Catalytic effect of active carbon black Chezacarb in wet oxidation of phenol

Article abstract of DOI:10.1135/cccc19961010

The catalytic effects of hydrophilic granulated active carbon black Chezacarb have been tested in the oxidation of dilute (5 000 ppm) aqueous solutions of phenol. The research was focused on combined utilization of sorption and chemical (oxygen functional groups) properties of carbon black which support formation of free radicals in oxidation and on the catalytic effects of metals (Fe, Ni and V) present in the sorbent mentioned. The oxidation was carried out in a stainless steel autoclave of 1.25 1 capacity. The reaction temperatures and the total pressures varied in the intervals of 120-160 °C and 3-5 MPa, respectively. The reaction mixture was analyzed by means of HPLC with UV detection (254 nm). The experimental data were evaluated by means of the ASPEN PLUS 9.2 program. The model of laboratory autoclave involved the reaction kinetics as well as the phase equilibria and dissolution of oxygen and carbon dioxide in the reaction mixture. Although the active carbon black shows a lower catalytic activity than the usual copper catalyst in the wet oxidation, it has the advantage of avoiding the undesirable releases of heavy metals in waste waters, which escapes accompany the application of copper catalysts.

Full text of DOI:10.1135/cccc19961010

1010
Tukac, Hanika:
CATALYTIC EFFECT OF ACTIVE CARBON BLACK CHEZACARB IN WET
OXIDATION OF PHENOL
1
2
Vratislav TUKAC and Jiri HANIKA
Department of Organic Technology, Prague Institute of Chemical Technology, 166 28 Prague 6,
Czech Republic; e-mail: 1 vratislav.tukac@vscht.cz, 2 jiri.hanika@vscht.cz
Received January 3, 1996
Accepted March 14, 1996
The catalytic effects of hydrophilic granulated active carbon black Chezacarb have been tested in the
oxidation of dilute (5 000 ppm) aqueous solutions of phenol. The research was focused on combined
utilization of sorption and chemical (oxygen functional groups) properties of carbon black which sup-
port formation of free radicals in oxidation and on the catalytic effects of metals (Fe, Ni and V)
present in the sorbent mentioned. The oxidation was carried out in a stainless steel autoclave of 1.25 l
capacity. The reaction temperatures and the total pressures varied in the intervals of 120–160 °C and
3–5 MPa, respectively. The reaction mixture was analyzed by means of HPLC with UV detection
(254 nm). The experimental data were evaluated by means of the ASPEN PLUS 9.2 program. The
model of laboratory autoclave involved the reaction kinetics as well as the phase equilibria and dis-
solution of oxygen and carbon dioxide in the reaction mixture. Although the active carbon black
shows a lower catalytic activity than the usual copper catalyst in the wet oxidation, it has the advantage
of avoiding the undesirable releases of heavy metals in waste waters, which escapes accompany the
application of copper catalysts.
Key words: Catalytic oxidation; Phenol in waste waters; Active carbon black Chezacarb.
Toxic or not readily biodegradable substances are often removed from waste waters by
sorption on active charcoal which is then regenerated or combusted. An alternative
procedure consists in the so-called wet oxidation of solutions of these substances at
enhanced temperatures and pressures giving biologically unobjectionable products or –
in the end – even carbon dioxide and water1. As the wet oxidation is also adopted as a
method for regeneration of sorbents based on charcoal, there arises an idea of application
of potential catalytic effects of these materials in oxidation reactions. The active carbon
black Chezacarb formed in the production of hydrogen by partial oxidation of crude oil
raw materials contains active groups at its surface (–SH, –OH, –OOH, –COOH) which
can support free radical reactions and, in addition to that, the presence of iron, nickel,
and vanadium oxides2 in the original crude oil enables catalytic effects.
The purpose of wet oxidation is liquidation of toxic and not readily biodegradable
substances in waste waters particularly those from chemical and pharmaceutical indus-
tries. The most frequently used typical model compound is phenol1 although the process
itself is designed for chloro-, nitro- (ref.3), sulfo-, and other derivatives of phenols and
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Catalytic Effect of Active Carbon Black
1011
aromatic hydroxy compounds4. The energy requirements of wet oxidation and the over-
all rate of the process can be improved by using a suitable catalyst. The catalysts most
frequently adopted involve the metals: Cu, Fe, Mn, Co, Zn, Ru, and Ce in the form of
ions, homogeneous complexes, or coatings on heterogeneous supports1,3–5. The proce-
dure is suitable for waste waters with the COD concentration interval of 15–150 kg/m3.
The problems are concentrated around the considerable energy demands of noncatalytic
processes, stability, and activity of potential catalysts, and corrosion of apparatus by
atomic oxygen and acid reaction intermediates4,5
.
The program packet ASPEN PLUS (ref.6) represents a sophisticated simulator of
technological operations and processes involving unit chemical-engineering operations
inclusive of separations, mass transport, and models of basic chemical reactors. How-
ever, the basic program is limited to the simulation of steady states of systems, where-
from follows its limited applicability. The program includes an extensive data base of
physico-chemical properties of pure compounds as well as polycomponent mixtures.
The involved optimization and regression procedures make it possible, e.g., to identify
the parameters of the kinetic equations chosen. The input of starting data is controlled
by an expert system.
The aim of the present work was to verify the presumption of catalytic processes of
the Chezacarb sorbent in the oxidation of phenol, determine the effects of reaction
conditions on the reaction rate, and make use of the simulation program ASPEN PLUS
for modelling a laboratory autoclave and identifying the rate constants.
EXPERIMENTAL
The chemicals used were of p.a. purity grade and were obtained from the following suppliers: phenol
and methanol (Penta Prague), distilled water (Prague Institute of Chemical Technology), and oxygen
(Technoplyn Prague). The catalyst used was active carbon black Chezacarb XE2 SH (Chemopetrol
Group, s.a. Litvinov), brittle granules of 0.5 mm diameter, the surface area of 950 m2/g, the bulk
weight of 125 g/l. The catalytic effect is enabled by the presence of metals: 0.08% V, 0.25% Ni, and
0.16% Fe. The active carbon black Chemviron Carbon WS4AWD (Germany) extruded (3.8 × 5 mm
pieces) and ground to powder of 0.25 mm grain size, specific surface area 1 049 m2/g.
The process was studied in a laboratory stainless steel autoclave with a magnetic stirrer with hor-
izontal partitions (Hoffer type) of 1.25 l capacity. The total pressure was maintained in the interval
of 3–5 MPa with the accuracy of 5%, the reaction temperature was controlled electronically in the
interval of 110–160 °C with the accuracy of 2 °C. The catalyst amounts varied in the interval of 0.1–1 g per
0.8 l reaction mixture. The initial concentration of phenol in water was 5 000 ppm.
The reaction mixtures were analyzed by means of HPLC with UV detector at 254 nm using Separon
SGX C18 7 µm and 40% aqueous methanol as the mobile phase; flow rate 0.6 ml/min. The identified
components involved phenol, hydroquinone, benzoquinone, maleic and oxalic acids; at the concentra-
tions above ca 20 mmol/l the acids could not easily be separated and were determined together as
dicarboxylic acids.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

