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Mesquite derived activated carbons: Microwave activation production and physical characterization

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MESQUITE DERIVED ACTIVATED CARBONS: MICROWAVE ACTIVATION
PRODUCTION AND PHYSICAL CHARACTERIZATION
A Thesis
by
ADITYA SINGH
Submitted to the College of Graduate Studies
Texas A&M University-Kingsville
In partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
December 2010
Major Subject: Environmental Engineering
UMI Number: 1504342
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UMI 1504342
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MESQULTE DERIVED ACTIVATED CARBONS: MICROWAVE ACTIVATION
PRODUCTION AND PHYSICAL CHARACTERIZATION
A Thesis
by
ADLTYA SINGH
Approved as to style and content by:
a
OC)CUMA
David Ramirez, Ph.D.
(Chairman of Committee)
Alvaro Martinez, Ph.D.
(Member)
i, Ph.D., P.E.
(Member)
im D. Jones, Ph.D., P.E.
(Head of the Department)
IAJ—S
w^Ay
Ambrose Anoruo, Ph.D.
(Associate Vi/e-President for Research and Graduate Studies)
December 2010
ABSTRACT
Mesquite Derived Activated Carbons: Microwave Activation Production and Physical
Characterization
(December 2010)
Aditya Singh, B.E., RTM University, Nagpur, Maharashtra State, INDIA
Chairman of Advisory Committee: Dr. David Ramirez
Activated carbon adsorbents are commonly used in industrial operations to remove
pollutants from contaminated air streams and water bodies. Activated carbon captures
pollutants by an adsorption process in which the pollutant adheres to the surface of the
adsorbent. The manufacture of activated carbon involves the carbonization of the raw
material at high temperature and the activation of the carbonized material. Activation can
occur through chemical or physical processes. Chemical activation is not an
environmentally friendly process because of the production of hazardous wastes. Physical
activation uses activation agents such as steam, CO2 to increase the porosity and surface
area of the materials.
Recent studies have shown that microwave heating can be used as activation technique
for producing activated carbons with large porosity and higher mechanical strength as
compared with the traditional steam activation. The objective of this study is to
manufacture activated carbon from mesquite barks using microwave. Mesquite tree barks
abundantly found in South Texas has high carbon content and this study shows that it can
potentially be effective adsorbents. Microwave activation was used to prepare the
mesquite-derived activated carbon (MDAC). These MDAC samples were then physically
iii
characterized in terms of the surface area, pore size, porosity and pore size distribution
using a high-speed surface area and pore size analyzer, bulk density and percent yield and
adsorption capacity of methanol. The MDAC samples were compared with commercially
available samples such as granular activated carbon and activated carbon monolith.
Experimental results for microwave activation indicated that the optimum conditions
were 200 W and microwave exposure time was 30 seconds when mesquite was
carbonized at 675 °C for 1 hour. N2-BET surface area and pore volume were found to be
673 m /g and 0.28 cm /g, respectively. An increase in carbonization temperature
increased the surface area of the mesquite-derived activated carbon. Optimum conditions
obtained for the manufactured MDAC through microwave activation was seen to be
particularly useful for short activation times and microwave power. These promising
results demonstrate that using mesquite for manufacturing activated carbons can help
recycle and reduce waste, and provides an environmentally friendly process for making
MDAC. Raw mesquite showed high porosity with a potential source for the manufacture
ofMDACs.
IV
DEDICATION
For my family & friends
ACKNOWLEDGEMENTS
I would like to express my deepest gratitude to my advisor, Dr. David Ramirez, for
his excellent guidance, caring, patience, and providing me with an excellent atmosphere
for doing research. The contribution of my advisor extends beyond the transfer of his
extensive technical skills which I cannot express in words. His commitment and true
kindness has drawn a sense of true reverence and admiration in me which will remain
unparalleled.
I would also like to extend my sincere thanks to all the committee members Dr.
Joseph O. Sai and Dr. Alvaro Martinez for their guidance and support during my
research.
Finally, I would also like to thank my parents, all of my friends and the staff for their
support and affection. They were always supporting me and encouraging me with their
best wishes.
vi
TABLE OF CONTENTS
ABSTRACT
Ill
DEDICATION
V
ACKNOWLEDGEMENTS
VI
TABLE OF CONTENTS
VII
LIST OF TABLES
IX
LIST OF FIGURES
X
CHAPTER I INTRODUCTION
1
1.1
BACKGROUND
1
1.2
ACTIVATED CARBON
2
1.3
MANUFACTURE OF ACTIVATED CARBONS
3
1.3.1
Carbonization
3
1.3.2
Activation
3
1.4
CHARACTERIZATION OF ACTIVATED CARBONS
6
1.5
RESEARCH OBJECTIVES
6
1.6
RESEARCH SIGNIFICANCE
6
1.7
HYPOTHESIS
8
CHAPTER II METHODOLOGY
2.1
2.2
9
MANUFACTURE OF MESQUITE-DERIVED ACTIVATED CARBON
9
2.1.1
Sample collection
9
2.1.2
Carbonization
10
2.1.3
Microwave activation
10
PHYSICAL CHARACTERIZATION
vn
11
2.2.1
Percent yield
12
2.2.2
Bulk density
12
2.2.3
Nitrogen adsorption surface area and pore size distribution
12
2.2.4
Methanol equilibrium adsorption capacity
13
2.2.5
Adsorption isotherm modeling
13
2.2.6
Morphology
15
CHAPTER III RESULTS AND DISCUSSIONS
16
3.1
MICROWAVE ACTIVATION
16
3.2
PHYSICAL CHARACTERIZATION
18
3.2.1
Percent yield
18
3.2.2
Bulk density
20
3.2.3
Nitrogen adsorption surface area and pore size distribution
20
3.2.4
Methanol equilibrium adsorption capacity
23
3.2.5
Morphology
26
CHAPTER IV SUMMARY AND CONCLUSIONS
28
REFERENCES
31
VITA
35
viii
LIST OF TABLES
Table 1. Process conditions and physical properties of MDAC and commercially
available ACs
17
Table 2: Freundlich and Langmuir model parameters at
ix
24
LIST OF FIGURES
