Carbon Vol. 34, No. 7, pp. 851-856, 1996
Copyright 0 1996 Elsevier Science Ltd
Printed in Great Britain. All rights reserved
0008-6223/96 $15.00 + 0.00
SOOOS-6223( 96)00053-X
HEATING ACTIVATED CARBON BY ELECTROMAGNETIC
INDUCTION
J. CH. BOURHIS~ and P. LE CLOIREC~*
Gtnie de I’Environnement
Industriel, 6, avenue de Clavitres, 30319
Al& Cedex, France
bEDF, Groupe Induction et Plasma, Centre des Renardikres, BP 1,77250 Moret sur Loing, France
“Ecole des Mines de Nantes, UMR Subatech, 4 rue Alfred Kastler, 44070 Nantes Cedex 03, France
P. MOCHO,’
“Ecole des Mines d’Alts, Laboratoire
(Received
5 January
1996; accepted
in revisedform
12 March 1996)
Abstract-The
purpose of this study is the use of electromagnetic
induction
to heat activated carbon.
The ultimate goal is to get an original process to regenerate adsorbants
loaded with the volatile organic
compounds
present in air or water.
The first step was to explore the possibilities of heating granular activated carbon with this technology.
In order to get the best operating parameters,
the methodology
of experimental
research was applied.
The carbon had to be selected according to its origin (coconut) and its granulometry
(average diameter
3.8 mm). In addition, the importance
of electric power and frequency was demonstrated.
The incorporation
of susceptors
into the carbon
really improves
the energy yield. Efficiency mainly depends on the
granulometry
of the activated carbon and the susceptors, for a given current at a frequency of 263 kHz.
In the second step, the heating mechanisms,
performed on a large pilot unit, followed mainly a surfacic
mode (circular motion) and a volumic mode (intra-particle).
A large size reactor has a positive effect on
the energy yield. The suceptor is used only to obtain a volumic mode of GAC filter heating and not to
improve the heating efficiency. Equations enabled us to model the electric power absorbed by the granular
medium. Experimental
and calculated data correlated well.
In industrial
applications
of the induction
heating, a lower frequency (from 20 to 263 kHz) can be
applied. The energy efficiency is found to be in the range of 40 to 80%. Copyright
0 1996 Elsevier
Science Ltd
Key Words-Adsorption,
activated
carbon,
regeneration,
water treatment, air treatment
induction
-
1. INTRODUCTION
Activated
carbon
als used
in water
number
of papers
is one of the most
and
on
wastewater
its
use,
adsorbent
treatment.
performance,
materi-
A large
-
mecha-
nisms and models have been written [l-4].
However,
little research has been carried out on material activation or regeneration
in an attempt to clean and/or
reuse activated carbon Cl]. In activated carbon production factories, regeneration
is currently carried
out thermally
by heating the reactor walls in an
oxidising atmosphere. Some processes has been developed to regenerate carbon in situ. Several approaches
have been studied and proposed: chemical or solvent
washing or biological regeneration
[ 1,2].
Two conventional
thermal methods are currently
used to desorb pollutants from activated carbon by
high pressure steam or preheating
fluid (air, nitrogen...) entering the reactor [S-9].
The compounds
fixed on the adsorbant
are then removed from the
solid to a fluid [IO-121.
In order to overcome problems found with conventional methods, a new process was considered with
the following approach:
*To whom correspondence
heating,
volatile
organic
compound,
good performance;
no contact
between adsorbent
source;
high energy density;
lower energy consumption;
safety procedure;
good environmental
conditions.
solvent,
and
heating
Given these different conditions, induction heating
seems to be an appropriate
method of generating in
situ the heating required to desorb molecules
fixed
on carbons. Induction heating is based on well known
physical phenomena:
electromagnetic
induction discovered by Faraday and the Joule effect.
Our investigations
into the regeneration
of granular activated carbon, using electromagnetic
induction
heating, are divided into two parts:
Determination
of the optimal heating conditions:
Whether induction processes should be used to get
very high temperatures
of continuous elements (iron,
steel...). There is not much research available on
heating at relatively low temperatures,
of reactors
packed with granular materials with a bad electrical
conductivity.
