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Anodic oxidation of methanol in a circulating flow batch reactor G.A. Al‐Enezi
a
a
Department of Chemical Engineering , University of Kuwait , P.O. Box 5969, Safat, 13060, Kuwait Published online: 15 Dec 2008.
To cite this article: G.A. Al‐Enezi (1990) Anodic oxidation of methanol in a circulating flow batch reactor, Journal of Environmental Science and Health . Part A: Environmental Science and Engineering and Toxicology: Toxic/ Hazardous Substances and Environmental Engineering, 25:1, 67-79, DOI: 10.1080/10934529009375540 To link to this article: http://dx.doi.org/10.1080/10934529009375540
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J . ENVIRON. SCI. HEALTH, A25(l), 67-79 (1990)
ANODIC OXIDATION OF METHANOL IN A CIRCULATING
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FLOW BATCH REACTOR
G.A. A l - E n e z i Department of Chemical Engineering University of Kuwait, P.O. Box 5969 13060 Safat - Kuwait
Abstract: The anodic oxidation of methanol in a circulating flow reactor
has
ditions
required
The
been
effects
of
studied
to determine
for a continuous applied
current,
experimental
fluidized
bed
initial
con-
reactor.
alcohol
con-
centration, and circulating flow rate on methanol conversion have been
investigated.
The
rate of methanol
conversion
increased with increasing applied current and decreased as initial alcohol concentration is increased.
A relationship
between methanol conversion and a combined effect of applied current and initial alcohol concentration has been derived. The
kinetics
second order.
of
the
reaction
has
been
found
to be of a
Values of up to 85% methanol conversion have
been achieved.
67 Copyright © 1990 by Marcel Dekker, Inc.
68
AL-ENEZI
Introduction: The anodic oxidation technique has been used with impressive success
for the removal of ethanol, ammonia, phenols, and
many other pollutants found in wastewater streams
.
The
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oxidation process can reduce unwanted organic or inorganic compounds to less toxic or harmless compounds. anode materials have been suggested.
Different
Lead, graphite, and
platinized titanium have been found to be the most effective anode materials . Several studies have been published on the anodic oxidation of methanol in acid electrolyte. studied
Faroque and Fahidy
the reaction products or reaction mechanisms. They
used a rotating tripolar wiper blade cell and they have presented
a plausible mechanism of the reactivated
reaction.
oxidation
They proposed the following two-step mechanism:
Step 1: Adsorption of Methanol CH3OH
< C H 3 0 H >ads
-
Step 2: Reaction with Surface Oxide ( C H 3 O H ) a d s + 3P t 0
+
3P t + C O 2 + 2H 2 O
where Step 2, which may be chemical or electrochemical or both in nature, is much taster compared to Step 1. al."
have
oxidation solution. detected
studied of
the
methanol
reaction on
a
Pt/ p t
products
of
electrode
Ota, et
the in
1M
anodic H2SO4
They reported that small amounts of HCOOCH3 were besides
CO2/
HCOOH
and
HCHO.
They
also
have
ANODIC OXIDATION OF METHANOL concluded the
69
that HCOOH and HCHO were produced mainly during
initial
period
of
the
reaction
and
their
production
rates in the initial 50 minutes increased when CH3OH concentration
increased, while
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significantly
depend
on
CO2 production
the
CH3OH
rates
concentration.
did
not
These
results are used in the definition of methanol conversion in this study. It is the objective of the current paper to present an experimental study of the anodic oxidation of methanol in a circulating batch reactor and to determine experimental conditions
required
fluidized out
under
for
methanol
bed reactor. different
a
continuous
The experimental study
is carried
reaction
oxidation
conditions
in
such
as
initial
alcohol concentration, applied current, and circulating flow rate.
To find the order of the reaction, kinetics of the
reaction process is also studied. nol
to formic acid
The conversion of metha-
is defined according
to the following
simplified equation:
„ . „ acid produced [HCOOH] Conversion - initial alcohol concentration or
X - £3.
(1)
Co Experimental: Figure 1 shows a simplified diagram of the experimental set-up used in this study.
