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INTRODUCTION TO CHEMICAL INTRODUCTION TO CHEMICAL REACTION ENGINEERINGREACTION ENGINEERING

(1)(1)

Marcel LacroixMarcel LacroixUniversitUniversitéé de Sherbrookede Sherbrooke

INTRODUCTION TO CHEMICAL REACTION ENGINEERING:INTRODUCTION TO CHEMICAL REACTION ENGINEERING:OBJECTIVESOBJECTIVES

1. TO DEFINE A CHEMICAL REACTION.2. TO DEFINE THE REACTION RATE.3. TO INTRODUCE MASS CONSERVATION IN

REACTOR SYSTEMS.4. TO INTRODUCE IDEAL REACTORS.5. TO FIND THE DESIGN EQUATIONS FOR EACH

REACTOR TYPE.

M. Lacroix Introduction 2

OVERVIEW OF CHEMICAL REACTION ENGINEERINGOVERVIEW OF CHEMICAL REACTION ENGINEERING

),,( contactingkineticsinputfoutput =M. Lacroix Introduction 3

EXAMPLES OF REACTIONSEXAMPLES OF REACTIONS

• CATALYTIC CRACKERS FOR OIL REFINING.• BLAST FURNACE FOR IRON MAKING.• ACTIVATED SLUDGE PONDS FOR SEWAGE

TREATMENT.• POLYMERIZATION TANKS FOR PLASTICS,

PAINTS, FIBERS.• PHARMACEUTICAL VATS FOR PRODUCING

DRUGS.• FERMENTATION JUGS FOR WINE MAKING.


HYDROGEN PRODUCTION: REFORMINGHYDROGEN PRODUCTION: REFORMING

CH4+2H2O +(chaleur) = 4H2+CO2


FUEL CELLFUEL CELL

Principe simplifiPrincipe simplifiéé du du fonctionnement dfonctionnement d’’une Pile une Pile àà

CombustibleCombustible


électricité

H2 2H+ +2e - 1/2 O 2+2H++2e - 2H 2O

H+

ÉlectrolyteSolide

Zonecatalytique

Zonecatalytique

CathodeAnode

e-- +

Air

Airappauvri en O2

H2

H2O

H2ORésidu de

H2

PYROLYSIS OF BIOMASSPYROLYSIS OF BIOMASS


Produit à traiter

θ

Résidu

Gaz

Conduction paroi-solide

Rayonnement

Convection

Chimie

Conduction et pertes à la paroi externe

Fumées

CHEMICAL REACTIONS: THREE TYPESCHEMICAL REACTIONS: THREE TYPES

• A CHEMICAL REACTION HAS TAKEN PLACE WHEN A DETECTABLE NUMBER OF MOLECULES OF ONE OR MORE SPECIES HAVE LOST THEIR IDENTITY AND ASSUMED A NEW FORM BY A CHANGE IN THE KIND OR NUMBER OF ATOMS IN THE COMPOUND AND/OR BY A CHANGE IN STUCTURE OR CONFIGURATION OF THESE ATOMS.

• THREE BASIC WAYS A SPECIES MAY LOSE ITS CHEMICAL IDENTITY:

1. DECOMPOSITION: MOLECULE IS BROKEN DOWN INTO SMALLER MOLECULES OR ATOMS:

2. COMBINATION3. ISOMERIZATION: MOLECULE CHANGES CONFIGURATION

)()()()( 63662356

PROPYLENEBENZENECUMENEHCHCCHCHHC

+→+→


CHEMICAL REACTIONS: DEFINITIONSCHEMICAL REACTIONS: DEFINITIONS

• HOMOGENEOUS REACTION: TAKES PLACE IN ONE PHASE ALONE.

• HETEROGENEOUS REACTION: REQUIRES THE PRESENCE OF AT LEAST TWO PHASES.

• CATALYTIC REACTION: RATE IS ALTERED BY MATERIALS THAT ARE NEITHER REACTANTS NOR PRODUCTS. THESE FOREIGN MATERIALS ARE CALLED CATALYSTS.


CLASSIFICATION OF CHEMICAL REACTIONSCLASSIFICATION OF CHEMICAL REACTIONS


CHEMICAL REACTION: RATE OF REACTION CHEMICAL REACTION: RATE OF REACTION

• THE RATE AT WHICH A GIVEN CHEMICAL REACTION PROCEEDS CAN BE EXPRESSED EITHER AS THE RATE OF DISAPPEARANCE OF REACTANTS OR THE RATE OF FORMATION OF PRODUCTS.

• THE RATE OF REACTION, , IS DEFINED AS THE NUMBER OF MOLES OF CHLORAL REACTING (DISAPPEARING) PER UNIT TIME PER UNIT VOLUME (mole/dm3s).

• THE RATE OF REACTION, , IS DEFINED AS THE NUMBER OF MOLES OF DDT REACTING (FORMING) PER UNIT TIME PER UNIT VOLUME (mole/dm3s).