Different approaches
to heating mechanisms
and
modelling:
Due to the original use of induction
heating, the equations modelling the phenomena have
to be re-investigated.
should be addressed.
851
P. MACHO et al.
852
2. MATERIALS
Table 2. Mean characteristics
AND METHOD
2.1 Activated carbon and graphite
The physical and electrical characteristics
of granular carbons have a great influence on their induction
heating. So, two different adsorbents
were tested in
order to define carbon parameters
necessary
for
working out the optimal heating conditions.
The
characteristics
of activated
carbon are shown in
Table 1. For activated carbon, the grains are assumed
to have the same conductivity.
Graphite used as a susceptor in this study have
the same size (OS-4 mm) as the activated carbons.
For some experiments, it is dispersed with the packing
material except for the determination
of temperature
distributions.
In this case the suceptor is put in the
center of the column. Graphite
is a non-porous
material with a polycrystallized
structure. Its specific
area is less than 10 m’*g-‘.
2.2 Experimental
equipment
Figure 1 shows the activated carbon heating equipment. Two different kinds of pilot unit were used
with the same schematic
presentation.
The mean
characteristics
of the experimental
equipement
are
given in Table 2. The columns are packed with the
two activated
carbons.
No special pressure
was
applied to the packing material in order to get the
same bed porosity for the different experiments.
A
high frequency generator supplies an oscillatory circuit. The frequency, fvaries with the number of turns
per unit length of the inductor and with the capacitor
values, Ci. Examples of unit I are given in Table 3.
Table 1. Some physical
Parameter/activated
characteristics
carbons
carbon
Origin
porosity
Specific area (m2.g-‘)
Porous volume (cm3*gm1)
Apparent density (kg*m3)
Real density (kg- m3)
Average diameter (mm)
Ash (%)
B
coconut
microporous
1240
0.55
500
2550
0.5-4
3
Wood
mesomacroporous
1750
1.26
300
1850
0.5-4
7
Reactor
Internal diameter (mm)
Height (mm)
Granular
activated carbon*
Mixture (A GAG/graphite)
Carbon weight (kg)
Frequency range (kHz)
Current power range (kW)
Coil diameter (D) (mm)
Coil height (L) (mm)
DIL
Water flow in the coil (leh-‘)
*The activated
carbons
presented
Table 3. Characteristics
Glass
120
180
A or B
90/10
1
140-263
O-10
150
180
0.8333
5
Glass
700
500
A
Coil
spirals
&)
9
6
6
6.5
2.89
2.89
of the oscillatory
f calculated
&J
165
165
90
(kW
153
230
312
I10
6.75-260
0 10
750
500
1.5
5
I.
circuit
f measured
(kW
140
227
263
The temperature
measurements
are taken either
with thermocouples
dispersed in the activated carbon
and connected
to a Schlumberger
Solartron
3430
computer (unit I) or with optical fibers (unit II). The
measurements
are taken after the high frequency field
has been turned off because the field disturbs the
functioning of the thermocouples
[ 133.
3. RESULTS AND DISCUSSION
The application of induction processes to granular
materials is relatively recent [ 14,151. To our knowledge, there have been no extensive studies in this
subject area. Therefore the methodology
of experimental research was applied to the study of activated
carbon heating by induction [ 161. Two level factorial
designs were chosen to determine
the influence of
some well-known factors on this type of heating. This
pattern takes into account the interactions
between
these factors.
w
c2
Cl E
-1
I+
Oscillatory circuit
t
ps5-j
FluId inlet
equipments
II
in Table
Fluid outlet
Fig. 1. Experimental
1
Pilot unit
of the two activated
A
of the pilot units.
used for the different
experiments
Line
Heating
activated
carbon
3.1 The injluence of the type of carbon on
induction heating
The experimental
pilot I, shown previously
in
Fig. 1, was used packed with the two different kinds
of activated carbon. The coil power used for induction
heating was 1.35 kW. The procedure used (experiment
no. 1) to get a clear picture of the influence of the
type of carbon is as follows: three parameters
Xi,
grain size, origin of the carbon and frequency were
chosen. The range of their values is given in Table 4.
