chapter 2: part ii process creation & synthesis · process creation & synthesis . learning...

Intro to PD - 1

CHAPTER 2: Part II Process Creation & Synthesis

Learning Outcomes

1. Understand how to go about assembling design data and creating/determining a preliminary data base.

2. To appreciate the principle of process synthesis; the steps in creating flowsheets involving reactions, separations, and T-P change operations.

3. Create synthesis tree for promising flowsheets.

4. Base-case design for promising alternatives.

5. Know how to select the principal pieces of equipment and to create a detailed process flowsheet, with a material and energy balance and a list of major equipment items.

On completing this part of the course, you should:

Preliminary Database Creation

• Thermophysical property data – physical properties

– phase equilibria (VLE data)

– Rate of reactions

• Environmental and safety data – toxicity data

– flammability data

– MSDS/CSDS-chemical safety data sheet

• Chemical Prices – Chemical Marketing Reporter

– Also Utilities Prices

• Experiments – to check on crucial items above

From lab or established database e.g. Knovel website, NIST chemical webpage, patents & etc.

http://www.nist.gov/public_affairs/chemical.htm

http://www.knovel.com/web/portal/home

Example of database information

1. Physical properties of all components 2. VLE data 3. The reactions and reaction conditions 4. Info about the rate of reaction & rate of catalyst deactivation 5. The desired product purity or some info about price vs purity 6. Information about safety, toxicity and environmental impact 7. Any processing constraints 8. Other plant and site data 9. Cost data for by-products, equipment and utilities 10. etc. 11.

Preliminary Process Synthesis (Seider, Seader and Lewin, 1999): Text Book

Process Synthesis involves:

Selection of processing mode: continuous or batch

Fixing the chemical state of raw materials, products, and by-products, noting the differences between them.

Process operations (unit operations) - flowsheet building blocks

Synthesis steps

Eliminate differences in molecular types (chemical reaction)

Distribute chemicals by matching sources and sinks (mixing & recycle)

Eliminate differences in composition (separation)

Eliminate differences in temperature, pressure and phase

Integrate tasks (combine tasks into unit operations)





Synthesis steps






Preliminary Process Synthesis

Continuous or batch processing?

Continuous

Batch

Fed-batch

Batch-product removal

Batch vs Continuous

• Continuous Process:

Dominant in chemical industry (esp commodity chemicals)

High production rate (World scale)

More efficient

• Batch and Semi-Batch Processes:

– Small production rate (quantity)

– High-priced chemical

e.g. pharmaceutical products,

foods, electronic materials

– Preferred if Product demands are irregular/intermittent

– Preferred for Hazardous and toxic chemical

– Reliable but inefficient

Batch vs Continuous

Batch vs Continuous

Production Rates Plant capacity < 1.0 x 106 kg/yr higher uncertainty, but simpler process more flexible, multi product, variable quality (e.g. paint)

Market Forces (Some) branded/trademark/proprietary products “Seasonal products” (e.g. fertilizer in cold climates) Short product lifetime, e.g. one or two year (e.g. pigments) High value, low production cost

Batch Process:

Operational Problems Slow kinetics (Large residence time) Severe fouling problems - frequent maintenance Process sensitive to upsets and variations





Synthesis steps







The Chemical State • Decide on the raw material and product specifications (states):

Mass (flow rate, capacity; esp important for commodity chemicals)

Composition (mole or mass fraction of each chemical species having a unique molecular type)

Phase (solid, liquid, or gas)

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

Temperature

Pressure

Others such as viscosity, color, odor, average MW etc.





Synthesis steps







QUESTIONS:

Discuss with person next to you on the various unit operation that both of you encountered during your industrial training.

Process Operations

Basic

operations

Chemical Reaction

Separation of

chemical mixtures

Phase separation

Change of

temperature Change of pressure

Change of phase

Mixing and splitting of

streams or batches

Operation on solids (size

reduction & enlargement)

Reactor

Separation & Recycle System

Heat Exchanger Network

Utilities

The “Onion” Model (Smith and Linnhoff, 1995)





Synthesis steps:







Synthesis Steps

Synthesis Step

Eliminate differences in molecular types

Distribute chemicals by matching sources and sinks

Eliminate differences in composition



Process Operation

Chemical reaction

Mixing/Recycle

Separation

Temperature, pressure and phase change

Combinations of all steps above (step 1-4)





Synthesis steps:








In other words, compare advantages and disadvantages of all type of reactions possible…

Level 1- Input Information - Reaction System

Reaction information:

1. The stoichiometry of all reactions that takes place. 2. The range of temperatures and pressures for the reactions 3. The phases of the reaction systems. 4. Information of conversion, selectivity and yield. 5. Information about product distribution versus conversion (& molar ratio of reactants, possibly reactor T and/or pressure). 6. Some info about conversion vs residence time/space velocity. 7. Some info about catalyst nature (homogeneous, slurry, packed bed, powder, etc.) method for regeneration (e.g. solvent wash).

