1. INTRODUCTION
The adsorption is
the process of accumulating substances that are in solution on a suitable
interface. Adsorption is a mass transfer operation in that a constituent in
liquid phase is transferred to the solid phase. The adsorbate is the substance
that is being removed from the liquid phase at the interface. The adsorbent is
the solid, liquid or gas phase on to which the adsorbate accumulates. The
adsorption process has not been used extensively in waste water treatment, but
demands for a better quality of treated waste water effluent, including
toxicity reduction, have led to an intensive examination and use of the process
of adsorption on activated carbon. Activated carbon treatment of waste water is
usually thought of as a polishing process for water that has already received
normal biological treatment. The carbon in this case is used to remove a
portion of the remaining dissolved organic matter.
Many volatile
organic compounds (VOCs) are toxic, posing a high risk to human health
as a result of their widespread use and occurrence in workplace
environments. Reducing the amount of organic vapours in ambient air is,
therefore, an important task. Activated carbons are the most versatile and
frequently used adsorbents, and fixed beds of activated carbon, in the
form of canisters or filters, are widely used for purifying contaminated
air. Its large internal surface area and pore volume, its ability to
absorb most organic vapours and low cost make activated carbon one of the
most practical adsorbents.
Activated carbon
beds eventually become exhausted after continuous exposure to
air contaminated with organic vapours. The time at which an organic vapour
of a defined concentration is able to penetrate the bed is known as the
breakthrough time of the adsorbate. In practical situations, a key factor
is the service life of the filter. This is defined as the time at which
the concentration of the compound penetrating the filter reaches
an unacceptable level. An accurate estimate of this service life is of great
importance to both users and manufacturers. A predictive model for filter
performance would reduce the need for time consuming filter tests, aid in
the design of filters with optimized performance and
provide knowledge of the service life of military filters exposed
to conditions and chemicals encountered in civilian situations.
2. ACTIVATED
CARBON
Activated carbon
is a solid, porous, black carbonaceous material. It is distinguished from
elemental carbon by the absence of both impurities and an oxidized surface. It
can be prepared from a large number of sources such as coconut, wood, peat,
coal, tar, sawdust, and cellulose residues. Any carbon source can be converted
into activated carbon via a number of methods. Usually, the process is divided
into carbonization and activation. During carbonization most of the non-carbon
elements are removed in gaseous form by the pyrolytic decomposition of the
source material. The porous structure is mainly developed during activation by
means of an activation agent that reacts with the carbon. Such agents may
include synthetic acids, bases, and other substances in a stream of activating
gases such as steam (H2O), nitrogen (N2) or carbon
dioxide (CO2).
Fig.1: Activated
carbon: surface and pores – scanning electron microscope image. Magnification
increases from left to right
Activated carbon
has an extraordinarily large surface area and pore volume, making it suitable
for a wide range of applications. It can be used as a decolorizing agent, a taste
and odour removing agent or as a purification agent in food processing. One
major use of activated carbon is in water purification, including the
production of potable water and the treatment of waste and ground waters.
The most important
property of activated carbon, the property that determines its usage, is the
pore structure. The total number of pores, their shape and size determine the
adsorption capacity and even the dynamic adsorption rate of the activated
carbon. IUPAC
classifies pores as follows:
macropores: d0 >
50nm
mesopores: 2 ≤ d0 ≤
50nm
micropores: d0 <
2nm
ultramicropores: d0 <
0.7nm
supermicropores:
0.7 < d0
where d0 is
the pore width for slit type pores or the pore diameter for cylindrical pores.
The macropores act
as transport pathways, through which the adsorptive molecules travel to the
mesopores, from where they finally enter the micropores. The micropores usually
constitute the largest proportion of the internal surface of the activated
carbon and contribute most to the total pore volume. Most of the adsorption of
gaseous adsorptives takes place within these micropores, where the attractive
forces are enhanced and the pores are filled at low relative pressures. Thus,
the total pore volume and the pore size distribution determine the adsorption
capacity.
