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L -33 -34 : Multiphase Reactors
Prof. K.K.Pant
Department of Chemical EngineeringIIT Delhi.
mailto:[email protected]:[email protected] -
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Packed Bed Reactor : Differential equation describing
diffusion for a first order reaction in a packed bed
Z=0 , CAb= CAb0
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Axial diffusion, can be neglected when
FAis very large
so
Finally, the conversion for
1storder reaction in PBR is
'
0 p A b p
a 0 Ab
U d -r d>>
D U C
2
Aba 2
d CD
dz
"
Ab b aAb
dC k S= - C
dz U
Remember the
forced
convection in
binary external
diffusion, JAisalso neglected
b a-( k"S L)/UAb
Ab0
CX = 1- = 1- e
C
Mass transfer and reaction in a packed bed
Ind engg. Chem res. 12, 412, 1973
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Overall Rate with in the pellet (-rA= (rAb)
For first order reaction : (-rAb;= (rAb)Sa= ksSa CAb
=>(-rA)= (rAb) = Sabk CAb
2
2
0s b bAb Ab
A
d C dC Da U k SaC
dz dz
Neglecting Axial Dispersion
( )s aAb
AbdC k S
Cdz U
CAb= CAb0 exp(- Sa bks Z/U)
X=1- CAb/ CAb0For design, Calculate and then to calculate rate.
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Determination of limiting situation from
reaction data
Type ofLimitation
Variation of Reaction Rate with:
Velocity
Particle
Size TemperatureExternaldiffusion
U (dp)-3/2
Rate= kcac CA
Linear
InternalDiffusionIndependent (dp)-1 Exponential
SurfaceReaction
IndependentIndepende
ntExponential
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Multiphase Reactors
Reactors in which Two or more phases areinvolved in Reactions.
Majority of reactions are : Gas liquid reactions
involving catalyst as solid material.
Hydrotreating, FT reaction, Hydrogenation
Reaction etc.
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Reactor Types
Two-Phase Reactors:
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Sasol slurry reactor for CO
hydrogenation Dia 5 m, height22m ,T240oC, P 22 atm.
FT synthesis
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Three-Phase Reactors:
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Three-phase reactors
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Multi-phase Reactors- Advantages and Disadvantages
Advantages Disadvantages
Catalytic FixedBed Reactor
The fluid flow regimesapproach plug flow, sohigh conversion can beachieved.
Pressure drop is low.
Owing to the high hold-up there is better radialmixing and channelingis not encountered.
High catalyst load perunit of reactor volume
The intra- articlediffusionresistance is veryhigh.
Comparatively low
Heat and masstransfer rates
Catalystreplacement isrelatively hard and
requires shutdown.
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Multi-phase Reactors- Advantages and Disadvantages
Advantages Disadvantages
Catalytic
Fluidized-bedReactor
The smooth, liquid-like flow of particles
allows continuous controlled operations
with ease of handling.
Near isothermal conditions due to the rapid
mixing of solids.
Small Intra-Particle resistance leads to a
better heat and mass transfer rate.
This violent particle motion of
particles tends to homogenize all
intensive properties of the bed.
Thus it is not generally possible to
provide an axial temperature
gradient which might be highly
desirable in some instances.
Erosion by abrasion of
particles can be serious.
Particle attrition
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Trickle Bed Reactors
Trickle-bed reactors are the most widely
used type of three-phase reactors. The
gas and liquid co-currently flow downward
over a fixed bed of catalyst particles.
Concurrent down-flow of gas and liquidover a fixed-bed of catalyst. Liquid trickles
down, while gas phase is continuous
In a trickle-bed, various flow regimes are
distinguished, depending on gas and liquid
flow rates, fluid properties and packing
characteristics.
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Three-phase Reactors- Advantages and Disadvantages
Advantages Disadvantages
Trickle-BedReactor
Gas and liquid flow regimesapproach plug flow; highconversion may be achieved.
Large catalyst particle, therefore,catalyst separation is easy.
Low liquid holdup, therefore liquidhomogenous reactions areminimized.
Low pressure drop
Flooding problems are notencountered.
High catalyst load per unit reactorvolume.
Poor distribution of theliquid-phase
Partial wettin of the catal st
Hi h intra- article resistance
Poor radial mixing
Temperature control isdifficult for highly exothermicreactions
Low gas-liquid interactiondecreases mass transfercoefficients.
