based on a thermodynamic analysis of the system (which contains four binary azeotropes and a ternary azeotrope, three of which are heterogeneous) and predictions of the product sequences. The effectiveness of these predictions is demonstrated by comparison with detailed simulations. It is found that quantities of products are important in the separation scheme as well as their relative sequence. The column hold-up seems to directly influence the period during which a given product may be obtained at the distillate. Therefore both thermodynamic and dynamic aspects must be considered in the design of optima1 operations.
KEY WORDS
Batch Distillation, Extraction, Heteroazeotrope,
Residue curves, Operating Policy
ODUCTION Batch distillation has been treated most extensively, covering modelling, simulation, optimization and control aspects (Bernot et al., 1990; Macchietto and Mujtaba, 1992; Bossen et al., 1993; Sorensen and Skogestad, 1994). Most works deal with mixtures which present rather simple thermodynamic behaviour. Mixtures with more complex behaviour (e.g. azeotropes), although of very high industrial interest, have been dealt with more sparsely in the research literature. The simulation and design of binary azeotropic batch distillation columns based on modified short-cut methods were discussed by Diwekar (1991). Bernot et al. (1991) present a rather general method for establishing the type and sequence of products which can be- obtained by batch distillation of multicomponent mixtures with azeotropes, based on simple batch distillation curves. They also discuss affecting the separation sequence by choosing suitable entrainers. Their results are based on the limiting behaviour of simple dynamic models which ignore, for example all holdups and apply only at large reflux ratios and a high number of separation stages. A case of extractive batch distillation has been discussed by Yatim et al. (1993). This paper considers the development of operation policies for the separation of a complex quaternary mixture, utilising (i) a priori estimations of product sequences based on the method of Bemot et al. and (ii) ways of modifying such a product sequence by means of solvent addition, to result in the entrainment of the selected species. A simple method is outlined for the a priori calculation of the required entrainer amounts. The effectiveness of these predictions is demonstrated by comparison with detailed simulations using the simulator “ProSimBatch” (Albet et al., 1991; ProSim, 1994). However, it is shown that in some cases, depending on the feed composition and product purity specifications, it is also possible to achieve the desired product purity using an alternative and very much simpler
1 Author to whom all correspondance
should be addressed SS89
European Symposium on Computer Aided Process Engineering-5
s590
cyclic operation. This operating policy cannot be found using the method of Bernot et al. since it does not consider dynamic aspects such as holdups. The results indicate that quantities of products are also important in the choice of the separation scheme, in addition to their relative sequence. These aspects are illustrated with reference to a typical quaternary mixture of industrial interest (with four binary azeotropes and a ternary azeotrope, some of which are heterogeneous) and utilising dynamic simulations with full dynamic models.
PROBLEM STATEMEN.%’ The quaternary system considered is a strongly non-ideal mixture of methanol, cyclohexane, ethanol and water with the initial compositions listed in Table 1. The objective of the distillation is to maximize the production of ethanol meeting the composition and time constraints presented in Table 2. The constraint on the methanol content of the product cut is particularly severe. Note that the mixture contains solid and heavy organic impurities (unmodelled) necessitating the removal of the desired ethanol cut in the distillate. The mixture is to be separated in a column made up of 50 theorical stages (including the reboiler and the condenser), with a hold-up of 30 1 in the condenser and a hold-up of 1.225 1 per plate. The condenser is a total condenser and the heat supplied to the boiler is held constant at 380 kW. Thermodynamic analysis using the NRTL model and the binary interaction coefficients in Table 3. revealed the azeotropes, heteroazeotropes and separatrices shown in Fig 1. There is a two phase region lying to the right hand side of the dotted lines.
Component
1 C2H50H
2 C6H12
3 H20
4 CH30H
Initial Molfraction
0.9208
0.0063
0.0620
0.0109
charge Mass (kg)
3037
38
80
25
Table 1: Composition
before batch distillation.
Component Molfraction
x2 C6H12
X3 H20
X4 CH30H
IO.0202
IO.0944
I 0.0008
Total time
I15 hours
time constraint Table 2 : Product composition
and time constraints.
,......... 65.07
E
1 0.530
1 hetero.
