TECHNOLOGY
AARMS
Vol. 7, No. 3 (2008) 533–550
Use of the microwave impregnation of active carbon
LÁSZLÓ HALÁSZ, ÁRPÁD VINCZE, JÓZSEF SOLYMOSI
Miklós Zrínyi National Defence University, Budapest, Hungary
Impregnated active carbon is used in different industrial and military air filters to
remove the dangerous vapours and gases from the air. The impregnation technologies
generally are using some toxic material and the energy consumption is significant and
sometimes changes the microstructure of activated carbon. A new impregnation method
combined with the use of microwave and a microwave drying and heat treatment
processes have been developed. The effectiveness of the new technology was proven
preparing a good quality NBC impregnated active carbon.
1. Introduction
The use of active carbon, which has been impregnated with metal compounds as a filter
to remove toxic gases has long been known. A number of impregnated carbon with
different compositions have been developed, which intend to provide effective defence
against the toxic gases. The generally used filter canister consists of a double filtration
mechanism, which have a primary and secondary filter system. The primary filter is on
the inlet side and its main function is to contain particles.
The secondary filter is impregnated active carbon. The main function of the carbon
is to adsorb chemicals from the air passing through. Adsorption is achieved through
physical and chemical adsorption. Physical adsorption involves weak Van der Waal’s
forces. Chemical adsorption is achieved through impregnation of the carbon with
copper, chromium and silver.
The impregnated carbon is produced by different technologies but the main steps are
common:
1. impregnation in salt solution,
2. drying with heat,
3. heat treatment to reach the final type of salts on carbon surface (generally used
salt form is the oxides).
This classical process requires a significant amount of heat. Using microwave in
both impregnation and heat treatment process has numerous advantages: on one hand
the energy requirement of production can be decreased and it gives better impregnation
results on the other hand. A new patented1 impregnation and heat treatment method will
be shown in this study.
Received: October 13, 2008
Address for correspondence:
LÁSZLÓ HALÁSZ
E-mail: [email protected]
L. HALÁSZ et al.: Use of the microwave impregnation of active carbon
2. Microwave impregnation process
The fundamental novelty of the developed impregnation procedure is, that during the
adsorption of the soluted substance the local temperature of granules of the adsorbent
cyclically makes change in with periodic application of a microwave field with a given
frequency and energy, and the temperature of adsorbent was kept on a higher value, than
the re-circulated and thermostated impregnationel solution. The difference in dielectric
properties of the adsorbent, solution and soluted material gives an opportunity onto the
heating with a microwave created locally. Thus it can be reached a decrease of the
resistance agains the diffusion process. It is necessary to cool down the recirculated
solution because thus the capillary pressure will change periodically which also can
help the effectiveness of impregnation.
The experimental setup is schematically shown in Figure 1 for the liquid/activated
carbon system. The liquid circulates through column (1) having an internal diameter of
D = 0.13 m. The particle stack in the column consists of 5 kg of Silicarbon 0.8 Supra
activated carbon in a space delimited by two grids. Data collection is initiated by
starting a circulation pump (2). The impregnation liquid (tap water) flows through a
filter (3) and a cooler (4), and is homogenized in a mixing tank (5) if required, or
recycled in the volume measured by a rotating flow meter (6). The liquid is fed
cyclically either at the top or at the bottom of the column. Four microwave devices (7)
were also installed along the height of the column with power output of 700 W each.
Initially, the fluidizing liquid enters the column from the top and inversely fluidizes
the activated carbon that is lighter than water. To impregnate the vesicles of the
activated carbon more efficiently, the direction of water flow was altered after 5 min,
and water was fed through the bed from the bottom for a further 5 min. At the same
time, the computer controlled microwave devices were switched on, one after the other,
for 5 s at a time from bottom to top. When the uppermost microwave device switches
off, the lowermost one is switched on again. The electromagnetic field intensifies the
exchange of air and the flowing liquid in the internal vesicle of the activated carbon.
