chapter 1_introduction to chemical reaction

Download CHAPTER 1_introduction to chemical reaction

Post on 04-Sep-2015

219 views

Category:

Documents

3 download

Embed Size (px)

DESCRIPTION

chemical reaction

TRANSCRIPT

Slide 1

1

CHE244CHAPTER 1 INTRODUCTION TO CHEMICAL REACTION ENGINEERINGPrepared by:ARBANAH MUHAMMADarbanah87@gmail.comCOURSE LEARNING OUTCOMEClassification of reactionsDefinition of reaction rateElementary and non-elementary reactionsMolecularity and order of reactionTemperature dependent term of a rate equation Arrhenius Law231.1 CHEMICAL REACTION ENGINEERING (CRE)

Study on the chemical kinetics with the reactor in which the rxns occur.

Chemical reactors are the heart of the majority of industrial chemical plant.

Operation in the safest and most efficient manner is the key to the economic success or failure of a chemical plant.

Chemical kinetics; tells how long it will take to achieve a specified level of conversion and what products will be formed.

41.1.1 (a) Typical Chemical Process

PRODUCT/ BY PRODUCTSEPARATORREACTORSEPARATORRAW MATERIAL51.1.1 (b) Five Aspects Involved in Reactor Design

HeatTransfer Mass TransferFluidMechanicsThermodynamicsREACTANT(S)PRODUCTREACTORReactionKinetics61.1.2 EXAMPLE OF CRE APPLICATION

Figure 1.1 : Gas-Liquid CSTR (left); batch reactor (right)

7Figure 1.2 : Bench scale reactor (courtesy of Shell Corp.)

8Figure 1.3 : Shell Cat-Cracker (left); All-riser Cracking FCC Unit (right)

91.1.2 (a) Ethylene Production Ethylene is used for manufacturing polyethylene - the world's most widely used plastic NOVA Chemicals and Dow Chemical at Joffre : The highest capacity of any ethylene production site in the world. Largest single ethane based cracker in the world. Rxn :

C2H6 C2H4 + H2 High-temperature tubular reactors101.1.2 (b) SMOG Modeling Allows to estimate the extent of smog formation ...

111.1.2 (c) Catalytic Converter Reduce CO, NO gasses and unburned fuel.

121.1.2 (d) Large-scale Growth of Stem Cells The challenge is to grow large quantities of viable cell.

131.1.2 (e) Pharmacokinetics CRE can be applied to describe human body-drug interaction

141.1.2. (f) Fuel Cells System

Fuel Cells

151.1. 2 (g) Micro-channel Reactors Compact reactors for compact fuel cells Production of hazardous chemicals in controlled quantities Potential application in bio-chemical systems.

channel

channel

Microchannels on a wafer16

1.2 CLASSIFICATION OF REACTIONS1.2.1 Types of Reactionsa. Homogenous and Heterogeneous Homogeneous rxn which involve in one phase. Heterogeneous rxn which involve more than 1 phase and occurs at the interface between the phases.Example : CO2 absorption into alkali (gas-liquid); coal combustion and automobile exhaust purification (gas-solid); coal liquefaction and oil hydrogenation (gas-liquid-solid).b. Irreversible and Reversible Irreversible rxn which proceed in 1 direction and continues until the reactant are consumed. Example :

Reversible rxn which occurs in backward direction. (depends on concentration of rxtants and products relative to the corresponding equilibrium concentration) Example :

c. Endothermic and Exothermic Endothermic rxn that absorbs heat. Exothermic rxn that releases heat.

d. Molecularity Molecularity of rxn the no of atoms, ions or molecules involved (colliding) in a rxn step. Terms unimolecular, bimolecular and termolecular rxn involved, one, two or three atoms (or molecules) interacting

17

or colliding in any rxn step. The common example : Unimolecular : Radioactive decay

Bimolecular : Involving free radicals

Termolecular : Almost non-exist

e. Single and Multiple Single stoichiometric equation and single rate equation represent the progress for single rxns. Example :

18

More than one kinetic expression to follow the changing composition of all the rxn components to represent the progress for single rxns. Multiple rxns may be classified : i. Series of rxns

ii. Parallel rxns

and

iii. Complicated scheme : rxn proceeds in parallel with respect to B, but in series with respect to A, R and S.

competitiveside by side19

1.2.2 Chemical Identity Chemical species: chemical component or element with a given identity. A chemical species is to be reacted when it has lost its chemical identity. The identity of a chemical species is determined by the kind, number, and configuration of that species atoms. Three ways a chemical species can lose its chemical identity : a. Decomposition (e.g. ) b. Combination (e.g. ) c. Isomerization e.g.