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Three -phase Reactors- Advantages and
Disadvantages
Advantages Disadvantages
BubbleFixed- BedReactor
High liquid holdup,therefore, catalyst arecompletely wetted, better
temperature control, and nochanneling problems.
Gas-liquid mass transfer ishigher than in Trickle beddue to higher gas-liquidinteraction.
Axial back mixing ishigher than trickle-beds, conversion is
lower.
Feasibility of liquid sidehomogeneousreactions
Pressure drop is high
Flooding problems mayoccur.
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Steps in Slurry Reactors
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Catalytic Fixed-Bed Reactor - Design Model
Mass Balance around the catalyst
Gas-Phase component mass balance (Plug Flow model)
Gas-Phase component mass balance (Dispersion model)
Energy Model
inetSGicc RiCCak )()()(
0.0)()( iSGiccGiG CCak
dzdCU
0.0)()(2
2
iSGiccGiGGiG CCakdz
dCU
zd
CdiD
)()( TaTUAjHRjdzdTCpU RGGG
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Reactions Steps in slurry reactors
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Rate of gas absorptions
Transport to the Catalyst Pellet
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Diffusion and Reaction in the Catalyst Pellet
m = mass of cata/vol of solution
Determination of RDS
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Comparison of Three Phase
Trickle- Bed and Bubble Fixed Bed
Reactors
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Comparison of Three Phase
Suspended Bed Reactors
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Approximate dimensions of commercial trickle-bed
reactors are a height of 10 m and a diameter of 2 m.
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Theory of Catalytic Gas- LiquidReactions
A(G) + B(L) C
Gaseous reactant A reacts with non-volatileliquid reactant B on solid catalyst sites.
Mechanism Of Three- Phase Reactions:-
Mass Transfer of component A from bulkgas to gas-liquid interface
Mass transfer of component A from gas-liquid interface to bulk liquid
Mass transfer of A& B from bulk liquid tocatalyst surface
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Intraparticle diffusion of species A&B through the catalyst pores to activesites.
Adsorption of both or one of thereactant species on catalyst activesites.
Surface reaction involving at leastone or both of the adsorbed species.
Desorption of products, reverse of
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1m-r =A H H H
1 A A A+ + +k a k a k a mk C AfcAg i Al i Ac sA B
First order rate constant for A
'-r =k C gvgA A
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Mole balance for A
Mole balance for B
dF
'A =r =-k C g
vgA AdW1 mol'-r = C
B B1 1 gcat.s+
k a nKCc p AS
-r =k CB vl B
dF dC'B B=v =-r =k Cvg1 B BdW dW
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REACTOR MODEL In kinetic models for trickle beds, the
reaction is often assumed to be first order
to both reactants
For the ideal case of plug flow and
completely wetted catalyst, the conversionfor a first-order reaction is given by:
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Conversion may be given as a function of
the liquid hourly space velocity (LHSV), and
the apparent rate constant, kapp, includes
the effect of partial wetting as well as the
effect of internal concentration gradients.
where
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Calculation of CatalyticEffectiveness Factor
Catalytic Effectiveness Factor:
where
- Thiele Modulus
1storder reaction rate:
Spherical Pellet
Cylindrical Pellet
Slab Pellet
1 1 = (Coth3 - 3
R = kSap/De3
R = kSap/De2
= L kSap/De
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Applications Trickle-bed reactors are employed in
petroleum, petrochemical and chemicalindustries, in waste water treatment andbiochemical and electrochemicalprocessing.
For Example: Residuum and vacuum residuum
desulfurization
Catalytic dewaxing of lubestock cuts
Hydrogenation of methyl styrene tocumene
Oxidation of glucose
Biochemical reactions and fermentations
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Three-Phase Gas-Liquid Catalytic
Reactor- Design Model
(Trickle-Bed, Fixed-upflow Bubble-
Bed, Bubble Slurry Bed,
3-Phase Fluidized Bed)Non-Volatile Liquid-phase mass balance:
2L, i L, i
L, i L c c i L, i S, i2
d C dCD - U -(K a ) (C - C )=0.0dzdz
Volatile Liquid-phase mass balance:
2g, iL, i L, i
L, i L L g i L, i c c i L, i S, i2
Cd C dCD - U +(K a ) ( - C )-(K a )(C - C )=0.0dz Hidz