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Chapter 9. Unit Operations and

Pollution Prevention

((((單元操作與污染預防單元操作與污染預防單元操作與污染預防單元操作與污染預防))))

潔淨生產技術潔淨生產技術潔淨生產技術潔淨生產技術 陳奕宏老師陳奕宏老師陳奕宏老師陳奕宏老師

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Content

� 9.1 Introduction 3

� 9.2 Pollution Prevention in Material Selection for Unit

Operation 4

� 9.3 Pollution Prevention For Chemical Reaction 8

� 9.3.1 Material Use and Selection for Reactors 10

� 9.3.2 Reaction Type and Reactor Choice 13

� 9.3.3 Reactor Operation 21

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9.1 Introduction

� In considering pollution for unit operations in the design

of chemical processes, the following considerations are

important.

�Material Selection

�Waster Generation Mechanisms

�Operating Conditions

�Material Storage and Transfer

�Energy Consumption

�Process Safety

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9.2 Pollution Prevention in Material

Selection for Unit Operation

� One important element of designing and modifying

process units for pollution prevention︰ Choice of

materials

� Questions regarding material selection include ︰

a) What are the environmental, toxicological, and safety

properties of the material

b) How do those properties compare to alternative choices

c) What extent does the material contribute to waste

generation or emission release in the process

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d) Are there alternative choices that generate that generate

less waste or emit less while maintaining or enhancing

the overall yield of the desired product

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Example 9.2-1

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9.3 Pollution Prevention For Chemical

Reaction

� These reactor considerations will be classified as ：

�Material use and selection

�Reaction type and reactor choice

�Reactor operation

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Table 9.2-1 summary of material selection issues for

unit operations in chemical processes.

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9.3.1 Material Use and Selection for Reactors

� The use of materials in a chemical reactor︰

�The choice of feed entering thee reactor

�Catalyst

�Solvents

�Diluents

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9.3.2 Reaction Type and Reactor Choice

� A general rule, in designing chemical reactor for

pollution prevention, one would like a reaction with a

very high conversion of the reactants, high selectivity

toward the desired product, and low selectivity toward

any byproducts.

� Yield︰ the ratio of the exiting concentration of

product to inlet reactant.

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� Selectivity︰ the ratio of exiting product concentration

to the undesired byproduct concentration.

� Modified selectivity︰ the ratio of exiting product

concentration to the sum of product and byproduct

concentrations.

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Figure 9.3-1 effect of kp/kw on the reactor outlet

concentrations of products, reactants, and byproducts, for

a simple irreversible first-order parallel reaction

mechsnism.

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Figure 9.3-2 Effect of product and waste reaction

rate constants on product and waste concentrations in a

first-order irreversible series reaction.

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Figure 9.3-3 Effect of product and waste reaction

rate constants on product yield and product modified

selectivity for a first-order irreversible series reaction.

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Figure 9.3-4 Product and byproduct profiles for

reversible parallel and series reaction.

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Figure 9.3-5 Process flow diagram illustrating the

concept of recycle to extinction for reversible reactions.

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9.3.3 Reactor Operation

� Reaction temperature︰

Influence the degree of conversion of reactants to products, the

product yield, and product selectivity.

Irreversible first-order parallel reaction︰

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As the temperature is changed to a new value (T1) above or below a given initial temperature (T0).

Selectivity =

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Figure 9.3-6 shows the expected change in the ratio

of product/byproduct rate constants when temperature

is changed above and below T0.

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� Mixing︰︰︰︰

The competitive consecutive reaction carried out in a constant

stirred tank reactor (CSTR).

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The mixing intensity for any second-order

competitive-consecutive reaction︰

Rearranging for τ from the equation, and

incorporating the Kolomogoroff universal equilibrium

theory for turbulent motion

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→→→→

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Example 9.3-1

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Example 9.3-2

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� Effect of Reactant Concentration︰︰︰︰

For a parallel irreversible reaction

Selectivity =