Chemical Reaction Engineering

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Chemical Reaction Engineering
Lecture 1
Lecturer ???
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Syllabus

• Fundamentals of CRE
• Ideal reactor types and design equations
• Interpretation of rate data
• Non-elementary homogeneous reactions
• Non-isothermal reactors
• Multiple reactions

• Non-ideal reactors
• Catalysis and catalytic reactors
• External diffusion effects on heterogeneous
  reactions
• Diffusion and reaction in porous catalysts
• Residence time distributions

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What is Chemical Reaction Engineering (CRE) ?
Understanding how chemical reactors work lies at
the heart of almost every chemical processing
operation. Design of the reactor is no
routine matter, and many alternatives can be
proposed for a process. Reactor design uses
information, knowledge and experience from a
variety of areas - thermodynamics, chemical
kinetics, fluid mechanics, heat and mass
transfer, and economics. CRE is the synthesis
of all these factors with the aim of properly
designing and understanding the chemical reactor.
Products
Chemical
Raw
Separation
Separation
By products
process
material
Process
Process
J. Wood at Bham Univ.
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Text book and Recommended Books

• Elements of Reaction Engineering, 4th
  Edition. H.Scott Fogler, Prentice Hall.
• Chemical Reaction Engineering, 3rd Edition.
  Octave Levenspiel, John Wiley and Sons.
• Reactor Design for Chemical Engineers. J.M.
  Winterbottom and M.B. King

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Fundamentals

• Ideal Reactors
• Perfectly mixed batch reactor (Batch)
• Continuous stirred tank reactor (CSTR) or Backmix
  reactor
• Plug flow reactor (PFR)
• Packed bed reactor (PBR)
• Chemical kinetics
• All reactions are presented as being homogeneous
  reactions.
• Multiple reactors
• Isothermal ideal Batch, CSTR, and PFR

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Chemical reaction

• 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
  structure or configuration of these atoms.
• Decomposition
• Combination
• Isomerization
• Rate of reaction
• How fast a number of moles of one chemical
  species are being consumed to form another
  chemical species.

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Rate law for rj

• rA the rate of formation of species A per unit
  volume e.g., mol/dm3-s
• -rA the rate of a disappearance of species A
  per unit volume
• Rate law for rj is a function of concentration,
  temperature, pressure, and the type of catalyst
  (if any)
• Rate law for rj is independent of the type of
  reaction system (batch, plug flow, etc.)
• Rate law for rj is an algebraic equation, not a
  differential equation

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Ideal Reactor Types

• It has neither inflow nor outflow of reactants or
  products which the reaction is being carried out.
• Perfectly mixed
• No variation in the rate of reaction throughout
  the reactor volume

BATCH
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Batch Reactor

• All reactants are supplied to the reactor at the
  outset. The reactor is sealed and the reaction is
  performed. No addition of reactants or removal of
  products during the reaction.
• Vessel is kept perfectly mixed. This means that
  there will be uniform concentrations. Composition
  changes with time.
• The temperature will also be uniform throughout
  the reactor - however, it may change with time.
• Generally used for small scale processes, e.g.
  Fine chemical and pharmaceutical manufacturing.
• Low capital cost. But high labour costs.
• Multipurpose, therefore allowing variable product
  specification.

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Example of a liquid phase batch reaction.
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Typical Laboratory Glass Batch Reactor
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Typical Laboratory High Pressure Batch Reactor
(Autoclave)
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Typical Commercial Batch Reactor
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Ideal Reactor Types

• Normally run at steady state.
• Quite well mixed
• Generally modelled as having no spatial
  variations in cencentration, temperature, or
  reaction rate throughout the vessel

CONTINUOUS STIRRED TANK REACTOR (CSTR) BACKMIX
REACTOR
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Backmixed, Well mixed or CSTR
FA0 (CA0)

• Usually employed for liquid phase reactions.
• Use for gas phase usually in laboratory for
  kinetic studies.

FA (CA)
CA
CA
Vr, g
CA
Assumption Perfect mixing occurs.
?
Schematic representation of a CSTR
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Characteristics

• Perfect mixing the properties of the reaction
  mixture are uniform in all parts of the vessel
  and identical to the properties of the reaction
  mixture in the exit stream (i.e. CA, outlet CA,
  tank)
• The inlet stream instantaneously mixes with the
  bulk of the reactor volume.
• A CSTR reactor is assumed to reach steady state.
  Therefore reaction rate is the same at every
  point, and time independent.
• What reactor volume, Vr , do we take?
• Vr refers to the volume of reactor contents.
• Gas phase Vr reactor volume volume contents
• Liquid phase Vr volume contents

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Cutaway view of a Pfaudler CSTR/ Batch Reactor
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Ideal Reactor Types

• Normally operated at steady state
• No radial variation in concentration
• Referred to as a plug-flow reactor
• The reactants are continuously consumed as they
  flow down the length of the reactor.

PLUG FLOW REACTOR (PFR), TUBULAR REACTOR
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PFR, Tubular reactor

• There is a steady movement of materials along the
  length of the reactor. No attempt to induce
  mixing of fluid element, hence at steady state
• At a given position, for any cross-section there
  is no pressure, temperature or composition change
  in the radial direction.
• No diffusion from one fluid element to another.
• All fluid element have same residence time.