The response to these conditions is the temperature
value at the centre of the granular medium after 15
minutes (Y). The heating rate, which is S”C.min-‘,
means convection
losses are negligible. The mathematical pattern is expressed as:
+B,,*X,*X,
+ B,,*X,*X,
+ B,,,.X,*X,-X,
with
Y= Response
Xi = Independent
parameter
The value of the coefficients Bi represents the effects
of Xi on Yin the experimental
field.
The results are presented
in Fig. 2. This figure
shows that the diameter (X,) and the type of carbon
especially (X,), i.e. the conductivity
of the material,
are of primary importance
in heating efficiency. The
best conditions
are activated carbon from coconut
and a grain diameter
as large as possible (d =
2.7 mm). Thus, granular size has an effect on the
surface available
for Foucault’s
currents
to flow
through each grain. In addition, the total electrical
Table 4. Experimental
experimental
Experiment
X,
XI
X,
X,
field for the two experiments
methodology
approaches
by
no. 1
Symbolic value
Grain size (mm)
Carbon origin
Frequency (kHz)
Experiment no. 2
Symbolic
XI
X,
Grain size
X,
Coil power
X,
Frequency
value
(mm)
(kW)
(kHz)
-1
0.8
Wood
140
-1
0.8
1.3
140
+1
2.7
Coconut
220
+1
2.7
2.3
220
Bi
by electromagnetic
induction
853
resistance of the bed of activated carbon increases as
granular size is reduced. The interactions
between
the different parameters, given by the bij or b,, values,
are found to be very weak. Then, their influences in
the response (Y) are negligible.
3.2 Electromagnetic
efSects
The same pilot, unit I, shown previously, is used.
Coconut charcoal is chosen in this study because of
its capacity to be heated by induction (Fig. 2). The
heating rate of activated
carbon is based on the
electrical power used and on the current frequency
in the coil. The effect of grain size is also studied.
The range of their values is given in Table 4 (experiment no. 2). As previously stated, the response to
these conditions is the temperature value at the centre
of the granular medium after 15 minutes (Y).
The results, presented in Fig. 3, show that electrical
power is the principal
parameter
in this type of
heating. The influences of granular size and current
frequency are less important,
but not negligible, in
this experimental
field. The current frequency is not
the only parameter to take into account, according
to classical induction heating. Grain size is as important as such a parameter.
The same effect of the
interaction coefficients are noted in this experiment.
3.3 Temperature distribution in the reactor
In order to receive good regeneration,
a uniform
temperature
distribution
is required, i.e. a uniform
heat through the reactor. So, two experiments
were
performed: one a small unit (I) and, to investigate
the scaling up of the filter, a second with a larger
pilot unit (II). This study also displayed the volume
characteristics
of heating activated carbon.
Two different pilot units (I and II) packed with
the activated carbon A (Table l), were used in this
study (Table 2).
Figure 4 shows a slight
3.3.1 The small unit
temperature
difference between the core and the
external part of the reactor. Superficial currents are
developed on each grain. The reactor is assumed to
be uniformly heated. So this heating process [ 17,l S]
enables the granular medium to be uniformly regenerated. The current frequency increase from 140 to
20
I8
16
14
12
10
8
6
4
2
0
16
14
12
10
8
6
4
2
0
XI
X2
X3 Xl2 X13 X23X123
Fig. 2. The effect on the core temperature
of the grain diameter (X,),
carbon
origin
(X,)
and
frequency
(X,).
(Experimental
methodology,
experiment
1).
Xl
X2
X3
X12
Xl3
X23X123
Fig. 3. The etfect on the core temperature
of the grain size
(X,), the coil power (X,) and the current frequency (X,).
(Experimental
methodology,
experiment 2).
P. MOCHO
854
cltcd.
T (“C)
T (“Cl
160
1 i=%Okhz
60
40
20
0
Tc, f = 140 kHz
n
Tp, f = 140 kHz
0
I-c, f = 221 kHz
0
Tp.
2
f=,Okhz
n
3
f=42kHz
0
4
f = 42 kHz wth
suceptor
f = 227 kHI.
I
0'
.
I
IO
20
30
40
50
60
Time (min)
Fig. 4. Temperature
distribution
in activated carbon reactor
140
or
frequencies:
I
(coconut
carbon
3.5 mm.
220 kHz). T, = core temperature;
Tp = peripheral temperature.