The electrochemical reactor is
70
AL-ENEZI Regulated D.C. Power Supply
Lead Anode
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Overflow
_L
VC Porous Dlaphram
Lead
Storage Tank
Stainless Steel Cathode
Spheres or Sheet Catholyte
Plastic Screen
•si
Pump
Fig. 1. Schematic diagram of the experimental set-up for the anodic oxidation of methanol.
divided The
into two compartments
anode
compartment
by a PVC porous
contained
the
diaphragm.
circulated
alcohol
(0.75-32 g/jt) in a sodium sulfate solution (10 g/l) .
The
anode is made of a lead sheet covered by an electrochemically deposited thin layer of lead dioxide. area
is used.
sheet.
The cathode
A 200 cm
anode
is made of a stainless steel
At the beginning, the solution is circulated for a
short period o£ time to insure complete mixing.
Power is
then
required
switched
current (1-4A).
on
and
is adjusted
to supply
the
During the one hour duration time for each
experimental run, samples are taken for analysis.
Titration
AKODIC OXIDATION OF METHANOL
71
with a standard sodium hydroxide solution is used to determine the concentration of the acid produced in each run. Results and Discussion;
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Anodic oxidation technique is used for the conversion of methanol in the presence of sodium sulfate.
Several oxida-
tion runs are conducted and the results are summarized and shown in figures for simplicity. Applied Current: The effect of applied current on methanol conversion is shown in Figure 2, where conversion is plotted versus time. It is clear from Figure 2 that % conversion to acid at a given
time
increased
as
the
applied
current
increased.
Methanol conversion of about 85% has been achieved at the following conditions: applied current 4A, 0.79 g/l initial alcohol concentration, 2.4 i/min circulating flow rate, and one hour experimental time. the
rate
of
methanol
It is observed, however, that
conversion
is
proportional
to
the
applied current and is doubled as the current is doubled, i.e., in the ranges O-1A and 3-4A, the effect on conversion were predominant and correspond about 27, while
in the ranges
to a conversion change of 1-2A and 2-3A, the rate of
conversion was almost constant and corresponds to a conversion change of about 13. Initial Alcohol Concentration: Figure
3 shows
the % conversion
versus time for five
initial methanol concentrations at a fixed applied current
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100:
Initial concentration
=0.79
g/l.
Circulating flow rate
= 2.4
l/min
80
30
40
50
60
Time > min. Fig. 2. Effect of applied current on methanol conversion.
70
ANODIC OXIDATION OF METHANOL
73
of 2A and a circulating flow rate 2.4 n/min.
As expected,
the % conversion at a given time increased as the initial alcohol
concentration
decreased.
These
results
favour of the local conditions where methanol
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small concentrations
are
in
is found in
in wastewater streams . These
results
are also in agreement with the results obtained by Abdo et al.
fori the
anodic oxidation of ethyl alcohol
same apparatus that is used in this study.
using
the
At higher con-
centrations, the rate of methanol conversion is almost flat. About 43% conversion is achieved at an initial alcohol concentration of 0.79 g/J. and one-hour methanol
conversion
to
acid
is
time.
From Figure 3,
easily
fitted
by
the
following relation:
X
=
a tb
(2)
Values of the constants a and b at a circulating flow rate of 2.4 £/min and four different alcohol concentrations and applied currents are listed in Table I. Combined
Effect
of
Applied
Current
and
Initial
Alcohol
Concentration: Initial Alcohol concentration and applied current effect
are on
found
the
relationship
to
be
conversion
has
been
the of
variables with methanol, and
approximated
at
a
the the
greatest following
circulating
flow
rate of 2.4 e/min and one-hour experimental time: 0.85 X
=
15.93 (-i-) c o
(3)
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60 Circulating flow rate = 2.4 1 /min Applied current
=2 A
50 0.79 g/l 40
c 2
30
o
20
,
3.2 g/l
10 16 g / l 32 g / l
10
30
40
50
60
70
Time/ min
s Fig. 3. Effect of initial alcohol concentration on conversion
PI to
ANODIC OXIDATION OF METHANOL
75 TABLE I
Values of the Constants a and b defined by Equation (2)
Concentration q/1
a
1
0.79
2.2210
0.663
1
16.00
0.0674
0.767
2
0.79
3.8010
0.561
2
1.60
1.9950
0.605
2
3.20
0.5831
0.763
2
16.00
0.1998
0.605
2
32.00
0.0466
0.809
3
0.79
1.4450
0.883
3
16.00
0.2025
0.656
4
0.79
8.3330
0.553
4
16.00
0.1679
0.768
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Current (A)
b
Circulating Flow Rate; The effect of increasing the flow rate on the % conversion
has
centration
been of
investigated
at
an
initial
16 g/l, and an applied
alcohol
current of
con-
2A, and
different flow rates of 1.5, 1.92, 2.4, and 2.76 l/min.