3. TYPES
OF ACTIVATED CARBON
3 1 Powdered Activated Carbon
PAC
for water treatment: The feed
location of PAC can be at any point prior to filtration. The most common
locations are in the flash mixer or flocculator since these pieces of equipment
will mix the PAC into the water very well. However, some plants feed PAC
just before filtration so that the PAC will form a layer on top of the filter
and ensure that all water comes in contact with the activated carbon.
Adding PAC just before filtration can cause problems, though, since the small
PAC particles can pass through the filters and cause dirty water complaints
from customers or can cake filters and result in reduced filter
runs. Powdered activated carbon, or PAC, is a form of activated
carbon with a very small particle size. Treatment involves adding PAC to
water, allowing the PAC to interact with contaminants in the water, then
removing the PAC by sedimentation or filtration.
Regardless of the
feed location, PAC can be added to water using either a dry feeder or as
slurry. Dry feeders are most often used in small plants when PAC is fed
at intervals in response to periodic taste and odour problems. In
contrast, slurries (mixtures of PAC with water) are used in larger
plants or when PAC is fed continuously. Since it is difficult to make the
PAC mix with water, the mixer should have an overhead spray system.
The effectiveness
of PAC in adsorbing tastes and odours depends on adequate mixing, contact time,
dosage, and on the cause and concentration of the taste/odour problem.
Mixing and contact time are determined by the location at which the PAC is
added in the treatment process, so adjustments made by the operator will
usually involve only dosage adjustments. The dosage usually ranges from 1
to 15 mg/L but must be much higher, in the range of 100 mg/L or more, when the
PAC is being used to remove trihalomethanes or trihalomethane precursors.
The operator chooses an appropriate dosage using jar tests and the results from
odour and taste tests.
3 2 Granular Activated Carbon
Granular activated
carbon, also known
as GAC, has a larger particle size than PAC with an associated
greater surface area. Like PAC, GAC can remove trihalomethane precursors
as well as taste and odour compounds.
4. APPLICATION
OF ACTIVATED CARBONGAC is used as a filter medium, either as a layer in a
rapid-sand filter or in a separate filter known as a contactor.
When contactors are used, the contactor is placed downstream of the filter so
that turbidity won't clog the contactor.
Although GAC filters can be operated like a rapid sand filter in most ways, backwashing and surface washing are not the only cleaning required for the units. The entire surface of the GAC will eventually become covered with contaminants, just as a softener's resin will become covered with magnesium and calcium ions. A GAC filter can typically operate for months or years before reaching this state, depending on the contaminant levels in the influent water. Once the GAC has reached its adsorption capacity, it must be regenerated using the same heating process used to activate the carbon.
Carbon adsorption
is used principally for the removal of refractory organic compounds, as well as
residual amounts of inorganic compounds such as nitrogen, sulphides and heavy
metals. The removal of taste and odour compounds from waste water is another
important application, especially in reuse applications. Both powdered and
granular activated carbon are used and appears to have low adsorption affinity
for low molecular weight polar organic species. Typical compounds that can be
removed by carbon adsorption are given in the table.
