chapter 02 - structure and synthesis of pfd

Design Project Procedure• Preliminary or quick estimate design

- based on approximate process methods- rough costs estimate

• Detailed estimate design- detailed analysis and calculations on cost as well as profitability analysis- complete design of equipment (chemical and mechanical design) is performed

CHAPTER 2

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The Structure and Synthesis of the Process Flow Diagram

Objectives of this section

• To learn the hierarchy of chemical process design

• To familiarize with the structure of continuous chemical processes

• To know the differences between batch and continuous processes

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Hierarchy of chemical process design• The important decision is the choice of chemical

syntheses or routes for production of desired product.

• The identification of alternatives process chemistries is being done at the very beginning of conceptual design.

• During evaluation of alternatives, cost of raw materials, value of by-products, complexity of synthesis, environmental impact of waste materials and pollutant produced are being taken into considerations.

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Douglas has proposed a hierarchical approach to conceptual design• Decide whether the process will be batch or

continuous• Identify and define the input/output structure of the

process• Identify and define the recycle structure of the

process• Identify and design the general structure of the

separation system (Chapter 12)• Identify and design the HEN or process energy

recovery system (Chapter 15)5

Step 1:Batch vs. Continuous

• Most chemical plants operates at continuous mode. Only unloading and loading of materials are batch wise.

• Batch operation – a finite quantity of product is made during a period of few hours or days.

• Continuous – feed is sent continuously to a series of equipment where product, by products and waste streams leave the process continuously

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Factors to decide

• The most important is size and flexibility. • Size– Batch < 500 tonne/yr ~ 1.5 tonne/day

(< 2 m3 of liquid or solid per day)– Continuous > 5000 tonne/yr

• Other factors- Table 2.1 textbook

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Copyright - R. Turton and J. Shaeiwitz, 2012 8

FACTOR ADV/DADV BATCH ADV/DADV CONTINUOUS

SIZE Small production capacity Large scale production

PRODUCT QUALITY For pharmaceutical and food products, where the manufacturing practices are strictly monitored by FDA

Potentially producing large quantities of off-spec products. But it can be recycled.

OPERATIONAL FLEXIBILITY The same equipment can be used for multiple operations

Often leads to inefficient use of capital

STANDARDIZED EQUIPMENT – multiple products

Can be easily modified to produce several different products

The product is fixed

PROCESSING EFFICIENCY High usage of utility due to no energy integration. Separation and reuse of materials are more difficult than continuous.

Energy integration, separation and reuse of materials are possible. The process becomes more efficient with high production capacity

MAINTENANCE AND OPERATING LABOUR Higher operating cost labour due to equipment cleaning and preparation time

Lower operating cost labour

FEEDSTOCK AVAILABILITY Batch operation is favoured when the feedstock is limited

Higher amount of feedstock is required


FACTOR ADV/DADV BATCH ADV/DADV CONTINUOUS

PRODUCT DEMAND Seasonal demand for products such as fertilizers etc. could be produced. Other product produce during the off-season

One plant for one product.

RATE OF RXN TO PRODUCE PRODUCTS For slow reaction rates and long residence time such as fermentation, biological rxn, aerobic/anaerobic wastewater treatment

Slow rxn requires very large equipment (reactor). So favour the high rxn rate

EQUIPMENT FOULING Cleaning of fouling is easy Fouling is a serious problem and difficult to handle

SAFETY Worker exposure to chemicals and operator error will be higher

The plant has excellent safety records and safety procedures are well established

CONTROLLABILITY The efficient scheduling of equipment becomes very important and the control is complicated

Easy to control

STEP 2: THE INPUT/OUTPUT STRUCTURE OF THE PROCESS

• Input represents feed stream and output represents products streams or waste streams

• To determine the process route through construction of process concept diagram.

• Uses the stoichiometry of the main reaction pathways to identify feed and product chemicals.

• Fig. 1.5 (toluene hydrodealkylation process)

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Input – Output Structure(Process Concept Diagram)

From Analysis, Synthesis, and Design of Chemical Processes, Fourth Edition, by Richard Turton, Richard C. Bailie, Wallace B. Whiting, Joseph Shaeiwitz, and Debangsu Bhattacharyya (ISBN-13: 978-0-13-261812-0) Copyright © 2012 Pearson Education, Inc. All rights reserved. 12

The input/output structure of the PFD

• The same overall input/output structure between Fig. 2.1 and 2.2

• The reactants streams, Stream 1 and 2• Benzene is clearly labelled in Stream 15 but no

identification of methane. Check with Table 1.5 on stream 16 – contain considerable amount of methane.

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Generic Structure of Process Flow Diagrams


C6H5CH3+H2 C6H6 + CH4

Input-Output on PFD


Input-Output – Utility Streams


• The generic block diagram – intermediate between the process concept diagram and PFD

• It contains logical building blocks for all processes

• Fig. 2.4(a) provides generic block diagram that shows chemical process broken into six basic areas;

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Generic Block Structure of Process Flow Diagrams

Other considerations for the input/output structure of the process flow sheet

• Feed purity and trace components • Addition of feeds required to stabilize

products or enable separations• Inert feed material to control exothermic

reactions• Addition of inert feed material to control

equilibrium reaction

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Information that can be obtained from input/output diagram of the process

• Basic economic analysis on profit margin – could decide on the pathway that is feasible

• What chemical components must enter with the feed and leave as products

• All the reactions, both desired and undesired that take place

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Profit Margin• If $ Products - $ Raw Material < 0,

then do not bother to pursue this process, but start looking for an alternate route

Toluene HDA vs. Toluene Disproportionation

C6H5CH3 + H2 C6H6 + CH4

Toluene benzene

2C6H5CH3 C6H6 +C6H4(CH3)2

Toluene benzene xylene


Toluene used more efficiently

STEP 3: THE RECYCLE STRUCTURE OF THE PROCESS

• Since raw materials make up from 25 to 75% of total operating costs, a plant should recover as much raw material as possible

• Furthermore, when the price is expensive • The unused reactants must be separated and

recycled • The extent of recycling depends on ease of

operation

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Efficiency of raw material usage

• Single pass conversion• Overall conversion• Yield


3 Basic Recycle Structures• Separate and purify unreacted feed from products

and then recycle, e.g., toluene• Recycle feed and products together and use a purge

stream, e.g., hydrogen with purge as fuel gas• Recycle feed and products together but do not use a

purge stream - must come to Equilibrium2C6H6 C12H10 + H2



Recycle Structure in PFD

Recycle without separation or purge


Recycle increases and equip. and op. costs increase

2C6H6 C12H10 + H2

Recycle with Separation (and Purge)


Extra tower with associated operating costs

2C6H6 C12H10 + H2

Other Issues on Recycle

• Number of recycle streams• Does excess reactant affect structure– Size of Recycle Loop

H2 : Toluene = 5 : 1• Number of Reactors– Separate and recycle to different reactors


Other Issues on Recycle (cont.)

• Do we need to purify prior to recycling?• Is recycling of inerts warranted?• Can recycling an unwanted inert material push

equilibrium to the right?– Gasification of coal – CO2 recycle


Other Issues on Recycle (cont.)

• Can recycling an unwanted inert control reaction– CO2 in Gasifier

• Phase of Recycle Stream?


chapter 02 - structure and synthesis of pfd

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