Lecture 7

Copyright © 2012 IA Karimi & Mukta Bansal 1

Process Synthesis – Learning Outcomes

• Define process synthesis & explain its significance

• Identify, compare, & evaluate design/process alternatives

• Make & justify design decisions

• State & rationalize heuristic rules for process synthesis

• Develop a chemical process for a desired objective


What is Process Synthesis?

• Developing a chemical process

Structure, unit operations, flows, operating conditions

• Scientific art that relies heavily on experience, heuristics, rules

of thumbs, …

Few exact mathematical models or automated procedures

Heat exchanger & separation networks are exceptions

• Why is it critical? Preliminary design costs 15-20% of the

entire project cost, but fixes about 80% of subsequent costs

Eastman Chemical received Kirkpatrick award in 1985 for a new

process for Methyl Acetate, which replaced a reactor plus several

separation units by one reactive distillation column

Huge reduction in cost made conventional processes non-competitive


Cyclohexane (Naphthene) Manufacture - I

How to

develop this

process

structure?

Is it good?

Is this the

only way?

Raw materials? Units? Operating conditions?


Cyclohexane Manufacture - II

What is the major difference from the previous process?

How to select?

PumpBenzene

Feed

Make-Up

Hydrogen

Flash

Stabilizer

Feed

Preheater

ReactorSteam

Boiler

Feed Water

(335 psia, 570 F)

Product

Cooler

Pump

Recycle

Compressor

Purge

Cyclohexane

Product

15 psia

Light

Ends


PFD vs. BFD

• Alternate schematic representations of a chemical process

Material/energy streams, their states, flow directions, and unit

operating conditions

• PFD (Process Flow Diagram) shows real processing equipment

(physical units)

Heat exchanger, pump, distillation unit, reactor, etc.

Often called process flowsheet

• BFD (Block Flow Diagram) shows process operations

Heating, pumping, separation, reaction, etc.


PFD for Cyclohexane

PumpBenzene

Feed

Make-Up

Hydrogen

Flash

Stabilizer

Feed

Preheater

ReactorSteam

Boiler

Feed Water

(335 psia, 570 F)

Product

Cooler

Pump

Recycle

Compressor

Purge

Cyclohexane

Product

15 psia

Light

Ends


BFD for Cyclohexane

H2 340 psia

120 F

V-Mix 340 psia

Pump 340 psia

Compress 340 psia

C6H12+C6H6

15 psia 100 F

Cool 100 F

Distil 20 psia

Light Ends

React 335-320 psia

570-430 F

Flash 320 psia

120 F

Cool 320 psia

DPT

Expand 20 psia

Purge 320 psia

Vaporize 340 psia

Heat 335 psia

570 F

C6H6 15 psia 100 F

L-Mix 340 psia

Pump 340 psia

Split 320 psia

Purge

Condense

320 psia

DPT-120 F

N2 + H2 + CH4

Heat

Expand 15 psia


Process Synthesis Problem

PFD? Fully specified

Raw Materials

Fully specified

Products

Each stream (material/energy) has a state (P, T, F, x, y, f, etc.)


Can a Stream Change its State by Magic?

Point A

P = 1 atm T = 30 C

x, y, f

Point B

P = 1 atm T = 90 C

x, y, f'

One single stream at two points in a process

What must happen between A & B?


Unit Operations Change Stream States in PFD

Changes in

Stream States

Unit

Operations

Plant Device/

Equipment

Rise in T Heating Heat Exchanger

A physical unit must exist for each unit operation

If we know state changes, we can know the operations

If we know the operations, we can know the units

Once we know the units, we can know the PFD!


Identify plausible alternative paths

via well-defined intermediate states

Our Approach to Synthesis

Define

States for

Start

Materials (Raw Materials)

Define

States for

End

Materials (Final Products)

State

2

State

4

State

1

State

6

State

3

State

5


Algorithm for Flowsheet Development

Recycle/purge

materials, as &

when needed

BFD

c-changes?

x-changes?

f-changes?

P-changes?

F-changes?

