activation of carbon produced from coconut shell by using fluidized bed reactor
TRANSCRIPT
Activated Carbon From Coconut Shell By
Using Pyrolysis And Fluidized Bed Reactors.
Prof. (Dr.) S. N SAHA, HOD, CHEMICAL ENGINEERING DEPARTMENT, IT,GGV,BILASPUR(C.G.)
RATAN MONDAL, VIIITH SEM STUDENT, CHEMICAL ENGINEERING DEPARTMENT,IT,GGV,BILASPUR(C.G.)
Abstract: A detailed study on production of chemically activated carbon from coconut
shells by Pyrolysis and fluidized bed reactor in India.The production process consists of a
pyrolysis stage and an activation stage. The effect of process variables such as void fraction,
particle size, area parameters, temperature of activation, andfluidizing velocity etc. on the
production and quality of activated carbon is studied. A study on change in variable when
other variables changes by using MATLAB programming on basis of data obtained from
industries which are using fluidized bed reactor for production of activated carbon.
Keywords: Activated carbon, Coconut shell, Fluidized bed reactor, Pyrolysis, MATLAB
programming.
I. INTRODUCTION
Activated carbon is a unique and effective agent for purification and for isolation and
recovery of trace materials. During the last two to three decades, treatment with active
carbon has become an important unit process for separations and purifications in the food,
pharmaceuticals, sugar, chemical and other processing industries.
Activated carbon is an amorphous form of elemental carbon prepared by destructive
distillation of any one of a variety of carbonaceous raw materials, including wood, coal or
coconut shells.
Global scenario:
According to Roskill, a market research firm, global activated carbon consumption was
about 650-kt in 2007, slightly over estimated production of 635-kt. Growth in consumption
in current markets is forecast by the report to be 5% per year through 2015.Noritcarbon
&Calgon carbon is the leading producersof activated carbon in the world.
Total world demand for activated carbon therefore has the potential to rise by nearly 10%
per year to 1.36-mtin 2015, with mercury emission control accounting for 30% of projected
total consumption.
Table: I
Major international manufacturers of activated carbon
Company Location_________
Norit Netherlands, Italy, UK & US
Calgon Carbon US, China
Carbochem Inc. US
CarboPur Technologies Canada
Carbon Activated Corp. US
CPL Carbon Link UK, Germany
Chemviron Carbon UK
Indian scenario:
Coconut shell is an important raw material for activated carbon manufacture, especially in
the southern states. Kochi-based, Indo German Carbons Ltd. claims to be the largest
player in the country and the third largest in the world in the production of coconut shell-
activated carbon. It is also planning to expand capacity to 20,000-tons per annum from the
present 14,000 tons per annum.
There are around 50 producers of Activated Carbon in India, mostly in Medium and SSI
sector. Total production capacity of India is about 80 kilotons.Total domestic demand for
activated carbon is about 50-kt, with the vegetables oil sector the largest endues sector,
accounting for some 35-kt of demand. Domestic demand growth is about 10% per annum
Table: II
Indian demand-supply scenario for
Activated carbon
Kilotons
Capacity 80
Production 70
Imports 5
Exports 25
Domestic demand 50
Table: III
Sector wise demand in India
Sector Demand in tons per
annum
Pharmaceutical 2630
Plasticizers 1750
Glucose/Dextrose Monohydrate/Sorbitol 1550
Vegetable Oil 32500
Miscellaneous sector 6100
Export Sector 400
Total 44930
Summary:
Due to their low ash content, high carbon content, and natural pore structure, coconut
shells are ideal for producing high quality activated carbon.
A review of these patents shows that a number of processes and a variety of industrial
equipment were used for the production of activated carbon, which include shaft kilns,
rotary kilns, moving grate stokers, multiple hearth furnaces, pile furnaces, vertically
stacked and connected crucibles, spaced perforated plates, dual pulsejetcombustion
systems, and fluidized beds. However, rotary kilns are most widely employed for the
manufacture of activated carbon.
