無投影片標題 - National Cheng Kung University
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Transcript 無投影片標題 - National Cheng Kung University
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
LEVEL I Decision: Batch vs. Continuous
Favor batch operation, if
1. Production rate
a ) less than 10×106 lb/yr (sometimes)
b ) less than
1×106
lb/yr (usually)
c ) multi-product plants
2. Market force
a ) seasonal production
b) short production lifetime
3. Scale-up problems
a ) very long reaction times
b ) handling slurries at low flow rates
c ) rapidly fouling materials.
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
Heuristics:
Recover more than 99% of all valuable materials.
assume
Completely recover and recycle all
valuable reactants
DECISIONS FOR THE INPUT/OUTPUT STRUCTURE
Flowsheet Alternatives
(1)
Feed streams
(2)
Feed streams
Products
by-products
no reactants
Process
Purge
Process
Products
By-Products
reasons:
a. inexpensive reactants, e.g. Air, Water.
b. gaseous reactants + (inert gaseous feed impurity or inert gaseous
reaction by-product)
LEVEL 2 DECISIONS:
1 ) Should we purify the feed streams before they enter the process?
2 ) Should we remove or recycle a reversible by-product?
3 ) Should we use a gas recycle and purge stream?
4 ) Should we not bother to recover and recycle some reactants?
5 ) How many product streams will there be?
6 ) What are the design variables for the input/output structure?
What economic trade-offs are associated with these variables?
Feeds
PROCESS
Products
&
By products
OR
Feeds
PROCESS
Purge
Products
&
By products
1 ) Purification of Feeds (Liquid/Vapor)
1 ) If a feed impurity is not inert and is present in significant quantities,
remove it.
2 ) If a feed impurity is present in large amount, remove it.
3 ) If a feed impurity is catalyst poison, remove it.
4 ) If a feed impurity is present in a gas feed, as a first guess, process the
impurity.
5 ) If a feed impurity is present as an azeotrope with a reactant, often it is
better to process the impurity.
6 ) If a feed impurity is inert, but it is easier to separate from the product than
the feed, it is better to process the impurity.
7 ) If a feed impurity in a liquid feed stream is also a byproduct or a product
component, usually it is better to feed the process through the separation
system.
Heat
Compressor
H2, CH4
Heat
Purge
H2
CH4
1150 ~1300
Reactor
Toluene
Coolant
Heat
Benzene
Product
Toluene
Flash
500 psia
Recycle
Heat
95F
Dipheny1
Toluene
H2, CH4
3 ) Gas Recycle and Purge
“Light” reactant
“Light” feed impurity, or
“Light” by-product produced by a reaction
Whenever a light reactant and either a light feed impurity or a light by-
product boil lower than propylene (-55ºF), use a gas recycle and purge
stream.
Lower boiling components normally cannot be condensed at high pressure
with cooling water.
A HIERARCHICAL APPROACH
Toluene + H2 Benzene + CH4
2 Benzene
Diphenyl + H2
1150 F ～ 1300 F
500 psia
4 ) Do not recover and recycle some reactants which are
inexpensive, e. g. air and H2O.
We could try to make them reacted completely, but often we feed them as an excess
to try to force some more valuable reactant to completion.
5 ) Number of Product 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.
EXAMPLE
b.p.
A
B
C
D
E
F
G
H
I
J
Waste
Waste
Recycle
Fuel
Fuel
Primary product
Recycle
Recycle
Valuable By-product
Fuel
A + B to waste
D + E to fuel stream # 1
F
to primary product
(storage for sale)
I
to valuable by-product (storage for sale)
J to fuel stream # 2
EXAMPLE
b.p.
-253C
-161
80
111
253
H2
CH4
Benzene
Toluene
Diphenyl
Recycle and Purge
Recycle and Purge
Primary Product
Recycle
Fuel
Purge : H , CH
2
4
H2 , CH4
Toluene
Process
Benzene
Diphenyl
5
H2 , CH4
1
Purge
H2 , CH4
3
Process
2
4
Toluene
Benzene
Diphenyl
Production rate = 265
Design variables: FE and x
Component
1
2
H2
FH2
0
CH4
FM
0
Benzene
0
0
Toluene
0
PB/S
Diphenyl
0
0
Temperature
100
100
Pressure
550
15
where S = 1 - 0.0036/(1 -x)1.544
3
0
Stream table
.