This effect is based on the different relative permeability of water and air as a function
of absorbed microwave energy at different extents. Active carbon itself behaves
differently in an electromagnetic field. The pressure exerted by the microwave
radiation, and consequently the exchange of material, is also increased by the gas
pressure fluctuation generated by the microwave heat, thus offsetting the hydrophobic
nature of activated carbon.
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Figure 1. Schematic diagram of the apparatus
(1) column, (2) pump, (3) filter, (4) cooler, (5) mixing tank, (6) flow meter, (7) microwave device
After 5 min. of the intensive treatment of microwave radiation, the flow was
directed downwards and the bed was inversely fluidized. Observations indicate that
approximately 50 min were required for a sufficient impregnation of the carbon
vesicles. Following this, the particle bed, having become heavier than water, displays
the homogeneous fluidization characteristics of the classical liquid/solid systems. Figure
2 shows the picture of the equipment.
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Figure 2. Microwave impregnation equipment
3. Microwave heat treatment method
After the impregnation the active carbon filtered or centrifugated from the solution and
a drying procedure needs to carry out. This has to remove from the surface physically
adhered water, and from the pores the chemisorbed water.
The drying process can be done with various procedures and equipment, and also
with a microwave treatment.
The conversion of electromagnetic energy into thermal energy is realized by the
electromagnetic characteristics of the materials and depends principally on the material,
temperature, and frequency. As generally only one frequency is used for the heating
process and the temperature dependency of the characteristics is not known, an
observation can only be carried out in terms of the material itself. The microwave drying
of the impregnated active carbon – and his heat treatment – signifies a special task.
The impregnated active carbon is a complex material system, since it consists of
active carbon, mix of the inorganic salts taken up in the course of the impregnation, possibly
from organic compounds and water. In the course of the drying the material quality
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changes with the outgoing water, partly due to the decrease of the quantity of the water,
partly due to the drying of impregnated salts on the surface of active carbon. As a result
of this process the dielectric constant of the sample, the conversion rate of applied energy
into heat, and the temperature of the sample changes. It makes the situation more
complicated that the dielectric constant of the sample either can increase or decrease
depending on the temperature. In the course of the microwave drying the relative
humidity grows quickly in the environment of the sample due to the fast increase of
temperature, so it is necessary to remove it continuously with an inert gas flow to
produce a relatively short drying time.
The application of microwave energy in the production of impregnated active
carbon has definite advantages in cases, when heat treatment is necessary after drying
because the two processes can be drawn together and carried out in the same equipment.
The microwave energy transfer has the next advantages:
• faster substance transformation, shorter operation time,
• warming with full mass, there is not wall effect,
• selective warming in the function of a material quality,
• well controled process management with fast and efficient reaction,
• less apparatus, continuous firm conduction,
• energy saving,
• environmentally friendly waste poor technological solution.
Figure 3 shows the laboratory heat treatment equipment
Figure 3. Laboratory microwave heat treatment equipment
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4. Optimization the working parameters of equipment and producing NBC type of
impregnated active carbon
The ASC type active charcoal, which has an excellent HCN adsorptivity contains Cu
(II), Cr (VI) salts, but unfortunately the Cr(VI) is carcinogenic thus the production and
use of this type active carbon is decreased nowadays. During the HCN adsorption, in
the first step Cu(CN)2 is formed, then it goes to CuCN and (CN)2 pieces and the last
one under the catalytic effect of Cr(VI) Cu(II) forms (CONH2)2 or HOCN in a
hydrolysis process and in the last step CO2 and NH3 are formed.
The ASZ type active carbon is impregnated by Cu (II), Zn (II) and Ag (I) salts. The
ASZM type active carbon contains some molybdenum or vanadium salts and the
ASZM-T is impregnated by triethylene-diamine (TEDA). The ASZ type active carbon
have a good HCN adsorptivity and they can also adsorbed arsine-hydrogene (AsH3),
chlor-cyane and phosgene.
One of the key steps of the production of the impregnated active carbons with good
efficiency presumably, that during the impregnation, drying and heat treatment it can be
maintained in the special position of CuO, ZnO and the others impregnating material on
the carrying active carbon. An ASZ type imregnated active carbon was planned to
produce with the new method.