The production of activated carbon from coconut shell involves two process. First process
consists of pyrolysis stage followed by activation stage.
In the pyrolysis process, the shells are crushed and sent to a pyrolysis unit. The shells are
held in the unit for two hours at 600 °C and 6 bar while recycled carbon dioxide flows
through the unit at a rate of 6 m3/min. During that period, the shells are carbonized.
Carbonization is the removal of volatiles and other impurities by thermal decomposition,
which results in carbon-rich char. Pyrolization of the coconut shells yields char, syngas,
bio-oils, and water. The bio-oils and syngas by-products are captured and sold in their
crude state.These by-products are marketable and sold in their crude state.
In the activation process, the char is further reduced in size and sent to a fluidized bed
Reactor(FBR). Fluidized bed reactors are well-known for their excellent gas-solid contact
and high heat and mass transfer rates. The vigorous gas-solid contact in a fluidized bed
aids the reaction and also removes the waste gaseous products from the vicinity of the
solids during reaction, thus exposing the solid reactant to the fresh incominggaseous
reactants. The char is activated by steam activation at 900 °C and 1.5 bar for one hour.
Steam activation is chosen over chemical activation because of the issues of
corrosion,Wastewater treatment,and high production costs associated with chemical
activation.
The FBR is the ideal reactor for activation because of its mixing capabilities and superior
heat distribution.Fluidized bed reactor is to maximize energy retention and recycling since
it is responsible for generating all the excess heat in the activation stage of the plant.
Oxygen is also fed to the reactor to enable the combustion of the carbon monoxide and
hydrogen formed during activation. The combustion reactions will convert the dangerous
species like carbon monoxide and hydrogen to steam and carbon dioxide. The steam is
condensed and recycled back to the Fluidised bed reactor.
The excess heat from the reactions is used to generate steam that is sent to the utility
grid.Whenever possible, hot streams are used to heat up cold streams in order to reduce
the amount of cooling water and energy input required. The plant uses large amount of
water reservoir to cool streams and then returns it back to the reservoir itself.
Process Description:
At start or pyrolization process, a compressor compresses CO2 from 1 bar to 6.2 bar which
is introduce in the furnace to eliminate or flush out all the air in the furnace. A conveyor
belt transport coconut shell from a jaw crusher to furnace which reduces the size of shell
from 100mm to 50 mm fragments which heats them up to 600 °C for 30 minutes at a rate of
20 °C/min. The shells remain in furnace for two hours at 600 °C with a continuous flow of
CO2.
Pyrolization of the coconut shells at 600 °C and 6 bar produces char with bio-oil
vapors,steam, and incondensable gases as by-products.The incondensable gasesconsist of
high amounts CO and small amounts of H2 and CO2, which form syngas. The following
reactions occur in pyrolyzer between the carbon, CO2, and moisture from the shells to
produce the syngas and some of the water.
3H2 (g) + CO (g) ↔ CH4 (g) + H2O(g) (1)
H2O(g) + CO (g) ↔ CO2 (g) + H2 (g) (2)
C (s) + 2H2 (g) ↔ CH4 (g) (3)
C (s) + CO2 (g) ↔ 2CO (g) (4)
C (s) + H2O(g) ↔ CO (g) + H2 (g) (5)
During the two and a half hour process, the unreacted carbon, bio-oil vapors, steam, and
syngas from pyrolyzer is send to cyclone separator to separates unreacted solid carbon
from gases. The unreacted solid carbon is collected in a vessel and recycle back to
pyrolyzer for combustion.
The bio-oil vapors, steam and syngas produced during pyrolization is first send to blower,
followed by series of heat exchanger to recover heat which is utilized to heat CO2 used in
pyrolyzer. The bio-oil vapors, steam and syngas leaves the heat exchanger at 620℃. It is
then passed through condenser which used ocean water to cool bio-oil vapors and steam to
95℃.
Then it is taken to gravity separators where bio-oil and water is separates from
syngas(liquid –gas separation). A compressor compresses the syngas to 4 bar and 331℃
and collected in vessels to be marketed.
