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Journal of Environmental Science and Health . Part A: Environmental Science and Engineering and Toxicology: Toxic/ Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa19

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

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