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Section summary |
---|

1. Definition of
bulk density |

2. Calculation of
bulk density |

3. Measurement of bulk density |

4. Applications |

5. Use of bulk density in process
design |

Bulk solids density should not be confused with particle density and skeletal density : you can find more information on these 2 important characteristics there :

Powder bulk density is the density of the bulk of the particles, included the voids in between each particles. 2 bulk densities are usually measured :

- Loose Density
- Tapped Density

Equation 1 : Solids Bulk Density

When measuring the loose density, the powder must be in its aerated state.

Figure 1 : Measuring loose bulk density

When measuring tapped density, a step of powder compaction is necessary.

Figure 2 : Measuring tapped bulk density

The sample volume is shown to be 100 cm3 in the example above because it helps a quick calculation of the bulk density, however it could be in principle any volume provided the ratio of weight of the sample over the volume of the sample is done correctly.

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Bulk density is used, among other purposes, to calculate the sizes of vessels in the process (hoppers, silos...)

*Example*

A weighing hopper must hold a maximum of 500 kg

The loose density of the powder dosed is 0.34 kg/l

The required useful volume of the hopper must be 500/0.34=1470
liters (be careful here to angle of repose of the powder to
calculate the total volume of the hopper)

WARNING : the loose and tapped density are critical bulk powder properties and should not be confused when designing an installation. It can lead to process limitations. It is especially the case in the following cases.

**Hopper design :** after tipping, pneumatic transport, or basically
movement of powder, the powder tends to be aerated and therefore
takes its loose density value. If hoppers are designed according
to tapped density, buffer volumes may be too small. When
considering a product with a CARR Index of 20 which is common, the
difference in between both bulk densities will be 20%, this can
make the difference between a process that operates
continuously... and a process that stops because the buffer size
was too low to contain the right weight of powder. The powder in
the hopper will tends towards the tapped density only with time or
vibration, which will make the powder more compact.

**Volumetric dosing :** such dosing
system is based on the assumption of a density of the
powder, if it is different, the feeding in terms of kg/h will not
be at target.