0
5
FE
0
0
FM + PB/S
PB
0
0
0
0
0
0
PB(1 - S)/(2S) 0
100
100
100
15
15
465
FH2 = FE + PB(1 + S)/2S
FM = (1 - yFH)[FE + PB(1 + S)/S]/ yFH
FIGURE 5.2-1
4
FG = FH2 + FE
Alternatives for the HDA Process
1. Purify the H2 feed stream.
2. Recycle diphenyl
3. Purify H2 recycle stream.
REACTOR PERFORMANCE
Conversion (x)
= (reactant consumed in the reactor)/(reactant
fed to the reactor)
Selectivity (S)
=[(desired product produced)/(reactant
consumed in the reactor)]*SF
Reactor Yield (Y)
=[(desired product produced)/(reactant fed to
the reactor)]*SF
STOICHIOMETRIC FACTOR
(SF)
The stoichiometric moles of reactant
required per mole of product
Material Balance of Limiting Reactant in Reactor
Toluene
unconverted
(1-x) mole
Toluene
feed
(1 mole)
Toluene
converted
x mole
recycle
Benzene
produced
Sx mole
Diphenyl
produced
(1-S)x / 2
Gas recycle
Purge
H2 , CH4
Toluene 1 x
Benzene
Sx
1
2
Benzene
Diphenyl (1 S) x
H2 , CH4
Reactor
system
Toluene
Separation
system
Sx
Dipheny1
x
1 x
Toluene recycle
Material Balance of the Limiting
Reactant (Toluene)
Assumption: completely recover and recycle the limiting reactant.
1
(1 S ) x
2
POSSIBLE DESIGN VARIABLES FOR LEVEL 2
For complex reactions:
Reactor conversion (x), reaction temperature (T) and pressure (P).
If excess reactants are used, due to reactant not recovered or gas recycle and purge, then
the excess amount is another design variable.
PROCEDURES FOR DEVELOPING OVERALL
MATERIAL BALANCE
1 ) Start with the specified production rate.
2 ) From the stoichiometry (and, for complex reactions, the correlation for product
distribution) find the by-product flows and the reactant requirements (in terms of the
design variables).
3 ) Calculate the impurity inlet and outlet flows for the feed streams where the reactant are
completely recovered/recycled.
4 ) Calculate the outlet flows of reactants in terms of a specific amount of excess for streams
where reactants are not recovered and recycled (recycle and purge, or air, or H2O)
5 ) Calculate the inlet and outlet flows for the impurities entering with the reactant streams
in Step 4).
Normally, it is possible to develop expressions for overall MB in terms of design
variables without considering recycle flows.
EXAMPLE
Purge ; H2 , CH4 , PG
FG , H2 , CH4
FFT , Toluene
relation
known
Benzene , PB
Diphenyl , PD
Process
design variable
given
SS( x )
= selectivity = given
PBB( mol/hr ) = production rate of Benzene =given
FFT( mol/hr ) = toluene feed to process ( limiting reactant ) = PB/S
PR , CH4
= methane produced in reaction = FFT = PB/S
PD
design
variable
= diphenyl produced in reaction = FFT (1 - S/2) = (PB/S)(1 - S/2)
Let FFEE = excess amount of H2 in purge stream= PH2
FE +
( PB/S ) - [( PB/S )( 1 - S )/2]
purge rate disapp. in reaction
y FF
= yFH
FH GG
of H2
where
FG = make-up gas stream flowrate (unknown)
y
FH =
mole fraction of H2 in FG
( known )
Let PCH4 = purge rate of CH4
methane in purge stream
( 1 - yFH ) FG + PB/S = PCH4
methane in feed
methane product in reaction
FH2
PG = total purge rate = PH2 + PCH4 = FE + (1 - yFH) FG + PB/S
= FG + ( PB/S )[( 1 - S )/2]
Define
y
PH =
purge composition of H2 = PH2/PG = FE/PG
design
variable
It can be derined that
PB [ 1- (1- yPH)(1-S)/2 ]
FG =
Known :
S (yFH - yPH)
design variable
Design Variable :
y
S (x)
x
FH
PB
PB/S
(PB/S)[(1-S)/2]
FE
PCH4
FCH4+PB/S
[(1- yFH)/ yFH]FH2
FCH4
FH2
PCH4+FE
PG
FN2+FCH4
FFT
FG
FE+[PB(1+S)/2S]
PD
Known : yFH
PB
Design Variables :
x, yPH
S(x)
PB/S
FFT
FFT(1-S)/2
PD
PB[1-(1- yPH)(1-S)/2
S(yFH - yPH)
FCH4
FCH4+PB/S
FE+PB(1+S)/2S
1- y
PH
y
PH
FH2
FH2
FE
(PH2)
PG yPH
PG
FG
FG+(PB/S)(1-S)/2
PCH4
6 ) ECONOMIC POTENTIAL AT LEVEL 2
EP2 = Annual profit if capital costs and utility costs are excluded
= Product Value + By-product Value - Raw-Material Costs
[EXAMPLE] HDA process
4 10^6
2 10^6
$/yr
-2 10^6
-4 10^6
y
0.1
0.3 0.5
PH
0.1
0.1
0.7
0.9
Douglas, J. M., “Process Synthesis for Waste
Minimization.” Ind. Eng. Chem. Res., 1992, 31, 238-243
If we produce waste by-products, then we have negative byproduct values.
Solid waste : land fill cost / lb
Contaminated waste water :
- sewer charge : $ / 1000 gal. (e.g. $0.2 / 1000 gal)
- waste treatment charge :
$ / lb BOD lb BOD / lb organic compound (e.g. $0.25 /lb BOD)
Solid or liquid waste to be incinerated :
$ 0.65 / lb
BOD - biological oxygen demand