Hierachy of desicion

8/15/16

LEVEL I Decision: Batch vs. Continuous

Hierarchy of decisions

Favor batch operation, if

1. Batch versus continuous
2. Input-output structure of the flowsheet

1. Production rate

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. 4
Ch.5

a ) less than 10×10 6 lb/yr (sometimes)
b ) less than

1×10 6

lb/yr (usually)

c ) multi-product plants
2. Market force

a ) seasonal production

Ch.6, Ch.7, Ch.16

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

Heuristics:
Ch. 4
Ch.5

Ch.6, Ch.7, Ch.16

Recover more than 99% of all valuable materials.
assume

Completely recover and recycle all
valuable reactants

LEVEL 2 DECISIONS:
DECISIONS FOR THE INPUT/OUTPUT STRUCTURE

2 ) Should we remove or recycle a reversible by-product?

• Flowsheet Alternatives

3 ) Should we use a gas recycle and purge stream?

(1)
Feed streams

4 ) Should we not bother to recover and recycle some reactants?
Products
by-products
no reactants

Process

(2)
Feed streams

1 ) Should we purify the feed streams before they enter the process?

Purge
Process

Products
By-Products

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?
Products
•
•
•
•
&
PROCESS
•
•
Feeds
By products

reasons:
a. inexpensive reactants, e.g. Air, Water.
b. gaseous reactants + (inert gaseous feed impurity or inert gaseous
reaction by-product)

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OR

Feeds

•
•
•

PROCESS

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•
•
•

Purge
Products
&
By products

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8/15/16

1 ) Purification of Feeds (Liquid/Vapor)
Heat

Compressor

H2, CH4

Purge
H2

Heat

CH4

Reactor

3 ) If a feed impurity is catalyst poison, remove it.

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.

Toluene
Heat

Coolant

Heat

Toluene

Flash

500 psia
Benzene
Recycle

4 ) If a feed impurity is present in a gas feed, as a first guess, process the
impurity.

95°F

1150° ~ 1300°

2 ) If a feed impurity is present in large amount, remove it.

H2, CH4

Product

1 ) If a feed impurity is not inert and is present in significant quantities,
remove it.

Dipheny1

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.

Toluene

A HIERARCHICAL APPROACH

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 byproduct 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.

Toluene + H2 ® Benzene + CH4
2 Benzene
Diphenyl + H2
1150 ° F ～ 1300 ° F
500 psia

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 ) 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.

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
should be
By-products to waste treatment

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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8/15/16

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

5

A + B to waste ①

H2 , CH4
1

D + E to fuel stream # 1 ②
F

to primary product ③
(storage for sale)

H2
CH4
Benzene
Toluene
Diphenyl

Recycle and Purge
Recycle and Purge
Primary Product
Recycle
Fuel

Process

1

H2

CH 4
Benzene
Toluene
Diphenyl
Temperature
Pressure

FH2
FM
0
0
0
100
550

Benzene
Diphenyl

4

2

3

0
0
0
P B/S
0
100

15

0
0
PB
0
0
100
15

①

where S = 1 - 0.0036/(1 -x) 1.544

②
③

FIGURE 5.2-1
Stream table

4

5

0
FE
0
F M + P B/S
0
0

0
0
P B(1 - S)/(2S) 0
100
100
15
465

F H2 = F E + P B(1 + S)/2S

F M = (1 - y FH)[F E + P B(1 + S)/S]/ y FH

① Purge : H2 , CH4
H2 , CH4
Toluene

Process

2

Production rate = 265
Design variables: F E and x
Component

I to valuable by-product (storage for sale) ④
J to fuel stream # 2 ⑤

EXAMPLE
b.p.
-253°C

-161
80
111
253

Toluene

Purge
H2 , CH4
3

F G = F H2 + F E

.

② Benzene
③ Diphenyl

REACTOR PERFORMANCE
Alternatives for the HDA Process
1. Purify the H2 feed stream.
2. Recycle diphenyl
3. Purify H2 recycle stream.