States
|
Purpose
|
Applications
|
Examples
|
|
Gas Phase
|
Recovery
|
Gasoline Vapour Recovery
|
Gasoline Fuel recovery, ELCD
|
|
Solvent Recovery
|
MEK, Cyclohexanone, CS2,
Furon, Trichloroethane
|
|||
Odour Removal
|
Room Odour Removal
|
Tobacco, CO, Room filters,
Toilet Odour, Pet Odour
|
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Refrigerator
|
Deodourizer
|
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Automobile
|
Cabin air filters
|
|||
Tobacco
|
Cigarette Filter
|
|||
Hospital
|
Anaesthetic gas removal
|
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Ozone Removal
|
Copiers, Laser Printers
|
|||
Harmful Gas
|
Closed Environment
|
Dioxin removal, Space Ships,
Underground CO2
|
||
Gas Separation
|
Nitrogen PSA
|
Nitrogen Gas Separation
|
||
Other PSA
|
Radio Active Gas
|
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Liquid Phase
|
Water Treatment
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Factory Waste Water
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Cleaning Waste Water
|
|
Drinking Water Treatment
|
||||
Decolourization of
Industrial Chemicals
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Industrial Use
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Sugar refinement,
Pharmaceutical use, Whisky distilment
|
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Medical Applications
|
Medical and Nursing
|
Kidney machine, Nursing
supplies, Respirators
|
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Electronics
|
Electrodes
|
Double Layer Capacitors,
Hardisks
|
||
Mineral Recovery
|
Gold Recovery
|
Gold Recovery
|
5. ACTIVATED CARBON
ADSOPTION SYSTEMS
5.1 Fixed bed type
A fixed bed column is used commonly for contacting
waste water with GAC. Fixed bed columns can be operated singly, in series or in
parallel. Granular medium filters are commonly used upstream of the activated
carbon contractors to remove the organics associated with the suspended solids
present in secondary effluent. The water to be treated is applied to top of the
column and withdrawn at bottom. The carbon is held in place with an under drain
system at the bottom of column. Provision for backwashing and surface washing
is often provided in waste water applications to limit the head loss build-up
due to the removal of particulate suspended solids within the carbon column.
The advantage of down flow design is that
adsorption of organics and filtration of suspended solids is accomplished in a
single step. Although upflow fixed bed reactors have been used, down flow beds
are used more commonly to lessen the chance of accumulating particulate matter
in the bottom of the bed, where the particulate matter would be difficult to
remove by backwashing. If soluble particles are not maintained at a high level,
more frequent regeneration of carbon may be required.
5.2 Expanded bed type
Expanded bed, moving bed and pulsed-bed carbon
contactors have also been developed to overcome the problems associated with
head loss build-up. In expanded bed system, the influent is introduced at
the bottom of the column and the activated carbon is allowed to expand, much as
a filter bed expands during backwash. When the adsorptive capacity of activated
carbon is exhausted, the bottom portion of carbon is removed, and an equivalent
amount of regenerated or virgin carbon is added to the top of the column. In
such a system, expanded-bed upflow contractors may have more carbon fines in
the effluent than down flow contractors because bed expansion leads to the
creation of fines as the carbon particles collide and abrade, and allows the
fines to escape through passageways created by the expanded bed.
5.3 Fluidized bed reactor type
A fluidized bed reactor (FBR) is a type
of reactor device that can be used to carry out a variety of multiphase chemical
reactions. In this type of reactor, a fluid (gas or liquid) is passed
through a granular activated carbon at high enough velocities to
suspend the solid and cause it to behave as though it were a fluid. This process,
known as fluidization, imparts many important advantages to the FBR. As a
result, the fluidized bed reactor is now used in many industrial applications.
The solid substrate (activated carbon) material in
the fluidized bed reactor is typically supported by a porous plate, known
as a distributor. The fluid is then forced through the distributor up
through the activated carbon. At lower fluid velocities, the solids remain in
place as the fluid passes through the voids in the material. This is known as
a packed bed reactor. As the fluid velocity is increased, the reactor
will reach a stage where the force of the fluid on the solids is enough to
balance the weight of the solid material. This stage is known as incipient
fluidization and occurs at this minimum fluidization velocity. Once this
minimum velocity is surpassed, the contents of the reactor bed begin to expand
and swirl around much like an agitated tank or boiling pot of water. The
reactor is now a fluidized bed. Depending on the operating conditions and
properties of solid phase various flow regimes can be observed in this reactor.
6. ADSORPTION
ISOTHERM
The quantity of adsorbate that can be taken up by
an adsorbent is a function of both the characteristics and concentration of
adsorbate and the temperature. The characteristics of the adsorbate that are of
importance include: solubility, molecular structure, molecular weight,
polarity, and hydrocarbon saturation. Generally, the amount of material
adsorbed is determined as a function of the concentration at a constant
temperature, and the resulting function is called an adsorption isotherm.
