Fluidized dense phase pneumatic conveying: a review

Abstract

The focus of investigations and research has been shifted in the past decade from conveying bulk materials in dilute phase flow at high velocity to dense phase flow at low conveying air velocity. Dense phase flow offers much less operational problems, such as wear and product attrition, and offers low specific power consumption. It, therefore, enhances the life of pneumatic conveying pipelines and associated components. However, the complete phenomenon of the flow mechanism of fine powder in fluidized dense phase pneumatic conveying has not been exactly explored. It necessitates an understanding of the flow behaviors of a fine powder, factors affecting the flow mechanisms, and approaches to model it. The present work describes the evaluation and assessment of the flow of fine powders in the fluidized dense phase. The performance of the pneumatic conveying system was based on modeling solids friction factor which was used for determining conveying line pressure drop. The predicted results provided by these approaches proved to be inaccurate compared to those of data collected from actual plants. Further, the numerical modeling methods to predict pressure drop for small length pipeline, difficulties, and recent progress in gas-solid prediction investigations are discussed as well. Finally, a Bypass dense phase conveying system which has many advantages over the conventional one has been presented.

Keywords:

• Dense phase conveying
• solids friction factor
• transient
• computational fluid dynamics
• bypass conveying

Nomenclature

Abbreviations
FDP:	=	fluidized dense phase
ECT:	=	electrical capacitance tomography
ESP:	=	electrostatic precipitator hoppers
PSD:	=	power spectral density
HHT:	=	Hilbert-Huang Transformation
CFD	=	Computational Fluid Dynamics

Greek symbols
${\lambda }_a$	=	Air/Gas friction factor
${\lambda }_s$	=	Solids friction factor
${\rho }_a$	=	Density of air (kg/m3)
ρam	=	Mean air density (kg/m3)
${\rho }_{ai}$	=	Inlet gas density (kg/m3)
${\rho }_s$	=	Particle density (kg/m3)
ρsus	=	Suspension density $\frac{\mathit{mass}\mathrm{ }of\mathrm{ }\mathit{solids}\hspace{0.17em}+\hspace{0.17em}\mathit{mass}\mathrm{ }of\mathrm{ }\mathit{gas}}{\mathit{Volume}\mathrm{ }of\mathrm{ }\mathit{solids}\hspace{0.17em}+\hspace{0.17em}\mathit{Volume}\mathrm{ }of\mathrm{ }\mathit{gas}}$ (kg/m3)
τ1	=	Relative contribution of non-suspension layer (Pa)
τ2	=	Relative contribution of suspension layer (Pa)
β0	=	Ratio of free settling velocity to superficial gas velocity

List of symbols
Fri,min	=	Minimum Froude number at the inlet
Fri	=	Inlet gas Froude number
Fr	=	Gas Froude number
Frs	=	Solid Froude number
m*	=	Solids loading ratio
K and a	=	Constants
$\Delta P$	=	Total pressure drop (Pa)
$\Delta P_a$	=	Air pressure drop (Pa)
$\Delta P_s$	=	Solids pressure drop (Pa)
$\Delta P_b$	=	Bend pressure drop (Pa)
$v_a$	=	Velocity of gas (m/s)
$v_s$	=	Velocity of particles (m/s)
$v_{ai}$	=	Gas/Air velocity at pipe inlet (m/s)
V	=	Superficial air/gas velocity (m/s)
L	=	Total length of the pipe (m)
D	=	Diameter of pipe (m)
dp	=	Mean particle diameter (m)
g	=	Acceleration due to gravity (m/s2)
Lev	=	Equivalent length of the vertical section (m)
Lv	=	Length of the vertical section (m)
Le	=	Equivalent length of the total pipeline (m)
$m_{s2}$	=	Solids mass flow rate in full-scale pilot plant (kg/s)
$m_{s1}$	=	Solids mass flow rate in test pilot plant (kg/s)
D2	=	Diameter of pipe of full-scale pilot plant (m)
D1	=	Diameter of pipe of test pilot plant (m)
VLR	=	Volumetric loading ratio {(ms/ρs)/(mf/ρ)}
wf0	=	Free settling velocity of an isolated particle (m/s)

Subscripts
a	=	Gas or air phase
s	=	Solids phase
b	=	Bend
ai	=	Inlet gas or air phase