Page 235 - Introduction to chemical reaction engineering and kinetics

8.6 catalyst Deactivation and Regeneration 217

For catalysts poisoned by sulfur, the metal-sulfur bond is usually broken in the pres-
ence of steam, as shown for nickel:

Ni-S + H,O -+ NiO + H,S
H$ + 2H,O = SO, + 3H,

The equilibrium for the second reaction favors H,S until extremely high temperatures
are reached (> 700°C). Thus, sintering of the catalyst could be a problem. Furthermore,
SO2 can act as a poison for some catalysts. If sintering or SO, poisoning precludes steam
treatment, it is usually possible to remove deposited sulfur by passing a sulfur-free gas
stream over the catalyst at moderate temperatures for an extended period of time.
Regeneration of coked catalysts may be accomplished by gasification with oxygen,
steam, hydrogen, or carbon dioxide:
c + 0, -+ co,
C+HH,O -+ CO+H,
C + 2H, + CH,
c+co,+2co

The first reaction is strongly exothermic, and may lead to high local temperatures
within the catalyst. Thus, temperature must be carefully controlled to avoid sintering.

A coked porous catalyst is to be regenerated by passage of a stream of CO, over it at 1000
K for reaction according to C(s) + CO,(A) + 2CO(B). From the data given below (Austin
and Walker, 1963), calculate the following characteristics of the regeneration process at the
conditions given: (a) the Thiele modulus, (b) the effectiveness factor, and (c) the (actual)
rate of regeneration, ( - rA)Obs.
Data: For the catalyst, D, = 0.10 cm2 s-l, L, = 0.7 cm; ckr (exterior surface
concentration) = 0.012 mol L-l. The reaction follows LH kinetics, with the intrinsic
rate given by

(-rA>i,t = kc,/(l + KAcA + KBcB)

where k = 3.8 x low4 s-l, KA = 340 L mol-‘, KB = 4.2 X lo6 L mol-‘, and ci is in
mol L-l.

SOLUTION

This example illustrates calculation of the rate of a surface reaction from an intrinsic-rate
law of the LH type in conjunction with determination of the effectiveness factor (7) from
the generalized Thiele modulus (&) and Figure 8.11 as an approximate representation of
the q--& relation. We first determine &, then q, and finally (-rJobs.
(a) From equation 8.5-22,

Le(-rA)intlcA, (8.522)
& = [2D, /p(-rA)intdCA]1'2

where ( - rA)i,+,, is the intrinsic rate evaluated at cAs. Since, from the stoichiome-
try, CB = 2(cA, - CA), we can eliminate cB from the LH expression, and express the