A possibility for this, which is in the literatures not known, is the impregnation in
the microwave space then drying and activation by microwave energy.
4.1. The examination of the microwave features of impregnated carbons
The dielectric constant (ε’) and the value of dielectric loss (ε”) of the impregnated and
not impregnated active carbons have been measured, and the morphological changes in
the active carbon resulting by the microwave treatment has been observed. The
temperature profil, temperature runaway and the spontaneous combustion during the
miocrowave treatment also have been determined.
4.1.1. The determination of dielectric constant (ε) and the value of dielectric loss (ε’)
For the determination of the values ε’ and ε” of impregnated active carbons a
measurement method has been developed. The sample was placed into an experimental
equipment where it was irradiated by big energy microwave (2.41 GHz, some 10 W).
The energy distribution was measured with 4 pieces of diode detector, which were
placed into a distance of λ/8 from each other. Those state was determined when the
longitudinal and reflected waves were in-phase in the equipment from the difference
sign of detectors with an odd and even order number. In this case, based on given
equations, the dielectric constant of the sample could be calculated.
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Figure 4. Equipment for the measurement of dielectric constant of active carbon
4.1.2. The effect of microwave energy on the morphology of impregnated and nonimpregnated active carbons
It has been established based on scanning electron microscopy that the thermal and
microwave drying and heat treatment have an identical effect onto the inner structure
and the morphology of active carbon. On the case of the amorphous active carbon the
changes drastic, the cavities were deepened and broadened out as the result of drying.
The changes on the inner surface of the sample are more significant. The inner channels
broadened out, shattered, they became amorphous.
In the case of extruded active carbons the changes were much smaller. On the
broken surface a minimal expansion of the channels could also be observed here, but it
did not report neither structural, neither morphological change. Their examinations
indicated that the extruded active carbons can be used much better to the production of
impregnated active carbons, than the amorphous carbons.
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a)
b)
c)
d)
Figure 5. Original and microwave dried active carbons
a) original amorphous, b) microwave dried amorphous, c) original extruded,
d) microwave dried extruded active carbon
There were not any data in scientific literature for the PIO and SIT values of
impregnated or non-impregnated active carbons determined in microwave space. It is
utmost important to know these values due to the safety of drying and heat treatment
processes. A method was developed to determine the value of PIO in microwave space
using equipment called CEM Discover.
The CEM Discover equipment has a cylindrical shape space and the microwave
power enters through the slits on cylindrical superficies of the equipment. Thus the
microwave space is very homogeneous. The measurement of temperature occurs by an
infrared thermometer at the bottom of the reaction space.
In the case of PIO value determination a given amount of sample was irradiated with
increasing microwave power and it was found an energy value where the microwave
power quickly decreased. In this point the oxidation of active carbon begins and the
temperature of active carbon is increasing independently of the microwave energy. It
can be considered this point as the PIO value of active carbon.
The PIO values of some active carbon are summarized in the following table.
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Table 1. PIO values of some impregnated active carbon
Impregnated active carbons
SiL-0,8
K5M (thermal))
SX (thermal)
CW (thermal))
SP-377/135 ( microwave)
Metals
none
Cu, Ag, Cr
Cu, Ag, Mo
Cu, Ag, Cr
Cu, Ag Zn
PIO point (°C)
215±2
210±1
220±1
210±1
217±3
4.2. Microwave drying and activation
The active carbone type Sil-08 has been impregnated with an ammonia solution
containing basic copper carbonate, zinc oxide, silver nitrate, ammonium carbonate and
ammonia solution at 25 °C. The air dried (its water content at this time is 39%)
impregnated active carbon, which was undergone a suitable microwave irradiation and
impregnationel residential time, has been dried with microwave energy, then activated.
To study the parameters of the microwave drying and activation a CEM Discover
apparatus was used with active carbon quantity of 1.5 g. The starting energy level was
70 W, thus the temperature of active carbon could not exceed the 135 °C. The increase
of temperature is quick till 80–90 °C, then turned into an almost linear region and in this
region considerable part of the water evaporates, then at 100–110 °C the temperature
increase is faster, and it is determined by the impregnating materials. This shows in
round I–II even better, where the temperature curves coincide quasi, on the case of the
third round somewhat faster increase difference can be observed, what it indicates the
chemical reaction of substances.
