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Pneumatic conveying of bulk solids

Dense-phase pneumatic conveying of solids offers many advantages over dilute-phase conveying. The lower air velocities, and, consequently, lower particle velocities, result in lower pipe wear and lower particle attrition. This paper describes an experimental program that has been undertaken to study the flow pattern of cohesionless solids in vertical transport and to measure the parameters influencing the pressure drop required to move a single plug of solids. Highspeed photographic techniques have been used to observe the flow pattern of polyethylene particles (diameter ≏ 3 mm) in the vertical riser section of a circulating unit constructed from pipes with an internal diameter of 50.8 mm. The flow pattern resembles that of square-nosed slugging in a fluidized bed. The solids move up as “plugs” of bulk solids that occupy the entire cross-section of the pipe. Particles are seen to “rain” down from the back of one plug and then to be collected by the front of the next plug. Collecting these particles causes a stress on the plug front which is transmitted by powder mechanics forces axially through the plug and radially to the wall. The pressure drop required to move a single plug of cohesionless solids through the transport pipeline was measured as a function of the plug length, particle properties, pipe diameter, and the frontal stress. The results of these experiments are compared with a theoretical model.



Le transport pneumatique en phase dense de solides presente de nombreux avantages pour le transport en phase diluee. Les vitesses d'air les plus basses et, en consequence, les vitesses de particules les plus basses, entraǐnent une faible usure des conduites et une faible attrition des particules. Nous decrivons dans le present article un programme experimental mis sur pied pour etudier le diagramme d'ecoulement de solides sans cohesion dans le transport vertical et pour mesurer les parametres qui influencent la perte de charge necessaire pour deplacer un seul bouchon de solides. Nous avons fait appel a des techniques photographiques a haute vitesse pour observer le diagramme d'ecoulement de particules de polyethylene (diametre 3 mm) dans la section montante verticale d'un systeme de circulation compose de conduites de 50,8 mm de diametre interieur. Le diagramme d'ecoulement ressemble au bouillonnage a profil carre dans un lit fluidise. Les solides se deplacent vers le haut comme des bouchons solides en occupant toute la section transversale de la conduite. II semble que les particules tombent en “pluie” de l'arriere d'un bouchon et qu'elles sont collectees a l'avant du bouchon suivant. La collecte de ces particules cause une tension a l'avant du bouchon qui se transmet par les forces de mechanique des poudres dans le sens axial dans le bouchon et dans le sens radial vers le mur. La perte de charge necessaire pour deplacer un simple bouchon de solides sans cohesion dans une conduite de transport a ete mesuree en fonction de la lonueur de conduit, des proprietes des particules, du diametre du conduit et de la contrainte frontale. Les resultats de ces experiences sont compares a un modele theorique.

Vertical pneumatic conveying of particle plugs

Determination of a particle size distribution criterion for predicting dense phase pneumatic conveying behaviour of granular and powder materials

This thesis outlines the use of powder properties, determined from bench tests to predict powder flow behaviour in pneumatic conveying particularly in dense phase and super dense phase systems. The bench test powder properties examined included, particle size and distribution, bulk density and particle density, surface characteristics, fluidization and deaeration characteristics, powder cohesiveness, tensile strength and wall friction characteristics. The surface characteristics of various powders pneumatically conveyed were observed by an Electron Scanning Microscope. This information provides a good insight into flow characteristics. Differences in shape and surface explain vastly different conveying characteristics of supposedly similar powders. Particle size and distribution of various powders were measured by a Malvern Particle Sizer. It was found that particle size determines, to a large extent, powder cohesiveness, fluidization and deaeration characteristics. Furthermore, the effect of particle density, bulk density, permeability and compressibility on flow behaviour were examined. These properties were determined using a Beckman Pycnometer, a Jenike Compressibility Tester and a Jenike Permeability Tester. The observed fluidization characteristics not only revealed the powder classification with respect to Geldart's diagram, but also revealed the extent of powder cohesiveness and ease of segregation. An estimate of a powder's cohesion was conveniently evaluated by use of an Arch Tester. This tester consists of a perspex silo with a variable width slotted outlet opened by a chain drive mechanism. The powder remains undisturbed as the outlet is opened. Each powder bed examined was subject to a set deaeration time before opening the outlet. These results were compared with the cohesion measured in a direct shear tester. It was found that arch length and deaeration characteristics assists prediction of powder behaviour and cohesion. Tensile strengths were measured using an Ajax W.S.L. Tensile Tester under different extents of consolidation. This property was found to indicate cohesiveness and indirectly the air retention capacity of powders. The deaeration characteristics of powders were observed in a perspex cylinder suitably instrumented. In particular, pressure transducers were connected at the middle

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Flow assessment of powders in pneumatic conveying : a bench top assessment

/pdf/low-velocity-pneumatic-transportation-of-bulk-solids-4mztawsbhp.pdf

Low-velocity pneumatic transportation of bulk solids

Low-velocity dense-phase pneumatic conveying promises low power consumption with reduced attrition and abrasion. However, under certain circumstances, particles wedge together to form immovable plugs. Experimental and theoretical results on plug behaviour are briefly reviewed, illustrating means of avoiding blockage. Some existing low-velocity systems of the turbulence-generating, plug-forming and plug-destroying types are then examined. The optimum choice depends on the properties of the conveyed material. .

Marcus R D

Papers

A review of low-velocity pneumatic conveying systems