chapter 1 edited student version

8/2/2019 Chapter 1 EDITED Student Version

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Chemical Engineering

Department

CBB REACTION ENGINEERING

CHAPTER 1: MOLE BALANCES

(part 1)

1


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1. Define the rate of chemical reaction

2. Apply the mole balance equation to batch reactor,

CSTR, PFR and PBR

3. Distinguish the difference between different types

of reactor system

2

OBJECTIVES OF CHAPTER 1


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LECTURE LEARNING OBJECTIVES

1

At the end of Lecture 1, students should be able to:

1. define chemical identity2. define reaction rate

3. describe the different types of industrial reactors

4. describe the general mole balance equation

5. apply the mole balance equation for different

reactor types

3

Learning objectives


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Why do you need to study REACTIONENGINEERING??

4

OVERVIEW ON REACTION ENGINEERING


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5


PHYSICALTREATMENT

STEP

CHEMICALTREATMENT

STEP

PHYSICALTREATMENT

STEP

RAWM

ATERIALS

PRODUCT

S

RECYCLE

REACT

OR


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6


ReactorDesign

Thermo-dynamics

Chemical

kinetics

Fluid

mechanics

Heat

transfer

Mass

transfer

Economics


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REACTORINPUT OUTPUT

PERFORMANCE

EQUATION

Equation relating

input to output

KINETICS

How fast a reaction

occur i.e. reactionrate

CONTACTING

PATTERN

How materials flow

into reactor

Mixing pattern

RTD of reactor


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Petrochemical processes (BASF)

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Some examples on reaction processes


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Steam cracking unit

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Olefins process route (UOP)

10



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Ammonia production unit

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Aromatics production

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AN OVERVIEW

CHEMICAL REACTION ENGINEERING

MOLEBALANCE

RATELAWS

STOICHIOMETRY

ISO

THERMAL

REAC

TORDESIGN

CATALYTIC REACTION

NON-IDEAL REACTOR

HEAT EFFECTS

MULTIPLE REACTION SYSTEM

DATA ANALYSIS


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Chemical Identity

Determined by the kind, numberand configuration ofthe species atom

CC

H H

CH3 CH3

Cis-2-butene

CC

H

HCH3

CH3

Trans-2-butene

Considered as 2 different species due to the different configuration even when

the numbers of atoms of elements are the same


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Chemical Identity

REACTION

OCCURRED

Changed in

number of

atoms

Changed in

structureChanged in

atom

configuration

Decomposition

22233CHCHHCHCH

Isomerisation

Combination

NO2ON22

2 5 2

2 3 2

C H CH CH

CH C CH


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Reaction rate indication on how fasta number of

moles of one chemical species being consumed to form

another chemical species (of differentchemical identity)

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Reaction rate, -rA

CONVERSION RATE

CHEMICAL

SPECIES A

CHEMICAL

SPECIES B


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Reaction rate, -rA

Defined as the rate at which a chemical species

reacts (or formed) per unitvolume

Express as: Rate of reactant disappearance

Rate of product formation


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Example:

AB

Rate of reaction is given by:

-rA = rate of disappearance of A

rB= rate of formation of B

For heterogeneous reaction, rate of reaction is express in

terms of catalyst volume or catalyst weight

Reaction rate, -rA


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Rate equation is an algebraic equation

Rate equation is an intensive properties depends on

concentration, temperature, pressure, or type of catalyst,

if any, present in a system

Rate equation is not influence by type of reactor used!!

NOTE: dCA/dt is not the definition for

reaction rate

Reaction rate, -rA


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Example: Is NaOHreacting? CSTR - operated at steady

state; inlet flow rate = outlet

flow rate

Perfectly well mixed system;

concentration of samplestaken at 10 a.m is the same as

concentration taken at 5 p.m

Therefore: dCA/dt = 0

Does this mean that -rA = 0; i.e.

no reaction occurs?

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Reaction rate, -rA


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Example: Is NaOHreacting? CSTR - operated at steady

state; inlet flow rate = outlet

flow rate

Perfectly well mixed system;

concentration of samplestaken at 10 a.m is the same as

concentration taken at 5 p.m

Therefore: dCA/dt = 0

Does this mean that -rA = 0; i.e.

no reaction occurs?