For demonstration of this after the second activation on 135 °C, the temperature of
third activation was 200 °C and the fourth activation occured again on 135 °C. It is
unambiguously clear that the activation occuring on the higher temperature a
considerable chemical change took place, because the temperature of the impregnated
active carbon increased more quickly.
Comparing the temperature increace curves of the microwave and thermally dried at
135 °C impregnated active carbons, they show an identical characterictics. It can be
concluded from this fact, that the impregnating material have an identical chemical
structure in the case of both drying and activation methods.
Thin layer cromatographic examination. 1 µl solution prepared with 1 ml 25%
amonia solution of the solid material of 50 mg, which was obtained with ammonia
dissolultion from the impregnated active carbon was applied onto a thin layer (Cellulose
F254 produced by Merck). The chromatographic reagent was 25% ammonia. The patch
of activated sample at 135 °C can be seen on the first column, the patch of the activated
sample at 200 °C is on the third column. Unambiguously, the two different copper
compounds formed on the different temperature during the activation.
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Figure 6. The thin layer chromatogram of the dissolved copper compounds from a the activated impregnated carbon
Thermogravimetry measurements (TG, DTG, TGA). According to the
thermogravimetry examination of the not impregnated active carbon type Sil-08 the
material did not suffer heat decomposition till 400 °C. The K5M impregnated active
carbon (ASC type carbon) but did, since one on its surface impregnating substances
catalyzes the oxidation of the carbon. The solid material obtained after the
evaporization of the impregnation solution which was used in our procedure consisted
of Cu(NH2)2CO3 and ZnO. This mixture was transformed into CuO, between 217 °C
and 312 °C with considerable mass decrease (46%).
The thermogravimetric behaviour of impregnated carbon activated at 135 °C and
200 °C differs from each other. During the microwave treatment of the carbon activated
on 135 °C first 0.66% of water leaves between 136.9 and 251.3 °C and a transformation
occurs in the structure of impregnating material with 1.8% of mass decrease, then the
active carbon begins reduce CuO into Cu2O and the carbon itself forms CO and CO2,
which means decrease of mass. During the microwave activation on 200 °C 1.19% of
water leaves and after 213 °C a slow decrease of mass occurs due to the oxidation of
active carbon. This facts means, that in the case of the activation on 200 °C, the
transformation of imregnating materials has already taken place, what is the lower
activation temperature yet not. This fact is in good agreement of the result of thin layer
chromatography that the dissolved substances from the active carbon treated on the two
different temperatures have different chemical composition.
XRD examination. The XRD examination gives information about the structure of
crystalline impregnating material on the surface of active carbon but it can not give any
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information about the amorphous material, thus the method can be used to determine
whether the impregnation material has any crystalline structure or not. After the
impregnation the active carbon (its water content is 39%) contains the complex metallic
salts in crystalline form. During the classic heat treatment between 135 and 175 °C this
crystalline structure remains unchanged.
The situation changes during the microwave drying and heat treatment. After the
drying (at 105 °C, with water contain 8–10%) the impregnating inorganic salts are in an
amorphous form, and the repeated moistening does not form a crystalline structure. The
activation (at 135–175 °C) can not modify the amorphous character of the impregnating
substances.
When the activation temperature increased to 200 °C, the composition of the
inorganic salts changes in both type of activation, crystalline CuO and Cu2O forms.
HCN adsorption. The measurement of HCN adsorption of impregnated active
carbons performed according to MSZ EN 141:2001. The characteristic parameters are
the breakthrough time and the equilibrum adsorption capacity according to the WheelerJonas equation.6,7
4.2.1. Small laboratory equipment and measurements
The heat treatment procedure consists of two consecutive steps, which cannot be
separated from each other sharply. The first step is the drying, where the water content
of active carbon needs to be reduced from the initial 27–30% to 3–5% . This occurs
when the temperature of the active carbon reaches 130 °C. In the course of the drying
the ammonium complex of basic copper carbonate, the basic zinc carbonate and the
silver nitrate decompose with the leave of ammonia and form copper- and zinc
carbonates and the silver nitrate, which remain on the exterior and inner surface of the
active carbon in crystalline or in amorphous form. These compounds can be removed
from the surface of the active carbon with only 25%-os ammonia or dilute soluition of
mineral acids.
