Transcript
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ABSTRACT
Modelling and Simulation of Reactive Distillation Columns
Reactive distillation (RD), simultaneous reaction and separation within a single unit,
representing an existing alternative to conventional reaction followed by separation
processes, leading to significantly reduction in initial investment and operating cost. The
modeling and simulation is done for both batch and continuous reactive distillation in the
production of ethyl acetate from ethyl alcohol and acetic acid. For the batch reactive
distillation done in 11 stages, the optimum reboiler heat duty is determined. For that
optimum reboiler heat duty, the dynamics of composition in reflux drum and reboiler is
studied until it reaches a steady state. In the continuous reactive distillation column the
reactants are fed at different trays, the optimum reboiler heat duty, feed plate location,
distillate flow rate, feed flow temperature are determined.
Keywords: Reactive distillation, modeling, simulation, ethyl acetate column.
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Chapter-1
1.1 Introduction and Literature survey
The combination of chemical reaction with distillation in only one unit is called reactive
distillation (RD). Reactive distillation has received increasing attention over the past
several years as a promising alternative to conventional processes. Although invented in
1921, the industrial application of reactive distillation did not take place before 1980s.
Especially, interesting equilibrium reactions suitable for reactive distillation are
esterification, ester hydrolysis reactions, etherification and transesterification. In recent
years, attention has been paid to ethyl acetate synthesis and hydrolysis, which serves as a
model for reactive distillation processes.
The performance of reaction with separation in one piece of equipment offers distinct
advantage over the conventional sequential approach. As a advantage of this integration,
chemical equilibrium limitations can be overcome, higher selectivities can be achieved,
the heat of reaction can be used for distillation, auxiliary solvents can be avoided and
azeotropic mixtures can be more easily separated than in conventional distillation. This
May lead to enormous reduction of capital and investments costs and may be important
for sustainable development due to lower consumption of resources. Some industrial
processes where reactive distillation is used are the esterification processes.
However reactive distillation is not suitable for every process where reaction and
separation steps occur. Operating conditions, such as pressure and temperature of the
reactive and separation processes and perhaps other requirements, must overlap in order
to assure the feasibility of the combined process. This limitation can be overcome by
fixing adequate operating conditions in the cases where this is possible.
In addition to the low capital investment, the RD column requires low operating costs
(energy, water, solvents, etc). however, the mathematical model for this piece of
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Chapter 2
Modelling and Simulation of Ethyl Acetate Reactive Distillation
Columns
2.1.Process Description:
2.1.1. For Batch Reactive Distillation Column:
The reboiler is fully charged, all the trays and the condensers are specified with
initial holdups. The reboiler is then heated. Vapor flows upwards in the
rectifying column and condenses at the top. The entire
condensate is returned to the column as reflux. This contacting
of vapor and liquid considerably improves the reaction. After
some time, a part of the overhead condensate is withdrawn
continuously as distillate and it is accumulated in the receivers,
and the other part is recycled into the column as reflux. Owing to
the differing vapour pressures of the distillate, there will be a
change in the overhead distillation with time.
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In the column the lighter (lower-boiling) component tend to concentrate in the vapor
phase, while the heavier (higher-boiling) components tend towards the liquid phase. The
result is the top product becomes richer in light components as the feed and the bottom
product becomes richer in heavy components as the feed.
Figure 2.2: Schematic ofa Continuous Reactive Distillation Column
The following is a reversible liquid phase reaction:
Acetic acid + Ethanol Ethyl acetate + Water
(1) (2) (3) (4)
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This is a two reactant- two product system. Among the two product water is the heaviest
one and it comes out as the bottom product. On the other hand, the main product, ethyl
acetate, comes out at the top section as the distillate product
2.2.Assumptions
Ethyl acetate RD column with trays numbered from bottom to top.
Liquid on the tray is perfectly mixed and incompressible.
Tray vapor holdups are negligible.
Variable liquid holdup in each tray.
No heat accumulation on each tray.
Vapor phase Murphree efficiency of 75% is considered.
Nonlinear Francis weir formula for tray hydraulics calculation.
