This document discusses sampling techniques and properties relevant to particulate materials. It covers the importance of representative sampling, basic sampling rules including sampling from a moving stream, and methods for sample splitting. The document also defines particle density and bulk density, and discusses techniques for measuring apparent density of porous particles. Finally, it discusses using the Hausner ratio, which is the ratio of tapped bulk density to aerated bulk density, as a measure of powder flowability and cohesiveness.

1. SAJJAD KHUDHUR ABBAS
Ceo , Founder & Head of SHacademy
Chemical Engineering , Al-Muthanna University, Iraq
Oil & Gas Safety and Health Professional – OSHACADEMY
Trainer of Trainers (TOT) - Canadian Center of Human
Development
Episode 37 : SAMPLING

2. SAMPLING
•As most laboratory tests use only a small sample,
this has to be taken from a production stream or from
an existing, stored material and it has to be
representative of the whole.
•Representative sampling absolutely critical for
the success and relevance of any subsequent
testing.

3. Segregation of
Free Flowing
Particulate
Materials.
•Sampling is therefore such an important
element of powder handling that it demands
careful scientific design and operation of the
sampling systems.

4. •The purpose is to collect a manageable mass of
material (- sample) which is representative of the
total mass of powder from which it was taken.
•This is achieved by taking many small samples
from all parts of the total which, when combined,
will represent the total with an acceptable degree
of accuracy.
•This means that all particles in the total must
have the same probability of being included in
the final sample. All parts of the total have to be
equally accessible.

5. • To satisfy the above requirements, the following
basic “ golden” rules of sampling should be
followed whenever practicable.
1. Sampling should be made preferably from a
moving stream (this applies to both powders and
suspensions) but powder on a stopped belt can be
sampled.
2. A sample of the whole of the stream should be
taken for many (equally-spaced) periods of time
rather than part of the stream for the whole of the
time.

6. •It is very likely that the re-combined, primary
sample taken from the whole is going to be too
large for most powder tests and it, therefore, needs
to be sub-divided into secondary or even tertiary
sub-samples.
•Allen(1981) reviewed and tested most methods
available for sample splitting and found the one
based on the spinning riffler to be the best.

9. DENSITY
•When processing and handling a particle, the
requisite density is the mass of the particle divided
by its volume, Fig. 2, and this differs from the
absolute or skeletal density if the particle is porous,
since the volume in question contains all pores,open
and closed.
•The defination of particle density, ρp
ρp
= Mass of a particle, M
Hydrodynamic envelope bounding particle volume,Vp

10. Open pores
Closed
pores
Hydrodynamic
envelope bounding
particle volume, Vp
Fig. 2 Defination of particle
density

11. •This "hydrodynamic" density is also given various
other ;names -apparent, particle, envelope,
effective, piece, density- and it is nither
straightforward to measure nor constant when the
particles are porous.
•The skeletal density is obtained from a gas
pycnometer.

12. •There are available seven methods for measuring the
apparent density of porous particles: (a) mercury
porosimeter, (b) caking end-point, (c)comparative, (d)
gas flow (Ergun, (e) photographic, (f) powder dis-
placement, (g) minimum fluidization velocity. All
except (b) can also be applied to non-porous materials.
•The methods vary considerably in the cost and
complexity of the equipment needed, in the time
required to complete an evaluation, and in the size of
particles for which they are most suitable.
•A qualitative comparison of the methods is given in
Table

14. .
The comparative method is based on the assumption
that the minimum packed bed voidage is virtually the
same for particles having the same narrow size range
and similar particle shape.
ε = Bed volume – Particle volume
Bed volume
i.e. ε = 1 - M/ρp
VB
(a)
or ε = 1 - ρB
/ρp
(b)
(εc
)min
= (εx
)min
(c)

15. A non-porous powder of known particle density,
ρpc
is used as a control powder (about 0.2-0.25
kg).
It is put into a cylinder with a volume of at least
l00 cm3, height ≈ diameter, fitted with an open-
ended plastic extension piece.

16. •This is overfilled and then tapped mechanically 480
times. When tapping ceases the extension is
carefully removed, the excess powder scraped off,
and the bulk density, ρBTC
determined (see
Abrahamsen and Geldart 1980).
•The procedure is repeated with the unknown porous
powder X to give ρBTX
. Then:
From (c), ρpx
= k (ρBTX
/ρBTC
)ρpc
(d)

17. The empirical factor k is introduced because
in practice it is not always possible to find a
control powder having the same particle
shape as that of the unknown powder.
k = 1 if x and c are approximately the same
shape
k≈ 0.82 if x is rounded or spherical and c is
angular
k≈ 1/0.8 if c is rounded or spherical and x is
angular

18. BULK DENSITY
•The bulk density of a powder is its mass divided by
the bulk volume it occupies.
•The volume includes the spaces between particles as
well as the envelope volumes of the particles
themselves.
•The bulk density depends very much on the state of
compaction and on the size, size distribution and
shape of the particles, and any changes to these
parameters caused by degradation will be reflected in
the bulk densities.

19. However, to be of any use for monitoring purposes, measurements
must be made in the standardised piece of equipment using a
standardised procedure which is operator-independent.

20. •The bulk densities which can be measured
are: aerated or most loosely
-packed bulk density, poured B.D. , tapped
B.D. , and compacted B.D.
•The two most useful bulk densities for
characterising powders are the aerate
and tapped B.D.s.
•For fine powders, the ratio of tapped to
aerated B.D. can give a measure of the
powder flowability/cohesivity.

21. Nisbah Hausner
Nisbah Hausner, HR ialah nisbah ketumpatan
pukal terketuk, ρBT
kepada ketumpatan pukal
terudara, ρBA
. Ia memberi satu ukuran
kebolehaliran sesuatu zarah dan kejelekitannya.
Menurut Geldart (1986), kriteria yang ditunjukkan
di Jadual 1.7 memberi pengelasan zarah mengikut
nisbah Hausner masing – masing.

22. Table 1.7: Particle classification based on Hausner ratio (Geldart
1976)
HR Particle
classification
HR > 1.4
1.25 < HR < 1.4
HR < 1.25
Group C
GroupAC
A, B, D

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