220 kHz improves
the induction
heating
yield by a
factor of 1.6.
In order to increase energy efficiency, the current
frequency is slightly increased and the addition of
susceptors is tried. Both activated carbon and graphite have similar structures (grains with similar diameter). Graphite
has a low resistivity
( 10m5 Q.m)
therefore a good capacity to be heated by induction
[ 193. The low value for the coil-reactor
coupling,
which produced unfavourable heating conditions, was
necessary in order to avoid overheating the graphite.
The granular size has an influence on the heating
efficiency. Improvements
obtained by the addition of
graphite are shown in Fig. 5. Energy efficiency is
increased by a factor of 1.7 in comparison with single
carbon performance.
Figure 6
3.3.2 The large pilot unit (II)
temperature
distribution.
shows
radial
the
Penetration
depth as a function of frequency has to
be noted. This phenomenon,
well known in induction
heating studies, was not found in the small unit due
to the low diameter value close to twice the penetraT (W
250
200
150
loo
PO
d
lb
5
1i
20
30
25
35
R (4
Fig. 6. Radial temperature
distribution
with and without
susceptor
addition
m unit II for different
frequencies
(422260 kHz). In this case the susceptor is put in the centre
of the column.
depth. So, temperature
distribution
seems to be
uniform. In order to avoid this problem, suceptors
(3% volume) were put in the centre of the column.
The results achieved in this instance are compared in
Fig. 6. A uniform temperature
distribution
is almost
achieved with this new packing material. For potential industrial applications. a conductor cylinder used
as a suceptor would be put in the center of the
reactor [20.21].
tion
3.4 Energy eficiency
The different energy efficiencies were determined
for the different experiments performed with the two
pilot units. The results are presented in Table 5. The
data are very interesting in terms of the scaling up.
A better yield is given by the larger of the two pieces
of experimental equipment. Yield increases according
to the increase in frequency.
3.5 Approaches
of‘mech~nisms
and modellkg
Induction heating mechanisms
and modelling are
well known for continuous elements. Maxwell’s equations are generally used successfully [ 19,22,23]. The
approach
is different for a granular material and
especially for activated carbon. Two hypotheses can
be formulated and these are shown schematically
in
SO
IO
0
II
GAC A (3.X mm) +
10 ‘7~ graphite (4 mm)
.
GAC A (3,8 mm) +
IO ‘% graphite (2 mm)
q
Fig.
5. Influence
20
Time (min)
GAC
Table 5. Energy efficency as a function
and frequency for the activated
30
Pilot
unit
of pilot unit scale
carbon A
II
I
Activated
carbon
A+
graphite
A
A
A (3.8 mm)
of the addition of polycrystallized
to the carbon.
graphite
f (kHz)
Yield (9%)
140
4
221
5.5
263
5.5
263
10
6.15
40
40
70
70
75
260
x0
Heating activated carbon by electromagnelic induction
Fig. 7. The induction heating follows a volumic mode
(intra-particle)
or surfacic mode (circular motion).
In order to investigate
the real mechanisms,
the
following experiments were performed. A carbon bed
was divided into several parts with plastic walls. The
part number of the column is different as shown in
Table 6. An example of a four division bed is presented
in Fig. 8. These kinds of divided reactors allow to cut
the induction current lines. The electric yields given
are presented in Table 6.
A decrease in yield is observed when the number
of parts increases. This data shows clearly that the
electric circuit is mainly a circular circuit around the
activated carbon filter, considering
the two mechanisms as independent.
Comparing the first experiment
with the fourth one gives an estimation of the weight
of the intraparticle
heating
mode: about
10%.
Previous experiments
(Figs 4 and 6) and the same
approach with pilot unit II confirms this mechanism.
Induction heating efficiency depends on the characteristics of the carbon. Electric power absorbed by
the granular media can be defined by the following
equations
855
[ 17,24,25]:
pH,‘SF
__
6
p=
where
H, =Magnetic
field intensity
(A-m-‘)
S = Surface area of grain exposed
the magnetic field (m’)
F = Induction
transmission
r = Resistivity
of GAC (Q.m)
to
factor
S = Nndh
(2)
where
N = Number
of grains
d = Median
diameter
h = Height
of cylinder
of GAC cylinder
(m)
3
(3)
where 6 = penetration
a=
depth (m)
p
J- nclf
(4)
where
lntra particle
circuit
f= Current
frequency
~~=Permeability
Fig. 7. Electric circulation
model in an activated
carbon
packed column. Examples of surfacic (circular motion) or
volumic modes (intra-particle).