Adsorption isotherms are developed by exposing a given amount of adsorbate in a
fixed volume of liquid to varying amounts of activated carbon. Typically more
than ten containers are used, and the minimum time allowed for the samples to
equilibrate where powdered activated carbon is used in seven days. If granular
activated carbon is used, it is usually powdered to minimize adsorption
times. At the end of the test period, the amount of absorbate remaining
in solution is measured. The adsorbate phase concentration data computed using
the equation, are then used to develop adsorption isotherms.
qe = [(C0 - Ce )/m] V
where,
qe = adsorbent phase concentration
after equilibrium, mg adsorbate/ g adsorbent
C0 = initial concentration of
adsorbate, mg/L
Ce = final concentration of
adsorbate after absorption has occurred, mg/L
V = volume of liquid in the reactor, L
m = mass of adsorbent, g
Laboratory evaluation of the adsorption isotherm
and the adsorption capacity is time consuming and may be affected by toxicity
or the availability of the adsorbate. For such difficult cases, a model that
can predict the adsorption capacity, making testing unnecessary, would be
highly desirable. A number of such models have been proposed for the adsorption
isotherm: the Freundlich isotherm equation; the Langmuir isotherm; BET-theory;
the Hacskaylo and Levan equation; the Dubinin Raduskevish equation; and a
modification of the DR equation developed by Stoeckli.
Dubinin Raduskevish Equation
Among the existing predictive equations, the DR
equation has been the most widely used to predict organic vapour adsorption
onto activated carbon. It has several advantages:
a) there is a good data fit over a wide
concentration range
b) temperature is included as a parameter
c) it is built around physical parameters
d) it is easy to apply.
Dubinin postulated that the amount of vapour
adsorbed (W) by an activated carbon source, at a relative pressure (P/Ps),
is a function of the thermodynamic potential (A), with A expressed
as
A = RT ln (Ps/P)
where R is the universal gas
constant, T is the absolute temperature, Ps is
the saturated vapour pressure at temperature T, and P is
the partial pressure of the adsorbate.
By examining the adsorption of simple organic
compounds, such as benzene, Dubinin concluded that the function was Gaussian.
This led to the classical expression of Dubinin and Radushkevich (the D-R
equation):
W = W0 * exp (A/βE0)2
Where, W0 is the
maximum amount adsorbed, E0 is the characteristic
adsorption energy for a reference vapour on a specific adsorbent. The parameter
β is called the affinity coefficient or similarity coefficient, and expresses
the ratio of the characteristic free energies of adsorption for the test and
reference vapours. Benzene is, by convention, used as the reference compound
for carbonaceous materials, and is, by definition, given the value β =
1.
Freundlich Isotherm
The Freundlich isotherm is used most commonly to
describe the adsorption characteristics of the activated carbon used in water
and waste water treatment. It was derived in 1912 and is defined as follows:
x/m = KfCe1/n
Where, x/m is the mass of adsorbate adsorbed per
unit mass of adsorbent, mg adsorbate/ g activated carbon
Kf = Freundlich capacity factor,
(mg absorbate/ g activated carbon)
Ce = equilibrium concentration of
adsorbate in solution after adsorption, mg/L
1/n = Freundlich intensity parameter
Langmuir Isotherm
Langmuir adsorption isotherm is derived by
assuming: (1) a fixed number of accessible sites are available on the
adsorption surface, all of which have the same energy, and (2) adsorption is
reversible. Equilibrium is reached when the rate of adsorption of molecules on
to the surface is same as the rate of desorption of molecules from the surface.
The rate at which adsorption proceedes is proportional to the driving force,
which is difference between the amount adsorbed at a particular concentration
and the amount can be adsorbed at that concentration. At the equilibrium
concentration, this difference is zero.
