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L -35 : Multiphase Reactors: Design

Approach

Prof. K.K.Pant

Department of Chemical EngineeringIIT Delhi.

kkpant@chemical.iitd.ac.in 

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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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3

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4

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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6

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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10

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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