Page 483 - Elements of Chemical Reaction Engineering Ebook

454 Steady-State Nonisothermal Reactor Design Chap. 8

Example 8-6 Liquid-Phase Isomerization of Normal Butane

Normal butane, C,H,,, is to be isomerized to isobutane in a plug-flow reactor. Isobutane
is a valuable product that is used in the manufacture of gasoline additives. For example,
isobutane can be further reacted to form isooctane. The 1996 selling price of n-butane
was 37.2 cents per gallon, while the price of isobutane was 48.5 cents per gallon.
The reaction is to be carried out adizbatically in the iiquid phase under high
pressure using essentially trace amounts of a liquid catalyst which gives a specific
reaction rate of 3 1.1 h-’ at 360 K. Calculate the PFR velum necessary to process
100,000 gal/day (163 kg mol/h) of a mixture 90 mol % n-butane and 10 mol % z-pen-
tane, which is considered an inert. The feed enters at 330 K.
Additional information:
The economic AHRx = -6900 J/mol* butane
incentive
$ = 48.5plgal Butane i-Pentane
vs. 37.2$/gal Cp,-B = 141 J/mol.K Cpi., = 161 J/mol. K

Cpi-B = 141 J/mol* K Activation energy = 65.7 kJ/mol
K, = 3.03 at 60°C
CAo = 9.3 g mol/dm3 = 9.3 kg mol/m3

Solution
i-C,H,o

Mole balance: (E8-6.1)

21
The algorithm Rate law: -rA=k C -- (E8-6.2)
( A

(E8-6.3)

(E8-6.4)
Stoichiometry (liquid phase, u = uo ):
CA = cAo(1 -x) (E8-6.5)
c, CAOX (E8-6.6)

Combine:
i 4x1
-rA =kCA, 1- 1 t - (E8-6.7)

Integrating Equation (E8-6.1) yields

(E8-6.8)