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

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