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1.1  Background to the Work

Mixing is the process of thoroughly combining different materials to produce a homogenous product. To formulate the requisite paint, a combination of dissimilar materials such as pigments, binders (resin), solvent (sometimes called thinner), and other additives are blended in a specified ratio. In other cases, a chemically homogenous material may be mixed to produce a uniform lot of the desired weight/volume with consistent particle size distribution, color, texture, viscosity and other required attributes [1]. Agitation is the principal means of stirring a solution to attain uniform mixture. It is a means by which mixing of phases can be accomplished and mass and heat transfer is enhanced between phases or with external surfaces [2]. In a more general sense, the process of mixing is concerned with all combinations of phases of which the most frequently occurring ones are:

i.                     Gases with gases.

ii. Gases into liquids by dispersion.

iii. Gases with granular solids by fluidization, pneumatic conveying, or drying. 

iv. Liquids into gases by spraying and atomization.

v. Liquids with liquids by dissolution, emulsification, or dispersion.

vi. Liquids with granular solids by suspension.

vii. Pastes with each other and with solids.

viii. Solids with solids by powder mixing.

The mixing processes involving liquids, namely; gases into liquids by dispersion, liquids with liquids by dissolution, or emulsification, or dispersion, and liquids with granular solids by suspension employ the same kind of equipment in the form of tanks in which the liquid is circulated and subjected to certain amount of shear. This kind of equipment has been studied most extensively [2]. Yet, some unusual cases of liquid mixing may require pilot plant/equipment. This study will concentrate mainly on these three processes involving liquids as it accommodates the nature of paint. Uniform blending of two or more ingredients is usually the reason for mixing. Lack of uniformity in requisite blending can affect the reaction, stability, durability efficiency and other parameters of the resultant product. Therefore it is important to consider the major factors in how uniformity is achieved and the consequences of achieving it. These factors include:

i.                      Time and techniques of loading, mixing, cleaning, and discharge.

ii.                    Maintenance downtime.

iii.                   Power consumption.

iv.                   Level of contact of product with mixer elements, and mixer design.

v.                    Level of exposure allowed for the product.

The type of operation and equipment used during mixing depends on the state of materials being mixed-liquid, semi-solid, or solid and the miscibility of the materials being processed [3]. In this context, mixing of paint solution of low viscous liquid is synonymous with stirring processes. Because of diversity of fluid mixing applications and variety of vessels, many different types of mixers are used in industrial applications. Mixer sizes vary in horse power ratings from as low as less than one horse power portable types to as large as 1000-horse power capacity types. Although mixers are normally viewed as a single piece of equipment, a typical mixer is composed of several individual components such as a motor, gear reducer, seal, shaft, impellers, and tank, which are designed and purchased separately [4]. The most common mixing equipment is the impeller type. It represents the largest category of general purpose mixing equipment for fluid processing applications such as paint mixing. From the process view, impeller type equipment is usually composed of blades mounted to a central hub and rotated by a drive shaft, which pushes and moves the material to be mixed. The mixing action results in fluid motion [4].   Figure 1.1 shows a typical impeller type mixing equipment and the dynamics of the fluid being mixed.

Fig 1.1: Impeller-type mixing agitator [3]

The major characteristics of effective mixing include the necessity of performing the process to make the liquid experience all kinds of movement inside the container from downwards to upwards and vice versa, cyclic, diagonal, etc [3]. Figure 1.2 shows various types of agitator flow patterns.

(a)                                                  (b)                                (c)                                      (d)

Figure 1.2: Various types of agitator flow patterns [2]

(a)    Axial or radial impellers without baffles produce vortexes, (b) Off-center location reduces the vortex, (c) Axial impeller with baffles, (d) Radial impeller with baffles.

Although impeller-type agitators and other forms of agitators are very effective in industries today, there exist some peculiar situations that require special design of agitators to meet agitation needs for which the existing types are not suitable. Two major factors for consideration when designing mixing equipment is the level of contact of product to be mixed with mixer elements and level of exposure of the product to environmental factors. It is very important to reduce or eliminate contact between products and mixer elements for products that are used intermittently and are always mixed each time before they are used. This reduces or eliminates the risk of contamination that will affect the mixture during its storage period. Also when designing mixer for fluids that are volatile or have volatile components, it is very important to reduce or eliminate exposure of such fluids during mixing. These two factors are very critical for consideration when designing mixer for mixing fluids such as paint to be used for a long period of time and are mixed each time they are to be used.  Mechanical agitator can be designed to induce motion of paint solution in a random way to stir it up and disperse it evenly within the container to cause the paint solution to maintain an approximately uniform viscosity without having any form of contact with the paint itself.

In the current economic and political climate in Nigeria, there have been enormous attention focused on the need to develop local technology that meet international standard in terms of efficiency safety and sophistication.  The mechanical paint tank agitator is borne out of the yearning to create something innovative to meet local needs using locally sourced material.

Agitation is a critical process in painting because the quality of the final product and its attributes are derived by the quality of the mix. Improper paint mixing results in non-homogenous product that lacks consistency with respect to desired attributes such as: chemical composition, colour, texture, reactivity, and particle size. It also has negative effect on the functionality of most painting tools such as the spray gun which experiences pulsating discharge of paint at the nozzles due to blockage by the un-dissolved paint as a result of improper mixing.