When the pyrolysis reaction is completed, a screw conveyor moves the coconut char in to a
cone crusher, the crusher reduces the char size from 25 mm to 10 mm. A conveyor belt,
transports the coconut char into the fluidized bed reactor, in the activation stage of the
production plant.
In the second stage of the plant, the coconut char is heated to 900 °C at a rate of 50 °C/min
for 17.5 minutes. The char then reacts with steam for one hour to produce activated
carbon. At this point, the coconut char consists entirely of elemental carbon, and some of
the carbon reacts with water to produce carbon monoxide and hydrogen gas. The gas
escapes from the solid char, leaving behind pores in the carbon solid. The endothermic
carbon-steam reaction takes place in the reactor.
C (s) + H2O (g) CO (g) + H2 (g) (6)
The carbon monoxide gas and hydrogen gas auto-combust to make water and carbon
dioxide gas by the following exothermic reactions:
CO (g) + 0.5 O2 (g) CO2 (g) (7)
H2 (g) + 0.5 O2 (g) H2O (g) (8)
A blower delivers pure oxygen for the combustion of carbon monoxide and hydrogen by
products.The conveyer belt transports the activated carbon out of the reactor at 900 °C.
The activated carbon cools down in transit to the storage location and releases its heat to
the surrounding air, which can be maintained at a cool temperature by fans and air
refrigeration units. Workers package the cooled activated carbon in airtight steel barrels
and the barrels are then shipped to the market.
Fig. I (Functional units )
Detailed Description:Fluidized Bed Reactor.
In fluidization, a gas or liquid is passed through abed of solid particles which is supported
on a perforated or porous plate. In the case of fluidized bed coating, air is passed through a
bed of polymer particles. When the frictional force acting on the particles, or pressure
drop, of the flowing air through the bed equals or exceeds the weight of the bed, the
powder particles become suspended and the bed exhibits liquid-like behavior.
Fluidized beds as chemical reactors offer many unique advantages such as large interfacial
surface areas between the fluid (gas or liquid) and particles, high fluid-particle contact
efficiency, excellent heat transfer, uniform bed temperature, and the ability to handle a
wide range of types of particles and a large quantity of particulate materials. Fluidized-bed
reactors include gas-solid, liquid-solid, and gas-liquid-solid fluidized- bed reactors in terms
of the fluid-particulate systems.Gas-liquid-solid fluidization has already been applied in
many biochemical reactors and in chemical processes where solid particles need to be
contacted with both liquid and gas, for example, aerobic wastewater treatment bioreactors,
sewage sludge pyrolysis and catalyzed reaction systems,etc.
IMPORTANT PARAMETERS INVOLVED
1.FLUIDIZING VELOCITY : It goes on decreasingas the density of the charge decreases
with reaction time due to loss of volatiles and product gases.The iodine number (mg of
iodine absorbed /gm of carbon)increases with an increase in fluidizing velocity whensteam
is used as the fluidizing medium for a particlesize of 1.55 mm at 850 °C. Iodine number
increases due to increase in velocity due to increase in reaction rate.Very high fluidizing
velocity may be poor due to vigorous bubbling of the bed. Most of the experiments are
carried out with fluidizing velocities ranging from 1 to 5 times the minimum fluidization
velocities. From the experimental data it is also observed that for small particles (dp=0.55
mm), the iodine number reaches a maximum around a fluidizing velocity of 8 times the
minimum fluidization velocity and
Decreasesbeyond this value. Since the rate of reactionis high for small particles, the
micropores would havecoalesced, resulting in macropores and thus reducingthe iodine
numbers. As the particlesize increases, the char-CO2 reaction may shift tothe mass transfer
controlled regime and hence anincrease in fluidizing velocity increases the rate ofreaction,
resulting in the formation of micropores, whichcontribute to the increase in iodine
numbers.
2.PARTICLESIZE: Iodine number increases with increase in particle size.As theparticle
size increases,the iodine numbers increase for both steam andCO2, Smaller particles burn
outearly due to faster reaction rates, resulting in porecoalescence and reducing iodine
numbers as comparedto particles of 1.55 mm diameter. Thus higher particlesizes are
required to give a matrix to the product withdeveloped pore structure.
