CHE594 Chemical Reaction EngineeringSHARMEELA MATALIRoom : PA-A11-8CPhone Number: 03-55436328/013-2990609

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INTRODUCTION TO CHEMICAL REACTION ENGINEERING

WHAT IS CHEMICAL REACTION ENGINEERING?

Chemical Reaction Engineering (CRE) is ..

the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place.Chemical reaction engineering is the heart of virtually every chemical process. It separates the chemical engineer from other engineers.

What is a chemical reaction?The change of a substance into a new substance which has new/different chemical identity i.e acid + metal a type of salt

2HCl + 2Mg 2MgCl + H2 2HCl + 2Na 2NaCl +H2 Mg(OH)2 + 2HCl MgCl2 + 2H2O

changes in physical effect i.e. emission of heat, formation of precipitation, colour change, etc.

In industrial chemical process, the typical situation of chemical process is shown below:

Generally, chemical reactions can be classified into homogeneous and heterogeneous reactions.1.Homogeneous reaction is a reaction that take place in one phase alone. i.e. reaction between 2 gases, 2 liquids, 2 solidsNOx formationNO (g) + O2 (g) NO2 (g)Ethylene ProductionC2H6 (g) C2H4 (g) + H2 (g)2.Heterogeneous reaction is a reaction that that requires the presence of at least 2 phases (or more) to proceed at certain reaction rate. i.e. reaction between gas and liquid, gas and solid, liquid and solidCoal combustionC (s) + O2 (g) CO2 (g)SO3(for sulphuric acid production) SO2 (g) + 1/2 O2 (g) SO3 (g) Vanadium catalyst (s)

Variables that affect rate of reaction:

In homogeneous systems the temperature, pressure, and composition are obvious variables.

In heterogeneous, the problem becomes more complex. Material may have to move from phase to phase during reaction; hence, the rate of heat and mass transfer can become important.E.g.: burning of coal briquette

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

A chemical species is said to have reacted when it has lost its chemical identity.

1. Decomposition2. Combination 3. Isomerization Example of reaction:

The reaction rate is the rate at which a species looses its chemical identity per unit volume.Some reactions can be very, very slow i.e. Sewage treatment plantsSome reactions can be very, very fast i.e. Reactions in rocket enginesThe rate of a reaction (mol/dm3/s) can be expressed as either the rate of Disappearance: -rA or as the rate of Formation (Generation): rA

Consider the isomerization ABrA = the rate of formation of species A per unit volume -rA = the rate of a disappearance of species A per unit volume rB = the rate of formation of species B per unit volume

EXAMPLE: AB

B is being formed at a rate of:rB = 0.2 mole/dm3/s

Then A is disappearing at the same rate:-rA= 0.2 mole/dm3/s

The rate of formation (generation of A) is rA= -0.2 mole/dm3/s

By definition, the reaction rate is the increase in molar concentration of a product of a reaction per unit time.

Consider this reaction

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)

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.

To evaluate:Reactor volume and volume in which reactiontakes place are identical. Thus,H2O produced = 108 kg/s X (1 kmol/18 kg) = 6 kmol/sThus, H2 used = 6 kmol/s and O2 used = 3 kmol/s

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 homogeneous systems 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 heterogeneous systems 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 rate of equation/ the rate law is an algebraic equation that depends on reacting materials and reaction conditions. It is independent of the type of reactor (batch or continuous).

k is rate constant which is temperature dependent

When there is no direct correspondence between stoichiometry and rate, 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:Non-elementary reactions: no direct correspondence between stoichiometry and rate

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

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

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

Non-elementary reaction always involve intermediate and multiple reactionsHowever, it is difficult to quantify the concentration of intermediate since it exists only for few minutes.

Types of intermediate can be grouped into free radicals, ions and polar substances, molecules, transition complexes, non-chain reactions and chain reactions

Testing of kinetic models

What is the possible reaction mechanism?

Temperature dependency on Arrhenius LawFor 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

It can be concluded that :

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).

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

k0 does not affect the temperature sensitivity.

Activation Energy and Temperature DependencyThe temperature dependency of reactions is determined by the activation energy and temperature level of the reaction, as illustrated in Table 2.1

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

Answer :Ea = 422 kJ/mol

WORK PROBLEMS: CHAPTER 1

1.1. Municipal waste water treatment plant. Consider a municipal water treatment plant for a small community (Fig. P1.1). Waste water, 32 000 m3/day, flows through the treatment plant with a mean residence time of 8 hr, air is bubbled through the tanks, and microbes in the tank attack and break down the organic materialA typical entering feed has a BOD (biological oxygen demand) of 200 mg O2/liter, while the effluent has a negligible BOD. Find the rate of reaction, or decrease in BOD in the treatment tanks.

1.3 Fluid catalytic crackers (FCC)FCC reactors are among the largest processing units used in the petroleum industry. Figure P1.3 shows an example of such units. A typical unit is 4-10 m ID and 10-20 m high and contains about 50 tons of porous catalyst ( = 800 kg/m3). It is fed about 38 000 barrels of crude oil per day (6000 m3/day at a density = 900 kg/m3), and it cracks these long chain hydrocarbons into shorter molecules. To get an idea of the rate of reaction; lets suppose that the feed consists of just C20 hydrocarbon:If 60% of the vaporized feed is cracked in the unit, what is the rate of reaction, expressed as r' (moles reacted/kg cat. s) and as r"' (moles reacted/m3 cat. s)?

THE END OF TOPIC 1