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Hierarchy of Decisions. Purge H 2 , CH 4. Reactor. Separation System. H 2 , CH 4. Benzene. Toluene. Diphenyl. LEVEL 2. LEVEL 3 DECISIONS 1 ) How many reactors are required ? Is there any separation between the reactors ? 2 ) How many recycle streams are required ?

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slide2
Purge

H2 , CH4

Reactor

Separation

System

H2 , CH4

Benzene

Toluene

Diphenyl

LEVEL 2

slide3
LEVEL 3 DECISIONS

1 ) How many reactors are required ?

Is there any separation between the reactors ?

2 ) How many recycle streams are required ?

3 ) Do we want to use an excess of one reactant at the reactor inlet ? Is there a

need to separate product partway or recycle byproduct ?

4 ) Should the reactor be operated adiabatically or with direct heating or cooling ?

Is a diluent or heat carrier required ?

What are the proper operating temperature and pressure ?

5 ) Is a gas compressor required ? costs ?

6 ) Which reactor model should be used ?

7 ) How do the reactor/compressor costs affect the economic potential ?

slide4
1 ) NUMBER OF REACTOR SYSTEMS

If sets of reactions take place at different T and P, or if they require different catalysts, then we use different reactor systems for these reaction sets.

Acetone  Ketene + CH4

Ketene  CO + 1/2C2H4

700C, 1atm

Ketene + Acetic Acid  Acetic Anhydride

80 C, 1atm

slide5
Number of Recycle Streams

TABLE 5.1-3

Destination codes and component classifications

Destination code Component classifications

1. Vent Gaseous by-products and feed impurities

2. Recycle and purge Gaseous reactants plus inert gases and/or gaseous by-products

3. Recycle Reactants

Reaction intermediates

Azeotropes with reactants (sometimes)

Reversible by-products (sometimes)

4.None Reactants-if complete conversion or unstable reaction intermediates

5.Excess - vent Gaseous reactant not recovered or recycles

6.Excess - vent Liquid reactant not recovered or recycled

7.Primary product Primary product

8.Fuel By-products to fuel

9.Waste By-products to waste treatment

should be minimized

A ) List all the components that are expected to leave the reactor. This list includes all

thecomponentsinfeedstreams, and allreactantsandproductsthatappearinevery

reaction.

B ) Classify each component in the list according to Table 5.1-3 and assign a destination

code to each.

C ) Order the components by their normal boiling points and group them with

neighboring destinations.

D ) The number of groups of all but the recycle streams is then considered to be the

number of product streams.

slide6
2 ) NUMBER OF RECYCLE STREAMS

EXAMPLE HDA Precess

ComponentNBP , CDestination

H2 -253 Recycle + Purge Gas

CH4 -161 Recycle + Purge Recycle

Benzene 80 Primary Product

Toluene 111 Recycle liq. Recycle

Diphenyl 255 By-product

Compressor

CH4 , H2(Purge)

(Gas Recycle)

Benezene

(PrimaryProduct)

Reactor

Separator

(Feed)H2 , CH4

(Feed) Toluene

Diphenyl

(By-product)

Toluene (liq. recycle)

slide7
2 ) NUMBER OF RECYCLE STREAMS

EXAMPLE

Acetone Ketene + CH4700C

Ketene  CO + 1/2C2H41atm

Ketene + Acetic Acid Acetic Anhydride

80 C, 1atm

ComponentNBP , CDestination

CO -312.6 Fuel By-product

CH4 -258.6 “

C2H4 -154.8 “

Ketene -42.1 Unstable

Acetone 133.2 Reactant

Acetic Acid 244.3 Reactant

Acetic Anhydride 281.9 Primary Product

CO , CH4 , C2H4

(By-product)

Acetic Acid (feed)

Acetone

(feed)

R1

R2

Separation

Acetic Anhydride

(primary product)

Acetic Acid (recycle to R2)

Acetone (recycle to R1)

slide8
3. REACTOR CONCENTRATION

(3-1) EXCESS REACTANTS

 shift product distribution

 force another component to be close to complete

conversion

 shift equilibrium

( molar ratio of reactants entering reactor )

is a design variable

slide9
( 1a ) Single Irreversible Reaction

force complete conversion

ex. C2H4 + Cl2 C2H4Cl2

excess

ex. CO + Cl2 COCl2

excess

( 1b ) Single reversible reaction

shift equilibrium conversion

ex. Benezene + 3H2 = Cyclohexane

excess

( 2 ) Multiple reactions in parallel producing byproducts

shift product distribution

type (3)

