Page 104 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: ZBU 2nd Revised Pages
Encyclopedia of Physical Science and Technology EN002G-87 May 19, 2001 20:3

502 Catalyst Characterization

catalyst. Reactions are typically carried out in batch auto-
claves under pressure. Processes involving hydrogenation,
alkylation, isomerization, and so on are commonly carried
out in slurry-phase reactors.
A wide variety of catalytic materials are used as slurry-
phase catalysts, most being metals supported on high sur-
face area alumina, carbon, and silica (Fig. 2, label 3). Phys-
ical properties such as density are important since these
catalysts must be suspended in the reaction mix. Since
rapid agitation could lead to abrasion and attrition of the
catalyst particles, strength is important.
Reactants and catalyst must be contacted; thus, a high
external surface area of the catalyst (smaller particle size)
is desirable to maximize reaction rate. The particle size of
the catalyst must be optimized to permit filterability for
ease of recycle while maintaining the high external surface
area needed for maximum reactant–catalyst contacting.
The internal structure, comprising pores and surface
area, is important for making the active catalytic sites ac-
cessible to the reactant molecules. The location of the
active species is important for minimizing diffusional re-
sistance since reactants must diffuse within the particle to
the active sites and products must diffuse away. Finally,
high catalytic surface area and high dispersion of active
species are advantageous for maximum reaction rate and
utilization of the catalytic components.
FIGURE 1 Schematic of the possible rate-controlling steps in a
heterogeneously catalyzed reaction.
2. Particulates
Particulate catalysts are commonly used in fixed-bed
real industrial catalysts. The final section, Section IV, reactors, in which the feed is passed through an
discusses nonroutine complementary techniques which immobilized bed of catalyst. Oxidation, hydrogenation–
when used in concert with those of Sections II and III dehydrogenation, isomerization, alkylation, and hydro-
provide fundamental property data. These, however, are treating are carried out in such reactors. Supported
not commonly used in practice in industry. catalysts are composed of an active catalytic species dis-
persed throughout the support matrix. The supports take
on many different sizes and shapes, all of which are de-
I. PHYSICAL FORMS OF termined by the reactor engineering. Spheres, extrudates,
HETEROGENEOUS CATALYSTS and tablets (Fig. 2, labels 2, 4, 6, and 7) are the most pop-
ular shapes; however, rings, stars, doughnuts, and others,
A. Supported Catalysts are also used for specialized applications.
They are usually charged to a reactor of a fixed volume,
1. Powders
and thus the bulk density influences the weight of catalyst
Powdered catalysts are used almost exclusively in slurry- present. Since these fixed-bed reactors can be very large
phase reactions, in which the catalyst powder is mixed (i.e., 15,000–20,000 lb per charge), a crush strength resis-
with a reactant. Vigorous agitation improves the contact- tance of some minimum value is often specified. Large
ing of reactant(s) and catalyst, and the rate of conversion of volumes of feed, frequently at high linear velocities, pass
reactant to product(s) is monitored by suitable analytical through the bed; thus, the resistance to abrasion must be
techniques. The agitation is provided by an internal im- considered.
peller; however, “rocking” or “shaking” reactors are also All of the internal properties such as pore size, surface
used. Separation of the catalyst from the reaction mix is area, catalytic species location, and catalytic surface area
usually accomplished by filtration, although sedimenta- are important since the five fundamental steps mentioned
tion can also be used depending on the settling rate of the in the opening paragraph are operative.