Hierarchy of Decisions

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Hierarchy of Decisions
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Purge H2 , CH4
Reactor
Separation System
Benzene
H2 , CH4
Diphenyl
Toluene
LEVEL 2
3
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
700?C, 1atm Ketene Acetic Acid ? Acetic
Anhydride 80 ?C, 1atm
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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
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
Compressor
CH4 , H2 (Purge)
(Gas Recycle)
Benezene (PrimaryProduct)
Reactor
Separator
(Feed)H2 , CH4
(Feed) Toluene
Diphenyl (By-product)
Toluene (liq. recycle)
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2 ) NUMBER OF RECYCLE STREAMS EXAMPLE
Acetone ? Ketene CH4
700?C 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
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)
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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
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( 3 ) Multiple reactions in series producing
byproducts type (3) shift
product distribution ex. CH3
H2 ?
CH4 excess 51
2
H2 ( 4 ) Mixed parallel and series reactions
? byproducts shift
product distribution ex. CH4
Cl2 ? CH3Cl HCl Primary
excess 101 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
H2O
H2O
SO2
REACT
ABSORB
REACT
ABSORB
O2 N2
H2SO4
H2SO4
( 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 ? 400?C ? Use of stainless steel is
severely limited ! T ? 260?C ? High
pressure steam ( 4050 bar)
provides heat at 250-265 ?C T
? 40?C ? Cooling water Temp 25-30?C

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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 )
yPH
Purge , PG
RG
FG , yFH
Benzene , PB
H2 , CH4
reactor
separator
FT ( 1-X )
FFT
PD
FT
Diphenyl
Toluene
LEVEL 3
FT ( 1-X )
LEVEL 2
always valid for limiting reactant when there is
complete recovery and recycle of the limiting
reactant
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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.
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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,


EP3EP2-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