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3.3.2. METHODS OF MATERIAL BALANCES
ON REACTIVE PROCESSES
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Material balances that involves chemical reactions
can be analyzed using three different methods:
Remember the seven general procedures of material
balances, i.e.,
1.
2.
3.
4.
5.
6.
7.
1. Molecular species balances method (the
approach always used for non-reactive
systems)
2. Atomic species balances method
3. Extent of reaction method
Each method leads to the same results, but any one
of them may be more convenient for a given
calculation. So it is important to become comfortable
w i t h a l l t h e t h r e e m e t h o d s .
Drawing the flowchart and labeling
Scaling up/down the flowchart
Choosing basis of calculation
Stating the problem
Putting all in one unit
Analyzing the degree of freedom (DOF)
Solving the problem
These procedures are also applied for balances on
reactive processes with the degree of freedom analysis
for reactive systems differing from the nonreactive ones.
1
2
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Independent Equations, Independent Species,
and Independent Chemical Reactions
Independent Equations & Independent Species…
EXAMPLE 1:
To carry out degree of freedom analysis for reactive systems,
it is must first to understand the concept of Independent
Equations, Independent Species, and Independent
Chemical Reactions.
Independent Equations & Independent Species
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4
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Independent Equations & Independent Species…
INDEPENDENT CHEMICAL REACTIONS
1. Chemical reactions are independent if the
stoichiometric equation of any one of them cannot be
obtained by adding and subtracting multiples of the
stoichiometric equations of the others.
EXAMPLE 2:
2. Applicable ONLY for molecular species balances and
extent of reaction methods.
EXAMPLE 1: Consider the reactions,
ANSWER:
5
6
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
INDEPENDENT CHEMICAL REACTIONS…
Molecular Species Balances Method
EXAMPLE 2:
• Must contain generation and/or consumption
terms
One generation or
consumption term
• DOF analysis in general is:
must either be
specified or calculated
for each independent
No. of unknown labeled variables
rxn, which is why each
rxn adds a degree of
freedom to the system.
+ No. of independent chemical reactions
− No. of independent molecular species balances
− No. of other equations relating the unknowns
ANSWERS:
= No. of Degree of Freedom
7
8
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Molecular Species Balances Method…
Example: Dehydrogenation of Ethane (C2H6
Molecular Species Balances Method…
SOLUTION: Dehydrogenation of Ethane (C2H6
C2H4 + H2)
9
C2H4 + H2)
10
5
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Atomic Species Balances Method…
Atomic Species Balances Method
Example: Dehydrogenation of Ethane (C2H6
DOF analysis in general is:
No. of unknown labeled variables
− No. of indep. atomic species balances
− No. molecular balances of indep. nonreactive species
− No. of other equations relating the unknowns
= No. of Degree of Freedom
C2H4 + H2)
Solution:
11
12
6
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Atomic Species Balances Method…
Atomic Species Balances Method…
Solution:
Solution:
(1)
13
14
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Extent of Reaction Method…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
Extent of Reaction Method
Example: Dehydrogenation of Ethane (C2H6
• Using expressions for flow rates (amounts) of
each species in terms of extent of reaction.
C2H4 + H2)
DOF analysis in general is:
No.of unknownlabeled variables
Solution:
+ No.of independent reactions(in termsof ξ )
− No.of ind. reactivespecies (in termsof ξ )
− No. molecularbalancesof indep.nonreactive species
− No.of other equationsrelating the unknowns
= No.of Degreeof Freedom
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
EXERCISE
SOLUTION:
17
18
9
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
SOLUTION:
SOLUTION:
19
20
10
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
SOLUTION:
SOLUTION:
21
22
11
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
SOLUTION:
SOLUTION:
23
24
12
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
SOLUTION:
SOLUTION:
25
26
13
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3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
3.3.2. MATERIAL BALANCES ON
REACTIVE PROCESSES…
SOLUTION:
SOLUTION:
27
28
14
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
Reactant Conversion…
Reactant Conversion
Two definitions of reactant conversion are used
in the analysis of chemical reactors with product
separation
and
recycle
of
unconsumed
reactants:
Overall Conversion:
REACTOR
SEP.
Overall Conversion =
reactant input to process − reactant output from process
reactant input to process
29
RECYCLE
30
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
Reactant Conversion…
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
Reactant Conversion…
Single-Pass Conversion =
reactant input to reactor − reactant output from reactor
reactant input to reactor
REACTOR
SEP.
RECYCLE
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32
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
Solution:
Example:
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34
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
Solution:
-1additional relation (n10 = 0.05n7)
Solution:
1
35
36
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
Solution:
Solution:
37
38
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
I N V O LV I N G R E C Y C L E
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
INVOLVING RECYCLE
Solution:
Solution:
39
40
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PURGE
A "purge" stream is a small stream bled off from a recycle loop to prevent
build-up of inerts or impurities in the system.
Often, the purge flow is so much smaller than the recycle flow that it can be
neglected in the steady state overall material balance.
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
INVOLVING RECYCLE & PURGE
PURGING
RECYCLE
PURGE
FEED
PROCESS
MIX
SPLIT
below
PRODUCT
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3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
INVOLVING RECYCLE & PURGE
PURGING…
3 . 3 . 3 . M AT E R I A L B A L A N C E S O N
REACTIVE PROCESSES
INVOLVING RECYCLE & PURGE
PURGING…
Fig.
43
44
22