Cre chapter 1 overview of chemical reaction engineering

KYÕ THUAÄT PHAÛN ÖÙNG Chemical Reaction Engineering 8/22/2013 1 Chapter 1 Overview of Chemical Reaction Engineering 8/22/2013 2 8/22/2013 3 Introduction Reactor design uses information, knowledge, and[.]

Chemical

Reaction

Engineering

Chapter 1

Overview of Chemical Reaction

Engineering

 Chemical reaction engineering is the synthesis

of all these factors with the aim of properly designing

a chemical reactor

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Performance equation

To find what a reactor is able to do we need to know the kinetics, the

contacting attern and the performance equation

Performance equation relates input to output

Contacting pattern or how materials

flow through and contact each other

in the reactor, how early or late they

mix, their clumpiness or state of

aggregation By their very nature

some materials are very clumpy-for

instance, solids and noncoalescing

liquid droplets

Kinetics or how fast things

happen If very fast, then equilibrium tells what will leave the reactor If not so fast, then the rate of chemical reaction, and maybe heat and mass transfer too, will determine what will happen

=f [input, kinetics, contacting]

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Classification of

Reactions

Homogeneous: takes place in one phase

Heterogeneous: it requires the presence

of at least two phases to proceed at the rate

Reduction of iron ore to iron and steel

Ammonia synthesis Oxidation of ammonia to produce nitric acid

Cracking of crude oil Oxidation of SO2 to SO3

Classification of

Reactions

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Definition of Reaction Rate

The rate of change in number of moles of this

component due to reaction is dNi /dt

Based on unit volume of reacting fluid:

Based on unit mass of solid in fluid-solid systems

Based on unit interfacial surface in two-fluid systems

or based on unit surface of solid in gas-solid systems

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Definition of Reaction Rate

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

Speed of Chemical Reactions

To carry out chemical reactions discontinuously operated reactors or

continuously operated reactors can be used

• Discontinuously: more frequently applied to produce fine chemicals

• Continuously: more advantageous for the production of larger

amounts of bulk chemicals

To study the different behavior of these types of reactors another important criterion serves to distinguish two limiting cases: mixed flow and plug flow behavior

For theoretical studies and to compare the different reactors, four different

ideal reactors can be defined using the above classification:

Reactor Classifying

a) Batch Reactor (BR, perfectly mixed, discontinuous operation):

Features:

• All components are in the reactor before the reaction starts

• Composition changes with time

• Composition throughout the reactor is uniformAdv.:

• Simple, flexible, high conversion…

Disadv.:

• Dead times for charging, discharging, cleaning,…

• Difficult to control and automate

• …

BR are applied in particular for:

• Relatively slow reactions

• Slightly exothermic reactions

Areas of application for BR are:

• Reactions in pharmaceutical industry

• Polymerisation reactions

• Dye production

• Speciality chemicals

b) Semi-batch Reactor (SBR): perfectly

mixed, semi continuous operation

Features:

• One reactant is introduced first and then the second is dosed in a controlled

manner

• Composition changes with time

• Composition throughout the reactor is uniform

c) Continuously Stirred Tank Reactor (CSTR): perfectly mixed, continuous operation

A,B A,B,products

Features:

• Reactants are continuously introduced, products (+ unconverted reactants) are continuously withdrawn

• Composition does not change with time

• Composition throughout the reactor is uniform

Adv.:

• Controlled heat generation

• Easy to control and automate

• No dead times

• Constant product quality,

Disadv.:

• Complicated

• Can become unstable

• Large investmnent cost,

d) Plug Flow Tubular Reactor (PFTR): no mixing, continuous operation

•No dead times

•Better to cool (compare to stirred tanks)

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To be reviewed by

students

∆HFi is the enthalpy of formation of component i

∆HR < 0, the reaction is exothermic

∆HR > 0, the reaction is endothermic

It is simple to calculate the reaction enthalpy at a certain standard state

∆HR0 from the corresponding standard enthalpies of formation ∆HFi0 The standard enthalpies of formation are available from databases for P = P0

= 1 bar and T = T0 = 298 K

For pure elements like C, H2,O2, : ∆HFi0 = 0

The reaction enthalpy is a state variable Thus, a change depends only

on the Initial and the end state of the reaction and does not dependent

on the reaction parthway

Temperature and pressure dependence of reaction enthalpy

T

H dP

P

H H

d

P

R T

The pressure dependence is usually very small For ideal gas

behaviour, the reaction enthalpy does not depend on pressure

  T H c   T dT

K T

The correlation of reaction enthalpy and temperature is related to the isobaric heat capacities of all species involved in the considered reaction, cPi

Assuming that the reactants and the products have different but temperature independent heat capacities, the temperarue

dependence of the reaction enthalpy can be estimated as follows:

  R0  0  P, products P, reactants

R T H T T c c



For constant pressure and temperature, the change of free Gibbs

enthalpy of reaction can be described as follows:

i

N

i i P

or

Changing of free Gibbs enthalpy

for a chemical reaction

The equilibrium is reached when the free

Gibbs enthalpy of reaction is minimum

Thus, for the chemical equilibrium:

Or dGR=0 (or in an integrated form: ∆GR = 0)

Thus, the equilibrium is characterized by:

Reation free Gibbs enthalpy, ∆G R

 free Gibbs energy of formation

Relation between ∆GR and ∆HR

 

2

0 0

T

H dT

T G

d  R    R

For a small temperature range, ∆HR is constant, thus:

         2  1  

0 1

ln

T T

R

H T

K T

Equilibrium constant and temperature dependence

Van‘t Hoff equation describing the temperature dependence of the equilibrium constant:

Example 1.1 THE ROCKET

ENGINE

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

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Example 1.2 THE LIVING

PERSON

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