1
1
I
WL is defined as : WL xiheaVy + (l_mL) xilight = xiliquid
I 0.900
I
-
I 0.100
0.383 molfraction bp=78.3 1°C cyclohexane bp = 80.64”C Fig 1 : Batch distillation
regions of the mixture (mole fractions).
-
I
0.617
II
European Symposium on Computer Aided Process Engineering-S
cyclohexane
ethanol
water
ethanol
water
802.840
806.800 -550.610
-81.5990 703.084
1393.80 761.770
2835.00 3125.00
0.2000
0.3060
0.2890
0.4376
0.2740
1125.55
s591
water
-114.840 cal. 1376.34 mol-1 0.2983
Table 3. Binary Interaction Coefficients used in the thermodynamic model, NRTL, 3 parameters
distillate projected curve from point 4 to plane I-2-3 residue curve
1 1
L’
Fig 2 : Distillate and residue curves - 4 hours of total reflux followed by distillation at R=30.
RETICALASPECTS Using the prediction method of Bemot et al. (1991) the residue path is expected to move towards the /$ 1 separatrix along the line that projects from the node with the lowest boiling point in the region, 6, and upon reaching the p-1 separatrix to travel towards node 1, pure ethanol. It should stay at this point until the charge
has been exhausted as the pure ethanol vertex corresponds to the node with the highest boiling point in the region. The distillate path and hence the sequence of cuts obtained is determined by the gradient of this curve. Hence the expected product sequence is 6, E, B, and finally 1. In order to verify this separation sequence a simulation with 4 hours of total reflux followed by more than 60 hours at R=30 was carried out. Such a simulation was chosen in order to attain a very good separation and maximize the purity and duration of the cuts obtained. The distillate and residue curves for this simulation are presented in Fig 2. From Fig 2 it may be seen that the product sequence is the heteroazeotrope 6, the ternary heteroazeotrope E mixed with methanol, the azeotrope j3, and finally pure ethanol. The distillate path clings to the 4-8-a-B face and then to the line p-1.This sequence .is not in agreement with the theory as the cut of heteroazeotrope E is not obtained on its own but instead as a mixture with methanol. From Fig 2 it can be seen that the principal diffkulty is therefore the removal of a sufftcient quantity of methanol from the system in order to meet the methanol composition constraint whilst minimising the ethanol lost with the methanol. The simulation shows that at the end of the period of total reflux the binary methanolcyclohexane heteroazeotrope, 6, is concentrated in the condenser. However, as soon as distillation begins the distillate composition quickly moves away from this unstable point (due to the very small amount of methanol in the initial charge - about 1%) and a mixture of the ethanol-cyclohexane-water ternary heteorazeotrope, a, and methanol is produced. The ethanol-water azeotrope, p, is next produced and finally as the water in the system is exhausted pure ethanol is removed in the distillate. At R=30 it takes 6 hours of distillation before all methanol has been removed from the system and 3 I hours before all water has been removed from the charge (the ethanol
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cut must be removed as distillate due to the heavy impurities contained in the initial charge). At this point the net loss of ethanol amounts to 1200 kg. The next section presents means of obtaining the theoretical sequence of cuts as predicted from the method of Bernot et al. and achieving pure ethanol production more rapidles.
Fig 3 : sm of Addition Method (states considered time invariant), (&$2=Xi,B3=Xi,B4 ; &2=&3=s&. Terminating event Step Total reflux time elapsed since the beginning of the step = 0.39 hr Methanol cut quantity of methanol removed from the charge > 23.4 kg Ethanol cut
time elapsed since the beginning of the simulation 2 15 hrs, or, boiler dry
Objective remove methanol ethanol product cut
Table 4 : Outline of steps for the Addition Method.
