Operation and Control of Azeotropic Distillation

advertisement
Azeotropic Distillation
Problem & solution principle
Ethanol cannot be obtained through simple distillation from an ethanol-water mixture because
of an azeotrope.
Different methods are available for this purpose. Here we will introduce azeotropic distillation
using n-pentane as entrainer (see fig. 1).
In a pre-treatment step, an ethanol-water mixture is concentrated to approx. 90% ethanol
content (see fig. 1, column 1).
Afterwards, the mixture is fed to a column together with the n-pentane, which results in a lowboiling ternary in the head (see fig. 1, column 2) that disintegrates into two liquid phases after
condensation.
The wet phase is then discharged into a flash and the arid phase returned to the column (see
fig. 1, flash 5).
Through formation of the ternary azeotrope and expulsion of the water, the ethanol turns into
a high boiler and can be removed from column 2 in almost pure form as bottom component.
Entrainer residues are removed from the water in column 4 and discharged afterwards.
Figure 1 Flow sheet of azeotropic distillation
focused on process simulation
Implementation of azeotropic distillation in CHEMCAD
Components
Thermodynamics
Feed streams k
Unit Operations
Ethanol
K: NRTL
Total flow = 1250
3 SCDS columns
Water
H: LATE
xE = 0.61, xW = 0.39
Multipurpose Flash
n-pentane
VLLE (global phase)
t = 30°C
p = 1 bar
Azeotropic distillation is a stationary process and can be simulated with CHEMCAD Steady
State. To do so, the feed streams and further design parameters are defined in the flow sheet
and the simulation is performed afterwards.
Once the chemical properties of water, ethanol and n-pentane have been selected from the
properties database, the three SCDS columns and a multipurpose flash are placed on the flow
sheet and connected.
For suitable selection of the feed quantities and the process structure, the thermodynamic
properties of the three-substance mixture are evaluated using a residue curve map and a
binodal plot. For this purpose, CHEMCAD contains the option [Plot][Binodal/Residue
Curves...]. Once the three components have been selected, whereby the first and the third
component form the miscibility gap, the previously entered atmospheric pressure is applied
and the distillate temperature of the second column set at approximately 32°C as binodal
temperature.
Now the finished triangular diagram (see fig. 2) is issued.
The boiling temperatures of the examined substances are stated in addition to the diagram,
together with the azeotropic points with the corresponding composition and the respective
boiling temperatures. CHEMCAD requires a suitable thermodynamic model for calculation of
the miscibility gap as well as the residue curves, which is why the gE model as well as the VLLE
phase characteristics must be set before plotting (both settings are located at
[Thermophysical][Thermodynamic Settings]).
Figure 2 illustrates the concentration gradient during the distillation process.
Separation within the first column takes place along the binary water-ethanol line,
whereby the red arrow (arrow 1) represents the rectifying section and the orange arrow (arrow
2) the stripping section. According to this, the two end-productions after the first distillation
step are pure water in the bottom and the composition of the binary azeotropic point in the
column head1.
1
The first distillation process represents a pre-treatment of the feed mixture and is omitted in case of a sufficiently high
ethanol concentration in the feed stream (>75%).
focused on process simulation
Page 2 of 12
Figure 2 Residue curves/binodal plot
In the next step, the bottom component of column 1 is fed into column 2 together with the
entrainer n-pentane. In this process, a mixture compound forms in proximity to the ternary
azeotropic point (green arrow tip, arrow 3).
In the downstream flash, the condensed distillate stream then disintegrates into two liquid
phases along the conode, due to the miscibility gap (blue arrow tips, arrow 4).
The organic phase (arrow 4a) is added again to the second column, which results in a
concentration shift of the ternary mixture across the distillation boundary (see [II] De Filliers,
French, Koplos; 2002). The distillation boundary progresses along the residue curves, starting at
the binary azeotropic points towards the ternary azeotrope. This way, after reflux of the recycle
stream of the organic phase, the material system is located in the distillation section limited by
the points binary azeotrope ethanol-n-pentane, pure ethanol and ternary azeotrope (see [I]
Ulrich; 14 et seq.).