The
% conversion has been found to decrease slightly as the flow rate increased, (i.e., % conversion the circulating
is a weak function of
flow rate at least, in the range of the
experimental conditions used in this study).
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I n i t i a l c o n c e n t r a t i o n =0.79
10
g/l
30
40
50
60
70
Time. min. Fig. 4. Test of second order kinetics for the anodic oxidation of alcohol.
i zw PI
ANODIC OXIDATION OF METHANOL
77 TABLE II
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Rate Constant of Methanol Reaction
KxlO 5
I
Co
1
0.79
500
1
16.00
0.985
2
0.79
844
2
1.60
191
2
16.00
1.60
2
32.00
0.411
3
0.79
1390
3
16.00
1.880
4
0.79
2780
4
16.00
2.630
Reaction Kinetics; The kinetics o£ the anodic oxidation reaction has been studied and is shown in Figure 4. the
reaction
It has been found that
is a second order reaction according
to the
following relationship:
I =
L+kt
(4)
Values of the rate constant k at different conditions are
78
AL-ENEZI
given in Table II.
From Table II, it is clear that the rate
constant is a function of the applied current and the initial alcohol concentration.
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Conclusion: 1.
Up to 85% conversion of methanol to formic acid can be achieved
successfully
by using
anodic oxidation
tech-
nique. 2.
The
reaction o£ methanol oxidation
is a second
order
reaction with the applied current and the initial alcohol concentration being the most affecting variables on such a mechanism. 3.
Flow rate has a negligible effect on methanol conversion in the range of the conditions used in this study.
4.
The usefulness of the anodic oxidation process for the conversion
of
methyl
alcohol
in
a
circulating
batch
reactor will be extended in a continuous fluidized bed reactor. Acknowledgement: The author wishes to thank A. Al-Shuwai, A. Al-Beteery, and N. Ahmed for their help in carrying out some experiments in
this
work.
The
author
would
also
like
to
thank
Dr. M.S.E. Abdo for his helpful discussion during the initial stages of
this research.
work by Kuwait University
Financial Support of this
Research Council
019) is gratefully acknowledged.
(Grant No. EDC
ANODIC OXIDATION OF METHANOL
79
Nomenclature;
C
=
Alcohol concentration r q/l
K = Rate constant l/g.min
Ca =
Acid produced, g/i
t = Time min
Co =
Initial alcohol concen-
X = % conversion defined
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tration, g/i I
=
by Equation
(1)
Applied current, A
References:
1.
Abdo, M.S.E., Al-Enezi, G.A., J.
2.
of
Environmental
Al-Haddad,
A.,
and
Science
and Al-Haddad, A.: and
Health,
Abdo, M.S.E.:
A21(6),
1986, World
1986, 487.
Congress
III of Chemical Eng., Tokyo, Japan. 3. 4.
Al-Enezi, G.A.,
1989, Working
Paper, Kuwait
University.
Abdo, M . S . E . , and Al-Enezi, G.A.: 1989, Paper accepted at the 10th European Conference on Environmental
Pollution,
Capri, Italy. 5.
De Sucre, V . S . , and Watkinson, A.P.: 1981, Cand. J. Chem. Eng., 59.
6.
Dart,
M.C.:
1963,
Appl.
Chemi,
13.
7. Faroque, H., and Fahidy, T.Z.: 1979, Electrochemica Acta, 24. 8.
Ota,
K.,
Nakagawa,
Y.,
and
Takahashi,
M.:
1984,
J.
Electro anal. Chem., 179. 9.
Shaban, H . I . , Al-Enezi, G.A., and Qader, A.: 1988, J. of Environmental Science and Health, A 2 3 ( 8 ) , 843.
Date Received: Date Accepted:
03/09/89 09/11/89