chemical reaction engineering
DESCRIPTION
Chemical Reaction Engineering. Dr. Yahia Alhamed. Kinetics and Reaction Rate. What is reaction rate? It is the rate at which a species looses its chemical identity per unit volume. The rate of a reaction can be expressed as:- - The rate of disappearance of a reactant or - PowerPoint PPT PresentationTRANSCRIPT
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ChemicalChemical Reaction Reaction EngineeringEngineering
Dr. Yahia AlhamedDr. Yahia Alhamed
SABIC Chair in Catalysis at KAUSABIC Chair in Catalysis at KAU
What is reaction rate?
It is the rate at which a species looses its chemical identity per unit volume.
The rate of a reaction can be expressed as:-
- The rate of disappearance of a reactant or
- The rate of appearance of a product.
Kinetics and Reaction RateKinetics and Reaction Rate
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Reaction RateReaction RateConsider species A:
-rA = the rate of formation of species A per unit volume
rB = the rate of formation of species B per unit volume
EXAMPLE: If B is being formed at 0.2 moles per decimeter cubed per second, ie, rB = 0.2 mole/dm3/s
Then A is disappearing at the same rate:-rA= 0.2 mole/dm3/sThe rate of formation (generation of A) is rA= -0.2 mole/dm3/s
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Reaction RateReaction RateConsider species j:
• rj is the rate of formation of species j per unit volume [e.g. mol/dm3*s]
• rj is a function of concentration, temperature, pressure, and the type of catalyst (if any)
• rj is independent of the type of reaction system (batch, plug flow, etc.)
• rj is an algebraic equation, not a differential equation
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Rate Law BasicsRate Law Basics
• A rate law describes the behavior of a reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration.
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Reaction Rate for solid catalytic reactionsReaction Rate for solid catalytic reactions
• For a catalytic reaction, we refer to -rA', which is the rate of disappearance of species A on a per mass of catalyst basis.
• -r'A = rA/bulk density of the catalyst (ρb)
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Rate Law BasicsRate Law Basics• A rate law describes the behavior of a reaction. The rate
of a reaction is a function of temperature (through the rate constant) and concentration.
• Power Law Modelk is the specific reaction rate (constant)
k is given by the Arrhenius Equation:
Where:E = activation energy (cal/mol)
– R = gas constant (cal/mol*K)
– T = temperature (K)
– A = frequency factor (units of A, and k, depend on overall reaction order)
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General General MoleMole Balance Balance
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Batch Reactor Mole BalanceBatch Reactor Mole Balance
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Constantly Stirred Tank Reactor Mole BalanceConstantly Stirred Tank Reactor Mole BalanceCSTR or MFRCSTR or MFR
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Plug Flow Reactor (PFR) Mole BalancePlug Flow Reactor (PFR) Mole Balance
V dFArAFA 0
FAThe integral form is:
This is the volume necessary to reduce the entering molar flow rate (mol/s) from FA0 to the exit molar flow rate of FA.
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Packed Bed Reactor Mole Packed Bed Reactor Mole BalanceBalance
PBR
The integral form to find the catalyst weight is:
W dFA
r AFA 0
FA
FA0 FA r AdW dNA
dt
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Space time and space velocitySpace time and space velocity FA0 = CAo vo
θ = is called space time (s) = V/vo
Space velocity = 1/θ, where;FA0 = Molar feed rate of key reactant A (mol/s)
CAo= Concentration of key reactant A in the feed (mol/m3)
vo=Volumetric flow rate of feed to the reactor (m3/s)
V = volume of the reactor
For constant volume systems v = vo where v is volumetric flow rate leaving the reactor
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Reactor Mole Balance SummaryReactor Mole Balance Summary
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Reactor Mole Balance Reactor Mole Balance SummarySummary
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Reactor Mole Balance Reactor Mole Balance SummarySummary
SABIC Chair in Catalysis at KAUSABIC Chair in Catalysis at KAU
Reactor Mole Balance Reactor Mole Balance SummarySummary
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Reactor Mole Balance Reactor Mole Balance SummarySummary
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ConversionConversion
Consider the general reaction: aA + bB -cC + dDWe will choose A as bases of calculation (i.e. Key reactant)The limiting reactant is usually taken as the key reactant Then: A + (b/a)B (c/a)C + (d/a)DXA = moles reacted/moles fed
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Batch Reactor ConversionBatch Reactor Conversion
dN Adt
rAV
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CSTR ConversionCSTR Conversion
Algebraic Form:
There is no differential or integral form for a CSTR.
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PFR ConversionPFR Conversion
PFR
dFAdV
rA
FA FA0 1 X
Differential Form:
Integral Form:
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Design EquationsDesign Equations
V
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Reactor Sizing (CSTR)Reactor Sizing (CSTR)
• Given -rA as a function of conversion, -rA=f(X), one can size any type of reactor.
• We do this by constructing a Levenspiel plot.
• Here we plot either as a function of X.
• volume of a CSTR is:
FA0 rA
or 1 rA
VFA0 X 0 rA EXIT
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Reactor Sizing (PFR)Reactor Sizing (PFR)
VPFR FA 0
rA0
X
dX
For PFR th evolume of the reactor needed is given by the area under the curve
=area
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SummarySummary
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Rate Law BasicsRate Law Basics
• A rate law describes the behavior of a reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration.
• Power Law Model
k is the specific reaction rate (constant)
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Examples of Rate LawsExamples of Rate Laws
62HCA
24262
kCr
HHCHC
T
1
1000
1
mol
82kcal1e0.072sk
• First Order Reactions
(1) Homogeneous irreversible elementary gas phase reaction
with
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• First Order Reactions
(1) Homogeneous irreversible elementary gas phase reaction
with
(2) Homogeneous reversible elementary reaction
with and
Examples of Rate LawsExamples of Rate Laws
104104 HCiHCn
CiCnCn KCCkr44
360T
360T790631.1expk
333T
333T830.33.03expKC
62HCA
24262
kCr
HHCHC
T
1
1000
1
mol
82kcal1e0.072sk
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• First Order Reactions
(1) Homogeneous irreversible elementary gas phase reaction
with
(2) Homogeneous reversible elementary reaction
with and
• Second Order Reactions
(1) Homogeneous irreversible non-elementary reaction
with and
This is first order in ONCB, first order in ammonia and overall second order.
Examples of Rate LawsExamples of Rate Laws
104104 HCiHCn
CiCnCn KCCkr44
360T
360T790631.1expk
333T
333T830.33.03expKC
62HCA
24262
kCr
HHCHC
T
1
1000
1
mol
82kcal1e0.072sk
3NHONCBA CkCr
kmol.min
m0.0017k
3
mol
cal11273E At 188˚C
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Examples of Rate LawsExamples of Rate Laws
2323 NCNHBrCHNHCHCNBr
23NHCHCNBrA CkCr s.mol
2.2dmk
3
with
• Second Order Reactions
(2) Homogeneous irreversible elementary reaction
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Examples of Rate LawsExamples of Rate Laws
with2323 NCNHBrCHNHCHCNBr
23NHCHCNBrA CkCr s.mol
2.2dmk
3
• Second Order Reactions
(2) Homogeneous irreversible elementary reaction
This reaction is first order in CNBr, first order in CH3NH2 and overall second order.
(3) Heterogeneous catalytic reaction: The following reaction takes place over a solid catalyst: