Page 412 - Chemical process engineering design and economics

392 Chapter 7

alumina is more widely used than the other materials [18]. Pellets are usually
molded or extruded into spheres, cylinders, or rings. Extrusion is a lower cost
operation than molding[18]. The most common pellet diameters are 1/32, 1/16,
and 1/8 in (0.794, 1.59, and 3.18 mm). Pellets should have a high compressive
strength to resist crushing and abrasion and a low pressure drop to minimize
compressor and power costs. Because pellets are packed in a bed, the bulk
crushing strength of the pellets limits the bed height. Trambouze et al. [8] de-
fine bulk crushing strength as the stress that produces 0.5 % fines as determined
by compressing the pellets in a press. Pellet strengths vary from 1.0 to 1.3 MPa
(145 to 189 psi) for several pellets tabulated by Trambouze et al. [8].
Selecting a pellet size, shape, and porosity (void fraction in the pellet) is a
trade-off between achieving high reactivity, high crushing strength, and low
pressure drop. Promoting high reactivity requires a porous pellet with a large
internal surface area, which requires small pores. Small pores, however, lower
the diffusion rate, reducing the pellet activity. The rate of diffusion increases
with increasing pore size, but the increased pore size reduces surface area and
therefore reactivity. Consequently, there is an optimum pore size that maximizes
pellet reactivity. Reactor reactivity increases if the pellet diameter is reduced,
allowing more pellets to be packed into a reactor, but then the pressure drop is
increased. Low pressure drop is achieved using large pellets, but' then this re-
duces the catalyst surface area for a unit volume of reactor. Also, crushing
strength decreases with increasing porosity particularly when the porosity is
above 50% [17].

Packed-Bed Reactor Selection

Catalyst pellets are contained in a reactor, as shown in Figure 7.4, in a single
bed, multiple beds in a single shell, several packed tubes in a single shell, or a
single bed with imbedded tubes. Deviation from the simple single bed may be
required because of the need to add or remove heat, to redistribute the flow to
avoid channeling, or to limit the bed height to avoid crushing the catalyst. In all
the reactors shown in Figure 7.4, the reacting gases flow downward through the
bed instead of upward to avoid fluidization and minimize entrainment of catalyst
in the exit gases.
The simplest packed-bed reactor is the adiabatic, single-bed reactor shown
in Figure 7.4a. According to Trambouze et al. [8], it is the most frequently used
reactor type. If the reactants must be cooled to limit catalyst fouling or deactiva-
tion, then select one of the other reactor types. In the reactor shown in Figure
7.4b, part of the feed stream is diverted and mixed with hot gases from the upper
bed before entering the lower bed. The methanol-synthesis reactor, discussed in
Chapter 3, uses this method of cooling. Adding an excess of one of the reactants
or an inert gas could also reduce the temperature rise of the reactants. These
gases are heat sinks, absorbing the enthalpy of reaction. In the reactor shown in
Figure 7.4c, the catalyst is packed in tubes, and a heat-transfer fluid flows in the

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