I
/start-up
Willate projected :urve from point 4 to plane l-2-3 residue curve
TION OF WUENCE
OF CUTS VIA SOLVENT ADDITION
Based on the evolution of the distillate curves shown in Fig 2 and upon the molar ratio of cyclohexane to (i) methanol in the binary heteroazeotrope, 6, and (ii) ethanol and water in the ternary heteorazeotrope, E, in the initial charge two points are apparent. Firstly, there is insufficient cyclohexane in the system to eliminate all methanol in the form of heteroazeotropes 6 or E. Secondly, because of the relative ratios of ethanol to water in the binary azeotrope, p, the small quantity of water that is contained in the system leads to a sizeable ethanol loss (1200kg at a R=30) before all water has been removed in the form of the azeotrope, p. Cyclohexane is therefore added to the initial charge as an extractive agent to serve two purposes : (i) to remove as much methanol in the initial stages of production as possible (by making up for the cyclohexane deficit in heteroazeotrope, 6) and (ii) to remove as much of the water in the form of the ternary heteroazeotrope, E, as possible and thereby stop the production of the binary azeotrope, @, (by making up for the cyclohexane deficit in the heteroazeotrope, E, and forcing the distillate path towards the binary heteroazeotope, a) The required quantity of the cyclohexane entrainer to be added to the initial charge may be approximately calculated by simple material balances on the the binary azeotrope, 6, and the ternary azeotrope, E. A total of
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14&i km01 or 1231.44 kg of cyclohexane is therefore required by the two azeotropes, 6 et E. However, since the initial charge already contains 38 kg of cyclohexane approximately 1194 kg of cyclohexanxe must be added to the initial charge. Note that for methanol removal only, the tequiered quantity of cyclohexane is about 4 kg. This procedure is represented schematically by the State Task Network, STN (Mujtatba and Macchietto, 1993), in Fig 3 which involves the (instantaneous) addition of the 1194 kg of cyclohexane to the initial charge followed by the heating of the new charge , filling of the plates and condenser, 24 minutes of infinite reflux, the methanol cut and finally the ethanol product cut. Table 4 lists the corresponding step termination events and conditions and the objective of each step. The results of this operating method are presented graphically in Figs 4 and 5 and the amounts of the components in the states defmed in Fig 2 are listed in Table 5 (the reflux ratio profile used is given in Fig 6). The rationale for the use of cyclohexane as an extra&e agent is supported by the distillate and residue curves, produced by simulation, shown in Figs 4 and 5. Initially as production begins after the period of total reflux the binary heteroazeotrope, 6, is produced. However, within 10 minutes the distillate composition progresses through the ternary heteroazeotrope, E, and the binary heteroazeotrope, a, (thedistillation region having been adjusted by the cyclohexane addition) and finally on towards pure ethanol. The ethanol cut begins as soon as most water (about 80%) has been removed. 1
mole fraction
.9
1
.8 .7 .6 .5
-----“”
0
.. ... .. ..
2
“..“..
I”..
4
. . . . . . . . . . . ..-
. 6
Fig 5. Evolution of the distillate composition. - addition method (Fig 3) -
OPmTING
Table 5. Components amounts. - addition method (Fig 3) -
POLICY. . Cw
An industrial operating policy, based on know-how, used in this case is a cyclic operation. Here use is made of the fact that it is possible to concentrate the binary heteroazeotrope., s, in the condenser at very high reflux ratios. The condenser is then effectively drained and a period of total reflux follows in which the heteroazeotrope, 6, is again concentrated in the condenser. The cyclic nature of this operation gets around the problem linked to the very low concentration of methanol in the initial charge whereby the heteroazeotrope, 6, disappears as soon as production begins at finite reflux (see Fig 2). This procedure is represented schematically by the State Task Network in Fig 6. After heating of the initial charge and filling of the plates and condenser, there follows four successive sets of an infinite reflux step for 2 hours combined with the subsequent total “‘drainage” of the condenser in 4 minutes (to concentrate the methanol in the condenser and then remove it from the system). Since the simulator requires a constant volumetric hold-up in the condenser this so called “drainage” is mimicked by operating at R=0.5. The consequences of this are discussed in the results section of this method. Finally the ethanol is removed from the system in the product cut. Table 6 lists the corresponding step termination events and conditions and the objective of each step. The results of this operating method are presented graphically in Figs 7 and 8 and the amounts of the components in the states defined in Fig 6 are listed in Table 7. Again the rationale for this operating policy is supported by the results of the simulation. As shown in Fig 7, the heteroazeotrope, 6, is concentrated in the condenser after the initial period of total reflux (t = 2.0 hrs) and, after the first and each successive “draining”, as the quantity of heteroazeotrope 6 in the system diminishes,
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Fig 6 : STN of Cyclic Method
(~,BO=&Bl=%.B2;
SB&Bl=sB2h(%,B3=&,B4;
@i,BS=Xi.B6;
sBFsB6).
