Page 241 - Elements of Chemical Reaction Engineering 3rd Edition

Chap. 4 Questions and Problems 21 3
conversion to the weight necessary to achieve the first 5% conversion (0 to
5%) in the reactor? Vary a and write a few sentences describing and explain-
ing your findings. What generalizations can you make? (Am.: W = 82g)
P4-19, The gas-phase dimerization
2A __j B
follows an elementary rate law and takes place isothermally in a PBR charged
with 1.0 kg of catqlyst. The feed, consisting of pure A, enters the PBR at a
pressure of 20 atm. The conversion exiting the PBR is 0.3, and the pressure at
the exit of the PBR is 5 atm.
(a) If the PBR were replaced by a “fluidized” CSTR with 1 kg of catalyst,
what will be the conversion at the exit of the CSTR? You may assume that
there is no pressure drop in the CSTR. (Am.: X = 0.4.)
(b) What would be the conversion in the PBR if the mass flow rate were
decreased by a factor of 4 and particle size were doubled? Assume turbu-
lent flow. (Final exam)
(c) Discuss what you learned from this problem as well as the strengths and
weaknesses of using this as a final exam problem.
P4-201, The decomposition of cumene,

is to be carried out at a high temperature in a packed-bed reactor. At this tem-
perature the reaction is internal diffusion limited and apparent first-order in
cumene. Currently, 1000 kg of catalyst is packed in a 4-cm diameter pipe.
The catalyst particles are 0.5 cm in diameter and the bulk density of the
packed catalyst is 1000 kg/m3. Currently, 6.4% conversion is realized ?when
pure cumene enters the reactor. The entering pressure is 20 atm and the pres-
sure at the exit of the reactor is 2.46 atm.
(a) What conversion would be achieved if the PBR were replaced by a fluid-
ized CSTR containing 8000 kg catalyst with negligible pressure drop?
(b) We know fromThapter 12 [e.g., Equation (12-35)] that for internal dif-
fusion limitations, the rate of reaction varies inversely with the catalyst
particle size. Consequently, one of the engineers suggests that the Icata-
lyst be ground up into a smaller size. She also notes there are three other
pipe sizes available into which the catalyst could be packed. These non-
corrosive heat-resistant pipes, which can be cut to any length, are 2 cm,
6 cm, and 8 cm in diameter. Should you change the catalyst size and pipe
diameter in which the catalyst is packed? If so, what are the appropriate
catalyst particle size, the appropriate pipe diameter, and the exiting con-
version? If nothing should be changed, explain your reasoning. Assume
that the flow is highly turbulent and that the bulk catalyst density is the
same for all catalyst and pipe sizes. Explain how your answers would
change if the flow were “laminar.”
(c) Discuss what you learned from this problem and what you believe to be
the point of the problem.
P4-2lC The first order irreversible gas phase reaction
Normal pentane --+ Iso-pentane
is to be carried out in a packed bed reactor. Currently 1000 kg of reform-ing
catalyst ae packed in a 4 cm diameter pipe. The catalyst particles are 0.5 cm
in’diameter and the bulk density of the packed catalyst is 1,000 kg/m3. C‘ur-