Raoults law for vapor-liquid equilibrium.
2.2.1 Bubble point calculation
For solving the vapor rates i.e. the energy balance equation, one requires the enthalpy
data; and to calculate the enthalpy, the temperature should be known. Therefore, it is
necessary to have the temperature-composition correlation. The vapor phase composition
in equilibrium with the liquid phase is given by,
i i iy x =
where k is equilibrium ratio, for this study k is calculated as follows,
s
ii
t
Pk
P=
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where is Pis the vapor pressure was calculated by using Antoine equation, and Pt is the
total pressure.
The Antoine equation is given as,
expiB
P AT C
= +
Where A, B, C are the constants and T is the temperature. Here the pressure is in
mmHgand the temperature in C.
Step-wise bubble point calculation
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2.2.2. Enthalphy Calculations
Assume Temperature
Calculate equilibrium ratio
Calculate yi
using xi
Check(T) = = y
i-1
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The liquid and vapor enthalpies were calculated based on the following equations (Yaws,
1996). Here the Temperature is in (K) and the enthalphy is in (J/mol.K).
*vH Cp T = (2.2.2 1)
2 3 4
Cp A BT CT DT ET = + + + + (2.2.2.2)l vH H= (2.2.2.3)v v
i iH y H = (2.2.2.4)
( )
2
2
i
i
aRT
b T
= +
(2.2.2.5)
where viH = Enthalpy of Vapor
l
iH = Enthalpy of liquid
= Latent heat.
R=gas constant
A,B,C,D,E, =constants
a , b =B,C of Antonie constants.
2.2.3. Liquid flow rate Calculations
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For the calculation of liquid flow rates the Francis weir formula (L.Wang.et,al.
Computers and Chemical engineering 27 (2003) 1485 - 1497). Liquid flow rate is a
function of holdup, weir height, area of the tray and the volume held.
liq tray
j jif h h> (2.2.3.1)
1.5liq tray
j j
j weir liq
j
h h
L lvol
=
(2.2.3.2)
0jelse L = (2.2.3.3)
liq
j jliq
j
tray
M vol where h
A= (2.2.3.4)
Where jL = Liquid flow rate (mole/s)
weirl = length of the weir (m).
, = constants.
trayjh =Height of the weir (m)
j = mass holdup (mole)
liq
jvol = Volume of mass held in the tray (m3).
trayA = Area of the tray (m2).
2.2.4. Rate of reaction
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2.4.1 : Batch Reactive Distillation
0 2 4 6 8 10 120.87
0.88
0.89
0.90
0.91
0.92
0.93
0.94
0.95
0.96
After 500 min
Heat duty QR=10 5 J/min
PurityofethylacetateinRefluxdrum(
mole
Figure:2.3 Effect of Ethyl acetate purity in Reflux Drum due to Reboiler heat duty
The effect of ethyl acetate purity in reflux drum due to reboiler heat duty is shown above.
With less than 5*105 J/min, the vapor produced is very less and hence there is less
interaction between the upcoming vapor and down coming condensed liquid and the
purity is low. With heat duty more than 5*105 J/min the vapor produced is very high,
there is less xi in the reaction, hence the purity is decreasing. The optimum high purity is
produced when the reboiler heat duty is 5*105 J/min.
