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L -33 -34 : Multiphase Reactors

Prof. K.K.Pant

Department of Chemical EngineeringIIT Delhi.

kkpant@chemical.iitd.ac.in

mailto:kkpant@chemical.iitd.ac.inmailto:kkpant@chemical.iitd.ac.in

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2

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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4

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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6

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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8

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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10

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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16

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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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22

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23

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24

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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