Chapter 4
Material Balances
Section 4.1—Process Classification
• Three different types of processes:
– Batch
• Material is fed in, process occurs, material is removed
– Continuous
• Constant flow of material (most common)
– Semibatch
• Just doesn’t fit in
• Processes can also be run assuming steady state
or transient conditions
– Steady state has no change in any operation
parameter
Section 4.2—Balances
• For all non-nuclear processes, the statement of conservation of
matter (total mass) will hold and is expressed in words as follows:
• General Mass Balance
– Input + Generation - Output - Consumption = Accumulation
• For all nonreactive processes (no chemical reaction):
– Accumulation = Input – Output
• For all nonreactive processes at steady state:
– Accumulation = 0
– Input = Output
Example
• A pipe containing a
hot sodium chloride
solution (25% by
mass) passes through
a heat exchanger with
a flow rate of 90
kg/min. There are
two output streams
one of which has a
flow rate of 35
kg/min.
Example #2
• One thousand kilograms/hr
of a 50%/50% mixture of
benzene and toluene are
fed into a distillation
column which separates it
into two fractions. The
mass flow rate of benzene
in the top stream is 450
kg/hr and that of toluene in
the bottom stream is 475
kg/hr. Determine the
unknown flow rates
assuming steady state.
Species Mass Balance (SMB)
• A species mass balance is basically the
application of the law of conservation of mass
with respect to a single component in an input
or output stream
– Input = Output; as applied to a single component
(species)
Formulation of the Problem
1. Draw a picture of the system. Show all streams
entering and leaving the system. Label all streams
with symbols. Where possible, put given values on
the appropriate variable.
*Define the particular system you are going to evaluate. May have to
repeat this step multiple times to finish entire problem.
2. To perform the material balance calculations, the
following info is necessary:
a)
b)
c)
Mass of all stream entering and leaving system
Composition of all streams
Chemical reactions involved
Formulation of the Problem
3. Determine the number of knowns and
unknowns. Generate a set of linearly independent
algebraic equations equal in number to the number
of unknowns.
4. Set of algebraic equations is generated by
applying statement of conservation of mass to the
problem.
5. Use algebra to get answer
Example
• A distillation column for the separation of a
mixture of methane, ethane, and propane is
constructed to handle a feed stream of 100
kg/h of composition 80 weight % CH4, 15
weight % C2H6, and the balance propane. Find
the flow rate and composition for each
stream.