CONVERSION = {Reactant* Consumed in the Reactor}

{Reactant* Fed to the Reactor}

*Limiting Reactant

• Main descriptor of a chemical reactor’s performance • For multiple rxns, Selectivity? Yield? • Need to decide what conversion the reaction system should have early during process synthesis • Affects the economics (capital + operating costs) of an entire process

Reactor Performance Indicators (RECALL BACK!!!!)

Reactor Temperature (RECALL BACK!!!!)

REACTION CAN BE

Endothermic ΔH = +ve

Exothermic ΔH = -ve

For example, if T r V

• Operating Temperature is Limited by :

• Safety

• Materials of Construction

• Process Constraints - e.g. decomposition, polymerization

• Catalyst Life

Single Reaction (RECALL BACK!!!!)

Feed1 + Feed2 Product

IRREVERSIBLE

An excess of one feed component would force another feed component to complete conversion.

Which feed in excess ? Cost Safety

REVERSIBLE

Feed1 + Feed2 Product + Byproduct

The maximum conversion is limited by the equilibrium conversion

Equilibrium conversion can be increased by

Adding access of one feed Remove byproduct

Computer-Aided Process Simulation Project Intro to PD - 25

Series Reactions (RECALL BACK!!!!)

WHAT IF SIDE REACTION IS REVERSIBLE? Q .

TO MAXIMIZE SELECTIVITY,

WE MUST INHIBIT THE BYPRODUCT FORMATION

By using excess of Feed2 will force Feed1 to complete conversion and inhibit the side reaction

Or, removing the product as reaction proceed will force the secondary reaction to LHS and inhibit the Byproduct formation

Feed1 + Feed2 Product

Feed1 + Product Byproduct ...............(2)

...............(1)

Example 1:

Production of Vinyl Chloride (VC)

Process Creation

About Vinyl Chloride:

• Extremely toxic chemicals

• Monomer for production of PVC (plastic pipe, fitting etc.)

• A commodity chemical

• Not a new product

• Established technology to manufacture

• Many option available for reaction pathways (routes)

Simple/Primitive Problem:

Your company is planning to expand PVC production in the existing petrochemical complex and therefore will require a steady supply of VC monomers (800 MM Ib/yr).

• Alternatives resulting from assessment of primitive problem

Alt 1: Buy from competitor VC plant. They will do expansion to cater for our needs and ship by train/truck. Design team need to project the purchase price and designs storage facilities.

Alt 2: Purchase and ship of chlorine using pipeline from nearby NaCl electrolysis plant. Cl2 will react with in-house ethylene. HCL is by-product.

Alt 3: Use HCl (byproducts of many processes in the complex). React with acetylene, or ethylene and O2 to produce 1,2-dichoroethane that could be cracked to produce VC

Alt 4: Electrolyze the HCl to produce H2 and Cl2. React chlorine according to Alt 2.

Computer-Aided Process Simulation Project Intro to PD - 31

Direct chlorination of ethylene:

Selection of pathway to VCM (1)

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.

HClClHCClHC 32242 --------- (3.1)

Hydrochlorination of acetylene:

Advantages: ü Exothermic reaction is a potential solution for the specific problem

denoted as Alternative 3.

ü Good conversion (98%) of C2H2 to VC in the presence of HgCl2

catalyst impregnated in activated carbon at atmospheric pressure.

ü Moderate reaction conditions, and hence, this reaction deserves

further study. Disadvantages:

– Flammability limits of C2H2 (2.5 to 100%)

ClHC HCl HC 3222 ------------- (3.2)


Thermal cracking of C2H4Cl2 from chlorination of Ethylene:

Advantages:

ü Conversion of ethylene to 1,2-dichloroethane in exothermic reaction (2.3) is

about 98% at 90 oC and 1 atm with a Friedel-Crafts catalyst such as FeCl3.

ü This intermediate is converted to vinyl chloride (Alt 2) by thermal cracking

according to the endothermic reaction (2.4), which occurs spontaneously at

500 oC with conversions as high as 65%.

Disadvantage:

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

which may not be sold easily.

