Vertical pneumatic conveying of particle plugs

Abstract

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 5 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 prksente de nombreux avantages pour le transport en phase diluke.Les vitesses d'air les plus basses et, en conskquence, les vitesses de particules les plus basses, entrainent une faible usure des conduites et une faible attrition des particules. Nous dtkrivons dans le prksent article un programme expinmental mis sur pied pour etudier le diagramme d'kcoulement de solides sans cohtsion dans le transport vertical et pour mesurer les paramktres qui influencent la perte de charge ntcessaire pour dkplacer un seul bouchon de solides. Nous avons fait appel B des techniques photographiques B haute vitesse pour observer le diagramme d'tcoulement de particules de polyethylkne (diamktre 3 mm) dans la section montante verticale d'un systkme de circulation compost de conduites de 50,8 mm de diamttre intkrieur. Le diagramme d'tcoulement resemble au bouillonnage a profil c a d dans un lit fluidist. Les solides se deplacent vers le haut comme des bouchons solides en occupant toute la section transversale de la conduite. I1 semble que les particules tombent en "pluie" de I'arritre d'un bouchon et qu'elles sont collectkes a I'avant du bouchon suivant. La collecte de ces particules cause une tension B I'avant du bouchon qui se transmet par les forces de mkchanique des poudres dans le sens axial dans le bouchon et dans le sens radial vers le mur. La perte de charge nkcessaire pour dkplacer un simple bouchon de solides sans cohtsion dans une conduite de transport a ktt mesurCe en fonction de la lonueur de conduit, des proprittes des particules, du diamktre du conduit et de la contrain...

Flow regime diagram for vertical pneumatic conveying and fluidized bed systems

“…2, that the classical and accumulated choking velocities predicted by Eqs. (7) and (8) are in reasonable agreement with the experimental results for a relatively small range of Archimedes numbers. The general accuracy of the classical choking velocity predicted by Eq.…”

“…The first is the plug flow regime, which is characterized by particles that are transported as plugs separated by air gaps. Sometimes these particles fall from the bottom of one plug and collect at the front of the consequent plug [8]. This phenomenon is known as "particle rain" and occurs when the cohesion force between the particles is smaller than the particle weight.…”

“…Note that the velocity which relates to the transition between the plug flow regime and blockage of the pipeline is absent from the diagram, that is because of a very poor theoretical and empirical basis presents in the literature. The literature proposes some theoretical models [8,50] conditions by comparing the friction, drag, gravity and adhesion forces, however the experimental basis of those models is inadequate. The non-dimensional terms of the diagram enable us to find a suitable flow state for any particle or gas property.…”

Fundamental investigation of plug conveying of cohesionless particles in a vertical pipe (pressure drop and friction of a stationary plug)

“…Many reports concern the results of conveying tests of particular systems. Some researchers have been studying plug flow from a basic viewpoint, for example, Konrad et al (1980), Tomita et al (1981), Konrad and Davidson (1984), Konrad and Totah (1989), Borzone and Klinzing (1987), Gu and Klinzing (1989). One of the present authors investigated plug flows before (Tsuji et al, 1982a, b), but the cases were rather special because the plug flows studied accompanied secondary air injection.…”

Plug conveying in a vertical tube

“…Unfortunately, current models [21,22] of plug conveying disagree even on the prediction of such basic quantities as the pressure drop and the total mass flow, and these quantities have a great impact in the industrial application. One of the reasons for the lack of valid models is that it is difficult to study plugs experimentally in a detailed way.…”