Chapter 1 introduction

PowerPoint Präsentation Mai Thanh Phong HCMUT FCE – HCMC University of Technology Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) Mai Thanh Phong, Ph D VIETNAM NATIONAL UNIVERS[.]

Chemical Reaction Engineering

(Homogeneous Reactions in Ideal Reactors)

Mai Thanh Phong, Ph.D.

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

FACULTY OF CHEMICAL ENGINEERING

1 Octave Levenspiel, “Chemical Reaction Engineering”, John Wiley&Sons, 2002.

2 H Scot Foggler, “Elements of Chemical Reaction Engineering”,International

students edition, 1989.

3 E.B.Nauman, “Chemical Reactor Design”, John Wiley & sons, 1987.

4 Stanley M Walas, “Reaction Kinetics for Chemical Engineers”,Int Student

Edition, 1990.

5 Coulson & Richardsons, “Chemical Engineering – Vol 6”,Elsevier, 1979.

6 Richard M Felder, “Elementary Principles of Chemical Processes”, John Wiley &

sons, 2000.

Chapter 1 Introduction

• Topic of the lecture „Chemical Reaction Engineering“ is the quantitative

assessment of chemical reactions The selection of suitable reactor types and their design will be discussed

• Reactor design uses information, knowledge, and experience from a variety of areas: thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics Chemical reaction engineering is the synthesis of all these factors with the aim of properly designing a chemical reactor

• Thermodynamics tell us in which direction a reaction system will develop and

how far it is from its equilibrium state

• Analyses of kinetics provide information about the rate with which the system will approach equilibrium

Chapter 1 Introduction

Description of the amount of a substance i:

i

i

i M

m

n 

Number of moles:

Molar concentration:

M i = molecular weight

V

n

Mole fraction:





j j

i i

n n x

Chapter 1 Introduction

Progress of chemical reactions:

Conversion:

0

0

i

i

i i

n

n

n

If V = const:

0

0

i

i

i

c

c

Extent of reaction:

i

i

i n

n



   0 Performance criteria:

Productivity:

time operating

P product of

amount

produced



P

n

Chapter 1 Introduction

II Stoichiometry of chemical reactions:

Stoichiometry is based on mass conservation and thus quantifies general laws

that must be fulfilled during each chemical reaction

Starting point of a quantitative analysis is the following formulation of a chemical

reaction:

0 1







N

i i i

A



This equation describes the change of the number of moles of N components

A1, A2, AN The νi are the stoichiometric coefficients of component i They have

to be chosen in such a way that the moles of all elements involved in the chemical reaction remain constant

A convention is that reactants have negative stoichiometric coefficients and products have positive stoichiometric coefficients

Chapter 1 Introduction

To calculate changes in the mole number of a component i due to reaction, the

following balance has to be respected:



i

i

i n

From this equation results the important stoichiometric balance:

k

k k

k k

i

i i

i

n





















0

Using the conversion X of a component k, the above equation becomes:

k

k k

i i









As an example the stoichiometric equation for the oxidation of carbon monoxide

is given by:

2CO + O2 → 2CO2 with νCO = -2, vO2 = -1, vCO2 = 2

Chapter 1 Introduction

III Chemical thermodynamics:

Chemical thermodynamics deal with equilibrium states of reaction system This

Section will concentrate on the following two essential areas:

a) The calculation of enthalpy changes connected with chemical reactions, and

b) The calculation of equilibrium compositions of reacting systems

3.1 Enthalpy of reaction

The change of enthalpy caused by a reaction is called reaction enthalpy ∆HR

This quantity can be calculated according to the following equation:









 N

i i Fi

H

1



∆HFi is the enthalpy of formation of component i

∆HR < 0, the reaction is exothermic

∆HR > 0, the reaction is endothermic

Chapter 1 Introduction

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

3.2 Temperature and pressure dependence of reaction enthalpy

T

H dP

P

H H

d

P

R T

R









































The pressure dependence is usually very small For ideal gas behaviour, the

reaction enthalpy does not depend on pressure

Chapter 1 Introduction

  T H c   T dT

K T

Pi

N

i i

R

R     



298 1

0 

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



Chapter 1 Introduction





i i





1

3.3 Chemical equilibrium

• Chemical reactions approach to an equilibrium, when the product and reactant

concentrations do not change anymore

• A reacting system is in chemical equilibrium if the reaction rates of the forward

and backward reactions are equal

• The basic quantity required to indentify the equilibrium state is the Gibbs free

enthalpy of reaction GR

• The change of this quantity becomes zero when the equilibrium is reached (i.e

dGR = 0)

For constant pressure and temperature, the change of free Gibbs enthalpy of

reaction can be described as follows:

i

N

i i P

T

R

d









1 ,

or

Chapter 1 Introduction

0

,











P T

R

d

dG



0

1





 i

N

i i





0

,















P T R

G

    , 0



P T R

G



0

,















P T R

G





Fig 1-1: Changing of free Gibbs enthalpy

for a chemical reaction

In Figure 1-1 is shown the course of free

Gibbs enthalpy of reaction as a function

of the extent of 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:

Chapter 1 Introduction

3.3.1 Reation free Gibbs enthalpy, ∆G R

 



i i Fi

G

1

0

0

Fi

G

 free Gibbs energy of formation

Relation between ∆GR and ∆HR

2

0 0

T

H dT

T G

Chapter 1 Introduction

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

0 1

ln

T T

R

H T

K T

3.3.2 Equilibrium constant and temperature dependence

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

constant:

 

2

0

ln

RT

H dT

K

GR0   ln









  



RT

G

K exp R0

Relationship between the free Gibss enthalpy and the equilibrium constant:

Chapter 1 Introduction

4 Reaction rate

Based on unit volume of reacting fluid:

dt

dn V

r i

R i

1

1



 • V• niR is mole number of component i is volume of the reaction mixture

• t is reaction time

If VR is constant:

dt

dc

r i

i



1

 • ci is molar concentration of component i

Based on unit mass of solid in solid-liquid systems:

dt

dn W

r i

i

1

1



 • W is mass of solid

Based on unit solid surface of solid-liquid or solid-gas systems:

dt

dn S

r i

i

1

1



 • S is solid surface area

Chapter 1 Introduction

5 Standard Reactors

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:

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 uniform

Chapter 1 Introduction

Adv.:

• 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

Chapter 1 Introduction

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

Adv.:

• Controlled reaction rate and heat generation

•

Disadv.:

• Same as BR

• More complicated than BR

• …

Chapter 1 Introduction

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,

Chapter 1 Introduction

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

products

tubular reactor

Features:

• Composition varies from point to point along a flow path

Adv.:

•High conversion

•Easy to automate

•No dead times

•Better to cool (compare to stirred tanks)

•…

Disadv.:

•Complicated

•Danger of “hot spot”

•…