242242 ClHC Cl HC

HCl ClHC ClHC 32242

HCl ClHC Cl HC 32242


------------------(3.3)

-------------(3.4)

-------------(3.1)


Thermal Cracking of C2H4Cl2 from Oxychlorination of ethylene:

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

C2H4Cl2 in the presence of CuCl2 catalyst, followed by pyrolysis

step (2.4) similar to Reaction Path 3.

ü Excellent candidate when cost of HCl is low

ü Solution for specific problem denoted as Alternative 3.

Disadvantages: – Economics dependent on cost of HCl

OH ClHC O HCl2 HC 22422 21

42

HCl ClHC ClHC 32242

OH ClHC O HCl HC 2322 21

42

-------------(3.5)

-------------(3.4)

-------------(3.6)


Balanced Process for Chlorination of Ethylene:

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

– All HCl produced in pyrolysis is consumed in oxychlorination rxn

– All Cl2 converted to VC

– No by-products!

OH ClHC O HCl2 HC 22422 21

42

HCl2 ClHC2 ClHC2 32242

242242 ClHC Cl HC

OH ClHC2 O Cl HC2 2 32221

242

-------------(3.3)

-------------(3.5)

-------------(3.4)

-------------(3.7)

Chemicals participating in VC Manufacture:


Evaluation of Alternative Pathways

Reaction Path is eliminated due its low selectivity and undesirable byproduct HCl.

This leaves four alternative paths, to be compared first in terms of Gross Profit or Economic Potential.

Gross Profit: The profit excluding equipment and operating costs.

Evaluation of Alternative Pathways

Chemical Cost (cents/lb)

Ethylene 18

Acetylene 50

Chlorine 11

Vinyl chloride 22

Hydrogen chloride 18

Water 0

Oxygen (air) 0

Chemical Bulk Prices per 1lb raw material:

Price from Chemical Marketing Reporter,

Computing Gross Profit

So, Gross profit = 22(1) + 18(0.583) - 18(0.449) - 11(1.134) = 11.94 cents/lb VC

Computing Gross Profit

Synthesis Tree:

Copyright@FKKKSA, UTM Computer-Aided Process Simulation Project Intro to PD - 41

Preliminary Flowsheet for Path

• 800 MM lb/year @ 330 days/yr or 100,000 lb/hr or 1600 lbmol/hr VC

• On the basis of this principal sink, the HCl sink and the sources can be computed (each flow is 1,600 lbmol/hr)

Next step involves distributing the chemicals by matching

sources and sinks.

Fig 3.3

Fig. 4.4





Synthesis steps:








In other words, taking into account the rxn conversion, how much

reactant do we need to obtain the desired production rate ?

Can we recycle unreacted reactant?

Let’s do for reaction path 3……

Distribute the chemicals

• A conversion of 100% of the C2H4 is assumed in the chlorination reaction.

Fig 4.4

Distribute the chemicals Also

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

according to Eqn. (3.4).

• To satisfy the overall material balance, 158,300 lb/hr of C2H4Cl2 must

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

• But a 60% conversion will only produce 60,000 lb/hr of VC.

• We need to supply excess C2H4Cl2

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

(158300/0.6)-158,300 = 263800-158300= 105,500 lb/hr.

• This is also the recycle stream.

• C2H4Cl2 entering pyrolysis reactor is

158300+105500 = 263,800 lb/hr.


• So, the effluent stream from the pyrolysis operation is the source for the C2H3Cl product, the HCl by-product, and the C2H4Cl2 recycle.

Fig 4.5

• Reactor pressure levels:

– Chlorination reaction: 1.5 atm is recommended, to eliminate the

possibility of an air leak into the reactor containing ethylene.

– Pyrolysis reaction: 26 atm is recommended by the B.F. Goodrich patent (1963) without any justification.

– This pressure probably to reduce the size of the pyrolysis furnace, although the tube walls must be considerably thicker and many precautions are necessary for operation at elevated pressures.

– The pressure level is also an important consideration in selecting the separation operations, as will be discussed in the next synthesis step.


Synthesis Tree





Synthesis steps:








In other words, how to separate desired and undesired product? Can we recycle to obtain

higher production?

Separation Selection - Liquid-Liquid system

Are key constituents present in high concentrations?

Is there a significant difference in volatility?

Are materials heat sensitive?

Distillation

Can compatible Ts be maintained @ Ps > 0.0001atm?

Vacuum distillation

Specialized techniques

Consider solid adsorption,

liquid extraction, precipitation,

ion exchange, bubble frac-

tionation, membrane processes

or other specialized techniques Yes

No

No No

No

Yes

Yes

Yes

See Next

Does one of the key components freeze @ temperature markedly

different from the others and at an absolute value above 230K?