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Tukac, Hanika:
CALCULATIONS
For the calculations it was necessary to define the parameters of the laboratory auto-
clave used, the stoichiometry of reactions and their main components, and assess the
form of the rate laws.
For the model of laboratory autoclave we adopted a model of batch reactor enabling
the calculation to involve also the phase equilibria between the liquid and vapour
phases of water, phenol, and other substances, as well as dissolution of oxygen in the
reaction mixture. The reactor parameters were chosen as follows: isothermal batch re-
actor at 141.5 °C, initial pressure of 5 MPa, and 1 g catalyst.
The complex reaction system involved a great number of parallel consecutive reac-
tions. For the calculation purposes, this number was limited to the reactions that could
be monitored analytically. The reaction scheme was simplified to a set of five consecu-
tive steps with the following products: hydroquinone, benzoquinone, maleic acid, oxalic
acid, carbon dioxide, water.
C6H6O + 1/2 O2 = C6H6O2
C6H6O2 + 1/2 O2 = C6H4O2 + H2O
C6H4O2 + H2O + 5/2 O2 = C4H4O4 + C2H2O4
C4H4O4 + 2 O2 = 2 C2H2O4
(1)
(2)
(3)
(4)
(5)
C2H2O4 + 1/2 O2 = 2 CO2 + H2O
The physico-chemical properties (e.g. solubility, volatility, and thermodynamic quan-
tities) of the reaction components were contained in the build-in data base.
For the kinetic description of reactions we chose the power rate laws with unit reac-
tion orders in the substrates and order 1/2 in oxygen.
RESULTS AND DISCUSSION
The catalytic action of the Chezacarb sorbent in the oxidation of dilute phenol solutions was
verified at not much demanding reaction conditions, viz. the temperature about 140 °C
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Catalytic Effect of Active Carbon Black
1013
and pressure 5 MPa. If the oxidation reaction did not occur and only sorption took
place, neither the reaction intermediates nor even the relatively stable dicarboxylic
acids would be observed. Figure 1 shows a typical time course of concentrations of
reaction products. The dicarboxylic acids (maleic and oxalic) are presented together
since it was difficult to separate them analytically. The concentration of benzoquinone
was very low, remaining below 1.3 mmol/l at its maximum.
The effect of temperature on the course of oxidation reactions is depicted in the
following figures. Figure 2 shows the dependence of phenol conversion upon the reac-
tion time at various temperatures. From the graph it can be seen that temperature affects
very strongly the length of induction period. At a low temperature, 120 °C, the oxida-
tion is very slow; the temperature of 140 °C represents a certain limit above which the
oxidation is much faster and probably involves a different oxidation mechanism (not a
radical one) since at 160 °C there is no induction period and the form of dependence of
phenol conversion corresponds to the zero order of reaction.
Similar conclusions can also be made for the oxidation of the relatively most stable
intermediates – dicarboxylic acids, maleic and oxalic – as it can be seen from Fig. 3.
The concentration profiles given for two reaction temperatures indicate a strong tem-
perature dependence of oxidation rate of the acids. At the lowest temperature, 120 °C,
the acids practically are not formed, their concentration remaining below 1.35 mmol/l;
at 140 °C there is an initial pause followed by a steep increase in their concentration;
and only at 160 °C the concentrations of acids show the maximum which is followed
by their decomposition to carbon dioxide and water.
1.0
60
3
x
c
0.8
3
40
20
0
0.6
0.4
2
0.2
0.0
1
1
2
0
40
80
120
160
200
0
40
80
120
160
200
t, min
t, min
FIG. 1
FIG. 2
Concentration profiles (c, mmol/l) of the reac-
tion components: 1 phenol, 2 hydroquinone,
3 dicarboxylic acids (0.5 g catalyst, 5 MPa,
141.5 °C)
Profiles of conversion of phenol at various tem-
peratures: 1 121 °C, 2 144 °C, 3 163 °C (0.5 g
catalyst, 5 MPa)
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

