Page 205 - Modelling in Transport Phenomena A Conceptual Approach

7.2. CONSERVATION OF CHEMICAL SPECIES 185

7.2 CONSERVATION OF CHEMICAL
SPECIES

The conservation statement for the mass of the ith chemical species is given by

Rate of mass ) - ( Rate of mass ) + ( Rate of generation
of i in of i out of mass i
= ( Rate of accumulation ) (7.2-1)
of mass i
For a system with a single inlet and a single outlet stream as shown in Figure 7.1,
Eq. (7.2-1) can be expressed as

Figure 7.1 Unsteady-state flow system exchanging mass with the surroundings.

The interphase mass transfer rate, (hi)int, is considered positive when mass is
added to the system and is expressed by

Substitution of Eq. (7.2-3) into Eq. (7.2-2) gives

In terms of molar basis, Eqs. (7.2-2) and (7.2-4) take the form
4n.l
z sys
(&)in - (&)out * (ki)int + vsys cyij Tj = - dt (7.2- 5)
j