Crystallization or freeze concentration

Liquid-liquid extraction or solid adsorption


Is there a significant difference in solubility in a

cheap, immiscible liquid or attraction to a solid adsorbent?

Consider Azeotropic or extractive distillation or other specialized techs.

No

Yes

Yes

No

No

Separation Selection - Liquid-Liquid system


Is one component preferentially soluble

in an inexpensive liquid from which it can be

readily desorbed?

Can one vapour be condensed at moderately

low temperatures?

Dehumidification?

Liquid absorption

Do economics justify ultra-low temperature

separation?

Cryogenic distillation

Consider solid adsorption, membrane

processes, thermal diffusion, or other

specialized techniques

Yes

No

No

No

No

Yes

Yes

Yes

Separation Selection - Gaseous System


Desorption Specialized techniques

Separation of a gas from a gas-liquid solutions

Yes No

Is solid present in appreciable concentrations?

Depending on relative economics

Evaporation Freeze crystallization,

membrane processes, or specialized techniques

Chemical precipitation or specialized techniques

Separation of a solid from a solid-liquid solutions

Yes

No

• The product of the chlorination reaction is nearly pure C2H4Cl2, and requires no purification.

• In contrast, the pyrolysis reactor conversion is only 60%.

• We have HCl, C2H3Cl and C2H4Cl2 in the pyrolysis reactor effluent.

• One or more separation operations are required to match the required purities in the C2H3Cl and HCl sinks.

Fig 4.5

Eliminate Differences in Composition

• One possible arrangement is given in the next slide. The data below explains the design decisions made.

Eliminate Differences in Composition

Chemical

Normal

boiling

point

(1atm,oC)

Boiling Points (oC) Critical constants

4.8 atm 12atm 26 atm Tc(oC) 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

• The effluent stream from the pyrolysis operation is the source for the C2H3Cl product, the HCl by-product (most volatile), and the C2H4Cl2 recycle (least volatile).

Before

Fig 4.5

To eliminate composition differences (separations), 2 distillation towers in series are inserted into the flowsheet.

Distillation is desirable because of large volatility differences among the 3 species.

Separation Selection- Liquid-Liquid system (1)



Are materials heat sensitive?

Distillation

Can compatible Ts be maintained @ Ps > 0.0001atm?

Vacuum distillation

Specialized techniques

Consider solid adsorption, liquid extraction, precipitation,

ion exchange, bubble frac- tionation, membrane processes or other specialized techniques

Yes

No

No No

No

Yes

Yes

Yes See Next

Setting Column Pressure

Economically, it is desirable to condense the overhead (top column) product against available cooling water. If the product is essentially pure, the necessary column P to ensure this is the saturation (vapor) pressure or bubble point pressure of the product at TCW + DT (~5-10 °C). However, this becomes undesirable if the pressure is too high.

Example:

A column separating CH3Cl (TBP = -24 °C, more volatle)

from CH2Cl2 (TBP = 40.5 °C) would operate at about 18 bar

(which is the svp of CH3Cl at 30 °C) ,

note: TCW + DT = 25 + 5 .

Tcw=25°C

Tbub,CH3Cl=30 °C

Tdew,CH3Cl > Tbub,CH3Cl

Column top section

Tdew,CH3Cl Tbub,CH3Cl

Tcw,out

(40°C) Condenser

Temp profile

DT=5°C

Liq. Distillate Overhead Vap

Setting Column Pressure

Example: A column separating CH3Cl (TBP = -24 °C, more volatle) from CH2Cl2 (TBP = 40.5 °C) would operate at about 18 bar (which is the svp of CH3Cl at 30 °C) , note: TCW + DT = 25 + 5 .

Tcw=25°C

Tbub,CH3Cl=30 °C

Tdew,CH3Cl > Tbub,CH3Cl

Column top section

Tdew,CH3Cl

Tcw,out

(40°C) Condenser Temp. profile

Overhead Vap

Tdew,C Tcw

Tbub,C

T bub,R

TST

T dew,R

Setting Column Pressure and Temperature

Primarily dictated by cooling water temp., Tcw

TCW Tbub,C Tdew,C Tbub,R Tdew,R TST

Select column pressure so that 1. T bub,C TCW 2. From the bubble pt eqn, Tbub increases with P. 3. Also choose P > 1 atm

Effect of Column P on Separation

• In reality,

a large change in P ==> significantly changes TBUBBLE

• Thus,

P partial P of overhead TBUB,condenser

product (PBUB,condenser) (overhead T)

chapter 2: part ii process creation & synthesis · process creation & synthesis . learning...

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