Fig. 8. Example
(1)
of the carbon bed division
the reactor II.
in four parts
in
balance in a divided reactor (unit I). An
example of the reactor division (four parts) is presented in
Figure 8
(Hz)
of GAC about
1.24 10e3 Hem-‘.
For the first time, the penetration
depth is calculated for different frequencies. The values are between
10 and 4 cm, respectively for f = 42 kHz and f =
263 kHz. These data confirm an homogenous
temperature on the reactor I and the temperature
distributions obtained with unit II and presented in Fig. 7.
Besides, the comparison
between the 6 values and
the adsorbant
diameter (d =4 mm) gives a ratio
between 0.04 < 6/d -c 0.1. These ratios confirm the
mechanism approach presented previously and shows
the heating is not due to the conduction through the
packed
column
but heating
by electomagnetic
induction.
The model put forward is also used to calculate
the power applied to the carbon bed (pilot unit I)
and is compared to the value determined
by experimentation. Table 7 gives the operating conditions and
Table 8 shows a comparison
between the experimental (P,,,) and calculated (Pcalc) data.
Experimental
and calculated values correlate well.
Table 6. Energy
Number of divisions
in the reactor
1
2
4
6
I
P, (kw)
1.1
1.1
1.1
1.1
p (W
Yield (%)
64
25
11
7
5.5
2.2
1
0.6
4. CONCLUSION
Induction heating was used to test the regeneration
of this material. The process efficiency depends on
the characteristics
of the carbon (type and grain size).
Coconut charcoal with a median diameter of 3.8 mm
is recommended.
This process with an appropriate
frequency (263 kHz) enables an homogeneous
heating
of the GAC. The energy yield was improved by the
P. MOCHO
X56
et cd.
Table 7. Data used for the simulation
Carbon*
A
A
A
A + graphite
*The activated
/‘(kHz)
d (m)
11(m)
5
S (m’)
140
227
263
263
0.005
0.005
0.005
0.003
0.004
0.004
0.004
0.004
21517
21517
21517
2152
1.35
I.35
1.35
0.08 1
carbon
shown
in Table
f (kH7)
t’, (kW)
140
227
263
263
I.1
1.1
I.1
1.1
A
A
A
A + graphite
p,,
’)
1.64.10 ’
1.64.10 A
1.64.10~-’
0.05
0.04
0.04
10 i
0.003
carbon
presented
1
H,
A.m
4500
4000
4000
4000
3.13.10~5
6.1.10 -’
6.1.10 ’
2.83.10 z
(W)
42
64
64
118
Design and use of GAC, C’onferrncc, Procertlings.
Cincinnati. Ohio, USA ( 1989).
V. Cocheo and S Bombi, Ant. Intl. Hyg. Alss. .I. 48(3),
injected
189 197 (1987).
M. Baudu, P. Le Cloirec and G. Martin.
P&(W)
2X
54
54
118
in Table 1.
addition
of graphite (10% weight) with a median
diameter of 4 mm in the granular media. In industrial
applications
of induction heating, a lower frequency
(from 20 to 263 kHz) can be applied. In this case, a
susceptor is put in the reactor to obtain a uniform
radial temperature distribution.
The energy efficiency
is found to be in the range of 40-80%.
The heating mechanisms
follow mainly a surfacic
mode (circular motion) and a volumic mode (intraparticle). Equations
allow us to model the electric
powder absorbed by the granular medium. A good
correlation is achieved.
Aclinowledgmlents-The
authors
thank Jean Reboux for
fruitful discussions
on the electromagnetic
induction
processes and Gkrard Dagois, Pica Company, Levallois, France,
for technical assistance on activated carbons.
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Lc chuujfuge
par inductiwl
principccs
t’r
ili~?zrn.\ronrlrmrrlr
drs inductam,
Notes EDF HE 121, NS
2366 (103)
24.
25.