3. STATIC BED HEIGHT: An increase in bed height decreases theiodine numbers, which
may be due to slugging behavior of the bed resulting in poor gas-solid contact.
4 TEMPERATURE: An increase intemperature results in an increase in iodine numbers.
Higher iodine numbers (>700) are obtainedwhen the experiments are conducted at 650 °C
and above, indicating insignificant activation at lower temperatures. On theeffect of
reaction time, at a particular temperature theiodine number increases with reaction time
and reachesa maximum. Due to pore coalescence or widening, theiodine number decreases
with further reaction. Thetime of occurrence of this maximum decreases with anincrease in
temperature, indicating faster pore coalescenceat lower reaction times due to the enhanced
rateof reaction.
5. PRESSURE DROP: The pressure drop through the bed is another important parameter
which controls the channel and slug formation and thereby mixing of the bed material with
the fluidizing fluid. At low flow rates in the packed bed, the pressure drop is approximately
proportional to gas velocity upto the minimum fluidization condition. With a further
increase in gas velocity, the packed bed suddenly unlocks (at the onset of minimum
fluidization condition), resulting in a decrease in pressure drop. With gas velocities beyond
minimum fluidization, the bed expands and gas bubbles are seen to rise resulting in non-
homogeneity in the bed. With the increase in gas flow, the pressure drop should remain
unchanged but due to bubbling and slugging there is always a fluctuation in the pressure
drop and it increases slightly.
6. BED EXPANSION RATIO:This term is used to describe the characteristics of bed
height during fluidization. This is quantitatively defined as the ratio of average height of a
fluidized bed to the initial static bed height at a particular flow rate of the fluidizing
medium above the minimum fluidizing velocity.
Average bed height is the arithmetic mean of highest and lowest level occupied by top of
the fluidized bed. It is denoted by “R”.
R=𝑯(𝒂𝒗𝒈)
𝑯(𝒔𝒕𝒂𝒕𝒊𝒄)
It is an important parameter for fixing the height of fluidized bed required for a particular
service. The expansion ratio of a fluidized bed depends on excess gas velocity, particle size
(dp), and initial bed height (Hs).
7. RAW MATERIALS: Activationdepends on the initial conditions of the solid
rawmaterial. Activation has been collected for three initially differentraw materials, in
terms of devolatilization, viz., rawcoconut shell (no devolatilization) and, coconut
shelldevolatilized in the presence of N2 in a fluidized bed anddevolatilized in an oxygen
deficient atmosphere.
Reaction with pure steam shows a maximumactivation in terms of the iodine number, and
the iodinenumber decreases with increasing CO2 composition inthe reacting gas at any
particular time. Even thoughindustrial processes use readily available flue gases withhigh
CO2 concentrations for economic benefits, steamactivation is preferable to obtain better
quality activatedcarbon at lower temperatures. A mixture of steam andCO2 may be used to
design a specific activated carbonto control pore structure.
Descriptive Behavior of a Fluidized Bed.
At gas flow rates above the point of minimum fluidization, a fluidized bed appears much
like a vigorously boiling liquid; bubbles of gas rise rapidly and burst on the surface, and
the emulsion phase is thoroughly agitated. The bubbles form very near the bottom of the
bed, very close to the distributor plate and as a result the design of the distributor plate has
a significant effect on fluidized-bed characteristics. Catalytic reactions in dense bubbling
fluidized beds usually use fine Geldart A solids that have a very small minimum fluidizing
velocity. Consequently, industrial operations are usually run at many multiples of umf, or
with u0/umf>>1, ub/umf>>1. For this situation, Kunii and Levenspiel proposed a
“bubbling bed model”. It is based on following assumptions:
Fresh feed gas containing reactant A at CAi enters the bed and, on contact with the
fine catalyst powder, reacts there according to a first-order reaction.