20

1.3 DEFINITION OF REACTION1.3.1 Rate of Reaction Example of simple reaction :

Rate of rxn (rA) the no. of moles of A (N) reacting/ disappearing per unit time per unit volume (V) (mol/dm3s).

-rA can describe about how fast a number of moles of one chemical species rxn to form another species.

+ if j is a product if j is a reactantwhere C = concentration (N/V = C at const. vol.)21

For heterogenenous rxn, (-rA) = the no. of moles of A reacting per unit time per unit mass of catalyst (mol/sg catalyst).

.

22

1.3.2 Rate Law Rate equation i.e. rate law ; function of the properties of the rxtant and rxn conditions (concentration, T or catalyst). However, it is independent to the type of reactor in which the rxn is carried out. Commonly, the rate law is influenced by the composition (concentration) and the energy of material for homogeneous rxn. The definition for energy of material is : i. Temperature (random kinetic energy of molecules) or ii. Light intensity within the system (effect the bond energy btw atoms) or iii. Magnetic field intensity. 23

However, for the heterogeneous rxn, rate law is depend to heat and mass transfers (surface area of catalyst). Function of rate law :

where k = specific reaction rate/ reaction rate constant Example 1 :

The rate law is :

Example 2 :

The rate law is :

concentrationTemperature24

1.3.3 Order of Reaction The order of a rxn refers to the powers to which the concentrations are raised in the kinetic law.

Example :

The rate law :

= order with respect to rxtant A = order with respect to rxtant B n = ( + ) order of rxn

In general :

25

Example :

Thus, the overall order is 2 + 3 = 5 Rate constants, dimension of k for the nth order :

Exercise : Unit of k = s-1(mol/dm3)1-n Zero-order (n = 0) = ________ 1st order (n = 1) = ________ 2nd order (n = 2) = _________

Reaction is 2nd order with respect to species AReaction is 3rdorder with respectto species B

26

1.3.4 Relative Rate of Reaction If the rate is known with respect to one species, the coefficients of the balanced chemical equation can be used to find the rates with respect to the other species. For rxn :

Note : Taken A species as basis of calculation.

Every mole of A consumed, c/a moles of C produced or formed:

27

Thus, rate of formation of C = c/a x (rate of disappearance of A) rc = c/a (rA) and so on

Consider the combustion of propane :

Compared to the rate with respect to propane :Rate with respect to oxygen is five times fasterRate with respect to carbon dioxide is three times fasterRate with respect to water is four times faster

28

Exercise : The liquid phase rxn :

i. Write down the relative of rxn rate. ii. State the rate law for the above rxn. iii. What is the rxn rate of A and B if the rate of formation of C = 4 mol/dm3s.

29

1.4 Elementary & Non-Elementary Reaction

The exponents in the rate law are generally unrelated to the chemical equations coefficients.

Note : Never simply assume the exponents and coefficients are the same!

The exponents must be determined from the results of experiments.

30Elementary ProcessIn general, the rxns mechanism is the series of simple rxns called elementary processes.

For any rxn shows equal rxn orders of its rxtants with the stoichiometric coefficients in its stochiometric equation

Thus, the rate law of an elementary process can be written based on its stoichiometric equation.

Examples : Stoichometric :

Rate Law :

31

Non-elementary ProcessThere is no direct correspondence between stoichiometry and rate stoichiometry does not match its kinetics.

Examples : Stoichometric :

Rate Law :

This nonmatch show a multisteps rxn model have to be developed to explains the kinetics.

32

In order to explain the non-elementary rxn, assume the sequence of elementary rxn is occurred but we can not measure or observe the intermediates.

Thus, we need to observe only the initial rxtants and final products appear in a single rxn formed they only present in very minute quantities.

33

1.4 TEMPERATURE DEPENDENT TERM OF A RATE EQUATION ARRHENIUSS LAW1.4.1 Parameters Affect For many rxns, the rate expression can be written as a product of a temperature-de