Page 198 - Elements of Chemical Reaction Engineering 3rd Edition

170 Isothermal Reactor Design Chap. 4

(4-39) make use of L and not L'. Thus, one does not need to adjust these for-
mulas to treat spherical reactors that have different amounts of empty space at
the entrance and exit (i.e., L # L'). Only the upper limit of integration needs
to be changed, zf = L + L' .

Example 4-8 Dehydrogenation Reactions in a Spherical Reactor

Reforming reactors are used to increase the octane number of petroleum. In a
reforming process 20,000 barrels of petroleum are to be processed per day. The cor-
responding mass and molar feed rates are 44 kg/s and 440 molls, respectively. In the
reformer, dehydrogenation reactions such as
paraffin + olefin -k H,

occur. The reaction is first-order in paraffin. Assume that pure paraffin enters the reac-
tor at a pressure of 2000 kPa and a corresponding concentration of 0.32 mol/dm3.
Compare the pressure'drop and conversion when this reaction is carried out in a tubu-
lar packed bed 2.4 m in diameter and 25 m in length with that of a spherical packed
bed 6 m in diameter. The catalyst weight is the same in each reactor, 173,870 kg.
-Ti = k' CA
-TA= pB(-Ti) = pc(l-$)(-?$.) = pc(l-$)k'C~

Additional information:
po = 0.032 kg/dm3
D, = 0.02 dm = 0.4
kt = 0.02 dm3kg cat . I.'. = 1.5 x kg/dm.s
L = L' = 27 dm pc = 2.6 kg/dm3

Solution
We begin by performing a mole balance over the cylindrical core of thickness Az
shown in Figure E4-8.1.

Figure E4-8.1 Spherical reactor.