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Tukac, Hanika:
The concentration increase of the acids is accompanied by increasing acidity of the
reaction mixture which presents considerable corrosion problems in the reactor and
apparatus. The corresponding pH profiles are given in Fig. 4. Only for the temperature
of 160 °C, the concentration profile of acids in Fig. 3 shows a maximum and the corre-
sponding pH profile in Fig. 4 shows a minimum.
The effect of initial pH value on the oxidation course of the dicarboxylic acids is
represented in Fig. 5. The initial pH value was adjusted at 10 (by adding sodium hy-
droxide) or 2 (by adding sulfuric acid). In the case of alkaline medium we can see a
steep initial increase in concentration of the acids, corresponding approximately to the
initial amount of phenolate; the curve shows an abrupt break followed by an only small
increase in the amount of acids. In contrast to this behaviour, the oxidation course in
the acidified solution of phenol is characterized by a slow increase in concentration of
the acids. The breaks on the curves and the final course of concentrations of the acids
which is the same for both the experiments are probably due to the fact that it was not
possible to analytically differentiate the consecutive product of oxidation of maleic acid
– oxalic acid – from the maleic acid in the reaction mixture. The steep section of the
curves is probably connected with the increase in the maleic acid content, whereas the
milder section could be ascribed to the subsequent oxidation of maleic into oxalic acid.
The ASPEN PLUS program made it possible to evaluate the rate constants of the
empirical power rate laws of the reactions (1) through (5) using a nonideal pseudoho-
mogeneous model of laboratory autoclave. Table I presents the values of rate constants
and activation energies for the individual reactions. The lowest rate constant (and,
hence, rate of oxidation) was found – in accordance with the data published1 – for
5
pH
1
60
4
1
c
3
2
3
40
20
0
2
1
0
2
0
40
80
120
160
t, min
200
0
40
80
120
160
200
t, min
FIG. 3
FIG. 4
Concentration profiles of dicarboxylic acids (c, mmol/l)
at various temperatures: 1 144 °C, 2 163 °C (0.5 g
catalyst, 5 MPa)
pH–time profiles of reaction mixtures at various
temperatures: 1 121 °C, 2 144 °C, 3 163 °C
(0.5 g catalyst, 5 MPa)
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Catalytic Effect of Active Carbon Black
1015
maleic acid, whereas benzoquinone was oxidized fastest. The activation energies of the
reactions are close to each other except for the oxidation of hydroquinone, which can
be ascribed to the well-known equilibrium between hydroquinone and benzoquinone
and formation of their complex (which is dependent on the reduction-oxidation poten-
tial of the system). This presumption is also supported by the presence of dark brown
to black colouration in the reaction solution at the beginning of the oxidation. In spite
of the fact that very distinctly limiting presumptions were made, the number of reac-
tions taking place was reduced to 5, and the component reaction orders fixed at 1 and
TABLE I
Kinetic parameters of oxidation reactions (Chezacarb catalyst concentration 0.63 g/l)
b
k(142 °C)a
Ea
Equation
103 mol–0.5 l0.5 s–1
kJ mol–1
1
2
3
4
5
0.094
0.60
1.4
28.0
9.9
35.2
33.1
33.6
0.036
0.11
a Rate constant; b activation energy.
50
c
20
2
c
4
40
1
15
30
10
20
10
0
3
5
0
2
1
0
100
200
300
0
40
80
120
160
t, min
200
t, min
FIG. 5
FIG. 6
Concentration profiles of dicarboxylic acids (c,
mmol/l) for various initial pH values: 1 pH 10,
2 pH 2 (0.5 g catalyst, 5 MPa)
Concentration profiles of dicarboxylic acids (c,
mmol/l) for Chezacarb carbon black; acids: 1
maleic, 2 oxalic and those for standard Chem-
viron charcoal; acids: 3 maleic, 4 oxalic (0.5 g
catalyst, 5 MPa, 143 °C)
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

