Page 428 - Modelling in Transport Phenomena A Conceptual Approach

408 CHAPTER 9. STEADY MICROSCOPIC BALANCES WITH GEN.

where M is the molecular weight of the reacting mixture. Substitution of Q.
(9.5150) into Eq. (9.5149) gives

TiRT
Q=- (9.5-151)
P
Therefore, Q remains constant when n and P do not change along the reactor.
The conditions for the constancy of Q are summarized in Table 9.4.

'Pable 9.4 Requirements for the constant volumetric flow rate for a plug flow
reactor operating under steady and isothermal conditions.
Liquid Phase Reactions Gas Phase Reactions
W No change in the total number of moles
Constant total mass density during reaction (5 = o)

H Negligible pressure drop across the reactor

When Q is constant, Eq. (9.5148) can be rearranged as

(9.5152)

Depending on the values of n the results are

n=O

n=l (9.5-153)

Q 1

9.5.3.2 Variable volumetric flow rate

When the volumetric flow rate is not constant, integration of Q. (9.5148) is
possible only after expressing both ni and Q in terms of 2. The following example
explains the procedure in detail.

Example 9.14 The irreversible gas phase reaction
A+B+C