Episode 55 : Conceptual Process Synthesis-Design

SAJJAD KHUDHUR ABBASCeo , Founder & Head of SHacademyChemical Engineering , Al-Muthanna University, IraqOil & Gas Safety and Health Professional – OSHACADEMYTrainer of Trainers (TOT) - Canadian Center of Human Development

Episode 55 : Conceptual Process Synthesis-Design

Conceptual Process Synthesis-Design

Process Flowsheet???Raw Materials Products

Process Flowsheet Synthesis: Method to determine a process flowsheet that satisfies all product, operational and other

requirements

Product Oriented Process Synthesis

Chemical Product

Process

Raw Material

What are chemical products?

What are the raw materials?

What is a chemical process?

Chemical Product-Process Relationship

Chemical Process ??

Raw Material Chemical Product

Chemical process synthesis-design is about finding a sustainable process that can convert the raw materials to the

desired chemical products

Chemical Process Design & Green EngineeringChemical Process

?? Chemical Product

Raw Material

Chemical process design is about finding a sustainable process that can convert the raw materials to the desired

chemical products

Sustainable: Economic, low environmental impact, low waste, efficient operation, correct raw material, …..

Example of a Chemical Process & Product

Oxychlori- nation Separa-

tion block 1

PyrolysisSepara- tion block 2

HCl (addition) Water HCl (product)

The production of VCM is usually done from ethylene. In this case the process is carried out in two steps, first reaction with either chlorine or hydrogen chloride in order to produce ethylene dichloride (EDC) and next pyrolysis (cracking) to form VCM. The compounds in the system are Ethylene, EDC, VCM, HCl, O2, Cl2 and H2O. The reactions involved is the process are:

Direct chlorination: C2H4 + Cl2 C2H4Cl2

Oxychlorination: C2H4+2HCl+½O2 C2H4Cl2 +H2O

Pyrolysis (cracking): C2H4Cl2 C2H3Cl + HCl

The two step process also requires two separation blocks. In the first step (formation of EDC) two different reactions are considered (Direct chlorination and oxychlorination). Oxychlorination produces water in

EDC

VCM(product)

HClOxygen

Ethylene

Chlorine

Recycle

addition to EDC and it aClsoomrepquutierresAaidreedcyPcrloe.cess Engineering - Lecture 7 (R. Gani)

Direct chlorination

Purge

Process design starts with defining the needs of the product that will be manufactured by the process!

The Product Tree

It is important to choose the right product and the corresponding raw material from which the product can be made. This also defines the path (process route)

A Scenario for Chemical Process Design1. Board of Directors’ Design Problem

2. Discovery of possible new projects

3. Feedback & customer reaction

4. Planning & organizational design

5. Preliminary process design

6. Layout & three dimensional modelling

7. Construction

8. Startup & commissioning

9. Plant Operation

10. Debottelnecking

11. Decommissioning

Stages in the life of a process !

Computer Aide

d Process Engineering - Le 9

Roles of different groups of people in the different stages of the life of a process - I

BU definition of1 obejctive

PE conceptual2 process

design

Chemical Research

rating mpany

Process C EngineeringBusiness Unit

CR preliminary1 study

PE preliminary1 study

BU decision on2 process

information about novel compound

proposal for process

improvement

idea for new compound/

market strategy

state of the market

objective idea of production

amount

schedule

description of the process PFD and pre-P&ID mass and energy balances datasheets for major units site and installation plan

onstruction Company

Ope Co

estimated costs estimated schedule patents description of synthesis

path first process description analysis methods for

product, product quality, etc.

first safety considerations

cture 7 (R. Gani

)

Roles of different groups of people in the different stages of the life of a process - II

Chemical Research

Operating Company

Construction Company

Process EngineeringBusiness Unit

PE 3

detailed process design

CC engineering1 design and

construction

CC 2

commissioning

hand-over

OC2

operation and

maintenance

OC1

commissioning, hand-over

OC decom-3 missioning

P&ID data sheets and

equipment drawings mass and energy

balances information about

process at steady state comments on startup

and shutdown plant plant documentation

P&ID data sheets equipment

drawings piping

specification installation plan building plan plant modell

BU decision on4 project

continuation

description of the process PFD and pre-P&ID mass and energy balances datasheets for major units site and installation plan

The Synthesis StepChemical Process

?? Chemical Product

Raw Material

Concept Generation

Generation of Alternatives

Analysis & Evaluation of Alternatives

Final Selection

The Synthesis Step

Chemical Process

?? Chemical Product

Raw Material

Problem specification

Approaches for design

Design alternatives

Performance

Cost, safety, …

Concept generation

Alternative generation

Analysis

Evaluation

Comparison & optimization

Abstract Description

Refined Description

Basic Steps in Flowsheet Synthesis

* Gathering Information* Representing Alternatives* Criteria for Assessing Preliminary Designs

Economic evaluation Environmental concerns Safety analysis Flexibility & controllability

* Generating & searching among alternatives

Generating & searching among alternatives: Methods

* Total enumeration* Evolutionary methods* Mathematical programming* Hybrid

Establish targets for the design

Generate (feasible) alternatives that will match the targets

Order all the feasible alternatives and select the most appropriate


* Total enumeration* Evolutionary methods* Mathematical programming* Hybrid

Concepts:

Superstructure & Alternatives

C[1]

C[2]

1

2

3

H[1]

T[in] T[out]

C[3]


* Total enumeration* Evolutionary methods* Mathematical programming

* HybridA, E

Concepts:

Superstructure & Alternatives

A, B, C, D, E, F

A / B, C, D, E, F

B, F

A, E, F / C, D, BReactor

CD A, F / F C / D, B

EF

Decomposition Strategies for Process Synthesis

• Bounding Strategies for Process Synthesis• Hierarchical Decomposition for Process

Synthesis

P1 = Cp Fp – Cr Fr = Original space = Profit bound

P2 = P1 – Constraints = Profit bound

P3 = P2 – Cop

P4 = P3 – CopP4 = P3 – Cop

Decomposition Strategies for Process Synthesis• Bounding Strategies for Process Synthesis• Hierarchical Decomposition for Process Synthesis

Level 1. Input Informationi problem definition Batch versus Continuous (1)

Level 2. Input-Output Structurei material selection i reaction pathways

Input-output structure of flowsheet (2)

Recycle structure of flowsheet (3)

Separation system synthesis (4)

Levels 3 & 4.i recycle i separation system

Levels 5 - 8.i energy integration i detailed evaluation

i control i safety

Heat/mass recovery network (5)

Design problem decomposed into a hierarchy of decisions

Hierarchical Decomposition for Process SynthesisLevel 1: Decisions on Batch versus Continuous

Consider,• Technical Information

• Does any apparatus work in batch mode? Is process sensitive to upsets & variations?

• Production Rate• High or low production rate? Only few days

production needed? Few days operational notice!• Product Lifetime

• One or two years or longer?• Value of Product

• Product value >> manufacturing cost?

Note: Several batch units can be combined to give a continuous production!

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Hierarchical Decomposition for Process Synthesis

Level 2: Decisions on Input-Output Structure

Consider,• Raw materials – what to do with impurities ?• How many product streams?• Recycle streams? Purge streams?• Reversible by-products? Recycle or recover?• Selectivity versus cost

Three Types of Decisions (Assumptions)

1. Decision that fix parts of the flowsheet

2. Decisions that fix some of the design variables

3. Decisions that fix connections to the environment



Raw Materials - Impurities?• If the impurities are inert, remove

them after reaction, if they are valuable

•If the impurities are inert, present in large amounts, and, can be easily separated, remove them before the reactor

•If the impurities have boiling points lower than reactants and products, and, they are also inert, recycle

them (note: purge unit will be needed!)•If the impurities are also products from the reactor,

place the feed stream before the unit that will remove theimpurities



Product streams?

Observe the following Rules• Recycle unreacted reactants

• Recycle intermediate reactants (products)• Recycle/remove azeotropes with reactants*

• Remove (recover) the main product• Remove (recover) the valuable by-products

• Remove (as waste)/recycle by-products that are not valuable



Recycle? Purge Units? Consider the following:• For < 100% conversion of reactants and reaction in

the gas phase,

Recycle of gases will be necessary

If impurities are present, purge units will be necessary

• For < 100% conversion and reaction in the liquid phase,

Recycle of liquids (reactants) will be necessary

If impurities are present, purge units will be necessary



Reversible by-products? Selectivity versus cost?

EP2 = Product value – Byproduct value – Raw material value

Function of conversion, selectivity, reaction T & P, ….


Level 3: Decisions on Recycle Structure & Reactor

Consider,• Number of reactors?• Number of recycle streams?• Need for compressor/pump?• Reactor type? Adiabatic or Isothermal?• Reaction equilibrium or kinetics?• Reactor cost (capital & operating)?

Hierarchical Decomposition for Process SynthesisLevel 3: Decisions on Recycle Structure & Reactor

• Number of reactors?• If more than one reaction is needed to get the desired product,

more than one reactor will be needed if the conditions are very different

• Number of recycle streams?• Depends on the number of raw materials and conversion of all

reactants• Need for compressor/pump?

• Compressor for gas recycle; pump for liquid recycle• Reactor type? Adiabatic or Isothermal?

• If temperature change is too high or low, heating or cooling will be needed

• Reaction equilibrium or kinetics?• Kinetically controlled or equilibrium reached?

• Reactor cost (capital & operating)?EP3 = EP2 – Compressor/pump cost – Reactor cost


Level 4: Decisions on Separation System

Vapour recovery and/or Liquid recovery? - Rules• If reactor stream is in liquid phase, use liquid recovery• If reactor stream has 2 phases (separate the 2 phases first)

• Use vapour recovery for vapour phase

• Use liquid recovery for liquid phase

• Reactor stream is gas (vapour) phase• Perform phase split (if possible) by reducing temperature (ie.,

condenser) and then separate the 2-phase. Otherwise, use vapour recovery

How to locate & perform the separation?

EP4 = EP3 – Vapour recovery cost – Liquid recovery cost


Level 4: Decisions on Separation System

Vapour Recovery System (VS) - Location• If vapour stream contains significant amount of valuable material

(reactants, impurities, product), place VS after purge unit• If vapour stream contains components that may slow down the

reaction or destroy or affect the catalyst, place VS on the recycle stream

• If both the above criteria are satisfied, place VS after a “phase split” unit

• If none of the above are satisfied, VS is most likely not necessary!

How to perform the separation?Condensation (high pressure and/or low temperature) Absorption (needs solvent and solvent recovery – try water!) Adsorption (needs adsorbent and regeneration of adsorbent) Membrane separation (suitable membrane, low flux, etc.)

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Hierarchical Decomposition for Process SynthesisLevel 4: Decisions on Separation System

Liquid Recovery System (LS) - Decisions on …• Which separations can be made with distillation?• Sequence of distillation columns• Removal of light ends (send to VS if valuable, otherwise, waste disposal)• Other types of separations possible?

Rule for selecting separation by distillation or phase split (V-L)• If > 1.1, between two key components, use distillation provided the key components do

not form azaotrope• If >>> 1.1, try phase split (condensation or vaporization)

Rules for sequencing of distillation columns• Recover lightest component first• Recover most plentiful component first• Make the most difficult separation last• Always try equimolar separations

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Method for Separation Process SynthesisSeparation Process Identification

Step 1: Mixture Analysis (number of components in mixture, mixture type, mixture state, temperature, pressure, number of binary pairs, azeotrope pairs, etc.)

Step 2: Generate binary ratio matrixR = rij = piA / piB for property I, binary pair j

compounds A & B in pair jStep 3: Identify separation technique

If, rmin < rij < rmax for separation technique k, select thistechnique for separation of the compounds in the binary pair j

Step 4: If more than one separation technique is feasible for binary pair j, select the separation technique with the largest rij value

Principle for separation is a driving force created by a difference in property; different separation techniques employ different properties!

Case Study: Maleic Anhydride (MA) Production

Level 1. Input / Output Information

Alternative 1Benzene Process

V2O5-MoO3

Alternative 2n-Butane Process

VPO

2C6 H 6 9O2 2C4 H 2O3 4CO2 4H 2O

C4 H 2 O3 O2 4CO H 2

O

2C6 H 6 9O2 12CO 6H 2

O

C4 H 2O3 3O2 4CO2 H 2O

2C6 H 6 15O2 12CO2 6H 2

O

2C4 H10 7O2 2C4 H 2O3 8H 2O

C4 H 2O3 O2 4CO H 2O

C4 H 2O3 3O2 4CO2 H 2

O

2C4 H10 9O2 8CO 10H 2O2C4 H10 13O2 8CO2 10H 2O

Benzene conversion, 95%MA Yield, 70% Air/Benzene, ~ 66 (moles) Temperature, 375°C Pressure, 150 kPa

n-butane conversion, 85%MA Yield, 60%Air/n-butane, ~ 62 (moles) Temperature, 400°C Pressure, 150 kPa

MA Production: IO AssumptionsLevel 1. Input / Output Information“Tier 1” Environmental Impact Analysis

CO2, H2O, air,traces of CO, MA

??

ReactorBenzene orn-butane Product

Recovery

Pollution Control

99% control

99% MA recovery

Design DecisionsUnreacted

Benzene orn-butane

CO, CO2 , H2O, air, MA

Air MA, CO,CO , H O2 2air

MA 50x10lb/yr

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Production rate

Initial Flowsheet for MA from n-C4Reactors

Air

n-Butane

AbsorberDistillation column

Compressor

MA

Vaporizer

Off-gas

Off-gas

Pump

Solvent

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Episode 55 : Conceptual Process Synthesis-Design

Engineering

Transcript of Episode 55 : Conceptual Process Synthesis-Design