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0 100 200 300 400 500 600335
340
345
350
355
360
365
Time(min)
Temperature(K)
ReboilerReflux drum
Figure:2.4 Dynamics of Temperature Profile in Reboiler and Reflux drum
The dynamics of temperature profile in the reboiler and distillate shows that, initially
both the temperatures are at the same point, as all trays including the reboiler and the
reflux drum have the same composition. As the heat is supplied from the reboiler the
heavier component (acetic acid and water) moves down from the reflux drum and the
lighter component (ethyl acetate and ethanol) moves up from the reboiler. Hence the
temperature of the distillate decreases and reaches steady state. While the reboiler
temperature decreases because of the removal of the lighter components and gradually
increases due to the addition of liquid component from the upper plates
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-50 0 50 100 150 200 250 300 350 400 450 500 550-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Compositioninrefluxdrum(
Time(min)
Aceticacid
Ethanol
Ethyl acetate
Water
0 100 200 300 400 500 6000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Reboilercomposition(molefr
Time(min)
Aceticacid
Ethanol
Ethyl acetate
Water
Figure 2.5. Start profile of Composition in Reflux drum and Reboiler respectively
The dynamics of the composition profile in the reflux drum in total reflux condition
during the initial start up is shown above. In the reflux drum the mole fraction of acetic
acid and water are decreasing since they are heavier components. The ethanol fraction
increases due to the addition of ethanol fractions from the bottom plates and then
decreases due to reaction.In the reboiler, the ethanol and acetic acid are decreasing and
the ethyl acetate and water are gradually increasing due to the reaction.
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2.4.2 : Continuous Reactive Distillation
1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.850.455
0.460
0.465
0.470
0.475
0.480
0.485
0.490
0.495
0.500
EthylacetatecompositioninDistillate(mol
Heat Duty in Reboiler (*106 J/min)
Figure:2.7 Effect of Ethyl acetate purity in Distillate due to Reboiler Heat Duty
The effect of ethyl acetate purity in distillate flow rate due to reboiler heat duty shows
that for the fixed feed flow rate of 25 gmole/min for ethanol and acetic acid given in 5 th
and 16th
feed plate respectively, when the reboiler heat duty was less than 1.5*106
J/min,the bottom flow rate was less than 25 gmole/min. For greater than 1.8*106 J/min of heat,
the bottom flow rate went more than 25 gmole/min. Within 1.5 and 1.8 *10 6 J/min, the
bottom flow rate was 25 gmole/min. The purity of ethyl acetate was high when the heat
duty was 1.5*106 J/min.
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295 300 305 310 315 320 325 330 3350.468
0.470
0.472
0.474
0.476
0.478
0.480
0.482
0.484
Ethylaceta
temolefractionindis
Temperature(K)
Figure 2.8 Effect of Ethyl acetate purity in Distillate due to Feed temperature
The effect of ethyl acetate purity in distillate due to feed temperatureis shown above.
When the feed temperature is increased from 298 K the purity is decreasing, because the
assumption is that the reaction completely takes place in liquid phase only.
8 10 12 14 16 18 20 22 24 260.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
Ethylacetatemolefr
actionind
DistillateFlowrate(gmole/min)
Figure 2.9. Effect of ethyl acetate purity of distillate due to distillate flow rate
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8 10 12 14 16 18 20 22 24 268.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
Amountofethylacetateindistillat
DistillateFlowrate(gmole/min)
Figure 2.10 Effect of amount of ethyl acetate in the distillate due to distillate flow rate.
Figure 2.9 and 2.10 depicts the effect of distillate purity and amount of distillate due to
distillate flow rate respectively. When the distillate flow rate is reduced, the reflux flow
rate increases, hence the residence time increases and the conversion increases followed
by the increased purity in the distillate. But this increased purity reduces the amount of
ethyl acetate in the distillate flow rate.
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50.48
0.49
0.50
0.51
0.52
0.53
0.54
0.55
0.56Acetic acid plate=16
Ethylacetatemolefractionindistillate
Feed Plate location for ethanol
Figure.2.11. Effect of Feed plate location for Ethanol
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15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.50.550
0.552
0.554
0.556
0.558
0.560
0.562
0.564
0.566
0.568
0.570Ethanol feedplate=2
Ethylacetatemolefractionindis
Acetic acidfeedplatelocation
Figure:2.12 Effect of Feed plate location for Acetic acid
The effect of feed plate location for ethanol and acetic acid are shown above in figure
2.11 and 2.12 respectively. When the location of the feed plate for ethanol is brought
downwards from 5th to 2nd, the reaction zone increases, hence the purity of ethyl acetate in
distillate increases. On the other hand, for the feed plate location of acetic acid is moved
upwards from 16th to 19th, the purity increases upto 18th and then decreases. This is due to
the fact that upto the 18th plate there is more reaction zone, when added on to 19 th plate
acetic acid goes into the distillate flow rate and reduces the purity.
