|3. How to reduce product breakage in pneumatic conveying|
When a solid is conveying in a pneumatic conveying transport, some of its physical properties will most probably be affected. The most common problem observed industrially is to break the material thus change the particle size distribution PSD of the product being conveyed. Generally large particles will break when the solids is hitting the conveying pipe, especially at bends where the flux of air and product needs to change direction. Because of this attrition, the PSD will be shifted towards smaller particles and fines will be generated.
The product breakage can be fully characterized by measuring the PSD of the material before and after transport. This PSD can be simply gathered through sieving or thanks to more complexed and precise methods like laser diffraction. However, industrially, a very quick estimation of the impact of the transport on the solid can be done by measuring the bulk density - tapped - before and after the transport. The more the bulk density increases, the more the breakage.
Equation 1 : calculation of solids breakage
With :- d1 = tapped bulk density of solids BEFORE transport (g/l)
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There is no typical value of breakage nor can it be calculated a priori. Trials should be carried out to estimate it. Now the attrition is directly linked to the impact of the solids towards the conveying pipe, thus it can be reduced and controlled by applying the following measures :
- Reduce the conveying velocity. In design, the conveying velocity should be minimized and confirm through pilot plant trials. For existing installation, it is often observed that the conveying velocity is way above its optimal, it is then possible, if the blower is on VFD, to progressively correct the air flow rate, observing the behavior of the line, and find an optimum.
- If possible, switch to dense phase conveying, which offers significantly lower air conveying velocity compared to dilute phase.
- Reduce the number of bends, especially in dilute phase. Bends are the main contributor to product breakage and every bends avoided will save the product from damages. At design stage, it is sometimes better to consider a slightly longer pipe layout but with less bends, rather than having a short pipe but turning around every equipment.
- Change the bend type. For some application, specific designs of bends can be chosen which are reducing the impact force and thus the breakage. Long radius bends are a solution, although not always the most efficient, while designs creating a layer of product in the bend, on which the product will softly impact, are generally quite efficient.
It is also important to note that particle degradation is sometimes
coupled with pipe wear, with the pipe getting damaged due to the
impact of the particles on the bends especially.