Hierarchy of Decisions
1. Batch versus continuous
2. Input-output structure of the flowsheet
3. Recycle structure of the flowsheet
4. General structure of the separation system
a. Vapor recovery system
b. Liquid recovery system
5. Heat-exchanger network
Ch.6, Ch.7, Ch.16
Ch. 4
Ch.5
Purge
H2 , CH4
H2 , CH4
Toluene
LEVEL 2
Reactor
Separation
System
Benzene
Diphenyl
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
1. Vent
2. Recycle and purge
3. Recycle
4.None
5.Excess - vent
6.Excess - vent
7.Primary product
8.Fuel
9.Waste
Component classifications
Gaseous by-products and feed impurities
Gaseous reactants plus inert gases and/or gaseous by-products
Reactants
Reaction intermediates
Azeotropes with reactants (sometimes)
Reversible by-products (sometimes)
Reactants-if complete conversion or unstable reaction intermediates
Gaseous reactant not recovered or recycles
Liquid reactant not recovered or recycled
Primary product
By-products to fuel
By-products to waste treatment
should be minimized
A ) List all the components that are expected to leave the reactor. This list includes all
the components in feed streams, and all reactants and products that appear in every
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
Component
H2
CH4
Benzene
Toluene
Diphenyl
NBP , C
-253
-161
80
111
255
(Gas Recycle)
(Feed)H2 , CH4
Destination
Recycle + Purge Gas
Recycle + Purge Recycle
Primary Product
Recycle
liq. Recycle
By-product
Compressor
CH4 , H2
(Purge)
Benezene
(PrimaryProduct)
Reactor
Separator
(Feed) Toluene
Diphenyl
(By-product)
Toluene (liq. recycle)
2 ) NUMBER OF RECYCLE STREAMS
EXAMPLE
Acetone  Ketene + CH4 700C
Ketene  CO + 1/2C2H4
1atm
Ketene + Acetic Acid  Acetic Anhydride
80 C, 1atm
NBP , C
Component
CO
CH4
C2H4
Ketene
Acetone
Acetic Acid
Acetic Anhydride
Destination
-312.6
-258.6
-154.8
-42.1
133.2
244.3
281.9
Fuel By-product
“
“
Unstable
Reactant
Reactant
Primary Product
CO , CH4 , C2H4
(By-product)
Acetic Acid (feed)
Acetone
(feed)
R1
R2
Separation
Acetic Acid (recycle to R2)
Acetone (recycle to R1)
Acetic Anhydride
(primary product)
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)
r2 k 2 a2  a1 b2 b1
 C FEED1C FEED2
r1 k1
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
CH3
ex.
O
+ H2  O
+ CH4
excess 5:1

2O
O + H2
 O
( 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
( 3-2 ) FEED INERTS TO REACTOR
( 1b ) Single reversible reaction

FEED PROD1 + PROD2
Cinert   Xfeed 
FEED1 + FEED2
keq =

Cp1Cp2
CF
PRODUCT
Cinert   Xfeed1 or Xfeed2 
keq =
CP
CF1CF2
( 2 ) Multiple reactions in parallel  byproducts
FEED1 + FEED2  PRODUCT

FEED1 + FEED2 BYPRODUCT
Cinert   Cbyproduct 
FEED1 + FEED2  PRODUCT
FEED1
 BYPROD1 + BYPROD2
Cinert   Cbyprod1-2 
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 +H2
Steam is usually used as the
C2H6 + H2 = 2CH4
diluent.
Example Styrene Production
EB = styrene +H2
EB  benzene +C2H4
EB + H2  toluene + CH4
Steam is also used.
( 3-3 ) PRODUCT REMOVAL DURING REACTION
to shift equilibrium + product distribution
( 1b ) single reversible reaction
ex. 2SO2 + O2 = 2SO3
H2O
H2O
SO2
REACT
O2 + N 2
ABSORB
REACT
H2SO4
( 3 ) multiple reactions in series  byproduct
FEED  PRODUCT
remove
PRODUCT = BYPRODUCT
remove
.
ABSORB
H2SO4
( 3-4 ) RECYCLE BYPRODUCT
to shift equilibrium + product distribution
CH3
O + H2  O
2 O
= O
+ CH4
O + H2
( 4-1 ) REACTOR TEMPERATURE
T   k   V
 Single Reaction :
- endothermic
AHAP !
- exothermic
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
* irreversible AHAP !
* reversible
continuously decreasing as conversion increases.
 Multiple Reaction
max. selectivity
( 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
FFT
Toluene
Benzene , PB
reactor
FT ( 1-X )
FT
FT ( 1-X )
separator
LEVEL 3
Diphenyl
LEVEL 2
always valid for limiting reactant
when there is complete recovery and
recycle of the limiting reactant
F
FT  FT
X
PD
RECYCLE MATERIAL BALANCE ( Quick Estimates !!! )
Example
HDA process
 other reactant : (Next )
molar ratio
FFT
y FH FG  y PH RG  ( MR )
X
extra design variable
FFT MR y FH
RG 

FG
X y PH y PH
RH 2  RG y PH
RCH 4  RG (1  y PH )
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,
FT  FFT
X  0, FFT  , $R  
X
F MR y FH
1 
F

RG  FT

FG 
 MR FT  y FH FG 
X y PH y PH
y PH 
X

yPH  0, RG  , $C  
EP3=EP2-annualized costs of reactors
-annualized costs of compressors
yPH
2  106
1  106
$/year
0
-1  106
0.1
0.3
0.5
0.7
0.2
0.4
0.6

-2  106
 does not include any separation or heating and cooling cost