General
proce
: edure for Single-Unit Process Material Balance Calculations
The problem-solving approach introduced in the previous sections and several additional pro-
cedural suggestions are summarized below. Given a description of a process. The values of sev-
eral process variables. And a list of quantities to be determined
1.
Choose as a basis of calculation an amount orjlow rate of one of the pro cess streams.
* If an amount or flow rate of a stream is given in the problem statement. It is usually
convenient to use it as the basis of calculation. Subsequently calculated quantities will
then be correctly scaled.
* If several stream amounts or now rates are given. Always use them collectively as the
basis.
* If no stream amount or flow rate is specified in the problem statement. Take as a basis
an arbitrary amount or flow rate of a stream with a known composition(e. g-. 100 kg or
100 kg/h if all mass fractions are known. or 100 mol or 100 mol/h if all mole fractions
are known).
2. Draw a Jlowchart and Jill in all known uariable ualues, including the basis of calcula-
tion Then label unknown stream uariables on the chart.
· The flowchart is cotnpletely labeled if volt can express the mass or mass flow rale (or
moles or motar flow rale) of e[i ch co
Labeled variables for each process stream should therefore include eilfzer
(a) the total mass le. g.. Ml(kg)] or mass flow rate llhl(kg/s)] and the mass fractions of
all stream components[e.g.. vcH+ (kg CH 4 kg)]. or
(b) the total moles le. g., n](kmol)] or motar now rate [nl(kmol/s)] and the mole frac-
tions of all stream components[e.g.. vcH. (kmol CH+ kmol)]. or
(c) for each stream component. The mass [e.g.. NtH (kg H.)]. Mass now rate [lhH
(kg SO,/s)]. Moles [nco(kmol CO)]. or motar now rate [nco(kmol CO/s)] ·
* I f you are given(or you can easily determine) either the amount or flow rate or any of
the component fractions for a stream. Iabel the total stream quantity or flow rate and
the component fractions (categories (a) and (b) in the preceding list). If you only
know the species present but have no quantitative information. Iabel component quan-
tities or flow rates (category (c) in the preceding list). Any labeling system will work
for any stream. But the algebra tends to be simpler if you follow these rules of thumb.
· Trv to incorporate given relationships between unknown quantities in the labeling. For
example. If you know that the motar flow rate of Stream 2 is double that of Stream 1.
label the flow rates n, and 2/l, rather than n, and 1l2·
· Ltrbel t· olumetric quantities only if they are gicen in the problem statement or yott(ire
asked to calculate filem. You will write mass or mole balances. But not volume balances.