Year

2009

Degree Name

Doctor of Philosophy

Department

Centre for bulk solids and particulate technologies - Faculty of Engineering

Abstract

Dense-phase pneumatic conveying of powders is becoming increasingly popular in various industries such as power, pharmaceutical, cement, alumina, chemical, limestone, refinery, and so on. Some of the reasons include: minimum gas flows and power consumption; improved product quality; increased workplace safety. However, due to the highly concentrated and turbulent mode of the solids-gas flow, only limited progress has been be achieved so far in understanding the fundamental transport mechanisms and accurately predicting pipeline pressure drop, which is a key system design parameter. This thesis aims to overcome the present limitations and provide the industry with a new validated modelling procedure for the accurate prediction and scale-up of pressure drop and optimal operating conditions for fluidised dense-phase pneumatic conveying systems.

Various popular/existing models (and model formats) for solids friction (for straight horizontal pipes) have been evaluated for scale-up accuracy and stability. It has been found that the models (and their use of parameter groupings) are generally not capable of accurately predicting pressure drop under scale-up conditions of pipeline diameter and/or length. Two new approaches and another method based on the parameters used by other researcher have been employed in this study as improved design techniques. One approach, derived by modifying an existing reliable dilute-phase model to make it suitable for dense-phase, has resulted in a substantial relative improvement in the overall accuracy of predictions under scale-up conditions for two types of fly ash, ESP dust, pulverised coal and fly ash/cement mixture. Another method has been derived using the concept of “two-layer” slurry flow modelling (i.e. suspension flow occurring on top of a non-suspension moving layer), and this has also resulted in similar improvements. The third method, using parameters that were mentioned by another researcher as providing better representation of the flow phenomenon, has also resulted in similar reliable predictions.

Three different popular/existing bend models have been evaluated to select an optimal (bend loss) model for dense-phase powder conveying. It has been found that the estimation of bend pressure drop can have a considerable impact towards correctly predicting the total pressure loss in a pneumatic conveying system.

An existing method of representing “minimum transport criteria” (based on superficial air velocity and solids loading ratio) has been found inadequate for predicting the unstable boundary, especially under diameter scale-up conditions. Based on the experimental data of various powders conveyed over a wide range of pipe lengths and diameters, it is found that with increase in pipe diameter, the requirement of minimum conveying air velocity increases. To capture the pipe diameter effect, a Froude number based approach has been introduced to reliably represent the minimum transport boundary.

The thesis also investigates the suitability of using a direct differential pressure (DP) measurement technique across a straight length of pipe for fine powder conveying in dense-phase. Standard Deviations (SD) of the DP, as well as the static pressure signals are presented. The trend shows the SD values are increasing with increase in pipe length from pipe inlet to exit (i.e. a dependence on tapping location).

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