IJCPE Vol.9 No. 2 (June 2008)

Iraqi Journal of Chemical and Petroleum Engineering

Vol.9 No.2 (June 2008) 17-23
ISSN: 1997-4884

Kinetic Study on Catalytic Wet Air Oxidation of Phenol in a Trickle Bed
Reactor

Wadood T. Mohammed
*
, Sama M. Abdullah

*’ **
Chemical Engineering Department - College of Engineering - University of Baghdad – Iraq

Abstract

Kinetics study on the phenol oxidation by catalytic wet air oxidation (CWAO) using CuO.NiO/Al2O3 as heterogeneous

catalyst is presented. 4 g/l phenol solution of pH 7.3 was oxidized in a trickle bed reactor with gas flow rate of 80%

stochiometric excess (S.E).. In order to verify the proposed kinetics, a series of CWAO experimental tests were done at

two temperatures (140 and 160° C), oxygen partial pressures (9 and 12 bar), and weight hourly space velocity (WHSV)

(1, 1.5, 2, 2.5, and 3 h
-1

). According to Power Law, the reaction orders are found to be approximately 1 and 0.5 with

respect to phenol concentration and oxygen solubility, respectively. These values favorably compare with those cited in

the literature for intrinsic kinetics, which indicates minimal mass transfer limitations in the trickle bed reacting system

used in this study.

Introduction

The petrochemical, chemical and pharmaceutical

industries produce waste waters containing organics, such

as phenols, which are extremely toxic to aquatic life [1].

The phenol and derivatives are generally toxic even at

very low concentrations [2]. In general, these pollutions

exist in concentration range of 500-10000 ppm, mostly

too toxic for conventional biotreatment methods, and too

low to treat by combustion [3]. Thus, chemical oxidation

emerges as a promising route for phenol removal at

intermediate concentrations [4]. Wet air oxidation

(WAO) is considered an emergent technology that

economically can depollute organic wastewaters in order

to meet the progressively more stringent environmental

regulations [5]. Normally a typical WAO process requires

elevated pressure (0.5-20 Mpa) and temperature (125-320

°C) in order to enhance the solubility of oxygen in

aqueous solution. In reality such requirements will

inevitably lead to higher equipment and operational costs

(6). Thus, CWO appears as an economically and

ecologically promising technique to convert refractory

organic compounds, such as phenol, into carbon dioxide

or harmless intermediates at mild pressure and

temperature conditions (7).

This work deals with kinetics study of the

catalytic phenol oxidation in aqueous phase using fixed

bed reactor working in trickle flow regime. Air was used

as oxidizing agent. Copper based catalyst supported on γ-

alumina was employed. The detailed examination of the

product dependence on the space time is provided in the

temperatures 140 and 160 °C, and between 9 and 12 bar

of oxygen partial pressure.

Theory

The ideal plug flow pseudo-homogeneous model

presented in the equation 1 that proposed by Froment and

Bkchoff (1990) was modified to describe the reactor in

terms of space time, instead of reactor length.

University of Baghdad

College of Engineering
Iraqi Journal of Chemical

and Petroleum Engineering

Kinetics Study on Catalytic Wet Air Oxidation of Phenol in a Trickle Bed Reactor

20
IJCPE Vol.9 No. 2 (June 2008)

 
0.

.


phb

phl
R

CUd


(1)

ph
R

d C





(2)

The first step is to consider only the phenol degradation

reaction described by equation 3

C6H6O + 7O2 → 6CO2 + 3H2O (3)

In agreement with observations in the literature (8, 9, 5,

10, 2, 11, 12), a simple power law was convenient to

accurately describe the phenol oxidation. Thus, the

following rate equation for phenol destruction was used:


phobph

CKr .
(4)


 2

..
O

ob
P

RT
kK 







 


(5)

phph
rR 

(6)




phob

ph
CK

dC
.

(7)

Equation 4 can be line raised in the following way:

   
phobph

LogCKLogrLog 
(8)

In the present study, the above expression for ob
K

was

modified to incorporate the oxygen mole fraction in the

liquid phase, 2O
X

instead of the partial pressure, leading

to:


 2

..
O

ob
X

E
kK 










(9)

i.e.


 phO

ph
CX

E
kr ...

