Page 244 - Elements of Chemical Reaction Engineering 3rd Edition

21 6 Isothermal Rerctor Design Chap. 4

such as ethene and propene. One of the first steps in the reforming process
involves the dehydration of methanol.

2CH3OH+CH2 = CH, +.2H20
This gas-phase reaction is carried out over a zeolite catalyst and follows an
elementary rate law. The catalyst is packed in a tubular PBR that is 2 m in
diameter and 22 m in length. Pure methanol is fed at a molar flow rate of 950
molls, a pressure of 1500 Ha, a concentration of 0.4 mol/ dm3, and 490°C.
The conversion and pressure at the exit are 0.5 and 375 kPa, respectively.
Flow throughout the bed is known to be such that the turbulent contribution
in the Ergun equation (i.e., G2) can be neglected. The tubular PBR is to be
replaced with a spherical PBR containing an equal amount of identical cata-
lyst. The spherical PBR measures 5.2 m in diameter and has screens placed
2 dm from each end (i.e., L = L’ = 24 dm).
(a) What conversion ahd exit pressure can we expect from the spherical
reactor? [Hint: What are the parameter values (e.g., a, k) for the PBR
(0.1 < k < I.O)? (Am.: X = 0.63.)
(b) By how much can the feed rate to the spherical PBR be increased and
still achieve the same conversion that was attained ,in the tubular PBR?
Assume that the flow is completely laminar up to FAo = 2000 mows.
(c) It is desired to minimize the pumping requirement for the feed to the
spherical PBR. How low can the entry pressure, Po. be and still achieve
a conversion of OS?
(d) Put two spherical PBRs in series. What is the conversion and pressure at
the exit? Experiment with putting more than two reactors in series. What
is the maximum attainable conversion?
[The data given in this problem are based on kinetics data given in H. Schoe-
nfelder, J. Hinderer, J. Werther, F.J. Keil, Methanol to olefins--prediction of
the performance of a circulating fluidized-bed reactor on the basis of kinetic
experiments in a fixed-bed reactor. Chem. Eng. Sci. 49, 5377 (1994).]
4-25, A very proprietary industrial waste reaction which we’ll code as A-+B -f- S to
be carried out in a 10-dm3 CSTR followed by 10-dm3 PFR. The reaction is
elementary, but A, which enters at a concentration of 0.001 moi/dm3 and a
molar flow rate of 20 mol/min, has trouble decomposing. The specific reac-
tion rate at 42°C (i.e., room temperature in the Mojave desert) is 0.0001 s-l.
However, we don’t know the activation energy; therefore, we catlnot carry out
this reaction in the winter in Michigan. Consequently this reaction, while
important, is not worth your time to study. Therefore, perhaps you want to
take a break and go watch a movie such as Dances with Wolves (a favorite of
the author), The Sixth Sense, Vertical Limit, or Billy Elliott.
P4-26* Pure butanol is to be fed into a semibatch reactor containing pure ethyl ace-
tate to produce butyl acetate and ethanol. The teaction
CH3COOC2H5 + C4H90H a CH3COOCdH9 + C2HSOH
is elementary and reversible. The reaction is carried out isothermally at
300 K. At this temperature the equilibrium constant is 1.08 and the specific
reaction rate is 9 X 10-5 dm3/mol 1 s . Initially, there is 200 dm3 of ethyl Ace-
tate in the vat and butanol is fed at a rate of 0.05 dm3/s. The feed and initial
concentrations of butanol and ethyl acetate are 10.93 mol/dm3 and 7.72
moYdm3, respectively.