# Material Balance Calculations

As suggested in the previous lesson, once the degrees of freedom
analysis yields a value of zero, you have a roadmap for solving the
material balance equation.

IMPORTANT

The equations suggested by the Degrees of Freedom
analysis (# of species = # of balance equations; # of process
specifications; # of physical constraints) MUST be the equations
used to solve the material balance!

### General Procedure for (single unit) Material Balances

STEP 1

Choose a basis of calculation from among the
given data. That is, choose one of the given stream amounts
(batch) or flowrates (continuous or semi-batch) OR (if no stream
amounts or flowrates are given) choose something convenient (like
100 mol(/s) or 100 g(/s)).

IMPORTANT

Its called conservation of MASS not VOLUME, do NOT
choose a volume as your basis!

STEP 2

Draw the flowchart. (Follow the procedure
previously outlined.)

IMPORTANT

Remember, to fully specify a stream, you must know
BOTH the total amount of the flow within the stream and the
composition of the stream (label what you DON'T know with
variables). (Alternatively, you can know the flow of each component
separately.)

STEP 3

Do the degrees of freedom analysis. (Follow
the procedure previously outlined.) Can you match the number of
unknowns with the number of equations? (i.e., is the DofF = 0?)

IMPORTANT

Remember, if they DO NOT match, you may have a
problem! If there are too few equations, you CAN'T solve it. If
there are too many, you MAY solve the problem, but then you must
CHECK THE ANSWER against your additional equations (the problem may
be overspecified).

STEP 4

If DofF is zero then simply do the algebra
for each of the equations suggested by the DofF!

OUTCOME:

Do the calculations suggested in the
degree-of-freedom analysis