Dear Promax,

The principles of fluidization are not sufficient for pneumatic conveying.

In fluidization, the gas, flowing through a particle bed, just keeps the particles lifted vertically., whereby the particles do not touch each other anymore.

(That is the reason for fluidized behavior with almost no internal friction)

The only energy used is for keeping the particles in suspension against gravity.

In pneumatic conveying, the particles also have to be kept in suspension, but also o be moved.

Therefore more energy is required to accelerate, lift and overcome collision losses.

Also keeping the particles afloat in a horizontal pipe is different from fluidizing.

The energy is delivered by the expanding air and therefore more air volume at a higher pressure is needed.

If you study pneumatic conveying in depth, you will find the principle rather simple, but the interaction between gas, solids and surroundings is very complex.

Success

teus ■

Geldart classification which is based on fluidisation is used to assess the material’s suitability for dense phase conveying. As far as my memory serves me it is discussed in G E Klinzing book on Pneumatic conveying. But the best reference is this paper “Characterisation of bulk solids to assess dense phaseconveying” G E Klinzing et al. published in Powder Technology 2003. ■

For system design in terms of dilute and dense phase conveying, and system selection, and also conveying line inlet air velocity, start with the bulk properties 'air retention' and 'permeability'.

If the material has good 'air retention' it is likely to be a candidate for low velocity dense phase conveying in sliding bed flow. If a high pressure gradient is available the material is likely to be conveyable at high values of solids loading ratio (say up to 100) and with a low conveying line inlet air velocity (say down to 3 m/s). For dilute phase suspension flow of such a material a conveying line inlet air velocity of about 14 m/s should be OK.

If the material has good 'permeability' it is likely to be a candidate for low velocity dense phase conveying in plug flow. Solids loading ratios will be up to about 30, but this is really irrelevant because of the permeability. Conveying line inlet air velocities can be down to 3 m/s for dense phase flow, but these materials generally have a 'pressure minimum point' which means that with pick-up velocities below about 13 m/s there will be a reduction in material flow rate with decrease in velocity. Then with pick-up velocities above about 13 m/s there will be a reduction in material flow rate with increase in velocity. For dilute phase conveying of these materials a minimum conveying air velocity of about 17 m/s is generally OK.

If the material has neither good air retention or permeability it is unlikely that it will be possible to convey the material in low velocity dense phase flow, in a conventional conveying system, even if a high pressure gradient is available. If low velocity conveying for such a material is required one of the 'innovatory' conveying systems could be considered, such as air addition with boosters, or pipe-in-pipe with a by-pass line. For dilute phase suspension flow of such a material the conveying line inlet air velocity will depend mainly upon the 'mean particle size', 'size distribution', 'particle shape' and 'particle density'.

Then you need to worry about 'abrasiveness', and the conveyed material damaging the conveying system; 'friability' and the conveying system damaging the conveyed material; 'dust explosion potential' and the use of inerting and a closed loop system, and similarly with 'toxicity'; etc. ■