Generally speaking it is possible to convey any powder in the dilute phase mode,
but because of the attractions of dense phase transport, there is great interest
in assessing the suitability of a powder for transport in this mode.
The most commonly used procedure is to undertake a series of tests
on a sample of the powder in a pilot plant. This is obviously expensive.
An alternative approach offered by Dixon (1979) is quite widely used.
Dixon recognised the similarities between gas fluidization and dense phase
transport and proposed a method of assessing the suitability of a powder
for transport in the dense phase based on Geldart's (1973) classification
of powders. Dixon proposed a 'slugging diagram' which allows prediction
of the possible dense phase flow patterns from a knowledge of particle size
and density. Dixon concluded that Geldart's Groups A and D were suitable
for dense phase transport whereas Groups B and C were generally not suitable.
Mainwaring and Reed (1987) claim that although Dixon's approach gives
a good general indication of the most likely mode of dense phase transport,
it is not the most appropriate means of determining whether a powder will
convey in this mode in the first instance. These authors propose an assessment
based on the results of bench-scale measurements of the permeability
and de-aeration characteristics of the powder. On this basis powders achieving
a sufficiently high permeability in the test would be suitable for
plug type dense phase transport and powders scoring high on air retention
would be suitable for transport in the rolling dune mode of dense phase flow.
According to the authors, powders satisfying neither of these criteria
are unsuitable for transport by conventional blow tank systems.
Flain (1972) offered a qualitative approach to matching the powder
to the system. He lists twelve devices for bringing about the initial
contact between gas and solids in a transport system and matches powder
characteristics to device. This is a useful starting point since certain
equipment can be excluded for use with a particular powder.