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SOURCE OF ENGINEER
SOURCE OF ENGINEER

Mixing - Blending - Conditioning
Mixing - Blending - Conditioning

- Granulating Technology

The mixing of solids in powder or granular form is a Depending on the loading and unloading conditions, the mixers
strictly physical unit operation, which has the aim of are classified as continuous and discontinuous (or batch). In the batch
homogenising the composition of a mixture consisting mixer the particles move randomly throughout (or almost throughout)
the mixing chamber, and tend to form a homogenous mixture as time
of two or more solids. goes by; on the other hand, in the case of a continuous mixer the
particles are subjected to random motion in a radial direction and
Mixing is brought about by three different mechanisms due to axial motion that is limited to the maximum. In batch mixers the mixing
diffusive phenomena, convective phenomena and cutting forces. The time is determined by the rotation time of the tools, while in continuous
equipment used for this unitary operation, indicated by the term mixers it varies according to the resistance to flow that the product
mixers, can be classified into moving casing mixers and fixed casing encounters while passing through the mixer; so theoretically, the stay
mixers. time is the same for all the particles.
A moving casing mixer, or rotating drum, consists of a casing of
varying shape (cylindrical, conical, double cone, etc.), sometimes The choice of a continuous or batch mixing system
fitted with internal baffles, which, by rotating around an axis, bring is determined by a large number of factors, mainly:
about the mixing mainly by means of diffusive phenomena. Mixing 1. The type of system in which the mixer is to be inserted
by diffusion is ideal for particulate solid mixtures in which the (continuous or batch);
constituents have the same physical characteristics (especially 2. The properties of the components, especially the size
particle size and density). distribution and the shape of the particles, their specific weight, the
For materials, which tend to show high degree of cohesion or flow characteristics, moisture content, the tendency to cohere or
form agglomerates, as well as materials subjected to segregation, agglomerate, their adhesiveness to the walls, possible degradation
the equipment adopted must be capable of high cutting and convection if subjected to cutting forces, the tendency to segregation phenomena;
actions, like a fixed casing mixer. In a fixed casing mixer, mixing is 3. The flexibility required by the plant and evaluation of possible
brought about by using internal moving tools. cross-contamination, which requires cleaning with every change in
the mixer operation;
The mixing quality is mainly affected by many 4. The space available.
factors; If the mixer is provided with a jacket for heat exchange, the drying,
Those factirs are including the average speed of the mass, the heating and cooling operations can be carried out simultaneously
characteristic dimension of mixing chamber, the length and shape of with the mixing process. A service fluid (water vapour, water or
tools, and density, dynamic viscosity, granulometry and adhesiveness diathermic oil) is made to circulate through the jacket which, in the
of the ingredients. presence of a suitable temperature gradient between solid product
Mixing of solids generally never gives a perfectly uniform and service fluid, guarantees heat exchange through the mixer walls.
distribution of the components; the aim is to create a global uniformity The humidification of solids can be brought about in different
of the mixture (random mixing). ways, depending on the type of operation to be carried out on the
A rotary blade mixer consists of a fixed cylinder inside which a solid product and the nature of liquid to be added; it can be done by:
rotating horizontal shaft is present, which is fitted with ploughshare 1) Injecting liquid into the solid material inlet section of the
or arc-shaped blades; the rotation of the blades allows the material mixer;
to be lifted from the drum walls and moved vigorously. Often, the 2) Injecting liquid in the mixing chamber;
mixer is provided with devices capable of breaking the lumps present. 3) Injecting liquid at the mixture outlet section of the mixer.
A ribbon mixer consists of a horizontal trough, which houses a If, however, it is necessary to add small quantities of
ribbon agitator, but usually, two concentric ribbon agitators contra liquid into the product to be distributed uniformly
pitches. throughout the mass, the liquid may be injected
Mixing takes place mainly by convection, based on a gentle radial together with air; this will improve nebulization of
movement; axial movement is limited and accomplished by some the ingredient and fluidisation of the mass.
shear. Special care must be taken while selecting
A double-shaft mixer consists of a mixing chamber inside which the material used for constructing the mixers; the
are placed two shafts rotating in opposite directions at different operating conditions, especially for
speeds, on which alternating blades are fitted, so as to create a fluid continuous fixed casing mixers
bed of material above the area of intersection of the two shafts, where operating at very high speeds, can
the mixing occurs mainly due to convection and diffusion. In this case, result in excessive wear of the
the material is treated gently, is conferred with limited amount of moving mechanical devices.
energy and subjected to limited abrasion.
62 FOOD FOCUS THAILAND JUN 2018