-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Distillatecomposition(molef
Time(min)
AceticacidEthanolEthyl acetateWater
Figure 2.13: Startup profile of distillate composition
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-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
Dynamics of bottomcomposition
Bottomcompos
ition(molefraction)
Time (min)
Acetic acid
Ethanol
Ethyl acetate
Water
Figure 2.14. Startup profile of bottom composition
The startup profile of distillate and composition is illustrated in the figure 2.13 and 2.14
respectively. In the reflux drum the ethanol mole fraction reaches a maximum due to the
addition from the trays below, then it reaches a steady state. Ethyl acetate on the other
hand reaches a maximum due to formation and goes with steady state. Acetic acid and
water mole fractions are decreasing because they are removed from the reflux drum to the
reboiler. In the reboiler initially the due to the reaction the ethyl acetate and water mole
fraction is increasing. Ethyl acetate and ethanol are removed to the top, hence they are
decreasing. The acetic acid and water are increasing and reaching to a steady state
because of their higher boiling point.
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-2 0 2 4 6 8 10 12 14 16 18 20 22340
345
350
355
360
365
370
375
Tem
perature(K)
TrayNumber
Fig 2.15: Temperature profile along the length of the column
The temperature profile along the length of the column illustrates three distinct regions,
the stripping zone, reaction zone and the rectifying zone. In the reaction zone between to
plate 2 and 18, the temperature is almost the same with slight variation. In the rectifying
zone the temperature decreases due to the removal of heavier component. In the stripping
section the temperature increases due to the heat from the reboiler.
0 2 4 6 8 10 12 14 16 18 20 22
0.0
0.1
0.2
0.3
0.4
0.5
0.6
molefraction
Platenumber
Aceticacid
Ethanol
Ethyl acetate
Water
Figure 2.16: Composition profile along the length of the column
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The composition profile along the length of the column illustrates that the acetic acid is at
the highest in its feed plate, decreases along the reaction zone due to reaction. It increases
in the stripping section due to its higher boiling point and decreases in the rectifying
section due to its removal. Ethanol likewise is high at its feed plate, decreases upto the
end of the reaction zone, then increases in the rectifying zone and decreases in the
stripping section due to its boiling point. Ethyl acetate and water are varying linearly in
the reaction zone due to the reaction, in the rectifying section the ethyl acetate increases
more rapidly, water mole fraction falls due to the adding and removal of lighter and
heavy component respectively. In the stripping section the water increases and ethyl
acetate mole fraction decreases due to the adding and removal of heavy and lighter
components respectively
0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.250.556
0.558
0.560
0.562
0.564
0.566
0.568
0.570
Ethyla
cetateinDistillate(mole
moleratio(ethanol/aceticacid)
Fig:2.17: Effect of mole ratio of the feed (ethanol: acetic acid)
The effect of mole ratio of the feed (ethanol: acetic acid), the conversion is more when
there is equal mole ratio for the ethanol and acetic acid feed. The conversion decreases if
the mole ratio is varied.
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-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500-5
0
5
10
15
20
25
30
35
40
45
Flowrate(gm
ole/min)
Time (min)
Distillate flowrate
Bottomflowrate
Fig 2.18: Dynamics of distillate and bottom flow rate
The startup profile of distillate and bottom flow rate is shown above. The distillate flow
rate is fixed as 25 gmole/min. The bottom flow rate is initially increases, oscillates and
then reaches steady state. Initially the value was high because the not enough vapors were
produced to boil the feed.
Following results are not included
1. Effect of feed flow rate keeping molar ratio fixed (F1=F2)
2. Dynamics response with respect to QRor F
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2.4. Conclusions
The study includes the development of equilibrium stage model for multistage
component batch and continuous. In the batch process the purity is nearly 95%. During
the production phase the purity is decreasing, so a control action is to be taken. The study
done for continuous reactive distillation reveals that for the feed flow rate of 50
mole/min, the reboiler load should be 1*10^6 J/min, feed plates are 2 and 16 for ethanol
and acetic acid respectively. Feed need to be cold feed. molar ratio of the two feeds
should be 1.0. the steady state reaches in nearly 1500 min.