The bed consists of three regions: bubble, cloud and emulsion, with the wake region
considered to be part of the cloud. We designate these regions by the letters b, c, and
e; we designate the reactant concentration at any level in these regions as CAb, CAc,
and CAe, respectively.
Since u0 >>umf, all the feed gas passes through the bed as bubbles, and flow
through the emulsion is negligible.
The gas interchange rate between bubble and cloud and between cloud and
emulsion are given by Kbc and Kce, respectively.
The mass of solids in the bed, Ws, is
𝑊𝑠= ρcAchs(1 −∈s) = ρcAch(1−∈). (9)
Where 𝐴𝑐 is the cross-sectional area of the batch, ℎ𝑠 is the height of the settled bed, h is the
Height of the bed at any time, 𝜖𝑠 is the porosity of the settled bed, 𝜖 is the porosity of the
expanded bed, and ρc is the density of the catalyst particles.
At low gas velocities in the range of fluidization, the rising bubbles contain very few solid
particles. The remainder of the bed has a much higher concentration of solids in it and is
known as the emulsion phase of the fluidized bed. The bubbles are shown as the bubble
phase. The cloud phase is an intermediate phase between the bubble and emulsion phases.
After the drag exerted on the particles equals the net gravitational force exerted on the
particles, that is
ΔP = g(ρc−ρg) (1 − ε)h. (10)
For the FBR, this is the density of the carbon char. In order to find 𝜖𝑠, the following
equation is used:
ϵs = 1 − (𝒓
𝟔). (11)
where r is the radius of the particles in question.
The umf is calculated from the following equation:
umf=[ (φDp)2 /150μ ] [g(ρc− ρg)] [(∈mf3/1 − ϵmf)]. (12) where μ is a pre-defined constant, g is the gravitational acceleration, ρcis the density of the
char particle, ρg is the density of the fluidizing gas.
As is defined as:
AS= 𝜋𝐷𝑝2 = 𝜋([6𝑉𝑝
𝜋]1/3)2
(13)
The variable, ϕ, is a dimensionless parameter defined as:
Φ = 𝐴𝑠
𝐴𝑝 =
𝜋([6𝑉𝑝
𝜋]1/3)2
𝐴𝑝(14)
Where,
Φ = sphericity of particle. (For spherical particle sphericity is unity and for
other particle its value is less than unity.) and ϵmf is equal to:
ϵmf=(0.071
Φ)1/3
(15)
Bubble velocity and Cloud size related for single bubble is given by Davidson and Harrison
as:
Ubr = 0.71(𝒈𝒅𝒃)𝟏/𝟐 (16)
The larger the value of u0, the faster should be the velocity of a gas bubble as it rises
through the bed. The higher the minimum fluidization velocity, the lower the velocity of the
rising bubble. Adopting an expression used in gas-liquid systems, Davidson and Harrison
proposed that the rate of bubble rise in a fluidized bed could be represented by simply
adding and subtracting these terms:
Ub=(Uo – Umf) +0.71(𝒈𝒅𝒃)𝟏/𝟐(17)
Fraction of Bed in the Bubble Phase:
Using the Kunii-Levenspiel model, the fraction of the bed occupied by the bubbles and
wakes can be estimated by material balances on the solid particles and the gas flows. The
parameter δ is the fraction of the total bed occupied by the part of the bubbles that does
not include the wake, and α is the volume of wake per volume of bubble. The bed fraction
in the wakes is therefore (αδ).
The bed fraction in the emulsion phase (which includes the clouds) is (1 – δ – αδ). Letting
Ac and ρc represent the cross-sectional area of the bed and the density of the solid
particles, respectively, a material balance on the solids gives:
Solids flowing downward in emulsion = Solids flowing upward in wakes
Ac𝝆c (1 – δ – αδ) = αδubAc𝝆c (18)
From above equation,
us= 𝛂𝛅𝐮𝐛
(𝟏 – 𝛅 – 𝛂𝛅)(19)
A material balance of gas flow in the reactor gives
Acuo =δubAc + Acϵmfαδub+ Acϵmf(1 – δ – αδ)ue(20)
The velocity of rise of gas in the emulsion phase is
ue = (umf/ ϵmf ) – us (21)
The fraction δ of the bed occupied by bubbles is given by following equation
δ =𝐔𝐨−𝐔𝐦𝐟
𝑼𝒃−𝑼𝒎𝒇(𝟏+𝜶 )(22)
Kunii and Levenspiel assume that the last equation can be simplified to
δ =𝐔𝐨−𝐔𝐦𝐟
𝑼𝒃(23)
which is valid for ub>>umf.
