07 Mixing - Food Process Engineering Technology

8/4/2019 07 Mixing - Food Process Engineering Technology

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7 Mixing

Mixer paling sederhana untuk fluida adalah

paddle mixer(Figure 7.1), terdiri atas satu atau

dua pasang pisat datar terikat pada poros.

Mixerpaddle dan anchorberoperasi pada

kecepatan rendah (puluhan rpm).

Dalam mixer turbin (Figure 7.3), impeller

terdiri atas empat pisau atau lebih yang terikat

pada poros (biasanya) vertikal.


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Kneading

Under this heading, we group intensive mixingoperations applied to dough and paste-likeproducts.

Mixers used for this type of applications arecharacterized by their ability to deliver highrotational momentum.

Despite the relatively low rotational speed,the energy input per unit mass of product isconsiderable.


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Since the mechanical energy transferred to

the material is finally converted to heat, it isoften necessary to provide kneading machines

with cooling jackets to prevent overheating.

Kneading machines include the planetarymixers (Figure 7.8), the horizontal dough

mixers (Figure 7.9), the sigma-blade mixers

(Figure 7.10) and the cutter mixers.

Mixing and kneading are also among the

effects performed by extruders.


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Equipment for mixing particulate

solids

Mixers for powders fall into two categories:

1. Diffusive (passive) mixers: in this type of mixer,homogeneity is achieved as a result of the randommotion of the particles when the particle is in flowunder the effect of gravity or vibration, withoutmechanical agitation.

2. Convective (active) mixers: in this type of mixer,mixing is achieved by mechanical agitation. Parts of

the bulk material are conveyed with respect toeach other by the action of impellers or turbulentgas flow.


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The drum blender (Figure 7.11) consists of a

horizontal cylinder rotating about its axis. Itsmixing action is essentially diffusive. The powder

to be mixed is placed inside the drum. As the

drum rotates, the powder is lifted up, until the

angle of repose is exceeded. At that point, the

powder falls back on the rest of the bulk and

enters a new cycle of lifting and falling. Diffusive

mixing takes place during the residence of thepowder in air, while falling. Continuous operation

can be made possible by tilting the drum.


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Tumbler mixers also belong to the category of

diffusive mixers. In the double-cone tumbler(Figure 7.12), the powder undergoes cycles of

expansion and compaction as the vessel

rotates.


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In the V-shaped tumbler (Figure 7.13), the

powder is subjected to cycles of division andassembly. Convectional elements such as

rotating or stationary flow distortion bars

(intensifier bars) are sometimes installed inboth types of mixers.


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Homogenization

Homogenization of dispersed systems(suspensions and emulsions) involves reductionof the size of the dispersed particles.

Such size reduction is achieved by the action ofshearing forces.

Shear can be applied to the fluid by mechanicalagitation, by forcing the fluid to flow at very high

velocity through a narrow passage, by shearingthe fluid between two surfaces moving one withrespect to the other or by ultrasonic vibrations.


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Homogenizers

The shear forces required for homogenization

can be generated in different ways.

Accordingly, different types of equipment are

used for homogenization. These are:

1. High shear mixers

2. Colloid mills

3. High pressure homogenizers


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Berk, Zeki (2009). Food Process Engineering

and Technology. Elsevier.

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07 Mixing - Food Process Engineering Technology