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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 Ch.5
a. Vapor recovery system
b. Liquid recovery system
5. Heat-exchanger network Ch.6, Ch.7, Ch.16
Ch. 4
ReactorSeparation
System
Purge
H2 , CH4
Benzene
Diphenyl
H2 , CH4
Toluene
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
700C, 1atmKetene + Acetic Acid Acetic Anhydride
80 C, 1atm
Number of Recycle Streams
TABLE 5.1-3Destination 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 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 NBP , C Destination
H2 -253 Recycle + Purge Gas
CH4 -161 Recycle + Purge Recycle Benzene 80 Primary Product Toluene 111 Recycle liq. Recycle Diphenyl 255 By-product
Reactor
Compressor
Separator
CH4 , H2 (Purge)
Benezene(PrimaryProduct)
Diphenyl(By-product)
(Feed)H2 , CH4
(Feed) Toluene
(Gas Recycle)
Toluene (liq. recycle)
2 ) NUMBER OF RECYCLE STREAMS
EXAMPLE Acetone Ketene + CH4 700C Ketene CO + 1/2C2H4 1atm Ketene + Acetic Acid Acetic Anhydride 80 C, 1atm
Component NBP , C Destination 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
R1 R2 Separation
Acetic Acid (feed)
Acetic Acid (recycle to R2)
Acetone (recycle to R1)
Acetone(feed)
CO , CH4 , C2H4
(By-product)
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)
if (a2 - a1) › (b2 - b1) then FEED2 excess
if (a2 - a1) ‹ (b2 - b1) then FEED1 excess
121221
1
2
1
2 bbFEED
aaFEED CC
k
k
r
r
( 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 +H2 Steam is usually used as the
C2H6 + H2 = 2CH4 diluent.
Example Styrene Production
EB = styrene +H2
EB benzene +C2H4 Steam 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
REACT ABSORB REACT ABSORB
H2O
H2SO4
H2O
H2SO4
SO2
O2 + N2
( 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 400C Use of stainless steel is severely
limited !
T 260C High pressure steam ( 40~50 bar) provides heat at 250-265 C
T 40C Cooling water Temp 25-30C
( 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 )
reactor separatorFT ( 1-X )
FT ( 1-X )
FTLEVEL 3
LEVEL 2
PDDiphenyl
Benzene , PB
Purge , PGRG
FFT
yPH
Toluene
H2 , CH4
FG , yFH
always valid for limiting reactant when there is complete recovery and
recycle of the limiting reactant
XFF FT
T
RECYCLE MATERIAL BALANCE ( Quick Estimates !!! )
Example HDA process
other reactant : (Next )
X
FMRRyFy FT
GPHGFH )(
molar ratio
extra design variable
GPH
FH
PH
FTG F
y
y
y
MR
X
FR
Note that details of separation system have not been specified at this level.
Therefore, we assume that reactants one recovered completely.
PHGH yRR 2
)1(4 PHGCH yRR
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,
GFHFT
PHG
PH
FH
PH
FTG
FTT
FyX
FMR
yF
y
y
y
MR
X
FR
XFF
1
,,0 FTFX $ R
,,0 GPH Ry $ C
EP3=EP2-annualized costs of reactors -annualized costs of compressors
0.2 0.4 0.6
PHy
0.1 0.3 0.5 0.7
$/year 0
2 106
1 106
-1 106
-2 106
does not include any separation or heating and cooling cost
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