DATA TABLES:
Table: IV
Different Biomass and their Pyrolysis Products
Table: V
Parameters (Sizes & Temperature)
Raw Coconut Shell Size (halfs) Approx. 100mm to 150mm
Crushed Coconut Shell Size 3mm to 4mm
Carbonization Temperature 500℃ to 600℃
Steam Activation Temperature 900℃ to1000℃
Table: VI
Characteristics of Coconut Shell Char
Parameter Analysis % By Mass
Fixed Carbon 62.95
Volatile Matter 18.61
Ash 13.69
Moisture 4.75
Calorific Vale
6221.00 (cal./g)
BET surface area
43.00 (m2 /g )
Now considering the particles are spherical in shape so the sphericity of particle,Φ = 1 .
For particles other than spherical particle the value of sphericity is always less than one. It
ranges from 0.5 to 1 (for particle other than spherical particle).
Density of char is 1317 kg/m3 as mentioned in above table. Density of fluidizing gas(steam)
is calculated by using steam table at 900℃ and 1.5 bar pressure. The porosity at minimum
fluidizing velocity is calculated by using equation no.(15) by using the sphericity of
particle.When the particles are large, the predicted εmfcan be much too small. If a value of
εmfbelow 0.40 is predicted, it should be considered suspect. Kunii and Levenspiel5 state
that εmfis an easily measurable value.Values of εmfaround 0.5 are typical.
Table: VII
Variable Parameters Related to FBR
Particle
SizeDp(
mm)
Volum
e
ofParti
cle Vp
SphericityofPar
ticleΦ
Porosity
at
minimu
m
Dens
ity
of
Char
Densityofflui
dizing gas𝝆g
(kg/m3)
Viscosity
of
fluidizin
ggas
Surfac
e Area
ofParti
cle
× 𝟏𝟎-
6(m3)
fluidizat
ion,
εmf
𝝆c
(kg/
m3)
μ(kg/m sec)
Ap×𝟏𝟎-
4(m2)
4mm 0.0335 1 0.5026
6mm 0.1130 1 1.1130
8mm 0.2680 1 2.0106
10mm 0.5235 1 3.1415
12mm 0.9047 1 0.414 1317 0.4643 1.3×10-5 4.5238
14mm 1.4367 1 6.1575
16mm 2.1446 1 8.0424
18mm 3.0536 1 10.178
20mm 4.1888 1 12.566
Fig. II(Steam density calculation)
The density of fluidizing gas (steam) is calculated by using steam table or by using the
calculator above for calculating all the parameters of superheated steam at 900℃ and 1.5
bar pressure.
CALCULATION USING MATLAB PROGRAMMING AND GRAPHICAL RELATION
BETWEEN VARIOUS PARAMETERS.
I. Programming For Calculation of Minimum Fluidizing Velocity(umf) : For
calculation of ( umf ) the following equation is used ,
umf=[ (φDp)2 /150μ ] [g(ρc− ρg)] [(∈mf3/1 − ϵmf)] All the other parameter in the above equation is calculated by using equation no. (13) to (15).
Fig. III(MATLAB Programming For Calculating umf)
In the above programme we have to input the values of volume of particle ,density
of char ,density of fluidizing gas ,viscosity of fluid and actual surface area of particle
which is available in Table VII .
For example putting the values for particle size 4mm, 6mm & 8mm from Table VII
in the programming to calculate umf, we get,
Fig. IV (Results)
Now preparing a table for the different values of ‘umf,based on different values of
particle size.
