Chemical Engineering March 2017 - 76

150
Material: LDPE
100
Pipeline:
Bore: 100 mm
Material: aluminum
Surface: sandblasted
ing. If the surface is too rough, however,
small pieces will be torn away
from the pellets instead, and a large
percentage of fines will result. It will
also have an adverse effect on the
pressure drop, and hence on material
conveying capacity.
Although the results presented in
50
Conveying conditions
Exit air velocity: 40 m/s
Material temperature: 50oC
2
4
6
Solids loading ratio
FIGURE 13. Solids-loading ratio can have an effect on the degradation of LDPE
degradation occurs. The transition
from zero degradation to total degradation,
however, becomes an increasingly
more gradual process as
the particle-impact angle decreases.
At impact angles of 15 and 20 deg,
it would appear that this transitional
process is spread over a very wide
range of velocity values. At an impact
angle of 10 deg, however, there
is a significant change once again, in
that no particle degradation was recorded
at all up to 30 m/s.
In Figure 11, an alternative plot of
the data in this program of tests is
presented. This is effectively a slice
taken from Figure 10 at a particle velocity
of 23 m/s. This graph shows
that tests were carried out at regular
increments of impact angle (about
10 deg) between 10 and 90 deg.
This plot shows quite clearly that at
impact angles below about 12 deg,
no degradation occurs, and that at
impact angles above about 55 deg,
the degradation remains essentially
constant at the maximum value for
this particular impact velocity.
Particle melting
Particle melting is a form of material
degradation that often occurs in
pneumatic conveying plants handling
plastic-type
materials, particularly
in pelletized form. If a conventional
pipeline is used, materials such as
polyethylene, nylon and polyesters
can form cobweb-like agglomerates.
They are variously given names like
" angel hairs, " " raffia, " " snake skins "
and " streamers. "
Such agglomerates frequently
cause blockages at line diverters and
filters, which require plant interruption
to remove them. Equipment is generally
installed at the terminating end of
the conveying system for this pur76
pose.
Such equipment is necessary
because streamers also cause material
rejection by customers (because
the presence of these contaminants
in the product is undesirable).
Mechanics of the process. The
streamers are caused by the pellets
impacting against pipe bends and
" sliding " along the inside surface of
the pipeline. A considerable amount
of energy is converted into heat by
the friction of the two surfaces when
they touch. If the surface of the pipe
is smooth, the pellet will slide. This
contact, though momentary, decelerates
the particle by friction, which
is transformed into heat. This friction
is generally sufficient to raise the temperature
at the surface of the pellet
to its melting point. To a certain extent,
this is analogous to the thermal
model proposed for erosive wear.
Influence of variables. The onset of
the formation of these angel hairs or
streamers is the result of a combination
of conditions. Particle velocity is
the most important, but it also depends
upon the temperature of the
pipeline, the temperature of the pellets,
and the solids-loading ratio of
the conveyed material. The influence
of conveying-line exit-air velocity for
low-density polyethylene is shown in
Figure 12 [5].
The influence of solids-loading ratio
for this same material is given in Figure
13. In each case, the degradation
of the material is expressed in terms
of the mass of streamers and fines
produced, in grams, per metric ton of
low-density polyethylene conveyed.
Pipeline treatment. The formation
of streamers and fines can be
reduced quite considerably by suitably
treating the pipe wall surface. A
roughened surface is necessary in
order to prevent the pellets from slid8
10
Figures
12 and 13 were obtained
from tests carried out with pipe surfaces
roughened by sandblasting,
this treatment is not recommended,
as it will result in the generation of
a large percentage of fines. Also,
this roughness is relatively shallow
in depth and an aluminium surface
will wear so that the pipe must be retreated
in six to twelve months.
A more recent innovation is to attach
a small-diameter wire to the
inner surface of the pipeline, arranged
in a spiral. This essentially acts as a
" tripwire " for any particles that are
sliding. On impact with the wire, any
particles impacting it will be thrown
back into the conveying air flow. n
Edited by Scott Jenkins
References
1. Mills, D., " Pneumatic Conveying Design Guide, " 3rd Ed.,
Butterworth-Heinemann (Elsevier Ltd.), 2016.
2. Mason, J.S. and Smith, B.V., The erosion of bends by
pneumatically conveyed suspensions of abrasive particles.,
Powder Technology, vol. 6, pp. 323-335, 1973.
3. Tilly, G.P., " Erosion caused by impact of solid particles.
Treatise on materials science and technology, " Academic
Press Inc., vol. 13, pp 287-319, 1979.
4. Mills, D., Safety Aspects of Pneumatic Conveying. Chem.
Eng., April 1999, pp. 84-91. April 1999.
5. Paulson, J., Effective means for reducing formation of
fines and streamers. presented at Process Conf. on Polyolefins,
Society of Plastics Engineers, Houston, 1978.
Author
David Mills is an academic
engineer and pneumatic conveying
consultant in the U.K. (9
Cherry Orchard, Old Wives Lees,
CT4 BQ8, Kent, Canterbury, U.K.;
Phone:
041-332-7090; Email:
profdavid.mills@outlook.com). He
was, for many years, Professor of
Bulk Solids Handling at Glasgow
Caledonian University in Scotland.
He is currently Conjoint Professor at the University of
Newcastle in New South Wales, Australia, and Adjunct
Professor at the University of the Witwatersrand in Johannesburg,
South Africa. His " Pneumatic Conveying
Design Guide " - started in 1979 with U.K. government
funding as it was recognised as a subject significantly
lacking in design capability and understanding
- is now in its third edition. He has contributed several
articles to Chemical Engineering since 1990. Mills
has expertise in system design and troubleshooting,
and also investigates product degradation and highpressure
blow-tank systems. He holds a Ph.D. in engineering
and is the author of over 80 papers on pneumatic
conveying, covering topics such as system
design and performance.
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MARCH 2017
Streamer generation, g/m.t
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Chemical Engineering March 2017

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