The wide variety and ever increasing complexity of mixing processes encountered in industrial applications require proper selection of agitators and scaled up designs to ensure effective and efficient mixing. Improved mixing efficiency leads to shorter batch cycle times and reduced operational costs. Today's competitive production lines necessitate robust equipment that are capable of fast blend times, lower power consumption, equipment flexibility, ease of cleaning, and a gamut of customized features. A paint mixer is not generic production tool, but a critical and decisive business tool. This is because profitability and competitive advantage are dependent upon subtle improvements in product quality through gains in mixing performance and efficiency. A recent study estimates world over annual cost of poor paint mixing to as high as 100 million dollars [1].

1.2  Problem Statement

In most painting plants, the economy volume for paint procurement is in drums of more or less 200-litre capacity, because of both cost and handling advantages during external and internal logistics. The paints in such drums are sometimes stored for durations of up to months or years in the drums and fetched therein and mixed in smaller quantities for paint work, with most paint still in stock in the drums. Whilst the paint at the top of the drum was being fetched for agitation in different equipment depending on the daily work need, the paint at the bottom of the drums tends to form sediments. When this happens the paint that is continued to be fetched from the top of the drums may not be assured of true composition and properties of the original and intended paint.  Defects such as; colour variation, loss of gloss, and low paint film thickness can be found on surfaces painted with paints from such stock. Observation has shown that though mixing of paint in smaller mixing pots are sufficient; there is often variation in the properties of the paint that is continued to be fetched from the drums and taken to the smaller mixing pot. To solve this problem there is need to properly agitate the whole batch of stored paint in each drum without adding solvent to ensure that the paint is evenly dispersed with consistent viscosity before fetching the paint to smaller mixing pot to be diluted to required viscosity and mixed for use. This will ensure that the paint fetched from the drum has the true composition and properties of the original and intended paint, also that the paint-solvent mixing ratio will be consistent in the smaller mixing pots.

Over the years various methods have been adopted for agitating paint before mixing it in smaller batches, most prominent of this, is turning out the whole drum of paint into a large cylindrical impeller type mixing pot to do the initial mixing of the paint without diluting it before transferring to smaller secondary mixing pot to dilute and carry out final mixing. However there have been many problems with this method, such as;

        i.            The much labour required to turn out a whole drum of paint to be mixed before taking just a small quantity to be diluted for final use and returning the rest to the drum for storage.

      ii.            The high of man hours required to carry out this operation in the pre-mixing operation.

    iii.            Susceptibility to loss of fluidity and development of lumps of congealed paint within the container when paints are frequently exposed to atmosphere. This affects the mixing efficiency and quality of the paint stored for future use.

    iv.            Loss of paint due to the process as some paint must be left in the larger mixing pot each time it is used and some pour off.

      v.            Capital intensiveness as it involves the use of additional equipment such as pumping facility to transfer paint back and front between the storage tank and the primary mixing equipment and final mixing pots.

These problems of paint agitation in small or large plants therefore remains challenge to design and produce a reliable and durable inexpensive agitator that will facilitate mixing paint in tanks as desired to reduce or eliminate wastages and costs and conserve quality of paint stored therein. 

1.3  Aims and Objectives of Study

The aim of this work is to design and simulate a mechanical agitator that can be used to supplement paint mixing with standard plant facilities for cases where there are mixing shortcomings that results in defects such as undesirable texture, gloss, colour, etc to paint finishes on works. The objectives of the work are:

  1. To design an agitator capable of mixing paint within its storage drum without having contact with the paint itself.
  2.  Reduce exposure period of the paint to the barest minimum to make it possible to store the paint as desired without experiencing changes in its properties and quality even with time to time taking out of pain portions for agitation in the standard agitator.
  3. To provide solution to a typical problem at Pan Nigeria Limited plant at Kaduna and pose the innovative idea and ability for solution to similar mixing problems elsewhere they exit.

1.4 Significant of the Study

The significant of the study includes:

  1. Enable paint finishing companies to procure paint at economy volume and store for use over a long period of time without fear of the paint losing its properties (quality) during usage.
  2. The study will enable the development of local technology using readily available material.
  3. The study will contribute to enhancement of modern engineering design skill that help reduce cost of research and development through the use of simulation testing instead of numerous trial production.
  4. The study will help to boost the economy by helping to reduce cost of production and expanding affordability.

1.5  Summary of Methodology

i.                    Conduct literature review on various agitation techniques and methods used for mixing different types and volumes of solutions in numerous areas that uniform mixture is of great importance.

ii.                  Selection of the most suitable and realistic approach among various agitation techniques and procedures for pre-mixing of paints used for surface painting.

iii.                Establishment of objective and criteria, synthesis, analysis, modelling and simulation testing of the agitator.

iv.                Specifically design an agitator for a drum and its content up to the mass of 200Kg.

v.                  Conduction of simulation tests of the agitator using a theory of failure criterion using the ‘Solidworks software 2014 version’ to validate its strength and overall integrity.

1.6 Scope of the Study

This study is limited to the design and simulation test of a mechanical paint agitator capable of the following functions:

  1. Agitate 200 litres of paint within its container
  2. Agitating the paint without having contact with the paint

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