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Chapter 4: Material Balances
MultipleUnit Processes
Recycle, Bypass, Purge Streams
Exam #1(Closed Book)
Wednesday, September 21, 2011
7:00  9:00 p.m.
PHSC Room 108
Review Session for Exam #1
Monday, September 19, 2011
6:00  8:00 p.m.
SEC N202A
Find the kg of recycle/kg fresh feed if the amount of waste (W) is 60 kg of A.
Basis: 60 kg of waste
Unknowns: F, P, R, G
F = P + W
F
=
P
+
60 kg
(Eq 1)
Plan:
Assumptions: No chemical rxn, steady state
Input  Output + Generation  Consumption = Accumulation
Input = Output
What system boundaries could we define?
Overall Mass Balance on Entire Process
Use Material Balance Equation
F
0.8 kg B
kg
=
P
0.95 kg B
kg
(Eq 2)
P
+
60 kg
(
)
0.8 kg B
kg
=
P
0.95 kg B
kg
(Eq 2)
48 kg B
=
P
0.15 kg B
kg
⇒
P = 320 kg, F = 380 kg
Substitute Eq 1 into Eq 2
Mass Balance on B for Entire Process
F + R = G
R
=
G  F = 506.7 kg  380 kg =126.7 kg
F
0.8 kg B
kg
=
G
0.60 kg B
kg
G =
380 kg
0.8 kg B
kg
0.60 kg B
kg
= 506.7 kg
R
F
=
126.7 kg
380 kg
=
0.333
kg recycle
kg feed
What’s another system
that we can define?
Overall Mass Balance at mixing point
Mass Balance on B for at mixing point
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Solving Recycle and Bypass Problems
The methods for solving recycle and bypass problems are basically the same.
In the steady state, there is no buildup or depletion of material within the system
or recycle stream of a properly designed and operated process.
Only three of these will be independent (the fourth is a linear combination).
In particular, when you write the balance around the entire process system,
terms describing the recycle/bypass stream do not appear; only the fresh feed
and the product are required.
Recycle
When solving, you can write balances (total material or component) around:
the entire process structure
the mixing point
the splitter
the processing unit (inside the recycle/bypass)
Stoichiometry
Objectives:
1. Write and balance chemical reaction equations
2. Calculate the stoichiometric quantities of reactants and
products given the chemical reaction.
3. Define excess reactant, limiting reactant, conversion, degree
of completion.
4. Know the products of common reactions given the reactants
We understand chemical reactions by keeping track of the number (not
mass) of molecules involved ….
.
Chemical Reactions
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 Fall '11
 STAFF
 Stoichiometry, Chemical reaction, kg, NH, mol heptane

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