Figure 1. (a) Granular activated carbon, (b) Activated carbon monolith
3
Figure 2. Bench-scale experimental set up for the manufacture of ACs by microwave
activation
10
Figure 3. Percent yield as a function of different carbonization temperatures and
microwave activation at 200 w and 30, 40 sec
19
Figure 4. Percent yield as a function of carbonization temperature of 650°C for 1 hr
and microwave activation at 200 W and 10, 30, 60 and 300 sec
19
Figure 5. Bulk density as a function of different carbonization temperatures and
microwave activation at 200 w and 30, 40 sec
20
Figure 6. Adsorption isotherm for microwave activated samples for microwave
activation at 200 w and 30, 40 sec
21
Figure 7 BET surface area as a function of different carbonization temperatures and
microwave activation at 200 w and 30, 40 sec
22
Figure 8. Pore size distribution of microwave activated samples for optimum
conditions using BJH method for mesopore region
23
Figure 9. Equilibrium adsorption capacities with methanol
25
Figure 10. Adsorption/Desorption Isotherm for 200 W, 30 sec and 675°C
25
Figure 11. Comparison of Freundlich and Langmuir adsorption isotherms with
experimental data
26
Figure 12. FE-SEM images of a) carbonized MDAC sample at 675°C andl hr, and bd) the microwave treated MDAC sample at 200W and 30 sec and carbonized
for 675°C andl hr
27
x
CHAPTERI
INTRODUCTION
1.1
BACKGROUND
As air pollution is growing day by day due to anthropogenic activities, there is an ever
growing demand of better emission control technologies. Adsorption that uses activated
carbon (AC) is a process that can be used for improving ambient air quality. AC is a
highly porous material useful in adsorption of organic vapors from waste gas streams and
water in industrial operations1. Adsorption of contaminants onto the ACs largely depends
on the pore size distribution of the adsorbent and geometry of the adsorbate . Previous
studies have shown that, a wide range of amorphous carbon-based materials including
lignin, cassava peel, nutshells, woodchips, waste biomass or tires can be used to produce
activated carbons which exhibit high degree of porosity and extended surface area2"7.
This property of ACs finds its applications in separation of gases, biomedical
engineering, catalyst supports, recovery of solvents, removal of organic pollutants from
i
o
drinking water etc ' . Success of adsorbents which are mostly microporous solids, vitally
depends on factors such as, pore size distribution, micropore volume and hence the
surface area. The pore size recommended by International Union for Pure and Applied
Chemistry (IUPAC) are: Micropores d < 2 nm, Mesopores 2 < d < 50 nm, Macropores d
>50nm.
Equilibrium adsorption capacity can be determined from isotherms by computing the
amount of pollutant adsorbed onto the adsorbent at equilibrium conditions. Gases
Style and format according to Journal of Environmental Science and Technology
1
2
adsorbed can be desorbed effectively by heating to high temperatures via steam or hot
combustion gases or by reducing the pressure to a low value. Previously, microporous
substances such as charcoal were used to remove organic vapors. With new
developments and recent technologies helped develop methods that produced adosrbents
such as activated carbon, zeolites, silica gel, and alumina. The ease of the whole process
and the comparatively cheap installation and operational costs makes the adsorption
technology more attractive.
1.2
ACTIVATED CARBON
Multiple varieties of activated carbon materials are commercially available today.
Surface area and attrition properties are the most critical properties based on which
activated carbon is selected. Some examples of commercially available ACs are granular
activated carbon (GAC), activated carbon monolith (ACM) and activated carbon fiber
cloth (ACFC) as shown Figure 1. GAC is being used since a long time and they are the
oldest form of adsorbent available. Internal surface area is large for GAC and it ranges
from 500 to 2000 m2/g9. ACM is a coal-based adsorbent prepared from powdered
activated carbon. Specific surface area of ACM ranges from 900-1100 m2/g and contains
4-6% by mass ash. The structure of monolith includes 400 longitudinal parallel open
channels of square section of 0.2 cm per side, and wall thickness between the channels of
0.05 cm10.
3
f.
.
f ;:
HsHBH
:•':
-
•
•
.
'
•
••.....
» • - :
(a)
'
•
,
>
(b)
Figure 1. (a) Granular activated carbon, (b) Activated carbon monolith
1.3
MANUFACTURE OF ACTIVATED CARBONS
1.3.1
Carbonization
Manufacture of AC involves the carbonization of the raw material at high temperature
and the activation of the carbonized material using an inert carrier gas 1 ' 5 . Activation
enhances pore volume and surface area of the sample material. Commonly used methods
for activation include chemical and physical activation.
1.3.2
Activation
Typically physical activation uses steam or carbon dioxide as the activating agents.
Steam activation has high temperature requirements as compared to CO2 which involves
less energetic reaction7. ACs produced by steam activation generally exhibit a 'fine'
(Microporous, <2 nm) to 'medium' (Mesoporous, 2-50 nm) pore structure which has a
significant effect on the adsorption properties of organic compounds8. Although pore size
distribution plays a major role in determining adsorption capacity of organic vapors on
AC, it is difficult to produce specific ACs for a given pore size distribution at low
4
temperatures using low cost precursors5. In a study, Eucalyptus kraft lignin was used for
the production of ACs by and it was found that physical activation of the precursor with
CO2 at 850°C for 20 hrs yielded a surface area of 1853 m2/g with a micropore volume of
0.57 cm3/g. this study also suggested that increase in activation time enhances
microporosity and widens of micropore size distribution6. In a study conducted by
Lehman et. al., activated carbons were prepared from waste tires using steam activation
and they exhibited N2-BET specific surface area as high as 1031 m /g .