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Tukac, Hanika:
1/2 in phenol and oxygen, respectively, the values of calculated concentrations agreed
very well with the experimental data.
The catalytic effect of traces of V, Ni, and Fe in Chezacarb carbon black was proved
by comparison with an experiment using a standard Chemviron charcoal. Whereas the
two catalysts do not much differ in the oxidation of the starting phenol (where predo-
minance of free radical mechanism is anticipated), the traces of metal oxides make
themselves felt in the oxidation of dicarboxylic acids. Figure 6 presents a comparison
of concentration profiles of maleic and oxalic acids in the oxidation with active carbon
black Chezacarb with those in an experiment with the Chemviron charcoal. The con-
centrations of both acids are always lower with the application of Chezacarb (approxi-
mately half in the case of oxalic acid) showing a typical profile of intermediate with a
maximum. With application of charcoal, the profile of maleic acid shows an almost
time-independent plateau. The said comparison supports the presumtions1,3 of metal
oxides acting as oxygen carriers in the oxidations of acids.
CONCLUSIONS
The catalytic action of Chezacarb sorbent in oxidations of diluted phenol solutions has
been verified at not much demanding reaction conditions, temperatures above 140 °C
and pressure of 5 MPa.
Temperature affects strongly the length of induction period. At low temperatures
(120 °C) the oxidation is very slow due to the induction period; at 160 °C no induction
period is present and the oxidation is much faster due probably to the effects of active
functional groups of Chezacarb. An independent experiment with the Chezacarb alone,
however, confirmed that the carbon black does not undergo oxidation in the tempera-
ture and pressure regions studied, i.e. the functional groups present do not make them-
selves felt.
The initial pH values affect the initial phase of the oxidation reaction. In the alkaline
solution, the concentrations of acids steeply increase at the beginning, whereas in an
acidified solution of phenol the concentrations of acids increase slowly.
The ASPEN PLUS program makes it possible to model laboratory-scale nonideal
reactors and determine the parameters of these models inclusive of the parameters of
rate equations.
The comparison experiments using a standard charcoal Chemviron supported the
presumptions of different oxidation mechanisms of phenol and dicarboxylic acids (free
radical and reduction-oxidation, respectively) and those about the catalytic effects of
the metal oxides present in Chezacarb particularly upon the oxidation of the acids.
The authors are indebted to the Grant Agency of the Czech Republic for supporting the study
described (Grant No. 104/96/0456).
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Catalytic Effect of Active Carbon Black
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1. Mishra V. S., Mahajani V. V., Joshi J. B.: Ind. Eng. Chem. Res. 34, 2 (1995).
2. Lederer J., Necesany F., Lamac M.: Proceedings Aprochem ’95 Conference, Kouty nad Desnou,
September 20–22, 1995, p. 225. ÇSCHI, Prague 1995.
3. Pintar A., Levec J.: Ind. Eng. Chem. Res. 33, 3070 (1994).
4. Horak O.: Chem.-Ing.-Tech. 62, 555 (1990).
5. Tukac V., Hanika J.: Collect. Czech. Chem. Commun. 60, 482 (1995).
6. ASPEN PLUS™ Release 9.2 User Guide. Aspen Technology, Inc., Cambridge (MA) 1995.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)