Page 125 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 3.3 Stoichiometric Tabie 97

I
which is identical to the concentration expression for a variable-volume batch
reactor.

Similarly, substitutiyg E and the appropriate 0’s into different concentra-
tion expressions for a flow system gives the same concentration expressions as
those in Table 3-61. for a variable-volume batch reaction in the gas phase.
One of the imajor objectives of this chapter is to learn how to express any
given rate law as a function of conversion. The schematic diagram in Fig-
ure 3-3 helps to summarize our discussion on this point. The concentration of
the key reactant, .A (the basis of our calculations), is expressed as a function of
conversion in both flow and batch systems, for various conditions of tempera-
ture, pressure, and volume.

Example 3-7 Anetermining = hi (X) for a Gas-Phase Reaction
A mixture of 28% SO, and 72% air is charged to a flow reactor in which SO, is
oxidized.
2s0, + 0, _j 2s0,
First, set up a stoichiometric table using only the symbols (i.e., e,, F,) and then
prepare a second stoichiometric table evaluating numerically as many symbols as
possible for the case when the total pressure is 1485 kPa and the temperature is con-
stant at 227°C.

Solution
Taking SO, as the basis of calculation, we divide the reaction through by the ,sto-
ichiometric coefficient of our chosen basis of calculation:
so,+;o, ___) so,

The initial stoichiometric table is given as Table E3-7.1. Initially, 72% of the total
number of moles is air containing 21% 0, and 79% N, .

FAO = (o.28) (FTO)
FBo = (0.72) (0.21) (FTO)

To write concentration in terms of conversion, we must express the volumetric flow
rate as a function of conversion.