Page 239 - Elements of Chemical Reaction Engineering 3rd Edition

n orthoxylene, metaxylene, and paraxylene in a reactor containing 1000 ft3 of

21 1
Questions and Problems
Chap. 4
P4-14, A total of 2500 gal/h of metaxylene IS being isomerized to a rnixrure of

catalyqt. The reaction is being carried out at 750°F and 300 psig. Under these
conditions, 37% of the metaxylene fed to the reactor is isomerized. At a flow
rate of 1667 galih, 50% of the metaxylene 15 isomerized at the Same temper-
ature and pressure. Energy changes are negligible.
It is now proposed that a second plant be built to process 5500 &:an of
metaxylene at the same temperature and pressure as described above. What size
reactor (i.e., what volume of catalyst) is required if conversion in the new plant
is to be 46% instead of 37%? Justify any assumptions made for the scale-up
calculation. (Am.: 293 1 ft3 of catalyst.) (From California Professional Engi-
6
neers Exam.) Make a list of the things you learned from this problem.
P4-15;A It is desired to carry out the gaseous reaction A --+ B in an existing
tubular reactor consisting of 50 parallel tubes 40 ft long with a 0.75-in. inside
diameter. Bench-scale experiments have given the reaction rate constaint for
this first-order reaction as 0.00152 s-I at 200°F and 0.0740 s-l at 3001°F. At
what temperature should the reactor be operated to give a conversion of A of
80% with a feed rate of 500 lbih of pure A and an operating pressure of
100 psig? A has a molecular weight of 73. Departures from perfect gas behav-
ior may be neglected, and the reverse reaction is insignificant at these condi-
tions. (14ns.: T = 275°F.) (From California Professional Engineers Exam.)
P4-14, An isothermal, constant-pressure plug-$ow reactor is designed to give a con-
version of 63.2% of A to B for the first-order gas-phase decomposition
k
A----+B
for a feed of pure A at a rate of 5 ft3/h. At the chosen operating temperature,
the first-order rate constant k = 5.0 h-l. However, after the reactor is installed
and in operation, it is found that conversion is 92.7% of the desired conver-
sion. This discrepancy is thought to be due to a flow disturbance in the reactor
that gives rise to a zone of intense backmixing. Assuming that this zone
behaves like a perfectly mixed stirred-tank reactor in series and in between
two plug-flow reactors, what fraction of the total reactor volume is occiupied
by this zone? (Ans.: 57%.)
P4-178, Currently, the herbicide atrozine found in the Des Plaines River is being
treated by passing part of the river arough a marsh, where it is degraded (Fig-
ure P4-17). The rate of degradation of atrazine, A, is assumed irreversible and
to follow first-order homogeneous kinetics.

A
ki f products
@ As the wastewater flows and reacts, it also evaporates at a constant rate (Q =
kmol waterlh * m2 ) from the surface. None of the toxic species are lost to the
Module
air by evaporation. You may assume that the reactor (marsh) is rectangular
and that the gentle downhill flow of the water can be modeled as plug fl~ow.
(a) Derive an equation for CA as a function of X and z.
Wetlands (Ans.: CA = CAo( 1 - X)/( 1 - UZ); U = QW/poZJo .)
(b) Derive an equation for X as a function of distance, z, down the wetlands.
(Ans.: X = 1 - (1 -a@; n = kDpo/Q.)
(c) Plot the conversion and rate of reaction as a function of distance for:
(1) no evaporation or condensation
(2) evaporation but no condensation