Page 22 - Principles of Catalyst Development

8 CHAPTER 1
1.2.3. Enzyme Catalysis

Enzymes are protein molecules of colloidal size, somewhere between
the molecular homogeneous and the macroscopic heterogeneous catalyst.
Thus they are neither but somewhere in between. Enzymes are, of course,
the driving force for biochemical reactions. Present in life processes, they
are characterized by tremendous efficiencies and selectivities. An enzyme,
9
catalase, decomposes H 20 2 10 times faster than any inorganic catalyst. l ]7)
There is great interest in harnessing enzyme catalysis for industrial
use. IIS ) Much depends on increasing the ability to withstand severe condi-
tions and developing techniques for anchoring enzymes to substrates for
use in packed beds.
With these brief comments, we leave homogeneous and enzyme cataly-
sis to others and concentrate the rest of this book on the development of
heterogeneous catalysts.

1.3. STRUCTURE AND TEXTURE OF CATALYST PARTICLES

Manufactured as a powder, the catalyst is formulated into particles,
whose shape and size are determined by the end use. Common types, with
a brief description of each, are given in Table 1.2, with more details to
follow in Chapter 6.
The first four types, pellets, extrudates, spheres, and granules, are
primarily used in packed bed operations. Generally, the larger the particle
diameter, the cheaper the catalyst. But this is usually not a significant factor
for the process designer. More important are uniform fluid flow, pressure
drop, and diffusional effects.
Uniform fluid flow through the bed is desirable for good utilization of
the catalyst and control of the process. To avoid channeling, the bed is
packed as evenly as possible. A rule of thumb dictates that the reactor to
particle diameter ratio should be from five to ten, with the reactor length
at least 50-100 times the particle diameter. (19) This ensures that the flow is
turbulent, uniform, and approximates plug flow. For most commercial
reactors these criteria are met. Only in the narrow-tube reactors found in
highly enda- or exothermic processes is there any concern.
Pressure drop is a consequence of flow through a packed bed. 121l1 An
extremely high i::.P requires energy-consuming compression and produces
undesirable pressure gradients in the bed. Bed voidage is important, and
shapes that increase this factor lower 11P. Figure 1.3 illustrates these prin-
ciples with pressure drop decreasing in the order spheres> pellets>
extrudates > rings > ~tars or lobes. Particle diameter has a much more