&BFxi,BS;
SB3=sB4), SBFSB8)
drain condenser
Table 6 : Outline of steps for the Cyclic Method
Table 7 : Material balances for STN - Cyclic method -
I =
...-.._.._ ....-..._ ._.. separatrix curve distillate curve
.
t = 6.;3 t= 8.60 hrs Idrainane 4 I I
distillate curve: Total reflux
/
distillate projected curve
Fig 7 : Evolution of distillate mole fractions-Cyclic
Rthll 1 -. 7 0.8 -0.6
ethanol
method -. drainage
Rtotlkl
0.8 -
_
0.4 -. /
+______ .__._ .._ A/-
0
5
__...I
-__-+.
10
Fig 8 : Evolution of the distillate composition - Cyclic method -
0 0
100
200
be 300 @conds)
Fig 9 : Evolution of the molar composition in the condenser during the first “‘drainage” period. - Cyclic method -
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more and more ethanol and water are concentrated in the condenser along with the methanol during the period of total reflux. Finally. sufficient methanol (24kg), cyclohexane and water are removed (at t=8.60 hrs) and the desired ethanol product cut can be made. In agreement with the theory of Bemot et al. the distillate path clings to the boundaries of the batch distillation zone in which the initial charge is contained. As already mentioned the simulator requires a constant volumetric hold-up in the condenser. The “drainage” of the condenser can therefore only be simulated by setting the reflux to a low value, R=O.5 during the “drainage” period. During the first “‘drainage”period the variation of the compositions of the four components with time is shown in Fig 9. At the end of the total reflux step the methanol-cyclohexane heteroaxeotrope 8 has been concentrated in the condenser. However, as soon as drainage begins ethanol and water start to dilute 8 and since the condenser is assumed to be perfectly mixed, this ethanol and water leaves with 8 as shown in Figs 8 and 9. This process continues throughout the Yrainage” operation, and tinally axcotrope B becomes established in the condenser. In the period of total reflux that follows the methanol remaining in the system becomes concentrated in the condenser. The simulation is therefore not an entirely accurate representation of the true drainage operation.
ONOFThe cyclic policy, simple to implement (requiring the reflux valve to be totally open or totally closed) offers tbe most advantages at an industrial level for this particular case where the constraint on water is not severe (see table 2). The addition policy, while effective, requires adding a large amount of entrainer in this case. The methodology behind the addition policy does however present a series of possibilities for the separation of other systems of mixtures requiring the addition of a smaller amount of an extractive agent and/or a high recovery of the agent in another column. The addition of a given component as entrainer (here: cyclohexane) to the initial charge before distillation may be used to steer the residue and hence the distillate paths by changing the distillation region. This effect is clearly shown by comparing the residue paths followed in Figs 2 and 7 with those in Fig 4. Without the addition of cyclohexane the residue path follows the p-1 separatrix whilst in the case of the addition the path is closer to the a-l separatrix and therefore as expected from the evolution of the gradient of this line the distillate path is forced towards heteroaxeotrope a. The effect of the three-phase nature of the system at certain compositions (see Fig 1) upon the separatrices and upon the operational policies is uncertain and requires further investigation (all simulations were performed on the basis of a liquid-vapor model). In the case of the cyclic operation with a period of total reflux followed by a period of zero reflux it is felt that the effect would be negligible (assuming liquid phases are well mixed). However, in the case of the addition method this is not so clear cut. Also worthy of note is the short lived production period of the methanol-cyclohexane heteroaxeotrope, 8, in the distillate. In both operational policies this hetereoaxeotrope, which has the lowest boiling point in the system, is concentrated in the condenser in the period of total reflux but as soon as production begins the distillate composition moves away from this point within a few minutes. The methanol and cyclohexane contained in the initial charge amounts to 25 kg and 38 kg respectively. By comparison of the ratio of methanol to cyclohexane it is found that only 23.3kg of this methanol can combine with the cyclohexane in the form of heteroazeotrope 8 if no cyclohexane is added to the system. The holdup in the system, especially in the condenser, is therefore very significant when compared to the quantity of 8 and it is for this reason that the period of production of 8 is so short. It seems that it is not only the distillation region within which the separation is being carried out that is important but the quantity of material that is to be separated. Plate and condenser holdups seem to directly influence the period over which a given cut is obtained.