Chemical activation is another method of manufacturing ACs, which requires lower
temperature and gives high yield, large surface area and develops higher micropore
volume as compared to physical activation11'12. In previous studies, it was reported that
chemical activation can be done in two or three step process in which sample is
impregnated with a chemical and then carbonized at high temperatures under constant
flow of nitrogen. After the sample has cooled down, it is washed with a dilute acid
solution and then with distilled water until it reaches neutral pH1' n '
13
.
Chemicals
generally used as activation agents are phosphoric acid (H3PO4), zinc chloride (ZnC^),
potassium hydroxide (KOH) and sodium hydroxide (NaOH). Based on the findings of
earlier investigations, steam activation is a more preferred method than chemical
activation to prepare ACs from waste-derived precursors as it consumes chemicals which
are expensive and generates undesirable residues which in turn creates waste disposal
problems '
. Also, in chemical activation there is concern regarding various aspects
such as corrosion or treatment of wastewater which further discouraged advancements of
this method . In a study conducted to manufacture ACs from mesquite wood chips in the
laboratory using physical activation, results showed that the surface area of wood-derived
5
activated carbon (WDAC) samples increased with the activation temperature and time
using steam activation1 . A more recent approach to manufacture ACs is by activating the
raw material using microwave heating.
Various technologies nowadays make use of microwaves for heating dielectric materials.
Recent investigations have shown promising results in application of microwave heating
for manufacture of waste-derived activated carbons17"20. Activation process in some cases
usually takes up to hours or even a day to produce ACs using conventional heating
systems, adding time, energy and cost to the whole process. Also conventional methods
generally do not ensure uniform heating for different shapes and sizes of precursors
resulting in a thermal gradient from the surface to the interior of the sample18. Microwave
activation of waste-derived precursor's supplies heat directly to the carbon sample
quickly and effectively as compared to conventional treatment where heating takes place
by convection or conduction, thus reducing treatment time and energy consumption
significantly " . So far there have been few publications which describe one step
preparation of ACs derived from wood using microwave. In recent investigations,
microwave-induced chemical activations were reported using chemical agents such as
ZnCb, K2CO3 or KOH where waste-derived precursors such as tobacco, coke were
impregnated with the chemical and dried under air to obtain a mixture of desired weight
ratio. The mixture was then placed in a suitably modified microwave furnace for different
time and power levels under a constant flow of nitrogen to obtain AC ' ' . In a recent
study ACs prepared from tobacco stems via microwave-induced K2CO3 achieved BET
surface area and pore volume as high as 2557 m2/g and 1.647 cm3/g respectively18.
6
1.4
CHARACTERIZATION OF ACTIVATED CARBONS
Characterization of ACs plays an important role as it describes the physical and chemical
properties of adsorbents with specific morphologies used to capture and recover organic
vapors from waste gas streams . Clear understanding of both the adsorptive and physical
characteristics of the material can assist in selecting an AC. The surface area, % yield,
bulk density, pore size distribution, average pore width and total pore volume of the
wood derived activated samples are determined to study the performance capabilities.
Adsorption capacity is not only determined by physical properties, but also by chemical
properties as the behavior of ACs attribute to the effects due to the presence of other
chemically bonded elements such as carbon, hydrogen and nitrogen in the raw material
used or activating gas .
1.5
RESEARCH OBJECTIVES
The objectives of this research are to:
1) Design an experimental setup for microwave activation to manufacture ACs to
study the effects of carbonization temperature and microwave heating on physical
properties of mesquite-derived activated carbon (MDAC).
2) Characterize the laboratory prepared MDAC materials in terms of physical and
adsorption properties and compare them with the commercially available ACs.
1.6
RESEARCH SIGNIFICANCE
Mesquite plants are thorny shrubs or tree of the legume family infest large region of the
Mexican-American border, New Mexico and Northern Mexico . Mesquite brush fills
7
over 50 million acres in the State of Texas. Previous investigations demonstrated the
suitability of mesquite wood as a carbon and energy source26. It is a hard wood, used for
heating homes, cooking, furniture and implements. Waste generated from such activities
add burden for its disposal.
Due to its abundance, South Texas home-grown mesquite woodchips have a great
potential to be used as precursors for the manufacture of ACs. Interest is growing in the
use of other low-cost precursors and abundantly available range of fruit and agricultural
wastes particularly in tropical countries for the manufacture of ACs. The use of these
wastes can provide a recycling path eliminating the problem of their disposal and also
derive incredible environmental and economical benefits. This requires developing
sustainable methods which can ensure both processes and products to be cost effective
and environment friendly. This study will have the following contributions:
1) Development of a procedure for producing highly efficient MDACs with high
porosity and surface area using microwave activation.
2) Establishment of a protocol to utilize the waste derived products and thus helps in
waste recycling and sustainable development.
3) Significant reduction in activation time to fabricate ACs thus reducing energy
consumption.
4) Determine the optimum conditions of microwave activation for the manufacture
of MDACs.
8
1.7
HYPOTHESIS
Mesquite trees are abundantly available in South Texas region, thus providing a good
source of raw material for the production of wood-derived activated carbon. In this study,
microwave activation is used instead of conventional furnace heating to manufacture ACs
as they can potentially consume less energy, and can produce mechanically strong ACs
with high surface area and large porosity. The proposed hypotheses are the following:
1. To manufacture MDAC using microwave activation that will have similar
physical and adsorption capacities when compared to commercially available
activated carbons such as granular activated carbon (GAC) and activated carbon
monolith (ACM).
2. With an increase in carbonization temperature for low microwave power levels
and exposure time, the surface area and pore volume of the activated carbon
adsorbent will be enhanced for shorter microwave activation times.
3. The physical properties such as surface area, pore volume, average pore radius of
the prepared ACs will be enhanced with increase in microwave power levels and
time of exposure.
CHAPTER II
METHODOLOGY
Activated carbons (ACs) were fabricated using mesquite wood collected from the Texas
A&M University-Kingsville (TAMUK) campus and broken to small pieces of size 2-5
mm using a chisel and a hammer. The manufacture process of AC samples involved two
main steps: Carbonization of the carbonaceous mesquite woodchips followed by
microwave activation. These samples were then physically characterized in terms of %
yield, bulk density, N2-BET surface area, pore volume and average pore size as described
below. Equilibrium adsorption capacities of methanol with prepared samples were
determined using symmetrical gravimetric organic vapor sorption analyzer. Morphology
of ACs was also studied using a field-emission scanning electron microscope. This
section of the report provides detail description of the production of mesquite-derived
activated carbons (MDACs) with microwave and their physical characterization process.
