Page 99 - Chemical Process Equipment - Selection and Design

5.2. PNEUMATIC CONVEYING 71
10 ' I I I I

. /='Ope = -0.5'

IO-' L- I I I I IO0
IO" 10'' 100 10' 102 io3 io4 to5 I
Shear rate, I/sec Shear rate, I/sec
(a) lbi
lFigme 5.2. NowNewtonian behavior of suspensions: (a) viscosity as a function of shear rate, 0.4 wt % polyacrylamide in water at room
temperature; (b) shear stress as a function of shear rate for suspensions of TiO, at the indicated vol % in a 47.1 wt % sucrose solution whose
viscosity is 0.01'7 Pa sec (Denn, Process Fluid Mechanics, Prentice-Hall, Englewood Cliffs, NJ, 1980).

and, for one-sized particles, 5.2. PNEUMATIC CONVEYING
(5.14) Granular solids of free-flowing natures may be conveyed through
ducts in any direction with high velocity air streams. In the normal
plant, such lines may be several hundred feet long, but dusty
These several pressure drop relations hardly appear consistent, and materials such as fly ash and cement have been moved over a mile
the numerical results of Example 5.1 based on them are only in this way. Materials that are being air-veyed include chemicals,
roughly in agreement. plastic pellets, grains, and powders of all kinds. The transfer of
From statements in the literature, it appears that existing slurry catalysts between regenerator and reactor under fluidized conditions
lines were designed on the basis of some direct pilot plant studies. is a common operation. Stoess (1983) has a list of recommendations
Nornettling slurries are formed with fine particles or plastics or for about 150 different materials, of which Table 5.1 is a selection.
fibers. Although their essentially homogeneous nature would Basic equipment arrangements are represented in Figure 5.3.
appear to make their flow behavior simpler than that of settling The performance of pneumatic conveyors is sensitive to several
slurries, they often possess non-Newtonian characteristics which characteristics of the solids, of which the most pertinent ones are
complicate their flow patterns. In Newtonian flow, the shear stress
is proportional to the shear strain, 1. bulk density, as poured and as aerated,
2. true density,
stress = strain), 3. coefficient of sliding friction (=tangent of the angie of repose),
4. particle size distribution,
but in other cases the relation between these two quantities is more 5. particle roughness and shape,
complex. Several classes of non-Newtonian behavior are recognized 6. moisture content and hygroscopicity, and
for suspensions. Pseudoplastic or power-law behavior is represented 7. characteristics such as friability, abrasiveness, flammability, etc.
by
stress = k(strain)", it < I, Sulfur, for example, builds up an electrostatic charge and may
introduce explosive risks.
where k is calleld the consistency index. Plastic or Bingham behavior In comparison with mechanical conveyors, pneumatic types
is represented by must be designed with greater care. They demand more power
input per unit weight transferred, but their cost may be less for
complicated paths, when exposure to the atmosphere is undesirable
stress = k, + q(strain), and when operator safety is a problem. Although in the final
analysis the design and operation of pneumatic conveyors demands
where 9 is calked the plastic viscosity. Data for some suspensions the attention of experienced engineers, a design for orientation
are given on Figure 5.2. purposes can be made by the inexpert on the basis of general
The constants of such equations must be found experimentally knowledge and rules of thumb that appear in the literature. An
over a range of conditions for each particular case, and related to article by Solt (1980) is devoted entirely to preventive trouble-
the friction factor with which pressure drops and power shooting.
requirements can be evaluated. The topic of nonsettling slurries is Some basic design features are the avoidance of sharp bends, a
treated by Bain and Bonnington (1970) and Clift (1980). Friction minimum of line fittings, provision for cleanout, and possibly
factors of power-law systems are treated by Dodge and Metzner electrical grounding. In many cases equipment suppliers may wish
(1959) and of fiber suspensions by Bobkowitz and Gauvin (1967). to do pilot plant work before making final recommendations. Figure