CONCLUSION A way of modifying the sequence of cuts for a batch separation of a quaternary mixture of ethanol, cyclohexane, water and methanol has been presented together with an industrial cyclic policy. Both methods offer a wide range of possible solutions for other systems. The best operational policy could be a combination of these two methods. Further investigations are necessary. With the cyclic method, operation might be improved by for example using a variable condenser holdup. The addition method becomes more favourable if the component to be added is needed in only small quantities and/or if this component can be recovered (e.g. via distillation or even decantation if the presence of heteroaxeotropes allows for it) and then recycled.
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The results also show that it is not only the distillation region within which the separation is being carried out that is important but the quantity of material that is to be separated. A given cut may only be obtained for a significant period of time if it is in a large enough quantity within the batch distillation region. If the quantity of plate and condenser hold-ups is significant next to the amount of the desired cut within the initial charge then the production of this product at the distillate is likely to be very short lived if not negligible. Column hold-ups therefore have a direct influence on the product sequence and the choice of an overall best operation requires careful optimisation of a variety of options and conditions.
We thank M. Marxullo of the Sociktk Chimique ROCHE (France) for the information they supply us.
The batch distillation operation is schematically represented as a State task Network @TN) where a state (denoted by a circle) represents a specified material, and a task (rectangular box) represents the operational step which transforms the input state(s) into the output states (Mujtaba and Macchietto, 1993) - see Figs 3 and 6. In these figures the letter B referres to the boiler and the letter D referres to the distillate. States are characterized by the following time invariant vectors and scalars : = mole fraction of species i in state s xi,s x0i.S
=
initial mole fraction of species i in state s at time 0 (initial)
= quantity of material in state s Ss Tasks are character&d by operational attributes such as the reflux ratio profile used during the step etc. R : corresponds to the external reflux ratio.
Albet J., Le Lann J.M., Joulia X. and Koehret B., Rigorous simulation of multicomponent multisequence batch reactive distillation. European Symposium on Computer Applications in Chemical Engineering, Process technology proceeding 10. pp. 75-80, COPE’9 1 Barcelona Spain October 14- 16, ( 199 1). Bemot C., Doherty M.F. and Malone M.F., Patterns of composition change in multicomponent batch distillation. Chem. Engng. Sci. 45. 1207-1221 (1990). Bemot C., Doherty M.F. and Malone M.F., Feasibility and separation sequencing in multicomponent batch distill&on. Chem. Engng. Sci. 46, NO 516, 1311-1326 (1991). Bossen B.S., Jorgensen S.B. and Gani R., Simulation, Design and Analysis of Azeotropic Distillation Operations. I&EC Research, 32, (1993). Diwekar U.M., An Eficient Design Method for Binary, Azeotropic, Batch Distillation Columns. AIChE Journal, 37, No. 10, (1991). Macchietto S., and Mujtaba LM., The Design of operations for batch distillation, proceedings, NATO ASI on Batch Processing, Antalya, Turkey (1992) [in press]. Mujtaba I.M. and Macchietto S., Optimal Operation of Multicomponent Batch Distillation - Multiperiod Formulation andSolution. Computers Chem. Engng., 17, No. 12, pp. 1191-1207, (1993). Pham H.N. and Doherty M.F., Design and synthesis of heterogeneous azeotropic distillations. I. Heterogeneous phase diagram II. Residue curve maps III. Columns sequences Chem. Engng. Sci. 45, 1823-1854, (1990). ProSim Batch, User guide (1994), ProSim S.A. , Toulouse FRANCE. Serensen E., Skogestad S., Optimal Operating Policies of Batch Distillation with Emphasis on the Cyclic operating Policy. Proceedings of the 4th International Symposium on Process Systems Engineering, 1, pp.449-456, (1994). Yatim H., Moszkowicx, P., Otterbein M. and Lang P., Dynamic Simulation of a Batch Extractive Distillation Process. Comput. Chem. Engng., 17S, pp. S57-62. (1993).