Pneumatic conveying - Dilute Phase - Lean Phase : an overview

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Section summary
1. Pressure dilute phase
2. Vacuum dilute phase
3. Dilute phase solid to air ratio
4. Pneumatic conveying systems suppliers

Section summary

1. Pressure dilute phase

2. Vacuum dilute phase

3. Dilute phase solid to air ratio

4. Pneumatic conveying systems suppliers

Pneumatic conveying consists in transporting a bulk solids (powder, granules) in a pipe thanks to an air flow created by an overpressure or a vacuum at one end of the pipe. Different technologies of pneumatic conveying exist, among them dilute phase conveying.

Dilute phase conveying, also called lean phase conveying, is the most common type of pneumatic transport for bulk solids across industries. With this type of pneumatic conveying system, the air quantity is high compared to the product quantity (there is much more air than material to transport) thus the conveying phase is dilute. Its simplicity and efficiency, along with a good containment of the product and dust, make it in many case the 1^st choice considered by factory operators when having to move bulk solids or powder from one point to another. Among other materials, it is often used to convey :

1. PRESSURE - Lean Phase pneumatic conveyor

1.1 What is dilute phase PRESSURE conveying ?

The air mover in pressure lean phase is most of the cases a Blower or compressor. The air conveyor blower most common type is to use a Roots Blower which combines good reliability, high air flow and can reach enough pressure for most of the industrial needs. This Blower will allow to reach max 1 bar g.

Another possibility is to use compressed air for some specific cases using a venturi to pick-up the powder. This solution is not very economical, therefore it should be kept for very specific cases and low throughput.

In order to introduce the powder to the conveying pipe, an Airlock rotary Valve should be used. This equipment will allow a rough dosing of the powder to the pipe, which is necessary to avoid blockages but will have as main function to isolate the area of high pressure (the pipe) to the area of low pressure (the hopper introducing the powder). This avoids to have the air going in the product and preventing its flow.

The receiver is most of the time equipped with an Airlock rotary Valve also in order to allow a separation of the air and the product. In some cases, it is possible to avoid such star valve if the receiver is big enough and the situation is safe (discharge directly to a silo).

1.2 Flow diagram of pressure dilute phase conveying

The essential pneumatic conveying equipment required for pressure dilute phase conveying are represented in the following flowsheet :

Figure 1 : Pressure lean phase typical process arrangement

2. VACUUM - Lean Phase pneumatic conveyor

2.1 What is dilute phase VACUUM conveying ?

In vacuum lean phase, the air mover is basically the same than in pressure lean phase, except that it is positioned at the end of the line thus sucking the air and creating a vacuum. The vacuum created is maximum around -400 / -500 mbar g at compressor suction.

At product pick up, there is the need of having an air inlet to allow the air sucked to flow in the pipe. On the product side, no Airlock rotary Valve is necessary here since the air will go in the direction of the product and will therefore not prevent it to flow. A system to control the product intake is however necessary (screw, simple valve) in order to avoid that a surge in product flow block the pipe.

At reception, the best practice is to position a cyclone with an Airlock rotary Valve. The design must take into account that the air flow from the downstream equipment will this time flow against the product. A proper degassing must be considered.

Among the key pneumatic conveying components required for a successful dilute phase conveying, the typical instrumentation should include the following : pressure sensor at compressor suction, differential pressure drop through filter, level sensor at reception.

2.2 Flow diagram of vacuum dilute phase conveying

The essential pneumatic material handling equipment required for vacuum dilute phase conveying are represented in the following flowsheet :

Figure 2 : Vacuum lean phase typical process arrangement

3. Dilute phase solid to air ratio

The dilute phase solid to air ratio is typically around 5 kg of material / kg of air.

4. Pneumatic conveying system suppliers

There are many companies proposing dilute phase conveying systems and one should be careful in selecting the right air conveyor manufacturer. The design of such system require indeed a certain degree of experience. Even if some design calculation methods have been published, specialized companies will have been able to refine those models and develop their own methodologies, ensuring successful application. Specialized companies will also be able to apply good design practices especially in terms of air velocity, pipe layout and bends.

Reputable suppliers will also be able to propose trials in their test plant when in doubt, which is a very important step to make sure the system will perform properly.