Chemical Engineering July 2022 - 45

Jenike & Johanson
FIGURE 3. Tubular drag conveyors pull material through a
pipe using pucks or discs attached to a continuous cable
or chain
partially suspended in the gas, at
high pressure drops and lower velocities,
and at much higher solids
loadings (typically 25-150 psig and
10-40 ft/s at pickup). Dense-phase
conveying also has two discrete regimes,
each of which are suitable
for very different materials and applications:
moving-bed ( " dune " flow),
which is often used for conveying
fine, aeratable and free-flowing powders
like alumina or cement, and
piston-plug flow, which is often used
for conveying coarse, highly permeable
materials like plastic beads or
catalyst pellets.
Both dilute- and dense-phase
can be done using either positive
pressure gas to " push " the material
through the line towards the delivery
point, or using a negative pressure
(vacuum) to " pull " the material
through the line. The main advantages
of pressure systems versus
vacuum systems are that pressure
systems can transport higher tonnages
(up to 300 ton/h for pressure,
compared to only up to 100 ton/h
for vacuum), over longer distances
(up to 1,000 ft for pressure versus
only about 200-300 ft for vacuum),
and at lower velocities, and pressure.
Also, pressure-based systems
are well-suited for conveying under
inert gases and preventing air/oxygen
ingress. The main advantages
of vacuum conveying are that all
leakage occurs inward, making it
highly attractive for containing toxic
dusts. Vacuum systems are also
very
well-suited when there are
many pickup points and a single
delivery point (for example, in dustcollection
systems).
Table 1 includes a comparison
of the main features, advantages
and disadvantages of dilute- and
dense-phase pneumatic transport,
along with how they compare with
a number of different common
mechanical conveying
technologies. One additional
potentially major disadvantage
of pneumatic systems to
consider is that they require
additional dedicated filter
equipment (such as cyclones,
baghouses, cartridge filters)
for separating the material out
of the conveying gas stream,
whereas mechanical systems
generally do not.
Mechanical conveyors
The major features and limitations of
several types of mechanical conveyors
are included in Table 1. Typical
applications for some of the most
common technologies are summarized
below.
Belt conveyors. Belt conveyors
are used for transporting a massive
range of products in a wide
variety of industries, from fine food
and chemical powders, up to heavy
coarse ores from mining operations.
One of the main strengths of standard
belts is their ability to convey
extremely high tonnages over long
distances, while some of their main
drawbacks are that they are generally
not very good at containing
dusts, are not very flexible from
a layout standpoint, and are not
suited for quickly elevating or lifting
material vertically. Typically, relatively
shallow inclines only up to about 15
deg are acceptable, meaning long
distances are required to lift material.
Specialized types of belts are
available (for example, chevron or
cleated belts, pocket belts, sandwich
belts, tube belts) which can
better contain dusts, may have
better layout flexibility and ability to
navigate around turns and corners,
and can convey up steeper angles.
Screw conveyors. Like belts, screw
conveyors are used in a huge array
of applications and with materials
across most industries, but they
are generally not suited for " heavy
duty " applications with heavy,
coarse, abrasive materials like ores
and minerals. The main strengths
of screws are their cost-effectiveness
and mechanical simplicity,
as well as their ability to contain
dusts, while their main drawbacks
are their limited length, their generally
poor layout flexibility, and their
relatively low capacity compared to
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other types of mechanical conveyors.
Also, solid material " grinds " as
it moves through the screw, which
can cause significant particle breakage
or degradation with some materials.
Standard screw conveyors
also tend to have more residual material
left in the conveyor than other
types of conveyors, even when
they're " empty " due to " dead " material
that remains at the bottom of
the screw trough, which can create
cross-contamination issues.
Screws come in a number of
specialized configurations, including
flexible screws that can convey
material up relatively steep inclines,
vertical screws that can elevate
some materials over relatively small
distances up to about 20 to 30 ft,
coreless, ribbon-flighted screws,
which are more resistant to buildup
of sticky materials. There are also
screws that have some blending
capability that can be used in some
applications, and some screws that
can be used for heating or cooling
during transport. Some screws are
also available with " drop-bottom "
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Chemical Engineering July 2022

Table of Contents for the Digital Edition of Chemical Engineering July 2022