2.1
MANUFACTURE OF MESQUITE-DERIVED ACTIVATED CARBON
2.1.1
Sample collection
Mesquite samples were collected from two different locations - the mesquite groove at
TAMUK campus (batch 1) and Brownsville landfill site (batch 2). Bark used from
mesquite tree branches was broken down to small wood chips using a chisel and a
hammer to obtain a size of 2-5mm.
9
10
2.1.2
Carbonization
The experimental apparatus consisted of compressed ultra-high purity nitrogen (UHP-N2,
Matheson, 99.999% pure) as the inert carrier gas and the gas flow rate (0.5 Lpm) was
controlled by a volumetric flow meter (Omega, FL-2060), a 3-zone temperature
controller furnace reactor (Lindberg/ Blue, Model STF55346C-1), and a temperature
monitoring and data logger system, for data collection as shown in Figure 2. Twenty-five
grams of dry mesquite wood sample was placed in a steel basket which was then placed
at the centre of the 3-zone tube furnace. Furnace was set to desired temperature and times
(600°C, 625°C, 650°C and 675°C for lhour).
Thermocouple - - •
Data
Logger
To The Exhaust
PC
A
[lit
S f e V ^ s i ! JESWI
31
•I
'Bltfef
V
jefel
(H
>«**.
v
t«»•• •
Furiuco
IP**
Flow
" Meter
Flow
Meter
Carbonized
Wood
CARBONIZATION
Control
Panel
Concealed
Reactor
ACTIVATION
Figure 2. Bench-scale experimental set up for the manufacture of ACs by microwave
activation.
2.1.3
Microwave activation
A commercial microwave oven (MW8119SBM, Emerson, 1000 watts, 2450 MHz) was
used to fabricate ACs as shown in Figure 2 . The oven has 10 power level controllers
11
(100 to 1000 watts). 0.5 g of carbonized mesquite samples were placed in a polyethylene
terephthalate PETE / or hig-density polyethylene (HDPE) container (used as a concealed
reactor). Nitrogen was then passed through the reactor in order to provide inert
environment in the reactor. The samples were then activated using microwave heating for
different time and power levels. As the temperature of the sample is nearly impossible to
measure due to the internal and volumetric nature of microwave heating, input power of
the microwave was used as process parameter instead of the sample temperature.
2.2
PHYSICAL CHARACTERIZATION
Ac samples were characterized by physical and adsorption properties. Physical properties
included bulk density and % yield. The other physical properties such as surface area
(using the Brunauer-Emmet and Teller (BET) method), total pore volume, pore size and
pore size distribution was determined using a high-speed surface area and pore size
analyzer (Quantachrome, Model: NOVA 2200e). Equilibrium adsorption capacities of
methanol with the MDAC samples were determined using a gravimetric method with a
symmetrical organic vapor sorption analyzer (VTI INC., MODEL SGA100). The
adsorbent morphology and shape was also characterized using a field-emission scanning
electron microscope (JEOL FE-SEM JSM-6701F). Visual characterization of the
adsorbent determined the pore size and shape of the activated carbon adsorbent. More
details of the experimental procedures are provided below.
12
2.2.1
Percent yield
It represents the overall efficiency of the resultant product after the activation process. It
was calculated as the ratio of final weight to initial weight of the sample and this ratio
was expressed in percentage.
% Yield = F i n a l W 6 i g h t x 100
Initial weight
2.2.2
(1)
Bulk density
It is defined as the ratio of mass of the material to the total volume of the material. The
total volume corresponds to the pore and inter-particle volume. Bulk density (p) of a
matter varies depending on its composition. To estimate the bulk density, sample was
weighed (m) and the total volume (V) of the sample was measured. A graduated cylinder
was used to determine the volume with tapping (3 times). It was then calculated using the
following formula:
2.2.3
m
n.
P= —
V
(2)
Nitrogen adsorption surface area and pore size distribution
N2-adsorption isotherms were evaluated at liquid nitrogen temperature (77K) using a
high-speed surface area and pore size analyzer (Quantachrome, Nova 2200e Series). Prior
to analysis, the samples were degassed for 2 hrs at 200°C to remove moisture or
unwanted gases from the sample. The N2 adsorption data obtained from the analysis was
used to find out the N2- BET surface area, pore width, total pore volume and the pore size
13
distribution. "The mesopore size distribution was determined using the BJH method
which is based on a model of the adsorbent as a collection of cylindrical pores. This
method accounts for capillary condensation in the pores using the Kelvin equation, which
in turn assumes a hemispherical liquid-vapor meniscus and a well-defined surface
tension".
2.2.4
Methanol equilibrium adsorption capacity
Equilibrium adsorption capacities of methanol for the MDAC samples were determined
using symmetrical gravimetric organic vapor sorption analyzer (VTI Inc., Model
SGA100). The three major sections of SGA are: 1) saturators, 2) heating chamber and 3)
An electronic microbalance. The microbalance is capable of quantifying weights ranging
from 0.1 ug to 100 g. Elongated chains in heating chamber is supported from the
microbalance which weighs the sample, dew point analyzer controls the relative humidity
for the water vapor adsorption experiments. Equilibrium adsorption isotherms were
computed for the adsorbate's relative pressures ranging from 0 to 0.95 and at 25°C16.
2.2.5
Adsorption isotherm modeling
This section describes Freundlich and the Langmuir adsorption models used in this study
to model the experimental data.
a) Freundlich isotherm model: This model is represented by a power law equation, for
modeling a single solute. The adsorption capacity for this model follows a linear behavior
with change in adsorbate concentration on logarithmic scales. The Freundlich equation is
represented as follows27:
14
q = KC 1/n
(3)
q - volume of adsorbate adsorbed/g of adsorbent.