Almost dry ethanol can now be extracted from column 2 as the bottom component (violet
arrow tip, arrow 5).
The wet part of the second phase (arrow 4b) in the flash is cleaned in a third distillation column
(see fig. 1, column 4), so that pure water can be extracted as bottom component and a waterethanol-n-pentane mixture as top component. This is then added to column 2 again.
focused on process simulation
Page 3 of 12
The feed stream is defined before the columns are
specified (see fig. 3).
The pressure is set to 1 bar and the temperature to
30°C. The feed concentration is selected arbitrarily in
the first distillation section (azeotropic point
water/ethanol - ternary azeotrope - pure water). The
mass flow is set to 250 water and 1000 ethanol,
which corresponds to an ethanol concentration of 61
%.
The feed composition can be selected with any
ethanol content below the azeotropic point waterethanol, due to the pre-treatment.
Figure 3 Feed definition
The layout of the first column is defined after the
feed definition. The feed stage is set to the 15th stage 1 at medium column height. Altogether,
30 stages 2 are simulated. With rigorous column design, numerous additional specification
options are available, such as pressure losses, pressures or temperatures within the column
(see fig. 4).
As the water-ethanol mixture is supposed to be advanced as close as possible to the azeotropic
point, the mole fraction in the distillate stream (distillate component mole fraction) is set to
0.9, while the mole fraction in the bottom (bottom component mole fraction) is set to 0.001
(see fig. 5).
1
The optimum feed stage has been calculated using a shortcut column as described by Fenske-Underwood-Gilliland. As this is
based on ideal mixtures and constant volatilities, deviations from the real optimum feed stage may occur.
2
With rigorous design, the number of stages can be selected by performing a sensitivity analysis or determining the theoretical
number of stages with the McCabe-Thiele diagram (see [III] Stephan, Schaber, Stephan, Mayinger; 460 et seq.).
focused on process simulation
Page 4 of 12
Figure 4 Design parameters of the first column
Figure 5 Design parameters of the first column
focused on process simulation
Page 5 of 12
Figure 6 Design parameters of the second column
The number of stages of column 2 is set to 151 (see fig. 6). The product stream of the first
column (stream 2) is fed at stage 82, while the recycle streams 4, 5 and 7 enter the column on
the three next higher stages.
An ethanol mole fraction of 0.999 is set as vaporizer specification to achieve the product
specification.
The condenser is specified with a reflux rate of 1, due to the minimum reflux rate of 0.9152. The
minimum value has been determined beforehand using a shortcut column.
Alternatively, graphic definition of the minimum reflux ratio using the McCabe-Thiele diagram is
also possible.
1
With rigorous design, the number of stages can be selected by performing a sensitivity analysis or determining the theoretical
number of stages with the McCabe-Thiele diagram (see [III] Stephan, Schaber, Stephan, Mayinger; 460 et seq.).
2
The optimum feed stage as well as the minimum reflux rate have been calculated using a shortcut column as described by
Fenske-Underwood-Gilliland. As this is based on ideal mixtures and constant volatilities, deviations from the real optimum feed
stage or from the minimum reflux rate may occur.
focused on process simulation
Page 6 of 12
Figure 7 Design parameters of the second column
Figure 8 Design parameters of the Multipurpose Flash
Once column 2 has been initialized, the flash mode is set to 1 in the "Multipurpose Flash" at
[Flash Mode] (see fig. 8). A low mole vapour fraction is selected in order to completely
condense a distillate while preventing convergence problems at the same time. The pressure
may remain unspecified as the inlet pressure of the flash is applied.
focused on process simulation
Page 7 of 12
In column 4, the number of stages is set to 30 and the feed stage centrally to 15
(see fig. 9). Both parameters have been calculated using a shortcut column.
