Chemical Reaction Engineering

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Chemical Reaction Engineering

• Dr. Yahia Alhamed

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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
• - The rate of appearance of a product.

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Reaction Rate
Consider 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/s The rate of formation
(generation of A) is rA -0.2 mole/dm3/s
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Reaction Rate

• Consider species j
• rj is the rate of formation of species j per unit
  volume e.g. mol/dm3s
• 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 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 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 (?b)

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Rate 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)
• k is given by the Arrhenius Equation
• WhereE activation energy (cal/mol)
• R gas constant (cal/molK)
• T temperature (K)
• A frequency factor (units of A, and k, depend
  on overall reaction order)

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General Mole Balance
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Batch Reactor Mole Balance
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Constantly Stirred Tank Reactor Mole BalanceCSTR
or MFR
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Plug Flow Reactor (PFR) Mole Balance
The 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 Balance
PBR
The integral form to find the catalyst weight is
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Space time and space velocity

• FA0 CAo vo
• ? is called space time (s) V/vo
• Space velocity 1/?, where
• FA0 Molar feed rate of key reactant A (mol/s)
• CAo Concentration of key reactant A in the feed
  (mol/m3)
• voVolumetric 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 Summary
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Reactor Mole Balance Summary
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Reactor Mole Balance Summary
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Reactor Mole Balance Summary
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Reactor Mole Balance Summary
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Conversion

• Consider the general reaction aA bB -?cC
  dD
• We 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)D
• XA moles reacted/moles fed

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Batch Reactor Conversion
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CSTR Conversion
Algebraic Form
There is no differential or integral form for a
CSTR.
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PFR Conversion
PFR
Differential Form
Integral Form
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Design Equations
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Reactor Sizing (CSTR)

• Given -rA as a function of conversion, -rAf(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

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Reactor Sizing (PFR)
For PFR th evolume of the reactor needed is
given by the area under the curve
area
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Summary
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Rate 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 Laws

• First Order Reactions
• (1) Homogeneous irreversible elementary gas
  phase reaction
• with

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Examples of Rate Laws

• First Order Reactions
• (1) Homogeneous irreversible elementary gas
  phase reaction
• with
• (2) Homogeneous reversible elementary
  reaction
• with
  and

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Examples of Rate Laws

• 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.

At 188C
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Examples of Rate Laws

• Second Order Reactions
• (2) Homogeneous irreversible elementary
  reaction

with
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Examples of Rate Laws

• 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

with