Page 200 - Principles of Catalyst Development

CATALYST DEACTIVATION 189
deactivation functions similar to equation (8.2), and computer programs
simulating particle-reactor behavior. Process designers profit since com-
puter predictions are rapid and inexpensive. The key is fundamental under-
standing of both reaction and deactivation mechanisms and kinetics.
Unfortunately, few processes are that advanced.
Deactivation regimes such as those in Fig. 8.2 are corn~cted in a number
of ways. First, harmful process conditions are avoided or changed. Decreas-
ing (or increasing in some cases) the temperature, increasing hydrogen
pressure, etc. are often sufficient to lower deactivation rates. This is often
an important consideration in designing process conditions. Another
approach is to maintain constant conversion by increasing temperature
gradually as the catalyst decays. This is limited by the sensitivity of the
process equipment to high temperatures, capacities of furnaces and heat
exchanges, and possible side reactions.
Second, poisons in the feed may be removed with up-stream processing
units or guard chambers. Catalytic reforming catalysts are poisoned by
sulfur compounds in naphthas, so the feed is hydrotreatl~d for desulfuriz-
ation. Sulfur in the natural gas feedstocks for steam reforming in ammonia
production is removed with zinc oxide guard beds. A clever and innovative
solution to lead poisoning of platinum-alumina automobile exhaust
catalysts is to formulate pellets with subsurface shells of platinum as shown
in Fig. 6.16, the outer alumina surface acting as a guard for the platinum.
Third, in processes designed for equilibrium conversion, extra large
catalyst beds may be used. The reaction occurs in a narrow zone at the
front of the bed. As the catalyst deactivates, this zone moves along the
reactor until slippage occurs. Catalyst charges are large enough to allow
operation for specified periods between shut-downs. Strongly adsorbing
poisons result in the best performance, as experienced with adiabatic proces-
ses such as low temperature shift and methanation. Figure 8.4 shows
temperature profiles for this type of process, indicating well-defined
poisoned, reaction, and clean zones, and differences between different types
of deactivation. (260)
Next, reactors may be designed to accommodate ralPid deactivation-
regeneration cycles. The best example is catalytic cracking where coke
deposition is so fast that decay takes only minutes. (261) Since, in this case,
the feed is the precursor to coke, treatment or guard chambers are not
practical. The only solution is to use fluidized beds, which provide reaction
and regeneration in a continuous cycle. Other cases are slurry and moving
bed reactors, when deactivation is not so rapid.
Last, but perhaps most important, is catalyst modifkation. Other sol-
utions should be attempted only when this avenue for c:atalyst control is
exhausted. The most common methods are as follows: