chapter 2b - steps in designing chemical processes

8/18/2019 Chapter 2b - Steps in Designing Chemical Processes

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EP426Chemical Process Design and Optimization

Chapter 2 - Synthesis of process flow diagram


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Student attainment

CLO2: Propose a chemical process flow diagram usingthe hierarchy of process design.C5 – Identify component concept and use component skillto solve a problem.

A3 – Proposed a plan for improvement.

PLO3 -Design/Development of Solutions.Design solutions for complex engineering problems and design systems,components or processes that meet specified needs with appropriateconsideration for public health and safety, cultural, societal, andenvironmental considerations

Note:

Teaching method - Lecture & Group Project

Assessment - Test, Final Exam and report presentation.


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Chapter 2

Synthesis of process flow

diagram (PFD)

Rule of thumb forprocess synthesis

Steps in Designing

ChemicalProcesses

Examples &Group Exercise


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EP426Chemical Process Design and Optimization

Chapter 2b – Synthesis of process flow diagram (PFD).Steps in Designing Chemical Processes


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Schedule The Design Process

• Primitive Design Problems

• Example

• Steps in Designing and Retrofitting Chemical Processes

• Assess Primitive Problem

• Process Creation

• Development of Base Case• Detailed Process Synthesis - Algorithmic Methods

• Process Controllability Assessment

• Detailed Design, Sizing, Cost Estimation, Optimization

• Construction, Start-up and Operation

• Environmental Protection

• Safety Considerations

Ref: Seider, Seader and Lewin (1999), Chapter 1

Section A

Section B

Section C

Initial

Final Design


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Steps in Process Design and Retrofit

Assess Primitive

Problem

Development ofBase-case

Detailed Process

Synthesis -Algorithmic

Methods

Plant-wide

ControllabilityAssessment

Detailed Design,Equipment sizing, Cap.

Cost Estimation,

Profitability Analysis,

Optimization

SECTION A


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Steps in Process Design and Retrofit

Process Creation• Preliminary Database Creation - to assemble data to

support the design.

• Experiments - often necessary to supply missingdatabase items or verify crucial data.

•

Preliminary Process Synthesiso top-down approach.

o to generate a “synthesis tree” of design alternatives.

o illustrated by the synthesis of processes for themanufacture. Ie. VCM.

Development of Base-case Design• focusing on the most promising alternative(s) from

the synthesis tree.

Ref: Seider, Seader and Lewin (1999), Chapter 2


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Process Creation


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Thermophysical property data


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Chemical Prices: http://www.icis.com/chemicals/channel-info-chemicals-a-z/


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Preliminary Process Synthesis

1. Selection of processing mode: continuous orbatch

2. Fixing the chemical state of raw materials,products, and by-products, noting the

differences between them.3. Process operations (unit operations) - flowsheet

building blocks


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1. Continuous or batch processing?

Continuous

Batch

Fed-batch

Batch-product removal


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2. The Chemical State

Decide on the raw material and productspecifications (states):

• Mass (flow rate)

• Composition (mole or mass fraction of each chemicalspecies having a unique molecular type)

• Phase (solid, liquid, or gas)

• Form (e.g., particle-size distribution and particle shape)

• Temperature• Pressure


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Temperature, pressureand phase change

Separation

Mixing

Chemical reaction

3. Synthesis Steps

Process Operation

Eliminate differences in molecular types

Distribute chemicals by matching sourcesand sinks

Eliminate differences in composition

Eliminate differences in temperature,pressure and phase

Integrate tasks (combine tasks into unit

operations)


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Process Operations (“Lego”)

• Chemical reaction• Positioning in the flowsheet involves many considerations

(conversion, rates, etc.), related to T and P at which thereaction are carried out.

• Separation of chemicals• needed to resolve difference between the desiredcomposition of a product stream and that of its source.

Selection of the appropriate method depends on thedifferences of the physical properties of the chemical speciesinvolved.

• Phase separation

• Change of temperature

• Change of pressure

• Change of phase

• Mixing and splitting of streams and branches


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Development ofBase-case Design


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Example:

Vinyl Chloride Manufacture

Process Creation and

Development of Base-case Design

Cl H C 32


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Reaction Path Overall Reaction

1 C2H4 + Cl2 = C2H3Cl + HCl

2 C2H2 + HCl = C2H3Cl

3 C2H4 +Cl2 = C2H3Cl + HCl

4 C2H4 + HCl + O2 = C2H3Cl + H2O

5 2C2H4 + Cl2 + O2 = 2C2H3Cl + H2O

Possible Reactions to produce Vinyl Chloride

Cl H C 32


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1. Eliminate differences in molecular types

ChemicalMolecular

weight

Chemical

formula

Chemical

structure

Acetylene 26.04 C2H2 H - C C - H

Chlorine 70.91 Cl2 Cl-Cl

1,2-Dichloroethane

98.96

C2H4Cl2

Cl Cl| |

H-C-C-H

| |

H H

Ethylene

28.05

C2H4

H H

C = C

H H

Hydrogen chloride 36.46 HCl H-Cl

Vinyl chloride

62.50

C2H3Cl

H Cl

C = C

H H

Chemicals participating in VC Manufacture:


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1. Direct chlorination of ethylene:

Selection of pathway to VCM ()

Advantages:– Attractive solution to the specific problem denoted as

Alternative 2 in analysis of primitive problem.– Occurs spontaneously at a few hundred oC.

Disadvantages:– Does not give a high yield of VC without simultaneously

producing large amounts of by-products such as

dichloroethylene– Half of the expensive chlorine is consumed to produce HCl by-

product, which may not be sold easily.

HClClHCClHC32242

(2.1)


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2. Hydrochlorination of acetylene:


Advantages:– This exothermic reaction is a potential solution for the specific

problem denoted as Alternative 3. It provides a goodconversion (98%) of C2H2 VC in the presence of HgCl2 catalystimpregnated in activated carbon at atmospheric pressure.

– These are fairly moderate reaction conditions, and hence, this

reaction deserves further study.

Disadvantages:– Flammability limits of C2H2 (2.5100%)

Cl H C HCl H C 3222

(2.2)


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3. Thermal cracking of C2H4Cl2 from chlorination of C2H4:


Advantages:

– Conversion of ethylene to 1,2-dichloroethane in exothermic

reaction (2.3) is 98% at 90 oC and 1 atm with a Friedel-Craftscatalyst such as FeCl3. This intermediate is converted to vinylchloride by thermal cracking according to the endothermicreaction (2.4), which occurs spontaneously at 500 oC with

conversions as high as 65% (Alternative 2).

Disadvantage:– Half of the expensive chlorine is consumed to produce HCl

by-product, which may not be sold easily.

242242ClHCClHC

HClClHCClHC32242

HClClHCClHC32242

(2.3)

(2.4)

(2.1)


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4. Thermal Cracking of C2H4Cl2 from Oxychlorination of C2H4:


Advantages:– Highly exothermic reaction (2.5) achieves a 95% conversion to

C2H4Cl2 in the presence of CuCl2 catalyst, followed by pyrolysis

step (2.4) as Reaction Path 3.

– Excellent candidate when cost of HCl is low

– Solution for specific problem denoted as Alternative 3.

Disadvantages:– Economics dependent on cost of HCl

(2.5)

(2.4)

(2.6)

OHClHCOHCl2HC224222

1

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HClClHCClHC32242

OHClHCOHCl HC23222

1

42


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5. Balanced Process for Chlorination of Ethylene:


Advantages:– Combination of Reaction Paths 3 and 4 - addresses Alternative 2.

– All Cl2 converted to VC

– No by-products!

(2.5)

(2.3)

(2.7)

OHClHCOHCl2HC224222

1

42

HCl2ClHC2ClHC232242

(2.4)

242242ClHCClHC

OHClHC2OClHC2 232221

242


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Evaluation of Alternative Pathways

Chemical Bulk Prices

•

Reaction Path is eliminated due its low selectivity.• This leaves four alternative paths, to be compared first in

terms of Gross Profit.

ChemicalChemical

formula Cost (cent/lb)

Acetylene C2H2 50

Chlorine Cl2 11

Ethylene C2H4 18

Hydrogen chloride HCl 18

Vinyl chloride C2H3Cl 22

Water H2O 0

Oxygen (air) O2 0


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Raw Materials

Process Flowsheet?

C2H4, Cl2

Products

C2H3Cl, HCl

Cl2113,400 lb/hr

C2H444,900 lb/hr

Direct

ChlorinationPyrolysis

C2H4Cl2

HCl58,300 lb/hr

C2H3Cl100,000 lb/hr

HCl

C2H3Cl

C2H4Cl2

C2H4Cl2 C2H3Cl + HClC2H4 + Cl2 C2H4Cl2

Preliminary Flowsheet for Path

• 800 MM lb/year @ 330 days/y 100,000 lb/hr VC

•

On the basis of this principalsink

, the HClsink

and reagentsources

can be computed (each flow is 1,600 lbmol/h)

Next step involves distributing the chemicals by matchingsources and sinks .


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Reactor pressure and temperature levels:• Chlorination reaction: 1.5 atm is recommended,

to eliminate the possibility of an air leak into thereactor containing ethylene and 90oC for highest

yield.• Pyrolysis reaction: 26 atm at 500oC are

recommended by the B.F. Goodrich patent(1963) without any justification. Since the

reaction is irreversible, the elevated pressuredoes not adversely affect the conversion.

2. Distribute the chemicals


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2. Distribute the chemicals• A conversion of 100% of the C

2

H4

is assumed in the

chlorination reaction.


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2. Distribute the chemicals• Only 60% of the C2H4Cl2 is converted to C2H3Cl with a byproduct of HCl,

according to Eqn. (2.4).

• To satisfy the overall material balance, 158,300 lb/h of C2H4Cl must

produce 100,000 lb/h of C2H3Cl and 58,300 lb/h of HCl.

• But a 60% conversion only produces 60,000 lb/h of VC.

• The additional C2H4Cl2 needed is computed by mass balance to equal:

[(1 - 0.6)/0.6] x 158,300 or 105,500 lb/h.

• Its source is a recycle stream from the separation of C2H3Cl from

unreacted C2H4Cl2, from a mixing operation, inserted to combine the two

sources, to give a total 263,800 lb/h.


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2. Distribute the chemicals• The effluent stream from the pyrolysis operation is the source for the

C2H3Cl product, the HCl by-product, and the C2H4Cl2 recycle.


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• The product of the chlorination reaction is nearly pure C2H4Cl2, and

requires no purification.• In contrast, the pyrolysis reactor conversion is only 60%, and one or

more separation operations are required to match the required puritiesin the C2H3Cl and HCl sinks.

• One possible arrangement is given in the next slide. The data below

explains the design decisions made.

3. Eliminate Differences in Composition

Boiling point (oC) Critical constants

Chemical 1 atm 4.8 atm 12 atm 26 atm T c,C Pc, atm

HCl -84.8 -51.7 -26.2 0 51.4 82.1C2H3Cl -13.8 33.1 70.5 110 159 56

C2H4Cl2 83.7 146 193 242 250 50


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3. Eliminate Differences in CompositionBoiling point (oC) Critical constants

Chemical 1 atm 4.8 atm 12 atm 26 atm T c,C Pc, atm

HCl -84.8 -51.7 -26.2 0 51.4 82.1

C2H3Cl -13.8 33.1 70.5 110 159 56

C2H4Cl2 83.7 146 193 242 250 50

There may be other, possibly better alternative configurations, as discussed

in (Chapter 3).


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4. Eliminate differences in T, P and phase


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5. Integrate tasks (tasks unit operations)


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Assembly of synthesis tree

Reaction

path

Distribution of

chemicals

Separations T, P and

phasechanges

Task

integration

Algorithmic methods are very effective for the synthesis, analysis and

optimization of alternative flowsheets. These will be covered in

Section B (Part II)


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Development of Base-case DesignDevelop one or two of the more promising flowsheets from the synthesis

tree for more detailed consideration.


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End

Next: Chapter 3 - Separation train synthesis

chapter 2b - steps in designing chemical processes

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