Vertical plug-flow pneumatic conveying from a fluidised bed

Experiments are described on the pneumatic conveying of 2.7 mm alumina particles up a vertical riser of internal diameter 46.4 mm or 71.4 mm. The particles entered the riser from a fluidised bed, via a short horizontal pipe and a bend of radius 75 mm. Measured variables included solids flow rates, air flow rates, inlet and outlet air pressures P1 and P2, and the pressure profile in the riser. The solids flow rate was consistent with some earlier models of similar systems, in which the plugs of packed solids move up at a velocity of about U − Umf, where U = superficial air velocity and Umf = incipient fluidising velocity. Solids–wall friction is significant and suppresses fluidisation. To model the system approximately, a conveying efficiency = (power for air compression) / (rate of gain of potential energy of solids) is defined and correlated against solids flux. It was found that the conveying efficiency tended to an asymptote just above 20%. The correlation led to a tentative design formula, Eq. (6), for predicting P1 − P2 at a given solids flow rate. P1 − P2 is typically between 50% and 100% of the pressure drop needed to support a column of solids of height equal to that of the riser.It was concluded that plug flow pneumatic conveying is a satisfactory technology for transporting coarse particles which cannot be conveyed in leaner regimes due to the possibility of pipeline erosion or solids attrition.

A vertical pneumatic conveying line was started from a fluidised bed. Particle diameter/density was high, causing square-nosed plugging. Solids and gas flow rates were linearly related and depended on line entry diameter. Transport efficiencies ranged from 5 to 25% and correlated well with solids flux, enabling prediction of the line pressure drop.Figure optionsDownload as PowerPoint slideHighlights
► High particle size and diameter suppress solids fluidisation.
► Solids and gas flow rates are linearly related and controlled by line entry diameter.
► Transport efficiency ranged from 4% to 22% and correlated well with solids flux.