SAJJAD KHUDHUR ABBASCeo , Founder & Head of SHacademyChemical Engineering , Al-Muthanna University, IraqOil & Gas Safety and Health Professional – OSHACADEMYTrainer of Trainers (TOT) - Canadian Center of Human Development

Episode 37 : SAMPLING

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.

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.

•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.

• 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.

•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.

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, p

 p = Mass of a particle, M

Hydrodynamic envelope bounding particle volume,Vp

Open pores

Closed pores

Hydrodynamic envelope bounding particle volume, Vp

Fig. 2 Defination of particle density

•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.

•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

1.

.

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.

= Bed volume – Particle volume

Bed volume

i.e. = 1 - M/pVB (a)

or = 1 - B/p (b)

(c)min = (x)min (c)

A non-porous powder of known particle density, pc 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.

•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, BTC determined (see Abrahamsen and Geldart 1980).

•The procedure is repeated with the unknown porous powder X to give BTX. Then:

From (c), px = k (BTX /BTC)pc (d)

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

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.

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.

•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 aerateand tapped B.D.s.

•For fine powders, the ratio of tapped to aerated B.D. can give a measure of the powder flowability/cohesivity.

Nisbah Hausner

  Nisbah Hausner, HR ialah nisbah ketumpatan pukal terketuk, BT kepada ketumpatan pukal terudara, BA. 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. 

Table 1.7: Particle classification based on Hausner ratio (Geldart 1976) 

HR Particle classification

HR > 1.41.25 < HR < 1.4HR < 1.25

Group CGroupACA, B, D

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