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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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Purge

H2 , CH4

Reactor

Separation

System

H2 , CH4

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 ?

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 ?


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


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.


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)


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)


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


( 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


( 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


( 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


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


( 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

.


( 3-4 ) RECYCLE BYPRODUCT

to shift equilibrium + product distribution

CH3

+ H2 + CH4

2 = + H2

O

O

O

O

O


( 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


( 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 ! )


Figure 2.5 Heat transfer to and from stirred tanks.


Figure 2.5 Heat transfer to and from stirred tanks.


Figure 2.5 Heat transfer to and from stirred tanks.


Figure 2.5 Heat transfer to and from stirred tanks.


Figure 2.6 Four possible arrangements for fixed-bed recators.


Figure 2.6 Four possible arrangements for fixed-bed reactors.


Figure 2.6 Four possible arrangements for fixed-bed recators.


Figure 2.6 Four possible arrangements for fixed-bed reactors.


( 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.


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


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.


5 ) COMPRESSOR DESIGN AND COST

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

recycle compressor.

Covered in “Unit Operation (I)”


6 ) EQUILIBRIUM LIMITATIONS

7 ) REACTOR DESIGN AND COSTS

Covered in

“Reactor Design and Reaction Kinetics”


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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