hierarchy of decisions reactor separation system purge h 2, ch 4 benzene diphenyl h 2, ch 4 toluene...
TRANSCRIPT
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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
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ReactorSeparation
System
Purge
H2 , CH4
Benzene
Diphenyl
H2 , CH4
Toluene
LEVEL 2
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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 ?
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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
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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.
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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)
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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)
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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
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( 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
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( 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
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( 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
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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
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( 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
.
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( 3-4 ) RECYCLE BYPRODUCT
to shift equilibrium + product distribution
CH3
+ H2 + CH4
2 = + H2
O O
O O O
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( 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
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( 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 ! )
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Figure 2.5 Heat transfer to and from stirred tanks.
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Figure 2.5 Heat transfer to and from stirred tanks.
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Figure 2.5 Heat transfer to and from stirred tanks.
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Figure 2.5 Heat transfer to and from stirred tanks.
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Figure 2.6 Four possible arrangements for fixed-bed recators.
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Figure 2.6 Four possible arrangements for fixed-bed reactors.
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Figure 2.6 Four possible arrangements for fixed-bed recators.
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Figure 2.6 Four possible arrangements for fixed-bed reactors.
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( 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.
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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
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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
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5 ) COMPRESSOR DESIGN AND COST
Whenever a gas-recycle stream is present, we will need a gas-
recycle compressor.
Covered in “Unit Operation (I)”
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6 ) EQUILIBRIUM LIMITATIONS
7 ) REACTOR DESIGN AND COSTS
Covered in
“Reactor Design and Reaction Kinetics”
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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