The Structure and Synthesis

of Process Flow Diagrams

Chapter 2 Part 1

Introduction

Objectives:

– Study the steps of evolution of any process.

– Provide a framework to generate alternative

PFDs for a given process.

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Introduction

• The choice of which chemical synthesis or routes should

be investigated to produce a desired product is an

important decision in the evolution of a process. In

considering alternative synthesis routes the following

should be considered:

• Cost of raw material.

• Value of by—products.

• Complexity of the synthesis.

• Environmental impacts (waste water & pollutants).

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Introduction

• Steps for design process:

– Batch vs. continuous process.

– Input – output structure.

– Recycle structure.

– General structure of the separation process.

– Heat exchanger network or process energy

recovery system.

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Batch VS. Continuous Processes

(Step 1)

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Batch VS. Continuous Processes

(Step 1)

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The Input – Output Structure of the Process

(Step 2)

• Process Concept Diagram:

– A single “cloud” is drawn to represent the concept of

the process. Within this cloud the stoichiometry for all

reactions that take place in the process is written.

– The reactant chemicals are drawn as streams

entering from the left.

– Product chemicals are drawn as streams leaving to

the right.

– Include all unwanted side reactions and unwanted

products.

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Step 2: Process Concept Diagram:

Example:

C6H5CH3 + H2 → C6H6 + CH4

Hydrogen

Toluene

Methane

Benzene

Figure 2.1 Input – Output Structure of the Process Concept Diagram for

the Toluene Hydro-dealkylation Process.

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Step 2: The input – Output Structure of the

Process Flow Diagram (PFD)

• The PFD shows the reactants or inert chemicals entering

from the left and shows the Products and the inert

chemicals leaving to the right

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Step 2: The input – Output Structure of the

Process Flow Diagram (PFD)

• There are other auxiliary streams shown on the PFD, such as utility

streams, but they are not part of the basic input – output structure.

• Utility streams are treated differently from process stream. They

usually provide or remove thermal energy or work

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Step 2: Generic Block Flow Process Diagram

• The “Generic Block Flow Diagram” is intermediate between the process concept diagram and the PFD. This diagram illustrates features, in addition to the basic input – output structure, which are common to all chemical processes.

• The generic block flow diagram breaks down the chemical process to six basic areas or blocks.

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Step 2: Generic Block Flow Process Diagram

• Reactor feed preparation: change the condition of the feed streams as required in the reactor.

• Reactor: all chemical reactions take place in this block.

• Separation feed preparation: alter reactor output stream to the condition required for effective separation.

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Step 2: Generic Block Flow Process Diagram

• Separator: separate products, by – products, unused reactants, and waste streams via a wide variety of physical processes such as distillation, absorption, extraction, and adsorption.

• Recycle: the return of un–reacted feed chemicals to the reactor for further reaction. Normally, this block contains a pump or compressor and perhaps a heat exchanger.

• Environmental control: to reduce significantly the waste emissions from a process and to render all non–product streams harmless to the environment.

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Step 2: Generic Block Flow Process Diagram

• The “Generic Block Flow Diagram” can contain more than six blocks where some blocks are repeated according to necessity.

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Step 2: Other Consideration

• Feed purity and trace components:

– If impurities are not presented in large quantities and these impurities do not react to form by-products do not separate.

– If the separation of the impurities is difficult do not separate.

– If the impurities foul or poison the catalyst purify the feed.

– If the impurity reacts to form difficult-to-separate or hazardous products purify the feed.

– If impurity in large quantity purify the feed, but if it is difficult –to-separate and the impurity acts as an inert separation may not be warranted.

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Step 2: Other Consideration

• Addition of feeds required to stabilize

products or enable separation:

Example: absorption of a water soluble chemical

from a gas stream, then this solvent is an

additional feed to the process.

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Step 2: Other Consideration

• Inert feed material to control exothermic reactions:

Example: the partial oxidation of propylene togive acrylic acid. The feeds consist ofnearly pure propylene, air, and steam. Allreactions take place are highly exothermic,not limited by equilibrium and potentiallyexplosive. To reduce the potential forexplosion, steam is fed to dilute the feedand provide thermal ballast to absorb heatof reaction and make control easier.

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Step 2: Other Consideration

• Addition of inert material to control equilibrium reactions:

Example: production of styrene via the catalytic dehydrogenation of ethyl benzene. The ethyl benzene is co-fed to the reactor with superheated steam, which provide heat for the reaction and dilutes the feed. As the ratio of steam to ethyl benzene increases, the equilibrium shifts favoring the products (LeChatelier’s principle).

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