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Type of Mixers and Factors that Affect theMixing OperationsPrepared by
Muhammad Zubair
SCME-NUST
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Type of Mixers
Introduction:
In process engineering, mixing is a process in which a
heterogeneous physical system changes in to homogeneous system with
the help of different types of mixers. It is a unite operation which involves
the movement of molecules of component A in the molecules of the
component B to form the homogeneous system. In the mixing process we
cannot achieve the 100 % homogeneity. In this way molecule of component
A and B does not appear in alternative manners at all. To find the mixing
efficiency of the system we generally use the spot sampling method. In this
method we take the number of spot sample from the system and calculate
the mixing index by calculating the Standard deviation. The mixing index
tells us the homogeneity of the entire batch.
In the mixing, three mixing phenomena are employed.
1: Convective Mixing
In convective mixing, a group of particles move from one
place to other place in the system. It is mostly use in large scale mixing
when we have a large quantity of both the components that are being
mixed.
2: Diffusion Mixing
In the phenomena, the partials are moved over a newly
formed interface. In the binging of the mixing when we have two
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components separately, we have two faces at this stage the mixing will
occur due to diffusion.
3: Shear Mixing
When slipping plane is formed in the mixing, then the mixing
will occur by the shear mixing.
According to our purpose of mixing of particles, powder and
solids, we select the mixers on base of these properties, Particle size,
density of the solid(bulk density ),density of the particle(true
density),viscosity, surface tension, moisture content, flow pattern, etc.
On the basic of our mixing there are two basic types of mixers.
1: Mixers for Cohesive solids
2: Mixers for non-cohesive solids
Mixers for Cohesive SolidsDue to the effect of the force of cohesion and behavior of the solids,
Selection of the cohesive solids mixer demands the special consideration.
Mixing of such solids is done by the compression folding, shearing and
stretching elements. The mixing of the types of solids required the large
scale power consumption.
Change-Can Mixer:
This mixer is use when we have to mix the high viscosity
liquids and light viscosity pasts like wood and paints. It have further two
types.
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1: Pony Mixer
In mixing these mixers rotates and produce the centrifugal
force. This centrifugal force is used as the mixing force.
2: Beater Mixer
In this mixer the mixing is done by the agitation of the
agitator. A shaft and blades or paddles are twisted and rotated in the mixer.
After the mixing, blades can be lifted out and material can remove.
Paddle Mixer:
These mixers are simply a open air chamber and the
paddle assembly. The paddles agitators scooping the material and drop
this material again in the open chamber. In this mixer we gain the mixing
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level by relocation of the material. The size of the blades is adjusted to the
desire space in the chamber. By the cross mixing paddles in this mixer we
gain the maximum homogeneous material. This mixer is good for thatmaterial whose densities differ very largely. The construction of the paddles
and blades is in such a way that we can get shear mixing as well.
Muller Mixers:
Muller mixer is mostly use for mixing of solids and pastes. It
mixes the material by rubbing, skidding and smearing to gain the maximum
level of mixing. Its working principle is like the mortar and pestle
arrangement. This mixer has many types. The most common types are
countercurrent pan Muller mixer, stationary pan mixer and rotating pan
mixer. The wheel in the pan rotates with the help of motor and mixing is
done by the shearing action .This shearing action cause the size reduction
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and mixing of the material
Kneader Mixers:
It is usually use to mix the plastic and any other deformable
mater by squashing the material flat and folding again and again. The
power consumption in this mixer is very large.
The kneader mixer with two arms having low power requirement is use for
suspensions and paste. A kneader with heavy body and large powe
consumption is mostly useful for the colors and additives.
The internal side of the mixing chamber is sealed during the working
condition and disperses the mixing material with water. The moving speed
of this mixer is 35 to 45 rpm. It works in batch and continuous conditions.
Mixers for Non-Cohesive Solids:
There are many types of mixer for Non-Cohesive Solids. These mixers are
used to mix the large variety of material form powder to the heavy high
density materials. The detail study of this type mixer is given below.
Ribbon Mixer:
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It is commonly used for dry mixing. Fine material, light and
sticky feed can b handle by this type of mixer. It operates in the continuous
manners as well as in batch. It consists of a horizontal shell with 2 ribbonsof mixers moving with the help of electric motor. The two ribbons moves
with the different velocities inter mix the batch of feed. It size could be large
to 34 meters and the length of the ribbon favor in the good mixing. The
cross sectional area, clearance, shell size and number of spiral are
important designing parameters in the ribbon mixer construction.
