Chapter 1

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REACTION ENGINEERINGCKB 20104

CHAPTER 1MOLE BALANCEDr. Kelly Yong Tau Len

Section of Chemical Engineering Technology UniKL MICETTel: 06-5512051, Email: [email protected],

E-learning: CKB20104 – Kelly Yong

Chapter 11.1 Definition of reaction rate1.2 The general mole balance1.3 Types of reactor in industry

ObjectivesUpon the completion of this chapter, students are able to:� Define the rate of chemical reaction� Apply mole balance equation in reactors systems� Identify commercial/industries reactor system

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Introduction3

When has a chemical reaction taken place?

When a chemical species lost its chemical identity and a new compound forms

Chemical Identity

Kind

Configuration

Number

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IntroductionWhat are the ways a species may lose its

chemical identities?

Combination

Decomposition

Isomerization

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Product

Reactant

Reaction

1.1 Definition of Reaction Rate

1. After reaction, reactant will disappear (reacted) while product will appear (formed).

2. The rate at which this occur is called RATE OF REACTION (or simply known as REACTION RATE) and it can be expressed in 2 ways either as:

The rate of DISAPPEARANCE of a reactantor

The rate of APPEARANCE of a product


❀ The rate of DISAPPEARANCE of reactant A, –rA is the number of moles of A reacting (disappearing) per unit time per unit volume (mol/volume.time).

❀ The rate of FORMATION of product B, +rB is the number of moles of B formed (appearing) per unit time per unit volume (mol/volume.time).

❀However, we usually use rate of DISAPPEARANCE of reactant A, –rA to evaluate rate of reaction.

❀Both are functions of concentration, temperature, pressure and types of catalyst (if any).

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A B

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Test Your Understanding 1

Give one example of unit for rate of

DISAPPEARANCE and rate of FORMATION

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A B


Rate of reaction can be evaluated in other ways too. +r means rate of formation formation and –r means rate of

disappearance.

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A B

A–rARate of DISAPPEARANCE of

reactant A

–rA value will be a positive (+) number because A disappear

E.g. –rA = 20 mol/dm3.s

+rARate of FORMATION of

reactant A

+rA value will be a negative (–) number because A disappear E.g. +rA = –20 mol/dm3.s

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Rate of reaction can be evaluated in other ways too. +r means rate of formation formation and –r means rate of

disappearance.

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A B

–rBRate of DISAPPEARANCE of

product B

–rB value will be a negative (–) number because B is formed E.g. –rB = –20 mol/dm3.s

+rBRate of FORMATION of

product B

+rB value will be a positive (+) number because B is formed E.g. +rB = 20 mol/dm3.s

B

Consider the reaction: A + 2B à C + 2D Given the rate of disappearance of A is 15 mol/L.min at the start of the reaction. At the start of the reaction:

1. Determine –rA2. Determine the rate of formation of B 3. Determine the rate of formation of C 4. Determine the rate of disappearance of C 5. Determine the rate of formation of A 6. Determine –rB

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Example 1

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1. Determine –rASolution: –rA is the rate of disappearance of A

−𝑟A=15 mol𝐿.𝑚𝑖𝑛

2. Determine the rate of formation of B Solution: For every 1 mol of A that disappears, 2 moles of B also disappear. Reactant B is being used up therefore its rate of formation is a negative number.

+𝑟B=−30 mol𝐿. 𝑚𝑖𝑛

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Example 1A + 2B à C + 2D

3. Determine the rate of formation of C Solution: C is a product that is being formed as fast as A is disappearing. Because C is being formed, its rate of formation is positive.

+𝑟C=15 mol𝐿. 𝑚𝑖𝑛

4. Determine the rate of disappearance of C Solution: The rate of disappearance of C is –rC. Because C is a product, its rate of disappearance, -rC, is a negative number

−𝑟C=−15 mol𝐿 .𝑚𝑖𝑛

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5. Determine the rate of formation of A Solution: A is a reactant that is being used up. Therefore, its rate of formation is a negative number

+𝑟A=−15 mol𝐿.𝑚𝑖𝑛

6. Determine –rBSolution: –rB is the rate of disappearance of B

−𝑟B=30 mol𝐿.𝑚𝑖𝑛

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A + 2B à C + 2D Given the rate of disappearance of A is 15 mol/L.min at the start of the reaction. At the start of the reaction:1. Determine the rate of formation of D2. Determine the rate of disappearance of D