OHCHCClClHCClHCCHOCCl 23246563 )(2 +→+

waterehloroethanphenyltricdichlorodienechlorobenzchloral +→+

Ar−DCBA +→+

Cr



• IN HETEROGENEOUS REACTION SYSTEMS, THE RATE OF REACTION IS USUALLY EXPRESSED IN MEASURES OTHER THAN VOLUME, SUCH AS A REACTION SURFACE AREA OR CATALYST MASS.

• FOR A GAS-SOLID CATALYTIC REACTION,THE DIMENSIONS OF THE RATE OF REACTION FOR THE REACTANT A, , ARE THE NUMBER OF MOLES OF A REACTED PER UNIT TIME PER UNIT MASS OF CATALYST (mole/g. catalyst s).

• WE SHALL SIMPLY SAY THAT IS THE RATE OF FORMATION OF SPECIES j PER UNIT VOLUME. IT IS THE NUMBER OF MOLES OF SPECIES j GENERATED PER UNIT VOLUME PER UNIT TIME.

'Ar−

jr



• THE RATE EQUATION FOR IS SOLELY A FUNCTION OF THE PROPERTIES OF THE REACTING MATERIALS (SPECIES CONCENTRATION), TEMPERATURE, PRESSURE, OR TYPE OF CATALYST (IF ANY) AT A POINT IN THE SYSTEM (BATCH OR CONTINUOUS FLOW) IN WHICH THE REACTION IS CARRIED OUT.

• HOWEVER, SINCE THE PROPERTIES OF THE REACTING MATERIALS CAN VARY WITH POSITION IN A CHEMICAL REACTOR, CAN IN TURN BE A FUNCTION OF POSITION AND CAN VARY FROM POINT TO POINT IN THE SYSTEM.

jr

jr



• THE REACTION RATE EQUATION (i.e., THE RATE LAW) IS ESSENTIALLY AN ALGEBRAIC EQUATION INVOLVING CONCENTRATION, NOT A DIFFERENTIAL EQUATION.

• FOR EXAMPLE, THE ALGEBRAIC FORM OF THE RATE LAW FOR THE REACTION MAY BE A LINEAR

FUNCTION OF CONCENTRATION, , OR IT MAY BESOME OTHER ALGEBRAIC FUNCTION OF CONCENTRATION, SUCH AS

• BY CONVENTION, IS THE RATE OF FORMATION OF A.

• THUS, IS THE RATE OF DISAPPEARANCE OF A.Ar−

jr productsA→AA kCr =−

2AA kCr =−

Ar


EXAMPLES OF RATES OF REACTIONEXAMPLES OF RATES OF REACTION


MASS CONSERVATION: STATEMENTMASS CONSERVATION: STATEMENT

ACCUMULATION = INLET – OUTLET + GENERATION


MASS CONSERVATION: MOLE BALANCE EQUATIONMASS CONSERVATION: MOLE BALANCE EQUATION

jjjj GFF

dtdN

+−= 0

RATE OF ACCUMULATION OF j IN SYSTEM(moles/time)

RATE OF FLOW OF j INTO SYSTEM(moles/time)

RATE OF FLOW OF j OUT OF SYSTEM(moles/time)

RATE OF GENERATION OF j BY CHEMICAL REACTION IN SYSTEM (moles/time)

∫=⋅= dVrVrG jjj

moles/(time volume)⋅ volume


BATCH REACTORBATCH REACTOR

• REACTANTS ARE CHARGED INTO A CONTAINER, WELL MIXED, LEFT TO REACT AND A MIXTURE IS DISCHARGED.

COMPOSITION UNIFORM BUT TIME-DEPENDENT


M. Lacroix 19

CHEMICAL CHEMICAL REACTORSREACTORS

CRUSHER

BATCH REACTOR

PLUG FLOW REACTOR

BATCH REACTOR: CHARACTERISTICSBATCH REACTOR: CHARACTERISTICS

• SMALL-SCALE OPERATION.• TEST NEW OR EXPENSIVE PROCESSES.• SIMPLE AND NEEDS LITTLE SUPPORTING

EQUIPMENT.• HIGH CONVERSIONS CAN BE OBTAINED BY

LEAVING THE REACTANT IN THE REACTOR FOR LONG PERIODS OF TIME.

• HIGH LABOR COST PER BATCH AND DIFFICULTY OF LARGE-SCALE PRODUCTION.


BATCH REACTOR: MASS BALANCEBATCH REACTOR: MASS BALANCE

00 == jj FFINFLOW OUTFLOW

∫= dVrdt

dNj

j⇒ IN GENERAL

Vrdt

dNj

j ⋅=⇒ PERFECTLY MIXED

REACTION MIXTURE


BATCH REACTOR: BATCH REACTOR: CONSTANT VOLUME VERSUS CONSTANT PRESSURECONSTANT VOLUME VERSUS CONSTANT PRESSURE

CONSTANT VOLUME

CONSTANT PRESSURE

jjjjj r

dtdV

VC

dtdC

dtVCd

VdtdN

V=⋅+=

⋅⋅=⋅

)(11j

jjj rdt

dCdt

VNddt

dNV

===⋅)/(1


CONTINUOUSCONTINUOUS--FLOW REACTORS:FLOW REACTORS:CONTINUOUSCONTINUOUS--STIRRED TANK REACTOR (CSTR)STIRRED TANK REACTOR (CSTR)

• IDEAL STEADY-STATE FLOW REACTOR

• THE EXIT STREAM FROM THIS REACTOR HAS THE SAME COMPOSITION AS THE FLUID WITHIN THE REACTOR.