Tumbling Mixers:
It construction is like the construction of the tumbling mill. It
consists of an empty vessel rotates about its on axis. First of all feed in
introduce in the mixer and it rotates by the help of the electric motor and
gear assembly. It can handle heavy solids and dense slurries. In the mixer,
mixing is done by the diffusion.
Baffles can be introduced in the internal surface of the mixer to gain
the desire level of mixing. This mixer is run for a specify time for mixing
because the Excess time may decrease the degree of mixing.
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Internal-Screw Mixer:
It consists of screw rotating and vessel arrangement. In this mixer,
mixing is done by the circulation and elevation of the material. This
arrangement results the mixing of the solid gain as well as shear action to
contact with the wall of vessel or screw. The feed is introduced from the
bottom of the vessel and it get mix and moves upward with the help of
screws. Conical and cylindrical shapes vessels are used in these types of
mixers. There are three further types of this mixer.
1:
It consists of conical vessel and the screw in the center of the vessel. The
screw rotates around its own axis.
2:
It consists of the same conical vessel and having screw in the center. The
conical tank is in the way that with the increase in the height swept area
increase.
3:
It consists of a cylindrical vessel and the screw in the center of the vessel.
The screw rotates around its own axis.
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Other Types:
Some other types used in the industries for a specific mixing:
Mobile Mixers
Vacuum Mixer
Jet Mixer
Double & Triple Shaft
High Shear Rotor Stator
Paddle
V Blender
Double Cone Blender
High Viscosity Mixer
Double and Triple Shaft
Planetary mixer
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Factors that affect the Mixing OperationsThere are the factors that affect the mixing of the solids. These factors
include:
Surface of The material:
If we have to mix two components which have different nature of
the surface .Then the particles of one component that have plan surface
can enter to the active sites of the particle of the other component which
have rough surface. In this way we can note get the suitable degree of
mixing.
Size of the particle:
Size of the particle plays an important role in the mixing operation.
If we have to mix the two component of different sizes. Then the particles ofsmall size can move from the spaces between the larger particles and can
settle down in the bottom. In this way we cannot achieve the good mixing.
The mixing of the power of the same size is very easy.
Shape of Particle:
The ideal shape of the particles for good mixing in the sphere. It is
difficult to mix the particles that have very less sphericity.
Density:
Density of the particles also effects the mixing. If there is a
difference in the densities in the particles then the particles having greater
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density eill move down and settle down. In this way we cannot get the good
mixing.
Charge on Particle:
Mixing is very difficult if we have to mix the particles which have
electrostatic forces of attraction. Then charges particles can be attract or
repel by each other and separation of particles can be occurred.
Proportion Of material:
We can get the ideal mixing if we have to mix the two materials inthe equivalent proportion. If the quantity of one component is very less than
the other. Then the lesser quantity cannot get mix with the other larger
quantity component. So we have to mix this type of material for a larger
time.
Time of mixing:
There is a suitable time of mixing for every mixture. If the mixing
time is very large then the particles can get separate as well. If the mixing
time is very low then we cannot get the suitable mixing. In the mixing, we
have to follow the graph of mixing index vs. time for the mixing time. If the
proper machine for mixing and proper capacity of material is chosen, then
the mixing time is not more than 15 minutes. There is always a machine for
every type of mixture which can mix the component in less than 15minutes.
Type of Mixer:
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For mixing of solid we have to choose the specific mixer. First of all
we check the nature of the material. If the material is cohesive then we
have to choose the mixer accordingly. On the other hand, if we have to mixthe non-cohesive material then we have other type of mixers. In fact
selection of mixer has its own importance in gaining the specific degree of
mixing.
Speed of mixer:
Speed of mixer also effects the mixing operation. For dens material,
low speed is considering the best. In fast speed the disturbance in the
system will be low and we cannot gain the good mixing.
Cleaning:
Cleaning of the mixer effect the mixing of the material in such a way
that if we dont clean the mixer after mixing operation. Then the material
can stuck in the mixer at different position like in corners, in seals and
stuffing boxes, in crevices at baffle supports. This material can affect the
next operation by mixing in the other material or it can reduce the space for
mixing in the equipment. In this way we cannot get the good mixing.
Type of mixing:
Basically there are two main types of mixing.
1. Dry mixing
2. Wet mixing
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Type of mixing also effect the mixing. If we have to mix that material, that
can burn at high temperature. Then we use wet mixing. In wet mixing first
of all we mix the material in a liquid then mix this mixture in other material.On the other side we use the dry mixing.