Page 39 - Principles of Catalyst Development

STRUCTURE OF CATALYSTS 25
process conditions. The forward reaction is steam reforming, an important
source of hydrogen for the ammonia and methanol synthesis, iron ore
reduction, and petroleum hydrotreating.(38) The reaction is endothermic and
equilibrium limited, so that high temperatures (700-1000°C) are needed for
high yields. Under these conditions, nickel sinters rapidly. But activity is
not important since kinetic rates at high temperatures are sufficient. High
dispersions of nickel are not necessary.
Because of the extreme endothermicity, the reaction is heat-transfer
limited. Narrow reactor tubes (10-20 cm) ensure high surface to volume
ratios and good heat transfer but need sufficient length for economic space
velocities (> 10,000 hr- ). Thus pressure drop is a problem. Large particles
I
are called for (Fig. 2.1), but in narrow tubes this causes flow problems if
the tube to particle diameter ratio falls below 5-10. Also, at these high
temperatures, reaction rates are so fast that the effectiveness factor is low.
Some relief is found by using ring-shaped particles, which not only increase
bed voidage but also give lower effective diameters. (38) But some degree of
compromise is necessary, leading to the use of particles about 2 cm in
diameter.
In addition, severe temperatures require thermal stability. Particles
must retain their physical and mechanical properties, and such things as
phase transitions and fracturing must be avoided. Thus, an important
feature in designing steam reforming catalysts is a suitable support giving
strength to the particle and stability to the nickel. Many solutions have
emerged, e.g., MgAI 20 4 - or CaAl 20 4-based systems.
The reverse of reaction (2.1) is methanation. Used to remove residual
CO traces from ammonia synthesis feedstocks, it was also developed as an
important source of substitute natural gas (SNG) in the synthetic fuels
industry. (4) Since this reaction is exothermic, equilibrium yields are better
at low temperatures (300-500°C). Thus, high activity is critical. Nickel must
be highly dispersed. Preparational methods are required to produce small
nickel crystallites. This high metal area must be maintained in the presence
of extreme exothermicity, so that sintering must be avoided. This is partially
accomplished through proper catalyst design, but process reactor type
must also be considered. (39) Recycle, fluidized, and slurry reactors are
appropriate.
Questions of lifetime stability are different for the two processes. We
have discussed thermal degradation and sintering. Also important are sulfur
poisoning and carbon fouling. Sulfur rapidly deactivates nickel sites by
adsorption of sulfur atoms. (40) For steam reforming, temperatures are high
enough that steam removes the sulfur, and higher levels may be tolerated. (38)
With methanation, sulfur poisoning is irreversible and the only protection
is to desulfurize the feed. (4)