Pneumatic conveying in the dense and dilute phase

Abstract

Pneumatic conveying is one of the backbones of chemical engineering as it has been used extensively in almost every industry. Yet many parameters involved in a usual conveying operation are not well understood. One important parameter is the solid concentration and solid flow behaviour after going through a pipe bend. The relationship between solid concentration and other parameters, like particle size, conveying velocity, pipe wall roughness etc, is a complex one. Computational fluid dynamics (CFD) was being used to investigate the relationship between these parameters, as well as observe the flow patterns that result. Two computational models were used, one with an Euler-Lagrange frame of reference and the other, an Euler-Euler one. The Euler-Lagrange model takes into account particle-wall collisions but not particle-particle interactions, and was only done in the dilute solid phase. The Euler-Euler model used a two fluid approach to model particle-particle as well as particle-wall interactions, with both dense and dilute conditions being modelled. Turbulence was also included for both models used. The two models were both validated quantitatively and qualitatively using data from previous experiments done in the department as well as from open literature. The results showed good agreement. For the Euler-Lagrange model, results for the solid behaviour after the bend was taken for the validation, while for the Euler-Euler model, the two-phase flow behaviour in the section before the bend was used. Four different geometries, of different radius of curvatures of the bends, were used for the Euler-Lagrange model. Other parameters varied using this model include particle size, inlet turbulence, orientation of the bend, particle shape, wall restitution coefficient, pipe wall roughness and conveying velocity. The results showed that of all factors considered, particle size was the most important in determining the concentration and flow pattern after the bend, especially rope formation and evolution. For the Euler-Euler model, only the sharp bend was used, in the horizontal to vertical orientation. Polypropylene particles 2.8mm in diameter were used in order to compare with previous experiments conducted. The results showed some qualitative agreement. The solid flow patterns observed in the simulations also had trends in the flow that were opposite to those observed by previous investigators. Sensitivity analysis with this model was done by varying inlet gas velocity. The results showed qualitatively similar trends.