The continuous dense phase in which the solids occupy the entire pipe
is virtually extrusion. Transport in this form requires very high gas pressures
and is limited to short straight pipe lengths and granular materials
(which have a high permeability).
Discontinuous dense phase flow can be divided into three fairly distinct flow patterns.
Discrete plug in which discrete plugs of solids occupy the full pipe cross section.
"Dune" flow in which a layer of solids settled at the bottom of the pipe move along
in the form of rolling dunes. A hybrid of discrete plug flow and dune flow
in which the rolling dunes completely fill the pipe cross-section but in which
there are no discrete plugs (also known as plug flow).
Salating flow is encountered at gas velocities just below the saltation velocity.
Particles are conveyed in suspension above a layer of settled solids.
Particles may be deposited and re-entrained from this layer. As the gas velocity
is decreased the thickness of the layer of settled solids increases
and eventually we have 'dune' flow.
It should be noted firstly that not all powders exhibit all these flow patterns
and secondly that within any transport line it is possible to encounter more than one regime.
The main advantages of dense phase transport arise from the low gas requirements
and low solids velocities. Low gas volume requirements generally mean low energy
requirements per kilogram of product conveyed, and also mean that smaller pipelines
and recovery and solids/gas separation are required. Indeed in some cases,
since the solids are not suspended in the transport gas, it may be possible
to operate without a filter at the receiving end of the pipeline.
Low solids velocities mean that abrasive and friable materials may be conveyed
without major pipeline erosion or product degradation.
It is interesting to look at the characteristics of the different dense phase
flow patterns with a view to selecting the optimum for a dense phase transport system.
The continuous dense phase flow pattern is the most attractive from the point of view
of low gas requirements and solid velocities, but has the serious drawback that
it is limited to use in the transport of granular materials along short straight pipes
and requires very high pressures. Saltating flow occurs at a velocity too close
to the saltation velocity and is therefore unstable. In addition this flow pattern
offers little advantage in the area of gas and solids velocity. We are then left with
the so-called discontinuous dense phase flow pattern with its plugs and dunes.
However, performance in this area is unpredictable, can give rise to complete pipeline
blockages and requires high pressures. Most commercial dense phase transport systems
operate in this flow pattern and incorporate some means of controlling plug length
in order to increase predicability and reduce the chance of blockages.
It is therefore necessary to consider how the pressure drop across a plug of solids
depends on its length. Unfortunately contradictory experimental evidence is reported
in the literature. Konrad (1986b) points out that the pressure drop across
a moving plug has been reported to increase (a) linearly with plug length,
(b) as the square of the plug length and (c) exponentially with plug length.
A possible explanation of these apparent contradictions is reported by Klintworth
and Marcus (1985) who cite the work of Wilson (1981) on the effect of stress
on the deformation within the plug.
Large cohesionless particles (typically Geldart Group D particles)
give rise to a permeable plug permitting the passage of a significant gas flow
at low pressure drops. In this case the stress developed in the plug
would be low and a linear dependence of pressure drop on plug length would result.
Plugs of fine cohesive particles (typically Geldart Group C)
would be virtually impermeable to gas flow at the pressures usually encountered.
In this case, the plug moves as a piston in a cylinder by purely mechanical means.
The stress developed within the plug is high. The high stress translates
to a high wall shear stress which gives rise to an exponential increase
in pressure drop with plug length.
Thus it is the degree of permeability of the plug which determines
the relationship between plug length and pressure drop. The pressure drop
across a plug can vary between a linear and exponential function
of the plug length depending on the permeability of the plug.
Large cohesionless particles form permeable plugs and are therefore suitable
for discontinuous dense phase transport. In other materials, where interaction
under the action of stress and interparticle forces give rise to low permeability plugs,
discontinuous dense phase transport is only possible if some mechanism is used
to limit plug length, avoiding blockages.
6.2: Equipment
In commercial systems, the problem of plug formation is tackled in three ways:
(i) Detect the plug at its formation and take appropriate action to either
a) use a bypass system in which the pressure build-up behind a plug causes more air
to flow around the by-pass line and break up the plug from its front end. (Figure 13)
iii. Fluidization - add extra air along the transport line in order to maintain
the aeration of the solids and hence avoid the formation of blockages.
Whatever the mechanism used to tackle the plug problem, all commercial dense phase
transport systems employ a blow-tank which may be with fluidizing element (Figures 19
and 20) or without (Figure 21).
Fig. 19: Dense-phase transport blow tank with fluidizing element
Fig. 20: Twin blow tanks equipped with fluidizing pads at the bottom
Fig. 21: Blow tank without fluidizing element
Video 6 [25M, 116 min to download at 28.8kbaud]:
Powder Flow in Blow Tank Operation
The blow tank is automatically taken through repeated cycles of filling, pressurising
and discharging (see Video 6). Since one third of the cycle time is used for filling
the blowtank, a system required to give a mean delivery rate of 20 tonnes/hour
must be able to deliver a peak rate of over 30 tonnes/hour. Dense phase transport
is thus a batch operation because of the high pressures involved, whereas dilute phase
transport can be continuous because of the relatively low pressures and the use
of rotary valves. The dense phase system can be made to operate in semi-continuous mode
by the use of two blow tanks in parallel.
In some cases where it is necessary to transfer solids over short horizontal distances,
an air slide is used. In this device, solids are fed on to a porous membrane,
through which air is passed to fluidize the solids. This allows the solids
to flow easily even when the membrane is inclined at only 4 or 5 degrees
to the horizontal (Figure 22 and Video 7).