C - Concentration of the adsorbate
K & n - constants
Taking log on both sides, the linear form of Freundlich equation can be represented as
follows:
Log q = (1/n) log C + log K
(4)
b) Langmuir isotherm model:. Langmuir equation is based on kinetic model of the
adsorption process. It assumes single-layer adsorption on a homogeneous surface.
Langmuir equation is represented as follows :
q
bC
(5)
" = 17c"
qm - maximum adsorption capacity of adsorbent for adsorbate,
b and qm - adsorption parameters
After simplifying the eq(5) a linear form of Langmuir equation can be obtained to
determine the model parameters as follows:
q q m blcj qm
A total relative error (TRE) concept was used to assess the accuracy and correlation of
the models to the experimentally obtained data. TRE is calculated as follows:
HI model
4experimental
H experimental
xlOO
(7)
15
2.2.6
Morphology
The adsorbent morphology and shape was also characterized using a field-emission
scanning electron microscope (JEOL FE-SEM JSM-6701F). Visual characterization of
the adsorbent determined the pore size, surface roughness and shape of the activated
carbon adsorbent.
CHAPTER III
RESULTS AND DISCUSSIONS
3.1
MICROWAVE ACTIVATION
Carbonization temperature and time, microwave activation power and time, the bulk
density, percent yield, N2 BET-surface area, total pore volume and average pore width of
the produced microwave treated samples and commercially available samples are
provided in Table 1. Three batches of experiment were conducted by varying
carbonization time of 600°C, 625°C, 650°C and 675°C for a constant temperature of 1 hr.
Microwave radiation power level and time was varied from 100 W to 600 W for 10, 20,
30, 40, 60 and 300 seconds for each batch of carbonized wood. Prepared samples were
analyzed for duplicates and their average values were reported in. The optimum
conditions for microwave activation were found to be 200 W and 30 seconds at a
carbonization temperature of 675°C for 1 hr achieving N2-BET surface area of 673 m2/g
which was comparable to activated carbon monolith (ACM,573 m /g). An increase in the
carbonization temperature (from 600°C to 675°C for a constant time of 1 hr) resulted in
higher surface area with and smaller bulk density due to burn-off of the carbonaceous
material.
Physical properties of granular activated carbon (GAC) and ACM are provided in Table
1. In general, N2-BET surface area, total pore volume and bulk density of GAC and ACM
were found to be higher as compared to the mesquite-derived activated carbon (MDAC)
samples.
16
17
Table 1. Process conditions and physical properties of MDAC and commercially
available ACs.
Sample
Carbonization
Temp
(°C)
Time
(min)
Microwave Activation
Power
(W)
100
.. 2Q0...
Mesquite,
Carbonized &
Microwaved3
300
600
60
400
500
600
Mesquite,
Carbonized &
Microwaved3
Time (sec)
20
40
60
20
40,..;..;
60
20
40
60
20
40
60
20
40
60
20
40
60
Bulk
Density
(g/mL)
0.13
0.13
0.12
0.15
1U.14 :
0.14
0.17
0.16
0.15
0.15
0.14
0.12
0.16
0.15
0.14
0.17
0.16
0.15
N2-BETTotal Pore Avg. Pore
Surface
Volume
Width
% Yield
Area
3
(cm
/g)
(A)
(m2/g)
100.0
229.9
13.8
0.13
100.0
253.8
13.4
0.13
100.0
14.1
273.9
0.14
100.0
237.8
14.2
0.13
4 frwwS
0.44 "
loojii moteA
20.7
99.7
0.34
249.9
17.5
97.3
195.6
0.20
97.3
288.6
0.24
15.6
88.7
296.3
0.14
13.0
99.7
12.9
244.6
0.12
89.3
304.0
0.14
12.8
81.7
12.2
317.8
0.14
85.7
12.7
277.2
0.13
83.3
306.2
0.14
12.3
81.7
304.5
0.14
12.5
90.0
195.4
13.0
0.09
0.12
13.4
85.3
234.9
74.0
162.2
14.3
0.09
*
625
60
10
650
60
Mesquite, No
Treatment
-
-
Mesquite,
Carbonized &
Microwaved"
675
60
100
200
300
400
500
600
500
GACC
ACMd
a
I" - !
J -
98.0
272.5
-r574'.0v
212.5
277.0
0.13
"--ro.23s*-
0.09
0.09
92.0
78.0
-
0.18
--
365.79
10
10
10
10
10
10
30311ft!
30
0.10
0.10
0.12
0.14
0.13
0.13
0.12
0.12
98.0
89.0
91.0
83.4
91.6
90.8
463.1
410.5
409.7
446.0
490.9
439.5
94.0
360.7
12.0
0.21
11.4
0.15
0.17
11.6
12.0
0.19
12.2
0.20
0.20
11.9
O.2:§M iSilJflli
11.6
0.15
93.IP
0.10
0.13
12.8
-• M:8
12.1
13.0
60
300
0.1156
11.7
-
--
-
-
0.20
-
425.9
0.15
11.6
—
--
—
-
-
0.45
1.04
—
-
1216
573
0.62
0.80
25.5
15.2
-
Batch 1
Batch 2
C
GAC: Granular activated carbon (Carbon BPL)
d
ACM: Activated carbon monolith (RIDC, Beijing)
b
*
1
•• "tao'trjs vmt&sd ^&W-
silKoo
Mesquite, No
Treatment
0.12
ft."
•••"i-in.*'
• *•
• 1
Mesquite,
Carbonized &
Microwaved3
' *
18
3.2
PHYSICAL CHARACTERIZATION
The physical properties of the prepared ACs obtained conformed as seen in previous
studies. Trends were observed for the optimum conditions of microwave activation with
different carbonization temperatures.