T-changes?

Add equipment

for each operation PFD

Identify “best”

intermediate states

Select & add operation to

effect each state change

Define start & end states


Example: Process for Naphthene • Product specs: 99 wt% Cyclohexane, 15 psia, 100 F

Defines the state of the final product stream

• Understand product (physicochemical properties)

MSDS maintained by OSHA (Occupational Safety & Health Agency)

Cyclohexane: MW = 84.16, Density@20 C = 0.779, BP = 81 C, hydrocarbon, immiscible with water, flammable

• Search literature & gather information

• We must decide/fix many details at various levels

Raw materials, reaction pathways, operating conditions, processing units, unit capacities, PFD, plant location, etc.

Vinyl Acetate example in your text book.


Naphthene via Benzene Hydrogenation • C6H6 (Benzene) + 3H2 C6H12 ΔH = –266 kJ/mol

Highly exothermic, equilibrium-controlled, near 100% conversion below 230 C

How do T & P affect equilibrium conversion? Rate of reaction?

• Catalysts: Ni/Alumina or Ni/Pd or Ni/Pt

Liquid phase: Catalyst suspended in liquid cyclohexane, 99.85% conversion, 20-30 bar, 160-220 C

Gas phase: Multi-stage, fixed bed, tubular reactors with inter-stage cooling, initial T < 370 C, exit T = 220-300 C

• Cyclohexane is recycled to the reactor for heat removal

• What should be the limiting reactant?

Heuristic: Use excess of other reactants to consume a valuable, toxic, or hazardous reactant


Cyclohexane PFD - Start & End States

Any c-change?

Hydrogen Feed

(340 psia, 120 F)

(97.5% H2, 2% CH4, 0.5% N2)

PFD?

Benzene Feed

(100% pure, 15 psia, 100 F)

Cyclohexane Product

(15 psia, 100 F, 99 wt%)


Cyclohexane PFD

Any more c-change?

Any x-change?

How to separate the reactor product to get C6H12 from PFD2?

H2

(340 psia, 120 F)

C6H12

15 psia

100 F C6H6

(15 psia, 100 F)

PFD1?

React

335-320 psia

570-430 F

PFD2?

N2

H2

CH4

C6H12

C6H6

Heat


How to Separate Reactor Product?

• What is the state of mixture?

V, L, S, VL, LS, VS, VLL, VLS?

• Heuristic: For vapor mixtures, use partial condensation,

absorption, adsorption, membrane separation, cryogenic

distillation, and/or desublimation.

• Heuristic: For liquid mixtures, use distillation, stripping,

enhanced distillation (extractive, azeotropic, reactive), liquid-

liquid extraction, crystallization, and/or adsorption.

• Heuristic: Condense or partially condense vapor mixtures using

cooling water or refrigerant (i.e. condense & flash)

Does the mixture have condensable components?

Are they desirable?


Add Flash for First-cut Separation

Can flash give high-purity products?

What should be in flash gas? liquid?

What should be pressure and temperature?

C6H12

15 psia

100 F

PFD4?

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD1?

React

335-320 psia

570-430 F

N2

H2

CH4

C6H12

C6H6

Heat

Flash

? PFD3?


Primary Purpose of Flash?

• Liquefy completely?

• Two ways of condensing C6H6 and C6H12?

• Heuristic: Avoid compression.

Fact: Cooling or heating is cheaper than compression

• Heuristic: Once attained, avoid losing pressure as long as

possible.

Is higher pressure better for separating C6H6 and C6H12?

• How low should flash temperature be?

BPT? DPT? < 0 C? 5 C? 20 C? 30 C? 50 C?

• Heuristic: Avoid refrigeration.


What Should be Flash T?

• Which cooling agent? What is its temperature?

• Heuristic: For closed-circuit cooling system, use 90 F (30 C)

for cooling water (CW) inlet and 120 F (45 C) for CW outlet.

Why restrict to 45 C?

• Heuristic: For open-circuit system, use 45 C for CW outlet.

