Mixing has been defined as an operation “in which two or more ingredients in separate or roughly mixed condition are treated so that each particle of any one ingredient is as nearly as possible adjacent to a particle of each of the other ingredients.”
Mixing
INTRODUCTION
Mixing
has been defined as an operation “in which two or more ingredients in separate
or roughly mixed condition are treated so that each particle of any one
ingredient is as nearly as possible adjacent to a particle of each of the other
ingredients.” (Perry and Chilton, 1999) The term “blending” is synonymous, and
“segregation” or “demixing” is the opposite.
Mixing
is a basic step in most process sequences, and it is normally carried out
1. to secure uniformity of composition, so that small
samples withdrawn from a bulk material represent the overall composition of the
mixture and
2. to promote physical or chemical reactions, such as
dissolution, in which natural diffusion is supplemented by agitation.
Danckwerts
(Dankwerts, 1953) classified mixing as follows:
Positive mixing, which applies to
systems that, given time, would spontaneously and completely mix. Examples are
provided by two gases or two miscible liquids, and mixing apparatus is used on
such systems to accelerate mixing.
Negative mixing, which is
demonstrated by suspensions of solids in liquids. Any two-phase system, in
which the phases differ in density, will separate unless continuously agitated.
Neutral mixing, which occurs when
neither mixing nor demixing takes place unless the system is acted upon by a
system of forces. Examples are found in the mixing of solids and of solids with
liquids when the concentration of the former is high.
Mixing
must embrace all combinations of the three states of matter. The theory of
mixing should be able, when the system to be mixed has been defined, to dictate
the type and design of the mixer, such as volume, shape and type of impeller,
and the process conditions, such as degree of agitation, and the time and power
required. Theoretical knowledge is, however, insufficient to predict the
performance of mixers. More commonly, choice is based on broad empirical
principles, which are then supported by practical tests.
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