Table: VIII
Different Values of Umf for Different Particle Size
Particle SizeDp(mm) Minimum Fluidization Velocity Umf(m/sec)
4mm 12.993
6mm 29.234
8mm 50.110
10mm 81.102
12mm 116.75
14mm 158.86
16mm 207.45
18mm 262.67
20mm 324.14
II. Programming For Graphical Relationship Between Various Parameters : This
programme is written in order to show the graphical representation of various
parameters of FBR and there variation with the other parameters.
It show the graphical relation between different parameters such as Diameter of
Particle, Minimum Fluidizing velocity, void fraction ratio and sphericity of particle.
The vector values of diameter of particles and the area parameter or actual surface
area of particle (Ap) is taken from Table VII .
The first graph show the relationship between diameter of particle and Umf.
The second graph show relationship between void fraction ratio and Umf.
The third graph show relationship between diameter of particle and void fraction
ratio.
The fourth graph show relationship between diameter of particle and sphericity of
particle.
Fig. V (MATLAB Programming ForGraphical Relationship Between Various
Parameter)
Fig. VI (Input of Variable Parameters)
Fig. VI (Graphical Relationship between Parameters)
Thegraph (1) above show there is parabolic relation between diameters of particles and
minimum fluidizing velocity. The graph (2) shows that with increase in minimum fluidizing
velocity the void fraction ratio decreases. The graph (3) shows the relationship between
diameters of particles and void fraction ratio which have same variation as that of graph
(2). The graph (4) shows the change in sphericity of particle with the diameters of particles.
The value of sphericity approaches to unity as spherical particles are considered. For
particle other than spherical particle the value is always between.5 to 1.
III. Programming For Velocity of Solid Driven by Wake and Fraction of Bed Occupied
by Bubble: For this equations no.(18) to equation no.(23) is used in the
programming.
The entering superficial velocity, u0, must be above theminimum fluidization
velocity but below the slugging ums and terminal, ut,velocities.
umf<u0 <ut
and
umf<u0 <ums
Both of these conditions must be satisfied for proper bed operation.
Here ut is given by following relation,
Ut= 𝜼 dp2 / 18𝝁 (for Re ≤ 0.4)
Ut=(1.78 ×10-2𝜼2 / ρg𝝁)1/3 (Dp) (for 0.4 ≤ Re≤500).
The superficial velocity of the tower (Us) is calculated by dividing the volumetric flow rate
of gas by area of the tower. The inlet velocity (Uo) is always greater than minimum
fluidization velocity so that the fluidization occurs. We have to find out the volume of wake
per volume of bubble.
The maximum bubble diameter( dbm ) is observed by following relationship,
dbm= 0.652 [Ac(uo-umf)]0.4
The initial bubble diameter (dbo) depend upon the type of distributor plate used,
For porous plate,
dbo=0.00376(uo-umf)2.
For perforated plate,
dbo=0.347[Ac(uo-umf) / 𝜼d]0.4.
For calculation of diameter of bubble following equation is used,
𝒅𝒃𝒎−𝒅𝒃
𝒅𝒃𝒎−𝒅𝒃𝒐 = 𝒆−𝟎.𝟑𝒉/𝑫𝒕
By finding out the diameter of bubble, we can easily calculate the volume of bubble. The
volume of wake is also calculated.
Fig. VI (Programming for Velocity Driven by Wake and Fraction of Bed occupied
by Wake).
By putting the values in the programme,we can find out the unknown parameter.
Conclusion: On the basis of the above data obtained, it can be concluded that the
fluidized bed reactor used for activation of the carbon produced from the coconut shell is
the best and simple method to produce higher yield of activated carbon. The fluidized bed
process gives better activation (high adsorption capacity of carbon) in less time and at a
lower temperature compared to the static bed process. Experimental data showed that the
increase in temperature and time resulted in a better activation. We can see above by
increasing the particle size, minimum fluidization velocity also increases.