Inlet depends on source and discharge governed by environmental

regulations

• Theoretical minimum temperature attainable in Singapore?

How much heat transfer area for that?

• How closely should fluid temperatures approach each other?

What is the optimal approach?


Minimum Temperature Approach

• Heuristic: Near-optimal min T-approaches are 10 F for below ambient, 20 F for up to 300 F (150 C), and 50 F for higher

• Heuristic: Near-optimal min T-approaches are 10 C for liquids (high heat transfer coefficient systems), 20 C for gases, and 5 C for refrigeration

• What should be flash temperature?

Heat

Exchanger

t1 t2

T1 T2

ΔTL = T2 – t1 ΔTR = T1 – t2

Minimum Temperature Approach = ΔTmin = min(ΔTL, ΔTR)


Fix Flash Conditions

Any x-change?

How to purify C6H12 in PFD4?

State of the mixture? Recall heuristic.

Is distillation easy for C6H6 & C6H12?

Recall: Used excess H2 to consume most of C6H6!

Heat

C6H12

15 psia

100 F

PFD4?

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD1?

React

335-320 psia

570-430 F

N2+H2+CH4

C6H6+C6H12

Flash

320 psia

120 F

PFD3?

H2+N2+CH4+vapors

C6H12+C6H6+gases


Add Distillation

C6H12+C6H6

15 psia

100 F PFD6?

H2+N2+CH4

Distil

P?

Top

Product?

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD1?

React

335-320 psia

570-430 F

N2

H2

CH4

C6H12

C6H6

Heat

Flash

320 psia

120 F

C6H12+C6H6

PFD3?

PFD5?

How to determine P & T?


Operating P & T for Distillation

• Heuristic: Distillation becomes easier at lower pressure, but

avoid vacuum

How do VLE characteristics change with P? Why avoid vacuum?

• Heuristic: Prefer atmospheric distillation (1-5 atm)

• Heuristic: Prefer CW in condenser, if possible

• Heuristic: If CW condenser not possible, then consider higher

pressure

• What is the top product? What is its state?

What type of condenser?

• What to do with top product?

• What to do with flash gas?


Fix Distillation P & Recycle Flash Gas

From where to recycle C6H12?

What are the two possible places?

Does it need to be pure?

Which is the best? Why? (P? benzene? Distil capacity?)

C6H12+C6H6

15 psia

100 F PFD6?

H2+N2+CH4

Distil

20 psia

Light

Ends

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD1?

React

335-320 psia

570-430 F

Heat

Flash

320 psia

120 F

PFD3?

PFD5?


Recycle Cyclohexane

Any x-change? What does PFD1 do? C6H12+C6H6

15 psia

100 F PFD6?

Distil

20 psia

Light

Ends

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD1?

React

335-320 psia

570-430 F

Heat

Flash

320 psia

120 F

PFD3?

PFD5? C6H12+C6H6+gases


Mixing in PFD1

• Fact: Mixing streams with very different viscosities is difficult

Dissolving sugar in water/tea

Dissolving a polymer in a solvent

• Fact: Mixing liquid and gas is difficult

Fuel injection in car engine

Hydrogen & benzene in the liquid-phase reactor for cyclohexane

• Heuristic: Avoid mixing a gas with liquid

Mix liquids or gases

• What are the states of streams into PFD1?

Liquid, gas, solid?


Expand PFD1 - Mix Liquid Streams

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD9?

L-Mix

P? PFD8?

PFD6?

Distil

20 psia

React

335-320 psia

570-430 F

Flash

320 psia

120 F

PFD3?

PFD5? PFD7?

C6H12+C6H6

15 psia

100 F

Light

Ends

What should be P & T of L-Mix?

15 psia? 320 psia? Intermediate?

Which heuristic rule can you invoke?


Fix L-Mix Pressure

H2

340 psia

120 F

C6H6

15 psia

100 F

PFD9?

L-Mix

320 psia PFD8?

PFD6?

Distil

20 psia

React

335-320 psia

570-430 F

Flash

320 psia

120 F

PFD3?

