Type of Mixers and the Factors That Effects the Mixing

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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 AND FACTORS THAT AFFECT THE

MIXING OPERATIONS]

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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.

Documents

Type of Mixers and the Factors That Effects the Mixing