Reference
Kirubakaran, C. J.; Krishnaiah, K.; Seshadri, S. K. Experimental Study of the
Production Of Activated Carbon From Coconut Shells in a Fluidized Bed Reactor.
Ind. Eng. Chem. Res. 1991,30, 2411.
Kunii, D.; Levenspiel, 0. Fluidization Engineering; Wiley: New
York, 1969 pp 8-11.
Apelsa Carbones. "Carbon Activation, Steam Activation, Chemical Activation."
ApelsaActivated Carbon Mexico. 23 Jan 2012
<http://www.carbonapelsa.com.mx/pages/english/activation.html>
Brown, Lee and Scott Fogler. Mechanics of Fluidized Beds. 2008. 20 April 2012
<http://www.engin.umich.edu/~cre/12chap/html/12prof2a.htm>.
Calgon Carbon. "Activated Carbon Principles." 2007. Calgon Carbon. 25 Jan 2012
<http://www.calgoncarbon.com/documents/ACPrinciples.pdf>.
Activated Carbon Products and Media. n.d. 23 January 2012
<http://www.calgoncarbon.com/carbon_products/index.html>.
Grimwood, Brian. Coconut Palm Products. Italy: Food and Agricultural
Organization of the United Nations, 1975
Sai, P.M. Satya et al. "Production of Activated Carbon from Coconut Shell Char in
a Fluidized Bed Reactor." Industrial and Engineering Chemistry Research
(1997): 3625-3630.
Wei, Su, et al. "Preparation of Microporous Activated Carbon from Raw Coconut
Shell by Twostep Procedure*." Chinese Journal of Chemical Engineering 14.2
(2006): 266-269 .
Zhang, Jieling, et al. "Product Analysis and Thermodynamic Simulations from the
Pyrolysis of Several Biomass Feedstocks." Energy and Fuels 21 (2007): 2373-2385.
The Freedonia Group, Inc. Activated Carbon. Cleveland: The Freedonia Group,
Inc., 2011.Turton, Richard. "Utilizing Experience- Based Principles to Confirm
Suitability of a Process Design." Analysis, Synthesis, and Design of Chemical
Processes. Prentice Hall, 2009. 363-389.
Siemens. "Comparing Coconut Shell-Based and Coal-Based Activated Carbons."
n.d.www.water.siemens.com. 12 April
2012<http://www.water.siemens.com/SiteCollectionDocuments/Product_Lines/West
ates_Carbon/Brochures/coconutvscoal.pdf>.
Sundaram, E. and E. Natarajan. "Pyrolysis of Coconout Shell: An Experimental
Invetigation." The Journal of Engineering Research 6.2 (2009): 33-39.
Tam, Man and Michael Antal. "Preparation of Activated Carbons from Macadamia
Nut Shell and Coconut Shell by Air Activation." Industrial and Engineeering
Chemistry Research 38 (1999): 4268-4276
Rubel, A.M., J.M. Stencel and S.N. Ahmed. "Activated Carbon for Selective Remval
of Nitrogen Oxide from Combustion Flue Gas." Symposium on Chemistry of Flue
Gas Cleanup Processes. Denver: American Chemical Society, 1993. 726-733.
Trambouze, P., & Euzen, J. „‟Chemical Reactors: From Design to Operation.‟‟ (R.
Bononno, Trans.). Paris: Editions Technip, 2004.
Hsiung; Thomas Hsiao-Ling (Emmaus, PA), Withers, Jr.; Howard Paul
(Breinigsville, PA)].; Abatement of F2 using small particle fluidized bed; US Patent
No.# 6,352,676; March 5, 2002
Arriagada R, García R, Molina-Sabio M and Rodríguez- Reinoso F (1997). Effect of
steam activation on the porosity and chemical nature of activated carbons from
Eucalyptus globulus and peach stones, Microporous Materials, 8: 123-130
Activated carbon-a techno-commercial profile: Chemical Weekly April 20, 2010
Laine J and Yunes S (1992). Effect of the preparation method on the pore size
distribution of activated carbon from coconut shell, Carbon, 30: 601-604.
1