What is Chemical Reaction Engineering?

Chemical reaction engineering (CRE) is the

discipline that

in relating reactor performance to

operating conditions and feed variables.

– deals with how fast a reaction proceeds (reaction

rates)

– deals with mechanism of reaction,

– deals with the effects of P,T, composition and

catalysts on reaction rates

– deals with size of reactor

– deals with type/configuration of reactor

– involves consideration of heat and mass transfer.

• Chemical species refers to any compound or element with

a given identity.

• The identity of a chemical species is determined by the

kind, number, and configuration of that species' atoms.

• A chemical species is said to have reacted when it has lost

its chemical identity.

• Three ways a chemical species can lose its chemical

identity:

• decomposition

• combination

• isomerization

Homogeneous vs Heterogeneous 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)

reactions that require the

presence of two distinct phasesCoal combustion

C (s) + O2 (g) CO2 (g)

SO3(for sulphuric acid production)

SO2 (g) + 1/2 O2 (g) SO3 (g) Vanadium catalyst (s)

•The reaction rate 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 as the rate of appearance of

a product. Consider species A: (rA; -rA; -rA’)

rA = the rate of formation of species A per unit volume

-rA = the rate of a disappearance of species A per unit v

volume

-rA’ = the rate of disappearance of species A on a per

mass of catalyst basis- for a catalytic reaction

Consider species j:

•rj is the rate of formation of species j per unit volume. It is the

number of moles of species j generated per unit volume per unit

time.

•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 can be also be function of position and can very from point to

point

– Continuous-Stirred Tank Reactor (CSTR)

– Plug Flow Reactor (PFR)

– Packed Bed Reactor (PBR)

– Membrane Reactor

– Fluidized Bed Reactor

, the typical

situation of chemical process is shown below:

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

– steady state operation;

used in series

– good mixing leads to

uniform concentration and

temperature

– mainly used for liquid

phase reaction

– suitable for viscous liquids

•PFR

– suitable for fast

reaction

– gas phase reaction

– temperature control is

difficult

– there are no moving

parts

CHARACTERISTICS OF COMMON REACTOR

Control Volume = V

FjO

Fj

Gj = (rate of formation of j) · V

= (rj)·V

dt

dN jF jO=G j

+F j-

Gj

Conc. changes with time

but is uniform within the

reactor. Reaction rate

varies with time.

Conc. inside the reactor is

uniform. rj is constant.

Exit conc = conc inside

reactor.

Differential

Equation

Algebraic

Equation

Integral

EquationRemarks

Vrdt

dNj

j)(

j

jO

N

N j

j

Vr

dNt

)(Batch

CSTR )(j

jjo

r

FFV

PFRj

jr

dV

dF

j

jO

F

F j

j

r

dFV

)(

Concentration and hence

reaction rates vary

spatially.

PBR 'j

jr

dW

dF

j

jO

F

F j

j

r

dFW

)'(

•The rate of equation/ the rate law is an that on reacting materials and

reaction conditions. It is of the type of reactor (batch or continuous).

•k is rate constant which is temperature dependent

Consider a single reaction with stoichiometric equation

The rate of disappearance of A is given by

Such reaction is called elementary reaction

the rate of equation corresponds to a stoichiometric equations

H2+I22HI -rH2=k[H2][I2]

When there is , then we have non-elementary reactions. The classical

example of a non-elementary reaction is that between hydrogen and bromine,

which has a rate expression

: no direct correspondence between stoichiometry and rate

• Elementary reactions are often represented by an equation showing both the molecularity and the rate constant.

For example

The rate of equation is:

• Consider this reaction

• Rate of equation that refers to B

• Rate of equation that refers to D

• Rate of equation that refers to T

• But from stoichiometry point of view, the equation will be

A non-elementary reaction is one whose stoichiometry does not match its kinetics. For example,

reaction always involve

•However, it is difficult to quantify the concentration of intermediate since it exists only for few minutes.

•Types of intermediate can be grouped into

• Simply put, reaction rates can be defined as speed of reactions.

• Some reactions can be very, i.e. Sewage treatment plants

• Some reactions can be very, i.e. Reactions in rocket engines

• The rate of a reaction can be expressed as the rate of of a reactant or as the rate of of a product

• The rate of a reaction can be expressed

as the rate of of a reactant

as the rate of of a product

Reaction rate is defined as changes in concentration over time

Unit SI is mol L-1s-1

Reaction rate can be quantified in terms of disappearing reactant or appearing product.

rate dCi

dt

• For relative rate of reactions, various species that involved in reaction can be obtained from stoichiometric coefficient:

rA

arB

brC

crD

d

aAbBcCdD

4 moles of A reacted with 8 moles of B to produce 4 moles of C and 4 moles of D

• If the rate of change is in number of moles of component i due to reaction, , the rate of reaction in various forms can be defined:

based on unit volume of reacting fluid

based on unit mass of solid in fluid-solid systems

(EQ 4)

(EQ 5)

based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems

based on unit volume of solid in gas-solid systems

based on unit volume of reactor, if different from the rate based on unit volume of fluid

(EQ 6)

(EQ 7)

(EQ 8)

In the volume of fluid in the reactor is often identical to the volume of reactor. In such a case V and Vr are identical and Eqs. 4 and 8 are used interchangeably.

In all the above definitions of reaction rate are encountered, the definition used in any particular situation often being a matter of convenience.

From Eqs. 4 to 8 these intensive definitions of reaction rate are related by:

(EQ 9)

• The molecularity of an elementary reaction is the number of molecules involved in the reaction, and this has been found to have the values of one, two, or occasionally three.

• Note that the molecularity refers only to an elementary reaction.

• Let us say, materials A, B, . . . , D, can be approximated by an expression of the following type:

The molecularity shows the power or the order of the reaction

• Testing of kinetic models

What is the possible reaction mechanism?

For many reactions, and particularly elementary reactions, the rate expression can be written as a product of a temperature-dependent term and a composition dependent term, or

This is practically well presented by Arrhenius’ Law

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

The temperature dependency of reactions is determined by the activation energy and temperature level of the reaction, as illustrated

These findings are summarized as follows:

1. From Arrhenius' law a plot of ln k vs 1/T gives a straight line, with large slope for large E and small slope for small E (slope = E/R).

2. Reactions with high activation energies are very temperature-sensitive; reactions with low activation energies are relatively temperature-insensitive.

3. k0 does not affect the temperature sensitivity.

Example 1

• Milk is pasteurized if it is heated to 63oC for 30 min, but if it is heated to 74°C it only needs 15 s for the same result. Find the activation energy of this sterilization process.

Example 2

• A rocket engine, Fig. El.l, burns a stoichiometric mixture of fuel (liquid hydrogen) in oxidant (liquid oxygen). The combustion chamber is cylindrical, 75 cm long and 60 cm in diameter, and the combustion process produces 108 kg/s of exhaust gases. If combustion is complete, find the rate of reaction of hydrogen and of oxygen.

Example 2

A human being (75 kg) consumes about 6000 kJ of food per day. Assume that the food is all glucose and that the overall reaction is

Find man's metabolic rate (the rate of living, loving, and laughing) in terms of moles of oxygen used per m3 of person per second.