3.1 constant-volume batch reactor 3.2 varying-volume batch...
TRANSCRIPT
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3.1 Constant-volume Batch Reactor
3.2 Varying-volume Batch Reactor
3.3 Temperature and Reaction Rate
3.4 The Search for a Rate Equation
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3.1 Constant Volume Batch Reactor
Constant volume of reaction mixture (Not the volume of reactor)
Meaning the rate of reaction of any component is given by
the rate of change of its concentration or partial pressure
For liquid phase system, it is easy to measure the concentration of each component i
However,
for gas phase system where the numbers of moles change,
it is impossible to measure the partial pressure of each component i directly.
Solution : calculation of partial pressure using total pressure () of the system
which can be measured directly
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How to calculate partial pressure of each component
General stoichiometric equation for a reaction in which the numbers of moles change
At any time t : total number =
Ideal gas law (pV = nRT)
Thus, the unknown variable
where = total pressure of the system
By measuring total pressure,
we can get partial pressure
Conversion fraction
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New terminology : Conversion (X)
Suppose that NAo is the initial amount of A in the reactor at time t = 0,
and that NA is the amount present at time t.
Then the conversion of A in the constant volume system
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Two Procedures for Analyzing Kinetic Data
1) Integral method
2) Differential method
In the integral method of analysis
we guess a particular form of rate equation and,
after appropriate integration and mathematical manipulation,
predict that the plot of a certain concentration function versus time should yield a straight line.
The data are plotted, and if a reasonably good straight line is obtained,
then the rate equation is said to satisfactorily fit the data.
In the differential method of analysis
we test the fit of the rate expression to the data directly and without any integration.
However, since the rate expression is a differential equation,
we must first find the value of (l/V)(dNldt) from the data before attempting the fitting procedure.
The integral method is easy to use and is recommended when testing specific mechanisms,
or relatively simple rate expressions
The differential method is useful in more complicated situations but requires more accurate
or larger amounts of data.
In general, integral analysis is attempted first, and,
if not successful, the differential method is tried.
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1) Integral Method of Analysis of Experimental Data
General Procedure :
The integral method of analysis always puts a particular rate equation to the test
by integrating and comparing the predicted C versus t curve
with the experimental C versus t data.
If the fit is unsatisfactory, another rate equation is guessed and tested.
Integral method is especially useful for fitting simple reaction types corresponding to
elementary reactions.
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a) Irreversible unimolecular- type first-order reaction
b) Irreversible bimolecular- type second-order reaction #1
c) Irreversible bimolecular- type second-order reaction #2
d) Irreversible bimolecular- type second-order reaction #3
e) Irreversible trimolecular- type third-order reaction
f) N th order reactions
g) Zero-order reactions
h) Irreversible reactions in parallel
i) Homogeneous Catalyzed Reactions
j) Autocatalytic reactions
k) Irreversible series reactions
l) First-order reversible reactions
m) Second-order reversible reactions
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a) Irreversible unimolecular- type first-order reaction
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b) Irreversible bimolecular- type second-order reaction #1
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c) Irreversible bimolecular- type second-order reaction #2
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d) Irreversible bimolecular- type second-order reaction #3
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e) Irreversible trimolecular- type third-order reaction
Other irreversible trimolecular- type third-order reactions
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f) Empirical rate equations of nth order
When the mechanism of reaction is not known,
we often attempt to fit the data with an nth-order rate equation of the form
The order n cannot be found explicitly from the equation,
so a trial-and-error solution must be made.
Select a value for n and calculate k.
the value of n which minimizes the variation in k is the desired value of n.
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g) Zero-order reactions
A reaction is of zero order
when the rate of reaction is independent of the concentration of materials
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h) Irreversible reactions in parallel (multiple reaction)
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i) Homogeneous catalyzed reactions (multiple reaction)
Catalyst concentration = constant
Making a series of runs with different catalyst concentrations allows us to find k1 and k2.
reaction would proceed even without a catalyst present and
reaction rate is directly proportional to the catalyst concentration.
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j) Autocatalytic reactions
One of the products of reaction acts as a catalyst
Because the total number of moles of A and
R remain unchanged as A is consumed
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k) Irreversible reactions in series
: consecutive unimolecular-type first-order reaction
if k2 is much larger than k1
if k1 is much larger than k2
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maximum concentration of R
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l) First-order reversible reactions
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m) Second-order reversible reactions
for the bimolecular-type second-order reactions
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Example 3.1 : Find a Rate Equation using the Integral Method
First-order kinetics Second-order kinetics nth order kinetics
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Trial 1 ) First-order kinetics
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Trial 2 ) Second-order kinetics
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Trial 3 ) n th-order kinetics
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REMIND) Two Procedures for Analyzing Kinetic Data
1) Integral method
2) Differential method
In the integral method of analysis
we guess a particular form of rate equation and,
after appropriate integration and mathematical manipulation,
predict that the plot of a certain concentration function versus time should yield a straight line.
The data are plotted, and if a reasonably good straight line is obtained,
then the rate equation is said to satisfactorily fit the data.
In the differential method of analysis
we test the fit of the rate expression to the data directly and without any integration.
However, since the rate expression is a differential equation,
we must first find the value of (l/V)(dNldt) from the data before attempting the fitting procedure.
The integral method is easy to use and is recommended when testing specific mechanisms,
or relatively simple rate expressions
The differential method is useful in more complicated situations but requires more accurate
or larger amounts of data.
In general, integral analysis is attempted first, and,
if not successful, the differential method is tried.
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2) Differential Method of Analysis of Experimental Data
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Example : If the curve pattern (-ra versus t) follow the M-M equation
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n th order kinetic : Solve it using differential method (without integration)
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3.2 Varying- Volume Batch Reactor
For isothermal constant pressure &
single stoichiometric reaction system,
the volume (V) is linearly related to the
conversion(X)
= the fractional change in volume of the system between
no conversion and complete conversion of reactant A
Example )
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3.3 Temperature and Reaction Rate
Arrhenius’ Law
[J/mol]
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Activation Energy and Temperature Dependency N
atur
al lo
garit
hm s
cale
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Example 3.4 : Correct and Incorrect E Values
1) Incorrect E value (using Pressure Units)
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2) Correct E value (using Composition Units)
At 400 K :
At 500 K :