COMPOSITION UNIFORM (WELL MIXED) AND TIME-INDEPENDENT


CSTR CHARACTERISTICSCSTR CHARACTERISTICS

• USED WHEN INTENSE AGITATION IS REQUIRED.

• EASY TO MAINTAIN.• GOOD TEMPERATURE CONTROL.• CONVERSION OF REACTANT PER VOLUME OF

REACTOR IS THE SMALLEST OF THE FLOW REACTORS.

• CONSEQUENTLY, LARGE REACTORS ARE NECESSARY TO OBTAIN HIGH CONVERSIONS.


CSTR: MASS BALANCECSTR: MASS BALANCE

∫+−= dVrFFdt

dNjjj

j0


0=dt

dN j VrdVr jj

⋅=∫

(NO SPATIAL VARIATIONS IN RATE OF REACTION)

(STEADY STATE)

vCF jj ⋅=NOTE:

Volume/timej

jj

rFF

V−−

= 0DESIGN EQUATION FOR CSTR

CONTINUOUSCONTINUOUS--FLOW REATORS:FLOW REATORS:PLUG FLOW REACTOR (PFR)PLUG FLOW REACTOR (PFR)

• IDEAL STEADY-STATE TUBULAR FLOW REACTOR

• FLOW HIGHLY TURBULENT AND NO RADIAL VARIATION IN CONCENTRATION.

• RESIDENCE TIME IN THE REACTOR IS THE SAME FOR ALL ELEMENTS OF FLUID.

COMPOSITION VARIES AXIALLY BUT IS TIME-INDEPENDENT


PFR CHARACTERISTICSPFR CHARACTERISTICS

• USED TO PROCESS LARGE QUANTITIES OF MATERIAL.

• PRODUCES THE HIGHEST CONVERSION PER REACTOR VOLUME OF ANY OF THE FLOW REACTORS.

• GOOD PRODUCT QUALITY.• EASY TO MAINTAIN (NO MOVING PARTS).• NEEDS SUPPORTING EQUIPMENT.• DIFFICULT TO CONTROL TEMPERATURE

WITHIN THE REACTOR (HOT SPOTS).


PFR: MASS BALANCEPFR: MASS BALANCE

∫+−= dVrFFdt

dNjjj

j0

0=dt

dN jjF∆− Vrj ∆⋅

(OVER A SUBVOLUME)(STEADY STATE)

jj r

dVdF

=⇒


PACKEDPACKED--BED REACTORBED REACTOR

• THE REACTION RATE IS BASED ON MASS OF SOLID CATALYST W RATHER THAN ON REACTOR VOLUME V: THE RATE OF REACTION OF A SUBSTANCE A IS

catalystgsreactedAmolesrA _

__'⋅

=−

IT INVOLVES FLUID-SOLID HETEROGENEOUS REACTIONS


PACKEDPACKED--BED REACTOR: MASS BALANCEBED REACTOR: MASS BALANCE

WrWWFWFAAA ∆+∆+−= ')()(0

'AA r

dWdF

= ∫=A

A

F

F A

A

rdFW

0'

CATALYST MASS


MOLE BALANCE ON FOUR COMMON REACTORSMOLE BALANCE ON FOUR COMMON REACTORS


ExampleExample nono. 1. 1

• The reaction is to be carried out isothermally in a continuous-flow reactor. Calculate both the CSTR and PFR reactor volumes necessary to consume 99% of A (i.e., CA=0.01CA0) when the entering molar flow is 5 mol/h, assuming the reaction rate –rA is :(a) –rA = k with k = 0.05 mol/h*dm3

(b) –rA = kCA with k = 0.0001 s-1

(c) –rA = k(CA)2 with k = 3 dm3/mol*hThe entering volume flow rate is 10 dm3/h. Note: FA=CAv. For a constant volumetric flow rate v=v0, then FA=CAv0. Also, CA0=FA0/v0=(5 mol/h)/(10 dm3/h)=0.5 mol/dm3.

BA→


ExampleExample nono. 2. 2

• The gas phase reaction is carried out isothermally in a 20-dm3 constant-volume batch reactor. Twenty moles of pure A is initially placed in the reactor. The reactor is well mixed. (a) If the reaction is first order: –rA = kCA with k = 0.865 min-1, calculate the time necessary to reduce the number of moles of A in the reactor to 0.2 mole (Note: NA=CAV)(b) If the reaction is second order: –rA = k(CA)2 with k = 2 dm3/(mol*min), calculate the time necessary to consume 19 moles of A.(c) If the temperature is 1270C, what is the initial total pressure? What is the final total pressure assuming the reaction goes to completion?

CBA +→


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