3.2.1
Percent yield
The activation degree of a sample is dependent on the microwave radiation, exposure
time and the carbonization conditions applied to the precursors. In this study the results
indicated that the % yield of the sample under the optimum conditions of microwave
activation decreased with increase in carbonization temperature as shown in Figure 3
ranging from 100% to 93.8% as expected. With higher carbonization and activation
degree, lower would be the yield as there would be more carbon burn off. Similarly
Figure 4 also shows a negative slope when the samples were carbonized at 650°C for lhr
and treated at a constant microwave input power level of 200 W and exposed for 10, 30,
60 and 300 sees.
19
100
y = -0.0832x+149.14
R2 = 0.8598
99
98
97
£ 96
95
94
93
92
575
600
650
625
675
700
Carbonization Temperature (°C)
Figure 3. Percent yield as a function of different carbonization temperatures and
microwave activation at 200 w and 30, 40 sec
100.00
y = -0.0637x + 96.874
R 2 = 0.9771
95.00
90.00
2
85.00
80.00
75.00
0
25
50
75
100
125
150
175
200
225
250
275
300
325
Activation Time (sec)
Figure 4. Percent yield as a function of carbonization temperature of 650°C for 1 hr and
microwave activation at 200 W and 10, 30, 60 and 300 sec
20
3.2.2
Bulk density
It was expected that an increase in carbonization temperature for a constant microwave
activation resulted in more mass of carbon being lost, subsequently resulting in a smaller
bulk density of the carbonaceous material. As seen in Figure 5, a slight increase was
observed in the sample corresponding to the carbonization temperature of 675°C.
0.15
y = -0.0003x + 0.3093
R2 = 0.7075
0.14
S
."S
m
C
0.13
M
0.12
<u
0.11
575
600
625
650
675
700
Carbonization Temperature (°C)
Figure 5. Bulk density as a function of different carbonization temperatures and
microwave activation at 200 w and 30,40 sec
3.2.3
Nitrogen adsorption surface area and pore size distribution
Nitrogen adsorption isotherm obtained for all the samples corresponding to the optimum
conditions with relative pressure ranging from 0.01 to 0.9 had a similar pattern
suggesting similar adsorption behavior and indicating increase in adsorption capacity
with increase in relative pressure (Figure 6). The adsorption isotherms were distinct from
21
each other and the amount adsorbed at relative pressure of 0.1 markedly increased for
200 W when treated for 30 seconds when carbonized at 675°C.
X
X
260
X
X
X
X
210
H
H
72
®
s
J"
160
Wt
m
S
•
B
X
#
B
B
B
•
B
B
•
•
•
•
•
•
•
•
X
B
X
•
•
s
>
n
•
•
•
X
•
?
•
•
X
•
110
X200W,40sec&600C
X
X
• 200W, 30 sec & 650 C
B200W, 30 sec & 675 C
en
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Relative Pressure, P/P0
Figure 6. Adsorption isotherm for microwave activated samples for microwave activation
at 200 w and 30, 40 sec
In Figure 7, the N2- BET surface area significantly increased with the increase of
carbonization temperature for the samples processed at 200 W and exposed for 30 sees.
Samples manufactured from location 1 & 2 exhibited surface area of as high as 672.7
m /g and 574.0 m /g respectively. Whereas the untreated samples showed low surface
area values of 365.8 m /g and 425.9 m /g for location 1 & 2 respectively.
22
700
650
M
a
y = 4.2091x-2165.9
R2 = 0.9995
600
550
<
V
u
S-
500
S
Cfl
H
W
ffl 450
400
350
575
600
625
650
675
700
Carbonization Temperature (°C)
Figure 7 BET surface area as a function of different carbonization temperatures and
microwave activation at 200 w and 30, 40 sec
Figure 8 shows the pore size distributions (PSD) of the prepared samples for the optimum
conditions analyzed by the BJH method (for mesopores). Mesopores were mainly found
in the range of 2 to 5 nm for these samples. Sample carbonized at 600°C for 1 hr and
microwave treated 200 W for 40 second showed larger mesopores as compared to the
ones treated for 30 seconds.
PSD for untreated mesquite and 30 second samples
indicated that it had a narrow range of mesopores as compared wide range of mesopores
for 40 second sample.
23
0 020
200 W, 40 sec & 600 C
0018
1
200 W, 30 sec & 675 C
200 W, 30 sec & 650 C
0 016
Raw mesquite wood
0014
41
b
0 012
>
0010
u
o
a.
0 008
!!
"-+3
£
5
0 006
0 004
0 002
0 000
Pore Radius (nm)
Figure 8. Pore size distribution of microwave activated samples for optimum conditions
using BJH method for mesopore region
3.2.4
Methanol equilibrium adsorption capacity
The methanol equilibrium adsorption capacities of MDAC, ACM and GAC were
calculated at relative pressures ranging from 0 to 0.95 at 25°C and are provided in Figure
9. The equilibrium adsorption capacities increased with increasing methanol relative
pressure. It was observed that the equilibrium adsorption capacity of methanol for
MDAC (microwave activated at 200 W and 30 sec), ACM and GAC ranged from 0-130
mg/g, 0-260 mg/g, 0-300 mg/g, respectively at 25°C. The methanol adsorption capacities
increased logarithmically with increase in relative pressure until 0.4 after which it started
to level off. The plot also depicts comparable adsorption capacities of the MDAC (~70
mg/g at relative pressure of 0.05) with ACM (-90 mg/g at relative pressure of 0.1) and
24
GAC (-90 mg/g at relative pressure of 0.05) at low relative pressures which are often the
sustaining industrial condition. It is expected that the methanol adsorption capacity of
ACM would be lower than 90 mg/g at relative pressure of 0.05.
Figure 10 provides the equilibrium adsorption and desorption isotherms of methanol on
the MDAC sample microwave activated at 200 W for 30 sec. Adsorption of methanol on
ACs increased logarithmically with increase in relative pressure. Desorption of methanol
followed a similar behavior and decreased logarithmically with the decrease in relative
pressure. The adsorption and desorption curves fit each other closely with a small gap
between the curves known as hysteresis effect. It occurs when the amount of organic
compound adsorbed is different then when it is being removed .
