CHE 309: Chemical Reaction EngineeringCHE 309: Chemical Reaction Engineering
LectureLecture--22LectureLecture--22
Module 1: Mole Balances, Conversion & Reactor Sizing(Chapters 1 and 2, Fogler)
Module 1: Mole Balances, Conversion & Reactor Sizing
• Topics to be covered in Module 1 (Lectures 2-6):– General definitions for homogeneous/heterogeneous reactions
– Reaction rate.
– Common Industrial and Laboratory Reactor types and their key
characteristicscharacteristics
– Development of general mole balance and its application to common
industrial reactors (batch and continuous)
– Development of reactor design equations in terms of conversions
– Application of design equations in reactor sizing
Topics to be covered in today’s lectureTopics to be covered in today’s lecture
• Homogeneous and Heterogeneous reactions
– Reaction Rates and Definitions [Fogler Section 1.1]
• General Mole Balance Equation (GMBE) [Fogler Section 1.2]
• Common Reactor Types and Their Characteristics
• GMBE for a Batch Reactor [Fogler Section 1.3]
Homogeneous Reactions: reactions that occur in a single
phase (gas or liquid)
NOx formation
NO (g) + O2 (g) ↔ NO2 (g)
Ethylene Production
C2H6 (g) ↔ C2H4 (g) + H2 (g)
Homogeneous & Heterogeneous Reactions
Heterogeneous Reactions: reactions that require the presence of
two distinct phases
Coal combustion
C (s) + O2 (g) ↔ CO2 (g)
SO3(for sulphuric acid production)
SO2 (g) + 1/2 O2 (g) ↔ SO3 (g) Vanadium catalyst (s)
Classification of chemical reactions useful in rxn design
Non-catalytic Catalytic
Homogeneous Most gas-phase rxn Most liquid phase rxn
Fast rxns such as
burning of flame
Rxns in colloidal
systems Enzyme and
microbial rxnsmicrobial rxns
Heterogeneous Burning of coal
Roasting of ores
Attack of solids by
acids
Gas-liquid absorption
with rxn
Reduction if iron ore
to iron and steel
Ammonia synthesis
Oxidation of ammonia to
produce nitric acid
Cracking of crude oil
Oxidation of SO2 to SO3
On Reaction Rates
Design of a reactor requires that reaction rates of participating
species be specified.
(– rA) = rate of consumption of species A
= moles of A consumed per unit volume (mass) per unit time
(rA) = rate of formation of species A
Note: “minus” sign denotes consumption or disappearance.
Reaction Rate (Reaction Rate (--rrAA) for Homogeneous Reactions) for Homogeneous Reactions
Note: “minus” sign denotes consumption or disappearance.
Units of (rA) or (– rA)
• moles per unit volume (mass) per unit time
• mol/L-s or kmol/m3-s, mol/g-s, kmol/kg-s
* Caution
Customarily, we use rA = dCA/dt. But rA & dCA/dt come from different
concepts. As will be shown later, rA = dCA/dt is valid for only a constant
volume batch reactor system with perfect mixing.
Intrinsic Specific
Reaction Rate for Heterogeneous ReactionsReaction Rate for Heterogeneous Reactions
For a heterogeneous reaction, rate of consumption of species
A is denoted as (-rA')
Heterogeneous reactions of interest are primarily catalytic in
nature. Consequently, the rates are defined in term of mass of
catalyst present.catalyst present.
Units of (-rA')
•mol per unit time per mass of catalyst
•mol/s-g or kmol/hr-kg catalyst
Is (Is (--rrAA) = dC) = dCAA/dt always true?/dt always true?
Neither CAO nor CA are
changing with time
Let us consider the example of this flow reactor and evaluate if
dCA/dt is equal to (rA).
Ethylene Oxide
CCAO CA
Steady State Operation
- no change with time
CAO
CA
CAO CA
(mol/L) (mol/L)
10:00 am 50.0 10.0
12:00 pm 50.0 10.0
3:00 pm 50.0 10.0
5:00 pm 50.0 10.0
More on …. Reaction RateMore on …. Reaction Rate
Reaction rate (intrinsic)• function of temperature and reactant concentrations
• independent of reactor type
• described by a kinetic expression or rate law.