Used for either gas phase or liquid phase
reactions.
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The plug flow assumptions tend to hold when there
is good radial mixing (achieved at high flow
rates Re gt104) and when axial mixing may be
neglected (when the length divided by the
diameter of the reactor gt 50 (approx.)) N.B. In
the case of a gas phase reaction, the pressure
history of the reaction must be noted in case the
number of moles change during the reaction. e.g.
A ? B C As the reaction progresses the number
of moles increases. Therefore at constant
pressure, fluid velocity must increase as
conversion increases.
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How can reaction rate be expressed ?

• Select one reaction component for consideration
  and define the rate in terms of this component,
  i.
• If the rate of change in number of moles of this
  component due to reaction is dNi/dt, then the
  rate of reaction in its various forms is defined
  as follows

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(a) Based on unit volume of reaction fluid
Ni moles of i V volume of fluid
e.g. CSTR (liquid) or Batch reactor (liquid)
(b) Based on unit volume of reactor
Vr reactor volume
(c) Based on unit interfacial surface area in
two-fluid system or based on per unit surface
area of solids in gas-solid systems
S interfacial area
(d) Based on unit mass of solid in fluid-solid
reactor
W mass of catalyst
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Conversion

• Conversion is defined to answer the questions
• How can we quantify how far a reaction has
  progressed?
• How many moles of product C are formed for every
  mole reactant A consumed?
• The conversion XA is the number of moles of A
  that have reacted per mole of A fed to the system

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Design equations for the ideal reactors based
on material balance
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A Mole Balance on Species j (isothermal)
Rate of accumulation of j within the
system moles/time
Rate of flow of j into the system moles/time
Rate of flow of j out of the system moles/time
Rate of generation of j by chemical rxn within
the system moles/time
Fj0
Fj
Gj
Fj0
Fj
Gj
System volume
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Mole Balance - Batch Reactor

• No material enters or leaves the reactor.
• If composition in uniform (i.e. perfect mixing) -
  material balance can be written over whole
  reactor.
• No flow in or out of reactor. Terms (1) and (2)
  0.

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Mole Balance - Batch Reactor
V Reactor volume, but really refers to the
volume of fluid in reactor.
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Rate of accumulation of A, moles/time
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Rate of disappearance of A, moles/time
If system is constant volume, then
Where CA0 is the initial concentration of A
(mol/m3)
Integrating the equation gives the design
equation for the batch reactor
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Mole Balance - CSTR
CSTR (at steady state) - NO ACCUMULATION.
Accumulation Input - Output
Generation by reaction 0 Fj0 - Fj

No spatial variation
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Mole Balance - CSTR
CSTR (at steady state) - NO ACCUMULATION.
Accumulation Input - Output -
Disappearance by reaction 0 FA0 - FA
- (-rA)Vr
FA FA0 (1-XA) FA0 FA (-rA)Vr
? FA0 XA (-rA) Vr
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Mole Balance tubular reactor
In a plug flow reactor the composition of the
fluid varies from point to point along a flow
path consequently, the material balance for a
reaction component must be made for a
differential element of volume ?V.
?V
FA0
FA
Fj0
Fj
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Mole Balance - PFR
INPUT Input of A, moles/time FA Conversion of A
XA
OUTPUT Output of A, moles/time FA
dFA Conversion of A XA dXA
Disappearance of A by reaction, moles/time
(-rA) dVr
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Accumulation Input - Output -
Disappearance by reaction 0 FA - (FAdFA)
- (-rA)dVr
PFR (at steady state) - NO ACCUMULATION.
- dFA (-rA)dVr
dFA -FA0 (dXA)
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Some issues

• Molar flow rates
• Space time
• Time necessary to process one reactor volume of
  fluid based on entrance conditions
• Also called the holding time or mean residence
  time

Liquid phase
Gas phase
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Some issues

• Space velocity
• Liquid-hourly space velocity (LHSV)
• Gas-hourly space velocity (GHSV)

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Factors Involved in Reactor Design

• Feedstock composition
• Single feedstock
• Reactant in a solvent
• Multi-component feedstock
• Scale of process
• output of product
• Process kinetics
• Effect of composition (concentration)
• Effect of temperature
• Catalyst
• Thermodynamics

• Reactor type
• Batch / continuous
• Semi batch / Semi continuous
• Isothermal, non-isothermal, adiabatic
• Single pass / recycle
• Multiple reactors
• Others
• Materials of construction
• instrumentation
• safety

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Mole balances on 4 common reactors
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Design equations

• Batch
• The conversion is a function of the time the
  reactants spend in the reactor.
• We are interested in determining how long to
  leave the reactants in the reactor to achieve a
  certain conversion X.

?
?
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Design equations

• CSTR
• We are interested in determining the size of the
  reactor to achieve a certain conversion X.

?
?
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Design equations

• PFR
• We are interested in determining the size of the
  reactor to achieve a certain conversion X.

