Page 170 - Instrumentation Reference Book 3E

I1 Particle sizing

W. L. SNOWSILL

11.1 Introduction 11.2 Characterization of
particles
The size of particles is an extremely important
factor in their behavior. To name but a few Most particles are not regularly shaped so that it
examples, it affects their chemical reactivity, their is not possible to describe the size uniquely. To
optical properties, their performance in a gas overcome this problem, the standard procedure is
stream, and the electrical charge they can acquire. to use the diameter of equivalent spheres. However,
The methods used for assessing size are often an irregularly shaped particle can have an almost
based on one or more of these effects. limitless number of different equivalent spheres
Particulate technology is a complex subject, depending on the particular parameter chosen for
and the major factor in this complexity is the equivalence.
variety of the physical and chemical properties For example, the diameter of a sphere with an
of the particles. What appears to the naked eye equivalent volume would be different from that
as a simple gray powder can be a fascinating with an equivalent surface area.
variety of shapes, colors, and sizes when viewed Consider a cubic particle with edge of length x.
under a microscope. Particles can be solid or The diameter of an equivalent volume sphere
hollow, or filled with gas. The surface structure, would be ~(6/7r)’/~, i.e., 1.24~. The diameter of an
porosity, specific gravity, etc., can have a profound equivalent surface area sphere would be x(6h) ”’,
effect on their behavior. Their ability to absorb Le., 1.38~. The chosen equivalent is usually related
moisture or to react with other chemicals in the to the method of analysis. It is sensible to select
environment or with each other can make hand- the method of analysis to suit the purpose of the
ling very difficult as well as actually affecting the measurement but in some cases this is compli-
size of the particles. The size analyst has to cated by practical and economic considerations.
combat the problem of particles adhering to each Sometimes, the equivalent diameter is not par-
other because of chemical reactions, mechanical ticularly relevant to the process, whereas the
bonding or electrostatic charging, and the prob- actual measurement made is relevant. In such
lem increases as the size decreases. At the same cases, the size is sometimes quoted in terms of
time he must be aware that the forces applied to the parameter measured. A good example of this
keep them separate may be enough with friable is terminal velocity (see Section 11.3). If, for
particles to break them. example, information is required to assess the
Sampling is a crucial factor when measure- aerodynamic effect of a gas stream on particles,
ments are made on particles. The essential points terminal velocity is more relevant than particle
are: size. Even if the particles are spherical, conver-
sion can be complicated by the possible variations
in particle density. The term “vel” is sometimes
(a) To be of any value at all, the sample must be used to denote particle size. A 1 -vel particle has a
representative of the source. free falling speed of lOmms-’ in still dry air at
(b) Steps must be taken to avoid the samplechang- s.t.p.
ing its character before or during analysis. It is important when equivalent diameters are
(c) Particulate material when poured, vibrated, quoted that the basis (equivalent mass, volume,
or moved in any way tends to segregate itself. surface area, projected area, etc.) is clearly stated.
The coarser particles tend to flow down the
outside of heaps, rise to the top of any vibrat-
ing regime, and be thrown to the outside 11.2.1 Statistical mean diameters
when leaving a belt feeder. These factors need
to be given careful consideration especially Microscopic examination of an irregularly shaped
when attempting to subdivide samples. particle suggests other methods of assessing