Hierarchy of Decisions 1. Batch ver ver sus continuous 2. Input-output Input-output struc tr ucture ture of the flowsheet flowsheet 3. Recycle structure of the flowsheet 4. General structure truct ure of t he separation separatio n system a. Vapor recovery reco very system b. Liquid recovery reco very system 5. Heat-exc Heat-exchang hanger er network 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 pr oper operating temperature and pressure ? 5 ) Is a gas compressor required ? costs ? 6 ) Which reactor model should be used ? 7 ) How do the reactor/compressor 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, 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 700C 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 type (3)
shift product distribution
A R (desired) and A S (waste) r R r S
k 1 k 2
a1 a 2
C A
if a1 › a2 keep CA high : high pressure, eliminate inerts, avoid recycle of products, use plug flow reactor
if a1 < a2 keep CA low : low pressure, add inerts, recycle of products, use CSTR
( 2 ) Multiple reactions in parallel producing byproducts type (3)
shift product distribution
A + B R (desired) and A + B S (waste) r R r S
k 1 k 2
a1 a2
C A
b1 b2
C B
if a1 › a2 and b1 › b2 keep CA & CB high
if a1 < a2 and b1 › b2 keep CA low, CB high
if a1 > a2 and b1 < b2 keep CA high, CB low
if a1 < a2 and b1 < b2 keep CA & CB low
( 3 ) Multiple reactions in series producing byproducts type (3)
shift product distribution CH3
ex.
O
+ H2 O excess 5:1 2O
O
+ CH4
O + H2
( 4 ) Mixed parallel and series reactions byproducts shift product distribution ex.
CH4 + Cl2 CH3Cl + HCl excess 10:1
Primary
CH3Cl + Cl2 CH2Cl2+ HCl CH2Cl2+ Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4
+ HCl
Secondary
( 3-2 ) FEED INERTS TO REACTOR ( 1b ) Single reversible reaction FEED
PROD1 + PROD2
Cinert Xfeed
keq =
FEED1 + FEED2
C p1C p2 CF
PRODUCT
Cinert Xfeed1 or Xfeed2
keq =
CP
CF1CF2 ( 2 ) Multiple reactions in parallel byproducts FEED1 + FEED2 PRODUCT FEED1 + FEED2
BYPRODUCT
Cinert C byproduct FEED1 + FEED2 PRODUCT FEED1
BYPROD1 + BYPROD2
Cinert C byprod1-2
Single reversible reaction A
B
+
C
Initial:
CA0
0
0
React:
CA0X
CA0X
CA0X
At equilibrium:
CA0(1-X)
CA0X
CA0X
C
n V
K eq
P RT
(C A0 X )(C A0 X ) C A0 (1 X )
C A0 X 2
(1 X )
Example: CA0=1, K eq = 4, then X = 0.828 If we increase the pressure (2 times) by reducing the volume of reactor V (2 times) OR increasing the number of moles n A0 (2 times), then CA0 = 2 CA0=2, K eq = 4, then X = 0.732 That is: P then X
A
B
+
C
Initial:
PA0
0
React:
PA0X
PA0X
PA0X
At equilibrium:
PA0(1-X)
PA0X
PA0X
C
n V
K eq
P RT
; Pt ( n) RT
( P A0 X )( PA0 X ) P A0 (1 X )
0
PA0 X 2
(1 X )
Example: Pt = PA0=1, K eq = 4, then X = 0.828 If we maintain the same initial pressure: P t = 1, but use inert with molar ratio n I n A 0
then PA0 = yA0*Pt = 0.5*1 = 0.5 PA0=0.5, K eq = 4, then X = 0.9 That is: P then X
1
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 + N2
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 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
* 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, F feed ) Q R = ( Heat of Reaction ) ( Fresh Feed Rate ) ……..for single reaction. ……..for HDA process ( approximation )
Adiabatic Temp. Change = T R, in - T R, out = Q R / FC P 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, ( T R, in - T R, 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
R G
Purge , PG
FG , yFH H2 , CH4 FFT Toluene
Benzene , PB reactor
FT ( 1-X )
FT FT ( 1-X )
separator Diphenyl
LEVEL 3 LEVEL 2
always valid for limiting reactant when there is complete recovery and recycle of the limiting reactant
F T
F FT
X
PD
RECYCLE MATERIAL BALANCE ( Quick Estimates !!! ) Example
HDA process
other reactant : (Next ) y FH F G
y PH RG
molar ratio
( MR)
F FT X
extra design variable
RG
F FT MR
X y PH
R H
2
RCH 4
y FH y PH
F G
RG y PH
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 gasrecycle compressor.
Covered in “Unit Operation (I)”
6 ) EQUILIBRIUM LIMITATIONS 7 ) REACTOR DESIGN AND COSTS
Covered in “Reactor Design and Reaction Kinetics ”
