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L 37:Multiphase Reactors

Prof. K.K.PantDepartment of Chemical Engineering

IIT Delhi.

kkpant@chemical.iitd.ac.in

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Overall or Apparent reaction rate (gas limiting)

Kinetic Factors

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3

rA= kgai (CA(g-CAi(g)/(1-) c = klai (CAi(g- CAb)/(1-) c

= kcac(CAbCAs) = k CAsCBs Moles/gcat.s

ai= interfacial area/ vol. of bed

., = bed void fraction

Eliminate intermediate concentrations

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Three-Phase Gas-Liquid Catalytic Reactor- Design

Model

(Trickle-Bed, Fixed-upflow Bubble-Bed, Bubble SlurryBed, 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

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'

'

AC g-r =

A H H H1 A A A+ + +k a k a k a k C fcAg i Al i Ac sA B

First order rate constant for A

'-r =k CvgA Ag

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Mole balance for A

Mole balance for B

dF

'A =r =-k C gvgA 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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Calculation of CatalyticEffectiveness Factor

Catalytic Effectiveness Factor:

where

- Thiele Modulus

1storder reaction rate:

Spherical Pellet

Cylindrical Pellet

Slab Pellet

1 1= (Coth3- 3

a pR = k'S / De3

a pR = k'S / De2

a p =L k'S /De

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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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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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WHICH KIND OF CONTACTOR TO USE

The selection of a good contactor depends on

where the controlling resistance lies in the rate expression,

the advantages of one contacting pattern over another, and the

difference in auxiliary equipment needed.The overall economics which accounts for these three factors

will determine which set up and reactor type is best.

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The Rate:

We should favor the contactor which favors the weakest

step in the rate.

For example,

if the main resistance lies in the G/L film, use a contactor

with large interfacial surface area.if the resistance lies at the L/S boundary, use a large

exterior surface of solid, thus large fs, or smal l part ic les.

if the resistance to pore diffusion intrudes, use tinyparticles.

Boosting the weakest step in the rate by a proper choice of

particle size, solid loading and reactor type can strongly affect

the overall economics of the process.

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HYDROGENATION OF ACETONE IN A PACKED BUBBLE COLUMN

Aqueous acetone (CB0, = 1000 mol/m31, vl = 10-4m3l /s ) and

hydrogen (1 atm, vg= 0.04 m3

g /s, HA = 36845 Pa.m3

l /mol) are fed to the bottom of a long, slender column (5-m

high, 0.1-m2cross section) packed with porous Raney nickel

catalyst (dp, = 5 x 10-3m cat, ps = 4500 kg /m3cat, fs= 0.6, De

= 8 X 10-10 m2 / s based on vol of l /m of cat) and kept at 140C.

At these conditions acetone is hydrogenated to propanolaccording to the reaction

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What will be the conversion of acetone in this unit?

Additional Data:The mass transfer rate constants are estimated to be the