?
?
PBR
Generally, the isothermal tubular reactor
volume is smaller than the CSTR for the same
conversion
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What is the relationship between X and rA ?
We need only -rA f (X) and FA0 to design a
variety of reactors !
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The heart of the design of an ideal
reactor (-rA) as a function of conversion
(concentration, partial pressure etc.)
We will discuss this issue in the later courses.
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Reactors in series

• To achieve the same overall conversion, the total
  volume for two CSTRs in series is less than that
  required for one CSTR.
• The overall conversion of two PFRs in series is
  the same as one PFR with the same total volumn.
• CSTRs in series A PFR can be modelled using a
  number of CSTR in series
• useful in modelling catalyst decay in a
  packed-bed reactor
• modelling transit heat effects in PFRs.

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Example Reactor Types

• Noncatalytic homogeneous gas reactor
• Homogeneous liquid reactor
• Liquid-liquid reactor
• Gas-liquid reactor
• Non-catalytic gas-solid reactor
• Fixed bed
• Fluidised bed
• Fixed bed catalytic reactor
• Fluid bed catalytic reactor
• Gas-liquid-solid reactor

• Ethylene polymerisation
• (high pressure)
• Mass polymerisation of styrene
• Saponification of fats
• Nitric acid production
• Iron production
• Chlorination of metals
• Ammonia synthesis
• Catalytic cracking (petroleum)
• Hydrodesulphurisation of oils

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Battery of two tubular reactors. Furnaces on
the back. Heat exchangers on the front.
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Selection of Reactors

• Batch
• small scale
• production of expensive products (e.g. pharmacy)
• high labor costs per batch
• difficult for large-scale production
• CSTR most homogeneous liquid-phase flow
  reactors
• when intense agitation is required
• relatively easy to maintain good temperature
  control
• the conversion of reactant per volume of reactor
  is the smallest of the flow reactors - very large
  reactors are necessary to obtain high conversions
• PFR most homogeneous gas-phase flow reactors
• relatively easy to maintain
• usually produces the highest conversion per
  reactor volumn (weight of catalyst if it is a
  packed-bed catalyze gas reaction) of any of the
  flow reactors
• difficult to control temperature within the
  reactor
• hot spots can occur
• Fluidised bed reactor (circulating fluidised bed
  CFB)

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Example 1-1 Consider the liquid phase cis-trans
isomerization of 2-butenewhich we will write
symbolically as A?B. The first order (-rAkCA)
reaction is carried out in a tubular reactor in
which the volumetric flow rate, v, is constant,
i.e., vv0. (a) Sketch the concentration profile.
(b) Derive an equation relating the reactor
volume to the entering and exiting concentration
of A, the rate constant k, and the volumetric
flow rate v.
PFR design equation
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Determine the reactor volume necessary to reduce
the exiting concentration to 10 of the entering
concentration when the volumetric flow rate is 10
dm3/min (i.e., liters/min) and the specific
reaction rate, k, is 0.23 min-1.
A reactor volume of 0.1m3 is necessary to convert
90 of species A entering into product B for the
parameter given.
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• Example 2-2 The reaction A?B described by the
  data in Table 2-2 is to be carried out in a CSTR.
  Species A enters the reactor at a molar flow rate
  of 0.4 mol/s. (a) Using the data in Table 2-2,
  calculate the volume necessary to achieve 80
  conversion in a CSTR.

(a) CSTR design equation
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(b) Shade the area in Figure 2-2 that would give
the CSTR volume necessary to achieve 80
conversion.
CSTR design equation
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Example 2-3 The reaction described by the data in
Table is to be carried out in a PFR. The entering
molar flow rate of A is 0.4 mol/s. (a) determine
the PFR reactor volume necessary to achieve 80
conversion.
PFR design equation
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(b) Next, shade the area in Figure 2-2 that would
give the PFR volume necessary to achieve 80
conversion.
PFR design equation
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(c) Sketch the profile of rA and X down the
length of the reactor
For X0.2, the corresponding reactor volume
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Example 2-4 From example 2-2 2-3, to achieve a
conversion of 80 with the entering molar flow
rate FA0 0.4 mol/s and the same feed
conditions, The CSTR volume was 6.4 m3 and the
PFR volume was 2.165 m3
For isothermal reactions greater than zero order,
the CSTR volume will usually be greater than the
PFR volume for the same conversion and reaction
conditions (temperature, flow rate, etc.)
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Example 2-5 For the two CSTRs in series, 40
conversion is achieved in the first reactor. What
is the volume of each of the two reactors
necessary to achieve 80 overall conversion of
the entering species A?
(a) CSTR design equation
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Example 2-6 calculate the reactor volumes V1 and
V2 for the plug-flow sequence when the
intermediate conversion is 40 and the final
conversion is 80. The entering molar flow rate
is the same as in the previous example, 0.4 mol/s.
PFR design equation
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Example 2-7 The isomerization of
butanenC4H10?iC4H10 was carried out
adiabatically in the liquid phase. The reactor
scheme shown in Figure E2-7.1 is used. Calculate
the volume of each of the reactors for an
entering molar flow rate of n-butane of 50
kmol/hr.