1.0 Introduction
Mixers are equipments used to prepare a uniform combination of two or more substances. It is used in mixing operation which is very common operation carried out in chemical and process industrial. It may involve the mixing of solids, liquids or gases in any possible combination involving two or more of the above.
Mixing operation is an operation that is performed in order to bring about homogeneity or reduce degree of non-uniformity of a system. These are achieved to another or from one part of the region to another part of the system region, this intend to effect distribution, intermingling and homogeneity of matter.
The material fed to mixer may be low or moderate viscosity liquids, highly viscous Newtonian or non-Newtonian liquid plastic and deformed solids like rubber or dry free flowing powders and the uniform combination of two or more material from a mixer may be either homogenous or heterogeneous.
Component of mixer
The component of mixer include:
- The shaft
- The impellers
- The vessel or tank
The Shafts: These are the connecting rods that hold electric motor with a belt. The diameter of the shaft varies with the point of attachment in the driving motor and its length of the shaft varies with the height of the vessel. Shafts are used to transmit an induced motion from the driving motor to the impeller arrangement.
Also they are used in the location of impeller in an agitated vessel.
The Impellers
These are usually in form of flat blade turbine type either centrally located in the vessel or nearer to the bottom entry of the required material. They are also in axial flow or radial flow depending on the angle which the blade makes with the plane of the impeller rotation.
Vessel
Here the degree of uniformity or homogeneity operation is carried out, depending on the material involved such as solid and liquid or required and gas.
1.2 Problems Encountered During Mixing
There are problems that are mainly encountered in this process the commonest one is the overtaking or mass swirling of the liquid.
Overtaking do occur in the mixing processing and it causes reduction in difference between the fluid velocity and the impeller velocity and thereby decreases the effectiveness of the mixing process. (Vortex occurs when there is gas liquid surface or phases).
If the mixing vessel or container is small in size overtaking can be prevented by installing the impeller of centre or at angle with the vertical or fixing the impeller to the side of the vessel as an angle.
These problems can be prevented by choosing an appropriate mixer and taking all the precautions in mixing to avoid overtaking.
Note: Vortex occurs when there is gas-liquid surface in mixing and is prevented by baffle.
1.3 Purposes / Aims of Mixing
Substances or materials are mixed for a number of purpose depending on the objectives of the processing step, they include:
- Suspending solid particles
- Blending miscible liquids
- Promoting heat transfer between the liquids and a heat exchanger (e.g. coil or jacket).
- Dispensing a gas through the liquid in the form of small bubbles.
- Reducing partial agglomerate.
- Contracting or dispersing immiscible liquids.
Mixers generally have a number of characteristics in common they are under the range of:
- Change can mixers
- Kneaders
- Dispersers
- Masticators
1.4 Significance of Mixing
Mixing is a critical process because the quality of the final product and its attribute are derived by the quality of the mixer. Improper mixing results in a non-homogenous product that lacks consistence in this respect to desire attributes like chemical composition, colour texture, flavor reactivity and particle size.
The wide variety and ever increasing complexity of mixing processes encountered in industrial applications requires careful selection, design and scale up to ensure effective and efficient mixing. Improved mixing efficiency leads to a shorter batch cycle time and operational costs. Today's competitive production lines necessitate robust equipment that are capable of fast blend times, lower power consumption, equipment flexibility ease of clearing and a gamut of customized features. In addition to blending components many moderate mixers are designed to combine different process steps in a single equipment example, coating, granulation, heat transfer, drying, etc.
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