HETEROGENEOUS AZEOTROPIC
DEHYDRATION OF ETHANOL TO OBTAIN
A CYCLOHEXANE-ETHANOL MIXTURE
Chemical Engineering Department
University of Alicante (Spain)
e-mail: [email protected]
Vicente Gomis
Mª Dolores Saquete
Alicia Font
Ricardo Pedraza
Victoria Pastor-Matea
OBJECTIVE
Study the viability of cyclohexane in
the ethanol dehydration to obtain an
ethanol + cyclohexane mixture from
an azeotropic distillation column.
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INTRODUCTION
Key renewable energy policy documents of the EU
1997 White Paper "Energy for the future"
•Doubling the share of renewable energy from 6%
(1997) to 12% (2010)
2003 EU Biofuels Directive (2003/30/EC)
•Target for biofuels in transport: 2% by 2005,
5.75% by 2010
2009 Directive „on the promotion of the use of
energy from renewable sources“ (2009/28/EC)
•Overall EU target : 20% renewable energy in
gross final energy consumption in 2020
•Target of 10% renewable energy in transport in
2020 for all member states
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INTRODUCTION
Ethanol Production
Sectored emissions in Europe
A griculture
8%
Waste
2%
Industry
8%
Benefits of biofuels
• Reduce GHG emissions
• Improve air quality
Energy
48%
• Reduce petroleum dependence
Transpo rt
34%
• Improve energy security
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INTRODUCTION
Ethanol Production
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INTRODUCTION
Ethanol dehydration
Pressure Swing Adsorption
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Azeotropic distillation
INTRODUCTION
Azeotropic distillation
TERNARY
AZEOTROPE (E)
C
0
1 00
Ethanol
D
G
(M)
FEED
A
25
75
(N)
(G)
50
50
Ethanol/Water (D)
+
Benzene (B)
25
75
ABSOLUTE
(C)
ALCOHOL
E
M
N
Heterogeneous region
0
100
Water
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B
100
75
50
25
0
Benzene
INTRODUCTION
Possible entrainers
Benzene (Young, 1902)
Acetone
Hexane
Heptane
Toluene
Isooctane
Cyclohexane
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Gasoline components
Pentane
INTRODUCTION
Conventional Process
Raw
materials
Ethanol
Production
Mixing
with
gasoline
Fuel
Proposed Process
Raw
materials
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Ethanol + Gasoline
Production
Fuel
INTRODUCTION
Cost diminution of:
- Mixing
Advantages
- Transportation
- Storage
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EXPERIMENTAL DESIGN
Study in an experimental
semi-pilot plant column
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Simulation of the industrial process
Semi-Pilot Plant Column study
Cyclohexane
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Semi-Pilot Plant Column study
Operation Variables
• Feed 1: pure cyclohexane. Temperature = 66 ± 1ºC
Flow rate = 41.00 g/min
• Feed 2: water + ethanol mixture (94% wt. of ethanol). Temperature: 63 ±1ºC
Flow rate = 4.38 g/min
• Condenser: Temperature = 35ºC
• Heat exchanger 3: Temperature of the stream leaving HE-3 = 66 ±1ºC
Simulation Variables
Simulated in Chemcad 6
Rigorous calculation using the SCDS module (simultaneous correction
method for rigorous fractionation simulation)
Thermodynamic model: UNIFAC
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Semi-Pilot Plant Column study
Bottoms Product
1.0
Weight Fraction
0.8
0.6
The trends
observed in the
experimental
results resemble
their simulated
counterparts
Ethanol Simulation
Cyclohexane Simulation
Cyclohexane
Ethanol
0.4
0.2
Optimal
0.0
0
50
100
150
200
250
Reboiler Heat Duty (W)
• The ethanol concentration depends on the heat duty
• Only values ranging from 80-100 W permit ethanol concentrations close to 5 % wt.
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Semi-Pilot Plant Column study
Bottoms Product
0.005
Water Simulation
Water
Weight Fraction
0.004
The trends
observed in the
experimental
results resemble
their simulated
counterparts
0.003
0.002
0.001
Too high
< 50ppm
0.000
0
50
100
150
200
250
Reboiler Heat Duty (W)
• The concentration of water in the residue stream does vary considerably with respect to
the reboiler heat duty
• As the heat duty increases, the concentration of the water gradually decreases, reaching
values lower than 50 ppm.
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Semi-Pilot Plant Column study
Aqueous phase
Water Simulation
Ethanol Simulation
Water Simulation
Water
Ethanol
Cyclohexane
1.0
Weight Fraction
0.8
0.6
0.4
0.2
0.0
0
50
100
150
Reboiler Heat Duty (W)
200
250
The simulation adequately
reproduces neither the flow
rate values of the bottom
product and aqueous layer
obtained experimentally nor
the composition of the
streams
• The composition of the aqueous layer is also dependent on the heat duty
• The composition tends to approach that of the plait point of the system.
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Semi-Pilot Plant Column study
Flows
50
Flow (g/min)
40
30
Simulation
Aqueous decant
Bottoms Product
20
10
0
0
50
100
150
Reboiler Heat Duty (W)
200
250
The simulation
adequately reproduces
neither the flow rate
values of the bottom
product and aqueous
layer obtained
experimentally nor the
composition of the
streams
• The flow rate of the residue is always higher than that of the aqueous layer
• Both flow rates become more similar when the reboiler heat duty increases.
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Semi-Pilot Plant Column study
0
10
0
Ethanol
UNIFAC non isothermal binodal curve
Experimental non isothermal binodal curve
75
25
50
50
75
25
UNIFAC phase split prediction
Plait Point
0
0
10
Water 100
75
50
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25
0
Cyclohexane
Semi-Pilot Plant Column study
0
100
Etanol
Experimental
UNIFAC
UNIFAC Dortmund
UNIFAC LLE
75
25
50
50
25
75
0
0
10
Agua
100
75
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50
25
0
Ciclohexano
Semi-Pilot Plant Column study
0
10
0
Etanol
Experimental
UNIQUAC
NRTL α constante
NRTL α variable
75
25
50
50
25
75
0
0
10
Agua
100
75
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50
25
0
Ciclohexano
CONCLUSIONS
It is possible, through azeotropic distillation, to obtain a mixture of cyclohexane
+ ethanol with concentrations of water lower than 50 ppm without the need
to distill absolute ethanol beforehand. Afterward, the mixture could be directly
employed as a carburant in car engines with no further modifications.
The most critical parameter of the process is the reboiler heat duty. At lower
values, this produces a mixture of cyclohexane + ethanol with excessive
amounts of water. Whereas, at higher values the azeotropic distillation column
does not work properly, since the top stream condenses giving only one liquid
phase.
Significant differences in some values are encountered between experimental
and simulated data which can be attributed to the calculation of the liquid-liquid
equilibrium. It is therefore necessary to improve the correlation of the
experimental equilibrium data for determined regions of the ternary system
diagram.
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CONCLUSIONS
The production of dry mixture of
ethanol + cyclohexane seems to be
technically and economically viable
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