CRE II -35
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L -35 : Multiphase Reactors: Design
Approach
Prof. K.K.Pant
Department of Chemical EngineeringIIT Delhi.
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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
inet S Gicc RiC C ak )()()(
0.0)()( iS GiccGiG C C ak dz dC U
0.0)()(2
2
iS GiccGi
GGi
G C C ak
dz
dC U
z d
C d i D
)()( TaT UA j H Rjdz dT CpU 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
Suspended Bed Reactors
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Theory of Catalytic Gas- Liquid
ReactionsA(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 εAf cAg i Al i Ac sA B
First order rate constant for A
'-r =k C gvg A A
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Mole balance for A
Mole balance for B
dF
' A =r =-k C gvg A AdW
1 mol'-r = CB B1 1 gcat.s
+k a nKCc p
AS-r =k C
B 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
1st order reaction rate:
Spherical Pellet
Cylindrical Pellet
Slab Pellet
1 1η = (Coth3φ -φ 3φ
Rφ = kSaρp/De3
Rφ = kSaρp/De2
φ = L kSaρp/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