Page 97 - Chemical Process Equipment - Selection and Design

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TRANSFER OF SOLIDS

n contrast to fluids which are transferred almost equipment. Most commonly, solids are carried on or pushed
exclusively through pipelines with pumps or blowers, a along by some kind of conveyor. Solids in granular form also
greater variety of igquipment is employed for moving are transported in pipelines as slurries in inert liquids or as
sokk to and from storage and between process suspensions in air or other gases.

5.1. SLURRY TRANWORT Aude, Seiter, and Thompson (1971),
C
In short process lines slurries are readily handled by centrifugal - exp(-2.55ut/ku@, (54
=
pumps with large clearances. When then: is a distribution of sizes, cll
the line particles effectively form a homogeneous mixture of high where
density in which the settling velocities of larger particles are less C=concentration of a particular sue at a level 92% of the
than in clear liquid. Turbulence in the line also helps to keep vertical diameter,
particles in suspension. It is essential, however, to avoid dead C, = concentration at the center of the pipe, assumed to be the
spaces in which solids clould accumulate and also to make provisions same as the average in the pipe,
for periodic cleaning of the line. A coal-oil slurry used as fuel and f = Fanning friction factor for pipe flow
acid waste neutralization with lime slurry are two examples of
process applications.
Many of the studies of slurry transfer have been made in (5.3)
connection with long distance movement of coal, limestone, ores,
and others. A few dozen such installations have been made, in At high Reynolds numbers, for example, Blasius’ equation is
length from several miles to several hundred miles.
Cod-water slurqr transport has been most thoroughly f = 0.0791/pkp, NRe 2 lo5 (5.4)
investigated and implemented. One of the earliest lines was 108
miles long, 10 nn. &a, 510-60 wt % solids up to 14 mesh, at velocities k in Eq. (5.2) is a constant whose value is given in this paper as
of 4.5-5.25 ftlsec, with positive displacement pumps at 30-mile 0.35, but the value 0.85 is shown in a computer output in a paper by
intervals. The longest line in the United States is 273 miles, Bin. Wasp, Thompson, and Snoek (1971, Fig. 9). With the latter value,
dia and handles 4.8-6.0 million tons/yr of coal; it is described in Eq. (5.2) becomes
detd by Jacques and Montfort (197’7). Other slurry pipeline
literature is by Wasp, Thompson, and Snoek (1971), Bain and c/c, = exp(-3.00u,/u@. (5.5)
Bonnington (19701, Ewing (1978), and Zandi (1971).
Principally, investigations have been conducted of suitable The latter paper also states that satisfactory Wow conditions prevail
linear velocities and power requirements. Slurries of 40-50 vol % when C/Clle0.7 for the largest particle size. On this basis, the
solids can be handled satisfactorily, with particle sizes less than minimum line velocity becomes
24-48 mesh or so (0.7-0.3mm). At low line velocities, particles
settle out and impede the flow of the slurry, and at high velocities
the frictional drag likewise increases. An intermediate condition
exists at which the pressure drop per unit distance is a minimum.
The velocity at this condition is called a critical velocity of which where u, is the settling velocity of the largest particle present.
one correlation is As Example 5.1 shows, the velocities predicted by Eqs. (5.1)
and (5.6) do not agree closely. Possibly an argument in favor of Eq.
(5.6) is that it is proposed by the organization that designed the
u; = 34.~,~u,~&Tj72, consistent units, successful 18 in., 273 mi Black Mesa coal slurry line.
Pressure drop in flow of aqueous suspensions sometimes has
where been approximated by multiplying the pressure drop of clear liquid
u, = critical flow velocity, at the same velocity by the specific gravity of the slurry. This is not
u, = terminal settling velocity of the particle, given by Figure borne out by experiment, however, and the multiplier has been
5.1, correlated by other relations of which Eq. (5.7) is typical:
C, = volume fraction of solids,
D = pipe diameter,
d = particle diameter, (5.71
s = ratio of densities of solid and liquid,
g = acceleration of gravity, 32.2 ft/sec2, or consistent units. This equation is a modification by Hayden and Stelson (1971) of a
series of earlier ones. The meanings of the symbols are
The numerical coefficient is due to Hayden and Stelson (1971). C,, = volume fraction occupied by the solids in the slurry,
Another criterion for selection of a flow rate is based on d = particle diameter,
considerations of the extent of sedimentation of particles of various D = pipe diameter,
sizes under flow conditions. This relation is developed by Wasp, s = ratio of specific gravities of solid and liquid.

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