01 Hetrogeneous Reactors

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Real Life Reactors. Real Life reactors are normally complex and their

design is based on coupled effect of kinetics andhydrodynamics.


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Kinetics & Hydrodynamics ?

CRE deals with relating output to input for variousreaction kinetics and flow-pattern / hydrodynamics

HETROGENEOUS REACTIONS Most of the real life reaction systems involve

heterogeneous catalyst .

For any heterogeneous systems phase boundariesare inherent and need to be delt-with for heat andmass transfer along with reaction kinetics

Thus physical processes affect the reactor design inmore than that for homogeneous reaction systems

However the starting point for both type of systems isto write mass and energy balance.

This could be more involved for Heterogeneousreaction as these equations may have to be developedfor each Phase


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GLOBAL RATE Mass balance as starting point to develop performance

equation for a reactor system include a kinetic termusing reaction rate as moles / second / volume ofthe reactor.

For Heterogeneous systems involving reaction fluidand solid catalyst particle, the proper rate to use inMB equation will include the effects of mass andenergy transfer processes from fluid to solid surface

and with in the solid particle.

Such rates are called Global Rate

Packed Bed Gas Solid Reactor

CAin

CAout

Global Rate Based onCA-in, CA-out, Catalyst

Weight and Flow rate


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GLOBAL RATE TO INCLUDE Global rate in terms of bulk properties should include

different steps contributing to the global rate. Following isthe Sequence of Steps for converting reactants to products

1. Transfer of Reactants from bulk to fluid-solid interface

2. Intra-particle Transport of reactants in to catalyst particle(if Porous Catalyst)

3. Adsorption of reactants at interior site

4. Chemical Reaction of Adsorbed reactant to adsorbedproduct

5. Desorption of adsorbed Product

6. Transport of products from the interior sites to outersurface of the catalyst

7. Transport of products from fluid-solid interface to bulk

Schematics follows..

8

Steps Involved . (Contd.):


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Typical Catalyst

Catalyst StructureCatalyst Support

(earth oxide, alumina, silica...)

Metal finely deposited on the support

responsible for the hydrogenationactivity and the selectivity of the

catalyst

The selected catalyst metal takes

into account impurities related tothe nature of the feedstock

Intra-catalyst Transfer

External surface ofExternal surface ofthe catalyst grainthe catalyst grain

MicroporesMicropores

MacroporesMacropores

Gas or liquid phaseGas or liquid phasecontaining thecontaining the reactivesreactives


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Adsorption Reaction DesorptionCatalysis Mechanism

Adsorption of reactant on specific active site

Chemical reaction on catalysts surface

Product desorption

ChemicalChemicalreactionreaction

Active siteActive site

Poreinside

thecatalyst

Poreinside

thecatalyst

grain

grain

Global Rate (Contd..) Global Rate can be developed using the

measurements at Steady State.

At Steady State the rates of all above defined sevensteps are equal.

Thus for a simplified situation, Global rates can bedefined using Bulk inlet and outlet concentrationat bulk temperature of the fluid.


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Concept of Global Rate

As already indicated various steps are involved and shouldbe considered to define GLOBAL REACTION RATE.

Let us consider a irreversible reaction

A(g) B(g)

on solid Catalyst at constant Temperature

Let for the sake of simplicity the Catalyst is non-porousso that intra catalyst transport does not take place and steps2 and 6 above are absent.

The problem is to formulate the rate of reaction per unitVolume of the bed (rv) in terms of bulk concentrations

and temperature. Note that Reaction rates per unit mass of the catalyst (rp )

are related to (rv) using catalyst bulk density

Concept Global Rate Contd. For non porous catalyst steps involved are

1, 3,4,5, and 7

Supposing that steps3, 4 and 5 can be represented bysingle first order reaction, the over all raction can bedescribed in three steps

1. Gas A transported from Bulk to Catalyst Surface

2. Reaction Occurs at catalyst Surface

3. Finally Product B is transferred from Catalyst Surfaceto Bulk.


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Concept Global Rate Contd.

