Separations analysis fundamentals - Principles of Chemical Separations with Environmental Applications - page 36

(d) moles of a chemical compound (chemical reactions are described in terms of moles,
not mass);
(e) mass of an atomic species (carbon balance, for example);
(f ) moles of an atomic species (appropriate if there are chemical reactions).
A mass balance does not apply directly to volume. If the densities of the materials
entering into each term are not the same, or mixing effects occur, then the volumes of the
material will not balance. Also, chemical reactions can result in a change in the number
of moles in the system (H 2 plus O 2 reacting to form H 2 O is one example). For gas-phase
reactions in which pressure and temperature remain constant, the volume can change (refer
to the ideal gas law). Non-constant volumes can also apply to liquid-phase systems. For
example, one classic mixing experiment is to mix equal volumes of alcohol and water.
The resulting solution does not have twice the volume.
There are assumptions which eliminate certain terms in a mass balance.
1 Steady state. This assumption implies that there is no change with respect to time within
the C.V. There may be changes with position. The result of this assumption is that the
accumulation term is zero.
2 No chemical reactions occur within the control volume, therefore there is no generation.
Material balances can be either microscopic or macroscopic. Which to use is primarily
dictated by the type of information desired. Figure 3.2 illustrates a hybrid process contain-
ing a distillation column and a membrane. The symbols indicate the different flowrates in
the system. This process will be used to illustrate the two types of balances.

Condenser

D

P

F 1 F 2

Membrane

R

Heat er

B

Figure 3.2 Hybrid process: a distillation column and a membrane.