Figure 11 compares experimental methanol equilibrium adsorption data with modeled
results from the Freundlich and Langmuir models. A total relative error (TRE) concept
was used to assess the accuracy and correlation of the models to the experimentally
obtained data. The TRE obtained between the Langmuir and experimental data was 2%
and 3% for the Freundlich and experimental data. This shows that the experimental data
fits closely with both Freundlich and Langmuir models. The model parameters for
Freundlich and Langmuir are as follows:
Table 2: Freundlich and Langmuir model parameters at
25°C for the activated carbon samples in this study
Sample Freundlich Model Langmuir Model
K
n
K
qm
MDAC
129.0
4.6
125.0
20.0
0.8
370.4
ACM
2.0
1.6E-05
GAC
2.0
2.7
322.6
8.3E-05
350
A
A
300
250
200
150
\f
X
x
100
£
x
•*•
X
x
*
B
X
x
X
AGAC
• ACM
50
X 200 W, 30 Sec & 675 C
om
00
01
02
03
04
05
06
07
08
09
10
Relative Pressure
Figure 9. Equilibrium adsorption capacities with methanol
13
12
11
o Adsorption
10
• Desorption
00
01
02
03
04
05
06
07
08
09
Relative Pressure (P/Po)
Figure 10. Adsorption/Desorption Isotherm for 200 W, 30 sec and 675°C
10
26
140
OX)
"630
X20
S
100
Experimental Data
«
c
Langmuir Model
S3
u
Freundlich Model
a
o
«P4
• * -
O.
i.
©
<:
s
0.00
0.20
0.40
0.60
Relative Pressure
0.80
1.00
1.20
(P/PQ)
Figure 11. Comparison of Freundlich and Langmuir adsorption isotherms with
experimental data
3.2.5
Morphology
The field emission-scanning electron microscope (FE-SEM) images of two of the MDAC
samples (Figure 12): a) an MDAC sample carbonized at 675°C for 1 hr and b-d) an
MDAC sample microwave activated at 200 W for 30 sec. Images were taken at a
magnification ranging from X500 (10 um scale, Figure 12b) to X5 0,000 (100 nm scale,
Figure 12d). Figure 12a shows the FE-SEM image of the surface of a carbonized sample
of mesquite woodchip. It was observed that the carbonized mesquite sample had an
amorphous and a non-uniform structure which had no observable pores. When the
carbonized mesquite was activated using microwave, opening up of pores were observed
as seen in Figure 12b-c at 500X and 1000X respectively.
•rawsMBeige**
\''*}
W^'f^'^-'tf^^'
tit? rjf
& f a
* + T*" ™
4&fr'-f
*
" '•-
><* *
'".
' ••
V
'»
—,
v -
«- .
-V
I
TOi t
'• .
m
,
V.-
'<•
-VAP*
.
^
-
V*
^-mp»i.'»(^^^j.^i ! i"»i,iif W » i <> 1 jji ; ~-' i ..,
a
•-
-
..
IX. V \ ° v
-J r!''
T T i H T H
Figure 12. FE-SEM images of a) carbonized MDAC sample at 675°C andl hr, and b-d)
the microwave treated MDAC sample at 200W and 30 sec and carbonized for 675°C
andl hr
CHAPTER IV
SUMMARY AND CONCLUSIONS
Mesquite tree barks are abundantly found in the state of Texas (estimated at 50 million
acres). This study has shown that the production of carbonaceous-based adsorbents from
mesquite wood provides a recycling path. The microwave activated mesquite-derived
activated carbon (MDAC) is a promising cost-effective adsorbent to adsorb gaseous
compounds with high adsorption capacities at low relative pressure at levels typical from
industrial emissions.
Previous studies suggested that the physical properties of activated carbons (ACs) were
enhanced such as mechanical strength as compared to the traditional steam activation.
Activating waste-derived precursors using microwave heating significantly reduces
processing time as compared to conventional heating, e.g. from 30 seconds to 6 hrs.
Even though the energy consumption in this study was not quantified, microwave
activation has a potential to significantly reduce consumption of energy of the whole
process. Mesquite woodchips were used as precursor for preparing ACs which was
carbonized for four different temperatures of 600°C, 625°C, 650 °C, 675 °C for 1 hr and
treated with microwave for different power levels (100 W to 600 W) and time of (10 sec
to 300 sec). Prepared MDACs were characterized and experimental results indicated that
the optimum conditions occurred when mesquite wood was carbonized at 675°C for 1 hr,
followed by microwave activation at 200 W with microwave exposure time of 30
'y
seconds. N2-BET surface area and pore volume reached values of 673 m /g and 0.28
cm /g, respectively and was comparable to activated carbon monolith (ACM).
28
29
Mesquite itself has a great potential to be an effective adsorbent. This study reports for
the first time that mesquite wood has an N2-BET surface area of 395 m /g. The results
obtained in this study are promising and additional test are required to further modify the
microwave setup to create more controlled experimental environment.
Optimum conditions obtained for the manufacture of MDACs through the microwave
heating was useful for shorter activation times. The microwave activation process has
potential reduction in energy consumption and provides a more environmental friendly
process. The short activation time and simplicity of the activation process demonstrate
that the microwave-activation method is a promising approach to convert waste mesquite
tree barks into useful adsorbent with high surface area and adsorption capacity.
Conclusion 1: The manufacture of MDAC using microwave activation for low
microwave power level and short activation times was found to have similar or higher
physical properties (surface area of 673 m2/g of MDAC as compared to 573 m2/g of
ACM) and adsorption capacities (70 mg/g of MDAC at relative pressure of 0.05 as
compared to 90 mg/g for both ACM and granular activated carbon, GAC, at relative
pressure of 0.1 and 0.05, respectively) when compared to commercially available
activated carbons such as ACM and GAC.
Conclusion 2: As hypothesized, with an increase in carbonization temperature from
600°C to 675°C, the surface area, pore volume and pore width of the activated carbon
adsorbent was enhanced for shorter activation times.
