Page 146 - Elements of Chemical Reaction Engineering 3rd Edition

118 Rate Laws and Stoichiometry Chap. 3

(c) For Problem 3-7(b) write the combined PFR mole balance on each spe-
cies and rate law solely in terms of the molar flow rates and rate law
parameters.
(d) For Problem 3-7(c), write the combined PFR mole balance and rate law
solely in terms of the molar flow rates for a PFR.
P3-10B For each of the following reactions and rate laws at low temperatures, suggest
a rate law at high temperatures. The reactions are highly exothermic and
therefore reversible at high temperatures.
(a) The reaction
A-B
is irreversible at low temperatures, and the rate law is
-rA = kCA
(b) The reaction
Af2B __j 2D
is irreversible at low temperatures and the rate law is
-rA = kcPCB

(c) The catalytic reaction
A+B ---+ C+D
is irreversible at low temperatures and the rate law is

In each case, make sure that the rate laws at high temperatures are ther-
modynamically consistent at equilibrium (cf. Appendix C).
P3-llB There were 820 million pounds of phthalic anhydride produced in the United
States in 1995. One of the end uses of phthalic anhydride is in the fiberglass
of sailboat hulls. Phthalic anhydride can be produced by the partial oxidation of
naphthalene in either a fixed or a fluidized catalytic bed. A flowsheet for the com-
mercial process is shown in Figure P3-11. Here the reaction is carried out in a
fixed-bed reactor with a vanadium pentoxide catalyst packed in 25-mm-diameter
tubes. A production rate of 3 1,000 tons per year would require 15,000 tubes.
Set up a stoichiometric table for this reaction for an initial mixture of
3.5% naphthalene and 96.5% air (mol %), and use this table to develop the
relations listed below. Po = 10 atm and To = 500 K.
(a) For an isothermal flow reactor in which there is no pressure drop, deter-
mine each of the following as a function of the conversion of naphtha-
lene, XN.
(1) The partial pressures of 0, and CO, (Am: Pcoz = 0.345 l5.8 - 912
X]/(1 - 0.0175 X))
(2) The concentrations of 0, and naphthalene (Ans.: C, = 0.084 (1 -
X)/(1 - 0.0175 X))
(3) The volumetric flow rate u
(b) Repeat part (a) when a pressure drop occurs in the reactor.
(c) If the reaction just happened to be first order in oxygen and second order
in naphthalene with a value of kN of 0.01 dm6/mo12 . s, write an equation
for -rN solely as a function of conversion for parts (a) and (b).