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Pneumatic conveying - Dense phase

Principle of operation, main characteristics of dense phase conveying systems with pressure or vacuum

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Section summary
1. Definition of dense phase conveying
2. Pressure dense phase
3. Vacuum dense phase

1. Dense phase pneumatic conveying systems : a definition

What is dense phase pneumatic conveying ?

Dense phase conveying system are a type of the pneumatic conveying system, which is using air to transport bulk solids / powder in pipes. Dense phase conveying differs from the other main technology of pneumatic conveying, dilute phase, by the fact to use only a small weight of air compared to the weight of product being transported. It means that dense phase systems operate at a high solids load ratio and at low air velocity. Dense phase conveying systems present many advantages, summarized below :

  • The air velocity in pipe is low, typically 3-8 m/s at the beginning of the line, which allows to reduce breakage of the solids being transported
  • Transporting at high density, with low air speed, reduce phenomena responsible for segregation of a mixture during pneumatic transport, which means that it is a method of choice for transporting powder after a mixer
  • If high solids load ratio are reached, the transport is usually more energy efficient than dilute phase conveying
  • Picking up the materials at the beginning of the line is easier, as it does not require a metering instrument such an airlock rotary valve or screw conveyor
Dilute phase and dense phase conveying comparison

Figure 1 :  Dilute phase conveying vs dense phase conveying

On the other side, because of a higher pressure drop, dense phase conveying will typically reach lower conveying length compared to dilute phase, except if very high pressure is used, and may lead to a higher initial investment.

2. PRESSURE - Dense Phase conveying

How works pressure dense phase pneumatic conveying ?

Pressure dense phase will require a specifically designed sender hopper in order to withstand pressures from 1 to 3-4 barg in most cases. Such a tank is called a pressure tank or pressure pot. It features a compressed air inlet at the top and another one after its discharge valve. It should also be equipped with a pressure safety valve in order to avoid any uncontrolled pressure increase. A degassing system with a filter must also be installed.

The receiver must be equipped with a filter and may also be equipped with a star valve if necessary. If the receiver is big enough and designed to work at atmospheric pressure, such equipment may not even be necessary.

As part of the instrumentation, pressure sensors in the tank, at the beginning of the pipe and at reception will be key since the whole system will be driven depending on the pressures. Level sensors in the sender hopper in order to detect the end of filling and the end of discharge are necessary while a level sensor at the receiver should be installed to avoid overfilling. Compressed air flowmeters are optional but constitute a good troubleshooting tool.

pneumatic conveying systemPneumatic conveying pressure dense phase

Figure 2 : Pressure dense phase typical process arrangement

3. VACUUM - Dense Phase conveying

How works vacuum dense phase pneumatic conveying ?

A vacuum dense phase system is quite similar to a lean phase setup except that a vacuum pump allowing to reach pressure < -900 mbarg should be installed.

The inlet of material is usually done through a sender hopper. No specific construction for the sender hopper is needed, which can be an advantage compared to pressure dense phase since the access is easier.

The receiver, on the contrary, must be designed for full vacuum, which means it must be reinforced and will account for some cost increase. It should be equipped with a star valve or with a butterfly valve if the transport is in batch

A pressure sensor at the receiver and at the pump suction is necessary. The pump should be on frequency drive if possible so that the operation can be flexible and energy efficient.

Pneumatic conveying vacuum dense phase

Figure 3 : Vacuum dense phase typical process arrangement

4. Materials that can be conveyed in dense phase

Which materials can be conveyed in dense phase ?

In order to be able to form slugs, the powder must generally have a low permeability, which means that air cannot go easily through the materials which would fluidize it too much and prevent to be pushed efficiently. Referring to the classification of Geldart, materials of category A or C are generally possible to convey dense phase (fine powders, powders channeling rather than fluidizing).

However it is only a generality as some materials with high permeability and fluidization properties can actually be conveyed dense phase if they are cohesive enough to keep together the slug of product during transport. [Coco] is giving the example of plastic pellets, a granular material thus with a high permeability to air, but able to convey dense phase thanks to the electrostatics forces keeping the particles together. On the other hand, such a conveying of materials have a priori adverse properties for dense phase conveying can be fragile, referring to the example of plastic pellets conveying, the humidity must be well controlled, as, if it increases, the electrostatic forces will decrease which can lead to a dislocation of the slugs and ultimately plugging of the pipe.

Source

[Coco] Understanding Dense Phase conveying, Ray Coco, PBE, 2018