2










(10)

This was done because the reaction actually takes place

in the liquid phase. Thus, the solubility of oxygen

characterizes the oxygen contribution to the kinetic

expression rather than the oxygen partial pressure.

Furthermore, the oxygen solubility is not only a function

of pressure but also of temperature. Therefore, the

oxygen mole fraction in the liquid phase was considered

to be more representive. This mole fraction was

calculated using Henry law
(13)

, Henry law is given by

equation 11.

222
.

OOO
XHP  (11)

Experimental Work

Material and catalyst preparation

The phenol used as reagent was purchased from Griffin.

High purity synthetic air was used as oxidant. Deionized

water was used to prepare catalyst and different aqueous

solutions. Glass balls were used in the tests with inert

material. γ-alumina was used as support for the copper.

Copper nitrate from Fluka was used as active component.

Nickel nitrate from BDH Chemicals Ltd. was used as

promoter.

Copper-based catalyst was prepared using γ-

alumina as support. The alumina, which was supplied as

spheres of 2.5 mm diameter, was dried for 4 h at 110°C.

The catalyst was made with a copper oxide loading of

10% and 2% nickel oxide prepared by the pore volume

impregnation method using aqueous solutions of copper

nitrate (26 g copper nitrate and 6.3 g nickel nitrate

dissolved in 45 ml hot deionized water) for impregnating

the support. Later, the catalyst was dried at 110°C

overnight, followed by calcining at 400°C for 8 h with

air. Table 1 lists the main physical characteristics of the

catalyst prepared and support used which were done in

the State Company of Geological Survey and Mining.

Fig. 1: Experimental setup

Wadood T. Mohammed and Sama M. Abdullah

21
IJCPE Vol.9 No. 2 (June 2008)

Table 1 The main physical characteristics of different

catalysts prepared and supports used.

Experimental set-up and procedure

The continuous oxidation of phenol was carried out in a

packed bed reactor. The fixed bed reactor consists of a

SS-316 tubular reactor, 80 cm long and 1.9 cm inner

diameter and controlled automatically by for sections of

15 cm height steel-jacket heaters. Independent inlet

systems for gas and liquid feed allow working at various

liquid to gas flow rate ratios.

The liquid feed is stored in a feed tank, which is

connected to a high-pressure metering pump (dosing

pump) that can dispense flow rates between 0 and 15

ml/min at constant pressure. The air oxidant comes from

a high pressure cylinder equipped with a pressure

controller to maintain the operating pressure constant. A

flow-meter coupled with a high precision valve is used to

measure and control the gas flow rate. The liquid and gas

streams are mixed and then entered to the reactor at the

required temperature. The mixture flows along the bed

packed with 85 cm3 (30 cm height) of the catalyst

enclosed between two layers of inert material (also a

flexible grid put at the top and bottom of the reactor to

prevent movement of particles).The exited solution goes

to a liquid-gas separation and sampling system, regularly,

liquid samples were withdrawn for analysis. Figure 1

illustrated the experimental setup.

To verify that only the catalyst causes the oxidation of the

phenol, test was made using an inert material (γ-alumina).

The phenol removal was negligible, less than 0.1 %,

which falls within the experimental error.

PH-adjustments

The pH of 4 g/l phenol solution is slightly acid, about 5.9.

However, for phenol solution of pH 7.3, the feed solution

was adjusted by adding sodium hydroxide solution. To

measure pH of the solution OAKION PH2100 Series was

used. The procedure was summarized as following:

pH meter was calibrated previous to use by using Buffer

solutions.

Measuring the pH of 4 g/l phenol solution.

Adding particular quantities of NaOH solution to the 4 g/l

phenol solution according to the titration method to

obtain solution of pH 7.3.

Products analysis

To analysis phenol concentration in the outlet samples,

Shimadzu model UV-160A ultraviolet/visible

spectrophotometer was used.

Estimation of reaction orders

According to the differential method, the derivatives

d

dC
ph

are evaluated from the experimental data to obtain

the reaction rate ( ph
r

) as shown in the figure 2. Then,

 
ph

rLog 
is plotted against

 
ph

CLog
in equation 8

as shown in the figure 3, to determine values

of and ob
K

. It should be noted that all above

calculations were done using computer. To calculate the

other unknown kinetic parameters 
k

, ob
E

and


equation 9 can be line raised in the following way:

E
XkK

Oob


2
lnlnln 

(12)

Alternative steps were done using different values of

ob
K

available at different temperatures and pressures to

evaluate these parameters.