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1.2 The General Mole Balance15

A AA

FA0

GA

FARate of flow of A into the system

(moles/time)INFA0

Rate of generationof A by chemical reaction within

system(moles/time)GENERATION

Rate of flow of A out of the system(moles/time)

OUTFA

+ – =Rate of

accumulationof A within system(moles/time)

ACCUMULATION

𝒅𝐍𝐀𝒅𝒕

NA represents the no. of moles of species A in the system at time t

Rate of generation, GA is a product of variation in the rate of reaction, rAthroughout the reactor

volume, V

𝐆𝐀 = : 𝒓𝑨𝑽

1.2 The General Mole Balance16

A AA

FA0

GA

FARate of flow of A into the system

(moles/time)INFA0

Rate of generationof A by chemical reaction within

system(moles/time)GENERATION

Rate of flow of A out of the system(moles/time)

OUTFA

+ – =Rate of

accumulationof A within system(moles/time)

ACCUMULATION

𝒅𝐍𝐀𝒅𝒕𝐆𝐀 = : 𝒓𝑨

𝑽

𝐅𝐀𝟎 + : 𝒓𝑨− 𝐅𝐀𝑽

=𝒅𝐍𝐀𝒅𝒕

Basic Equation for Chemical Reaction

Engineering

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1.3 Types of Reactor in IndustryBatch Reactors17

• Used for small scale operation.• For testing new process. • Manufacturing of expensive products.• Processes that are difficult to convert to continuous operations.

Advantages:• High conversions obtained by leaving reactant in the reactor for long periods of time.

Disadvantages:• High labor costs per batch.• Variability of product from batch to batch.• Difficulty large scale production.

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Source: http://www.youtube.com/watch?v=ftnLJ6VDwS8

1.3 Types of Reactor in IndustryBatch Reactors

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Key Characteristics• Unsteady-state operation because there is NO flow through the system.

• No spatial variation of concentration and temperature (well-mixed).

• Mainly used for small scale operation.

• Suitable for slow reactions• Has no inflow and no outflowof reactants or products while reaction is carried out.

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1.3 Types of Reactor in IndustryBatch Reactors

1.3 Types of Reactor in IndustryContinuous Flow Reactors20

Continuous Flow

Reactors

Continuous Stirred Tank Reactor (CSTR)

Plug Flow Reactor (PFR)

Packed Bed

Reactor (PBR)

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• Commonly used in industrial processing.• Also referred to as backmix reactor.• Used primarily for liquid phase reaction.

Source: http://www.youtube.com/watchv=sSjn7doP550&list=PL4bHyGq-6VBol0o9LMUC7Z2EntbS-4Iv9

1.3 Types of Reactor in IndustryContinuous Stirred Tank Reactor (CSTR)

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Key Characteristics• Steady state operation (flow through the system), hence no accumulation.

• Can be used in series configurations.

• No spatial variation of concentration or temperature (well-mixed).

• Mainly used for liquid phase reaction.

• Suitable for viscous liquid.• Reactants are continuously introduced into the reactor while products are continuously removed.

1.3 Types of Reactor in IndustryContinuous Stirred Tank Reactor (CSTR)

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1.3 Types of Reactor in IndustryPlug Flow Reactor (PFR)

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• Commonly used in industrial processing.• Consists of a cylindrical pipe• Used more often for gas phase reaction

Source: http://www.youtube.com/watch?v=I241zL4vJ7A

Key Characteristics• Steady-state operation (flow through the system), hence no accumulation• Consists of a cylindrical pipe which reactants and products can flow through

• Spatial variation in axial direction but not in radial direction• Suitable for fast reaction mainly used for gas phase reaction• Temperature control may be difficult• There are no moving parts

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1.3 Types of Reactor in IndustryPlug Flow Reactor (PFR)

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1.3 Types of Reactor in IndustryPacked Bed Reactor (PBR)

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Key Characteristics• Similar to PFR. Can be thought of as PFR packed with solid particles, which are almost always catalysts.

• Steady-state operation.• Spatial variation.• Mainly used for gas phase catalytic reaction although examples for liquid phase reaction are also known.

• Temperature control may be difficult.• There are no moving parts.• Pressure drop across the packed bed is an important consideration.

IN OUT


Determine which reactor you will recommend for this reaction.

ProductReactant

Reaction

Chapter 1

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