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Table 1
Antonie Constants: Unit of vapor pressure and temperature
Component A B C
Acetic acid 16.8080 3405.57 -56.34Ethanol 18.912 3803.98 -41.68
Ethyl acetate 16.1516 2690.52 -57.160
Water 18.3086 3816.44 -46.19
Table 2
Constants for vapor enthalphy: Unit of Cp and temperature
Components A B C D E
Acetic acid 34.85 3.7626*102 2.83*10-4 -3.0*10-7 9.2*10-11
Ethanol 27.091 1.10*10-1 1.09*10-4 -1.5*10-7 4.6*10-11
Ethyl acetate 69.848 8.23*10-2 3.71*10-4 -4.1*10,-7 1.2*10-10
Water 39.933 -8.4*10-3 2.99*10-5 -1.7*10-8 3.6*10-12
Table 3
Molecular weight and Density
Component Molecular weight (gm/gmole) Density (kg/m3
)Acetic acid 60.052 1049
Ethanol 46.069 791.8
Ethyl acetate 88.107 668
Water 18 1000
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Notation
s
BM =Liquid holdup in reboiler (mol)
DM =Liquid holdup in reflux drum (mol)
nM = Liquid holdup in tray n (mol)
Tn =Top most tray.
D =Distillate flow rate (gmole/min)
B =Bottom flow rate (gmole/min).
BV =Vapor from the reboiler (gmole/min).
TVn =Vapor in top most tray (gmole/min)
TLn =Liquid in top most tray (gmole/min).
nL =Liquid from nth tray
nV =Vapor from nth tray.
,B iy = Vapor mole fraction in reboiler
,B ix =Liquid mole fraction in reboiler.
,D ix =Liquid mole fraction in distillate.
,n ix =Liquid mole fraction in stage n.
,n iy =Vapor mole fraction in stage n.
l
nH = Liquid enthalpy in stage n.
vnH =Vapor enthalpy in stage n.
F=Feed flow rate (gmole/min).l
fH = Enthalpy of feed
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Modelling Equations of batch or continuous RD
Mass Balance:
Reboiler
1B
B
dML V B
dt=
Plate 1
1
2 1 1B
dMV L V L
dt= +
Plate 2 to below top plate
1 1n
n n n n
dMF V L V L
dt +
= + +
Top most plate
1T
T T T
dMn Lo V L V n n ndt
= +
Reflux drum
D TdM
V Lo Dndt
=
Component balance:
Reboiler:
4
,
1 1. , , , ,
1 1
( ) cB B ii B B i B i f m n m n n
m i
d M xL x V y Bx r R
dt= =
= +
Plate 1
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4
1 1,
, 2 2 1 1, 1 1, , ,
1 1
( ) ci B B i ,i i i f m n m n n
m i
d M xV y L x V y L x r R
dt= =
= + + Plate 2 to below top plate
4,
, 1 1, 1 1, , , , ,
1 1
( ) cn n in n i n n i n n i n n i n n i f m n m n n
m i
d M x
F x V y L x V y L x r Rdt + +
= == + + + Top most plate:
4
,
, , ,1 1, , ,
1 1
( ) cT T i
D i f m n m n nT T i T T i T T i
m i
d M xn nLox V y L x V y r Rn n n n n n
dt
= =
= + +
Reflux drum
4,
, , , ,,
1 1
( ) cD D iD i D i f m n m n nT T i
m i
d M x V y Lox Dx r Rn ndt
= =
= +
Energy Balance
Reboiler:
1 1
( )l l v lB B B B B R
d M HL H V H BH Q
dt= +
1 1( )l lR B
B v
B
L H Q BH V
H
+ =
Plate 1
1 12 2 1 1 1 1
( )l v l v l B B
d M HV H L H V H L H
dt= +
2 2 1 111
v l l
B B
v
V H L H L H V
H
+ =
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