Catalyst γ-Al2O3 Cat.

Active phase (CuO), % - 10

Promoter (NiO), % - 2

Support - γ-Al2O3

Calcinations temperature,
o
C - 400

Pore volume, cm
3
/g 0.44 0.352

Bulk density, g/cm
3
0.68 0.748

Surface area, m
2
/g 289 237

Kinetics Study on Catalytic Wet Air Oxidation of Phenol in a Trickle Bed Reactor

22
IJCPE Vol.9 No. 2 (June 2008)

-10

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

Space time, h

C
p

h
,m

m
o

l/
l

Results and Discussion

A first reaction order for phenol concentration given by

equation 10 was found, in agreement with findings of the

other studies conducted in slurry reactor (14, 15) or

trickle bed reactor (TBR) (10, 16, 11, 12).

A 0.5 oxygen order was found on reaction rate as found

by (14, 15, 10, 11, 12). This results explained that as

disscociative oxygen adsorption is an elementary step

during CWAO of phenol over copper oxide catalyst and

reported 0.5 oxygen order (15), while reported 0.5

(  0.1)(9).
The observed activation energy for phenol destruction

over CuO.NiO.Al2O3 was found to be 78.5 kJ/mol. Falls

in the range of the 85(  2) kJ/mol (9) and 77.1(  4)
kJ/mol (10) in the same TBR using the Cu0803 Catalyst,

while for stirred slurry reactors intrinsic kinetic values of

85 kJ/mol (17) and 84 kJ/mol (15) for different copper

oxide catalysts with similar characteristics. Considerably,

the oxidation reactions of phenol to 4-HBA (equation 13)

and p-benzoquinone (equation 14) have activation

energies of 82.4 and 72 kJ/mol were obtained in the TBR

over active carbon as catalyst (12)

C6H6O + CO2 → C7H6O3 13

C6H6O + O2 → C6H4O2 + H2O 14

It is well known that the big gas/liquid ratio employed by

this type of reactor permits better contact between the gas

and liquid phases thus improving the mass transfer

between phases and suggesting that the mass transfer

limitation can indeed be neglected.

The frequency factor was found 3.2×1011.36 L / kg

Cat. h close to the values 1011 L/kg Cat. h (9), 1011.36

L/kg Cat. h (10) and 1014.35 and 1013.66 respectively

(12).

When those results are compared with the once in this

study, it can be seen that the kinetic parameters are

slightly different or closed to the values given in the

literature. The difference in the results may arise from the

reaction conditions and the type of catalyst used. Finally

the reaction rate equation is:

5.011

2
.

5.78538
.102.3

Ophph
XC

RT
r 








 15

Fig.2: Concentration of phenol versus space time at

various conditions.

Reaction conditions: type of catalyst= Cat.4, feed

solution pH=7.3, S.E. = 80%, and initial phenol

concentration= 4 g/l.

Fig. 3: Log (-rph) versus Log Cph at various conditions

140° C and 9 bar
160°C and 9 bar

160° C and 12 bar

Best Fit

0.5

1.5

2.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Log Cph

L
o

g
(-

r p
h
)

140° C and 9 bar
160°C and 9 bar

160° C and 12 bar

Best Fit

Wadood T. Mohammed and Sama M. Abdullah

23
IJCPE Vol.9 No. 2 (June 2008)

CONCLUSIONS

The experimental results of CWAO of phenol showed

that the oxidation reaction of phenol is first order with

respect to phenol concentration and o.5 order with respect

to oxygen solubility, observed activation energy equal to

78.5 kJ/mol and pre-exponential factor equal to 3.2

×1011 L/kg Cat. h.

Nomenclature

Cph : Concentration of phenol mmol /l

Ea : Activation energy J / mol

H : Henry constant

ko : Frequency factor (case dependent units)

Kob : Observed rate constant (case dependent

units)

2O
P

: Oxygen partial pressure bar

R : Universal gas constant, 8.314 J / mol. K

rph : Phenol reaction rate mol / kg. s

T : Temperature K

2
O

x
: Mole-fraction of oxygen

z : Reactor length (case dependent units)

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α Oxygen order

β Organic order

Ρb Bed density kg / m
3

τ Space time h