Figure 9 Design parameters of the third column
Figure 10 Design parameters of the third column
focused on process simulation
Page 8 of 12
Due to the low n-pentane fraction of the wet phase, a water-mole concentration of 0.999 is
selected as vaporizer specification (see fig. 10). At the same time, the distillate is concentrated
until an ethanol concentration of 85% is achieved so that the fraction of water recycled in the
second column is not too large.
Once all "Unit Operations" have been entered, linked and
specified, the start values of the cut stream (see fig. 1, stream 3)
have to be initialized.
The simulation can be started with "run all", and an immediate
convergence of the "Unit Operations" as well as of the material
streams should be achieved. In case of convergence problems
with the recycle stream, it may be necessary to restart the
simulation.
Cut Stream (stream 3)
p = 1 bar
T = 33°C
̇
Water
= 100
̇
Ethanol
= 1000
̇
N-Pentane
= 20000
focused on process simulation
Page 9 of 12
Assessment of the simulation results
In order to obtain a simple overview of the material streams during the process, a list of
characteristics of selected streams can be displayed in real time at [Format][Add Stream Box]
(see fig. 11).
Figure 11 Stream box with selected streams
The approx. 21.7
ethanol fraction in the global feed can be removed almost dry at product
stream
9.
The
process
runs
in
a
bar
(absolute)
here.
The product specification has thus been achieved.
The entrainer n-pentane is completely recycled without loss in the simulation model. In real
operation, slight entrainer losses must be compensated for through constant addition. In
CHEMCAD, this compensation is realized with the unit operation "Controller".
focused on process simulation
Page 10 of 12
Optimization of azeotropic distillation
The energy input and the type of entrainer can be optimized.
In order to reduce the energy input, economisers can be used and the purity of the top and
bottom components reduced. CHEMCAD provides the option to perform the sensitivity study to
realize optimization.
In addition, selection of a suitable entrainer can influence the energy expenditure and the
entrainer quantity. When using cyclohexane, for example, less entrainer is required due to the
higher ethanol fraction in the tertiary azeotrope. However, as the boiling temperatures of the
tertiary azeotropes as well as the impacts of the different organic substances on the
environment vary, a suitable compromise must be found in practice. Benzol, for example, is no
longer used due to its high degree of toxicity despite its good entrainer characteristics.
Usually, the activity coefficients at infinite dilution are determined first when selecting a
suitable entrainer, and the effects on the relative volatility of the substances to be separated
are investigated (see [IV] Gmehling, Kolbe, Kleiber, Rarey; 2012; 512 pp.).
The simulation discussed in this document was generated in CHEMCAD 6.5.3 and can be used
with all versions as of CHEMCAD 5.
Are you interested in further tutorials, seminars or other solutions with CHEMCAD?
Then please contact us:
Mail: [email protected]
Phone:
+49 (0)30 20 200 600
www.chemstations.eu
Authors:
Daniel Seidl
Meik Wusterhausen
Armin Fricke
Sources
focused on process simulation
Page 11 of 12
I. Ulrich, Jan
"Operation and Control of Azeotropic Distillation Column Sequences"
Diss. ETH No. 14890, Swiss Federal Institute of Technology, Zurich, 2002
II. De Villiers, French, Koplos
"Navigate Phase Equilibria via Residue Curve Maps"
2002, http://people.clarkson.edu/~wwilcox/Design/rescurve.pdf
(Accessed on 10.09.2013)
III. Stephan, Schaber, Stephan, Mayinger
"Thermodynamik"
Band 2 Mehrstoffsysteme und chemische Reaktionen, 15th edition, Springer
IV. Gmehling, Kolbe, Kleiber, Rarey
"Chemical Thermodynamics for Process Simulation"
2012, Wiley-VCH
focused on process simulation
Page 12 of 12
Download