Gas–solid separation devices:Separation requirements and Separation mechanisms.

Gas–solid separation devices

Introduction

Gas–solid separation devices associated with pneumatic conveying systems have two functions. The first is to recover as much as possible of the conveyed material for the next stage of the handling or treatment process. The second is to minimize pollution of the working environment by the material. A number of devices are available that meet these requirements. Particle size is the main parameter to be considered here in terms of system selection and air flow rate in terms of system sizing.

Separation requirements

The first of these functions is principally a matter of economics, in that the more valuable the material, the more trouble should be taken to ensure total recovery. However, the avoidance of environmental pollution is potentially more important, particularly since the introduction of more stringent Health and Safety at Work Legislation. Where the material is known to be potentially dangerous, of course, extreme measures must be taken to prevent its escape into the atmosphere from the handling plant. This is particularly the case with toxic and explosive materials.

The choice of gas–solid disengaging system to be used on any given application will be influenced by a number of factors, notably the amount of bulk particulate material involved, the particle size range of the material, the collecting efficiency required, and the capital and running costs. In general, the finer the particles that have to be collected, the higher will be the cost of a suitable separation system.

Separation mechanisms

Where a bulk material consists of relatively large and heavy particles, with no fine dust, it may be sufficient to collect the material in a simple bin, the solid material falling under gravity to the bottom of the bin, while the gas is taken off through a suitable vent. However, with a bulk solid of slightly smaller particle size it may be advisable to enhance the gravitational effect, and the most common method of achieving this is to impart spin to the gas–solid stream so that the solid particles are thrown outwards while the gas is drawn off from the centre of the vortex. This is basically the principle on which the cyclone separator operates.

Where fine particles are involved, especially if they are also of low density, separation in a cyclone may not be fully effective, and in this case the gas–solid stream may be vented through a fabric filter. Many different types of fabric filter are in use and selection depends mainly upon the nature of the solid particles being collected and the proportion of solids in the gas stream.

For materials containing extremely fine particles or dust, further refinement in the filtration technique may be necessary, using wet washers or scrubbers, or electrostatic precipitators, for example. While this last group of gas–solid separation devices are used in industry, they are generally used in association with a process plant, and are very rarely used in conjunction with a pneumatic conveying system, and so no further reference will be made to any of these devices.

Pressure drop considerations

The separation device should not present a high pressure drop to the system if maximum material flow rate is to be achieved for a given overall pressure drop. This is particularly the case in low pressure fan systems, where the pressure drop across the separation unit could be a significant percentage of the total pressure drop available. Regular maintenance of separation equipment is important. The pressure drop across fabric filters will increase rapidly if they are not cleaned regularly, or if the fabric is not replaced when cleaning is no longer effective. If cyclones are used for separation, wear will reduce the separation efficiency.

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