Langmuir adsorption isotherm can be expressed as:
x/m =(abCe)/(1+bCe)
x/m = mass of adsorbate adsorbed per unit mass of
adsorbent, mg adsorbate/g activated carbon
a, b = empirical constants
Ce = equilibrium concentration of adsorbate in
solution after adsorption, mg/L
Adsorption isotherm
|
7. ADSORPTION
KINETICS
Initially, when a contaminated gas stream passes
through a packed carbon bed, most of the contaminant, the adsorbate, is
adsorbed in the vicinity of the inlet to the bed. The gas then passes on with
little further adsorption taking place. Later, when the inlet part of the
adsorbent becomes saturated, adsorption takes place deeper inside the bed. As
more gas passes through and adsorption proceeds, the so-called mass transfer
zone (MTZ) moves forward until the breakthrough point is reached. If the flow
of gas is continued, the exit concentration from the bed will rise gradually
until it reaches the level of the inlet concentration. At this point, the bed
is fully saturated. The service life of the filter bed is regarded as the time
when the exit concentration has reached an unacceptable level.
Adsorption kinetics
|
Several models to predict the breakthrough curves
for physisorption of organic vapours have been proposed. Among these, the
Wheeler-Jonas equation is the most widely used to estimate the breakthrough
time of organic compounds on activated carbon. It has a simple form, with some
parameters readily available from the literature or from carbon manufacturers,
and it is known to yield good predictions for breakthrough times. It has
recently been shown that the Wheeler-Jonas equation has a wider scope of application
than just physisorption for a constant flow pattern.
The Wheeler-Jonas equation takes the form
where
tb = time to reach the breakthrough fraction b = Cx / C0 (min)
C0 = bed inlet concentration (g/cm3)
Cx = chosen breakthrough
concentration (g/cm3)
We = equilibrium adsorption
capacity of the carbon for a given vapour (g/g carbon)
Q = volumetric flow rate (cm3/min)
ρB = bulk density of the
carbon bed (g carbon/cm3)
To use this equation, two parameters, We and kv, must
be determined. This can be done
either experimentally or by extrapolation from
measurements using a reference adsorbate. The first parameter We,
the adsorption capacity, is usually calculated from an adsorption isotherm
equation; the Dubinin-Radushkevich equation is often used in the case of
organic vapour adsorption.
8. ADVANTAGES OF ACTIVATED CARBON ADSORPTION
· Highly
effective at removing non-polar organic chemicals from water.
· Applicable
to a wide variety of organic compounds
· Very
effective at removing colours from waste streams.
· Effective
at removing low levels (ppb range) of inorganic pollutants.
· Thermal
regeneration of the carbon destroys the adsorbed waste solute.
· Very
flexible system allows rapid start-up and shut down as needed.
· System
can be designed so that it is portable, to be taken to waste sites.
9. LIMITATIONS OF ACTIVATED CARBON ADSORPTION
· Limited
to wastes with low organic concentrations (< 5%).
· Limited
to wastes with very low inorganic concentrations (< 1%).
· Unable
to remove highly soluble organics, or those with low molecular weights.
· Systems
cannot tolerate suspended solids in the influent stream (due to clogging).
· High
operating costs due to carbon costs system requirements.
· Disposal
of contaminated carbon can be problematic if it is not regenerated.
REFERENCES
1.
Armenante, M Piero, ‘Adsorption with Granular Activated Carbon (GAC)’,
NJIT
2. Congjin Chen,
Xin Li, 2014, ‘Modification process optimization, characterization and
adsorption property of granular fir-based activated carbon’, Applied
Surface Science, 1-9
3. Fabio
Montagnaro, Ana Silvestre-Albero et. al., 2014, ‘Post-combustion CO2 adsorption
on activated carbons with different textural properties’, Microporous
and Mesoporous Materials, 1 – 8
4. Fuat Guzel,
2006, ‘Characterisation of the micropore structure of activated
carbons by adsorption of nitrogen and some hydrocarbons’,Separation Science
and Technology, 590 -594
5. Jufang
Wu, 2004, ‘Modelling adsorption of organic compounds on
activated carbon’, 1-20
6. M. Balsamo, T.
Budinova et. al., 2013, ‘CO2 adsorption onto synthetic
activated carbon: Kinetic, thermodynamic and regeneration studies’, Microporous
and Mesoporous Materials, 1 – 8
7. Metcalf and
Eddy, 2003, ‘Waste Water Engineering: Treatment and Reuse’, 1138
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