Doctor of Philosophy
Department of Mechanical Engineering
Desai, Mukeshchandra Kantilal, Flow assessment of powders in pneumatic conveying : a bench top assessment, Doctor of Philosophy thesis, Department of Mechanical Engineering, University of Wollongong, 1992. http://ro.uow.edu.au/theses/1579
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 and bottom of the cylinder fitted with permeable and impermeable bases, respectively. For effective measurement of deaeration characteristics, it was found that the fill rate should be as fast as possible. These characteristics are important for assessing air retention characteristics of powders.
Knowledge of the powder velocity is very important parameter in pneumatic conveying. For instance, if powders travel too slow, they drop out from the suspension and settle at the bottom of the pipe. This may lead to a blockage. Hence, it is necessary to convey powders above the critical settling velocity and they should not be conveyed with excessive velocity, which leads to powder degradation, pipeline wear and increased energy consumption.
To effect velocity measurement, an optical fibre probe was developed on the cross-correlation principle using two sensing probes, a fixed distance apart. The probe consisted of eight fibres in total. Six projector fibres are connected to a light source, which emits light to the powder passing through a sight glass fitted in the conveying line. In this case, the conveying line formed part of an actual pilot scale pneumatic conveying rig. Reflected light from the travelling powder was transmitted by two receiver fibres and cross-correlated, using a HP3721A correlator, to determine the particle transit time between the two fibres and thus predict the powder velocity.
Wall friction is another important factor contributing to the pressure drop in dense phase pneumatic conveying. The frictional properties of powders have an adverse effect in pneumatic conveying. These properties were evaluated under aerated conditions in a perspex tube by pushing powders upwards for different column lengths.
Due to the importance of cohesion in governing a powder's dense phase flow characteristics or whether it can be transported by pneumatic conveying, the cohesion strengths predicted by the Arch Tester, Jenike Shear Tester and the Tensile Tester. Hence, cohesiveness ranking of the various powders tested was possible.
A new phase diagram incorporating powder properties is proposed to predict a powder's an optimal pneumatic conveying mode.
This thesis concludes by correlating the described bench measured powder properties with reported actual powder pneumatic conveying characteristics. The identified correlations provide useful information for future pneumatic conveying system designs.