Page 232 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:46 PM Page 228
228 3. Heterogeneous Processes and Reactor Analysis

3.9.1 Surface area

The solid provides an extended surface to the reaction or adsorption to take place. The area
xterior
provided by the solid is the sum of the e and interior ones. Consequently the solid ,
surface includes not only the geometrical one as determined from the solid’s shape but also
the interior surface that is the result of its porous structure. The surface area is expressed as
specific surface area in units of m 2 /g. Its value may be from a few m 2 /g up to hundreds of
m 2 /g. In the case of a porous solid, the interior surface constitutes the greatest percentage of
alues of specific surface area may be achieved. Specifically, the total surface, and high v the
specific area of an activated carbon can reach the value of 1500 m 2 /g. So, the a ailable area v
for a hydrocarbon to react on 4 g of activated carbon is equal to that of a football field.

3.9.2 Particle shape

Sphericity is a measure of how close a particle is to being a sphere and is defined as

r
a
re
me
s
e
surface a of a sphe with sa volume a the pa rtic le
(4.554)
S
actual su rface r area of the p i c art e l
For granular particles, typical values of sphericity range between 0.6 and 0.95. In T able
3.13, the sphericity of selected materials is presented.
3.9.3 Particle size
The size (diameter) and the shape of the solid particles affect the mass transfer phenom-
ena within the particle, and thus the efeness in the case of a catalyst. The follo v fecti wing
shapes and sizes are frequently used in applications:

• 20–100 µm diameter spheres for fluidized-bed reactors
• 0.3–0.7 cm diameter spheres for f ed-bed reactors ix
• 0.3–1.3 cm diameter cylinders with a length-to-diameter ratio of 3–4
• up to 2.5 cm diameter holloylinders or rings w c

When particles of irregular shape are inolved, the particle diameter corresponding to a v
sphere of the same v olume d is used in many equations. For instance, a cubic particle
sph
with a side of 5 mm has a volume of 5 3 , and thus the d is
sph
4 d  sph  3
V 125 d 6.20 mm
sphere   sph
3 2  
gular
However, in the typical case of an irre-shaped particle, it is not easy to calculate its
volume and thus d is taken equal to the mean nominal diameter measured by sieve analy-
sph
sis d . In the present book, d is considered to be equal to the a e diameter . v v erage sie
p p