The second process the activation, where the water tied removes at 130–200 °C,
then in the course of additional heating those reactions are carrying out in which copper,
zinc and silver oxides are forming and these compound are determining in the
chemisorption processes.
During the microwave heat treatment procedure it is necessary to provide the suitable
microwave energy, continous removement and adsorption of the water vapour and
ammonia. These tasks can be solved with a suitable microwave equipment, (for example
microwave oven), a water vapour condensator and sulfuric acid ammonia absorber.
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The impregnation of active carbon can be done by two methods. One of them is the
classical so-called soaking method. Its feature, that the active carbon is stirring in a
twofold amount impregnating solution in 60 min, then the unnecessary solution is
removed with filtering, and it is dried in 24 hours on room temperature in a layer with a
thickness of 1 cm. At this time the water content of the impregnated carbon decreases to
the values of 34–38%. The other method is the impregnation with microwave. Its
feature, that the carbon to be impregnated is poured into a reactor which is in a
microwave space, the impregnating solution flow through the carbon bed with suitable
velocity, that lets the carbon be in constant mixing. Particularly using a microwave
radiation the quantity of the adsorbed impregnating material is increased. After the
Impregnation process the impregnated carbon is dried with air flow at 24 hours. The
water content of the carbon gained in this manner 38–41%. The drying and activation
were made by the above mentioned two methods prepared impregnated active carbon.
Drying and activation experiments were also performed thermal heating. In the
course of the thermal treatment two methods, the classical and the vacuum drying were
applied. During the classic drying the impregnated carbon to be treated was spread out
in 1 cm of thickness, then it was kept at 100 °C for 60 minutes, then at 135 °C or
175 °C for 60 minutes. In the case of the vacuum drying the impregnated carbon to be
treated was kept at 95 °C, for 60 minute, then at 135 °C or 175 °C-on for 60 minutes.
The vacuum of 20 Torr was applied in all cases in the course of a treatment.
The characterisation of the different type of impregnated active carbons is
performed using XRD and HCN breakthroug measurements. Then XRD experiments
have shown the amorphous or crystalline structure of the impregnating material on the
surface of impregnated active carbon. If the impregnating material was crystalline its
composition also could be determined. The commercial impregnated carbons (K5M,
ABEK) which were used as reference materials all of them contain amorphous type
impregnating materials. It is important to know, that a given technology parameter what
kind of effect onto the evolving morphological relations of impregnating material. The
HCN absorption capacity can be characterized with the time when measurable HCN
quantity attains the 10 ppm value after the breakthrough and with the calculated
equilibrium adsorption capacity. The bigger these values are, the more efficient is the
given impregnated active carbon.
The first parameter to be examined was the influencing effect of the water content of
the impregnated carbon. The water content of impregnated carbon after the impregnation
was 48%, at this time it adhered, and it could not be treated at in the additional
technological processes. A room temperature air flow was used for 12 , 24 and 48 hours
and the water content decreased continuously at this time to 40, 34, or 24%, respectively.
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In this manner dried impregnated carbon became non-adhesive granular form already after
12 hours, and it could be well treated in the additional processes. A crystalline structure
formed on the case of all three samples, that they did not differ from each other, and the
XRD examination in all cases the one with a next formula (Cu0.3Zn0.7)5(CO3)2OH mixed
basic-copper-zinc carbonate was pointed out, it means, that the original water content had
not any effect on the morphological structure of impregnating material. (see 377/138-141
experiments). The additional experiments were performed by the samples containing
30–40% water, prepared with drying on 24 hours.
The second parameter to be examined is the air flow velocity in the course of the
drying. The microwave energy was set onto 640 W (it is 80% of the original value), the
temperature was 135 °C and the air flow velocities were 2000, 1700, 1200, and 700 l/h,
respectively during the drying and activation. (see 377/145-150 experiments). The data
show, that the increase of air flow velocity will increase the efficiency of HCN
absorption (the equilibrum adsorption capacity increases).
The experimental data are summarized in Table 2.
Table 2. Experimental results in the caseof small laboratory experiments
Number of
experiment
Drying
min. MW
Activation
min. MW
Humidity
begin end
XRD*
Information
HCN equil.
adsorption
capacity (g/g)
377/139
20 640
30 800
34.2 <1
0.007
377/140
20 640
30 800
40.8 <1
377/141
16 640
30 800
20.3 <1
377/147
700-800 l/h
377/146
1200 l/h
377/145
1700 l/h
377/150
2000 l/h
377/151
2000 l/h
377/200vt-1
377/200vt-2
377/200vt-3
377/134 T-T
56 640
30 400-640
135 °C
30 400-640
135 °C
30 400-640
135 °C
30 400-640
135 °C
30+30 640
175 °C
60–95 °C
60–135 °C
60–175 °C
60–135 °C
37.25 1.64
crystalline
(Cu0.3Zn0.7)5(CO3)2OH
crystalline
(Cu0.3Zn0.7)5(CO3)2OH
crystalline
(Cu0.3Zn0.7)5(CO3)2OH
crystalline
(Cu0.3Zn0.9)5(CO3)2OH
crystalline
(Cu0.1Zn0.9)5(CO3)2OH
crystalline
(Cu0.3Zn0.7)5(CO3)2OH
amorphous
52 640
50 640
43 640
43 640
60
60
60
60
therm**
therm**
therm**
therm**
37.25 1.45
38 1.09
32 1.19
32 1.05
32
32
32
32
crystalline
(Cu0.1Zn0.9)5(CO3)2OH
amorphous
amorphous
amorphous
crystalline
Cu2(CO3)(OH)2
Cu0.1Zn0.9)5(CO3)2OH
CuO
0.008
0.007
0.008
0.006
0.0202
0.030
0.0261
0.036
0.041
0.038
0.020
* Determined from the intensity of XRD lines.
** Vacuum drying at in a pressure of 20 Torr.
*** Drying in drying box in a layer of 1 cm.
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The XRD examinations render it probable, that growing velocity of air flow
decreases the crystalline character of impregnating material and certain velocity of air
flow causes forming only amorphous state. The explanation of the phenomenon may be
that the increase of air velocity – in the case of an identical microwave energy level
(identical W/g) – causes an increases of the drying velocity which is appropriate for the
development of the amorphous state. For proving the abovementioned theory a
continuous sampling and determination of water content were carried out during the
drying process. The content of water also in the case of thermal drying with air (sample
377/134 TT) and thermal drying with vacuum were also determined (sample
377/200vt). The XRD pictures of impregnated carbons also have been taken up. The
structure of impregnating material is almost amophous in the case of vacuum drying
and crystalline one in the case of thermal air drying.
Looking for a context the running down of the drying curves and HCN equilibrum
adsorption capacity the biggest one has been found in the case of the vacuum drying
(sample 377/200vt 1-3), the structure of impregnating material was amorphous
according to XRD measurements. The decrease of the drying velocity which was
accomplished with the decrease of air flow (samples 377/147-149) crystalline structure
of impregnating material formed what reduced the HCN adsorption capacity. From the
microwave drying curve only the curve obtained by of 2000 l/h was found as
permanently better than the vacuum drying curve, the structure of impregnating material
was amorphous according to an XRD examination and here is the biggest equuilibrum
asdsorption capacity. The explanation of the phenomenon, that in the crystalline form of
impregnating substances can not, or only minimaly can step into a reaction with HCN
gas. The XRD examinations had been taken up after HCN breakthrough measurement
proved that the crytalline form of (Cu0.3Zn0.7)5(CO3)2OH and CuO after a HCN
treatment was unchanged. Summarizing the results it can be stated that the structure of
impregnating material on the surface of active carbon would be amorphous if the
velocity of air flow is enough to the effective drying.