Rate Law (Rate Equation)
Note: a more appropriate description of functionality should be in terms
of “activities” rather than concentration.
Rate Law (Rate Equation)rate law is an algebraic equation that relates reaction rate to species
concentration via a reaction rate constant --- a constitutive relationship.
(-rA) = k ·[concentration terms]
e.g. (-rA) = k CA or (-rA) = k CA2 where, k is rate constant [k=f(T)]
Common Industrial & Laboratory Reactor TypesCommon Industrial & Laboratory Reactor Types
• Batch Reactor
• Continuous-Flow Reactors
– Continuous-Stirred Tank Reactor (CSTR)
– Tubular Reactor ⊃ Plug Flow Reactor (PFR)– Tubular Reactor ⊃ Plug Flow Reactor (PFR)
– Packed Bed Reactor (PBR)
• Other Reactor Types
– Membrane Reactor
– Fluidized Bed Reactor
Characteristics of Key Reactor TypesCharacteristics of Key Reactor Types
• Batch Reactor
– mainly used for small scale operation
– suitable for slow reactions
– mainly used for liquid-phase reaction
– charge-in/clean-up times can be large
• CSTR (Continuous-Stirred Tank Reactor)
– steady state operation; used in series
Loading
(t < 0)Reaction
(t ≥ 0)
Discharge
(t = tf)
– steady state operation; used in series
– good mixing leads to uniform conc. and temp.
– mainly used for liquid phase reaction
– suitable for viscous liquids
• PFR (Plug Flow Reactor)
– suitable for fast reaction
– gas phase reaction
– temperature control is difficult
– there are no moving parts
CAO
CA
Reactants Products
Characteristics of Other Reactor TypesCharacteristics of Other Reactor Types
• Membrane Reactor
– Perfect solid-liquid separation
– Short hydraulic retention time
– Long solid retention time
– Facility compactness
– Suitable for coating, combustion process– Suitable for coating, combustion process
Characteristics of Other Reactor TypesCharacteristics of Other Reactor Types
• Fluidized Bed Reactor
– The smooth, liquidlike flow of particles
– Easy handling
– Isothermal condition
– Large-scale operation
– Suitable for coating, combustion process– Suitable for coating, combustion process
Spraying Wetting Solidifying
coating droplets
particle coated particle
Drag force by upward moving gas = Weight of particles
[Ref.] Daizo Kunii & Octave Levenspiel, “ Fluidization Engineering”, John Wiley & Sons, Inc
Reactant enters Reactant leaves
Element of reactor volume
Reactant disappears by
chemical rxn within the
elementReactant accumulates
within the element
General Mole Balance Equation (GMBE)General Mole Balance Equation (GMBE)
INPUT Rate – OUTPUT Rate + Rate of GENERATION – Rate of Consumption =
Rate of ACCUMULATION
General mole balance equation is the foundation of reactor design.
You have used the following form of mole balance
Rate of ACCUMULATION
{ rate of reactant flow into element of volume} –{rate of reactant flow out element of
volume} +{rate of reactant loss due to chemical rxn within the element of volume}
={rate of accumulation of reactant in element of volume}
{ rate of heat flow into element of volume} –{rate of heat flow out element of volume}
+{rate of disapperance due to chemical rxn within the element of volume}
={rate of accumulation of heat in element of volume}
General Mole Balance EquationGeneral Mole Balance Equation
INPUT Rate - OUTPUT Rate + Rate of GENERATION =
Rate of ACCUMULATION
FA
dt
dN AFAO=GA
+FA−
Control Volume = V
FAO
FA
GA = (rate of Generation of A) · V
= (rA)·VIn case of perfect mixing
In a general case Vd r∆V rlim∆GlimG
V
Ai
M
1i
jiM
M
1i
jiM
A ∫∑∑ ====
∞→=
∞→
General Mole Balance for a Batch ReactorGeneral Mole Balance for a Batch Reactor
Reaction: A → Products
General Mole Balance Equation (GMBE) for Batch Reactor may be
written in differential form as
written in integral form as
FA0 = FA = 0 and GA = (rA)·V (as a result of perfect mixing)
1
Key information: Time to reduce NA from NA0 to NA1.
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