Review of Dense Phase Pneumatic Conveying

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D J Mason, M G Jones, and Miss L Buchanan
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Powder Flow
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United Kingdom

Pneumatic conveying systems have been used by many industries to transport dry bulk particulate materials. The majority of installed systems are so called dilute phase systems where the particles that comprise the material are transported at low concentrations, suspended in the transport gas. Compared with other bulk handling systems they provide a simple means of enclosing the material and great flexibility in routing. The downside to this is that they are often less energy efficient and more hostile to the material being transported. The development of dense phase pneumatic conveying systems was aimed at reducing these disadvantages.

This review examines the state of the art of dense phase pneumatic conveying. The review is divided into five sections:

- Material capability for dense phase conveying.

- Conventional dense phase conveying systems.

- Special systems for low-velocity dense phase conveying.

- Modelling dense phase flow.

- Future research directions.

There has been a certain amount of confusion about the definition of dense phase conveying. The first section of the review examines the nature of dense phase flow, A gas-solids flow in a horizontal pipe is defined as being dense phase when the majority of particles are not suspended in the conveying gas. The possible modes of dense phase flow are identified, and the relationship between the shape of the operating envelope of the pneumatic conveying system and these modes of flow is presented. From this three classes of bulk material can be identified:

- Those only capable of dilute phase flow.

- Those capable of a moving-bed mode of dense phase flow.

- Those capable of a wave-like mode of dense phase flow.

Measurement of the gas diffusion properties (permeability and de-aeration) and cohesiveness of a bulk material provides a means of classifying materials according to the type of dense phase flow that they can achieve. This classification provides a means of assessing the suitability of the material for dense phase transport and the likely benefits in terms of system performance.

If a bulk material is naturally capable of dense phase flow then the selection of system components is similar to that for a dilute phase system. The second section of the review examines those systems that have most commonly employed for naturally dense phase capable bulk materials. Systems using pressure vessels as the solids feed device, blow tank systems, are probably the most common and feature throughout this section. Another important class of system employ so called ‘high pressure’ rotary valves. These permit a V IFPRI Report On Dense Phase Pneumatic Conveying Glasgow Caledonian University continuous solids feed while minimising the air leakage suffered by all rotary valves.

Although many of the feeders employed in these dense phase systems are capable of operating in systems with high pipeline pressure drops, the equation dense phase = high pressure is not always true.

Many users of dilute phase pneumatic conveying systems wish to obtain the benefits of conveying in a dense phase mode, but have materials that are not naturally capable of any mode of dense phase flow. The third section reviews the types of novel systems that have been developed to meet the demands of these applications. These systems may be classified as either:

- special feeders;

- special pipelines - air bypass and air injection system.

Special feeders attempt to introduce the solids into the pipeline in such a manner as to promote &nse phase flow. Special pipelines aim to maintain the correct conditions in the pipeline in order to sustain dense phase flow. From the previous discussion it is clear that the special pipeline systems have the best chance of achieving dense phase flow with a bulk material that is not naturally capable of such flow. When a bulk material is transported at a velocity below its natural minimum it will form a blockage. Most special pipelines permit operation below the natural minimum velocity by providing an automatic means of clearing pipeline blockages.

Air injection systems achieve this by injecting air into the blockage to clear the problem. Air bypass systems achieve this by providing an alternate route for the air flow. This directs air to the downstream end of the blockage to reduce the length of the blockage and hence aid its removal. Air bypass systems have the advantage of being less complex in construction, and do not change the total mass flow rate of gas in the pipeline.

The forth section reviews the types of model that have been developed to describe gas-solids flow in pipes and assesses them in terms of their suitability for predicting kzse phase flows. The aim of any model is to describe the behaviour of a physical process, in this case the model must be able to predict: 

-Straight Pipe: A FALL in the pressure drop for a constant solids flow rate as the gas flow rate is increased over the range of velocities normally employed (2-30m/ s)

-Inclined Straigh Pipe: For constant gas and solids flow rates, a maximum pressure drop which occurs at an angle less than 90” (vertically up).

-Bends: A RISE in the pressure drop for a constant solids flow rate as the gas flow rate is increased over the range of velocities normally employed (2-30m/ s)

The modelling techniques developed may be classified as either:

- Inside describe the physical processes that occur inside the Pipe

- Outside describe the behaviour of system components, usually derived from experimental correlations

Outside models have the advantage of simplicity, but are dependent upon experimentally determined parameters. The quality of the experimental data limits the application of these models in terms of scale-up and the range of bulk materials that can be modelled.

The development of inside models provides a means of understanding the physical phenomena that occur during transport. The disadvantage of this type of model is the computational effort required to solve an industrial scale problem.

The final section suggests some areas for further investigation. The key to a successful outcome of this work is the strength of the links between the various strands of the programme (experimental - measurement - theoretical).

David J. Mason