chpr4406 reactions lecture 1

8/12/2019 CHPR4406 Reactions Lecture 1

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REACTION ENGINEERING

Introduction:

Raw physical Chemical physicalMaterials treatment treatment treatment products

steps steps steps

Recycle

Physical Treatment Step:

Eg: removal of impurities and undesirable compounds from rawmaterials or reactants;

Chemical Treatment Step:

Reactants in reactor react to form products

Physical Treatment Step

Eg: purifying the products

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The performance equation:

),,( contacting kineticsinput f output =

With this equation we can compare different designs and conditions,find which is best and then scale up to larger units

Classification of Reactions Homogeneous (one phase) Heterogeneous ( two phases)

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Variables Affecting the Rate of Reaction Homogeneous

Temperature, pressure, composition

Heterogeneous As above plus

Heat transfer, mass transfer, etc

For reaction involving a series of steps, the sloweststep determines the overall reaction rate.

In burning of a coal briquette, oxygen needs to bemoved through the gaseous film (products of coalcombustion) and the ashes layer to get to theburning or reaction zone. If this is the limiting stepthan mass transfer or diffusion is the ratecontrolling step.

Definition of Reaction Rate

B A

dt dN

V dt dN

V r B A B

11==

also ,V

N A A

=C

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Speed of Chemical Reactions

Figure 1.3 shows the relative rates at which reactions occur

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Thermodynamic

Two important pieces of information for Reactor Design Heat of reaction (exothermic or endothermic )r H

Maximum extent of reaction for reversible rxn

cC bBaA + veor

ve H r +

=

ve H r = , heat liberated, exothermic rxn

ve H r +=

, heat adsorbed, endothermic rxn

Extent of rxn for reversible rxn

C B A +

Equilibrium constant, ][

]][[

A

C B K =

which gives the maximum attainable yield of the products rxn tobe estimated .

K RT G o ln=

where is change of the standard Gibbs Free energy.oG

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Kinetic of Homogeneous

r =f(temperature, pressure, composition)

r =f(temperature, composition)

Concentration-Dependent Term of a Rate Equation

1. Single and Multiple Reactions

Single reaction: a single stoichiometric equation and singlerate equation to represent the progress of a reaction.

B A

Multiple reaction : when more than one stoichiometricequation is used to represent the observed changes, thenmore than one kinetic expression is needed to follow thechanging composition.

Rxn in series: S R A

Rxn in parallel: R

A

and others

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2. Elementary and Non-elementary Reactions

elementary R B A +

B A A C kC r =

non-elementary

HBr Br H 222 +

]/[][

]][[

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Br HBr k

Br H k r HBr +

=

3. Kinetic View of Equilibrium for Elementary Reactions

Consider the elementary reversible rxns:k 1

S R B A ++ k 2

Rate of formation of : R B A forward R C C k r 1, =

Rate of disappearance of : R S Rbackward R C C k r 2, =

At equilibrium:

backward R forward R r r ,, =

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2

1

2

1

21

k k K

C C C C

k k

C C k C C k

C

B A

S R

S R B A

=

=

=

At equilibrium, 0= G

4. Molecularity and Order of Reaction

The molecularity of an elementary reaction is the number of

molecules involved in the reaction. It was found to have a valueof 1,2 and sometime 3.

d D

b B

a A A C C kC r ......=

the order of reaction : nd ba =+++ .... It is:

a th order with respect to Ab th order with respect to Bn th order overall

5. Rate Constant k

The dimension or units of k ( ) ( ) nionconcentrat time 11

for example for a first order reaction, the unit

( ) 1time

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6. Representation of a Reaction Rate

For liquid phase elementary rxn:

R B A + B A A R C kC r r ==

For gaseous phase elementary rxn:

B A A R pkpr r ==

A p = partial pressure of A Kinetic Models for Nonelementary Reactions: an example

Free radicals chain reaction mechanism :

HBr Br H 222 +

with experimental rate

]/[][]][[

2

2/1221

Br HBr k Br H k

r HBr +=

can be explained by the following scheme:

Br Br 22 initiation and termination++ H HBr H Br 2 propagation++ Br HBr Br H 2 propagation

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Temperature-Dependent Term of a Rate Equation

Temperature Dependency from Arrhenius Law

)(.)(.)(

2

21

ncompositio f k r ncompositio f temp f r

i

i

=

=

According to Arrhenius Law:

RT E o ek k

/=

where is the frequency factor and is the activationenergy of reaction. This expression fits experiment well overwide temperature ranges and hence is a good approximation.

ok E

At the same concentration, but at two different temperatures, Arrhenius Law indicates that

)11

(lnln212

1

1

2

T T R E

k k

r r

==

Comparison of Theories with Arrhenius' Law

This expression

RT E mo eT k k

/' = 10 m

summaries the predictions of the simpler versions of collisionand transition theories for the temperature dependency of therate constant. For more complicated versions, is as large as3 or 4.

m

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The exponential term is so much more temperature-sensitivethan the T term, the variation of caused by the latter iseffectively masked, and we have:

m k

RT E o

RT E

ek k

ek /

/

=

or we can look at it this way

22

)(ln RT

E mRT RT

E T m

dT k d +

=+=

As for most reactions studied, we have E mRT


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Activation Energy and Temperature Dependency

From Arrhenius law a plot of versus 1 gives astraight line , with large slope for large and small slopefor small .

k ln T / E

E Reactions with high activation energies are very

temperature-sensitive; reactions with low activation energies

are relatively temperature- insensitive . A given reaction is much more temperature-sensitive atlow temperature than at high temperature

From the Arrhenius law the frequency factor does notaffect the temperature sensitivity of a reaction. In an actualreaction there may be a slight temperature dependency;however this minor and can be ignored.

ok

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chpr4406 reactions lecture 1

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