STOICHIOMETRIC FACTOR
(SF)
The stoichiometric moles of reactant
required per mole of product

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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

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

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8/15/16

Gas recycle

Purge
H2 , CH4

Toluene 1 x
Benzene Sx

H2 , CH4

1
2

Diphenyl (1 S) x

Reactor
system

Toluene

POSSIBLE DESIGN VARIABLES FOR LEVEL 2
Benzene

Sx

Separation
system

x

Dipheny1

1
(1
2

1 x

For complex reactions:
Reactor conversion (x), reaction temperature (T) and pressure (P).

S)x

If excess reactants are used, due to reactant not recovered or gas recycle and purge, then
the excess amount is another design variable.

Toluene recycle

Material Balance of the Limiting
Reactant (Toluene)
Assumption: completely recover and recycle the limiting reactant.

EXAMPLE

PROCEDURES FOR DEVELOPING OVERALL
MATERIAL BALANCE
1 ) Start with the specified production rate.

relation
known

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)

SS( x )
= selectivity = given
PPBB ( mol/hr ) = production rate of Benzene =given
ÚFFT ( mol/hr ) = toluene feed to process ( limiting reactant ) = PB /S
PR , CH 4
= methane produced in reaction = FFT = PB /S
ÚPD

design
variable

= diphenyl produced in reaction = FFT (1 - S /2 ) = (PB /S)(1 - S /2 )
Let FFEE = excess amount of H2 in purge stream= PH 2
FE +

Ú

( PB /S ) - [( PB /S )( 1 - S )/2]

purge rate
of H2

= yFH FG

disapp. in reaction

FH 2

where
FG = make-up gas stream flowrate (unknown)
yFH =

5 ) Calculate the inlet and outlet flows for the impurities entering with the reactant streams
in Step 4).

mole fraction of H2 in FG
( known )

Let PCH 4 = purge rate of CH4

methane in purge stream

( 1 - yFH ) FG + PB /S = PCH 4

Ú

Normally, it is possible to develop expressions for overall MB in terms of design

variables without considering recycle flows.

Benzene , PB
Diphenyl , PD

Process

design variable

given

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).

Purge ; H2 , CH4 , PG

FG , H2 , CH4
FFT , Toluene

methane product in reaction

methane in feed

Ú PG = total purge rate = PH2 + PCH4 = FE + (1 - yFH) FG + PB/S
= FG + ( PB/S )[( 1 - S )/2]

Known : yFH
PB
Design Variables :

y
x, PH

Define
yPH =

purge composition of H2 = PH2/PG = FE/PG

S(x)

P B/S

F FT(1-S)/2

FFT

PD
PB[1-(1- yPH)(1-S)/2
S( yFH - yPH)

design
variable

It can be derined that
FG =
Known :

FCH4

PB [ 1- (1- yPH)(1-S)/2 ]

S (yFH - yPH)

F CH4+P B/S

S (x)

x

PB
PCH4

FCH4+PB/S

FN2+FCH4

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FG

FG
F G+(P B/S)(1-S)/2

6 ) ECONOMIC POTENTIAL AT LEVEL 2
EP2 = Annual profit if capital costs and utility costs are excluded

FFT

[(1- yFH)/ yFH]FH2
FE+[PB(1+S)/2S]
FCH4

FH2
PD

PCH4+FE
PG

PB/S

(PB/S)[(1-S)/2]

FE

PG

PCH4

design variable

Design Variable :

yFH

1- yPH
yPH F H2

F E+P B(1+S)/2S
FH2
FE
(PH2) P G yPH

= Product Value + By-product Value - Raw-Material Costs
[EXAMPLE] HDA process
4 ´10^6
2 ´10^6
$/yr
-2 ´10^6
-4 ´10^6

yPH
0.1

0.3

Ỵ

0.5

0.1

/>
0.1
0.7
0.9

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8/15/16

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

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