if (a2 - a1) › (b2 - b1) then FEED2 excess

if (a2 - a1) ‹ (b2 - b1) then FEED1 excess

slide10
( 3 ) Multiple reactions in series producing byproducts

type (3) shift product distribution

ex. CH3

+ H2 + CH4

excess 5:1

2+ H2

( 4 ) Mixed parallel and series reactions  byproducts

shift product distribution

ex. CH4 + Cl2  CH3Cl + HCl Primary

excess 10:1

CH3Cl + Cl2 CH2Cl2+ HCl

CH2Cl2+ Cl2  CHCl3 + HCl Secondary

CHCl3 + Cl2  CCl4 + HCl

O

O

O

O

O

slide11
( 3-2 ) FEED INERTS TO REACTOR

( 1b ) Single reversible reaction

FEED PROD1 + PROD2

Cinert   Xfeed  keq =

FEED1 + FEED2 PRODUCT

Cinert   Xfeed1 or Xfeed2  keq =

( 2 ) Multiple reactions in parallel  byproducts

FEED1 + FEED2  PRODUCT

FEED1 + FEED2 BYPRODUCT

Cinert   Cbyproduct 

FEED1 + FEED2  PRODUCT

FEED1 BYPROD1 + BYPROD2

Cinert   Cbyprod1-2 

Cp1Cp2

CF

CP

CF1CF2

slide12
Some of the decisions involve introducing a new component into the flowsheet, e.g. adding a new component to shift the product distribution, to shift the equilibrium conversion, or to act as a heat carrier. This will require that we also remove the component from the process and this may cause a waste treatment problem.

Example Ethylene production

C2H6= C2H4 +H2Steam is usually used as the

C2H6 + H2 = 2CH4diluent.

Example Styrene Production

EB = styrene +H2

EB  benzene +C2H4Steam is also used.

EB + H2 toluene + CH4

slide13
( 3-3 ) PRODUCT REMOVAL DURING REACTION

to shift equilibrium + product distribution

( 1b ) single reversible reaction

ex. 2SO2 + O2= 2SO3

H2O

H2O

SO2

REACT

ABSORB

REACT

ABSORB

O2 + N2

H2SO4

H2SO4

( 3 ) multiple reactions in series  byproduct

FEED  PRODUCT

remove

PRODUCT = BYPRODUCT

remove

.

slide14
( 3-4 ) RECYCLE BYPRODUCT

to shift equilibrium + product distribution

CH3

+ H2 + CH4

2 = + H2

O

O

O

O

O

slide15
( 4-1 ) REACTOR TEMPERATURE

T   k   V

 Single Reaction :

- endothermic

AHAP !

- exothermic

* irreversible AHAP !

* reversible

continuously decreasing as conversion increases.

 Multiple Reaction

max. selectivity

T  400C  Use of stainless steel is severely

limited !

T  260C  High pressure steam ( 40~50 bar)

provides heat at 250-265 C

T  40C  Cooling water Temp 25-30C

slide16
( 4-2 ) REACTOR HEAT EFFECTS

Reactor heat load = f ( x, T, P, MR, Ffeed)

QR = ( Heat of Reaction )  ( Fresh Feed Rate )

……..for single reaction.

……..for HDA process ( approximation )

Adiabatic Temp. Change = TR, in - TR, out = QR / FCP

 If adiabatic operation is not feasible, then we can try to use indirect heating or cooling. In general,

Qt, max  6 ~ 8  106 BTU / hr

 Cold shots and hot shots.

 The temp. change, ( TR, in- TR, out), can be moderated by

- recycle a product or by-product ( preferred )

- add an extraneous component.

( separation system becomes more complex ! )

slide26
( 4-3 ) REACTOR PRESSURE ( usually 1-10 bar )

 VAPOR-PHASE REACTION

- irreversible as high as possible

P     V 

r 

- reversible single reaction

* decrease in the number of moles

AHSP

* increase in the number of moles

continuously decreases as conversion increases

- multiple reactions

 LIQUID-PHASE REACTION

prevent vaporization of products

allow vaporization of liquid so that it can be condensed and refluxed as a

means of removing heat of reaction.

allow vaporization of one of the components in a reversible reaction.

slide27
RECYCLE MATERIAL BALANCE ( Quick Estimates !!! )

Example HDA process

 Limiting Reactant : Toluene ( first )

yPH

RG

Purge , PG

FG, yFH

H2 , CH4

Benzene , PB

reactor

separator

FFT

FT ( 1-X )

PD

Toluene

FT

Diphenyl

LEVEL 3

FT ( 1-X )

LEVEL 2

always valid for limiting reactant

when there is complete recovery and

recycle of the limiting reactant

slide28
RECYCLE MATERIAL BALANCE ( Quick Estimates !!! )

Example HDA process

 other reactant : (Next )

molar ratio

extra design variable

Note that details of separation system have not been specified at this level. Therefore, we assume that reactants one recovered completely.

slide29
5 ) COMPRESSOR DESIGN AND COST

Whenever a gas-recycle stream is present, we will need a gas-

recycle compressor.

Covered in “Unit Operation (I)”

slide30
6 ) EQUILIBRIUM LIMITATIONS

7 ) REACTOR DESIGN AND COSTS

Covered in

“Reactor Design and Reaction Kinetics”

slide31
ECONOMIC POTENTIAL AT LEVEL 3

Note,

$

$

EP3=EP2-annualized costs of reactors

-annualized costs of compressors

2  106

1  106

0.2

0.4

0.6

$/year 0

0.1

0.3

0.5

0.7

-1  106

-2  106

 does not include any separation or heating and cooling cost

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