The increase of the activation temperature to 175 °C caused a decrease of the
equilibrum adsorption capacity in the both in the case of microwave and thermal
activation (samples 377/151 and 377/200vt-3).
4.2.2. Production of a microwave drying and heat treatment equipment
On the basis of experience gathered with the small laboratory equipment a big
laboratory microwave dryer has been designed. The microwave rotational tubular
furnace is suitable equipment for the microwave heat treatment of solid granular
substances (drying, heat treatment, chemical and biochemical transformations). The
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main elements of the equipment are the more, multimodal microwave apparatus
(practically domestic microwave oven) suitable connecting in the line in which a
borsilicate glass tube is placed, and drived exterior motor with a constant rotation. The
suitable tilt of the glass tube, and his turning velocity insures the continuous flow of
substance. The substance to be treated in the microwave space continuously circulates
and it gets mixed continuously in a glass tube.
Additional task was to solve the continuous regulation of the microwave
performance of the single microwave ovens, in this manner each single microwave oven
can be regulated independently, between 0 and 850 W. A closed hopper in point of view
of microwave has been developed and this hopper could feed the active carbon with
great accuracy into the microwave space.
Figure 7. Microwave rotational tubular furnace
Similarly to the feeding a closed outlet in the point of view of microwave has been
developped with a thermometer stub, a water vapour and an ammonia sucking stubs.
The leaving water vapour condensed a laboratory cooler, the ammonia was neutralized
in two glass absorber containing 5% sulphuric acid. The necessary air flow was insured
with a domestic wet vacuum cleaner. Figure 7 shows the picture of equipment.
The developed big laboratory equipment continuously worked for 8 hours without
any problem. The function parameters were optimized, the optimal tilt angle of the
tubular furnace, its rotational velocity and the feeding velocities were defined. An
experiment was made to dry a not impregnated active carbon containing 30% water
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with a material volume flow velocity of 7500 cm3/h and with air flow of 2000 l/h. The
charge of the rotational tubular reactor was 1500±200 ml. A drying process was carried
out with impregnated active carbon with the optimised working parameters and after
85 min working time the remaining water content of the active carbon was 0.82% and
the temperature of active carbon was 135 °C.
Production of ASZ-TEDA type active carbons in the experimental plant
In the first step the moisture content decrease was examined in the case of 340 W
microwave energy and air flow velocities of 3000 l/h (sample number is 377/ 209) and
10000 l/h (sample is 377/208).
Table 3. Production of ASZ and ASZ-TEDA type impregnated active carbon in the experimental plant
Number of
experiments
Drying
min. MW
Activation
min. MW/°C
1377/200
31 340
3000 l/h
60 340/160
3000 l/h
1377/201
50 255
3000 l/h
58 255/150
3000 l/h
34.03 1.20
377/203
70 255
7000 l/h
38 340
7000 l/h
35 225
3000 l/h
33 255/150
7000 l/h
50 255/155
7000 l/h
65 225/135
3000 l/h
34.33 0.67
377/206
33 340
8000 l/h
26 255/135
8000 l/h
33.8 2.21
377/207
32 340
10000 l/h
28 255/150
10000 l/h
33.99 0.76
377/210
26 340
5000 l/h
56 255/150
10000 l/h
33.99 0.57
377/211**
28 225
3000 l/h
–
5.14 2.58
377/212
29 340
7000 l/h
29 340
7000 l/h
27 340
7000 l/h
39 255/155
7000 l/h
40 255/155
7000 l/h
36 255/155
7000 l/h
33.10 1.30
377/204
377/205
377/213
377/214
Water content
(%)
begin end
34.84 1.40
33.10 0.76
33.37 1.76
33.10 2.21
33.10 1.20
XRD*
Information
Crystalline
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Crystalline
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Almost amorphous
(Cu0.3Zn0.7)5(CO3)2 OH
Almost amorphous
(Cu0.3Zn0.7)5(CO3)2 OH
Crystalline
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Crystalline
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Crystalline
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Crystalline
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Almost amorphous
Cu2(CO3)(OH)2
(Cu0.3Zn0.7)5(CO3)2 OH
Almost amorphous
(Cu0.3Zn0.7)5(CO3)2 OH
Almost amorphous
(Cu0.3Zn0.7)5(CO3)2 OH
Almost amorphous
(Cu0.3Zn0.7)5(CO3)2 OH
HCN equil.
adsorption
capacity (g/g)
–
0.0219
0.0343
0.0358
0.0190
0.0217
0.0213
0.0215
0.0142
0.0169
0.0285
0.0235
* Determined from the intensity of XRD lines.