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Reaction rate, -rA


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Consider the reaction

A + 2B C

in which the rate of disappearance of A is 5 moles of A per

dm3 per second at the start of the reaction.

At the start of the reaction

(a) What is the rate of disappearance of A?

(b) What is the rate of disappearance of B?

(c) What is the rate of formation of C?

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SELF TEST


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Types of Reactor:

1. Batch reactor

2. Continuous-Stirred Tank Reactor (CSTR)

3. Plug Flow Reactor (PFR) or Tubular Reactor

4. Packed Bed Reactor (PBR)


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Industrial reactors

Types of reaction

Liquid phase reaction Gas phase reaction

Batch / Semi batch reactor

CSTR

PFR

Tubular reactorPacked bed reactor


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Different types of reactor

1) Batch reactor

Physical shape: Tank

Used for: small scale operation

process that is not suitable for continuous operation.

Advantage: High conversion longer residence time

Disadvantage High cost Product variability

Not for large-scale operation


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Different types of reactor:

2) Continuous-Stirred Tank Reactor (CSTR)

Physical shape: Tank

Continuous Flow, Steady state, Perfectly mixed

Used for: Liquid phase reaction

process that is suitable for continuous operation.

Advantage:

Continuous operation Disadvantage

Not for non-ideal mixing


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Different types of reactor:

3) Plug Flow Reactor (PFR)

Physical shape: Cylindrical pipe


Used for: Gas phase reaction

Reaction rate varies axially NOT radially.

Reactant Product


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Different types of reactor

4) Packed Bed Reactor (PBR)

Physical shape: Cylindrical


Used for: Fluid-solid heterogeneous reaction (catalyst)

Reactant Product


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Photos of real reactor systems

Batch reactor


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CSTR



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PFR


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General Mole Balance Equation

For species A:Any REACTOR with volume V

Number of moles ofspecies A in a system

Rate of generation of speciesA in a system


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General Mole Balance Equation

0

0

A

A A A

V A

A A A

dNF F G

dt

dNF F r dV dt


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Mole Balance For Different Reactor Type

Batch reactor

dt

dNVr

A

A


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Continuous stirred

tank reactor (CSTR)


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Plug Flow Reactor(PFR)


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Packed Bed Reactor (PBR)-rA = mol A reacted

time. mass of catalyst


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SUMMARY

Reactor Mole Balance Remarks

Batch Well mix, nospatialvariation,unsteady state

CSTR Well mix, nospatialvariation,steady state

PFR Steady state

PBR Steady state

dt

dNVr

A

A


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Example 1-1: How large is it?

Consider the reaction: AB (first order reaction)

The reaction is carried out in a tubular reactor with constant

volumetric flow rate.1. Derive the equation to relate the reactor volume to the

entering and exiting concentration of A, the rate constant, k,

and the volumetric flow rate, v.

2. Determine the reactor volume required to reduce the exit

concentration to 10% of the entering concentration. Data:

volumetric flow rate = 10 dm3/min, k= 0.23 min-1

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EXAMPLE


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Reaction: A B

Data : Isothermal, liquid phase

Inlet molar flow rate = 5 mol/hrInlet volumetric flow rate: 10 dm3/hr

-rA = kCA2 with k= 3 dm3/mol.h

Calculate the volume of required to consume 99.9% ofreactant A for operation in a CSTR

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EXAMPLE


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Problem

The irreversible liquid phase second order reaction

is carried out in a CSTR. The entering concentration ofA, CA0, is 2 molar and the exit concentration of A, CA is

0.1 molar. The entering and exiting volumetric flow

rate, vo, is constant at 3 dm3/s. What is the

corresponding reactor volume?

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Formative assessment 1


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Reaction: A B+C

Data: a. Isothermal, liquid phase reaction

b. First order reaction, with k= 0.865 min-1

c. Reactor: Well mixed batch reactor of 20dm3 volume

d. 20 moles of pure A is placed in the reactor initially

Calculate the time necessary to reduce the number of moles of Ain the reactor to 0.2 mol.

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Formative assessment 2


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END OF LECTURE

chapter 1 edited student version

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