PFD5? PFD7?

C6H12+C6H6

15 psia

100 F

Light

Ends

What about gas streams?


Expand Purge for Disposal

Any change by magic? What about heat from the reactor?

H2 340 psia

120 F

G-Mix 340 psia

Pump 340 psia

Compress 340 psia

C6H12+C6H6

15 psia 100 F

Cool 100 F

Distil 20 psia

Light Ends

React 335-320 psia

570-430 F

Flash 320 psia

120 F

Cool 320 psia

DPT

Expand 20 psia

Purge 320 psia

Vaporize 340 psia

Heat 335 psia

570 F

C6H6 15 psia 100 F

L-Mix 340 psia

Pump 340 psia

Split 320 psia

Purge

Condense

320 psia

DPT-120 F

N2 + H2 + CH4

Heat

Expand 15 psia


Assign Units to Operations

• React: Multi-stage, gas-phase, catalytic, fixed bed tubular

reactors with inter-stage coolers that produce steam

Boiler feed water (BFW) on shell sides of coolers & reactors

• Mix: Pipeline joints

• Expand: Expansion valves (Any other option? Why not?)

• Heat, cool, vaporize: Appropriate shell & tube exchangers with

cooling water or steam

• Flash: Vertical flash drum

• Distillation: Column with a partial condenser & reboiler

• Purge / Split: Pipe splits

• Pump / Compress: Centrifugal pump or compressor


Cyclohexane PFD

PumpBenzene

Feed

Make-Up

Hydrogen

Flash

Stabilizer

Feed

Preheater

ReactorSteam

Boiler

Feed Water

(335 psia, 570 F)

Product

Cooler

Pump

Recycle

Compressor

Purge

Cyclohexane

Product

15 psia

Light

Ends


Reading

• Tutorial exercises involve synthesis + simulation

Do not rush to simulate without developing a PFD on paper

• Read SSL Chapters 1, 3, & 5

Basics of PFD development - Vinyl Chloride (Ch 1)

Heuristic rules for design & synthesis (Ch 5)

References on synthesis (Ch 1, SSL)

Handbooks, encyclopedias, Hydrocarbon Processing, etc.


Practice Exercise • Formaldehyde (HCHO) is fully soluble in water, alcohols and other

polar solvents. However, aqueous formaldehyde is not a simple physical solution of formaldehyde in water, but a complex mixture of various hydrated chemical species. Develop a process to produce 40 wt% aqueous formaldehyde (1 atm, 40 C) using methanol (CH3OH, 2 atm, 40 C) and ambient air as raw materials via the following gas-phase, catalytic reactions at 1.5 atm and 600-650 C:

CH3OH + 0.5O2 HCHO + H2O DH = -156 kJ

CH3OH HCHO + H2 DH = +85 kJ

• The conversion of methanol in the reactor is almost 100%. 50-60% of the formaldehyde is produced via reaction (1), making the overall reaction highly exothermic. Boiling points of methanol and aqueous formaldehyde are 65 C and 98.9 C respectively. High-pressure (HP) steam is available at 400 C in the plant. Assume other utilities, as you need and justify your steps.


Improving a PFD

• Energy integration leads to alternate PFDs

Match hot streams with cold streams, under what conditions?

Where can you conserve energy in this process?

• Alternate reaction pathways, separation methods, etc.

Simulators, heuristic rules, simple costing, experience, etc. useful in

selecting the best

• Optimize operating conditions via process simulators

Unit/stream conditions may change, but PFD largely unchanged

Simulators handy in estimating properties and conditions (e.g. Flash,

BPs of Benzene and C6H12)

On-line optimization using simulators


Other Heuristics

• Estimated ΔPs for heat exchangers are

1 psi for boiling & condensing

3 psi for a gas

5 psi for a low-viscosity liquid

7-9 psi for a high-viscosity liquid

20 psi for a fluid through furnace

• Other units (reactors, columns, etc.)

• Flow velocities (gas & liquid)

Determine pipe sizes

• Standard pipe and tube dimensions

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