Conclusion 3: The physical properties such as surface area, pore volume, average
pore radius of the prepared MDACs enhanced considerably for shorter microwave power
levels (200 W) and shorter time of exposure (30 sec).
30
Comparatively, so far there has been little work published about microwave activation
and there is considerable scope for more detailed studies on the production,
characterization
and
adsorption
properties
of
MDACs.
Following
are
the
recommendations for future purposes in extension to his study:
\/ Microwave activation: Prepare more samples with a two step sequential activation
using chemicals such as K2CO3. Samples can be prepared under different power levels
ranging from 100 W to 600 W for exposure times of 10-30 seconds.
V Consumption of energy during microwave activation may be quantified and compared
with other available activation methods.
V Elemental analysis may be conducted to determine the composition of carbon,
nitrogen and hydrogen of the prepared samples using the Leco TruSpec (CHN)
elemental analyzer.
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1. Hayashi, J. i.; Kazehaya, A.; Muroyama, K.; Watkinson, A. P., Preparation of
activated carbon from lignin by chemical activation. Carbon 2000, 38, (13), 18731878.
2. Diaz-Diez, M. A.; Gomez-Serrano, V.; Fernandez Gonzalez, C ; Cuerda-Correa, E.
M.; Macias-Garcia, A., Porous texture of activated carbons prepared by phosphoric
acid activation of woods. Applied Surface Science
APHYS'03 Special Issue 2004, 238, (1-4), 309-313.
3. Lehmann, C. M. B.; Rostam-Abadi, M.; Rood, M. J.; Sun, J., Reprocessing and Reuse
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Technology 2006, 97, (5), 734-739.
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7. Wei, S.; Li, Z.; Yaping, Z., Preparation of Microporous Activated Carbon from Raw
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31
32
8. Zhu, Z.; Li, A.; Yan, L.; Liu, F.; Zhang, Q., Preparation and characterization of
highly mesoporous spherical activated carbons from divinylbenzene-derived polymer
by ZnC12 activation. Journal of Colloid and Interface Science 2007, 316, (2), 628634.
9. Hassler, J. W., Activated Carbon. Chemical Publishing Co.: New York, 1963; p 397.
10. Luo, L.; Ramirez, D.; Rood, M. J.; Grevillot, G.; Hay, K. J.; Thurston, D. L.,
Adsorption and electrothermal desorption of organic vapors using activated carbon
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Technology 4th French Meeting on Powder Science and Technology 2005, 157, (1-3),
48-56.
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pitch by ZnC12. Applied Surface Science 2006, 252, (17), 5976-5979.
13. Srinivasakannan, C ; Zailani Abu Bakar, M., Production of activated carbon from
rubber wood sawdust. Biomass and Bioenergy 2004, 27, (1), 89-96.
14. Adinata, D.; Wan Daud, W. M. A.; Aroua, M. K., Preparation and characterization of
activated carbon from palm shell by chemical activation with K2C03. Bioresource
Technology 2007, 98, (1), 145-149.
15. Wu, F.-C; Tseng, R.-L.; Hu, C.-C; Wang, C.-C, Physical and electrochemical
characterization of activated carbons prepared from firwoods for supercapacitors.
Journal of Power Sources 2004,138, (1-2), 351-359.
33
16. Sakaray, A.; Gangupomu, R. H. In Manufacture and Characterization of Mesquite
Wood Derived Activated Carbon for Air Quality Control Applications, Air & Waste
Management Association 101st Annual Conference & Exhibition, Portland, Oregon,
2008; Portland, Oregon, 2008.
17. Guo, J.; Lua, A. C , Preparation of activated carbons from oil-palm-stone chars by
microwave-induced carbon dioxide activation. Carbon 2000, 38, (14), 1985-1993.
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activated carbons from tobacco stems with K2C03 activation using microwave
radiation. Industrial Crops and Products 2008, 27, (3), 341-347.
19. Li, W.; Peng, J.; Zhang, L.; Yang, K.; Xia, H.; Zhang, S.; Guo, S.-h., Preparation of
activated carbon from coconut shell chars in pilot-scale microwave heating equipment
at 60 kW. Waste Management 2009, 29, (2), 756-760.
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carbon from waste tea by chemical activation with microwave energy. Fuel 2008, 87,
(15-16), 3278-3285.
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microwave heating KOH activation. Applied Surface Science 2007, 254, (2), 506-512.
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from wood via microwave-induced ZnC12 activation. Carbon 2009, 47, (7), 18801883.
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USA, 1988.
24. Marsden, J.; House, I., The chemistry of gold extraction. 2 ed.; SME: 2006; p 651.
34
25. Taylor, M. J., The Mesquite Economy in the Mexican-American Borderlands.
Journal of Latin American Geography 2008, 7, (1), 133-149.
26. Thayer, D. W.; Murray, J. O., Physiological, Biochemical and Morphological
Characteristics of Mesquite Wood-digesting Bacteria
10.1099/00221287-101-1-71. Journal of General Microbiology 1977, 101, (1), 71-77.
27. Frank, L. S., Adsorption Technology- A step by step Approach to Process Evaluation
and Application. 1985.
28. Anne, A. N.; Adrina, S. F.; Leandro, S. O., Activated carbons from waste biomass:
An alternative use for biodiesel production solid residues. Bioresource Technology
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VITA
Aditya Singh received his Bachelor's degree in Civil Engineering in the year 2007 from
Shri RamdeoBaba Kamla Nehru Engineering College (S.R.K.N.E.C) affiliated to
Rashtrakant Tukodoji Maharaj Nagpur University in Maharashtra, India. In spring 2008,
he joined the Environmental Engineering Master's Program at Texas A&M UniversityKingsville. Since then he has been working with Dr. David Ramirez, assistant professor
on the area of manufacture and characterization of Activated Carbon Adsorbent using
novel methods of activation from waste Mesquite tree barks for air pollution control as a
graduate research assistant. 23 Baji-Prabhu Nagar, Near Ram Nagar, Nagpur-440033,
Maharashtra, INDIA.
35
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