** Impregnating with TEDA.
548
AARMS 7(3) (2008)
L. HALÁSZ et al.: Use of the microwave impregnation of active carbon
Comparing the results with those, which were given by the vacuum drying, where
HCN equiluibrum adsorption capacity was the biggest one, it was found that the velocity
of the decrease of water content at the 3000 l/h air flow velocity was similar to one of
vacuum drying, thus this air velocity was the first data which was used. The experiments
in which the air flow velocity was 3000 l/h indicated that (sample number is 377/200) the
drying very fast, the impregnating substances will be in a crystalline form. The drying
time was increased by the reduction of the microwave energy at constant value of air flow
velocity (sample number is 377/201, 205), but at this time crystalline impregnating
substances formed. These results indicated that it is necessary to optimize the working
parameters of experimental plant. The air flow velocity was increased to 7000 l/h and the
microwave energy was kept on constant value, then in the course of the drying and
activation the impregnating substances got into an amorphous state.
Continuing the optimizing process, the air flow velocity was decreased to 5000 l/h
during the drying and increased to 10000 l/h during the activation, unfortunately
crystalline impregnating material formed.
5. Comparison of the characteristic of produced NBC impregnated carbon with
some commercial product
The characteristics of the produced impregnated active carbon (377/213, 204), which
were called Sovecarbon NBC-1 and NBC-2 respectively) was compared the parameters
of K5M (Russian), Pleisch CW and Pleisch NBC-T impregnated active carbons as it can
be shown in Table 4.
It can be observed that the impregnated active carbons produced with the new
technology have the better or similar parameters than the commercial active carbons.
Table 4. Characteristics of different NBC impregnated active carbons
Parameter
Bulk density g/cm3
Moisture content (%)
BET surface (m2)
Pore volume (cm3/g)
HCN adsorption (g/g)
ClCN adsorption (g/g)
CCl4 adsorption (g/g)
Benzene adsorption (g/g)
AARMS 7(3) (2008)
Sovecarbon
NBC-1
0.455
2
1424
0.64
0.0343
0.028
0.128
0.230
Sovecarbon
NBC-2
0.452
2
1388
0.76
0.0358
0.033
0.130
0.178
Pleisch
CW
0.452
3
1295
0.64
0.036
0.028
0.057
0.095
Pleisch
NBC-T
0.460
2
1305
0.67
0.028
0.027
0.053
0.210
549
L. HALÁSZ et al.: Use of the microwave impregnation of active carbon
6. Conclusion
A new technology was developed for the impregnation of active carbon. This new
technology uses microwave in both the impregnation and both the heat treatment stage
of the process. The advantage of the use of microwave is a more effective impregnation
process and a homogenous and lower temperature heat treatment. The energy
requirement of the all process is lower than the classical technology. The new
technology can be used for production of different impregnated active carbons and the
heat treatment unit also can be used for regeneration of active carbon.
7. References
1.
2.
3.
4.
5.
6.
7.
550
GY. BUCSKY, L. HALÁSZ, J. SOLYMOSI, A. UJHIDY, A. VASS, Á. VINCZE: Hungarian patent, P0401518, 2004.
ROSSIN, J. A.: Carbon, 29(2) 197–205 (1991).
ROSSIN, J. A.: Carbon, 27(4) 611–3 (1989).
US Patent 4 531 953 (1985).
US Patent 4801311 (1989).
WHEELER, A.: J. Catal., 13 (1969) 299.
JONAS, L. A., REHRMANN, J. A.: Carbon, 1 (1973) 59.
AARMS 7(3) (2008)