Since for irreversible reaction Presence of B does notinfluence the rate the disappearance of A can berepresented byfirst 2 steps onlye.g.

Rate of Transfer of A from bulk to the catalyst surface

Or Rate of reaction at the surface

km is the mass transfer rate per unit area and am is

the transfer area per unit catalyst wt. k is the reaction rate per unit mass of catalyst

At steady state both the rates are the same

)( sbmmp CCakr =

sp Ckr =

Concept Global Rate Contd.

Because Cs < Cb to facilitate transfer of A

Actual rate is lower than that it would be for Cb .

Therefore Mass Transfer resistance is reducing theoverall reaction rate


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Global Rate contd This is the expression for the Global rates.

Of course it is subjected to many simplifications asstated in the previous slides, however fullydemonstrates the concept of a Global Rate.

Subsequent modules addresses the less simplified

situations to describe Global Rate.

Coupled effect of Mass and HeattransferAbove hypothetical example considers resistances to

mass transfer (1/km am) as well as Chemical Reaction(1/k).

Incase of exo- or endothermic reactions, resistance toheat transfer is also important.

There are examples that Higher temperature at thereaction site on the catalyst in case of exothermic

reaction, there could be an offset of 350% in theestimated of reaction rate, if estimated using intrinsickinetics.

Therefore methodical analysis is important forheterogeneous reaction systems


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Types of Heterogeneous Reactions Types of Heterogeneous Reaction systems can be

classified depending on the types of phases invoved..

Gas Solid heterogeneous reaction systems arecommonly encountered and could be Catalytic andNon-Catalytic.

Non-Catalytic Heterogeneous reactions involvesreaction of Gas phase with at least one solid reactant.

Hydro fluorination of Uranium di-oxide or Smelting ofores (ZnS to ZnO) are such examples.

Contd Example Non-Catalytic G-S reactions are

Hydroflorination of Uranium Di Oxide

UO2 (s) + 4HF(g) UF4 (s) + 2 H2O(g)

Smelting of Ores

ZnS(S) + 3/2 O2(g) ZnO (s) + SO2 (g)

Making HCl in Transport Reactor

2NaCl(s) + SO3(g) + H2O(g)Na2SO4(s)+2HCl(g)

Of course Non Catalytic gas-solid reactions are dynamic innature due to consumption of solid reactant which is notthe case with Solid catalyst based Gas-Solid Heterogeneousreaction Systems.


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Contd Gas Liquid Heterogeneous systems present the

example of Steady State Non-Catalytic Heterogeneoussystems where reactant from gas phase are absorbedand continuously react with Liquid Phase Component.

Bubble Column Reactors fall in this category whichcan operate co-current or Counter current manner.

Chlorination of Para-cresol is one of the examples

Co2 Absorption fro gas using Amine solution isanother example

Contd Liquid-Solid Reactions where solid is a catalyst are

often encountered in Petroleum industry. Examplesare:

1. Alkylation with AlCl3

Such systems are not simple to analyze andcomponent of empiricism increases to have a design

model. Three Phase (G-L-S - Slurry Reactors) systems are

also employed in Petroleum industry for exampleResidue up-gradation through hydro Cracking.


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Contd Liquid-Solid Reactions where solid is a catalyst are

often encountered in Petroleum industry. Examplesare:

1. Alkylation with AlCl3

Such systems are not simple to analyze andcomponent of empiricism increases to have a designmodel.

Three Phase (G-L-S - Slurry Reactors) systems arealso employed in Petroleum industry for exampleResidue up-gradation through hydro Cracking.

Contd Gas-Liquid -Solid reaction systems as packed bed in the

form of Trickle bed reactors are also employed extensivelyfor large scale hydro-treatments in Petroleum refining.

Liquid Liquid multiphase reactors are alsoencountered. Alkylation of Hydrocarbons with Sulfuric

Acids or HF is an example.

Solid Solid non catalytic reactions are relevant forceramics industry. However, diffusion process/ resistanceare difficult to define and little is known about these.

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01 Hetrogeneous Reactors